Hole Fermi surface in Bi2Se3 probed by quantum oscillations

NASA Astrophysics Data System (ADS)

Piot, B. A.; Desrat, W.; Maude, D. K.; Orlita, M.; Potemski, M.; Martinez, G.; Hor, Y. S.

2016-04-01

Transport and torque magnetometry measurements are performed at high magnetic fields and low temperatures in a series of p-type (Ca-doped) Bi2Se3 crystals. The angular dependence of the Shubnikov-de Haas and de Haas-van Alphen quantum oscillations enables us to determine the Fermi surface of the bulk valence band states as a function of the carrier density. At low density, the angular dependence exhibits a downturn in the oscillations frequency between 0∘ and 90∘, reflecting a bag-shaped hole Fermi surface. The detection of a single frequency for all tilt angles rules out the existence of a Fermi surface with different extremal cross sections down to 24 meV. There is therefore no signature of a camelback in the valence band of our bulk samples, in accordance with the direct band gap predicted by G W calculations.

Splitting Fermi Surfaces and Heavy Electronic States in Non-Centrosymmetric U3Ni3Sn4

NASA Astrophysics Data System (ADS)

Maurya, Arvind; Harima, Hisatomo; Nakamura, Ai; Shimizu, Yusei; Homma, Yoshiya; Li, DeXin; Honda, Fuminori; Sato, Yoshiki J.; Aoki, Dai

2018-04-01

We report the single-crystal growth of the non-centrosymmetric paramagnet U3Ni3Sn4 by the Bridgman method and the Fermi surface properties detected by de Haas-van Alphen (dHvA) experiments. We have also investigated single-crystal U3Ni3Sn4 by single-crystal X-ray diffraction, magnetization, electrical resistivity, and heat capacity measurements. The angular dependence of the dHvA frequencies reveals many closed Fermi surfaces, which are nearly spherical in topology. The experimental results are in good agreement with local density approximation (LDA) band structure calculations based on the 5f-itinerant model. The band structure calculation predicts many Fermi surfaces, mostly with spherical shape, derived from 12 bands crossing the Fermi energy. To our knowledge, the splitting of Fermi surfaces due to the non-centrosymmetric crystal in 5f-electron systems is experimentally detected for the first time. The temperature dependence of the dHvA amplitude reveals a large cyclotron effective mass of up to 35 m0, indicating the heavy electronic state of U3Ni3Sn4 due to the proximity of the quantum critical point. From the field dependence of the dHvA amplitude, a mean free path of conduction electrons of up to 1950 Å is detected, reflecting the good quality of the grown crystal. The small splitting energy related to the antisymmetric spin-orbit interaction is most likely due to the large cyclotron effective mass.

Surface effects in the unitary Fermi gas

NASA Astrophysics Data System (ADS)

Salasnich, L.; Ancilotto, F.; Toigo, F.

2010-01-01

We study the extended Thomas-Fermi (ETF) density functional of the superfluid unitary Fermi gas. This functional includes a gradient term which is essential to describe accurately the surface effects of the system, in particular with a small number of atoms, where the Thomas-Fermi (local density) approximation fails. We find that our ETF functional gives density profiles which are in good agreement with recent Monte Carlo results and also with a more sophisticated superfluid density functional based on Bogoliubov-de Gennes equations. In addition, by using extended hydrodynamics equations of superfluids, we calculate the frequencies of collective surface oscillations of the unitary Fermi gas, showing that quadrupole and octupole modes strongly depend on the number of trapped atoms.

Broken rotational symmetry on the Fermi surface of a high-Tc superconductor

DOE PAGES

Ramshaw, B. J.; Harrison, N.; Sebastian, S. E.; ...

2017-02-13

Broken fourfold rotational (C 4) symmetry is observed in the experimental properties of several classes of unconventional superconductors. It has been proposed that this symmetry breaking is important for superconducting pairing in these materials, but in the high-T c cuprates this broken symmetry has never been observed on the Fermi surface. Here we report a pronounced anisotropy in the angle dependence of the interlayer magnetoresistance of the underdoped high transition temperature (high-T c) superconductor YBa 2Cu 3O 6.58, directly revealing broken C 4 symmetry on the Fermi surface. Moreover, we demonstrate that this Fermi surface has C 2 symmetry ofmore » the type produced by a uniaxial or anisotropic density-wave phase. This establishes the central role of C 4 symmetry breaking in the Fermi surface reconstruction of YBa 2Cu 3O 6+δ , and suggests a striking degree of universality among unconventional superconductors.« less

Superconductivity across Lifshitz transition and anomalous insulating state in surface K-dosed (Li0.8Fe0.2OH)FeSe.

PubMed

Ren, Mingqiang; Yan, Yajun; Niu, Xiaohai; Tao, Ran; Hu, Die; Peng, Rui; Xie, Binping; Zhao, Jun; Zhang, Tong; Feng, Dong-Lai

2017-07-01

In iron-based superconductors, understanding the relation between superconductivity and electronic structure upon doping is crucial for exploring the pairing mechanism. Recently, it was found that, in iron selenide (FeSe), enhanced superconductivity ( T c of more than 40 K) can be achieved via electron doping, with the Fermi surface only comprising M-centered electron pockets. By using surface K dosing, scanning tunneling microscopy/spectroscopy, and angle-resolved photoemission spectroscopy, we studied the electronic structure and superconductivity of (Li 0.8 Fe 0.2 OH)FeSe in the deep electron-doped regime. We find that a Γ-centered electron band, which originally lies above the Fermi level ( E F ), can be continuously tuned to cross E F and contribute a new electron pocket at Γ. When this Lifshitz transition occurs, the superconductivity in the M-centered electron pocket is slightly suppressed, and a possible superconducting gap with a small size (up to ~5 meV) and a dome-like doping dependence is observed on the new Γ electron pocket. Upon further K dosing, the system eventually evolves into an insulating state. Our findings provide new clues to understand superconductivity versus Fermi surface topology and the correlation effect in FeSe-based superconductors.

Superconductivity across Lifshitz transition and anomalous insulating state in surface K–dosed (Li0.8Fe0.2OH)FeSe

PubMed Central

Ren, Mingqiang; Yan, Yajun; Niu, Xiaohai; Tao, Ran; Hu, Die; Peng, Rui; Xie, Binping; Zhao, Jun; Zhang, Tong; Feng, Dong-Lai

2017-01-01

In iron-based superconductors, understanding the relation between superconductivity and electronic structure upon doping is crucial for exploring the pairing mechanism. Recently, it was found that, in iron selenide (FeSe), enhanced superconductivity (Tc of more than 40 K) can be achieved via electron doping, with the Fermi surface only comprising M-centered electron pockets. By using surface K dosing, scanning tunneling microscopy/spectroscopy, and angle-resolved photoemission spectroscopy, we studied the electronic structure and superconductivity of (Li0.8Fe0.2OH)FeSe in the deep electron-doped regime. We find that a Γ-centered electron band, which originally lies above the Fermi level (EF), can be continuously tuned to cross EF and contribute a new electron pocket at Γ. When this Lifshitz transition occurs, the superconductivity in the M-centered electron pocket is slightly suppressed, and a possible superconducting gap with a small size (up to ~5 meV) and a dome-like doping dependence is observed on the new Γ electron pocket. Upon further K dosing, the system eventually evolves into an insulating state. Our findings provide new clues to understand superconductivity versus Fermi surface topology and the correlation effect in FeSe-based superconductors. PMID:28740865

3D Quantum Hall Effect of Fermi Arc in Topological Semimetals

NASA Astrophysics Data System (ADS)

Wang, C. M.; Sun, Hai-Peng; Lu, Hai-Zhou; Xie, X. C.

2017-09-01

The quantum Hall effect is usually observed in 2D systems. We show that the Fermi arcs can give rise to a distinctive 3D quantum Hall effect in topological semimetals. Because of the topological constraint, the Fermi arc at a single surface has an open Fermi surface, which cannot host the quantum Hall effect. Via a "wormhole" tunneling assisted by the Weyl nodes, the Fermi arcs at opposite surfaces can form a complete Fermi loop and support the quantum Hall effect. The edge states of the Fermi arcs show a unique 3D distribution, giving an example of (d -2 )-dimensional boundary states. This is distinctly different from the surface-state quantum Hall effect from a single surface of topological insulator. As the Fermi energy sweeps through the Weyl nodes, the sheet Hall conductivity evolves from the 1 /B dependence to quantized plateaus at the Weyl nodes. This behavior can be realized by tuning gate voltages in a slab of topological semimetal, such as the TaAs family, Cd3 As2 , or Na3Bi . This work will be instructive not only for searching transport signatures of the Fermi arcs but also for exploring novel electron gases in other topological phases of matter.

Theoretical reconsideration of antiferromagnetic Fermi surfaces in URu2Su2

NASA Astrophysics Data System (ADS)

Yamagami, Hiroshi

2011-01-01

In an itinerant 5f-band model, the antiferromagnetic (AFM) Fermi surfaces of URu2Si2 are reconsidered using a relativistic LAPW method within a local spin-density approximation, especially taking into account the lattice parameters dependent on pressures. The reduction of the z-coordinate of the Si sites results in the effect of flattening the Ru-Si layers of URu2Si2 crystal structure, thus weakening a hybridization/mixing between the U-5f and Ru-4d states in the band structure. Consequently the 5f bands around the Fermi level are more flat in the dispersion with decreasing the z-coordinate, thus producing three closed Fermi surfaces like "curing-stone", "rugby-ball " and "ball". The origins of de Haas-van Alphen branches can be qualitatively interpreted from the obtained AFM Fermi surfaces.

Shubnikov-de Haas Oscillations in LaTiO3/SrTiO3 Heterostructures

NASA Astrophysics Data System (ADS)

Veit, Michael; Ramshaw, Brad; Chan, Mun; Suzuki, Yuri

Emergent metallic behavior in heterostructures of the Mott insulator LaTiO3 and the band insulator SrTiO3 was observed for the first time more than a decade ago. It has often been compared to other oxide systems which have a two-dimensional Fermi surface, but there have been few studies probing the dimensionality of the metallicity in this system. We have studied the transport properties of thin films of LaTiO3 on SrTiO3 substrates. Our measurements have indicated that the entirety of the LaTiO3 film is conductive with an additional contribution near the interface. When the film thickness is on the order of 3-4 unit cells, we observe two sets of Shubnikov-de Haas oscillations - low frequency oscillations with a frequency of 2T and high frequency of 36T. We attribute the observation of these two sets of oscillations to a Rashba splitting which creates a smaller inner Fermi pocket and a larger outer Fermi pocket. These results are consistent with our measurements of in plane anisotropic magnetoresistance and a weak antilocalization correction to the magnetoconductance Further measurements on the angular dependence of the oscillations indicate that their frequency does not change, thus indicating that the Fermi surface is more three-dimensional.

Quasiparticles and Fermi liquid behaviour in an organic metal

PubMed Central

Kiss, T.; Chainani, A.; Yamamoto, H.M.; Miyazaki, T.; Akimoto, T.; Shimojima, T.; Ishizaka, K.; Watanabe, S.; Chen, C.-T.; Fukaya, A.; Kato, R.; Shin, S.

2012-01-01

Many organic metals display exotic properties such as superconductivity, spin-charge separation and so on and have been described as quasi-one-dimensional Luttinger liquids. However, a genuine Fermi liquid behaviour with quasiparticles and Fermi surfaces have not been reported to date for any organic metal. Here, we report the experimental Fermi surface and band structure of an organic metal (BEDT-TTF)3Br(pBIB) obtained using angle-resolved photoelectron spectroscopy, and show its consistency with first-principles band structure calculations. Our results reveal a quasiparticle renormalization at low energy scales (effective mass m*=1.9 me) and ω2 dependence of the imaginary part of the self energy, limited by a kink at ~50 meV arising from coupling to molecular vibrations. The study unambiguously proves that (BEDT-TTF)3Br(pBIB) is a quasi-2D organic Fermi liquid with a Fermi surface consistent with Shubnikov-de Haas results. PMID:23011143

Fermi arc mediated entropy transport in topological semimetals

NASA Astrophysics Data System (ADS)

McCormick, Timothy M.; Watzman, Sarah J.; Heremans, Joseph P.; Trivedi, Nandini

2018-05-01

The low-energy excitations of topological Weyl semimetals are composed of linearly dispersing Weyl fermions that act as monopoles of Berry curvature in the bulk momentum space. Furthermore, on the surface there exist topologically protected Fermi arcs at the projections of these Weyl points. We propose a pathway for entropy transport involving Fermi arcs on one surface connecting to Fermi arcs on the other surface via the bulk Weyl monopoles. We present results for the temperature and magnetic field dependence of the magnetothermal conductance of this conveyor belt channel. The circulating currents result in a net entropy transport without any net charge transport. We provide results for the Fermi arc mediated magnetothermal conductivity in the low-field semiclassical limit as well as in the high-field ultraquantum limit, where only chiral Landau levels are involved. Our work provides a proposed signature of Fermi arc mediated magnetothermal transport and sets the stage for utilizing and manipulating the topological Fermi arcs in thermal applications.

Fermi arcs vs. fermi pockets in electron-doped perovskite iridates

DOE PAGES

He, Junfeng; Hafiz, H.; Mion, Thomas R.; ...

2015-02-23

We report on an angle resolved photoemission (ARPES) study of bulk electron-doped perovskite iridate, (Sr 1-xLa x)₃Ir₂O₇. Fermi surface pockets are observed with a total electron count in keeping with that expected from La substitution. Depending on the energy and polarization of the incident photons, these pockets show up in the form of disconnected “Fermi arcs”, reminiscent of those reported recently in surface electron-doped Sr₂IrO₄. Our observed spectral variation is consistent with the coexistence of an electronic supermodulation with structural distortion in the system.

Towards a complete Fermi surface in underdoped high Tc superconductors

NASA Astrophysics Data System (ADS)

Harrison, Neil

The discovery of magnetic quantum oscillations in underdoped high Tc superconductors raised many questions, and initiated a quest to understand the origin of the Fermi surface the like of which had not been seen since the very first discovery of quantum oscillations in elemental bismuth. While studies of the Fermi surface of materials are today mostly assisted by computer codes for calculating the electronic band structure, this was not the case in the underdoped high Tc materials. The Fermi surface was shown to reconstructed into small pockets, yet there was no hint of a viable order parameter. Crucial clues to understanding the origin of the Fermi surface were provided by the small value of the observed Fermi surface cross-section, the negative Hall coefficient and the small electronic heat capacity at high magnetic fields. We also know that the magnetic fields were likely to be too weak to destroy the pseudogap and that vortex pinning effects could be seen to persist to high magnetic fields at low temperatures. I will show that the Fermi surface that appears to fit best with the experimental observations is a small electron pocket formed by connecting the nodal `Fermi arcs' seen in photoemission experiments, corresponding to a density-wave state with two different orthogonal ordering vectors. The existence of such order has subsequently been detected by x-ray scattering experiments, thereby strengthening the case for charge ordering being responsible for reconstructing the Fermi surface. I will discuss new efforts to understand the relationship between the charge ordering and the pseudogap state, discussing the fate of the quasiparticles in the antinodal region and the dimensionality of the Fermi surface. The author acknowledges contributions from Suchitra Sebastian, Brad Ramshaw, Mun Chan, Yu-Te Hsu, Mate Hartstein, Gil Lonzarich, Beng Tan, Arkady Shekhter, Fedor Balakirev, Ross McDonald, Jon Betts, Moaz Altarawneh, Zengwei Zhu, Chuck Mielke, James Day, Doug Bonn, Ruixing Liang, Walter Hardy. Supported by BES ``Science of 100 tesla'' program.

The electronic structure of the high-TC cuprates within the hidden rotating order

NASA Astrophysics Data System (ADS)

Azzouz, M.; Ramakko, B. W.; Presenza-Pitman, G.

2010-09-01

The doping dependence of the Fermi surface and energy distribution curves of the high-TC cuprate materials La2 - xSrxCuO4 and Bi2Sr2CaCu2O8 + δ are analyzed within the rotating antiferromagnetism theory. Using three different quantities; the k-dependent occupation probability, the spectral function, and the chemical potential (energy spectra), the Fermi surface is calculated and compared to experimental data for La2 - xSrxCuO4. The Fermi surface we calculate evolves from hole-like pockets in the underdoped regime to large electron-like contours in the overdoped regime. This is in agreement with recent findings by Sebastian et al for the α-pocket of Y Ba2Cu3O6 + x (2010 Phys. Rev. B 81 214524). In addition, the full width at half maximum of the energy distribution curves is found to behave linearly with their peak position in agreement with experiment for Bi2Sr2CaCu2O8 + δ. The effect of scattering on both the Fermi surface and energy distribution curves is examined.

Response of the Shockley surface state on Cu(111) to an external electrical field: A density-functional theory study

NASA Astrophysics Data System (ADS)

Berland, Kristian; Hyldgaard, Per; Einstein, T. L.

2011-03-01

We study the response of the Cu(111) Shockley surface state to an external electrical field E by combining a density-functional theory calculation for a finite slab geometry with an analysis of the Kohn-Sham wavefunctions to obtain a well-converged characterization. We find that the surface state displays isotropic dispersion, quadratic until the Fermi wave vector but with a significant quartic contribution beyond. We find that the shift in band minimum and effective mass depend linearly on E. Most change in electrostatic potential profile, and charge transfer occurs outside the outermost copper atoms, and most of the screening is due to bulk electrons. Our analysis is facilitated by a method used to decouple the Kohn-Sham states due to the finite slab geometry, using a rotation in Hilbert space. We discuss applications to tuning the Fermi wavelength and so the many patterns attributed to metallic surface states. Supported by (KB and PH) Swedish Vetenskapsrådet VR 621-2008-4346 and (TLE) NSF CHE 07-50334 & UMD MRSEC DMR 05-20471.

Electronic topological transitions in the AgPd system

NASA Astrophysics Data System (ADS)

Skorodumova, N. V.; Simak, S. I.; Smirnova, E. A.; Vekilov, Yu. Kh.

1995-02-01

“First-principles” LMTO-CPA calculations of the Fermi surfaces and thermodynamic properties of AgPd random alloys are presented. We show that there are at least four electronic topological transitions (ETT) in the system. The changes of the Fermi surface topology lead to the appearance of peculiarities in the concentration dependence of the thermodynamic (ground state) properties.

Secondary electron emission yield dependence on the Fermi level in Silicon

NASA Astrophysics Data System (ADS)

Urrabazo, David; Goeckner, Matthew; Overzet, Lawrence

2013-09-01

Secondary Electron Emission (SEE) by ion bombardment plays a key role in determining the properties of many plasmas. As a result, significant efforts have been expended to control the SEE coefficient (increasing or decreasing it) by tailoring the electron work function of surfaces. A few recent publications point to the possibility of controlling the SEE coefficient of semiconductor surfaces in real time through controlling the numbers of electrons in the conduction band near the surface. Large control over the plasma was achieved by injecting electrons into the semiconductor just under the cathode surface via a subsurface PN junction. The hypothesis was that SEE is dependent on the numbers of electrons in the conduction band near the surface (which is related to the position of the Fermi level near the surface). We are testing the validity of this hypothesis. We have begun fundamental ion beam studies to explore this possible dependence of SEE on the Fermi energy level using Si. Various doping levels and dopants are being evaluated and the results of these tests will be presented. This work was supported in part by US Dept. of Energy. Acknowledgement to Dr. L. Raja at UT Austin.

Phenomenological view at the two-component physics of cuprates

NASA Astrophysics Data System (ADS)

Teitel'baum, G. B.

2017-08-01

In the search for mechanisms of high- T c superconductivity it is critical to know the electronic spectrum in the pseudogap phase from which superconductivity evolves. The lack of ARPES data for every cuprate family precludes an agreement as to its structure, doping and temperature dependence and the role of charge ordering. No approach has been developed yet to address the issue theoretically, and we limit ourselves by the phenomenological analysis of the experimental data. We argue that, in the Fermi-liquid-like regime ubiquitous in underdoped cuprates, the spectrum consists of holes on the Fermi arcs and an electronic pocket in contrast to the idea of the Fermi surface reconstruction via charge ordering. At high temperatures, the electrons are dragged by holes while at lower temperatures they get decoupled. The longstanding issue of the origin of the negative Hall coefficient in YBCO and Hg1201 at low temperature is resolved: the electronic contribution prevails, as its mobility becomes temperature independent, while the mobility of holes, scattered by the shortwavelength charge density waves, decreases.

Quasiparticle interference of the Fermi arcs and surface-bulk connectivity of a Weyl semimetal.

PubMed

Inoue, Hiroyuki; Gyenis, András; Wang, Zhijun; Li, Jian; Oh, Seong Woo; Jiang, Shan; Ni, Ni; Bernevig, B Andrei; Yazdani, Ali

2016-03-11

Weyl semimetals host topologically protected surface states, with arced Fermi surface contours that are predicted to propagate through the bulk when their momentum matches that of the surface projections of the bulk's Weyl nodes. We used spectroscopic mapping with a scanning tunneling microscope to visualize quasiparticle scattering and interference at the surface of the Weyl semimetal TaAs. Our measurements reveal 10 different scattering wave vectors, which can be understood and precisely reproduced with a theory that takes into account the shape, spin texture, and momentum-dependent propagation of the Fermi arc surface states into the bulk. Our findings provide evidence that Weyl nodes act as sinks for electron transport on the surface of these materials. Copyright © 2016, American Association for the Advancement of Science.

Fermi-surface topologies and low-temperature phases of the filled skutterudite compounds CeOs 4 Sb 12 and NdOs 4 Sb 12

DOE PAGES

Ho, Pei Chun; Singleton, John; Goddard, Paul A.; ...

2016-11-28

We use MHz conductivity, torque magnetometer, and magnetization measurements to report on single crystals of CeOs 4 Sb 12 and NdOs 4 Sb 12 using temperatures down to 0.5 K and magnetic fields of up to 60 tesla. The field-orientation dependence of the de Haas-van Alphen and Shubnikov-de Haas oscillations is deduced by rotating the samples about the [ 010 ] and [ 0more » $$\\bar{1}$$ 1 1 ] directions. Our results indicate that NdOs 4 Sb 12 has a similar Fermi surface topology to that of the unusual superconductor PrOs 4 Sb 12 , but with significantly smaller effective masses, supporting the importance of local phonon modes in contributing to the low-temperature heat capacity of NdOs 4 Sb 12 . By contrast, CeOs 4 Sb 12 undergoes a field-induced transition from an unusual semimetal into a high-field, high-temperature state characterized by a single, almost spherical Fermi-surface section. Furthermore, the behavior of the phase boundary and comparisons with models of the band structure lead us to propose that the field-induced phase transition in CeOs 4 Sb 12 is similar in origin to the well-known α - γ transition in Ce and its alloys.« less

Quantum oscillations and nodal pockets from Fermi surface reconstruction in the underdoped cuprates

NASA Astrophysics Data System (ADS)

Harrison, Neil

2012-02-01

Fermiology in the underdoped high Tc cuprates presents us with unique challenges, requiring experimentalists to look deeper into the data than is normally required for clues. Recent measurements of an oscillatory chemical potential affecting the oscillations at high magnetic fields provide a strong indication of a single type of carrier pocket. When considered in conjunction with photoemission and specific heat measurements, a Fermi surface comprised almost entirely of nodal pockets is suggested. The mystery of the Fermi surface is deepened, however, by a near doping-independent Fermi surface cross-sectional area and negative Hall and Seebeck coefficients. We explore ways in which these findings can be reconciled, taking an important hint from the diverging effective mass yielded by quantum oscillations at low dopings. The author wishes to thank Suchitra Sebastian, Moaz Atarawneh, Doug Bonn, Walter Hardy, Ruixing Liang, Charles Mielke and Gilbert Lonzarich who have contributed to this work. The work is supported by the NSF through the NHMFL and by the DOE project ``Science at 100 tesla.''

Evidence of Cross-correlation between the CMB Lensing and the γ-Ray Sky

NASA Astrophysics Data System (ADS)

Fornengo, Nicolao; Perotto, Laurence; Regis, Marco; Camera, Stefano

2015-03-01

We report the measurement of the angular power spectrum of cross-correlation between the unresolved component of the Fermi-LAT γ-ray sky maps and the cosmic microwave background lensing potential map reconstructed by the Planck satellite. The matter distribution in the universe determines the bending of light coming from the last scattering surface. At the same time, the matter density drives the growth history of astrophysical objects, including their capability at generating non-thermal phenomena, which in turn give rise to γ-ray emissions. The Planck lensing map provides information on the integrated distribution of matter, while the integrated history of γ-ray emitters is imprinted in the Fermi-LAT sky maps. We report here the first evidence of their correlation. We find that the multipole dependence of the cross-correlation measurement is in agreement with current models of the γ-ray luminosity function for active galactic nuclei and star-forming galaxies, with a statistical evidence of 3.0σ. Moreover, its amplitude can in general be matched only assuming that these extragalactic emitters are also the bulk contribution of the measured isotopic γ-ray background (IGRB) intensity. This leaves little room for a big contribution from galactic sources to the IGRB measured by Fermi-LAT, pointing toward direct evidence of the extragalactic origin of the IGRB.

Origins of Fermi-level pinning on GaN and InN polar and nonpolar surfaces

NASA Astrophysics Data System (ADS)

Segev, D.; Van de Walle, C. G.

2006-10-01

Using band structure and total energy methods, we study the atomic and electronic structures of the polar (+c and - c plane) and nonpolar (a and m plane) surfaces of GaN and InN. We identify two distinct microscopic origins for Fermi-level pinning on GaN and InN, depending on surface stoichiometry and surface polarity. At moderate Ga/N ratios unoccupied gallium dangling bonds pin the Fermi level on n-type GaN at 0.5 0.7 eV below the conduction-band minimum. Under highly Ga-rich conditions metallic Ga adlayers lead to Fermi-level pinning at 1.8 eV above the valence-band maximum. We also explain the source of the intrinsic electron accumulation that has been universally observed on polar InN surfaces. It is caused by In-In bonds leading to occupied surface states above the conduction-band minimum. We predict that such a charge accumulation will be absent on the nonpolar surfaces of InN, when prepared under specific conditions.

GaAsSb bandgap, surface fermi level, and surface state density studied by photoreflectance modulation spectroscopy

NASA Astrophysics Data System (ADS)

Hwang, J. S.; Tsai, J. T.; Su, I. C.; Lin, H. C.; Lu, Y. T.; Chiu, P. C.; Chyi, J. I.

2012-05-01

The bandgap, surface Fermi level, and surface state density of a series of GaAs1-xSbx surface intrinsic-n+ structures with GaAs as substrate are determined for various Sb mole fractions x by the photoreflectance modulation spectroscopy. The dependence of the bandgap on the mole composition x is in good agreement with previous measurements as well as predictions calculated using the dielectric model of Van Vechten and Bergstresser in Phys. Rev. B 1, 3551 (1970). For a particular composition x, the surface Fermi level is always strongly pinned within the bandgap of GaAs1-xSbx and we find its variation with composition x is well described by a function EF = 0.70 - 0.192 x for 0 ≦ x ≦ 0.35, a result which is notably different from that reported by Chouaib et al. [Appl. Phys. Lett. 93, 041913 (2008)]. Our results suggest that the surface Fermi level is pinned at the midgap of GaAs and near the valence band of the GaSb.

Magnetotransport properties of MoP 2

DOE PAGES

Wang, Aifeng; Graf, D.; Stein, Aaron; ...

2017-11-02

We report magnetotransport and de Haas–van Alphen (dHvA) effect studies on MoP 2 single crystals, predicted to be a type- II Weyl semimetal with four pairs of robust Weyl points located below the Fermi level and long Fermi arcs. The temperature dependence of resistivity shows a peak before saturation, which does not move with magnetic field. Large nonsaturating magnetoresistance (MR) was observed, and the field dependence of MR exhibits a crossover from semiclassical weak-field B 2 dependence to the high-field linear-field dependence, indicating the presence of Dirac linear energy dispersion. In addition, a systematic violation of Kohler's rule was observed,more » consistent with multiband electronic transport. Strong spin-orbit coupling splitting has an effect on dHvA measurements whereas the angular-dependent dHvA orbit frequencies agree well with the calculated Fermi surface. The cyclotron effective mass ~1.6m e indicates the bands might be trivial, possibly since the Weyl points are located below the Fermi level.« less

Fermi Surface as a Driver for the Shape-Memory Effect in AuZn

NASA Astrophysics Data System (ADS)

Lashley, Jason

2005-03-01

Martensites are materials that undergo diffusionless, solid-state transitions. The martensitic transition yields properties that depend on the history of the material and if reversible can allow it to recover its previous shape after plastic deformation. This is known as the shape-memory effect (SME). We have succeeded in identifying the operative electronic mechanism responsible for the martensitic transition in the shape-memory alloy AuZn by using Fermi-surface measurements (de Haas-van Alphen oscillations) and band-structure calculations. Our findings suggest that electronic band structure gives rise to special features on the Fermi surface that is important to consider in the design of SME alloys.

Potentiometric Titrations for Measuring the Capacitance of Colloidal Photodoped ZnO Nanocrystals.

PubMed

Brozek, Carl K; Hartstein, Kimberly H; Gamelin, Daniel R

2016-08-24

Colloidal semiconductor nanocrystals offer a unique opportunity to bridge molecular and bulk semiconductor redox phenomena. Here, potentiometric titration is demonstrated as a method for quantifying the Fermi levels and charging potentials of free-standing colloidal n-type ZnO nanocrystals possessing between 0 and 20 conduction-band electrons per nanocrystal, corresponding to carrier densities between 0 and 1.2 × 10(20) cm(-3). Potentiometric titration of colloidal semiconductor nanocrystals has not been described previously, and little precedent exists for analogous potentiometric titration of any soluble reductants involving so many electrons. Linear changes in Fermi level vs charge-carrier density are observed for each ensemble of nanocrystals, with slopes that depend on the nanocrystal size. Analysis indicates that the ensemble nanocrystal capacitance is governed by classical surface electrical double layers, showing no evidence of quantum contributions. Systematic shifts in the Fermi level are also observed with specific changes in the identity of the charge-compensating countercation. As a simple and contactless alternative to more common thin-film-based voltammetric techniques, potentiometric titration offers a powerful new approach for quantifying the redox properties of colloidal semiconductor nanocrystals.

Anisotropy of the Fermi surface, Fermi velocity, many-body enhancement, and superconducting energy gap in Nb

DOE Office of Scientific and Technical Information (OSTI.GOV)

Crabtree, G.W.; Dye, D.H.; Karim, D.P.

1987-02-01

The detailed angular dependence of the Fermi radius k/sub F/, the Fermi velocity v/sub F/(k), the many-body enhancement factor lambda(k), and the superconducting energy gap ..delta..(k), for electrons on the Fermi surface of Nb are derived with use of the de Haas--van Alphen (dHvA) data of Karim, Ketterson, and Crabtree (J. Low Temp. Phys. 30, 389 (1978)), a Korringa-Kohn-Rostoker parametrization scheme, and an empirically adjusted band-structure calculation of Koelling. The parametrization is a nonrelativistic five-parameter fit allowing for cubic rather than spherical symmetry inside the muffin-tin spheres. The parametrized Fermi surface gives a detailed interpretation of the previously unexplained kappa,more » ..cap alpha..', and ..cap alpha..'' orbits in the dHvA data. Comparison of the parametrized Fermi velocities with those of the empirically adjusted band calculation allow the anisotropic many-body enhancement factor lambda(k) to be determined. Theoretical calculations of the electron-phonon interaction based on the tight-binding model agree with our derived values of lambda(k) much better than those based on the rigid-muffin-tin approximation. The anisotropy in the superconducting energy gap ..delta..(k) is estimated from our results for lambda(k), assuming weak anisotropy.« less

Anisotropy of the Fermi surface, Fermi velocity, many-body enhancement, and superconducting energy gap in Nb

NASA Astrophysics Data System (ADS)

Crabtree, G. W.; Dye, D. H.; Karim, D. P.; Campbell, S. A.; Ketterson, J. B.

1987-02-01

The detailed angular dependence of the Fermi radius kF, the Fermi velocity vF(k), the many-body enhancement factor λ(k), and the superconducting energy gap Δ(k), for electrons on the Fermi surface of Nb are derived with use of the de Haas-van Alphen (dHvA) data of Karim, Ketterson, and Crabtree [J. Low Temp. Phys. 30, 389 (1978)], a Korringa-Kohn-Rostoker parametrization scheme, and an empirically adjusted band-structure calculation of Koelling. The parametrization is a nonrelativistic five-parameter fit allowing for cubic rather than spherical symmetry inside the muffin-tin spheres. The parametrized Fermi surface gives a detailed interpretation of the previously unexplained κ, α', and α'' orbits in the dHvA data. Comparison of the parametrized Fermi velocities with those of the empirically adjusted band calculation allow the anisotropic many-body enhancement factor λ(k) to be determined. Theoretical calculations of the electron-phonon interaction based on the tight-binding model agree with our derived values of λ(k) much better than those based on the rigid-muffin-tin approximation. The anisotropy in the superconducting energy gap Δ(k) is estimated from our results for λ(k), assuming weak anisotropy.

New Theoretical Estimates of the Contribution of Unresolved Star-Forming Galaxies to the Extragalactic Gamma-Ray Background (EGB) as Measured by EGRET and the Fermi-LAT

NASA Technical Reports Server (NTRS)

Venters, Tonia M.

2011-01-01

We present new theoretical estimates of the contribution of unresolved star-forming galaxies to the extragalactic gamma-ray background (EGB) as measured by EGRET and the Fermi-LAT. We employ several methods for determining the star-forming galaxy contribution the the EGB, including a method positing a correlation between the gamma-ray luminosity of a galaxy and its rate of star formation as calculated from the total infrared luminosity, and a method that makes use of a model of the evolution of the galaxy gas mass with cosmic time. We find that depending on the model, unresolved star-forming galaxies could contribute significantly to the EGB as measured by the Fermi-LAT at energies between approx. 300 MeV and approx. few GeV. However, the overall spectrum of unresolved star-forming galaxies can explain neither the EGRET EGB spectrum at energies between 50 and 200 MeV nor the Fermi-LAT EGB spectrum at energies above approx. few GeV.

Optically induced Lifshitz transition in bilayer graphene

NASA Astrophysics Data System (ADS)

Iorsh, I. V.; Dini, K.; Kibis, O. V.; Shelykh, I. A.

2017-10-01

It is shown theoretically that the renormalization of the electron energy spectrum of bilayer graphene with a strong high-frequency electromagnetic field (dressing field) results in the Lifshitz transition—the abrupt change in the topology of the Fermi surface near the band edge. This effect substantially depends on the polarization of the field: The linearly polarized dressing field induces the Lifshitz transition from the quadruply connected Fermi surface to the doubly connected one, whereas the circularly polarized field induces the multicritical point where the four different Fermi topologies may coexist. As a consequence, the discussed phenomenon creates a physical basis to control the electronic properties of bilayer graphene with light.

Friedel oscillation near a van Hove singularity in two-dimensional Dirac materials

NASA Astrophysics Data System (ADS)

Lu, Chi-Ken

2016-02-01

We consider Friedel oscillation in the two-dimensional Dirac materials when the Fermi level is near the van Hove singularity. Twisted graphene bilayer and the surface state of topological crystalline insulator are the representative materials which show low-energy saddle points that are feasible to probe by gating. We approximate the Fermi surface near saddle point with a hyperbola and calculate the static Lindhard response function. Employing a theorem of Lighthill, the induced charge density δ n due to an impurity is obtained and the algebraic decay of δ n is determined by the singularity of the static response function. Although a hyperbolic Fermi surface is rather different from a circular one, the static Lindhard response function in the present case shows a singularity similar with the response function associated with circular Fermi surface, which leads to the δ n\\propto {{R}-2} at large distance R. The dependences of charge density on the Fermi energy are different. Consequently, it is possible to observe in twisted graphene bilayer the evolution that δ n\\propto {{R}-3} near Dirac point changes to δ n\\propto {{R}-2} above the saddle point. Measurements using scanning tunnelling microscopy around the impurity sites could verify the prediction.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Saperstein, E. E., E-mail: saper@mbslab.kiae.ru; Tolokonnikov, S. V.

Recent results obtained on the basis of the self-consistent theory of finite Fermi systems by employing the energy density functional proposed by Fayans and his coauthors are surveyed. These results are compared with the predictions of Skyrme–Hartree–Fock theory involving several popular versions of the Skyrme energy density functional. Spherical nuclei are predominantly considered. The charge radii of even and odd nuclei and features of low-lying 2{sup +} excitations in semimagic nuclei are discussed briefly. The single-particle energies ofmagic nuclei are examined inmore detail with allowance for corrections to mean-field theory that are induced by particle coupling to low-lying collective surfacemore » excitations (phonons). The importance of taking into account, in this problem, nonpole (tadpole) diagrams, which are usually disregarded, is emphasized. The spectroscopic factors of magic and semimagic nuclei are also considered. In this problem, only the surface term stemming from the energy dependence induced in the mass operator by the exchange of surface phonons is usually taken into account. The volume contribution associated with the energy dependence initially present in the mass operator within the self-consistent theory of finite Fermi systems because of the exchange of high-lying particle–hole excitations is also included in the spectroscopic factor. The results of the first studies that employed the Fayans energy density functional for deformed nuclei are also presented.« less

Bulk contribution to magnetotransport properties of low-defect-density Bi2Te3 topological insulator thin films

NASA Astrophysics Data System (ADS)

Ngabonziza, P.; Wang, Y.; Brinkman, A.

2018-04-01

An important challenge in the field of topological materials is to carefully disentangle the electronic transport contribution of the topological surface states from that of the bulk. For Bi2Te3 topological insulator samples, bulk single crystals and thin films exposed to air during fabrication processes are known to be bulk conducting, with the chemical potential in the bulk conduction band. For Bi2Te3 thin films grown by molecular beam epitaxy, we combine structural characterization (transmission electron microscopy), chemical surface analysis as function of time (x-ray photoelectron spectroscopy) and magnetotransport analysis to understand the low defect density and record high bulk electron mobility once charge is doped into the bulk by surface degradation. Carrier densities and electronic mobilities extracted from the Hall effect and the quantum oscillations are consistent and reveal a large bulk carrier mobility. Because of the cylindrical shape of the bulk Fermi surface, the angle dependence of the bulk magnetoresistance oscillations is two dimensional in nature.

Composition-dependent magnetic response properties of Mn1 -xFexGe alloys

NASA Astrophysics Data System (ADS)

Mankovsky, S.; Wimmer, S.; Polesya, S.; Ebert, H.

2018-01-01

The composition-dependent behavior of the Dzyaloshinskii-Moriya interaction (DMI), the spin-orbit torque (SOT), as well as anomalous and spin Hall conductivities of Mn1 -xFexGe alloys have been investigated by first-principles calculations using the relativistic multiple scattering Korringa-Kohn-Rostoker (KKR) formalism. The Dxx component of the DMI exhibits a strong dependence on the Fe concentration, changing sign at x ≈0.85 in line with previous theoretical calculations as well as with experimental results demonstrating the change of spin helicity at x ≈0.8 . A corresponding behavior with a sign change at x ≈0.5 is predicted also for the Fermi-sea contribution to the SOT, because this is closely related to the DMI. In the case of anomalous and spin Hall effects it is shown that the calculated Fermi-sea contributions are rather small and the composition-dependent behavior of these effects are determined mainly by the electronic states at the Fermi level. The spin-orbit-induced scattering mechanisms responsible for both these effects suggest a common origin of the minimum of the anomalous Hall effect and the sign change of the spin Hall effect conductivities.

Effect of the particle-hole channel on BCS–Bose-Einstein condensation crossover in atomic Fermi gases

PubMed Central

Chen, Qijin

2016-01-01

BCS–Bose-Einstein condensation (BEC) crossover is effected by increasing pairing strength between fermions from weak to strong in the particle-particle channel, and has attracted a lot of attention since the experimental realization of quantum degenerate atomic Fermi gases. Here we study the effect of the (often dropped) particle-hole channel on the zero T gap Δ(0), superfluid transition temperature Tc, the pseudogap at Tc, and the mean-field ratio 2Δ(0)/, from BCS through BEC regimes, using a pairing fluctuation theory which includes self-consistently the contributions of finite-momentum pairs and features a pseudogap in single particle excitation spectrum. Summing over the infinite particle-hole ladder diagrams, we find a complex dynamical structure for the particle-hole susceptibility χph, and conclude that neglecting the self-energy feedback causes a serious over-estimate of χph. While our result in the BCS limit agrees with Gor’kov et al., the particle-hole channel effect becomes more complex and pronounced in the crossover regime, where χph is reduced by both a smaller Fermi surface and a big (pseudo)gap. Deep in the BEC regime, the particle-hole channel contributions drop to zero. We predict a density dependence of the magnetic field at the Feshbach resonance, which can be used to quantify χph and test different theories. PMID:27183875

Quantum oscillations and upper critical magnetic field of the iron-based superconductor FeSe

NASA Astrophysics Data System (ADS)

Audouard, Alain; Duc, Fabienne; Drigo, Loïc; Toulemonde, Pierre; Karlsson, Sandra; Strobel, Pierre; Sulpice, André

2015-01-01

Shubnikov-de Haas (SdH) oscillations and upper critical magnetic field (Hc2) of the iron-based superconductor FeSe (Tc = 8.6 \\text{K}) have been studied by tunnel diode oscillator-based measurements in magnetic fields of up to 55 T and temperatures down to 1.6 K. Several Fourier components enter the SdH oscillations spectrum with frequencies definitely smaller than predicted by band structure calculations indicating band renormalization and reconstruction of the Fermi surface at low temperature, in line with previous ARPES data. The Werthamer-Helfand-Hohenberg model accounts for the temperature dependence of (Hc2) for magnetic field applied both parallel (\\textbf{H} \\| ab) and perpendicular (\\textbf{H} \\| c) to the iron conducting plane, suggesting that one band mainly controls the superconducting properties in magnetic fields despite the multiband nature of the Fermi surface. Whereas Pauli pair breaking is negligible for \\textbf{H} \\| c , a Pauli paramagnetic contribution is evidenced for \\textbf{H} \\| ab with Maki parameter α = 2.1 , corresponding to Pauli field HP = 36.5 \\text{T} .

Surface electronic structure of the topological Kondo-insulator candidate correlated electron system SmB6.

PubMed

Neupane, M; Alidoust, N; Xu, S-Y; Kondo, T; Ishida, Y; Kim, D J; Liu, Chang; Belopolski, I; Jo, Y J; Chang, T-R; Jeng, H-T; Durakiewicz, T; Balicas, L; Lin, H; Bansil, A; Shin, S; Fisk, Z; Hasan, M Z

2013-01-01

The Kondo insulator SmB6 has long been known to exhibit low-temperature transport anomalies whose origin is of great interest. Here we uniquely access the surface electronic structure of the anomalous transport regime by combining state-of-the-art laser and synchrotron-based angle-resolved photoemission techniques. We observe clear in-gap states (up to ~4 meV), whose temperature dependence is contingent on the Kondo gap formation. In addition, our observed in-gap Fermi surface oddness tied with the Kramers' point topology, their coexistence with the two-dimensional transport anomaly in the Kondo hybridization regime, as well as their robustness against thermal recycling, taken together, collectively provide strong evidence for protected surface metallicity with a Fermi surface whose topology is consistent with the theoretically predicted topological Fermi surface. Our observations of systematic surface electronic structure provide the fundamental electronic parameters for the anomalous Kondo ground state of correlated electron material SmB6.

Is BaCr 2 As 2 symmetrical to BaFe 2 As 2 with respect to half 3 d shell filling?

DOE PAGES

Richard, P.; van Roekeghem, A.; Lv, B. Q.; ...

2017-05-25

We have performed an angle-resolved photoemission spectroscopy study of BaCr 2As 2, which has the same crystal structure as BaFe2As2, a parent compound BaFe 2As 2 of Fe-based superconductors. We determine the Fermi surface of this material and its band dispersion down to 5 eV below the Fermi level. Very moderate band renormalization (1.35) is observed for only two bands. We attribute this small renormalization to enhanced direct exchange as compared to Fe in BaFe 2As 2, and to a larger contribution of the eg orbitals in the composition of the bands forming the Fermi surface.

Composite Fermi surface in the half-filled Landau level with anisotropic electron mass

NASA Astrophysics Data System (ADS)

Ippoliti, Matteo; Geraedts, Scott; Bhatt, Ravindra

We study the problem of interacting electrons in the lowest Landau level at half filling in the quantum Hall regime, when the electron dispersion is given by an anisotropic mass tensor. Based on experimental observations and theoretical arguments, the ground state of the system is expected to consist of composite Fermions filling an elliptical Fermi sea, with the anisotropy of the ellipse determined by the competing effects of the isotropic Coulomb interaction and anisotropic electron mass tensor. We test this idea quantitatively by using a numerical density matrix renormalization group method for quantum Hall systems on an infinitely long cylinder. Singularities in the structure factor allow us to map the Fermi surface of the composite Fermions. We compute the composite Fermi surface anisotropy for several values of the electron mass anisotropy which allow us to deduce the functional dependence of the former on the latter. This research was supported by Department of Energy Office of Basic Energy Sciences through Grant No. DE-SC0002140.

Magnetic field-induced Fermi surface reconstruction and quantum criticality in CeRhIn 5

DOE PAGES

Jiao, Lin; Weng, Z. F.; Smidman, Michael; ...

2017-02-06

Here, we present detailed results of the field evolution of the de Haas–van Alphen (dHvA) effect in CeRhIn 5. A magnetic field-induced reconstruction of the Fermi surface is clearly shown to occur inside the antiferromagnetic state, in an applied field of around B* ≃ 30 T, which is evidenced by the appearance of several new dHvA branches. The angular dependence of the dHvA frequencies reveals that the Fermi surfaces of CeRhIn 5 at B > B* and CeCoIn5 are similar. The results suggest that the Ce-4f electrons in become itinerant at B > B* due to the Kondo effect, priormore » to the field-induced quantum critical point (QCP) at Bc0 ≃ 50 T. The electronic states at the field-induced QCP are therefore different from that of the pressure-induced QCP where a dramatic Fermi surface reconstruction occurs exactly at the critical pressure, indicating that multiple types of QCP may exist in CeRhIn 5.« less

Effective mass and Fermi surface complexity factor from ab initio band structure calculations

NASA Astrophysics Data System (ADS)

Gibbs, Zachary M.; Ricci, Francesco; Li, Guodong; Zhu, Hong; Persson, Kristin; Ceder, Gerbrand; Hautier, Geoffroy; Jain, Anubhav; Snyder, G. Jeffrey

2017-02-01

The effective mass is a convenient descriptor of the electronic band structure used to characterize the density of states and electron transport based on a free electron model. While effective mass is an excellent first-order descriptor in real systems, the exact value can have several definitions, each of which describe a different aspect of electron transport. Here we use Boltzmann transport calculations applied to ab initio band structures to extract a density-of-states effective mass from the Seebeck Coefficient and an inertial mass from the electrical conductivity to characterize the band structure irrespective of the exact scattering mechanism. We identify a Fermi Surface Complexity Factor: Nv*K* from the ratio of these two masses, which in simple cases depends on the number of Fermi surface pockets (Nv* ) and their anisotropy K*, both of which are beneficial to high thermoelectric performance as exemplified by the high values found in PbTe. The Fermi Surface Complexity factor can be used in high-throughput search of promising thermoelectric materials.

Origin of the pressure-dependent Tc valley in superconducting simple cubic phosphorus

NASA Astrophysics Data System (ADS)

Wu, Xianxin; Jeschke, Harald O.; Di Sante, Domenico; von Rohr, Fabian O.; Cava, Robert J.; Thomale, Ronny

2018-03-01

Motivated by recent experiments, we investigate the pressure-dependent electronic structure and electron-phonon (e-ph) coupling for simple cubic phosphorus by performing first-principles calculations within the full potential linearized augmented plane-wave method. As a function of increasing pressure, our calculations show a valley feature in Tc, followed by an eventual decrease for higher pressures. We demonstrate that this Tc valley at low pressures is due to two nearby Lifshitz transitions, as we analyze the band-resolved contributions to the e-ph coupling. Below the first Lifshitz transition, the phonon hardening and shrinking of the γ Fermi surface with s -orbital character results in a decreased Tc with increasing pressure. After the second Lifshitz transition, the appearance of δ Fermi surfaces with 3 d -orbital character generate strong e-ph interband couplings in α δ and β δ channels, and hence lead to an increase of Tc. For higher pressures, the phonon hardening finally dominates, and Tc decreases again. Our study reveals that the intriguing Tc valley discovered in experiment can be attributed to Lifshitz transitions, while the plateau of Tc detected at intermediate pressures appears to be beyond the scope of our analysis. This strongly suggests that aside from e-ph coupling, electronic correlations along with plasmonic contributions may be relevant for simple cubic phosphorus. Our findings hint at the notion that increasing pressure can shift the low-energy orbital weight towards d character, and as such even trigger an enhanced importance of orbital-selective electronic correlations despite an increase of the overall bandwidth.

Orbital-dependent electron correlation effects in iron-based superconductors

NASA Astrophysics Data System (ADS)

Yi, Ming

The iron chalcogenide superconductors constitute arguably one of the most intriguing families of the iron-based high temperature superconductors given their ability to superconduct at comparable temperatures as the iron pnictides, despite the lack of similarities in their magnetic structures and Fermi surface topologies. In particular, the lack of hole Fermi pockets at the Brillouin zone center posts a challenge to the previous proposal of spin fluctuation mediated pairing via Fermi surface nesting. In this talk, using angle-resolved photoemission spectroscopy measurements, I will present evidence that show that instead of Fermi surface topology, strong electron correlation observed in electron bandwidth is an important ingredient for superconductivity in the iron chalcogenides. Specifically, I will show i) there exists universal strong orbital-selective renormalization effects and proximity to an orbital-selective Mott phase in Fe1+yTe1-xSex, AxFe2-ySe2, and monolayer FeSe film on SrTiO3, and ii) in RbxFe2(Se1-zSz)2 , where sulfur substitution for selenium continuously suppresses superconductivity down to zero, little change occurs in the Fermi surface topology while a substantial reduction of electron correlation is observed in an expansion of the overall bandwidth, implying that electron correlation is one of the key tuning parameters for superconductivity in these materials.

Twisted Fermi surface of a thin-film Weyl semimetal

NASA Astrophysics Data System (ADS)

Bovenzi, N.; Breitkreiz, M.; O'Brien, T. E.; Tworzydło, J.; Beenakker, C. W. J.

2018-02-01

The Fermi surface of a conventional two-dimensional electron gas is equivalent to a circle, up to smooth deformations that preserve the orientation of the equi-energy contour. Here we show that a Weyl semimetal confined to a thin film with an in-plane magnetization and broken spatial inversion symmetry can have a topologically distinct Fermi surface that is twisted into a figure-8—opposite orientations are coupled at a crossing which is protected up to an exponentially small gap. The twisted spectral response to a perpendicular magnetic field B is distinct from that of a deformed Fermi circle, because the two lobes of a figure-8 cyclotron orbit give opposite contributions to the Aharonov-Bohm phase. The magnetic edge channels come in two counterpropagating types, a wide channel of width β {l}m2\\propto 1/B and a narrow channel of width {l}m\\propto 1/\\sqrt{B} (with {l}m=\\sqrt{{\\hslash }/{eB}} the magnetic length and β the momentum separation of the Weyl points). Only one of the two is transmitted into a metallic contact, providing unique magnetotransport signatures.

Stability of rhombohedral phases in vanadium at high-pressure and high-temperature: first-principles investigations

NASA Astrophysics Data System (ADS)

Wang, Yi X.; Wu, Q.; Chen, Xiang R.; Geng, Hua Y.

2016-09-01

The pressure-induced transition of vanadium from BCC to rhombohedral structures is unique and intriguing among transition metals. In this work, the stability of these phases is revisited by using density functional theory. At finite temperatures, a novel transition of rhombohedral phases back to BCC phase induced by thermal electrons is discovered. This reentrant transition is found not driven by phonons, instead it is the electronic entropy that stabilizes the latter phase, which is totally out of expectation. Parallel to this transition, we find a peculiar and strong increase of the shear modulus C44 with increasing temperature. It is counter-intuitive in the sense that it suggests an unusual harding mechanism of vanadium by temperature. With these stability analyses, the high-pressure and finite-temperature phase diagram of vanadium is proposed. Furthermore, the dependence of the stability of RH phases on the Fermi energy and chemical environment is investigated. The results demonstrate that the position of the Fermi level has a significant impact on the phase stability, and follows the band-filling argument. Besides the Fermi surface nesting, we find that the localization/delocalization of the d orbitals also contributes to the instability of rhombohedral distortions in vanadium.

Nonadiabatic Dynamics May Be Probed through Electronic Coherence in Time-Resolved Photoelectron Spectroscopy.

PubMed

Bennett, Kochise; Kowalewski, Markus; Mukamel, Shaul

2016-02-09

We present a hierarchy of Fermi golden rules (FGRs) that incorporate strongly coupled electronic/nuclear dynamics in time-resolved photoelectron spectroscopy (TRPES) signals at different levels of theory. Expansion in the joint electronic and nuclear eigenbasis yields the numerically most challenging exact FGR (eFGR). The quasistatic Fermi Golden Rule (qsFGR) neglects nuclear motion during the photoionization process but takes into account electronic coherences as well as populations initially present in the pumped matter as well as those generated internally by coupling between electronic surfaces. The standard semiclassical Fermi Golden Rule (scFGR) neglects the electronic coherences and the nuclear kinetic energy during the ionizing pulse altogether, yielding the classical Condon approximation. The coherence contributions depend on the phase-profile of the ionizing field, allowing coherent control of TRPES signals. The photoelectron spectrum from model systems is simulated using these three levels of theory. The eFGR and the qsFGR show temporal oscillations originating from the electronic or vibrational coherences generated as the nuclear wave packet traverses a conical intersection. These oscillations, which are missed by the scFGR, directly reveal the time-evolving splitting between electronic states of the neutral molecule in the curve-crossing regime.

Resistivity scaling and electron relaxation times in metallic nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Moors, Kristof, E-mail: kristof@itf.fys.kuleuven.be; Imec, Kapeldreef 75, B-3001 Leuven; Sorée, Bart

2014-08-14

We study the resistivity scaling in nanometer-sized metallic wires due to surface roughness and grain-boundaries, currently the main cause of electron scattering in nanoscaled interconnects. The resistivity has been obtained with the Boltzmann transport equation, adopting the relaxation time approximation of the distribution function and the effective mass approximation for the conducting electrons. The relaxation times are calculated exactly, using Fermi's golden rule, resulting in a correct relaxation time for every sub-band state contributing to the transport. In general, the relaxation time strongly depends on the sub-band state, something that remained unclear with the methods of previous work. The resistivitymore » scaling is obtained for different roughness and grain-boundary properties, showing large differences in scaling behavior and relaxation times. Our model clearly indicates that the resistivity is dominated by grain-boundary scattering, easily surpassing the surface roughness contribution by a factor of 10.« less

Fermi surface in the hidden-order state of URu2Si2 under intense pulsed magnetic fields up to 81 T

NASA Astrophysics Data System (ADS)

Scheerer, G. W.; Knafo, W.; Aoki, D.; Nardone, M.; Zitouni, A.; Béard, J.; Billette, J.; Barata, J.; Jaudet, C.; Suleiman, M.; Frings, P.; Drigo, L.; Audouard, A.; Matsuda, T. D.; Pourret, A.; Knebel, G.; Flouquet, J.

2014-04-01

We present measurements of the resistivity ρx ,x of URu2Si2 high-quality single crystals in pulsed high magnetic fields up to 81 T at a temperature of 1.4 K and up to 60 T at temperatures down to 100 mK. For a field H applied along the magnetic easy axis c, a strong sample dependence of the low-temperature resistivity in the hidden-order phase is attributed to a high carrier mobility. The interplay between the magnetic and orbital properties is emphasized by the angle dependence of the phase diagram, where magnetic transition fields and crossover fields related to the Fermi surface properties follow a 1/cosθ law, θ being the angle between H and c. For H ∥c, a crossover defined at a kink of ρx ,x, as initially reported in [Shishido, Phys. Rev. Lett. 102, 156403 (2009), 10.1103/PhysRevLett.102.156403], is found to be strongly sample dependent: its characteristic field μ0H* varies from ≃20 T in our best sample with a residual resistivity ratio RRR = ρx ,x(300K)/ ρx ,x(2K) of 225 to ≃25 T in a sample with a RRR of 90. A second crossover is defined at the maximum of ρx ,x at the sample-independent low-temperature (LT) characteristic field μ0Hρ,maxLT≃30 T. Fourier analyses of Shubnikov-de Haas oscillations show that Hρ,maxLT coincides with a sudden modification of the Fermi surface, while H* lies in a regime where the Fermi surface is smoothly modified. For H ∥a, (i) no phase transition is observed at low temperature and the system remains in the hidden-order phase up to 81 T, (ii) quantum oscillations surviving up to 7 K are related to a new orbit observed at the frequency Fλ≃1350 T and associated with a low effective mass mλ*=(1±0.5)m0, where m0 is the free electron mass, and (iii) no Fermi surface modification occurs up to 81 T.

Magnetic and Fermi Surface Properties of EuGa4

NASA Astrophysics Data System (ADS)

Nakamura, Ai; Hiranaka, Yuichi; Hedo, Masato; Nakama, Takao; Miura, Yasunao; Tsutsumi, Hiroki; Mori, Akinobu; Ishida, Kazuhiro; Mitamura, Katsuya; Hirose, Yusuke; Sugiyama, Kiyohiro; Honda, Fuminori; Settai, Rikio; Takeuchi, Tetsuya; Hagiwara, Masayuki; Matsuda, Tatsuma D.; Yamamoto, Etsuji; Haga, Yoshinori; Matsubayashi, Kazuyuki; Uwatoko, Yoshiya; Harima, Hisatomo; Ōnuki, Yoshichika

2013-10-01

We grew a high-quality single crystal EuGa4 with the tetragonal structure by the Ga self-flux method, and measured the electrical resistivity, magnetic susceptibility, high-field magnetization, specific heat, thermoelectric power and de Haas--van Alphen (dHvA) effect, together with the electrical resistivity and thermoelectric power under pressure. EuGa4 is found to be a Eu-divalent compound without anisotropy of the magnetic susceptibility in the paramagnetic state and to reveal the same magnetization curve between H \\parallel [100] and [001] in the antiferromagnetic state, where the antiferromagnetic easy-axis is oriented along the [100] direction below a Néel temperature TN=16.5 K. The magnetization curve is discussed on the basis of a simple two-sublattice model. The Fermi surface in the paramagnetic state was clarified from the results of a dHvA experiment for EuGa4 and an energy band calculation for a non-4f reference compound SrGa4, which consists of a small ellipsoidal hole--Fermi surface and a compensated cube-like electron--Fermi surface with vacant space in center. We observed an anomaly in the temperature dependence of the electrical resistivity and thermoelectric power at TCDW=150 K under 2 GPa. This might correspond to an emergence of the charge density wave (CDW). The similar phenomenon was also observed in EuAl4 at ambient pressure. We discussed the CDW phenomenon on the basis of the present peculiar Fermi surfaces.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ho, Pei Chun; Singleton, John; Goddard, Paul A.

We use MHz conductivity, torque magnetometer, and magnetization measurements to report on single crystals of CeOs 4 Sb 12 and NdOs 4 Sb 12 using temperatures down to 0.5 K and magnetic fields of up to 60 tesla. The field-orientation dependence of the de Haas-van Alphen and Shubnikov-de Haas oscillations is deduced by rotating the samples about the [ 010 ] and [ 0more » $$\\bar{1}$$ 1 1 ] directions. Our results indicate that NdOs 4 Sb 12 has a similar Fermi surface topology to that of the unusual superconductor PrOs 4 Sb 12 , but with significantly smaller effective masses, supporting the importance of local phonon modes in contributing to the low-temperature heat capacity of NdOs 4 Sb 12 . By contrast, CeOs 4 Sb 12 undergoes a field-induced transition from an unusual semimetal into a high-field, high-temperature state characterized by a single, almost spherical Fermi-surface section. Furthermore, the behavior of the phase boundary and comparisons with models of the band structure lead us to propose that the field-induced phase transition in CeOs 4 Sb 12 is similar in origin to the well-known α - γ transition in Ce and its alloys.« less

Chemical contrast in STM imaging of transition metal aluminides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Duguet, T.; Thiel, Patricia A.

2012-08-01

The present manuscript reviews recent scanning tunnelling microscopy (STM) studies of transition metal (TM) aluminide surfaces. It provides a general perspective on the contrast between Al atoms and TM atoms in STM imaging. A general trend is the much stronger bias dependence of TM atoms, or TM-rich regions of the surface. This dependence can be attenuated by the local chemical arrangements and environments. Al atoms can show a stronger bias dependence when their chemical environment, such as their immediate subsurface, is populated with TM. All this is well explained in light of combined results of STM and both theoretical andmore » experimental electronic and crystallographic structure determinations. Since STM probes the Fermi surface, the electronic structure in the vicinity of the Fermi level (EF) is essential for understanding contrast and bias dependence. Hence, partial density of states provides information about the TM d band position and width, s–p–d hybridization or interactions, or charge transfer between constituent elements. In addition, recent developments in STM image simulations are very interesting for elucidating chemical contrast at Al–TM alloy surfaces, and allow direct atomic identification, when the surface does not show too much disorder. Overall, we show that chemically-specific imaging is often possible at these surfaces.« less

Thermodynamics of GaN(s)-NH3(v)+N2(v)+H2(v) system - Electronic aspects of the processes at GaN(0001) surface

NASA Astrophysics Data System (ADS)

Kempisty, Pawel; Strak, Pawel; Sakowski, Konrad; Krukowski, Stanislaw

2017-08-01

Comprehensive analysis of GaN(0001) surface in equilibrium with ammonia/hydrogen mixture was undertaken using results of ab initio calculations. Adsorption energies of the species derived from ammonia and molecular hydrogen and their stable sites were obtained. It was shown that the adsorption process type and energy depend on the position of Fermi level at the surface. Hydrogen decomposes into two separate H atoms, always adsorbed in the positions on top of the surface Ga atoms (On-top). Ammonia adsorption at GaN(0001) surface proceeds molecularly to ammonia in the On-top position or dissociatively into NH2 radicals in bridge (NH2-bridge) or On-top positions or into NH radicals in H3 (NH-H3) site. Presence of these species affects Fermi level pinning at the surface due to creation of new surface states. The Fermi level pinning in function of the surface attached species concentration was determined using extended electron counting rule (EECR). Results of ab initio calculations fully proved validity of the EECR predictions. Thermodynamic analysis of the surface in equilibrium with molecular hydrogen and ammonia vapor mixture is made giving the range of ammonia and hydrogen pressures, corresponding to Fermi level pinned at Ga-broken bond state for NH-H3&H and NH3&H and NH2-bridge&H coverage and at VBM for NH3 & H coverage. As the region of Fermi level pinned at Ga broken bond state corresponds to very low pressures, at pressures close to normal, GaN(0001) surface is almost totally covered by H, NH3 and NH2 located in On-top positions. It is also shown however that dominant portion of the hydrogen and ammonia pressures corresponds to Fermi level not pinned. Among them are these corresponding to MOVPE and HVPE growth conditions in which the surface is almost fully covered by NH3, NH2 and H species in On-top positions.

Doping dependence of charge order in electron-doped cuprate superconductors

NASA Astrophysics Data System (ADS)

Mou, Yingping; Feng, Shiping

2017-12-01

In the recent studies of the unconventional physics in cuprate superconductors, one of the central issues is the interplay between charge order and superconductivity. Here the mechanism of the charge-order formation in the electron-doped cuprate superconductors is investigated based on the t-J model. The experimentally observed momentum dependence of the electron quasiparticle scattering rate is qualitatively reproduced, where the scattering rate is highly anisotropic in momentum space, and is intriguingly related to the charge-order gap. Although the scattering strength appears to be weakest at the hot spots, the scattering in the antinodal region is stronger than that in the nodal region, which leads to the original electron Fermi surface is broken up into the Fermi pockets and their coexistence with the Fermi arcs located around the nodal region. In particular, this electron Fermi surface instability drives the charge-order correlation, with the charge-order wave vector that matches well with the wave vector connecting the hot spots, as the charge-order correlation in the hole-doped counterparts. However, in a striking contrast to the hole-doped case, the charge-order wave vector in the electron-doped side increases in magnitude with the electron doping. The theory also shows the existence of a quantitative link between the single-electron fermiology and the collective response of the electron density.

Unconventional Electron Pairing and Topological Superconductivity in Proximitized HgTe Quantum Wells

NASA Astrophysics Data System (ADS)

Ren, Hechen; Hart, Sean; Kosowsky, Michael; Ben-Shach, Gilad; Leubner, Philipp; Brüne, Christoph; Buhmann, Hartmut; Molenkamp, Laurens; Halperin, Bertrand; Yacoby, Amir

Coupling s-wave superconductors to systems with exotic Fermi surface spin textures has been recently proposed as a way to manipulate the nature of the paired state, in some cases even leading to a topological phase transition. Recently, we studied the behavior of Fraunhofer interference in HgTe quantum well-based Josephson junctions, in the presence of a magnetic field applied in the plane of the quantum well. Here we theoretically analyze our system and compare the predicted behavior to our experimental results. We find that the in-plane magnetic field tunes the momentum of Cooper pairs in the quantum well, directly reflecting the response of the spin-dependent Fermi surfaces. This momentum tuning depends crucially on the type of spin-orbit coupling in the system. In the high electron density regime, the induced superconductivity evolves with electron density in agreement with our model based on the Hamiltonian of Bernevig, Hughes and Zhang. This agreement provides a quantitative value for g/vF, where g is the effective g-factor and vF is the Fermi velocity. Our new understanding of the interplay between spin physics and superconductivity introduces a way to spatially engineer the order parameter from singlet to triplet pairing, and in general allows investigation of electronic spin texture at the Fermi surface of materials. NSF DMR-1206016; STC Center for Integrated Quantum Materials under NSF Grant No. DMR-1231319; NSF GRFP under Grant DGE1144152, Microsoft Corporation Project Q.

Pair momentum distribution in Bi2Sr2CaCu2O(8+delta) measured by positron annihilation - Existence and nature of the Fermi surface

NASA Astrophysics Data System (ADS)

Chan, L. P.; Lynn, K. G.; Harshman, D. R.; Massidda, S.; Mitzi, D. B.

1991-09-01

The first measurement is reported of the position-electron momentum density in superconducting single-crystal Bi2Sr2CaCu2O(8+delta)(Tc roughly 90 K). The observed anisotropy exhibits a twofold (rather than fourfold) symmetry, which is attributed to the superlattice modulation along the b axis of the BiO2 layers. Subtraction of the superlattice contribution also reveals a pair momentum distribution consistent with the CuO2 and BiO2 Fermi surfaces, and in reasonable agreement with the theoretical pair momentum density derived from band theory.

Fermi Surface of Metallic V_{2}O_{3} from Angle-Resolved Photoemission: Mid-level Filling of e_{g}^{π} Bands.

PubMed

Lo Vecchio, I; Denlinger, J D; Krupin, O; Kim, B J; Metcalf, P A; Lupi, S; Allen, J W; Lanzara, A

2016-10-14

Using angle resolved photoemission spectroscopy, we report the first band dispersions and distinct features of the bulk Fermi surface (FS) in the paramagnetic metallic phase of the prototypical metal-insulator transition material V_{2}O_{3}. Along the c axis we observe both an electron pocket and a triangular holelike FS topology, showing that both V 3d a_{1g} and e_{g}^{π} states contribute to the FS. These results challenge the existing correlation-enhanced crystal field splitting theoretical explanation for the transition mechanism and pave the way for the solution of this mystery.

On the widths of Stokes lines in Raman scattering from molecules adsorbed at metal surfaces and in molecular conduction junctions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gao, Yi, E-mail: yig057@ucsd.edu; Galperin, Michael, E-mail: migalperin@ucsd.edu; Nitzan, Abraham, E-mail: nitzan@post.tau.ac.il

Within a generic model we analyze the Stokes linewidth in surface enhanced Raman scattering (SERS) from molecules embedded as bridges in molecular junctions. We identify four main contributions to the off-resonant Stokes signal and show that under zero voltage bias (a situation pertaining also to standard SERS experiments) and at low bias junctions only one of these contributions is pronounced. The linewidth of this component is determined by the molecular vibrational relaxation rate, which is dominated by interactions with the essentially bosonic thermal environment when the relevant molecular electronic energy is far from the metal(s) Fermi energy(ies). It increases whenmore » the molecular electronic level is close to the metal Fermi level so that an additional vibrational relaxation channel due to electron-hole (eh) exciton in the molecule opens. Other contributions to the Raman signal, of considerably broader linewidths, can become important at larger junction bias.« less

Positron-annihilation study of the electronic structure of URu2Si2

NASA Astrophysics Data System (ADS)

Rozing, G. J.; Mijnarends, P. E.; Menovsky, A. A.; de Chtel, P. F.

1991-04-01

Measurements of the two-dimensional angular correlation of annihilation radiation (2D-ACAR) were performed on oriented single crystals of URu2Si2. The spectra, obtained with integration along four different symmetry directions, display anisotropic structure in fair agreement with a previous calculation of the two-photon momentum distribution. In particular, the contribution of the f-ligand hybridized electron states is clearly observed and reasonably well described by the band calculation. The 2D-ACAR distribution remains unchanged as the temperature is increased from 6 K in the Fermi-liquid state to 72 K, which is just above the coherence temperature. The inhomogeneity of the positron density in the unit cell complicates the Lock-Crisp-West (LCW) analysis of the experiments in terms of Fermi-surface features. Nevertheless, the disagreement between theory and experiment after LCW folding indicates that the Fermi surface as predicted by local-density-approximation band theory does not apply.

Stability of rhombohedral phases in vanadium at high-pressure and high-temperature: first-principles investigations

PubMed Central

Wang, Yi X.; Wu, Q.; Chen, Xiang R.; Geng, Hua Y.

2016-01-01

The pressure-induced transition of vanadium from BCC to rhombohedral structures is unique and intriguing among transition metals. In this work, the stability of these phases is revisited by using density functional theory. At finite temperatures, a novel transition of rhombohedral phases back to BCC phase induced by thermal electrons is discovered. This reentrant transition is found not driven by phonons, instead it is the electronic entropy that stabilizes the latter phase, which is totally out of expectation. Parallel to this transition, we find a peculiar and strong increase of the shear modulus C44 with increasing temperature. It is counter-intuitive in the sense that it suggests an unusual harding mechanism of vanadium by temperature. With these stability analyses, the high-pressure and finite-temperature phase diagram of vanadium is proposed. Furthermore, the dependence of the stability of RH phases on the Fermi energy and chemical environment is investigated. The results demonstrate that the position of the Fermi level has a significant impact on the phase stability, and follows the band-filling argument. Besides the Fermi surface nesting, we find that the localization/delocalization of the d orbitals also contributes to the instability of rhombohedral distortions in vanadium. PMID:27581551

Fermi surface in the absence of a Fermi liquid in the Kondo insulator SmB6

NASA Astrophysics Data System (ADS)

Hartstein, M.; Toews, W. H.; Hsu, Y.-T.; Zeng, B.; Chen, X.; Hatnean, M. Ciomaga; Zhang, Q. R.; Nakamura, S.; Padgett, A. S.; Rodway-Gant, G.; Berk, J.; Kingston, M. K.; Zhang, G. H.; Chan, M. K.; Yamashita, S.; Sakakibara, T.; Takano, Y.; Park, J.-H.; Balicas, L.; Harrison, N.; Shitsevalova, N.; Balakrishnan, G.; Lonzarich, G. G.; Hill, R. W.; Sutherland, M.; Sebastian, Suchitra E.

2018-02-01

The search for a Fermi surface in the absence of a conventional Fermi liquid has thus far yielded very few potential candidates. Among promising materials are spin-frustrated Mott insulators near the insulator-metal transition, where theory predicts a Fermi surface associated with neutral low-energy excitations. Here we reveal another route to experimentally realize a Fermi surface in the absence of a Fermi liquid by the experimental study of a Kondo insulator SmB6 positioned close to the insulator-metal transition. We present experimental signatures down to low temperatures (<<1 K) associated with a Fermi surface in the bulk, including a sizeable linear specific heat coefficient, and on the application of a finite magnetic field, bulk magnetic quantum oscillations, finite quantum oscillatory entropy, and substantial enhancement in thermal conductivity well below the charge gap energy scale. Thus, the weight of evidence indicates that despite an extreme instance of Fermi liquid breakdown in Kondo insulating SmB6, a Fermi surface arises from novel itinerant low-energy excitations that couple to magnetic fields, but not weak DC electric fields.

Large optical conductivity of Dirac semimetal Fermi arc surface states

NASA Astrophysics Data System (ADS)

Shi, Li-kun; Song, Justin C. W.

2017-08-01

Fermi arc surface states, a hallmark of topological Dirac semimetals, can host carriers that exhibit unusual dynamics distinct from that of their parent bulk. Here we find that Fermi arc carriers in intrinsic Dirac semimetals possess a strong and anisotropic light-matter interaction. This is characterized by a large Fermi arc optical conductivity when light is polarized transverse to the Fermi arc; when light is polarized along the Fermi arc, Fermi arc optical conductivity is significantly muted. The large surface spectral weight is locked to the wide separation between Dirac nodes and persists as a large Drude weight of Fermi arc carriers when the system is doped. As a result, large and anisotropic Fermi arc conductivity provides a novel means of optically interrogating the topological surfaces states of Dirac semimetals.

Positron trapping in Y1-xPrxBa2Cu3O7-δ and the Fermi surface of YBa2Cu3O7-δ

NASA Astrophysics Data System (ADS)

Shukla, A.; Hoffmann, L.; Manuel, A. A.; Walker, E.; Barbiellini, B.; Peter, M.

1995-03-01

Temperature-dependent positron lifetime measurements in ceramic Y1-xPrxBa2Cu3O7-δ samples reveal positron trapping, in particular at low temperature and for small x. Positrons appear to be completely delocalized for T~400 K and higher. At high temperatures the lifetime for YBa2Cu3O7-δ and PrBa2Cu3O7-δ is identical (~165 ps) and close to the theoretical value. For these reasons a two-dimensional angular correlation of annihilation radiation (2D-ACAR) spectrum was measured in YBa2Cu3O7 at T=400 K. The spectrum width confirms the delocalization of the positron and the 2D-ACAR shows, apart from the one-dimensional Fermi surface due to CuO chains, a smaller Fermi surface sheet centered around the S point, in the first Brillouin zone.

Fermi surfaces in Kondo insulators

NASA Astrophysics Data System (ADS)

Liu, Hsu; Hartstein, Máté; Wallace, Gregory J.; Davies, Alexander J.; Ciomaga Hatnean, Monica; Johannes, Michelle D.; Shitsevalova, Natalya; Balakrishnan, Geetha; Sebastian, Suchitra E.

2018-04-01

We report magnetic quantum oscillations measured using torque magnetisation in the Kondo insulator YbB12 and discuss the potential origin of the underlying Fermi surface. Observed quantum oscillations as well as complementary quantities such as a finite linear specific heat capacity in YbB12 exhibit similarities with the Kondo insulator SmB6, yet also crucial differences. Small heavy Fermi sections are observed in YbB12 with similarities to the neighbouring heavy fermion semimetallic Fermi surface, in contrast to large light Fermi surface sections in SmB6 which are more similar to the conduction electron Fermi surface. A rich spectrum of theoretical models is suggested to explain the origin across different Kondo insulating families of a bulk Fermi surface potentially from novel itinerant quasiparticles that couple to magnetic fields, yet do not couple to weak DC electric fields.

Fermi surface in the absence of a Fermi liquid in the Kondo insulator SmB 6

DOE PAGES

Hartstein, M.; Toews, W. H.; Hsu, Y. -T.; ...

2017-10-23

The search for a Fermi surface in the absence of a conventional Fermi liquid has thus far yielded very few potential candidates. Among promising materials are spin-frustrated Mott insulators near the insulator–metal transition, where theory predicts a Fermi surface associated with neutral low-energy excitations. In this paper, we reveal another route to experimentally realize a Fermi surface in the absence of a Fermi liquid by the experimental study of a Kondo insulator SmB 6 positioned close to the insulator–metal transition. We present experimental signatures down to low temperatures (<<1 K) associated with a Fermi surface in the bulk, including amore » sizeable linear specific heat coefficient, and on the application of a finite magnetic field, bulk magnetic quantum oscillations, finite quantum oscillatory entropy, and substantial enhancement in thermal conductivity well below the charge gap energy scale. Finally, the weight of evidence indicates that despite an extreme instance of Fermi liquid breakdown in Kondo insulating SmB 6, a Fermi surface arises from novel itinerant low-energy excitations that couple to magnetic fields, but not weak DC electric fields.« less

First-Principles Momentum Dependent Local Ansatz Approach to the Momentum Distribution Function in Iron-Group Transition Metals

NASA Astrophysics Data System (ADS)

Kakehashi, Yoshiro; Chandra, Sumal

2017-03-01

The momentum distribution function (MDF) bands of iron-group transition metals from Sc to Cu have been investigated on the basis of the first-principles momentum dependent local ansatz wavefunction method. It is found that the MDF for d electrons show a strong momentum dependence and a large deviation from the Fermi-Dirac distribution function along high-symmetry lines of the first Brillouin zone, while the sp electrons behave as independent electrons. In particular, the deviation in bcc Fe (fcc Ni) is shown to be enhanced by the narrow eg (t2g) bands with flat dispersion in the vicinity of the Fermi level. Mass enhancement factors (MEF) calculated from the jump on the Fermi surface are also shown to be momentum dependent. Large mass enhancements of Mn and Fe are found to be caused by spin fluctuations due to d electrons, while that for Ni is mainly caused by charge fluctuations. Calculated MEF are consistent with electronic specific heat data as well as recent angle resolved photoemission spectroscopy data.

Fermi surface properties of NbAs2 studied by de Haas-van Alphen oscillation

NASA Astrophysics Data System (ADS)

Singha, Ratnadwip; Mandal, Prabhat

2018-04-01

We have grown high quality single crystal of NbAs2, a member of the transition metal dipnictide family and measured magnetotransport properties. Very large magnetoresistance ˜1.3×105 % has been observed at 2 K with 9 T magnetic field. The Fermi surface properties have been studied by de Haas-van Alphen oscillation technique. The Fermi surface is highly anisotropic and consists of multiple Fermi pockets. From quantum oscillation results, different Fermi surface related parameters have been quantified.

Thermally Driven Electronic Topological Transition in FeTi

NASA Astrophysics Data System (ADS)

Yang, F. C.; Muñoz, J. A.; Hellman, O.; Mauger, L.; Lucas, M. S.; Tracy, S. J.; Stone, M. B.; Abernathy, D. L.; Xiao, Yuming; Fultz, B.

2016-08-01

Ab initio molecular dynamics, supported by inelastic neutron scattering and nuclear resonant inelastic x-ray scattering, showed an anomalous thermal softening of the M5- phonon mode in B 2 -ordered FeTi that could not be explained by phonon-phonon interactions or electron-phonon interactions calculated at low temperatures. A computational investigation showed that the Fermi surface undergoes a novel thermally driven electronic topological transition, in which new features of the Fermi surface arise at elevated temperatures. The thermally induced electronic topological transition causes an increased electronic screening for the atom displacements in the M5- phonon mode and an adiabatic electron-phonon interaction with an unusual temperature dependence.

Visualizing Type-II Weyl Points in Tungsten Ditelluride by Quasiparticle Interference.

PubMed

Lin, Chun-Liang; Arafune, Ryuichi; Liu, Ro-Ya; Yoshimura, Masato; Feng, Baojie; Kawahara, Kazuaki; Ni, Zeyuan; Minamitani, Emi; Watanabe, Satoshi; Shi, Youguo; Kawai, Maki; Chiang, Tai-Chang; Matsuda, Iwao; Takagi, Noriaki

2017-11-28

Weyl semimetals (WSMs) are classified into two types, type I and II, according to the topology of the Weyl point, where the electron and hole pockets touch each other. Tungsten ditelluride (WTe 2 ) has garnered a great deal of attention as a strong candidate to be a type-II WSM. However, the Weyl points for WTe 2 are located above the Fermi level, which has prevented us from identifying the locations and the connection to the Fermi arc surface states by using angle-resolved photoemission spectroscopy. Here, we present experimental proof that WTe 2 is a type-II WSM. We measured energy-dependent quasiparticle interference patterns with a cryogenic scanning tunneling microscope, revealing the position of the Weyl point and its connection with the Fermi arc surface states, in agreement with prior theoretical predictions. Our results provide an answer to this crucial question and stimulate further exploration of the characteristics of WSMs.

Anisotropic Fermi surface and quantum limit transport in high mobility three-dimensional Dirac semimetal Cd 3As 2

DOE PAGES

Zhao, Yanfei; Liu, Haiwen; Zhang, Chenglong; ...

2015-09-16

Three-dimensional (3D) topological Dirac semimetals have a linear dispersion in the 3D momentum space and are viewed as the 3D analogues of graphene. Here, we report angle dependent magnetotransport on the newly revealed Cd 3As 2 single crystals and clearly show how the Fermi surface evolves with crystallographic orientations. Remarkably, when the magnetic field lies in [112] or [44more » $$\\bar{1}$$] axis, magnetoresistance oscillations with only single period are present. However, the oscillation shows double periods when the field is applied along [1$$\\bar{1}$$0] direction. Moreover, aligning the magnetic field at certain directions also gives rise to double period oscillations. We attribute the observed anomalous oscillation behavior to the sophisticated geometry of Fermi surface and illustrate a complete 3D Fermi surfaces with two nested anisotropic ellipsoids around the Dirac points. Additionally, a sub-millimeter mean free path at 6 K is found in Cd 3As 2 crystals, indicating ballistic transport in this material. By measuring the magnetoresistance up to 60 T, we reach the quantum limit (n = 1 Landau level) at about 43 T. Lastly, these results improve the knowledge of the Dirac semimetal material Cd 3As 2, and also pave the way for proposing new electronic applications based on 3D Dirac materials.« less

Orbital selectivity causing anisotropy and particle-hole asymmetry in the charge density wave gap of 2 H -TaS2

NASA Astrophysics Data System (ADS)

Zhao, J.; Wijayaratne, K.; Butler, A.; Yang, J.; Malliakas, C. D.; Chung, D. Y.; Louca, D.; Kanatzidis, M. G.; van Wezel, J.; Chatterjee, U.

2017-09-01

We report an in-depth angle-resolved photoemission spectroscopy study on 2 H -TaS2 , a canonical incommensurate charge density wave (CDW) system. This study demonstrates that just as in related incommensurate CDW systems, 2 H -TaSe2 and 2 H -NbSe2 , the energy gap (ΔCDW) of 2 H -TaS2 is localized along the K -centered Fermi surface barrels and is particle-hole asymmetric. The persistence of ΔCDW even at temperatures higher than the CDW transition temperature TCDW in 2 H -TaS2 , reflects the similar pseudogap behavior observed previously in 2 H -TaSe2 and 2 H -NbSe2 . However, in sharp contrast to 2 H -NbSe2 , where ΔCDW is nonzero only in the vicinity of a few "hot spots" on the inner K -centered Fermi surface barrels, ΔCDW in 2 H -TaS2 is nonzero along the entirety of both K -centered Fermi surface barrels. Based on a tight-binding model, we attribute this dichotomy in the momentum dependence and the Fermi surface specificity of ΔCDW between otherwise similar CDW compounds to the different orbital orientations of their electronic states that participate in the CDW pairing. Our results suggest that the orbital selectivity plays a critical role in the description of incommensurate CDW materials.

Ab initio calculation of the G peak intensity of graphene: Laser-energy and Fermi-energy dependence and importance of quantum interference effects

NASA Astrophysics Data System (ADS)

Reichardt, Sven; Wirtz, Ludger

2017-05-01

We present the results of a diagrammatic, fully ab initio calculation of the G peak intensity of graphene. The flexibility and generality of our approach enables us to go beyond the previous analytical calculations in the low-energy regime. We study the laser and Fermi energy dependence of the G peak intensity and analyze the contributions from resonant and nonresonant electronic transitions. In particular, we explicitly demonstrate the importance of quantum interference and nonresonant states for the G peak process. Our method of analysis and computational concept is completely general and can easily be applied to study other materials as well.

Components of the Extragalactic Gamma-Ray Background

NASA Technical Reports Server (NTRS)

Stecker, Floyd W.; Venters, Tonia M.

2011-01-01

We present new theoretical estimates of the relative contributions of unresolved blazars and star-forming galaxies to the extragalactic gamma-ray background (EGB) and discuss constraints on the contributions from alternative mechanisms such as dark matter annihilation and truly diffuse gamma-ray production. We find that the Fermi source count data do not rule out a scenario in which the EGB is dominated by emission from unresolved blazars, though unresolved star-forming galaxies may also contribute significantly to the background, within order-of-magnitude uncertainties. In addition, we find that the spectrum of the unresolved star-forming galaxy contribution cannot explain the EGB spectrum found by EGRET at energies between 50 and 200 MeV, whereas the spectrum of unresolved flat spectrum radio quasars, when accounting for the energy-dependent effects of source confusion, could be consistent with the combined spectrum of the low-energy EGRET EGB measurements and the Fermi-Large Area Telescope EGB measurements.

Lifshitz transitions and zero point lattice fluctuations in sulfur hydride showing near room temperature superconductivity

NASA Astrophysics Data System (ADS)

Bianconi, Antonio; Jarlborg, Thomas

2015-11-01

Emerets's experiments on pressurized sulfur hydride have shown that H3S metal has the highest known superconducting critical temperature Tc = 203 K. The Emerets data show pressure induced changes of the isotope coefficient between 0.25 and 0.5, in disagreement with Eliashberg theory which predicts a nearly constant isotope coefficient.We assign the pressure dependent isotope coefficient to Lifshitz transitions induced by pressure and zero point lattice fluctuations. It is known that pressure could induce changes of the topology of the Fermi surface, called Lifshitz transitions, but were neglected in previous papers on the H3S superconductivity issue. Here we propose thatH3S is a multi-gap superconductor with a first condensate in the BCS regime (located in the large Fermi surface with high Fermi energy) which coexists with second condensates in the BCS-BEC crossover regime (located on the Fermi surface spots with small Fermi energy) near the and Mpoints.We discuss the Bianconi-Perali-Valletta (BPV) superconductivity theory to understand superconductivity in H3S since the BPV theory includes the corrections of the chemical potential due to pairing and the configuration interaction between different condensates, neglected by the Eliashberg theory. These two terms in the BPV theory give the shape resonance in superconducting gaps, similar to Feshbach resonance in ultracold fermionic gases, which is known to amplify the critical temperature. Therefore this work provides some key tools useful in the search for new room temperature superconductors.

Fermi surfaces of the pyrite-type cubic AuSb2 compared with split Fermi surfaces of the ullmannite-type cubic chiral NiSbS and PdBiSe

NASA Astrophysics Data System (ADS)

Nishimura, K.; Kakihana, M.; Nakamura, A.; Aoki, D.; Harima, H.; Hedo, M.; Nakama, T.; Ōnuki, Y.

2018-05-01

We grew high-quality single crystals of AuSb2 with the pyrite (FeS2)-type cubic structure by the Bridgman method and studied the Fermi surface properties by the de Haas-van Alphen (dHvA) experiment and the full potential LAPW band calculation. The Fermi surfaces of AuSb2 are found to be similar to those of NiSbS and PdBiSe with the ullmannite (NiSbS)-type cubic chiral structure because the crystal structures are similar each other and the number of valence electrons is the same between two different compounds. Note that each Fermi surface splits into two Fermi surfaces in NiSbS and PdBiSe, reflecting the non-centrosymmetric crystal structure.

Effects of impurity and Bose-Fermi interactions on the transition temperature of a dilute dipolar Bose-Einstein condensation in trapped Bose-Fermi mixtures

NASA Astrophysics Data System (ADS)

Yavari, H.; Mokhtari, M.

2014-03-01

The effects of impurity and Bose-Fermi interactions on the transition temperature of a dipolar Bose-Einstein condensation in trapped Bose-Fermi mixture, by using the two-fluid model, are investigated. The shift of the transition temperature consists of four contributions due to contact, Bose-Fermi, dipole-dipole, and impurity interactions. We will show that in the presence of an anisotropic trap, the Bose-Fermi correction to the shift of transition temperature due to the excitation spectra of the thermal part is independent of anisotropy factor. Applying our results to trapped Bose-Fermi mixtures shows that, by knowing the impurity effect, the shift of the transition temperature due to Bose-Fermi interaction could be measured for isotropic trap (dipole-dipole contributions is zero) and Feshbach resonance technique (contact potential contribution is negligible).

Angle-Dependent Magnetoresistance in Organic Metals

NASA Astrophysics Data System (ADS)

Blundell, Stephen J.; Singleton, John

1996-12-01

Recent experimental studies of the angle-dependent magnetoresistance in various organic metals have been remarkably successful in elucidating the nature of the low-temperature ground state and providing information about the Fermi surface shape which is hard or impossible to obtain using other techniques. We review various theoretical approaches to describe angel-dependent magnetoresistance and a number of important experimental results which have been obtained.

Probing topological Fermi-Arcs and bulk boundary correspondence in the Weyl semimetal TaAs

NASA Astrophysics Data System (ADS)

Batabyal, Rajib; Morali, Noam; Avraham, Nurit; Sun, Yan; Schmidt, Marcus; Felser, Claudia; Stern, Ady; Yan, Binghai; Beidenkopf, Haim

The relation between surface Fermi-arcs and bulk Weyl cones in a Weyl semimetal, uniquely allows to study the notion of bulk to surface correspondence. We visualize these topological Fermi arc states on the surface of the Weyl semi-metal tantalum arsenide using scanning tunneling spectroscopy. Its surface hosts 12 Fermi arcs amongst several other surface bands of non-topological origin. We detect the possible scattering processes of surface bands in which Fermi arcs are involved including intra- and inter arc scatterings and arc-trivial scatterings. Each of the measured scattering processes entails additional information on the unique nature of Fermi arcs in tantalum arsenide: their contour, their energy-momentum dispersion and its relation with the bulk Weyl nodes. We further identify a sharp distinction between the wave function's spatial distribution of topological versus trivial bands. The non-topological surface bands, which are derived from the arsenic dangling bonds, are tightly bound to the arsenic termination layer. In contrast, the Fermi-arc bands reside on the deeper tantalum layer, penetrating into the bulk, which is predominantly derived from tantalum orbitals.

Adsorption of ammonia at GaN(0001) surface in the mixed ammonia/hydrogen ambient - a summary of ab initio data

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kempisty, Paweł; Krukowski, Stanisław; Interdisciplinary Centre for Materials Modelling, Warsaw University, Pawińskiego 5a, 02-106 Warsaw

Adsorption of ammonia at NH{sub 3}/NH{sub 2}/H-covered GaN(0001) surface was analyzed using results of ab initio calculations. The whole configuration space of partially NH{sub 3}/NH{sub 2}/H-covered GaN(0001) surface was divided into zones of differently pinned Fermi level: at the Ga broken bond state for dominantly bare surface (region I), at the valence band maximum (VBM) for NH{sub 2} and H-covered surface (region II), and at the conduction band minimum (CBM) for NH{sub 3}-covered surface (region III). The electron counting rule (ECR) extension was formulated for the case of adsorbed molecules. The extensive ab intio calculations show the validity of themore » ECR in case of all mixed H-NH{sub 2}-NH{sub 3} coverages for the determination of the borders between the three regions. The adsorption was analyzed using the recently identified dependence of the adsorption energy on the charge transfer at the surface. For region I ammonia adsorbs dissociatively, disintegrating into a H adatom and a HN{sub 2} radical for a large fraction of vacant sites, while for region II adsorption of ammonia is molecular. The dissociative adsorption energy strongly depends on the Fermi level at the surface (pinned) and in the bulk (unpinned) while the molecular adsorption energy is determined by bonding to surface only, in accordance to the recently published theory. Adsorption of Ammonia in region III (Fermi level pinned at CBM) leads to an unstable configuration both molecular and dissociative, which is explained by the fact that broken Ga-bonds are doubly occupied by electrons. The adsorbing ammonia brings 8 electrons to the surface, necessitating the transfer of these two electrons from the Ga broken bond state to the Fermi level. This is an energetically costly process. Adsorption of ammonia at H-covered site leads to the creation of a NH{sub 2} radical at the surface and escape of H{sub 2} molecule. The process energy is close to 0.12 eV, thus not large, but the direct inverse process is not possible due to the escape of the hydrogen molecule.« less

Implementation of a method for calculating temperature-dependent resistivities in the KKR formalism

NASA Astrophysics Data System (ADS)

Mahr, Carsten E.; Czerner, Michael; Heiliger, Christian

2017-10-01

We present a method to calculate the electron-phonon induced resistivity of metals in scattering-time approximation based on the nonequilibrium Green's function formalism. The general theory as well as its implementation in a density-functional theory based Korringa-Kohn-Rostoker code are described and subsequently verified by studying copper as a test system. We model the thermal expansion by fitting a Debye-Grüneisen curve to experimental data. Both the electronic and vibrational structures are discussed for different temperatures, and employing a Wannier interpolation of these quantities we evaluate the scattering time by integrating the electron linewidth on a triangulation of the Fermi surface. Based thereupon, the temperature-dependent resistivity is calculated and found to be in good agreement with experiment. We show that the effect of thermal expansion has to be considered in the whole calculation regime. Further, for low temperatures, an accurate sampling of the Fermi surface becomes important.

Pressure dependence of critical temperature of bulk FeSe from spin fluctuation theory

NASA Astrophysics Data System (ADS)

Hirschfeld, Peter; Kreisel, Andreas; Wang, Yan; Tomic, Milan; Jeschke, Harald; Jacko, Anthony; Valenti, Roser; Maier, Thomas; Scalapino, Douglas

2013-03-01

The critical temperature of the 8K superconductor FeSe is extremely sensitive to pressure, rising to a maximum of 40K at about 10GPa. We test the ability of the current generation of fluctuation exchange pairing theories to account for this effect, by downfolding the density functional theory electronic structure for each pressure to a tight binding model. The Fermi surface found in such a procedure is then used with fixed Hubbard parameters to determine the pairing strength using the random phase approximation for the spin singlet pairing vertex. We find that the evolution of the Fermi surface captured by such an approach is alone not sufficient to explain the observed pressure dependence, and discuss alternative approaches. PJH, YW, AK were supported by DOE DE-FG02-05ER46236, the financial support of MT, HJ, and RV from the DFG Schwerpunktprogramm 1458 is kindly acknowledged.

Fermi arc plasmons in Weyl semimetals

NASA Astrophysics Data System (ADS)

Song, Justin C. W.; Rudner, Mark S.

2017-11-01

In the recently discovered Weyl semimetals, the Fermi surface may feature disjoint, open segments—the so-called Fermi arcs—associated with topological states bound to exposed crystal surfaces. Here we show that the collective dynamics of electrons near such surfaces sharply departs from that of a conventional three-dimensional metal. In magnetic systems with broken time reversal symmetry, the resulting Fermi arc plasmons (FAPs) are chiral, with dispersion relations featuring open, hyperbolic constant frequency contours. As a result, a large range of surface plasmon wave vectors can be supported at a given frequency, with corresponding group velocity vectors directed along a few specific collimated directions. Fermi arc plasmons can be probed using near-field photonics techniques, which may be used to launch highly directional, focused surface plasmon beams. The unusual characteristics of FAPs arise from the interplay of bulk and surface Fermi arc carrier dynamics and give a window into the unusual fermiology of Weyl semimetals.

Surface to bulk Fermi arcs via Weyl nodes as topological defects

PubMed Central

Kim, Kun Woo; Lee, Woo-Ram; Kim, Yong Baek; Park, Kwon

2016-01-01

A hallmark of Weyl semimetal is the existence of surface Fermi arcs. An intriguing question is what determines the connectivity of surface Fermi arcs, when multiple pairs of Weyl nodes are present. To answer this question, we show that the locations of surface Fermi arcs are predominantly determined by the condition that the Zak phase integrated along the normal-to-surface direction is . The Zak phase can reveal the peculiar topological structure of Weyl semimetal directly in the bulk. Here, we show that the winding of the Zak phase around each projected Weyl node manifests itself as a topological defect of the Wannier–Stark ladder, energy eigenstates under an electric field. Remarkably, this leads to bulk Fermi arcs, open-line segments in the bulk spectra. Bulk Fermi arcs should exist in conjunction with surface counterparts to conserve the Weyl fermion number under an electric field, which is supported by explicit numerical evidence. PMID:27845342

Fermi-surface-free superconductivity in underdoped (Bi,Pb)(Sr,La) 2CuO 6+δ (Bi2201)

DOE PAGES

Mistark, Peter; Hafiz, Hasnain; Markiewicz, Robert S.; ...

2015-06-18

Fermi-surface-free superconductivity arises when the superconducting order pulls down spectral weight from a band that is completely above the Fermi energy in the normal state. Here, we show that this can arise in hole-doped cuprates when a competing order causes a reconstruction of the Fermi surface. The change in Fermi surface topology is accompanied by a characteristic rise in the spectral weight. Finally, our results support the presence of a trisected superconducting dome, and suggest that superconductivity is responsible for stabilizing the (π,π) magnetic order at higher doping.

Observation of universal strong orbital-dependent correlation effects in iron chalcogenides

DOE PAGES

Yi, M.; Liu, Z. -K.; Zhang, Y.; ...

2015-07-23

Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide superconductors. Here, we use angle-resolved photoemission spectroscopy to measure three representative iron chalcogenides, FeTe 0.56Se 0.44, monolayer FeSe grown on SrTiO 3 and K 0.76Fe 1.72Se 2. We show that these superconductors are all strongly correlated, with an orbital-selective strong renormalization in the dxy bands despite having drastically different Fermi surface topologies. Furthermore, raising temperature brings all three compounds frommore » a metallic state to a phase where the dxy orbital loses all spectral weight while other orbitals remain itinerant. As a result, these observations establish that iron chalcogenides display universal orbital-selective strong correlations that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase, hence placing strong constraints for theoretical understanding of iron-based superconductors.« less

Observation of universal strong orbital-dependent correlation effects in iron chalcogenides

PubMed Central

Yi, M.; Liu, Z-K; Zhang, Y.; Yu, R.; Zhu, J.-X.; Lee, J.J.; Moore, R.G.; Schmitt, F.T.; Li, W.; Riggs, S.C.; Chu, J.-H.; Lv, B.; Hu, J.; Hashimoto, M.; Mo, S.-K.; Hussain, Z.; Mao, Z.Q.; Chu, C.W.; Fisher, I.R.; Si, Q.; Shen, Z.-X.; Lu, D.H.

2015-01-01

Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide superconductors. Here, we use angle-resolved photoemission spectroscopy to measure three representative iron chalcogenides, FeTe0.56Se0.44, monolayer FeSe grown on SrTiO3 and K0.76Fe1.72Se2. We show that these superconductors are all strongly correlated, with an orbital-selective strong renormalization in the dxy bands despite having drastically different Fermi surface topologies. Furthermore, raising temperature brings all three compounds from a metallic state to a phase where the dxy orbital loses all spectral weight while other orbitals remain itinerant. These observations establish that iron chalcogenides display universal orbital-selective strong correlations that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase, hence placing strong constraints for theoretical understanding of iron-based superconductors. PMID:26204461

π and 4 π Josephson Effects Mediated by a Dirac Semimetal

NASA Astrophysics Data System (ADS)

Yu, W.; Pan, W.; Medlin, D. L.; Rodriguez, M. A.; Lee, S. R.; Bao, Zhi-qiang; Zhang, F.

2018-04-01

Cd3As2 is a three-dimensional topological Dirac semimetal with connected Fermi-arc surface states. It has been suggested that topological superconductivity can be achieved in the nontrivial surface states of topological materials by utilizing the superconductor proximity effect. Here we report observations of both π and 4 π periodic supercurrents in aluminum-Cd3As2 -aluminum Josephson junctions. The π period is manifested by both the magnetic-field dependence of the critical supercurrent and the appearance of half-integer Shapiro steps in the ac Josephson effect. Our macroscopic theory suggests that the π period arises from interference between the induced bulk superconductivity and the induced Fermi-arc surface superconductivity. The 4 π period is manifested by the missing first Shapiro steps and is expected for topological superconductivity.

Surface structure of neutron stars with high magnetic fields

NASA Technical Reports Server (NTRS)

Fushiki, I.; Gudmundsson, E. H.; Pethick, C. J.

1989-01-01

The equation of state of cold dense matter in strong magnetic fields is calculated in the Thomas-Fermi and Thomas-Fermi-Dirac approximations. For use in the latter calculation, a new expression is derived for the exchange energy of the uniform electron gas in a strong magnetic field. Detailed calculations of the density profile in the surface region of a neutron star are described for a variety of equations of state, and these show that the surface density profile is strongly affected by the magnetic field, irrespective of whether or not matter in a magnetic field has a condensed state bound with respect to isolated atoms. It is also shown that, as a consequence of the field dependence of the screening potential, magnetic fields can significantly increase nuclear reaction rates.

Scanning tunneling spectroscopy of the surface states of Dirac fermions in thermoelectrics based on bismuth telluride

NASA Astrophysics Data System (ADS)

Lukyanova, L. N.; Makarenko, I. V.; Usov, O. A.; Dementev, P. A.

2018-05-01

The morphology of the interlayer van der Waals surface and differential tunneling conductance in p-Bi2‑xSbxTe3‑ySey solid solutions were studied by scanning tunneling microscopy and spectroscopy in dependence on compositions. The topological characteristics of the Dirac fermion surface states were determined. It was shown that the thermoelectric power factor and the material parameter enhance with the shift of the Dirac point to the top of the valence band with the increasing of atomic substitution in these thermoelectrics. A correlation between topological characteristics, power factor and material parameters was found. A growth contribution of the surface states is determined by an increase of the Fermi velocity for large atomic substitutions of Bi at x > 1.5 and small substitutions in the Te sublattice (y = 0.06). In compositions with smaller substitutions at x = (1–1.3) and y = (0.06–0.09), similar effect of the surface states is determined by raising the surface concentration of charge carriers.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hartstein, M.; Toews, W. H.; Hsu, Y. -T.

The search for a Fermi surface in the absence of a conventional Fermi liquid has thus far yielded very few potential candidates. Among promising materials are spin-frustrated Mott insulators near the insulator–metal transition, where theory predicts a Fermi surface associated with neutral low-energy excitations. In this paper, we reveal another route to experimentally realize a Fermi surface in the absence of a Fermi liquid by the experimental study of a Kondo insulator SmB 6 positioned close to the insulator–metal transition. We present experimental signatures down to low temperatures (<<1 K) associated with a Fermi surface in the bulk, including amore » sizeable linear specific heat coefficient, and on the application of a finite magnetic field, bulk magnetic quantum oscillations, finite quantum oscillatory entropy, and substantial enhancement in thermal conductivity well below the charge gap energy scale. Finally, the weight of evidence indicates that despite an extreme instance of Fermi liquid breakdown in Kondo insulating SmB 6, a Fermi surface arises from novel itinerant low-energy excitations that couple to magnetic fields, but not weak DC electric fields.« less

Thermally Driven Electronic Topological Transition in FeTi

DOE PAGES

Yang, F. C.; Muñoz, J. A.; Hellman, O.; ...

2016-08-08

In this paper, ab initio molecular dynamics, supported by inelastic neutron scattering and nuclear resonant inelastic x-ray scattering, showed an anomalous thermal softening of the M 5 - phonon mode in B2-ordered FeTi that could not be explained by phonon-phonon interactions or electron-phonon interactions calculated at low temperatures. A computational investigation showed that the Fermi surface undergoes a novel thermally driven electronic topological transition, in which new features of the Fermi surface arise at elevated temperatures. Finally, the thermally induced electronic topological transition causes an increased electronic screening for the atom displacements in the M 5 - phonon mode andmore » an adiabatic electron-phonon interaction with an unusual temperature dependence.« less

Positron Annihilation Studies of the Electronic Structure of Selected High-Temperature Cuprate and Organic Superconductors.

NASA Astrophysics Data System (ADS)

Chan, Lie Ping

The understanding of the electronic structure of the high-T_{c} superconductors could be important for a full theoretical description of the mechanism behind superconductivity in these materials. In this thesis, we present our measurements of the positron -electron momentum distributions of the cuprate superconductors Bi_2Sr_2CaCu _2O_8, Tl _2Ba_2Ca _2Cu_3O_ {10}, and the organic superconductor kappa-(BEDT)_2Cu(NCS) _2. We use the positron Two-dimensional Angular Correlation of Annihilation Radiation technique to make the measurements on single crystals and compare our high-statistics data with band structure calculations to determine the existence and nature of the respective Fermi surfaces. The spectra from unannealed Bi _2Sr_2CaCu _2O_8 exhibit effects of the superlattice modulation in the BiO_2 layers, and a theoretical understanding of the modulation effects on the electronic band structure is required to interpret these spectra. Since the present theory does not consider the modulation, we have developed a technique to remove the modulation effects from our spectra, and the resultant data when compared with the positron -electron momentum distribution calculation, yield features consistent with the predicted CuO_2 and BiO_2 Fermi surfaces. In the data from unannealed Tl_2Ba _2Ca_2Cu_3 O_{10}, we only observe indications of the TlO Fermi surfaces, and attribute the absence of the predicted CuO_2 Fermi surfaces to the poor sample quality. In the absence of positron-electron momentum calculations for kappa-(BEDT)_2Cu(NCS) _2, we compare our data to electronic band structure calculations, and observed features suggestive of the predicted Fermi surface contributions from the BEDT cation layers. A complete positron-electron calculation for kappa-(BEDT)_2 Cu(NCS)_2 is required to understand the positron wavefunction effects in this material.

Thermoelectric Properties in Fermi Level Tuned Topological Materials (Bi1-xSnx)2Te3

NASA Astrophysics Data System (ADS)

Lin, Chan-Chieh; Shon, Won Hyuk; Rathnam, Lydia; Rhyee, Jong-Soo

2018-03-01

We investigated the thermoelectric properties of Sn-doped (Bi1-xSnx)2Te3 (x = 0, 0.1, 0.3, 0.5, and 0.7%) compounds, which is known as topological insulators. Fermi level tuning by Sn-doping can be justified by the n- to p-type transition with increasing Sn-doping concentration, as confirmed by Seebeck coefficient and Hall coefficient. Near x = 0.3 and 0.5%, the Fermi level resides inside the bulk band gap, resulting in a low Seebeck coefficient and increase of electrical resistivity. The magnetoconductivity with applying magnetic field showed weak antilocalization (WAL) effect for pristine Bi2Te3 while Sn-doped compounds do not follow the WAL behavior of magneto-conductivity, implying that the topological surface Dirac band contribution in magneto-conductivity is suppressed with decreasing the Fermi level by Sn-doping. This research can be applied to the topological composite of p-type/n-type topological materials by Fermi level tuning via Sn-doping in Bi2Te3 compounds.

High surface conductivity of Fermi-arc electrons in Weyl semimetals

NASA Astrophysics Data System (ADS)

Resta, Giacomo; Pi, Shu-Ting; Wan, Xiangang; Savrasov, Sergey Y.

2018-02-01

Weyl semimetals (WSMs), a new type of topological condensed matter, are currently attracting great interest due to their unusual electronic states and intriguing transport properties such as chiral anomaly induced negative magnetoresistance, a semiquantized anomalous Hall effect, and the debated chiral magnetic effect. These systems are close cousins of topological insulators (TIs) which are known for their disorder-tolerant surface states. Similarly, WSMs exhibit unique topologically protected Fermi-arc surface states. Here, we analyze electron-phonon scattering, a primary source of resistivity in metals at finite temperatures, as a function of the shape of the Fermi arc where we find that the impact on surface transport is significantly dependent on the arc curvature and disappears in the limit of a straight arc. Next, we discuss the effect of strong surface disorder on the resistivity by numerically simulating a tight-binding model with the presence of quenched surface vacancies using the coherent potential approximation and Kubo-Greenwood formalism. We find that the limit of a straight arc geometry is remarkably disorder tolerant, producing surface conductivity that is one to two orders of magnitude larger than a comparable setup with surface states of TI. This is primarily attributed to a significantly different hybridization strength of the surface states with the remaining electrons in two systems. Finally, a simulation of the effects of surface vacancies on TaAs is presented, illustrating the disorder tolerance of the topological surface states in a recently discovered WSM material.

Depletion region surface effects in electron beam induced current measurements.

PubMed

Haney, Paul M; Yoon, Heayoung P; Gaury, Benoit; Zhitenev, Nikolai B

2016-09-07

Electron beam induced current (EBIC) is a powerful characterization technique which offers the high spatial resolution needed to study polycrystalline solar cells. Current models of EBIC assume that excitations in the p - n junction depletion region result in perfect charge collection efficiency. However we find that in CdTe and Si samples prepared by focused ion beam (FIB) milling, there is a reduced and nonuniform EBIC lineshape for excitations in the depletion region. Motivated by this, we present a model of the EBIC response for excitations in the depletion region which includes the effects of surface recombination from both charge-neutral and charged surfaces. For neutral surfaces we present a simple analytical formula which describes the numerical data well, while the charged surface response depends qualitatively on the location of the surface Fermi level relative to the bulk Fermi level. We find the experimental data on FIB-prepared Si solar cells is most consistent with a charged surface, and discuss the implications for EBIC experiments on polycrystalline materials.

Facet-specific interaction between methanol and TiO2 probed by sum-frequency vibrational spectroscopy.

PubMed

Yang, Deheng; Li, Yadong; Liu, Xinyi; Cao, Yue; Gao, Yi; Shen, Y Ron; Liu, Wei-Tao

2018-04-24

The facet-specific interaction between molecules and crystalline catalysts, such as titanium dioxides (TiO 2 ), has attracted much attention due to possible facet-dependent reactivity. Using surface-sensitive sum-frequency vibrational spectroscopy, we have studied how methanol interacts with different common facets of crystalline TiO 2 , including rutile(110), (001), (100), and anatase(101), under ambient temperature and pressure. We found that methanol adsorbs predominantly in the molecular form on all of the four surfaces, while spontaneous dissociation into methoxy occurs preferentially when these surfaces become defective. Extraction of Fermi resonance coupling between stretch and bending modes of the methyl group in analyzing adsorbed methanol spectra allows determination of the methanol adsorption isotherm. The isotherms obtained for the four surfaces are nearly the same, yielding two adsorbed Gibbs free energies associated with two different adsorption configurations singled out by ab initio calculations. They are ( i ) ∼-20 kJ/mol for methanol with its oxygen attached to a low-coordinated surface titanium, and ( ii ) ∼-5 kJ/mol for methanol hydrogen-bonded to a surface oxygen and a neighboring methanol molecule. Despite similar adsorption energetics, the Fermi resonance coupling strength for adsorbed methanol appears to depend sensitively on the surface facet and coverage.

Comparison between two scalar field models using rotation curves of spiral galaxies

NASA Astrophysics Data System (ADS)

Fernández-Hernández, Lizbeth M.; Rodríguez-Meza, Mario A.; Matos, Tonatiuh

2018-04-01

Scalar fields have been used as candidates for dark matter in the universe, from axions with masses ∼ 10-5eV until ultra-light scalar fields with masses ∼ Axions behave as cold dark matter while the ultra-light scalar fields galaxies are Bose-Einstein condensate drops. The ultra-light scalar fields are also called scalar field dark matter model. In this work we study rotation curves for low surface brightness spiral galaxies using two scalar field models: the Gross-Pitaevskii Bose-Einstein condensate in the Thomas-Fermi approximation and a scalar field solution of the Klein-Gordon equation. We also used the zero disk approximation galaxy model where photometric data is not considered, only the scalar field dark matter model contribution to rotation curve is taken into account. From the best-fitting analysis of the galaxy catalog we use, we found the range of values of the fitting parameters: the length scale and the central density. The worst fitting results (values of χ red2 much greater than 1, on the average) were for the Thomas-Fermi models, i.e., the scalar field dark matter is better than the Thomas- Fermi approximation model to fit the rotation curves of the analysed galaxies. To complete our analysis we compute from the fitting parameters the mass of the scalar field models and two astrophysical quantities of interest, the dynamical dark matter mass within 300 pc and the characteristic central surface density of the dark matter models. We found that the value of the central mass within 300 pc is in agreement with previous reported results, that this mass is ≈ 107 M ⊙/pc2, independent of the dark matter model. And, on the contrary, the value of the characteristic central surface density do depend on the dark matter model.

Complementary views on electron spectra: From fluctuation diagnostics to real-space correlations

NASA Astrophysics Data System (ADS)

Gunnarsson, O.; Merino, J.; Schäfer, T.; Sangiovanni, G.; Rohringer, G.; Toschi, A.

2018-03-01

We study the relation between the microscopic properties of a many-body system and the electron spectra, experimentally accessible by photoemission. In a recent paper [O. Gunnarsson et al., Phys. Rev. Lett. 114, 236402 (2015), 10.1103/PhysRevLett.114.236402], we introduced the "fluctuation diagnostics" approach to extract the dominant wave-vector-dependent bosonic fluctuations from the electronic self-energy. Here, we first reformulate the theory in terms of fermionic modes to render its connection with resonance valence bond (RVB) fluctuations more transparent. Second, by using a large-U expansion, where U is the Coulomb interaction, we relate the fluctuations to real-space correlations. Therefore, it becomes possible to study how electron spectra are related to charge, spin, superconductivity, and RVB-like real-space correlations, broadening the analysis of an earlier work [J. Merino and O. Gunnarsson, Phys. Rev. B 89, 245130 (2014), 10.1103/PhysRevB.89.245130]. This formalism is applied to the pseudogap physics of the two-dimensional Hubbard model, studied in the dynamical cluster approximation. We perform calculations for embedded clusters with up to 32 sites, having three inequivalent K points at the Fermi surface. We find that as U is increased, correlation functions gradually attain values consistent with an RVB state. This first happens for correlation functions involving the antinodal point and gradually spreads to the nodal point along the Fermi surface. Simultaneously, a pseudogap opens up along the Fermi surface. We relate this to a crossover from a Kondo-type state to an RVB-like localized cluster state and to the presence of RVB and spin fluctuations. These changes are caused by a strong momentum dependence in the cluster bath couplings along the Fermi surface. We also show, from a more algorithmic perspective, how the time-consuming calculations in fluctuation diagnostics can be drastically simplified.

Spin-split fermi surfaces in CexLa1-xB6 and PrxLa1-xB6

NASA Astrophysics Data System (ADS)

Isshiki, T.; Endo, M.; Sugi, M.; Kimura, N.; Nakamura, S.; Nojima, T.; Aoki, H.; Kunii, S.

2006-05-01

We have performed the dHvA measurements on CexLa1-xB6 and PrxLa1-xB6 compounds to study spin splitting of the Fermi surfaces. In PrB 6 we have found new frequency branches to confirm that the Fermi surface splits into up and down spin Fermi surfaces, whereas no spin splitting has been found for x=0.25,0.5,0.75. We have also found several new frequency branches in CeB6. The new frequency branches imply that the Fermi surfaces of up and down spin conduction electrons are significantly different in CeB6 as well as in PrB6.

Importance of nonlocal electron correlation in the BaNiS2 semimetal from quantum oscillations studies

NASA Astrophysics Data System (ADS)

Klein, Yannick; Casula, Michele; Santos-Cottin, David; Audouard, Alain; Vignolles, David; Fève, Gwendal; Freulon, Vincent; Plaçais, Bernard; Verseils, Marine; Yang, Hancheng; Paulatto, Lorenzo; Gauzzi, Andrea

2018-02-01

By means of Shubnikov-de Haas and de Haas-van Alphen oscillations, and ab initio calculations, we have studied the Fermi surface of high-quality BaNiS2 single crystals, with mean free path l ˜400 Å . The angle and temperature dependence of quantum oscillations indicates a quasi-two-dimensional Fermi surface, made of an electronlike tube centered at Γ , and of four holelike cones, generated by Dirac bands, weakly dispersive in the out-of-plane direction. Ab initio electronic structure calculations, in the density functional theory framework, show that the inclusion of screened exchange is necessary to account for the experimental Fermi pockets. Therefore, the choice of the functional becomes crucial. A modified HSE hybrid functional with 7% of exact exchange outperforms both GGA and GGA +U density functionals, signaling the importance of nonlocal screened-exchange interactions in BaNiS2, and, more generally, in 3 d compensated semimetals.

Observation of the spin-polarized surface state in a noncentrosymmetric superconductor BiPd

DOE Office of Scientific and Technical Information (OSTI.GOV)

Neupane, Madhab; Alidoust, Nasser; Hosen, M. Mofazzel

Recently, noncentrosymmetric superconductor BiPd has attracted considerable research interest due to the possibility of hosting topological superconductivity. Here in this paper we report a systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES study of the normal state electronic and spin properties of BiPd. Our experimental results show the presence of a surface state at higher-binding energy with the location of Dirac point at around 700 meV below the Fermi level. The detailed photon energy, temperature-dependent and spin-resolved ARPES measurements complemented by our first-principles calculations demonstrate the existence of the spin-polarized surface states at high-binding energy. The absence of suchmore » spin-polarized surface states near the Fermi level negates the possibility of a topological superconducting behaviour on the surface. Our direct experimental observation of spin-polarized surface states in BiPd provides critical information that will guide the future search for topological superconductivity in noncentrosymmetric materials.« less

Observation of the spin-polarized surface state in a noncentrosymmetric superconductor BiPd

DOE PAGES

Neupane, Madhab; Alidoust, Nasser; Hosen, M. Mofazzel; ...

2016-11-07

Recently, noncentrosymmetric superconductor BiPd has attracted considerable research interest due to the possibility of hosting topological superconductivity. Here in this paper we report a systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES study of the normal state electronic and spin properties of BiPd. Our experimental results show the presence of a surface state at higher-binding energy with the location of Dirac point at around 700 meV below the Fermi level. The detailed photon energy, temperature-dependent and spin-resolved ARPES measurements complemented by our first-principles calculations demonstrate the existence of the spin-polarized surface states at high-binding energy. The absence of suchmore » spin-polarized surface states near the Fermi level negates the possibility of a topological superconducting behaviour on the surface. Our direct experimental observation of spin-polarized surface states in BiPd provides critical information that will guide the future search for topological superconductivity in noncentrosymmetric materials.« less

Do the surface Fermi arcs in Weyl semimetals survive disorder?

NASA Astrophysics Data System (ADS)

Wilson, Justin H.; Pixley, J. H.; Huse, David A.; Refael, Gil; Das Sarma, S.

2018-06-01

We theoretically study the topological robustness of the surface physics induced by Weyl Fermi-arc surface states in the presence of short-ranged quenched disorder and surface-bulk hybridization. This is investigated with numerically exact calculations on a lattice model exhibiting Weyl Fermi arcs. We find that the Fermi-arc surface states, in addition to having a finite lifetime from disorder broadening, hybridize with nonperturbative bulk rare states making them no longer bound to the surface (i.e., they lose their purely surface spectral character). Thus, we provide strong numerical evidence that the Weyl Fermi arcs are not topologically protected from disorder. Nonetheless, the surface chiral velocity is robust and survives in the presence of strong disorder, persisting all the way to the Anderson-localized phase by forming localized current loops that live within the localization length of the surface. Thus, the Weyl semimetal is not topologically robust to the presence of disorder, but the surface chiral velocity is.

Momentum density and Fermi surface of Nd2-xCexCuO4-δ

NASA Astrophysics Data System (ADS)

Shukla, A.; Barbiellini, B.; Hoffmann, L.; Manuel, A. A.; Sadowski, W.; Walker, E.; Peter, M.

1996-02-01

High-temperature positron two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurements have recently been succesfully applied to map parts of the Fermi surface of YBa2Cu3O7-δ. Using the same principle, we have been able to observe with a bulk sensitive method, the Fermi surface of Nd2-xCexCuO4-δ. Although positron trapping by defects and correlation effects are strong, positron 2D-ACAR measurements provide a signal from the Fermi surface which agrees with band-structure calculations, confirming earlier surface sensitive photoemission experiments.

Electron-phonon relaxation and excited electron distribution in gallium nitride

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhukov, V. P.; Donostia International Physics Center; Tyuterev, V. G., E-mail: valtyut00@mail.ru

2016-08-28

We develop a theory of energy relaxation in semiconductors and insulators highly excited by the long-acting external irradiation. We derive the equation for the non-equilibrium distribution function of excited electrons. The solution for this function breaks up into the sum of two contributions. The low-energy contribution is concentrated in a narrow range near the bottom of the conduction band. It has the typical form of a Fermi distribution with an effective temperature and chemical potential. The effective temperature and chemical potential in this low-energy term are determined by the intensity of carriers' generation, the speed of electron-phonon relaxation, rates ofmore » inter-band recombination, and electron capture on the defects. In addition, there is a substantial high-energy correction. This high-energy “tail” largely covers the conduction band. The shape of the high-energy “tail” strongly depends on the rate of electron-phonon relaxation but does not depend on the rates of recombination and trapping. We apply the theory to the calculation of a non-equilibrium distribution of electrons in an irradiated GaN. Probabilities of optical excitations from the valence to conduction band and electron-phonon coupling probabilities in GaN were calculated by the density functional perturbation theory. Our calculation of both parts of distribution function in gallium nitride shows that when the speed of the electron-phonon scattering is comparable with the rate of recombination and trapping then the contribution of the non-Fermi “tail” is comparable with that of the low-energy Fermi-like component. So the high-energy contribution can essentially affect the charge transport in the irradiated and highly doped semiconductors.« less

Electronic properties of Cr-N codoped rutile TiO2(110) thin films

NASA Astrophysics Data System (ADS)

Cheng, Zhengwang; Zhang, Lili; Dong, Shihui; Ma, Xiaochuan; Ju, Huanxin; Zhu, Junfa; Cui, Xuefeng; Zhao, Jin; Wang, Bing

2017-12-01

We report our investigation on the electronic properties of Cr-N codoped rutile TiO2(110) single crystal thin films, homoepitaxially grown by pulsed-laser-deposition method, and characterized using scanning tunneling microscopy and spectroscopy (STM/STS), X-ray/ultraviolet photoemission spectroscopy (XPS/UPS), in combination with first-principles calculations. Our results show that the bandgap reduction of the TiO2(110) surface is mainly contributed by the delocalized states whose position is at 2.0 eV below the Fermi level, introduced by the substitutional codoped Cr-2N pair, which is evidenced by the accordance of the results between the STS spectra and the calculated DOS. The codoped Cr-N pair contributes the gap state at about 0.8 eV below the Fermi level, in consistent with the theoretical calculations. While, the monodoped Cr contributes the states either close to the valence band maximum or the conduction band minimum, which should not contribute to the bandgap reduction too much. Our experimental results joint with theoretical calculations provide an atomic view of the bandgap reduction of the rutile TiO2(110) surface, which indicates that the excess substitutional N atoms should be important to efficiently narrow the bandgap by introducing the Cr-2N pairs.

Systematics of the temperature-dependent interplane resistivity in Ba(Fe 1-xM x)₂As₂ (M=Co, Rh, Ni, and Pd)

DOE PAGES

Tanatar, M. A.; Ni, N.; Thaler, A.; ...

2011-07-27

Temperature-dependent interplane resistivity ρ c(T) was measured systematically as a function of transition-metal substitution in the iron-arsenide superconductors Ba(Fe 1-xM x)₂As₂, M=Ni, Pd, Rh. The data are compared with the behavior found in Ba(Fe 1-xCo x)₂As₂, revealing resistive signatures of pseudogap. In all compounds we find resistivity crossover at a characteristic pseudogap temperature T* from nonmetallic to metallic temperature dependence on cooling. Suppression of T* proceeds very similarly in cases of Ni and Pd doping and much faster than in similar cases of Co and Rh doping. In cases of Co and Rh doping an additional minimum in the temperature-dependentmore » ρ c emerges for high dopings, when superconductivity is completely suppressed. These features are consistent with the existence of a charge gap covering part of the Fermi surface. The part of the Fermi surface affected by this gap is notably larger for Ni- and Pd-doped compositions than in Co- and Rh-doped compounds.« less

Superconductivity mediated by quantum critical antiferromagnetic fluctuations: the rise and fall of hot spots

NASA Astrophysics Data System (ADS)

Wang, Xiaoyu; Schattner, Yoni; Berg, Erez; Fernandes, Rafael

The maximum transition temperature Tc observed in the phase diagrams of several unconventional superconductors takes place in the vicinity of a putative antiferromagnetic quantum critical point. This observation motivated the theoretical proposal that superconductivity in these systems may be driven by quantum critical fluctuations, which in turn can also promote non-Fermi liquid behavior. In this talk, we present a combined analytical and sign-problem-free Quantum Monte Carlo investigation of the spin-fermion model - a widely studied low-energy model for the interplay between superconductivity and magnetic fluctuations. By engineering a series of band dispersions that interpolate between near-nested and open Fermi surfaces, and by also varying the strength of the spin-fermion interaction, we find that the hot spots of the Fermi surface provide the dominant contribution to the pairing instability in this model. We show that the analytical expressions for Tc and for the pairing susceptibility, obtained within a large-N Eliashberg approximation to the spin-fermion model, agree well with the Quantum Monte Carlo data, even in the regime of interactions comparable to the electronic bandwidth. DE-SC0012336.

The electronic structures and work functions of (100) surface of typical binary and doped REB6 single crystals

NASA Astrophysics Data System (ADS)

Liu, Hongliang; Zhang, Xin; Xiao, Yixin; Zhang, Jiuxing

2018-03-01

The density function theory been used to calculate the electronic structures of binary and doped rare earth hexaborides (REB6), which exhibits the large density of states (DOS) near Fermi level. The d orbital elections of RE element contribute the electronic states of election emission near the Fermi level, which imply that the REB6 (RE = La, Ce, Gd) with wide distribution of high density d orbital electrons could provide a lower work function and excellent emission properties. Doping RE elements into binary REB6 can adjust DOS and the position of the Fermi energy level. The calculated work functions of considered REB6 (100) surface show that the REB6 (RE = La, Ce, Gd) have lower work function and doping RE elements with active d orbital electrons can significantly reduce work function of binary REB6. The thermionic emission test results are basically accordant with the calculated value, proving the first principles calculation could provide a good theoretical guidance for the study of electron emission properties of REB6.

Direct k-space mapping of the electronic structure in an oxide-oxide interface.

PubMed

Berner, G; Sing, M; Fujiwara, H; Yasui, A; Saitoh, Y; Yamasaki, A; Nishitani, Y; Sekiyama, A; Pavlenko, N; Kopp, T; Richter, C; Mannhart, J; Suga, S; Claessen, R

2013-06-14

The interface between LaAlO(3) and SrTiO(3) hosts a two-dimensional electron system of itinerant carriers, although both oxides are band insulators. Interface ferromagnetism coexisting with superconductivity has been found and attributed to local moments. Experimentally, it has been established that Ti 3d electrons are confined to the interface. Using soft x-ray angle-resolved resonant photoelectron spectroscopy we have directly mapped the interface states in k space. Our data demonstrate a charge dichotomy. A mobile fraction contributes to Fermi surface sheets, whereas a localized portion at higher binding energies is tentatively attributed to electrons trapped by O vacancies in the SrTiO(3). While photovoltage effects in the polar LaAlO(3) layers cannot be excluded, the apparent absence of surface-related Fermi surface sheets could also be fully reconciled in a recently proposed electronic reconstruction picture where the built-in potential in the LaAlO(3) is compensated by surface O vacancies serving also as a charge reservoir.

Strong Rashba effect in the localized impurity states of halogen-doped monolayer PtSe2

NASA Astrophysics Data System (ADS)

Absor, Moh. Adhib Ulil; Santoso, Iman; Harsojo, Abraha, Kamsul; Kotaka, Hiroki; Ishii, Fumiyuki; Saito, Mineo

2018-05-01

The recent epitaxial growth of the 1 T phase of the PtSe2 monolayer (ML) has opened the possibility for novel applications, in particular for a spintronics device. However, in contrast to the 2 H phase of transition-metal dichalcogenides (TMDs), the absence of spin splitting in the PtSe2 ML may limit the functionality for spintronics application. Through fully relativistic density-functional theory calculations, we show that large spin splitting can be induced in the PtSe2 ML by introducing a substitutional halogen impurity. Depending on the atomic number Z of the halogen dopants, we observe an enhancement of the spin splitting in the localized impurity states (LIS), which is due to the increased contribution of the p -d orbital coupling. More importantly, we identify very large Rashba splitting in the LIS near the Fermi level around the Γ point characterized by hexagonal warping of the Fermi surface. We show that the Rashba splitting can be controlled by adjusting the doping concentration. Therefore, this work provides a possible way to induce significant Rashba splitting in the two-dimensional TMDs, which is useful for spintronic devices operating at room temperature.

Evolution of electronic states in n-type copper oxide superconductor via electric double layer gating

NASA Astrophysics Data System (ADS)

Jin, Kui; Hu, Wei; Zhu, Beiyi; Kim, Dohun; Yuan, Jie; Sun, Yujie; Xiang, Tao; Fuhrer, Michael S.; Takeuchi, Ichiro; Greene, Richard. L.

2016-05-01

The occurrence of electrons and holes in n-type copper oxides has been achieved by chemical doping, pressure, and/or deoxygenation. However, the observed electronic properties are blurred by the concomitant effects such as change of lattice structure, disorder, etc. Here, we report on successful tuning the electronic band structure of n-type Pr2-xCexCuO4 (x = 0.15) ultrathin films, via the electric double layer transistor technique. Abnormal transport properties, such as multiple sign reversals of Hall resistivity in normal and mixed states, have been revealed within an electrostatic field in range of -2 V to + 2 V, as well as varying the temperature and magnetic field. In the mixed state, the intrinsic anomalous Hall conductivity invokes the contribution of both electron and hole-bands as well as the energy dependent density of states near the Fermi level. The two-band model can also describe the normal state transport properties well, whereas the carrier concentrations of electrons and holes are always enhanced or depressed simultaneously in electric fields. This is in contrast to the scenario of Fermi surface reconstruction by antiferromagnetism, where an anti-correlation is commonly expected.

Fermi surface and quantum well states of V(110) films on W(110)

NASA Astrophysics Data System (ADS)

Krupin, Oleg; Rotenberg, Eli; Kevan, S. D.

2007-09-01

Using angle-resolved photoemission spectroscopy, we have measured the Fermi surface of V(110) films epitaxially grown on a W(110) substrate. We compare our results for thicker films to existing calculations and measurements for bulk vanadium and find generally very good agreement. For thinner films, we observe and analyse a diverse array of quantum well states that split and distort the Fermi surface segments. We have searched unsuccessfully for a thickness-induced topological transition associated with contact between the zone-centre jungle gym and zone-boundary hole ellipsoid Fermi surface segments. We also find no evidence for ferromagnetic splitting of any bands on this surface.

Electronic structure in high temperature superconducting oxides

NASA Astrophysics Data System (ADS)

Howell, R. H.; Sterne, P.; Solal, F.; Fluss, M. J.; Tobin, J.; Obrien, J.; Radousky, H. B.; Haghighi, H.; Kaiser, J. H.; Rayner, S. L.

1991-08-01

We have performed measurements on entwined single crystals of YBCO using both photoemission and positron angular correlation of annihilation radiation and on single crystals of LSCO using only angular correlation. Fermi surface features in good agreement with band theory were found and identified in all of the measurements. In photoemission, the Fermi momentum was fixed for several points and the band dispersion below the Fermi energy was mapped. In positron angular correlation measurements, the shape of the Fermi surface was mapped for the CuO chains (YBCO) and the CuO planes (LSCO). Demonstration of the existence of Fermi surfaces in the HTSC materials points a direction for future theoretical considerations.

Quasiparticle interference, quasiparticle interactions, and the origin of the charge density wave in 2 H – NbSe 2

DOE PAGES

Arguello, C. J.; Rosenthal, E. P.; Andrade, E. F.; ...

2015-01-21

We show that a small number of intentionally introduced defects can be used as a spectroscopic tool to amplify quasiparticle interference in 2H-NbSe₂ that we measure by scanning tunneling spectroscopic imaging. We show, from the momentum and energy dependence of the quasiparticle interference, that Fermi surface nesting is inconsequential to charge density wave formation in 2H-NbSe₂. We demonstrate that, by combining quasiparticle interference data with additional knowledge of the quasiparticle band structure from angle resolved photoemission measurements, one can extract the wave vector and energy dependence of the important electronic scattering processes thereby obtaining direct information both about the fermiologymore » and the interactions. In 2H-NbSe₂, we use this combination to confirm that the important near-Fermi-surface electronic physics is dominated by the coupling of the quasiparticles to soft mode phonons at a wave vector different from the charge density wave ordering wave vector.« less

Quasiparticle interference, quasiparticle interactions, and the origin of the charge density wave in 2H-NbSe2.

PubMed

Arguello, C J; Rosenthal, E P; Andrade, E F; Jin, W; Yeh, P C; Zaki, N; Jia, S; Cava, R J; Fernandes, R M; Millis, A J; Valla, T; Osgood, R M; Pasupathy, A N

2015-01-23

We show that a small number of intentionally introduced defects can be used as a spectroscopic tool to amplify quasiparticle interference in 2H-NbSe2 that we measure by scanning tunneling spectroscopic imaging. We show, from the momentum and energy dependence of the quasiparticle interference, that Fermi surface nesting is inconsequential to charge density wave formation in 2H-NbSe2. We demonstrate that, by combining quasiparticle interference data with additional knowledge of the quasiparticle band structure from angle resolved photoemission measurements, one can extract the wave vector and energy dependence of the important electronic scattering processes thereby obtaining direct information both about the fermiology and the interactions. In 2H-NbSe2, we use this combination to confirm that the important near-Fermi-surface electronic physics is dominated by the coupling of the quasiparticles to soft mode phonons at a wave vector different from the charge density wave ordering wave vector.

Temperature dependence of magnetoresistance in copper single crystals

NASA Astrophysics Data System (ADS)

Bian, Q.; Niewczas, M.

2018-03-01

Transverse magnetoresistance of copper single crystals has been measured in the orientation of open-orbit from 2 K to 20 K for fields up to 9 T. The experimental Kohler's plots display deviation between individual curves below 16 K and overlap in the range of 16 K-20 K. The violation of the Kohler's rule below 16 K indicates that the magnetotransport can not be described by the classical theory of electron transport on spherical Fermi surface with a single relaxation time. A theoretical model incorporating two energy bands, spherical and cylindrical, with different relaxation times has been developed to describe the magnetoresistance data. The calculations show that the electron-phonon scattering rates at belly and neck regions of the Fermi surface have different temperature dependencies, and in general, they do not follow T3 law. The ratio of the relaxation times in belly and neck regions decreases parabolically with temperature as A - CT2 , with A and C being constants.

Visualizing weakly bound surface Fermi arcs and their correspondence to bulk Weyl fermions

PubMed Central

Batabyal, Rajib; Morali, Noam; Avraham, Nurit; Sun, Yan; Schmidt, Marcus; Felser, Claudia; Stern, Ady; Yan, Binghai; Beidenkopf, Haim

2016-01-01

Fermi arcs are the surface manifestation of the topological nature of Weyl semimetals, enforced by the bulk-boundary correspondence with the bulk Weyl nodes. The surface of tantalum arsenide, similar to that of other members of the Weyl semimetal class, hosts nontopological bands that obscure the exploration of this correspondence. We use the spatial structure of the Fermi arc wave function, probed by scanning tunneling microscopy, as a spectroscopic tool to distinguish and characterize the surface Fermi arc bands. We find that, as opposed to nontopological states, the Fermi arc wave function is weakly affected by the surface potential: it spreads rather uniformly within the unit cell and penetrates deeper into the bulk. Fermi arcs reside predominantly on tantalum sites, from which the topological bulk bands are derived. Furthermore, we identify a correspondence between the Fermi arc dispersion and the energy and momentum of the bulk Weyl nodes that classify this material as topological. We obtain these results by introducing an analysis based on the role the Bloch wave function has in shaping quantum electronic interference patterns. It thus carries broader applicability to the study of other electronic systems and other physical processes. PMID:27551687

Depletion region surface effects in electron beam induced current measurements

DOE Office of Scientific and Technical Information (OSTI.GOV)

Haney, Paul M.; Zhitenev, Nikolai B.; Yoon, Heayoung P.

2016-09-07

Electron beam induced current (EBIC) is a powerful characterization technique which offers the high spatial resolution needed to study polycrystalline solar cells. Current models of EBIC assume that excitations in the p-n junction depletion region result in perfect charge collection efficiency. However, we find that in CdTe and Si samples prepared by focused ion beam (FIB) milling, there is a reduced and nonuniform EBIC lineshape for excitations in the depletion region. Motivated by this, we present a model of the EBIC response for excitations in the depletion region which includes the effects of surface recombination from both charge-neutral and chargedmore » surfaces. For neutral surfaces, we present a simple analytical formula which describes the numerical data well, while the charged surface response depends qualitatively on the location of the surface Fermi level relative to the bulk Fermi level. We find that the experimental data on FIB-prepared Si solar cells are most consistent with a charged surface and discuss the implications for EBIC experiments on polycrystalline materials.« less

Origin of the quasiparticle peak in the spectral density of Cr(001) surfaces

NASA Astrophysics Data System (ADS)

Peters, L.; Jacob, D.; Karolak, M.; Lichtenstein, A. I.; Katsnelson, M. I.

2017-12-01

In the spectral density of Cr(001) surfaces, a sharp resonance close to the Fermi level is observed in both experiment and theory. For the physical origin of this peak, two mechanisms were proposed: a single-particle dz2 surface state renormalized by electron-phonon coupling and an orbital Kondo effect due to the degenerate dx z/dy z states. Despite several experimental and theoretical investigations, the origin is still under debate. In this work, we address this problem by two different approaches of the dynamical mean-field theory: first, by the spin-polarized T -matrix fluctuation exchange approximation suitable for weakly and moderately correlated systems; second, by the noncrossing approximation derived in the limit of weak hybridization (i.e., for strongly correlated systems) capturing Kondo-type processes. By using recent continuous-time quantum Monte Carlo calculations as a benchmark, we find that the high-energy features, everything except the resonance, of the spectrum are captured within the spin-polarized T -matrix fluctuation exchange approximation. More precisely, the particle-particle processes provide the main contribution. For the noncrossing approximation, it appears that spin-polarized calculations suffer from spurious behavior at the Fermi level. Then, we turned to non-spin-polarized calculations to avoid this unphysical behavior. By employing two plausible starting hybridization functions, it is observed that the characteristics of the resonance are crucially dependent on the starting point. It appears that only one of these starting hybridizations could result in an orbital Kondo resonance in the presence of a strong magnetic field like in the Cr(001) surface. It is for a future investigation to first resolve the unphysical behavior within the spin-polarized noncrossing approximation and then check for an orbital Kondo resonance.

Electron-hole pairing of Fermi-arc surface states in a Weyl semimetal bilayer

NASA Astrophysics Data System (ADS)

Michetti, Paolo; Timm, Carsten

2017-03-01

The topological nature of Weyl semimetals (WSMs) is corroborated by the presence of chiral surface states, which connect the projections of the bulk Weyl points by Fermi arcs (FAs). We study a bilayer structure realized by introducing a thin insulating spacer into a bulk WSM. Employing a self-consistent mean-field description of the interlayer Coulomb interaction, we propose that this system can develop an interlayer electron-hole pair condensate. The formation of this excitonic condensate leads to partial gapping of the FA dispersion. We obtain the dependence of the energy gap and the critical temperature on the model parameters, finding, in particular, a linear scaling of these quantities with the separation between the Weyl points in momentum space. A detrimental role is played by the curvature of the FAs, although the pairing persists for moderately small curvature. A signature of the condensate is the modification of the quantum oscillations involving the surface FAs.

Effective field theories for superconducting systems with multiple Fermi surfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Braga, P.R., E-mail: pedro.rangel.braga@gmail.com; Granado, D.R., E-mail: diegorochagrana@uerj.br; Department of Physics and Astronomy, Ghent University, Krijgslaan 281-S9, 9000 Gent

2016-11-15

In this work we investigate the description of superconducting systems with multiple Fermi surfaces. For the case of one Fermi surface we re-obtain the result that the superconductor is more precisely described as a topological state of matter. Studying the case of more than one Fermi surface, we obtain the effective theory describing a time reversal symmetric topological superconductor. These results are obtained by employing a general procedure to construct effective low energy actions describing states of electromagnetic systems interacting with charges and defects. The procedure consists in taking into account the proliferation or dilution of these charges and defectsmore » and its consequences for the low energy description of the electromagnetic response of the system. We find that the main ingredient entering the low energy characterization of the system with more than one Fermi surface is a non-conservation of the canonical supercurrent triggered by particular vortex configurations.« less

Topological semimetals carrying arbitrary Hopf numbers: Fermi surface topologies of a Hopf link, Solomon's knot, trefoil knot, and other linked nodal varieties

NASA Astrophysics Data System (ADS)

Ezawa, Motohiko

2017-07-01

We propose a type of Hopf semimetal indexed by a pair of numbers (p ,q ) , where the Hopf number is given by p q . The Fermi surface is given by a preimage of the Hopf map, which consists of loops nontrivially linked for a nonzero Hopf number. The Fermi surface forms a torus link, whose examples are a Hopf link indexed by (1 ,1 ) , Solomon's knot (2 ,1 ) , a double Hopf link (2 ,2 ) , and a double trefoil knot (3 ,2 ) . We may choose p or q to be a half integer, where the Fermi surface is a torus knot, such as a trefoil knot (3 /2 ,1 ) . It is even possible to make the Hopf number an arbitrary rational number, where a semimetal whose Fermi surface forms open strings is generated.

Angular studies of the magnetoresistance in the density wave state of the quasi-two-dimensional purple bronze KMo6O17

NASA Astrophysics Data System (ADS)

Guyot, H.; Dumas, J.; Kartsovnik, M. V.; Marcus, J.; Schlenker, C.; Sheikin, I.; Vignolles, D.

2007-07-01

The purple molybdenum bronze KMo6O17 is a quasi-two-dimensional compound which shows a Peierls transition towards a commensurate metallic charge density wave (CDW) state. High magnetic field measurements have revealed several transitions at low temperature and have provided an unusual phase diagram “temperature-magnetic field”. Angular studies of the interlayer magnetoresistance are now reported. The results suggest that the orbital coupling of the magnetic field to the CDW is the most likely mechanism for the field induced transitions. The angular dependence of the magnetoresistance is discussed on the basis of a warped quasi-cylindrical Fermi surface and provides information on the geometry of the Fermi surface in the low temperature density wave state.

Effects of disordered Ru substitution in BaFe2As2: possible realization of superdiffusion in real materials.

PubMed

Wang, Limin; Berlijn, Tom; Wang, Yan; Lin, Chia-Hui; Hirschfeld, P J; Ku, Wei

2013-01-18

An unexpected insensitivity of the Fermi surface to impurity scattering is found in Ru substituted BaFe(2)As(2) from first-principles theory, offering a natural explanation of the unusual resilience of transport and superconductivity to a high level of disordered substitution in this material. This robustness is shown to originate from a coherent interference of correlated on-site and intersite impurity scattering, similar in spirit to the microscopic mechanism of superdiffusion in one dimension. Our result also demonstrates a strong substitution dependence of the Fermi surface and carrier concentration and provides a resolution to current discrepancies in recent photoelectron spectroscopy. These effects offer a natural explanation of the diminishing long-range magnetic, orbital, and superconducting orders with high substitution.

Quantum chaos on a critical Fermi surface.

PubMed

Patel, Aavishkar A; Sachdev, Subir

2017-02-21

We compute parameters characterizing many-body quantum chaos for a critical Fermi surface without quasiparticle excitations. We examine a theory of [Formula: see text] species of fermions at nonzero density coupled to a [Formula: see text] gauge field in two spatial dimensions and determine the Lyapunov rate and the butterfly velocity in an extended random-phase approximation. The thermal diffusivity is found to be universally related to these chaos parameters; i.e., the relationship is independent of [Formula: see text], the gauge-coupling constant, the Fermi velocity, the Fermi surface curvature, and high-energy details.

Unconventional and conventional quantum criticalities in CeRh0.58Ir0.42In5

NASA Astrophysics Data System (ADS)

Luo, Yongkang; Lu, Xin; Dioguardi, Aadm P.; Rosa, Priscila F. S.; Bauer, Eric D.; Si, Qimiao; Thompson, Joe D.

2018-03-01

An appropriate description of the state of matter that appears as a second order phase transition is tuned toward zero temperature, viz. quantum-critical point (QCP), poses fundamental and still not fully answered questions. Experiments are needed both to test basic conclusions and to guide further refinement of theoretical models. Here, charge and entropy transport properties as well as AC specific heat of the heavy-fermion compound CeRh0.58Ir0.42In5, measured as a function of pressure, reveal two qualitatively different QCPs in a single material driven by a single non-symmetry-breaking tuning parameter. A discontinuous sign-change jump in thermopower suggests an unconventional QCP at pc1 accompanied by an abrupt Fermi-surface reconstruction that is followed by a conventional spin-density-wave critical point at pc2 across which the Fermi surface evolves smoothly to a heavy Fermi-liquid state. These experiments are consistent with some theoretical predictions, including the sequence of critical points and the temperature dependence of the thermopower in their vicinity.

Renormalization of Coulomb interactions in a system of two-dimensional tilted Dirac fermions

NASA Astrophysics Data System (ADS)

Lee, Yu-Wen; Lee, Yu-Li

2018-01-01

We investigate the effects of long-ranged Coulomb interactions in a tilted Dirac semimetal in two dimensions by using the perturbative renormalization-group (RG) method. Depending on the magnitude of the tilting parameter, the undoped system can have either Fermi points (type I) or Fermi lines (type II). Previous studies usually performed the renormalization-group transformations by integrating out the modes with large momenta. This is problematic when the Fermi surface is open, like type-II Dirac fermions. In this work we study the effects of Coulomb interactions, following the spirit of Shankar [Rev. Mod. Phys. 66, 129 (1994), 10.1103/RevModPhys.66.129], by introducing a cutoff in the energy scale around the Fermi surface and integrating out the high-energy modes. For type-I Dirac fermions, our result is consistent with that of the previous work. On the other hand, we find that for type-II Dirac fermions, the magnitude of the tilting parameter increases monotonically with lowering energies. This implies the stability of type-II Dirac fermions in the presence of Coulomb interactions, in contrast with previous results. Furthermore, for type-II Dirac fermions, the velocities in different directions acquire different renormalization even if they have the same bare values. By taking into account the renormalization of the tilting parameter and the velocities due to the Coulomb interactions, we show that while the presence of a charged impurity leads only to charge redistribution around the impurity for type-I Dirac fermions, for type-II Dirac fermions, the impurity charge is completely screened, albeit with a very long screening length. The latter indicates that the temperature dependence of physical observables are essentially determined by the RG equations we derived. We illustrate this by calculating the temperature dependence of the compressibility and specific heat of the interacting tilted Dirac fermions.

Millisecond Pulsars at Gamma-Ray Energies: Fermi Detections and Implications

NASA Technical Reports Server (NTRS)

Harding, Alice K.

2011-01-01

The Fermi Gamma-Ray Space Telescope has revolutionized the study of pulsar physics with the discovery of new populations of radio quiet and millisecond gamma-ray pulsars. The Fermi Large Area Telescope has so far discovered approx.20 new gamma-ray millisecond pulsars (MSPs) by both folding at periods of known radio MSPs or by detecting them as gamma-ray sources that are followed up by radio pulsar searches. The second method has resulted in a phenomenally successful synergy, with -30 new radio MSPs (to date) having been discovered at Fermi unidentified source locations and the gamma-ray pulsations having then been detected in a number of these using the radio timing solutions. Many of the newly discovered MSPs may be suitable for addition to the collection of very stable MSPs used for gravitational wave detection. Detection of such a large number of MSPs was surprising, given that most have relatively low spin-down luminosity and surface field strength. I will discuss their properties and the implications for pulsar particle acceleration and emission, as well as their potential contribution to gamma-ray backgrounds and Galactic cosmic rays.

Polarized Fermi Condensates with Unequal Masses: Tuning the Tricritical Point

DOE Office of Scientific and Technical Information (OSTI.GOV)

Parish, M. M.; Marchetti, F. M.; Simons, B. D.

We consider a two-component atomic Fermi gas within a mean-field, single-channel model, where both the mass and population of each component are unequal. We show that the tricritical point at zero temperature evolves smoothly from the BEC to BCS side of the resonance as a function of mass ratio r. We find that the interior gap state proposed by Liu and Wilczek is always unstable to phase separation, while the breached pair state with one Fermi surface for the excess fermions exhibits differences in its density of states and pair correlation functions depending on which side of the resonance itmore » lies. Finally, we show that, when r > or appro. 3.95, the finite-temperature phase diagram of trapped gases at unitarity becomes topologically distinct from the equal mass system.« less

Origin of the transition voltage in gold-vacuum-gold atomic junctions.

PubMed

Wu, Kunlin; Bai, Meilin; Sanvito, Stefano; Hou, Shimin

2013-01-18

The origin and the distance dependence of the transition voltage of gold-vacuum-gold junctions are investigated by employing first-principles quantum transport simulations. Our calculations show that atomic protrusions always exist on the electrode surface of gold-vacuum-gold junctions fabricated using the mechanically controllable break junction (MCBJ) method. The transition voltage of these gold-vacuum-gold junctions with atomically sharp electrodes is determined by the local density of states (LDOS) of the apex gold atom on the electrode surface rather than by the vacuum barrier shape. More specifically, the absolute value of the transition voltage roughly equals the rising edge of the LDOS peak contributed by the 6p atomic orbitals of the gold atoms protruding from the electrode surface, whose local Fermi level is shifted downwards when a bias voltage is applied. Since the LDOS of the apex gold atom depends strongly on the exact shape of the electrode, the transition voltage is sensitive to the variation of the atomic configuration of the junction. For asymmetric junctions, the transition voltage may also change significantly depending on the bias polarity. Considering that the occurrence of the transition voltage requires the electrode distance to be larger than a critical value, the interaction between the two electrodes is actually rather weak. Consequently, the LDOS of the apex gold atom is mainly determined by its local atomic configuration and the transition voltage only depends weakly on the electrode distance as observed in the MCBJ experiments.

Stokes paradox in electronic Fermi liquids

NASA Astrophysics Data System (ADS)

Lucas, Andrew

2017-03-01

The Stokes paradox is the statement that in a viscous two-dimensional fluid, the "linear response" problem of fluid flow around an obstacle is ill posed. We present a simple consequence of this paradox in the hydrodynamic regime of a Fermi liquid of electrons in two-dimensional metals. Using hydrodynamics and kinetic theory, we estimate the contribution of a single cylindrical obstacle to the global electrical resistance of a material, within linear response. Momentum relaxation, present in any realistic electron liquid, resolves the classical paradox. Nonetheless, this paradox imprints itself in the resistance, which can be parametrically larger than predicted by Ohmic transport theory. We find a remarkably rich set of behaviors, depending on whether or not the quasiparticle dynamics in the Fermi liquid should be treated as diffusive, hydrodynamic, or ballistic on the length scale of the obstacle. We argue that all three types of behavior are observable in present day experiments.

Thermodynamic foundations of applications of ab initio methods for determination of the adsorbate equilibria: hydrogen at the GaN(0001) surface.

PubMed

Kempisty, Pawel; Strąk, Paweł; Sakowski, Konrad; Kangawa, Yoshihiro; Krukowski, Stanisław

2017-11-08

Thermodynamic foundations of ab initio modeling of vapor-solid and vapor-surface equilibria are introduced. The chemical potential change is divided into enthalpy and entropy terms. The enthalpy path passes through vapor-solid transition at zero temperature. The entropy path avoids the singular point at zero temperature passing a solid-vapor transition under normal conditions, where evaporation entropy is employed. In addition, the thermal changes are calculated. The chemical potential difference contribution of the following terms: vaporization enthalpy, vaporization entropy, the temperature-entropy related change, the thermal enthalpy change and mechanical pressure is obtained. The latter term is negligibly small for the pressure typical for epitaxy. The thermal enthalpy change is two orders smaller than the first three terms which have to be taken into account explicitly. The configurational vaporization entropy change is derived for adsorption processes. The same formulation is derived for vapor-surface equilibria using hydrogen at the GaN(0001) surface as an example. The critical factor is the dependence of the enthalpy of evaporation (desorption energy) on the pinning of the Fermi level bringing a drastic change of the value from 2.24 eV to -2.38 eV. In addition it is shown that entropic contributions considerable change the hydrogen equilibrium pressure over the GaN(0001) surface by several orders of magnitude. Thus a complete and exact formulation of vapor-solid and vapor-surface equilibria is presented.

Topological Nodal Cooper Pairing in Doped Weyl Metals

NASA Astrophysics Data System (ADS)

Li, Yi; Haldane, F. D. M.

2018-02-01

We generalize the concept of Berry connection of the single-electron band structure to that of a two-particle Cooper pairing state between two Fermi surfaces with opposite Chern numbers. Because of underlying Fermi surface topology, the pairing Berry phase acquires nontrivial monopole structure. Consequently, pairing gap functions have topologically protected nodal structure as vortices in the momentum space with the total vorticity solely determined by the pair monopole charge qp. The nodes of gap function behave as the Weyl-Majorana points of the Bogoliubov-de Gennes pairing Hamiltonian. Their relation with the connection patterns of the surface modes from the Weyl band structure and the Majorana surface modes inside the pairing gap is also discussed. Under the approximation of spherical Fermi surfaces, the pairing symmetry are represented by monopole harmonic functions. The lowest possible pairing channel carries angular momentum number j =|qp|, and the corresponding gap functions are holomorphic or antiholomorphic functions on Fermi surfaces. After projected on the Fermi surfaces with nontrivial topology, all the partial-wave channels of pairing interactions acquire the monopole charge qp independent of concrete pairing mechanism.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xiang, Z.; Lawson, B.; Asaba, T.

The Kondo insulator samarium hexaboride (SmB 6) has been intensely studied in recent years as a potential candidate of a strongly correlated topological insulator. One of the most exciting phenomena observed in SmB 6 is the clear quantum oscillations appearing in magnetic torque at a low temperature despite the insulating behavior in resistance. These quantum oscillations show multiple frequencies and varied effective masses. The origin of quantum oscillation is, however, still under debate with evidence of both two-dimensional Fermi surfaces and three-dimensional Fermi surfaces. Here, we carry out angle-resolved torque magnetometry measurements in a magnetic field up to 45 Tmore » and a temperature range down to 40 mK. With the magnetic field rotated in the (010) plane, the quantum oscillation frequency of the strongest oscillation branch shows a fourfold rotational symmetry. However, in the angular dependence of the amplitude of the same branch, this fourfold symmetry is broken and, instead, a twofold symmetry shows up, which is consistent with the prediction of a two-dimensional Lifshitz-Kosevich model. No deviation of Lifshitz-Kosevich behavior is observed down to 40 mK. Our results suggest the existence of multiple light-mass surface states in SmB 6, with their mobility significantly depending on the surface disorder level.« less

Bulk Rotational Symmetry Breaking in Kondo Insulator SmB 6

DOE PAGES

Xiang, Z.; Lawson, B.; Asaba, T.; ...

2017-09-25

The Kondo insulator samarium hexaboride (SmB 6) has been intensely studied in recent years as a potential candidate of a strongly correlated topological insulator. One of the most exciting phenomena observed in SmB 6 is the clear quantum oscillations appearing in magnetic torque at a low temperature despite the insulating behavior in resistance. These quantum oscillations show multiple frequencies and varied effective masses. The origin of quantum oscillation is, however, still under debate with evidence of both two-dimensional Fermi surfaces and three-dimensional Fermi surfaces. Here, we carry out angle-resolved torque magnetometry measurements in a magnetic field up to 45 Tmore » and a temperature range down to 40 mK. With the magnetic field rotated in the (010) plane, the quantum oscillation frequency of the strongest oscillation branch shows a fourfold rotational symmetry. However, in the angular dependence of the amplitude of the same branch, this fourfold symmetry is broken and, instead, a twofold symmetry shows up, which is consistent with the prediction of a two-dimensional Lifshitz-Kosevich model. No deviation of Lifshitz-Kosevich behavior is observed down to 40 mK. Our results suggest the existence of multiple light-mass surface states in SmB 6, with their mobility significantly depending on the surface disorder level.« less

Temperature-induced Lifshitz transition in WTe 2

DOE PAGES

Wu, Yun; Jo, Na Hyun; Ochi, Masayuki; ...

2015-10-12

In this study, we use ultrahigh resolution, tunable, vacuum ultraviolet laser-based, angle-resolved photoemission spectroscopy (ARPES), temperature- and field-dependent resistivity, and thermoelectric power (TEP) measurements to study the electronic properties of WTe 2, a compound that manifests exceptionally large, temperature-dependent magnetoresistance. The Fermi surface consists of two pairs of electron and two pairs of hole pockets along the X–Γ–X direction. Using detailed ARPES temperature scans, we find a rare example of a temperature-induced Lifshitz transition at T≃160 K, associated with the complete disappearance of the hole pockets. Our electronic structure calculations show a clear and substantial shift of the chemical potentialmore » μ(T) due to the semimetal nature of this material driven by modest changes in temperature. This change of Fermi surface topology is also corroborated by the temperature dependence of the TEP that shows a change of slope at T≈175 K and a breakdown of Kohler’s rule in the 70–140 K range. Our results and the mechanisms driving the Lifshitz transition and transport anomalies are relevant to other systems, such as pnictides, 3D Dirac semimetals, and Weyl semimetals.« less

TOMOGRAPHY OF THE FERMI-LAT γ-RAY DIFFUSE EXTRAGALACTIC SIGNAL VIA CROSS CORRELATIONS WITH GALAXY CATALOGS

DOE PAGES

Xia, Jun-Qing; Cuoco, Alessandro; Branchini, Enzo; ...

2015-03-24

Building on our previous cross-correlation analysis (Xia et al. 2011) between the isotropic γ-ray background (IGRB) and different tracers of the large-scale structure of the universe, we update our results using 60 months of data from the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi). For this study, we perform a cross-correlation analysis both in configuration and spherical harmonics space between the IGRB and objects that may trace the astrophysical sources of the IGRB: QSOs in the Sloan Digital Sky Survey (SDSS) DR6, the SDSS DR8 Main Galaxy Sample, luminous red galaxies (LRGs) in the SDSS catalog, infrared-selected galaxies in the Two Micron All Sky Survey (2MASS), and radio galaxies in the NRAO VLA Sky Survey (NVSS). The benefit of correlating the Fermi-LAT signal with catalogs of objects at various redshifts is to provide tomographic information on the IGRB, which is crucial in separating the various contributions and clarifying its origin. The main result is that, unlike in our previous analysis, we now observe a significant (>3.5σ) cross-correlation signal on angular scales smaller than 1° in the NVSS, 2MASS, and QSO cases and, at lower statistical significance (~3.0σ), with SDSS galaxies. The signal is stronger in two energy bands, E > 0.5 GeV and E > 1 GeV, but it is also seen at E > 10 GeV. No cross-correlation signal is detected between Fermi data and the LRGs. These results are robust against the choice of the statistical estimator, estimate of errors, map cleaning procedure, and instrumental effects. Finally, we test the hypothesis that the IGRB observed by Fermi-LAT originates from the summed contributions of three types of unresolved extragalactic sources: BL Lacertae objects (BL Lacs), flat spectrum radio quasars (FSRQs), and star-forming galaxies (SFGs). Finally, we find that a model in which the IGRB is mainly produced by SFGs (more » $$72_{-37}^{+23}$$% with 2σ errors), with BL Lacs and FSRQs giving a minor contribution, provides a good fit to the data. We also consider a possible contribution from misaligned active galactic nuclei, and we find that, depending on the details of the model and its uncertainty, they can also provide a substantial contribution, partly degenerate with the SFG one.« less

Spin-density wave state in simple hexagonal graphite

NASA Astrophysics Data System (ADS)

Mosoyan, K. S.; Rozhkov, A. V.; Sboychakov, A. O.; Rakhmanov, A. L.

2018-02-01

Simple hexagonal graphite, also known as AA graphite, is a metastable configuration of graphite. Using tight-binding approximation, it is easy to show that AA graphite is a metal with well-defined Fermi surface. The Fermi surface consists of two sheets, each shaped like a rugby ball. One sheet corresponds to electron states, another corresponds to hole states. The Fermi surface demonstrates good nesting: a suitable translation in the reciprocal space superposes one sheet onto another. In the presence of the electron-electron repulsion, a nested Fermi surface is unstable with respect to spin-density-wave ordering. This instability is studied using the mean-field theory at zero temperature, and the spin-density-wave order parameter is evaluated.

The relationship between anisotropic magnetoresistance and topology of Fermi surface in Td-MoTe2 crystal

NASA Astrophysics Data System (ADS)

Lv, Yang-Yang; Li, Xiao; Pang, Bin; Cao, Lin; Lin, Dajun; Zhang, Bin-Bin; Yao, Shu-Hua; Chen, Y. B.; Zhou, Jian; Dong, Song-Tao; Zhang, Shan-Tao; Lu, Ming-Hui; Chen, Yan-Feng

2017-07-01

Layered transition-metal dichalcogenides have been recently attracted a lot of attention because of their unique physical properties, such as extremely large and anisotropic magnetoresistance (MR) in WTe2. In this work, we observed the abnormally anisotropic MR on Td-MoTe2 crystal that is strongly dependent on the temperature, as well as the orientations of both magnetic field B and electric field E with respect to crystallographic axes of Td-MoTe2. When E//a-axis and B//c-axis, MR is parabolically dependent on B and is as high as 520% under 9 T and 2 K conditions; the MR is quasi-linearly dependent on B when E//a-axis and B//b-axis (E//b-axis and B//c-axis), and the corresponding MR is only 130% (220%); MR is initially parabolically dependent on B, then linearly on B, and finally shows a saturate trend under E//B//a-axis (or E//B//b-axis) conditions, and the MR is about 16% (30%). These anisotropic MR behaviors can be qualitatively explained by the features of the Fermi surface of Td-MoTe2. This work may demonstrate the rich anisotropic physical behavior in layered transition-metal dichalcognides.

The novel metallic states of the cuprates: Topological Fermi liquids and strange metals

NASA Astrophysics Data System (ADS)

Sachdev, Subir; Chowdhury, Debanjan

2016-12-01

We review ideas on the nature of the metallic states of the hole-doped cuprate high temperature superconductors, with an emphasis on the connections between the Luttinger theorem for the size of the Fermi surface, topological quantum field theories (TQFTs), and critical theories involving changes in the size of the Fermi surface. We begin with the derivation of the Luttinger theorem for a Fermi liquid, using momentum balance during a process of flux insertion in a lattice electronic model with toroidal boundary conditions. We then review the TQFT of the ℤ spin liquid, and demonstrate its compatibility with the toroidal momentum balance argument. This discussion leads naturally to a simple construction of "topological" Fermi liquid states: the fractionalized Fermi liquid (FL*) and the algebraic charge liquid (ACL). We present arguments for a description of the pseudogap metal of the cuprates using ℤ-FL* or ℤ-ACL states with Ising-nematic order. These pseudogap metal states are also described as Higgs phases of a SU(2) gauge theory. The Higgs field represents local antiferromagnetism, but the Higgs-condensed phase does not have long-range antiferromagnetic order: the magnitude of the Higgs field determines the pseudogap, the reconstruction of the Fermi surface, and the Ising-nematic order. Finally, we discuss the route to the large Fermi surface Fermi liquid via the critical point where the Higgs condensate and Ising nematic order vanish, and the application of Higgs criticality to the strange metal.

Optical conductivity of three and two dimensional topological nodal-line semimetals

NASA Astrophysics Data System (ADS)

Barati, Shahin; Abedinpour, Saeed H.

2017-10-01

The peculiar shape of the Fermi surface of topological nodal-line semimetals at low carrier concentrations results in their unusual optical and transport properties. We analytically investigate the linear optical responses of three- and two-dimensional nodal-line semimetals using the Kubo formula. The optical conductivity of a three-dimensional nodal-line semimetal is anisotropic. Along the axial direction (i.e., the direction perpendicular to the nodal-ring plane), the Drude weight has a linear dependence on the chemical potential at both low and high carrier dopings. For the radial direction (i.e., the direction parallel to the nodal-ring plane), this dependence changes from linear into quadratic in the transition from low into high carrier concentration. The interband contribution into optical conductivity is also anisotropic. In particular, at large frequencies, it saturates to a constant value for the axial direction and linearly increases with frequency along the radial direction. In two-dimensional nodal-line semimetals, no interband optical transition could be induced and the only contribution to the optical conductivity arises from the intraband excitations. The corresponding Drude weight is independent of the carrier density at low carrier concentrations and linearly increases with chemical potential at high carrier doping.

Spin-imbalanced pairing and Fermi surface deformation in flat bands

NASA Astrophysics Data System (ADS)

Huhtinen, Kukka-Emilia; Tylutki, Marek; Kumar, Pramod; Vanhala, Tuomas I.; Peotta, Sebastiano; Törmä, Päivi

2018-06-01

We study the attractive Hubbard model with spin imbalance on two lattices featuring a flat band: the Lieb and kagome lattices. We present mean-field phase diagrams featuring exotic superfluid phases, similar to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, whose stability is confirmed by dynamical mean-field theory. The nature of the pairing is found to be richer than just the Fermi surface shift responsible for the usual FFLO state. The presence of a flat band allows for changes in the particle momentum distributions at null energy cost. This facilitates formation of nontrivial superfluid phases via multiband Cooper pair formation: the momentum distribution of the spin component in the flat band deforms to mimic the Fermi surface of the other spin component residing in a dispersive band. The Fermi surface of the unpaired particles that are typical for gapless superfluids becomes deformed as well. The results highlight the profound effect of flat dispersions on Fermi surface instabilities, and provide a potential route for observing spin-imbalanced superfluidity and superconductivity.

Methods for modeling non-equilibrium degenerate statistics and quantum-confined scattering in 3D ensemble Monte Carlo transport simulations

NASA Astrophysics Data System (ADS)

Crum, Dax M.; Valsaraj, Amithraj; David, John K.; Register, Leonard F.; Banerjee, Sanjay K.

2016-12-01

Particle-based ensemble semi-classical Monte Carlo (MC) methods employ quantum corrections (QCs) to address quantum confinement and degenerate carrier populations to model tomorrow's ultra-scaled metal-oxide-semiconductor-field-effect-transistors. Here, we present the most complete treatment of quantum confinement and carrier degeneracy effects in a three-dimensional (3D) MC device simulator to date, and illustrate their significance through simulation of n-channel Si and III-V FinFETs. Original contributions include our treatment of far-from-equilibrium degenerate statistics and QC-based modeling of surface-roughness scattering, as well as considering quantum-confined phonon and ionized-impurity scattering in 3D. Typical MC simulations approximate degenerate carrier populations as Fermi distributions to model the Pauli-blocking (PB) of scattering to occupied final states. To allow for increasingly far-from-equilibrium non-Fermi carrier distributions in ultra-scaled and III-V devices, we instead generate the final-state occupation probabilities used for PB by sampling the local carrier populations as function of energy and energy valley. This process is aided by the use of fractional carriers or sub-carriers, which minimizes classical carrier-carrier scattering intrinsically incompatible with degenerate statistics. Quantum-confinement effects are addressed through quantum-correction potentials (QCPs) generated from coupled Schrödinger-Poisson solvers, as commonly done. However, we use these valley- and orientation-dependent QCPs not just to redistribute carriers in real space, or even among energy valleys, but also to calculate confinement-dependent phonon, ionized-impurity, and surface-roughness scattering rates. FinFET simulations are used to illustrate the contributions of each of these QCs. Collectively, these quantum effects can substantially reduce and even eliminate otherwise expected benefits of considered In0.53Ga0.47 As FinFETs over otherwise identical Si FinFETs despite higher thermal velocities in In0.53Ga0.47 As. It also may be possible to extend these basic uses of QCPs, however calculated, to still more computationally efficient drift-diffusion and hydrodynamic simulations, and the basic concepts even to compact device modeling.

Electron-positron momentum density in Tl 2Ba 2CuO 6

NASA Astrophysics Data System (ADS)

Barbiellini, B.; Gauthier, M.; Hoffmann, L.; Jarlborg, T.; Manuel, A. A.; Massidda, S.; Peter, M.; Triscone, G.

1994-08-01

We present calculations of the electron-positron momentum density for the high- Tc superconductor Tl 2Ba 2CuO 6, together with some preliminary two-dimensional angular correlation of the annihilation radiation (2D-ACAR) measurements. The calculations are based on the first-principles electronic structure obtained using the full-potential linearized augmented plane wave (FLAPW) and the linear muffin-tin orbital (LMTO) methods. We also use a linear combination of the atomic orbitals-molecular orbital method (LCAO-MO) to discuss orbital contributions to the anisotropies. Some agreement between calculated and measured 2D-ACAR anisotropies encourage sample improvement for further Fermi surface investigations. Indeed, our results indicate a non-negligle overlap of the positron wave function with the CuOo 2 plane electrons. Therefore, this compound may be well suited for investigating the relevant CuO 2 Fermi surface by 2D-ACAR.

Electronic properties of two inequivalent surfaces in MoTe2 studied by quasi-particle interference

NASA Astrophysics Data System (ADS)

Iaia, Davide; Shichao, Yan; Madhavan, Vidya

MoTe2 has received renewed interest due to its topological properties. At a temperature below 250 K, MoTe2 is a type II Weyl semimetal hosting three-dimensional (3D) linearly dispersing states with well defined chirality. Nodes in this 3D dispersion are called Weyl points. Weyl points of opposite chirality are expected to be connected by topologically protected Fermi arcs. In this talk we discuss low temperature scanning tunneling microscopy studies of the electronic structure of MoTe2. The electronic properties are studied using quasi-particle interference technique which allows us to resolve Fermi arcs features and to clearly distinguish between two inequivalent MoTe2 surfaces. Our results provide important contributions to further our understanding of the electronic properties of this new and exotic class of materials. National Science Foundation (NSF).

First principle study of structural, electronic and fermi surface properties of aluminum praseodymium

NASA Astrophysics Data System (ADS)

Shugani, Mani; Aynyas, Mahendra; Sanyal, S. P.

2018-05-01

We present a structural, Electronic and Fermi surface properties of Aluminum Praseodymium (AlPr) using First-principles density functional calculation by using full potential linearized augmented plane wave (FP-LAPW) method within generalized gradient approximation (GGA). The ground state properties along with electronic and Fermi surface properties are studied. It is found that AlPr is metallic and the bonding between Al and Pr is covalent.

First-Order Antiferromagnetic Transition and Fermi Surfaces in Semimetal EuSn3

NASA Astrophysics Data System (ADS)

Mori, Akinobu; Miura, Yasunao; Tsutsumi, Hiroki; Mitamura, Katsuya; Hagiwara, Masayuki; Sugiyama, Kiyohiro; Hirose, Yusuke; Honda, Fuminori; Takeuchi, Tetsuya; Nakamura, Ai; Hiranaka, Yuichi; Hedo, Masato; Nakama, Takao; Ōnuki, Yoshichika

2014-02-01

We grew high-quality single crystals of the antiferromagnet EuSn3 with the AuCu3-type cubic crystal structure by the Sn self-flux method and measured the electrical resistivity, magnetic susceptibility, high-field magnetization, specific heat, thermal expansion, and de Haas-van Alphen (dHvA) effect, in order to study the magnetic and Fermi surface properties. We observed steplike changes in the electrical resistivity and magnetic susceptibility, and a sharp peak of the specific heat and thermal expansion coefficient at a Néel temperature TN = 36.4 K. The first-order nature of the antiferromagnetic transition was ascertained by the observation of thermal hysteresis as well as of latent heat at TN. The present antiferromagnetic transition is found to be not a typical second-order phase transition but a first-order one. From the results of dHvA experiment, we clarified that the Fermi surface is very similar to that of the divalent compound YbSn3, mainly consisting of a nearly spherical hole Fermi surface and eight ellipsoidal electron Fermi surfaces. EuSn3 is possibly a compensated metal, and the occupation of a nearly spherical hole Fermi surface is 3.5% in its Brillouin zone, indicating that EuSn3 is a semimetal.

Fermi surface properties of paramagnetic NpCd11 with a large unit cell

NASA Astrophysics Data System (ADS)

Homma, Yoshiya; Aoki, Dai; Haga, Yoshinori; Settai, Rikio; Sakai, Hironori; Ikeda, Shugo; Yamamoto, Etsuji; Nakamura, Akio; Shiokawa, Yoshinobu; Takeuchi, Tetsuya; Yamagami, Hiroshi; Ōnuki, Yoshichika

2010-03-01

We succeeded in growing a high-quality single crystal of NpCd11 with the cubic BaHg11-type structure by the Cd-self flux method. The lattice parameter of a = 9.2968(2) Å and crystallographic positions of the atoms were determined by x-ray single-crystal structure analysis. From the results of the magnetic susceptibility and specific heat experiments, this compound is found to be a 5f-localized paramagnet with the singlet ground state in the crystalline electric field (CEF) scheme. Fermi surface properties were measured using the de Haas-van Alphen (dHvA) technique. Long-period oscillations were observed in the dHvA frequency range of 9.1 x 105 to 1.9 x 107 Oe, indicating small cross-sectional areas of Fermi surfaces, which is consistent with a small Brillouin zone based on a large unit cell. From the results of dHvA and magnetoresistance experiments, the Fermi surface of NpCd11 is found to consist of many kinds of closed Fermi surfaces and a multiply-connected-like Fermi surface, although the result of energy band calculations based on the 5f-localized Np3+(5f4) configuration reveals the existence of only closed Fermi surfaces. The corresponding cyclotron effective mass is small, ranging from 0.1 to 0.7 m0, which is consistent with a small electronic specific heat coefficient γ ≅ 10mJ/K2·mol, revealing no hybridization between the 5f electrons and conduction electrons.

Topological Properties and the Dynamical Crossover from Mixed-Valence to Kondo-Lattice Behavior in the Golden Phase of SmS.

PubMed

Kang, Chang-Jong; Choi, Hong Chul; Kim, Kyoo; Min, B I

2015-04-24

We have investigated temperature-dependent behaviors of electronic structure and resistivity in a mixed-valent golden phase of SmS, based on the dynamical mean-field-theory band-structure calculations. Upon cooling, the coherent Sm 4f bands are formed to produce the hybridization-induced pseudogap near the Fermi level, and accordingly the topology of the Fermi surface is changed to exhibit a Lifshitz-like transition. The surface states emerging in the bulk gap region are found to be not topologically protected states but just typical Rashba spin-polarized states, indicating that SmS is not a topological Kondo semimetal. From the analysis of anomalous resistivity behavior in SmS, we have identified universal energy scales, which characterize the Kondo-mixed-valent semimetallic systems.

Nonmagnetic impurity resonances as a signature of sign-reversal pairing in FeAs-based superconductors.

PubMed

Zhang, Degang

2009-10-30

The energy band structure of FeAs-based superconductors is fitted by a tight-binding model with two Fe ions per unit cell and two degenerate orbitals per Fe ion. Based on this, superconductivity with extended s-wave pairing symmetry of the form cosk(x)+cosk(y) is examined. The local density of states near an impurity is also investigated by using the T-matrix approach. For the nonmagnetic scattering potential, we found that there exist two major resonances inside the gap. The height of the resonance peaks depends on the strength of the impurity potential. These in-gap resonances are originated in the Andreev's bound states due to the quasiparticle scattering between the hole Fermi surfaces around Gamma point with positive order parameter and the electron Fermi surfaces around M point with negative order parameter.

Pressure variation of Rashba spin splitting toward topological transition in the polar semiconductor BiTeI

NASA Astrophysics Data System (ADS)

Ideue, T.; Checkelsky, J. G.; Bahramy, M. S.; Murakawa, H.; Kaneko, Y.; Nagaosa, N.; Tokura, Y.

2014-10-01

BiTeI is a polar semiconductor with gigantic Rashba spin-split bands in bulk. We have investigated the effect of pressure on the electronic structure of this material via magnetotransport. Periods of Shubunikov-de Haas (SdH) oscillations originating from the spin-split outer Fermi surface and inner Fermi surface show disparate responses to pressure, while the carrier number derived from the Hall effect is unchanged with pressure. The associated parameters which characterize the spin-split band structure are strongly dependent on pressure, reflecting the pressure-induced band deformation. We find the SdH oscillations and transport response are consistent with the theoretically proposed pressure-induced band deformation leading to a topological phase transition. Our analysis suggests the critical pressure for the quantum phase transition near Pc=3.5 GPa.

Unconventional Fermi surface associated with novel quasiparticles in the Kondo insulator SmB6

NASA Astrophysics Data System (ADS)

Sebastian, Suchitra

The search for a Fermi surface in the absence of a Fermi liquid has endured for decades. We present evidence for the realisation of such a state in the Kondo Insulator SmB6, which is an extreme example of Fermi liquid breakdown. Experimental results are presented from complementary techniques including quantum oscillations, specific heat capacity, thermal conductivity, and oscillatory entropy down to low temperatures. An experimental comparison is made with alternative theoretical models that associate novel quasiparticles with the unconventional Fermi surface we uncover in SmB6. A new paradigm for the realisation of a Fermi surface in the absence of conventional quasiparticles is proposed in the vicinity of a Kondo insulator transition. This work was performed in collaboration with M. Hartstein, W. H. Toews, Y.-T. Hsu, B. Zeng, X. Chen, M. Ciomaga Hatnean, Q. R. Zhang, S. Nakamura, A. S. Padgett, G. Rodway-Gant, J. Berk, M. K. Kingston, G. H. Zhang, M. K. Chan, S. Yamashita, T. Sakakibara, Y. Takano, J. -H. Park, L. Balicas, N. Harrison, N. Shitsevalova, G. Balakrishnan, G. G. Lonzarich, R. W. Hill, and M. Sutherland.

Magnetism in thin transition metal alloys

NASA Astrophysics Data System (ADS)

Janke-Gilman, Nathaniel; Reade

Magnetic linear dichroism measurements allowed us to measure atomic moments and spin order in alloy magnetic systems with chemical specificity and surface sensitivity. The width of the dichroism spectrum is a measure of the atomic moment via the local exchange, while the dichroism amplitude is a measure of the elemental contribution to magnetic order in the alloy via the dipole selection rules. A novel method has been introduced to systematically determine the dichroism width and amplitude. Changing magnetic moments have been tracked with changing alloy composition, along with changes in the magnetic easy axis and Curie temperature. Measurements have been made of the bandstructure and band topology near the Fermi energy. Well defined spin and k states are selected using high energy and k resolution. The 'Stoner gap' in d bands near the Fermi energy is equal to the minimum energy spin-flip excitation available to d electrons in particular symmetry states. The size and shape of the sp band Fermi surface in momentum space determines the periodicity of oscillatory magnetic coupling. The exchange splitting in the sp bands is one measure of changing magnetization in a magnetic alloy, while the spin dependent mean free path is the inverse of the band width dk. The strong variation of these effects from one magnetic impurity to another supports the concept of magnetic impurity doping in magnetoelectronic devices. When the thickness of a magnetic system is sufficiently reduced, the finite size effect leads to reduction in the critical temperature Tc with decreasing thickness n according to the power law 1 - Tc(n)/Tc(bulk) = b n^lambda.Deviations from this power law have been observed by many authors in the ultrathin film limit (2--3 monolayers or less). We have shown that these deviations from power law behavior arise when the film thickness becomes less than the mean range of spin-spin interactions in the magnetic film, at which point the reduced surface free energy term dominates. The quantity b provides a measure of this range of spin-spin interactions. The range of magnetic interactions scales with the mean free path of minority spins.

Berry phase and anomalous transport of the composite fermions at the half-filled Landau level

NASA Astrophysics Data System (ADS)

Pan, W.; Kang, W.; Baldwin, K. W.; West, K. W.; Pfeiffer, L. N.; Tsui, D. C.

2017-12-01

The fractional quantum Hall effect (FQHE) in two-dimensional electron systems is an exotic, superfluid-like matter with an emergent topological order. From the consideration of the Aharonov-Bohm interaction between electrons and magnetic field, the ground state of a half-filled lowest Landau level is mathematically transformed to a Fermi sea of composite objects of electrons bound to two flux quanta, termed composite fermions (CFs). A strong support for the CF theories comes from experimental confirmation of the predicted Fermi surface at ν = 1/2 (where ν is the Landau level filling factor) from the detection of the Fermi wavevector in semi-classical geometrical resonance experiments. Recent developments in the theory of CFs have led to the prediction of a π Berry phase for the CF circling around the Fermi surface at half-filling. In this paper we provide experimental evidence for the detection of the Berry phase of CFs in the fractional quantum Hall effect. Our measurements of the Shubnikov-de Haas oscillations of CFs as a function carrier density at a fixed magnetic field provide strong support for the existence of a π Berry phase at ν = 1/2. We also discover that the conductivity of composite fermions at ν = 1/2 displays an anomalous linear density dependence, whose origin remains mysterious yet tantalizing.

Electrical properties of metal/Al2O3/In0.53Ga0.47As capacitors grown on InP

NASA Astrophysics Data System (ADS)

Ferrandis, Philippe; Billaud, Mathilde; Duvernay, Julien; Martin, Mickael; Arnoult, Alexandre; Grampeix, Helen; Cassé, Mikael; Boutry, Hervé; Baron, Thierry; Vinet, Maud; Reimbold, Gilles

2018-04-01

To overcome the Fermi-level pinning in III-V metal-oxide-semiconductor capacitors, attention is usually focused on the choice of dielectric and surface chemical treatments prior to oxide deposition. In this work, we examined the influence of the III-V material surface cleaning and the semiconductor growth technique on the electrical properties of metal/Al2O3/In0.53Ga0.47As capacitors grown on InP(100) substrates. By means of the capacitance-voltage measurements, we demonstrated that samples do not have the same total oxide charge density depending on the cleaning solution used [(NH4)2S or NH4OH] prior to oxide deposition. The determination of the interface trap density revealed that a Fermi-level pinning occurs for samples grown by metalorganic chemical vapor deposition but not for similar samples grown by molecular beam epitaxy. Deep level transient spectroscopy analysis explained the Fermi-level pinning by an additional signal for samples grown by metalorganic chemical vapor deposition, attributed to the tunneling effect of carriers trapped in oxide toward interface states. This work emphasizes that the choice of appropriate oxide and cleaning treatment is not enough to prevent a Fermi-level pinning in III-V metal-oxide-semiconductor capacitors. The semiconductor growth technique needs to be taken into account because it impacts the trapping properties of the oxide.

Fermiology of Ce2 Rh3 Ge5

NASA Astrophysics Data System (ADS)

Wartenbe, Mark

The competition between localized and delocalized f electrons in heavy fermion materials produces a wide variety of interesting physical phenomena. Among these compounds is Ce2Rh3Ge5. This heavy-fermion system undergoes an antiferromagnetic transition below 4K and exhibits an angle dependent magnetic phase transition around 25 tesla. In addition, RF conductivity measurements in pulsed field (65T) have revealed quantum oscillations. Temperature dependence at fixed angle indicates relatively heavy effective masses of values ranging from ~3me on up to ~10me. This indicates that the narrow f-electron density of states is partially hybridized close to the Fermi energy, but also places strict cryogenic constraints upon the measurement (3Helium temperatures are required). Fermi surface calculations have produced complex figures which lend validation to such rich behavior. Presented are updated measurements including magnetization and revised theoretical calculations..

Terahertz spectroscopic evidence of non-Fermi-liquid-like behavior in structurally modulated PrNi O3 thin films

NASA Astrophysics Data System (ADS)

Phanindra, V. Eswara; Agarwal, Piyush; Rana, D. S.

2018-01-01

The intertwined and competing energy scales of various interactions in rare-earth nickelates R Ni O3 (R =La to Lu) hold potential for a wide range of exotic ground states realized upon structural modulation. Using terahertz (THz) spectroscopy, the low-energy dynamics of a novel non-Fermi liquid (NFL) metallic phase induced in compressive PrNi O3 thin film was studied by evaluating the quasiparticle scattering rate in the light of two distinct models over a wide temperature range. First, evaluating THz conductivity in the framework of extended Drude model, the frequency-dependent scattering rate is found to deviate from the Landau Fermi liquid (LFL) behavior, thus, suggesting NFL-like phase at THz frequencies. Second, fitting THz conductivity to the multiband Drude-Lorentz model reveals the band-dependent scattering rates and provides microscopic interpretation of the carriers contributing to the Drude modes. This is first evidence of NFL-like behavior in nickelates at THz frequencies consistent with dc conductivity, which also suggests that THz technology is indispensable in understanding emerging electronic phases and associated phenomena. We further demonstrate that the metal-insulator transition in nickelates has the potential to design efficient THz modulators.

Role of defects in the carrier-tunable topological-insulator (Bi1 -xSbx )2Te3 thin films

NASA Astrophysics Data System (ADS)

Scipioni, Kane L.; Wang, Zhenyu; Maximenko, Yulia; Katmis, Ferhat; Steiner, Charlie; Madhavan, Vidya

2018-03-01

Alloys of Bi2Te3 and Sb2Te3[(Bi1-xSbx) 2Te3] have played an essential role in the exploration of topological surface states, allowing us to study phenomena that would otherwise be obscured by bulk contributions to conductivity. Despite intensive transport and angle resolved photoemission (ARPES) studies, important questions about this system remain unanswered. For example, previous studies reported the chemical tuning of the Fermi level to the Dirac point by controlling the Sb:Bi composition ratio, but the optimum ratio varies widely across various studies. Moreover, it is unclear how the quasiparticle lifetime is affected by the disorder resulting from Sb/Bi alloying. In this work, we use scanning tunneling microscopy and spectroscopy to study the electronic structure of epitaxially grown (Bi,Sb) 2Te3 thin films at the nanoscale. We study Landau levels (LLs) to determine the effect of disorder on the quasiparticle lifetime as well as the position of the Dirac point with respect to the Fermi energy. A plot of the LL peak widths shows that despite the intrinsic disorder, the quasiparticle lifetime is not significantly degraded. We further determine that the ideal Sb concentration to place the Fermi energy to within a few meV of the Dirac point is x ˜0.7 , but that postannealing temperatures can have a significant effect on the crystallinity and Fermi level position. Specifically, high postgrowth annealing temperature can result in better crystallinity and surface roughness, but also produces a larger Te defect density which adds n -type carriers. Finally, in combination with quasiparticle interference imaging, the dispersion is revealed over a large energy range above the Fermi energy, in a regime inaccessible to ARPES. Interestingly, the surface state dispersion for the x ˜0.7 sample shows great similarity to pristine Bi2Te3 . This work provides microscopic information on the role of disorder and composition in determining carrier concentration, surface state dispersion, and quasiparticle lifetime in (Bi1 -xSbx )2Te3 .

Dissecting the Gamma-Ray Background in Search of Dark Matter

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cholis, Ilias; Hooper, Dan; McDermott, Samuel D.

2014-02-01

Several classes of astrophysical sources contribute to the approximately isotropic gamma-ray background measured by the Fermi Gamma-Ray Space Telescope. In this paper, we use Fermi's catalog of gamma-ray sources (along with corresponding source catalogs at infrared and radio wavelengths) to build and constrain a model for the contributions to the extragalactic gamma-ray background from astrophysical sources, including radio galaxies, star-forming galaxies, and blazars. We then combine our model with Fermi's measurement of the gamma-ray background to derive constraints on the dark matter annihilation cross section, including contributions from both extragalactic and galactic halos and subhalos. The resulting constraints are competitivemore » with the strongest current constraints from the Galactic Center and dwarf spheroidal galaxies. As Fermi continues to measure the gamma-ray emission from a greater number of astrophysical sources, it will become possible to more tightly constrain the astrophysical contributions to the extragalactic gamma-ray background. We project that with 10 years of data, Fermi's measurement of this background combined with the improved constraints on the astrophysical source contributions will yield a sensitivity to dark matter annihilations that exceeds the strongest current constraints by a factor of ~ 5 - 10.« less

The nuclear Thomas-Fermi model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Myers, W.D.; Swiatecki, W.J.

1994-08-01

The statistical Thomas-Fermi model is applied to a comprehensive survey of macroscopic nuclear properties. The model uses a Seyler-Blanchard effective nucleon-nucleon interaction, generalized by the addition of one momentum-dependent and one density-dependent term. The adjustable parameters of the interaction were fitted to shell-corrected masses of 1654 nuclei, to the diffuseness of the nuclear surface and to the measured depths of the optical model potential. With these parameters nuclear sizes are well reproduced, and only relatively minor deviations between measured and calculated fission barriers of 36 nuclei are found. The model determines the principal bulk and surface properties of nuclear mattermore » and provides estimates for the more subtle, Droplet Model, properties. The predicted energy vs density relation for neutron matter is in striking correspondence with the 1981 theoretical estimate of Friedman and Pandharipande. Other extreme situations to which the model is applied are a study of Sn isotopes from {sup 82}Sn to {sup 170}Sn, and the rupture into a bubble configuration of a nucleus (constrained to spherical symmetry) which takes place when Z{sup 2}/A exceeds about 100.« less

The Nuclear Thomas-Fermi Model

DOE R&D Accomplishments Database

Myers, W. D.; Swiatecki, W. J.

1994-08-01

The statistical Thomas-Fermi model is applied to a comprehensive survey of macroscopic nuclear properties. The model uses a Seyler-Blanchard effective nucleon-nucleon interaction, generalized by the addition of one momentum-dependent and one density-dependent term. The adjustable parameters of the interaction were fitted to shell-corrected masses of 1654 nuclei, to the diffuseness of the nuclear surface and to the measured depths of the optical model potential. With these parameters nuclear sizes are well reproduced, and only relatively minor deviations between measured and calculated fission barriers of 36 nuclei are found. The model determines the principal bulk and surface properties of nuclear matter and provides estimates for the more subtle, Droplet Model, properties. The predicted energy vs density relation for neutron matter is in striking correspondence with the 1981 theoretical estimate of Friedman and Pandharipande. Other extreme situations to which the model is applied are a study of Sn isotopes from {sup 82}Sn to {sup 170}Sn, and the rupture into a bubble configuration of a nucleus (constrained to spherical symmetry) which takes place when Z{sup 2}/A exceeds about 100.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xia, Jun-Qing; Cuoco, Alessandro; Branchini, Enzo

Building on our previous cross-correlation analysis (Xia et al. 2011) between the isotropic γ-ray background (IGRB) and different tracers of the large-scale structure of the universe, we update our results using 60 months of data from the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi). For this study, we perform a cross-correlation analysis both in configuration and spherical harmonics space between the IGRB and objects that may trace the astrophysical sources of the IGRB: QSOs in the Sloan Digital Sky Survey (SDSS) DR6, the SDSS DR8 Main Galaxy Sample, luminous red galaxies (LRGs) in the SDSS catalog, infrared-selected galaxies in the Two Micron All Sky Survey (2MASS), and radio galaxies in the NRAO VLA Sky Survey (NVSS). The benefit of correlating the Fermi-LAT signal with catalogs of objects at various redshifts is to provide tomographic information on the IGRB, which is crucial in separating the various contributions and clarifying its origin. The main result is that, unlike in our previous analysis, we now observe a significant (>3.5σ) cross-correlation signal on angular scales smaller than 1° in the NVSS, 2MASS, and QSO cases and, at lower statistical significance (~3.0σ), with SDSS galaxies. The signal is stronger in two energy bands, E > 0.5 GeV and E > 1 GeV, but it is also seen at E > 10 GeV. No cross-correlation signal is detected between Fermi data and the LRGs. These results are robust against the choice of the statistical estimator, estimate of errors, map cleaning procedure, and instrumental effects. Finally, we test the hypothesis that the IGRB observed by Fermi-LAT originates from the summed contributions of three types of unresolved extragalactic sources: BL Lacertae objects (BL Lacs), flat spectrum radio quasars (FSRQs), and star-forming galaxies (SFGs). Finally, we find that a model in which the IGRB is mainly produced by SFGs (more » $$72_{-37}^{+23}$$% with 2σ errors), with BL Lacs and FSRQs giving a minor contribution, provides a good fit to the data. We also consider a possible contribution from misaligned active galactic nuclei, and we find that, depending on the details of the model and its uncertainty, they can also provide a substantial contribution, partly degenerate with the SFG one.« less

Unified mechanism of the surface Fermi level pinning in III-As nanowires.

PubMed

Alekseev, Prokhor A; Dunaevskiy, Mikhail S; Cirlin, George E; Reznik, Rodion R; Smirnov, Alexander N; Kirilenko, Demid A; Davydov, Valery Yu; Berkovits, Vladimir L

2018-08-03

Fermi level pinning at the oxidized (110) surfaces of III-As nanowires (GaAs, InAs, InGaAs, AlGaAs) is studied. Using scanning gradient Kelvin probe microscopy, we show that the Fermi level at oxidized cleavage surfaces of ternary Al x Ga 1-x As (0 ≤ x ≤ 0.45) and Ga x In 1-x As (0 ≤ x ≤ 1) alloys is pinned at the same position of 4.8 ± 0.1 eV with regard to the vacuum level. The finding implies a unified mechanism of the Fermi level pinning for such surfaces. Further investigation, performed by Raman scattering and photoluminescence spectroscopy, shows that photooxidation of the Al x Ga 1-x As and Ga x In 1-x As nanowires leads to the accumulation of an excess of arsenic on their crystal surfaces which is accompanied by a strong decrease of the band-edge photoluminescence intensity. We conclude that the surface excess arsenic in crystalline or amorphous forms is responsible for the Fermi level pinning at oxidized (110) surfaces of III-As nanowires.

Fermi Surface Properties, Metamagnetic Transition and Quantum Phase Transition of CeRu2Si2 and Its Alloys Probed by the dHvA Effect

NASA Astrophysics Data System (ADS)

Aoki, Haruyoshi; Kimura, Noriaki; Terashima, Taichi

2014-07-01

This article describes the Fermi surface properties of CeRu2Si2 and its alloy systems CeRu2(SixGe1-x)2 and CexLa1-xRu2Si2 studied by the de Haas-van Alphen (dHvA) effect. We pay particular attention to how the Fermi surface properties and the f electron state change with magnetic properties, in particular how they change associated with metamagnetic transition and quantum phase transition. After summarizing the important physical properties of CeRu2Si2, we present the magnetic phase diagrams of CeRu2(SixGe1-x)2 and CexLa1-xRu2Si2 as a function of temperature, magnetic field and concentration x. From the characteristic features of the magnetic phase diagram, we argue that the ferromagnetic interaction in addition to the antiferromagnetic interaction and the Kondo effect is responsible for the magnetic properties and that the metamagnetic transitions in these systems are relevant to the ferromagnetic interaction. We summarize the Fermi surface properties of CeRu2Si2 in fields below the metamagnetic transition where the f electron state is now well understood theoretically as well as experimentally. We present experimental results in fields above the metamagnetic transitions in CeRu2(SixGe1-x)2 and CexLa1-xRu2Si2 as well as CeRu2Si2 to show that the Fermi surface properties above the metamagnetic transitions are significantly different from those below in many important aspects. We argue that the Fermi surface properties above the metamagnetic transitions are not appropriately described in terms of either itinerant or localized f electron. The experimental results in fields below the metamagnetic transitions in CeRu2(SixGe1-x)2 and CexLa1-xRu2Si2 are presented to discuss the f electron state in the ground state. The Fermi surface properties of dilute Kondo alloys of CexLa1-xRu2Si2 have been revealed as a function of Ce concentration and temperature. We show that the f electron state can be regarded as itinerant in the ground state together with the definition of the term "itinerant" in this case. The Fermi surface properties are measured also in high concentration alloys of CeRu2(SixGe1-x)2 and CexLa1-xRu2Si2 as a function of x. With the help of the angle resolved photoemission spectroscopy studies, we show that the f electron nature does not change at the quantum phase transition between the paramagnetic and antiferromagnetic phases. However, the picture for the f electron state may be ambiguous and depend on which property one considers in the magnetic states of these systems. The ambiguity and confusion of the f electron state may come from the inherent dual nature of the f electron and we would like to point out that it is sometimes misleading and may not be fruitful to discriminate the f electron state either as itinerant or localized without any clear definition for the terms "itinerant" and "localized".

Fermi level pinning at the Ge(001) surface—A case for non-standard explanation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wojtaszek, Mateusz; Zuzak, Rafal; Godlewski, Szymon

2015-11-14

To explore the origin of the Fermi level pinning in germanium, we investigate the Ge(001) and Ge(001):H surfaces. The absence of relevant surface states in the case of Ge(001):H should unpin the surface Fermi level. This is not observed. For samples with donors as majority dopants, the surface Fermi level appears close to the top of the valence band regardless of the surface structure. Surprisingly, for the passivated surface, it is located below the top of the valence band allowing scanning tunneling microscopy imaging within the band gap. We argue that the well known electronic mechanism behind band bending doesmore » not apply and a more complicated scenario involving ionic degrees of freedom is therefore necessary. Experimental techniques involve four point probe electric current measurements, scanning tunneling microscopy, and spectroscopy.« less

Magnetic-Field Dependences of Thermodynamic Quantities in the Vortex State of Type-Ii Superconductors

NASA Astrophysics Data System (ADS)

Watanabe, Koichi; Kita, Takafumi; Arai, Masao

2006-08-01

We develop an alternative method to solve the Eilenberger equations numerically for the vortex-lattice states of type-II superconductors. Using it, we clarify the magnetic-field and impurity-concentration dependences of the magnetization, the entropy, the Pauli paramagnetism, and the mixing of higher Landau levels in the pair potential for two-dimensional s- and dx2-y2-wave superconductors with a cylindrical Fermi surface.

Anomalous Quasiparticle Reflection from the Surface of a ^{3}He-^{4}He Dilute Solution.

PubMed

Ikegami, Hiroki; Kim, Kitak; Sato, Daisuke; Kono, Kimitoshi; Choi, Hyoungsoon; Monarkha, Yuriy P

2017-11-10

A free surface of a dilute ^{3}He-^{4}He liquid mixture is a unique system where two Fermi liquids with distinct dimensions coexist: a three-dimensional (3D) ^{3}He Fermi liquid in the bulk and a two-dimensional (2D) ^{3}He Fermi liquid at the surface. To investigate a novel effect generated by the interaction between the two Fermi liquids, the mobility of a Wigner crystal of electrons formed on the free surface of the mixture is studied. An anomalous enhancement of the mobility, compared with the case where the 3D and 2D systems do not interact with each other, is observed. The enhancement is explained by the nontrivial reflection of 3D quasiparticles from the surface covered with the 2D ^{3}He system.

Quasi-continuous transition from a Fermi liquid to a spin liquid in κ-(ET)2Cu2(CN)3.

PubMed

Furukawa, Tetsuya; Kobashi, Kazuhiko; Kurosaki, Yosuke; Miyagawa, Kazuya; Kanoda, Kazushi

2018-01-22

The Mott metal-insulator transition-a manifestation of Coulomb interactions among electrons-is known as a discontinuous transition. Recent theoretical studies, however, suggest that the transition is continuous if the Mott insulator carries a spin liquid with a spinon Fermi surface. Here, we demonstrate the case of a quasi-continuous Mott transition from a Fermi liquid to a spin liquid in an organic triangular-lattice system κ-(ET) 2 Cu 2 (CN) 3 . Transport experiments performed under fine pressure tuning have found that as the Mott transition is approached, the Fermi liquid coherence temperature continuously falls to the scale of kelvins, with a divergent quasi-particle decay rate on the metal side, and the charge gap continuously closes on the insulator side. A Clausius-Clapeyron analysis provides thermodynamic evidence for the extremely weak first-order nature of the transition. These results provide additional support for the existence of a spinon Fermi surface, which becomes an electron Fermi surface when charges are delocalized.

Bulk Fermi Surfaces of the Dirac Type-II Semimetallic Candidates M Al3 (Where M =V , Nb, and Ta)

NASA Astrophysics Data System (ADS)

Chen, K.-W.; Lian, X.; Lai, Y.; Aryal, N.; Chiu, Y.-C.; Lan, W.; Graf, D.; Manousakis, E.; Baumbach, R. E.; Balicas, L.

2018-05-01

We report a de Haas-van Alphen (dHvA) effect study on the Dirac type-II semimetallic candidates M Al3 (where, M =V , Nb and Ta). The angular dependence of their Fermi surface (FS) cross-sectional areas reveals a remarkably good agreement with our first-principles calculations. Therefore, dHvA supports the existence of tilted Dirac cones with Dirac type-II nodes located at 100, 230 and 250 meV above the Fermi level ɛF for VAl3 , NbAl3 and TaAl3 respectively, in agreement with the prediction of broken Lorentz invariance in these compounds. However, for all three compounds we find that the cyclotron orbits on their FSs, including an orbit nearly enclosing the Dirac type-II node, yield trivial Berry phases. We explain this via an analysis of the Berry phase where the position of this orbit, relative to the Dirac node, is adjusted within the error implied by the small disagreement between our calculations and the experiments. We suggest that a very small amount of doping could displace ɛF to produce topologically nontrivial orbits encircling their Dirac node(s).

Schottky-barrier-free contacts with two-dimensional semiconductors by surface-engineered MXenes

DOE PAGES

Liu, Yuanyue; Xiao, Hai; Goddard, III, William A.

2016-11-22

Two-dimensional (2D) metal carbides and nitrides, called MXenes, have attracted great interest for applications such as energy storage. Here we demonstrate their potential as Schottky-barrier-free metal contacts to 2D semiconductors, providing a solution to the contact-resistance problem in 2D electronics. Based on first principles calculations, we find that the surface chemistry strongly affects the Fermi level of MXenes: O termination always increases the work function with respect to that of bare surface, OH always decreases it, while F exhibits either trend depending on the specific material. This phenomenon originates from the effect of surface dipoles, which together with the weakmore » Fermi level pinning, enable Schottky-barrier-free hole (or electron) injection into 2D semiconductors through van der Waals junctions with some of the O-terminated (or all the OH-terminated) MXenes. Furthermore, we suggest synthetic routes to control the surface terminations based on the calculated formation energies. Finally, this study enhances the understanding of the correlation between surface chemistry and electronic/transport properties of 2D materials, and also gives practical predictions for improving 2D electronics.« less

Contactless electroreflectance study of the Fermi level pinning on GaSb surface in n-type and p-type GaSb Van Hoof structures

NASA Astrophysics Data System (ADS)

Kudrawiec, R.; Nair, H. P.; Latkowska, M.; Misiewicz, J.; Bank, S. R.; Walukiewicz, W.

2012-12-01

Contactless electroreflectance (CER) has been applied to study the Fermi-level position on GaSb surface in n-type and p-type GaSb Van Hoof structures. CER resonances, followed by strong Franz-Keldysh oscillation of various periods, were clearly observed for two series of structures. This period was much wider (i.e., the built-in electric field was much larger) for n-type structures, indicating that the GaSb surface Fermi level pinning position is closer to the valence-band than the conduction-band. From analysis of the built-in electric fields in undoped GaSb layers, it was concluded that on GaSb surface the Fermi-level is located ˜0.2 eV above the valence band.

Superconductivity in doped Dirac semimetals

NASA Astrophysics Data System (ADS)

Hashimoto, Tatsuki; Kobayashi, Shingo; Tanaka, Yukio; Sato, Masatoshi

2016-07-01

We theoretically study intrinsic superconductivity in doped Dirac semimetals. Dirac semimetals host bulk Dirac points, which are formed by doubly degenerate bands, so the Hamiltonian is described by a 4 ×4 matrix and six types of k -independent pair potentials are allowed by the Fermi-Dirac statistics. We show that the unique spin-orbit coupling leads to characteristic superconducting gap structures and d vectors on the Fermi surface and the electron-electron interaction between intra and interorbitals gives a novel phase diagram of superconductivity. It is found that when the interorbital attraction is dominant, an unconventional superconducting state with point nodes appears. To verify the experimental signature of possible superconducting states, we calculate the temperature dependence of bulk physical properties such as electronic specific heat and spin susceptibility and surface state. In the unconventional superconducting phase, either dispersive or flat Andreev bound states appear between point nodes, which leads to double peaks or a single peak in the surface density of states, respectively. As a result, possible superconducting states can be distinguished by combining bulk and surface measurements.

Defect characterization in Mg-doped GaN studied using a monoenergetic positron beam

NASA Astrophysics Data System (ADS)

Uedono, A.; Ishibashi, S.; Tenjinbayashi, K.; Tsutsui, T.; Nakahara, K.; Takamizu, D.; Chichibu, S. F.

2012-01-01

Vacancy-type defects in Mg-doped GaN grown by metalorganic vapor phase epitaxy were probed using a monoenergetic positron beam. For a sample fabricated with a high H2-flow rate, before post-growth annealing the major defect species detected by positrons was identified as vacancy-clusters. Evidence suggested that other donor-type defects such as nitrogen vacancies also existed. The defects increased the Fermi level position, and enhanced the diffusion of positrons toward the surface. The annihilation of positrons at the top surface was suppressed by Mg-doping. This was attributed to the introduction of a subsurface layer (<6 nm) with a low defect concentration, where the Fermi level position was considered to decrease due to partial activation of Mg. For samples after annealing, the trapping of positrons by residual vacancy-type defects was observed, and the sample crystal quality was found to depend on that before annealing.

Three-component fermions with surface Fermi arcs in tungsten carbide

NASA Astrophysics Data System (ADS)

Ma, J.-Z.; He, J.-B.; Xu, Y.-F.; Lv, B. Q.; Chen, D.; Zhu, W.-L.; Zhang, S.; Kong, L.-Y.; Gao, X.; Rong, L.-Y.; Huang, Y.-B.; Richard, P.; Xi, C.-Y.; Choi, E. S.; Shao, Y.; Wang, Y.-L.; Gao, H.-J.; Dai, X.; Fang, C.; Weng, H.-M.; Chen, G.-F.; Qian, T.; Ding, H.

2018-04-01

Topological Dirac and Weyl semimetals not only host quasiparticles analogous to the elementary fermionic particles in high-energy physics, but also have a non-trivial band topology manifested by gapless surface states, which induce exotic surface Fermi arcs1,2. Recent advances suggest new types of topological semimetal, in which spatial symmetries protect gapless electronic excitations without high-energy analogues3-11. Here, using angle-resolved photoemission spectroscopy, we observe triply degenerate nodal points near the Fermi level of tungsten carbide with space group P 6 ¯m 2 (no. 187), in which the low-energy quasiparticles are described as three-component fermions distinct from Dirac and Weyl fermions. We further observe topological surface states, whose constant-energy contours constitute pairs of `Fermi arcs' connecting to the surface projections of the triply degenerate nodal points, proving the non-trivial topology of the newly identified semimetal state.

Orbital selectivity causing anisotropy and particle-hole asymmetry in the charge density wave gap of 2 H - TaS 2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhao, J.; Wijayaratne, K.; Butler, A.

We report an in-depth angle-resolved photoemission spectroscopy study on 2H-TaS2, a canonical incommensurate charge density wave (CDW) system. This study demonstrates that just as in related incommensurate CDW systems, 2H-TaSe2 and 2H-NbSe2, the energy gap (triangle(CDW)) of 2H-TaS2 is localized along the K-centered Fermi surface barrels and is particle-hole asymmetric. The persistence of triangle(CDW) even at temperatures higher than the CDW transition temperature T-CDW in 2H-TaS2, reflects the similar pseudogap behavior observed previously in 2H-TaSe2 and 2H-NbSe2. However, in sharp contrast to 2H-NbSe2, where triangle(CDW) is nonzero only in the vicinity of a few "hot spots" on the innerK-centered Fermimore » surface barrels, triangle(CDW) in 2H-TaS2 is nonzero along the entirety of both K-centered Fermi surface barrels. Based on a tight-binding model, we attribute this dichotomy in the momentum dependence and the Fermi surface specificity of triangle(CDW) between otherwise similar CDW compounds to the different orbital orientations of their electronic states that participate in the CDW pairing. Our results suggest that the orbital selectivity plays a critical role in the description of incommensurate CDW materials.« less

Distinct evolutions of Weyl fermion quasiparticles and Fermi arcs with bulk band topology in Weyl semimetals

NASA Astrophysics Data System (ADS)

Xu, Nan; Autes, Gabriel; Matt, Christian; Lv, Baiqing; Bisti, Federico; Strocov, Vladimir; Gawryluk, Dariusz; Pomjakushina, Ekaterina; Conder, Kazimierz; Plumb, Nicholas; Radovic, Milan; Qian, Tian; Yazyev, Oleg; Mesot, Joel; Ding, Hong; Shi, Ming

By performing ARPES and first-principle calculations, we demonstrate that Weyl fermions quasiparticles in bulk and Fermi arc on surface show distinct evolutions with the bulk band topology in transition-metal monophosphides. While Weyl fermion quasiparticles exist only when the chemical potential is located between two saddle points of the Weyl cone features, the Fermi arc states extend in a larger energy scale and are robust across the bulk Lifshitz transitions associated with the recombination of two non-trivial Fermi surfaces enclosing one Weyl point into a single trivial Fermi surface enclosing two Weyl points of opposite chirality. Therefore, in some systems (NbP), Fermi arc states are preserved even if Weyl fermion quasiparticles are absent in the bulk. Our findings not only provide insight into the relationship between the exotic physical phenomena and the intrinsic bulk band topology in Weyl semimetals, but also resolve the apparent puzzle of the different magneto-transport properties observed in TaAs, TaP and NbP, where the Fermi arc states are similar. The Sino-Swiss Science and Technology Cooperation (No. IZLCZ2138954), NCCR-MARVEL funded by the Swiss National Science Foundation.

Surface-state depopulation on small Ag(111) terraces.

PubMed

Morgenstern, Karina; Braun, Kai-Felix; Rieder, Karl-Heinz

2002-11-25

The dependence of the local density of states near the Fermi energy E(F) on the width of terraces T is investigated by tunneling scanning spectroscopy on Ag(111) at 7 K. With decreasing T, the electronic density in the occupied surface state shifts monotonically towards E(F), leading to a depopulation at T=3.2 nm in quantitative agreement with a Fabry-Pérot model. Depopulation coincides with a switch from confinement by terrace modulation to step modulation.

High Magnetic Field Properties of Ce2Rh3Ge5

NASA Astrophysics Data System (ADS)

Wartenbe, Mark

2015-03-01

The competition between localized and delocalized f electrons in heavy fermion materials produces a wide variety of interesting physical phenomena. Among these compounds is Ce2Rh3Ge5. This heavy-fermion system undergoes an antiferromagnetic transition below 4K and exhibits an angle dependent magnetic phase transition around 25 tesla. In addition, RF conductivity measurements in pulsed field (65T) have revealed quantum oscillations. Temperature dependence at fixed angle indicates relatively heavy effective masses of values ranging from around 3me on up to 10me. This indicates that the narrow f-electron density of states is partially hybridized close to the Fermi energy, but also places strict cryogenic constraints upon the measurement (3Helium temperatures are required). Fermi surface calculations have produced complex figures which lend validation to such rich behavior. Ryan Baumbach, Andrew Gallagher, Eric Bauer, Ross McDonald, Kuan-Wen Chen,David Graf.

Short-period oscillations in photoemission from thin films of Cr(100)

NASA Astrophysics Data System (ADS)

Vyalikh, Denis V.; Zahn, Peter; Richter, Manuel; Dedkov, Yu. S.; Molodtsov, S. L.

2005-07-01

Angle-resolved photoemission (PE) study of thin films of Cr grown on Fe(100) reveals thickness-dependent short-period oscillations of the PE intensity close to the Fermi energy at k‖˜0 . The oscillations are assigned to quantum-well states (QWS) caused by the nesting between the Fermi-surface sheets around the Γ and the X points in the Brillouin zone of antiferromagnetic Cr. The experimental data are confirmed by density-functional calculations applying a screened Korringa-Kohn-Rostoker Green’s function method. The period of the experimentally observed QWS oscillations amounts to about 2.6 monolayers and is larger than the fundamental 2-monolayer period of antiferromagnetic coupling in Cr.

Bulk Fermi surface of the Weyl type-II semimetallic candidate γ - MoTe 2

DOE PAGES

Rhodes, D.; Schönemann, R.; Aryal, N.; ...

2017-10-17

The electronic structure of semi-metallic transition-metal dichalcogenides, such as WTemore » $$_2$$ and orthorhombic $$\\gamma-$$MoTe$$_2$$, are claimed to contain pairs of Weyl points or linearly touching electron and hole pockets associated with a non-trivial Chern number. For this reason, these compounds were recently claimed to conform to a new class, deemed type-II, of Weyl semi-metallic systems. A series of angle resolved photoemission experiments (ARPES) claim a broad agreement with these predictions detecting, for example, topological Fermi arcs at the surface of these crystals. We synthesized single-crystals of semi-metallic MoTe$$_2$$ through a Te flux method to validate these predictions through measurements of its bulk Fermi surface (FS) via quantum oscillatory phenomena. We find that the superconducting transition temperature of $$\\gamma-$$MoTe$$_2$$ depends on disorder as quantified by the ratio between the room- and low-temperature resistivities, suggesting the possibility of an unconventional superconducting pairing symmetry. Similarly to WTe$$_2$$, the magnetoresistivity of $$\\gamma-$$MoTe$$_2$$ does not saturate at high magnetic fields and can easily surpass $$10^{6}$$ \\%. Remarkably, the analysis of the de Haas-van Alphen (dHvA) signal superimposed onto the magnetic torque, indicates that the geometry of its FS is markedly distinct from the calculated one. The dHvA signal also reveals that the FS is affected by the Zeeman-effect precluding the extraction of the Berry-phase. A direct comparison between the previous ARPES studies and density-functional-theory (DFT) calculations reveals a disagreement in the position of the valence bands relative to the Fermi level $$\\varepsilon_F$$. Here in this paper, we show that a shift of the DFT valence bands relative to $$\\varepsilon_F$$, in order to match the ARPES observations, and of the DFT electron bands to explain some of the observed dHvA frequencies, leads to a good agreement between the calculations and the angular dependence of the FS cross-sectional areas observed experimentally. However, this relative displacement between electron- and hole-bands eliminates their crossings and, therefore, the Weyl type-II points predicted for $$\\gamma-$$MoTe$$_2$$« less

Bulk Fermi surface of the Weyl type-II semimetallic candidate γ -MoTe2

NASA Astrophysics Data System (ADS)

Rhodes, D.; Schönemann, R.; Aryal, N.; Zhou, Q.; Zhang, Q. R.; Kampert, E.; Chiu, Y.-C.; Lai, Y.; Shimura, Y.; McCandless, G. T.; Chan, J. Y.; Paley, D. W.; Lee, J.; Finke, A. D.; Ruff, J. P. C.; Das, S.; Manousakis, E.; Balicas, L.

2017-10-01

The electronic structure of semimetallic transition-metal dichalcogenides, such as WTe2 and orthorhombic γ -MoTe2 , are claimed to contain pairs of Weyl points or linearly touching electron and hole pockets associated with a nontrivial Chern number. For this reason, these compounds were recently claimed to conform to a new class, deemed type-II, of Weyl semimetallic systems. A series of angle-resolved photoemission experiments (ARPES) claim a broad agreement with these predictions detecting, for example, Fermi arcs at the surface of these crystals. We synthesized single crystals of semimetallic MoTe2 through a Te flux method to validate these predictions through measurements of its bulk Fermi surface (FS) via quantum oscillatory phenomena. We find that the superconducting transition temperature of γ -MoTe2 depends on disorder as quantified by the ratio between the room- and low-temperature resistivities, suggesting the possibility of an unconventional superconducting pairing symmetry. Similarly to WTe2, the magnetoresistivity of γ -MoTe2 does not saturate at high magnetic fields and can easily surpass 106%. Remarkably, the analysis of the de Haas-van Alphen (dHvA) signal superimposed onto the magnetic torque indicates that the geometry of its FS is markedly distinct from the calculated one. The dHvA signal also reveals that the FS is affected by the Zeeman effect precluding the extraction of the Berry phase. A direct comparison between the previous ARPES studies and density-functional-theory (DFT) calculations reveals a disagreement in the position of the valence bands relative to the Fermi level ɛF. Here, we show that a shift of the DFT valence bands relative to ɛF, in order to match the ARPES observations, and of the DFT electron bands to explain some of the observed dHvA frequencies, leads to a good agreement between the calculations and the angular dependence of the FS cross-sectional areas observed experimentally. However, this relative displacement between electron and hole bands eliminates their crossings and, therefore, the Weyl type-II points predicted for γ -MoTe2 .

The 7 × 1 Fermi Surface Reconstruction in a Two-dimensional f -electron Charge Density Wave System: PrTe 3

DOE PAGES

Lee, Eunsook; Kim, D. H.; Kim, Hyun Woo; ...

2016-07-25

The electronic structure of a charge density wave (CDW) system PrTe 3 and its modulated structure in the CDW phase have been investigated by employing ARPES, XAS, Pr 4 f RPES, and first-principles band structure calculation. Pr ions are found to be nearly trivalent, supporting the CDW instability in the metallic Te sheets through partial filling. Finite Pr 4 f spectral weight is observed near the Fermi level, suggesting the non-negligible Pr 4 f contribution to the CDW formation through the Pr 4 f -Te 5p hybridization. The two-fold symmetric features in the measured Fermi surface (FS) of PrTe 3more » are explained by the calculated FS for the assumed 7 × 1 CDW supercell formation in Te sheets. The shadow bands and the corresponding very weak FSs are observed, which originate from both the band folding due to the 3D interaction of Te sheets with neighboring Pr-Te layers and that due to the CDW-induced FS reconstruction. The straight vertical FSs are observed along k z, demonstrating the nearly 2D character for the near-EF states. The observed linear dichroism reveals the in-plane orbital character of the near-E F Te 5p states.« less

Electronic structure of R Sb ( R = Y , Ce, Gd, Dy, Ho, Tm, Lu) studied by angle-resolved photoemission spectroscopy

DOE PAGES

Wu, Yun; Lee, Yongbin; Kong, Tai; ...

2017-07-15

Here, we use high-resolution angle-resolved photoemission spectroscopy (ARPES) and electronic structure calculations to study the electronic properties of rare-earth monoantimonides RSb (R = Y, Ce, Gd, Dy, Ho, Tm, Lu). The experimentally measured Fermi surface (FS) of RSb consists of at least two concentric hole pockets at the Γ point and two intersecting electron pockets at the X point. These data agree relatively well with the electronic structure calculations. Detailed photon energy dependence measurements using both synchrotron and laser ARPES systems indicate that there is at least one Fermi surface sheet with strong three-dimensionality centered at the Γ point. Duemore » to the “lanthanide contraction”, the unit cell of different rare-earth monoantimonides shrinks when changing the rare-earth ion from CeSb to LuSb. This results in the differences in the chemical potentials in these compounds, which are demonstrated by both ARPES measurements and electronic structure calculations. Interestingly, in CeSb, the intersecting electron pockets at the X point seem to be touching the valence bands, forming a fourfold-degenerate Dirac-like feature. On the other hand, the remaining rare-earth monoantimonides show significant gaps between the upper and lower bands at the X point. Furthermore, similar to the previously reported results of LaBi, a Dirac-like structure was observed at the Γ point in YSb, CeSb, and GdSb, compounds showing relatively high magnetoresistance. This Dirac-like structure may contribute to the unusually large magnetoresistance in these compounds.« less

Electronic structure of R Sb ( R = Y , Ce, Gd, Dy, Ho, Tm, Lu) studied by angle-resolved photoemission spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, Yun; Lee, Yongbin; Kong, Tai

Here, we use high-resolution angle-resolved photoemission spectroscopy (ARPES) and electronic structure calculations to study the electronic properties of rare-earth monoantimonides RSb (R = Y, Ce, Gd, Dy, Ho, Tm, Lu). The experimentally measured Fermi surface (FS) of RSb consists of at least two concentric hole pockets at the Γ point and two intersecting electron pockets at the X point. These data agree relatively well with the electronic structure calculations. Detailed photon energy dependence measurements using both synchrotron and laser ARPES systems indicate that there is at least one Fermi surface sheet with strong three-dimensionality centered at the Γ point. Duemore » to the “lanthanide contraction”, the unit cell of different rare-earth monoantimonides shrinks when changing the rare-earth ion from CeSb to LuSb. This results in the differences in the chemical potentials in these compounds, which are demonstrated by both ARPES measurements and electronic structure calculations. Interestingly, in CeSb, the intersecting electron pockets at the X point seem to be touching the valence bands, forming a fourfold-degenerate Dirac-like feature. On the other hand, the remaining rare-earth monoantimonides show significant gaps between the upper and lower bands at the X point. Furthermore, similar to the previously reported results of LaBi, a Dirac-like structure was observed at the Γ point in YSb, CeSb, and GdSb, compounds showing relatively high magnetoresistance. This Dirac-like structure may contribute to the unusually large magnetoresistance in these compounds.« less

Observation of two-dimensional Fermi surface and Dirac dispersion in the new material YbMnSb2

NASA Astrophysics Data System (ADS)

Kealhofer, Robert; Jang, Sooyoung; Griffin, Sinead; John, Caolan; Doyle, Spencer; Neaton, Jeffrey; Analytis, James G.; Denlinger, J. D.; Benavides, Katherine; Chan, Julia

We present the synthesis, crystal structure, electronic structure, and transport properties of the new material YbMnSb2. Our measurements reveal that this system is a low-carrier-density semimetal with a 2D Fermi surface arising from a 3D Dirac dispersion. This Fermi surface is consistent with the predictions of antiferromagnetic density functional theory calculations and the Fermi surface observed via angle-resolved photoemission spectroscopy. The quantitative agreement between these measurements and calculations indicates that YbMnSb2 may be a new topological semimetal in the presence of magnetic order. R. K. is supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1106400. C. J., J. G. A., and much of this work received support from the Gordon and Betty Moore Foundation Grant No. GBMF4374.

Breakdown of the Migdal approximation at Lifshitz transitions with giant zero-point motion in the H3S superconductor.

PubMed

Jarlborg, Thomas; Bianconi, Antonio

2016-04-20

While 203 K high temperature superconductivity in H3S has been interpreted by BCS theory in the dirty limit here we focus on the effects of hydrogen zero-point-motion and the multiband electronic structure relevant for multigap superconductivity near Lifshitz transitions. We describe how the topology of the Fermi surfaces evolves with pressure giving different Lifshitz-transitions. A neck-disrupting Lifshitz-transition (type 2) occurs where the van Hove singularity, vHs, crosses the chemical potential at 210 GPa and new small 2D Fermi surface portions appear with slow Fermi velocity where the Migdal-approximation becomes questionable. We show that the neglected hydrogen zero-point motion ZPM, plays a key role at Lifshitz transitions. It induces an energy shift of about 600 meV of the vHs. The other Lifshitz-transition (of type 1) for the appearing of a new Fermi surface occurs at 130 GPa where new Fermi surfaces appear at the Γ point of the Brillouin zone here the Migdal-approximation breaks down and the zero-point-motion induces large fluctuations. The maximum Tc = 203 K occurs at 160 GPa where EF/ω0 = 1 in the small Fermi surface pocket at Γ. A Feshbach-like resonance between a possible BEC-BCS condensate at Γ and the BCS condensate in different k-space spots is proposed.

Reduction of Fermi level pinning and recombination at polycrystalline CdTe surfaces by laser irradiation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Simonds, Brian J.; Kheraj, Vipul; Department of Applied Physics, S. V. National Institute of Technology, Surat 395 007

2015-06-14

Laser processing of polycrystalline CdTe is a promising approach that could potentially increase module manufacturing throughput while reducing capital expenditure costs. For these benefits to be realized, the basic effects of laser irradiation on CdTe must be ascertained. In this study, we utilize surface photovoltage spectroscopy (SPS) to investigate the changes to the electronic properties of the surface of polycrystalline CdTe solar cell stacks induced by continuous-wave laser annealing. The experimental data explained within a model consisting of two space charge regions, one at the CdTe/air interface and one at the CdTe/CdS junction, are used to interpret our SPS results.more » The frequency dependence and phase spectra of the SPS signal are also discussed. To support the SPS findings, low-temperature spectrally-resolved photoluminescence and time-resolved photoluminescence were also measured. The data show that a modest laser treatment of 250 W/cm{sup 2} with a dwell time of 20 s is sufficient to reduce the effects of Fermi level pinning at the surface due to surface defects.« less

Fermi surfaces, spin-mixing parameter, and colossal anisotropy of spin relaxation in transition metals from ab initio theory

NASA Astrophysics Data System (ADS)

Zimmermann, Bernd; Mavropoulos, Phivos; Long, Nguyen H.; Gerhorst, Christian-Roman; Blügel, Stefan; Mokrousov, Yuriy

2016-04-01

The Fermi surfaces and Elliott-Yafet spin-mixing parameter (EYP) of several elemental metals are studied by ab initio calculations. We focus first on the anisotropy of the EYP as a function of the direction of the spin-quantization axis [B. Zimmermann et al., Phys. Rev. Lett. 109, 236603 (2012), 10.1103/PhysRevLett.109.236603]. We analyze in detail the origin of the gigantic anisotropy in 5 d hcp metals as compared to 5 d cubic metals by band structure calculations and discuss the stability of our results against an applied magnetic field. We further present calculations of light (4 d and 3 d ) hcp crystals, where we find a huge increase of the EYP anisotropy, reaching colossal values as large as 6000 % in hcp Ti. We attribute these findings to the reduced strength of spin-orbit coupling, which promotes the anisotropic spin-flip hot loops at the Fermi surface. In order to conduct these investigations, we developed an adapted tetrahedron-based method for the precise calculation of Fermi surfaces of complicated shape and accurate Fermi-surface integrals within the full-potential relativistic Korringa-Kohn-Rostoker Green function method.

Pseudogap and Fermi-Surface Topology in the Two-Dimensional Hubbard Model

NASA Astrophysics Data System (ADS)

Wu, Wei; Scheurer, Mathias S.; Chatterjee, Shubhayu; Sachdev, Subir; Georges, Antoine; Ferrero, Michel

2018-04-01

One of the distinctive features of hole-doped cuprate superconductors is the onset of a "pseudogap" below a temperature T* . Recent experiments suggest that there may be a connection between the existence of the pseudogap and the topology of the Fermi surface. Here, we address this issue by studying the two-dimensional Hubbard model with two distinct numerical methods. We find that the pseudogap only exists when the Fermi surface is holelike and that, for a broad range of parameters, its opening is concomitant with a Fermi-surface topology change from electronlike to holelike. We identify a common link between these observations: The polelike feature of the electronic self-energy associated with the formation of the pseudogap is found to also control the degree of particle-hole asymmetry, and hence the Fermi-surface topology transition. We interpret our results in the framework of an SU(2) gauge theory of fluctuating antiferromagnetism. We show that a mean-field treatment of this theory in a metallic state with U(1) topological order provides an explanation of this polelike feature and a good description of our numerical results. We discuss the relevance of our results to experiments on cuprates.

Recent Developments in Non-Fermi Liquid Theory

NASA Astrophysics Data System (ADS)

Lee, Sung-Sik

2018-03-01

Non-Fermi liquids are unconventional metals whose physical properties deviate qualitatively from those of noninteracting fermions due to strong quantum fluctuations near Fermi surfaces. They arise when metals are subject to singular interactions mediated by soft collective modes. In the absence of well-defined quasiparticles, universal physics of non-Fermi liquids is captured by interacting field theories which replace Landau Fermi liquid theory. However, it has been difficult to understand their universal low-energy physics due to a lack of theoretical methods that take into account strong quantum fluctuations in the presence of abundant low-energy degrees of freedom. In this review, we discuss two approaches that have been recently developed for non-Fermi liquid theory with emphasis on two space dimensions. The first is a perturbative scheme based on a dimensional regularization, which achieves a controlled access to the low-energy physics by tuning the codimension of Fermi surface. The second is a nonperturbative approach which treats the interaction ahead of the kinetic term through a non-Gaussian scaling called interaction-driven scaling. Examples of strongly coupled non-Fermi liquids amenable to exact treatments through the interaction-driven scaling are discussed.

Parity-violating hybridization in heavy Weyl semimetals

NASA Astrophysics Data System (ADS)

Chang, Po-Yao; Coleman, Piers

2018-04-01

We introduce a simple model to describe the formation of heavy Weyl semimetals in noncentrosymmetric heavy fermion compounds under the influence of a parity-mixing, onsite hybridization. A key aspect of interaction-driven heavy Weyl semimetals is the development of surface Kondo breakdown, which is expected to give rise to a temperature-dependent reconfiguration of the Fermi arcs and the Weyl cyclotron orbits which connect them via the chiral bulk states. Our theory predicts a strong temperature-dependent transformation in the quantum oscillations at low temperatures. In addition to the effects of surface Kondo breakdown, the renormalization effects in heavy Weyl semimetals will appear in a variety of thermodynamic and transport measurements.

High-temperature fusion of a multielectron leviton

NASA Astrophysics Data System (ADS)

Moskalets, Michael

2018-04-01

The state of electrons injected onto the surface of the Fermi sea depends on temperature. The state is pure at zero temperature and is mixed at finite temperature. In the case of a single-electron injection, such a transformation can be detected as a decrease in shot noise with increasing temperature. In the case of a multielectron injection, the situation is subtler. The mixedness helps the development of quantum-mechanical exchange correlations between injected electrons, even if such correlations are absent at zero temperature. These correlations enhance the shot noise, which in part counteracts the reduction of noise with temperature. Moreover, at sufficiently high temperatures, the correlation contribution to noise predominates over the contribution of individual particles. As a result, in the system of N electrons, the apparent charge (which is revealed via the shot noise) is changed from e at zero temperature to N e at high temperatures. It looks like the exchange correlations glue electrons into one particle of total charge and energy. This point of view is supported by both charge noise and heat noise. Interestingly, in the macroscopic limit, N →∞ , the correlation contribution completely suppresses the effect of temperature on noise.

First-principles study of electronic structure and Fermi surface in semimetallic YAs

DOE PAGES

Swatek, Przemys?aw Wojciech

2018-03-23

In the course of searching for new systems, which exhibit nonsaturating and extremely large positive magnetoresistance, electronic structure, Fermi surface, and de Haas-van Alphen characteristics of the semimetallic YAs compound were studied using the all-electron full-potential linearized augmented-plane wave (FP–LAPW) approach in the framework of the generalized gradient approximation (GGA). In the scalar-relativistic calculation, the cubic symmetry splits fivefold degenerate Y- d orbital into low-energy threefold-degenerate and twofold degenerate doublet states at point around the Fermi energy. Furthermore one of them, together with the threefold degenerate character of As-p orbital, render the YAs semimetal with a topologically trivial band ordermore » and fairly low density of states at the Fermi level. Including spin–orbit (SO) coupling into the calculation leads to pronounced splitting of the state and shifting the bands in the energy scale. Consequently, the determined four different 3-dimensional Fermi surface sheets of YAs consists of three concentric hole-like bands at and one ellipsoidal electron-like sheet centred at the X points. In full accordance with the previous first-principles calculations for isostructural YSb and YBi, the calculated Fermi surface of YAs originates from fairly compensated multi-band electronic structures.« less

First-principles study of electronic structure and Fermi surface in semimetallic YAs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Swatek, Przemys?aw Wojciech

In the course of searching for new systems, which exhibit nonsaturating and extremely large positive magnetoresistance, electronic structure, Fermi surface, and de Haas-van Alphen characteristics of the semimetallic YAs compound were studied using the all-electron full-potential linearized augmented-plane wave (FP–LAPW) approach in the framework of the generalized gradient approximation (GGA). In the scalar-relativistic calculation, the cubic symmetry splits fivefold degenerate Y- d orbital into low-energy threefold-degenerate and twofold degenerate doublet states at point around the Fermi energy. Furthermore one of them, together with the threefold degenerate character of As-p orbital, render the YAs semimetal with a topologically trivial band ordermore » and fairly low density of states at the Fermi level. Including spin–orbit (SO) coupling into the calculation leads to pronounced splitting of the state and shifting the bands in the energy scale. Consequently, the determined four different 3-dimensional Fermi surface sheets of YAs consists of three concentric hole-like bands at and one ellipsoidal electron-like sheet centred at the X points. In full accordance with the previous first-principles calculations for isostructural YSb and YBi, the calculated Fermi surface of YAs originates from fairly compensated multi-band electronic structures.« less

Relativistic Thomas-Fermi treatment of compressed atoms and compressed nuclear matter cores of stellar dimensions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rotondo, M.; Rueda, Jorge A.; Xue, S.-S.

The Feynman-Metropolis-Teller treatment of compressed atoms is extended to the relativistic regimes. Each atomic configuration is confined by a Wigner-Seitz cell and is characterized by a positive electron Fermi energy. The nonrelativistic treatment assumes a pointlike nucleus and infinite values of the electron Fermi energy can be attained. In the relativistic treatment there exists a limiting configuration, reached when the Wigner-Seitz cell radius equals the radius of the nucleus, with a maximum value of the electron Fermi energy (E{sub e}{sup F}){sub max}, here expressed analytically in the ultrarelativistic approximation. The corrections given by the relativistic Thomas-Fermi-Dirac exchange term are alsomore » evaluated and shown to be generally small and negligible in the relativistic high-density regime. The dependence of the relativistic electron Fermi energies by compression for selected nuclei are compared and contrasted to the nonrelativistic ones and to the ones obtained in the uniform approximation. The relativistic Feynman-Metropolis-Teller approach here presented overcomes some difficulties in the Salpeter approximation generally adopted for compressed matter in physics and astrophysics. The treatment is then extrapolated to compressed nuclear matter cores of stellar dimensions with A{approx_equal}(m{sub Planck}/m{sub n}){sup 3}{approx}10{sup 57} or M{sub core}{approx}M{sub {circle_dot}}. A new family of equilibrium configurations exists for selected values of the electron Fermi energy varying in the range 0

Optical Asymmetry and Nonlinear Light Scattering from Colloidal Gold Nanorods.

PubMed

Lien, Miao-Bin; Kim, Ji-Young; Han, Myung-Geun; Chang, You-Chia; Chang, Yu-Chung; Ferguson, Heather J; Zhu, Yimei; Herzing, Andrew A; Schotland, John C; Kotov, Nicholas A; Norris, Theodore B

2017-06-27

A systematic study is presented of the intensity-dependent nonlinear light scattering spectra of gold nanorods under resonant excitation of the longitudinal surface plasmon resonance (SPR). The spectra exhibit features due to coherent second and third harmonic generation as well as a broadband feature that has been previously attributed to multiphoton photoluminescence arising primarily from interband optical transitions in the gold. A detailed study of the spectral dependence of the scaling of the scattered light with excitation intensity shows unexpected scaling behavior of the coherent signals, which is quantitatively accounted for by optically induced damping of the SPR mode through a Fermi liquid model of the electronic scattering. The broadband feature is shown to arise not from luminescence, but from scattering of the second-order longitudinal SPR mode with the electron gas, where efficient excitation of the second order mode arises from an optical asymmetry of the nanorod. The electronic-temperature-dependent plasmon damping and the Fermi-Dirac distribution together determine the intensity dependence of the broadband emission, and the structure-dependent absorption spectrum determines the spectral shape through the fluctuation-dissipation theorem. Hence a complete self-consistent picture of both coherent and incoherent light scattering is obtained with a single set of physical parameters.

Structure of the charge density wave in cuprate superconductors: Lessons from NMR

NASA Astrophysics Data System (ADS)

Atkinson, W. A.; Ufkes, S.; Kampf, A. P.

2018-03-01

Using a mix of numerical and analytic methods, we show that recent NMR 17O measurements provide detailed information about the structure of the charge-density wave (CDW) phase in underdoped YBa2Cu3O6 +x . We perform Bogoliubov-de Gennes (BdG) calculations of both the local density of states and the orbitally resolved charge density, which are closely related to the magnetic and electric quadrupole contributions to the NMR spectrum, using a microscopic model that was shown previously to agree closely with x-ray experiments. The BdG results reproduce qualitative features of the experimental spectrum extremely well. These results are interpreted in terms of a generic "hot-spot" model that allows one to trace the origins of the NMR line shapes. We find that four quantities—the orbital character of the Fermi surface at the hot spots, the Fermi surface curvature at the hot spots, the CDW correlation length, and the magnitude of the subdominant CDW component—are key in determining the line shapes.

Resistivity in the Vicinity of a van Hove Singularity: Sr2RuO4 under Uniaxial Pressure

NASA Astrophysics Data System (ADS)

Barber, M. E.; Gibbs, A. S.; Maeno, Y.; Mackenzie, A. P.; Hicks, C. W.

2018-02-01

We report the results of a combined study of the normal-state resistivity and superconducting transition temperature Tc of the unconventional superconductor Sr2 RuO4 under uniaxial pressure. There is strong evidence that, as well as driving Tc through a maximum at ˜3.5 K , compressive strains ɛ of nearly 1% along the crystallographic [100] axis drive the γ Fermi surface sheet through a van Hove singularity, changing the temperature dependence of the resistivity from T2 above, and below the transition region to T1.5 within it. This occurs in extremely pure single-crystals in which the impurity contribution to the resistivity is <100 n Ω cm , so our study also highlights the potential of uniaxial pressure as a more general probe of this class of physics in clean systems.

Discovery of Weyl Fermion Semimetals and Topological Fermi Arc States

NASA Astrophysics Data System (ADS)

Hasan, M. Zahid; Xu, Su-Yang; Belopolski, Ilya; Huang, Shin-Ming

2017-03-01

Weyl semimetals are conductors whose low-energy bulk excitations are Weyl fermions, whereas their surfaces possess metallic Fermi arc surface states. These Fermi arc surface states are protected by a topological invariant associated with the bulk electronic wave functions of the material. Recently, it has been shown that the TaAs and NbAs classes of materials harbor such a state of topological matter. We review the basic phenomena and experimental history of the discovery of the first Weyl semimetals, starting with the observation of topological Fermi arcs and Weyl nodes in TaAs and NbAs by angle and spin-resolved surface and bulk sensitive photoemission spectroscopy and continuing through magnetotransport measurements reporting the Adler-Bell-Jackiw chiral anomaly. We hope that this article provides a useful introduction to the theory of Weyl semimetals, a summary of recent experimental discoveries, and a guideline to future directions.

Hydrogenated borophene as a stable two-dimensional Dirac material with an ultrahigh Fermi velocity.

PubMed

Xu, Li-Chun; Du, Aijun; Kou, Liangzhi

2016-10-05

The recent synthesis of monolayer borophene (triangular boron monolayer) on a substrate has opened the era of boron nanosheets (Science, 2015, 350, 1513), but the structural instability and a need to explore the novel physical properties are still open issues. Here we demonstrated that borophene can be stabilized by full surface hydrogenation (borophane), from first-principles calculations. Most interestingly, our calculations show that borophane has direction-dependent Dirac cones, which are mainly caused by the in-plane p x and p y orbitals of boron atoms. The Dirac fermions possess an ultrahigh Fermi velocity of up to 3.5 × 10 6 m s -1 under the HSE06 level, which is 4 times higher than that of graphene. The Young's moduli are calculated to be 190 and 120 GPa nm along two different directions, which are comparable to those of steel. The ultrahigh Fermi velocity and good mechanical features render borophane ideal for nanoelectronic applications.

Iron layer-dependent surface-enhanced raman scattering of hierarchical nanocap arrays

NASA Astrophysics Data System (ADS)

Chen, Lei; Sun, Huanhuan; Zhao, Yue; Gao, Renxian; Wang, Yaxin; Liu, Yang; Zhang, Yongjun; Hua, Zhong; Yang, Jinghai

2017-11-01

In this report, we fabricated the multi-layer Ag/Fe/Ag sandwich cap-shaped films on monolayer non-closed packed (ncp) polystyrene colloidal particle (PSCP) templates through a layer-by-layer (LBL) depositing method. This research focused on the surface-enhanced Raman scattering (SERS) effect of the thickness of the deposited Fe film which was controlled by the sputtering time. The SERS intensities were increased firstly, and then decreased as the thickness of Fe layer grows gradually, which is attributed to the charge transition from the Fermi level of the Ag NPs to Fe layer. The use of multi-layer Ag/Fe/Ag sandwich cap-shaped films enables us to evaluate the contribution of surface plasmon resonance and charge distribution between Ag and Fe to SERS enhancement. Our work introduced a novel system (Ag/Fe/Ag) for high performance SERS and extended the SERS application of Fe. Furthermore, we have designed the Ag/Fe/Ag SERS-active substrate as the immunoassay chip for quantitative determination of AFP-L3 which is the biomarker of hepatocellular carcinoma (HCC). The proposed research demonstrates that the SERS substrates with Ag/Fe/Ag sandwich cap-shaped arrays have a high sensitivity for bioassay.

The Effect of Blazar Spectral Breaks on the Blazar Contribution to the Extragalactic Gamma-Ray Background

NASA Technical Reports Server (NTRS)

Venters, Tonia M.; Pavlidou, Vasiliki

2011-01-01

The spectral shapes of the contributions of different classes of unresolved gamma-ray emitters can provide insight into their relative contributions to the extragalactic gamma-ray background (EGB) and the natures of their spectra at GeV energies, We calculate the spectral shapes of the contributions to the EGB arising from BL Lacertae type objects (BL Lacs) and flat-spectrum radio quasars (FSRQs) assuming blazar spectra can be described as broken power laws, We fit the resulting total blazar spectral shape to the Fermi Large Area Telescope measurements of the EGB, finding that the best-fit shape reproduces well the shape of the Fermi EGB for various break scenarios. We conclude that a scenario in which the contribution of blazars is dominant cannot be excluded on spectral grounds alone, even if spectral breaks are shown to be common among Fermi blazars. We also find that while the observation of a featureless (within uncertainties) power-law EGB spectrum by Fermi does not necessarily imply a single class of contributing unresolved sources with featureless individual spectra, such an observation and the collective spectra of the separate contributing populations determine the ratios of their contributions. As such, a comparison with studies including blazar gamma-ray luminosity functions could have profound implications for the blazar contribution to the EGB, blazar evolution, and blazar gamma-ray spectra and emission.

Anisotropies in the diffuse gamma-ray background from dark matter with Fermi LAT: A closer look

DOE PAGES

Cuoco, A.; Sellerholm, A.; Conrad, J.; ...

2011-06-21

We perform a detailed study of the sensitivity to the anisotropies related to dark matter (DM) annihilation in the isotropic gamma-ray background (IGRB) as measured by the Fermi Large Area Telescope ( Fermi LAT). For the first time, we take into account the effects of the Galactic foregrounds and use a realistic representation of the Fermi LAT. We implement an analysis pipeline which simulates Fermi LAT data sets starting from model maps of the Galactic foregrounds, the Fermi-resolved point sources, the extragalactic diffuse emission and the signal from DM annihilation. The effects of the detector are taken into account bymore » convolving the model maps with the Fermi LAT instrumental response. We then use the angular power spectrum to characterize the anisotropy properties of the simulated data and to study the sensitivity to DM. We consider DM anisotropies of extragalactic origin and of Galactic origin (which can be generated through annihilation in the Milky Way substructures) as opposed to a background of anisotropies generated by sources of astrophysical origin, blazars for example. We find that with statistics from 5 yr of observation, Fermi is sensitive to a DM contribution at the level of 1–10 per cent of the measured IGRB depending on the DM mass m χ and annihilation mode. In terms of the thermally averaged cross-section , this corresponds to ~10 –25 cm 3 s –1, i.e. slightly above the typical expectations for a thermal relic, for low values of the DM mass m χ≲ 100 GeV. As a result, the anisotropy method for DM searches has a sensitivity comparable to the usual methods based only on the energy spectrum and thus constitutes an independent and complementary piece of information in the DM puzzle.« less

Random walk numerical simulation for hopping transport at finite carrier concentrations: diffusion coefficient and transport energy concept.

PubMed

Gonzalez-Vazquez, J P; Anta, Juan A; Bisquert, Juan

2009-11-28

The random walk numerical simulation (RWNS) method is used to compute diffusion coefficients for hopping transport in a fully disordered medium at finite carrier concentrations. We use Miller-Abrahams jumping rates and an exponential distribution of energies to compute the hopping times in the random walk simulation. The computed diffusion coefficient shows an exponential dependence with respect to Fermi-level and Arrhenius behavior with respect to temperature. This result indicates that there is a well-defined transport level implicit to the system dynamics. To establish the origin of this transport level we construct histograms to monitor the energies of the most visited sites. In addition, we construct "corrected" histograms where backward moves are removed. Since these moves do not contribute to transport, these histograms provide a better estimation of the effective transport level energy. The analysis of this concept in connection with the Fermi-level dependence of the diffusion coefficient and the regime of interest for the functioning of dye-sensitised solar cells is thoroughly discussed.

Unconventional aspects of electronic transport in delafossite oxides

NASA Astrophysics Data System (ADS)

Daou, Ramzy; Frésard, Raymond; Eyert, Volker; Hébert, Sylvie; Maignan, Antoine

2017-12-01

The electronic transport properties of the delafossite oxides ? are usually understood in terms of two well-separated entities, namely the triangular ? and (? layers. Here, we review several cases among this extensive family of materials where the transport depends on the interlayer coupling and displays unconventional properties. We review the doped thermoelectrics based on ? and ?, which show a high-temperature recovery of Fermi-liquid transport exponents, as well as the highly anisotropic metals ?, ?, and ?, where the sheer simplicity of the Fermi surface leads to unconventional transport. We present some of the theoretical tools that have been used to investigate these transport properties and review what can and cannot be learned from the extensive set of electronic structure calculations that have been performed.

Controlled finite momentum pairing and spatially varying order parameter in proximitized HgTe quantum wells

NASA Astrophysics Data System (ADS)

Hart, Sean; Ren, Hechen; Kosowsky, Michael; Ben-Shach, Gilad; Leubner, Philipp; Bruene, Christoph; Buhmann, Hartmut; Molenkamp, Laurens; Halperin, Bertrand; Yacoby, Amir

Conventional s-wave superconductivity arises from singlet pairing of electrons with opposite Fermi momenta, forming Cooper pairs with zero net momentum. Recent studies have focused on coupling s-wave superconductors to systems with an unusual configuration of electronic spin and momentum at the Fermi surface, where the nature of the paired state can be modified and the system may even undergo a topological phase transition. Here we present measurements on Josephson junctions based on HgTe quantum wells coupled to aluminum or niobium superconductors, and subject to a magnetic field in the plane of the quantum well. We observe that the in-plane magnetic field modulates the Fraunhofer interference pattern, and that this modulation depends both on electron density and on the direction of the in-plane field with respect to the junction. However, the orientation of the junction with respect to the underlying crystal lattice does not impact the measurements. These findings suggest that spin-orbit coupling plays a role in the observed behavior, and that measurements of Josephson junctions in the presence of an in-plane field can elucidate the Fermi surface properties of the weak link material. NSF DMR-1206016; STC Center for Integrated Quantum Materials under NSF Grant No. DMR-1231319; NSF GRFP under Grant DGE1144152, Microsoft Corporation Project Q.

Fermi level dependence of hydrogen diffusivity in GaN

NASA Astrophysics Data System (ADS)

Polyakov, A. Y.; Smirnov, N. B.; Pearton, S. J.; Ren, F.; Theys, B.; Jomard, F.; Teukam, Z.; Dmitriev, V. A.; Nikolaev, A. E.; Usikov, A. S.; Nikitina, I. P.

2001-09-01

Hydrogen diffusion studies were performed in GaN samples with different Fermi level positions. It is shown that, at 350 °C, hydrogen diffusion is quite fast in heavily Mg doped p-type material with the Fermi level close to Ev+0.15 eV, considerably slower in high-resistivity p-GaN(Zn) with the Fermi level Ev+0.9 eV, while for conducting and semi-insulating n-GaN samples with the Fermi level in the upper half of the band gap no measurable hydrogen diffusion could be detected. For these latter samples it is shown that higher diffusion temperature of 500 °C and longer times (50 h) are necessary to incorporate hydrogen to appreciable depth. These findings are in line with previously published theoretical predictions of the dependence of hydrogen interstitials formation in GaN on the Fermi level position.

Observation of an electron band above the Fermi level in FeTe₀.₅₅Se₀.₄₅ from in-situ surface doping

DOE PAGES

Zhang, P.; Richard, P.; Xu, N.; ...

2014-10-27

We used in-situ potassium (K) evaporation to dope the surface of the iron-based superconductor FeTe₀.₅₅Se₀.₄₅. The systematic study of the bands near the Fermi level confirms that electrons are doped into the system, allowing us to tune the Fermi level of this material and to access otherwise unoccupied electronic states. In particular, we observe an electron band located above the Fermi level before doping that shares similarities with a small three-dimensional pocket observed in the cousin, heavily-electron-doped KFe₂₋ xSe₂ compound.

Electronic structure of dense Pb overlayers on Si(111) investigated using angle-resolved photoemission

NASA Astrophysics Data System (ADS)

Choi, W. H.; Koh, H.; Rotenberg, E.; Yeom, H. W.

2007-02-01

Dense Pb overlayers on Si(111) are important as the wetting layer for anomalous Pb island growth as well as for their own complex “devil’s-staircase” phases. The electronic structures of dense Pb overlayers on Si(111) were investigated in detail by angle-resolved photoemission. Among the series of ordered phases found recently above one monolayer, the low-coverage 7×3 and the high-coverage 14×3 phases are studied; they are well ordered and form reproducibly in large areas. The band dispersions and Fermi surfaces of the two-dimensional (2D) electronic states of these overlayers are mapped out. A number of metallic surface-state bands are identified for both phases with complex Fermi contours. The basic features of the observed Fermi contours can be explained by overlapping 2D free-electron-like Fermi circles. This analysis reveals that the 2D electrons near the Fermi level of the 7×3 and 14×3 phases are mainly governed by strong 1×1 and 3×3 potentials, respectively. The origins of the 2D electronic states and their apparent Fermi surface shapes are discussed based on recent structure models.

Electron and positron states in HgBa2CuO4

NASA Astrophysics Data System (ADS)

Barbiellini, B.; Jarlborg, T.

1994-08-01

Local-density-calculations of the electronic structure of HgBa2CuO4 have been performed with the self-consistent linear muffin-tin orbital method. The positron-density distribution and its sensitivity due to different potentials are calculated. The annihilation rates are computed in order to study the chemical bonding and to predict the Fermi-surface signal. Comparisons are made with previous calculations on other high-Tc copper oxides concerning the Fermi-surface properties and electron-positron overlap. We discuss the possibility of observing the Fermi surface associated with the Cu-O planes in positron-annihilation experiments.

Spectroscopic Visualization of Inversion and Time-Reversal Symmetry Breaking Weyl Semi-metals

NASA Astrophysics Data System (ADS)

Beidenkopf, Haim

A defining property of a topological material is the existence of surface bands that cannot be realized but as the termination of a topological bulk. In a Weyl semi-metal these surface states are in the form of Fermi-arcs. Their open-contour Fermi-surface curves between pairs of surface projections of bulk Weyl cones. Such Dirac-like bulk bands, as opposed to the gapped bulk of topological insulators, land a unique opportunity to examine the deep notion of bulk to surface correspondence. We study the intricate properties both of inversion symmetry broken and of time-reversal symmetry broken Weyl semimetals using scanning tunneling spectroscopy. We visualize the Fermi arc states on the surface of the non-centrosymmetric Weyl semi-metal TaAs. Using the distinct structure and spatial distribution of the wavefunctions associated with the different topological and trivial bands we detect the scattering processes that involve Fermi arcs. Each of these imaged scattering processes entails information on the unique nature of Fermi arcs and their correspondence to the topological bulk. We further visualize the magnetic response of the candidate magnetic Weyl semimetal GdPtBi in which the magnetic order parameter is coupled to the topological classification. European Research Council (ERC-StG no. 678702, TOPO-NW\\x9D), the Israel Science Foundation (ISF), and the United States-Israel Binational Science Foundation (BSF).

Detecting Fermi-level shifts by Auger electron spectroscopy in Si and GaAs

NASA Astrophysics Data System (ADS)

Debehets, J.; Homm, P.; Menghini, M.; Chambers, S. A.; Marchiori, C.; Heyns, M.; Locquet, J. P.; Seo, J. W.

2018-05-01

In this paper, changes in surface Fermi-level of Si and GaAs, caused by doping and cleaning, are investigated by Auger electron spectroscopy. Based on the Auger voltage contrast, we compared the Auger transition peak energy but with higher accuracy by using a more accurate analyzer and an improved peak position determination method. For silicon, a peak shift as large as 0.46 eV was detected when comparing a cleaned p-type and n-type wafer, which corresponds rather well with the theoretical difference in Fermi-levels. If no cleaning was applied, the peak position did not differ significantly for both wafer types, indicating Fermi-level pinning in the band gap. For GaAs, peak shifts were detected after cleaning with HF and (NH4)2S-solutions in an inert atmosphere (N2-gas). Although the (NH4)2S-cleaning in N2 is very efficient in removing the oxygen from the surface, the observed Ga- and As-peak shifts are smaller than those obtained after the HF-cleaning. It is shown that the magnitude of the shift is related to the surface composition. After Si-deposition on the (NH4)2S-cleaned surface, the Fermi-level shifts back to a similar position as observed for an as-received wafer, indicating that this combination is not successful in unpinning the Fermi-level of GaAs.

Thermal rectification in mass-graded next-nearest-neighbor Fermi-Pasta-Ulam lattices

NASA Astrophysics Data System (ADS)

Romero-Bastida, M.; Miranda-Peña, Jorge-Orlando; López, Juan M.

2017-03-01

We study the thermal rectification efficiency, i.e., quantification of asymmetric heat flow, of a one-dimensional mass-graded anharmonic oscillator Fermi-Pasta-Ulam lattice both with nearest-neighbor (NN) and next-nearest-neighbor (NNN) interactions. The system presents a maximum rectification efficiency for a very precise value of the parameter that controls the coupling strength of the NNN interactions, which also optimizes the rectification figure when its dependence on mass asymmetry and temperature differences is considered. The origin of the enhanced rectification is the asymmetric local heat flow response as the heat reservoirs are swapped when a finely tuned NNN contribution is taken into account. A simple theoretical analysis gives an estimate of the optimal NNN coupling in excellent agreement with our simulation results.

Temperature-driven Topological Phase Transition in MoTe2

NASA Astrophysics Data System (ADS)

Notis Berger, Ayelet; Andrade, Erick; Kerelsky, Alex; Cheong, Sang-Wook; Li, Jian; Bernevig, B. Andrei; Pasupathy, Abhay

The discovery of several candidates predicted to be weyl semimetals has made it possible to experimentally study weyl fermions and their exotic properties. One example is MoTe2, a transition metal dichalcogenide. At temperatures below 240 K it is predicted to be a type II Weyl semimetal with four Weyl points close to the fermi level. As with most weyl semimetals, the complicated band structure causes difficulty in distinguishing features related to bulk states and those related to topological fermi arc surface states characteristic of weyl semimetals. MoTe2 is unique because of its temperature-driven phase change. At high temperatures, MoTe2 is monoclinic, with trivial surface states. When cooled below 240K, it undergoes a first order phase transition to become an orthorhombic weyl semimetal with topologically protected fermi arc surface states. We present STM and STS measurements on MoTe2 crystals in both states. In the orthorhombic phase, we observe scattering that is consistent with the presence of the Fermi-arc surface states. Upon warming into the monoclinic phase, these features disappear in the observed interference patterns, providing direct evidence of the topological nature of the fermi arcs in the Weyl phase

Reconstruction de la surface de Fermi dans l'etat normal d'un supraconducteur a haute Tc: Une etude du transport electrique en champ magnetique intense

NASA Astrophysics Data System (ADS)

Le Boeuf, David

Des mesures de resistance longitudinale et de resistance de Hall en champ magnetique intense transverse (perpendiculaire aux plans CuO2) ont ete effectuees au sein de monocristaux de YBa2Cu3Oy (YBCO) demacles, ordonnes et de grande purete, afin d'etudier l'etat fondamental des supraconducteurs a haute Tc dans le regime sous-dope. Cette etude a ete realisee en fonction du dopage et de l'orientation du courant d'excitation J par rapport a l'axe orthorhombique b de la structure cristalline. Les mesures en champ magnetique intense revelent par suppression de la supraconductivite des oscillations magnetiques des resistances longitudinale et de Hall dans YBa2Cu 3O6.51 et YBa2Cu4O8. La conformite du comportement de ces oscillations quantiques au formalisme de Lifshitz-Kosevich, apporte la preuve de l'existence d'une surface de Fermi fermee a caractere quasi-2D, abritant des quasiparticules coherentes respectant la statistique de Fermi-Dirac, dans la phase pseudogap d'YBCO. La faible frequence des oscillations quantiques, combinee avec l'etude de la partie monotone de la resistance de Hall en fonction de la temperature indique que la surface de Fermi d'YBCO sous-dope comprend une petite poche de Fermi occupee par des porteurs de charge negative. Cette particularite de la surface de Fermi dans le regime sous-dope incompatible avec les calculs de structure de bande est en fort contraste avec la structure electronique presente dans le regime surdope. Cette observation implique ainsi l'existence d'un point critique quantique dans le diagramme de phase d'YBCO, au voisinage duquel la surface de Fermi doit subir une reconstruction induite par l'etablissement d'une brisure de la symetrie de translation du reseau cristallin sous-jacent. Enfin, l'etude en fonction du dopage de la resistance de Hall et de la resistance longitudinale en champ magnetique intense suggere qu'un ordre du type onde de densite (DW) est responsable de la reconstruction de la surface de Fermi. L'analogie de la phenomenologie entourant le comportement des resistances longitudinale et de Hall dans YBa2Cu3Oy, avec des systemes dans lesquels l'existence d'un ordre du type DW est etablie, notamment des cuprates a structure tetragonale a basse temperature ("Low Temperature Tetragonal", LTT), indique que l'ordre causant la reconstruction de la surface de Fermi est stabilise au voisinage du dopage p = 1/8, et est en competition directe avec la supraconductivite.

Hall number across a van Hove singularity

DOE PAGES

Maharaj, Akash V.; Esterlis, Ilya; Zhang, Yi; ...

2017-07-24

In this paper, in the context of the relaxation time approximation to Boltzmann transport theory, we examine the behavior of the Hall number n H of a metal in the neighborhood of a Lifshitz transition from a closed Fermi surface to open sheets. We find a universal nonanalytic dependence of n H on the electron density in the high-field limit, but a nonsingular dependence at low fields. Finally, the existence of an assumed nematic transition produces a doping dependent n H similar to that observed in recent experiments in the high-temperature superconductor YBa 2Cu 3O 7-x.

Observation of non-Fermi liquid behavior in hole-doped LiFe 1-x V xAs

DOE PAGES

Xing, L. Y.; Shi, X.; Richard, P.; ...

2016-09-28

Here we synthesized a series of V-doped LiFe 1₋xV xAs single crystals. The superconducting transition temperature T c of LiFeAs decreases rapidly at a rate of 7 K per 1% V. The Hall coefficient of LiFeAs switches from negative to positive with 4.2% V doping, showing that V doping introduces hole carriers. This observation is further confirmed by the evaluation of the Fermi surface volume measured by angle-resolved photoemission spectroscopy (ARPES), from which a 0.3 hole doping per V atom introduced is deduced. Interestingly, the introduction of holes does not follow a rigid band shift. We also show that themore » temperature evolution of the electrical resistivity as a function of doping is consistent with a crossover from a Fermi liquid to a non-Fermi liquid. Our ARPES data indicate that the non-Fermi liquid behavior is mostly enhanced when one of the hole d xz/dyz Fermi surfaces is well nested by the antiferromagnetic wave vector to the inner electron Fermi surface pocket with the d xy orbital character. In conclusion, the magnetic susceptibility of LiFe 1₋xV xAs suggests the presence of strong magnetic impurities following V doping, thus providing a natural explanation to the rapid suppression of superconductivity upon V doping.« less

Observation of non-Fermi liquid behavior in hole-doped LiFe 1-x V xAs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xing, L. Y.; Shi, X.; Richard, P.

Here we synthesized a series of V-doped LiFe 1₋xV xAs single crystals. The superconducting transition temperature T c of LiFeAs decreases rapidly at a rate of 7 K per 1% V. The Hall coefficient of LiFeAs switches from negative to positive with 4.2% V doping, showing that V doping introduces hole carriers. This observation is further confirmed by the evaluation of the Fermi surface volume measured by angle-resolved photoemission spectroscopy (ARPES), from which a 0.3 hole doping per V atom introduced is deduced. Interestingly, the introduction of holes does not follow a rigid band shift. We also show that themore » temperature evolution of the electrical resistivity as a function of doping is consistent with a crossover from a Fermi liquid to a non-Fermi liquid. Our ARPES data indicate that the non-Fermi liquid behavior is mostly enhanced when one of the hole d xz/dyz Fermi surfaces is well nested by the antiferromagnetic wave vector to the inner electron Fermi surface pocket with the d xy orbital character. In conclusion, the magnetic susceptibility of LiFe 1₋xV xAs suggests the presence of strong magnetic impurities following V doping, thus providing a natural explanation to the rapid suppression of superconductivity upon V doping.« less

Heavy surface state in a possible topological Kondo insulator: Magnetothermoelectric transport on the (011) plane of SmB 6

DOE PAGES

Luo, Yongkang; Chen, Hua; Dai, Jianhui; ...

2015-02-25

Motivated by the high sensitivity to Fermi surface topology and scattering mechanisms in magnetothermoelectric transport, we have measured the thermopower and Nernst effect on the (011) plane of the proposed topological Kondo insulator SmB 6. These experiments, together with electrical resistivity and Hall effect measurements, suggest that the (011) plane also harbors a metallic surface with an effective mass on the order of 10–10 2 m 0. The surface and bulk conductances are well distinguished in these measurements and are categorized into metallic and nondegenerate semiconducting regimes, respectively. As a result, electronic correlations play an important role in enhancing scatteringmore » and also contribute to the heavy surface state.« less

Density dependence of the saturated velocity in graphene

NASA Astrophysics Data System (ADS)

Ferry, D. K.

2016-11-01

The saturated velocity of a semiconductor is an important measure in bench-marking performance for either logic or microwave applications. Graphene has been of interest for such applications due to its apparently high value of the saturated velocity. Recent experiments have suggested that this value is very density dependent and can even exceed the band limiting Fermi velocity. Some of these measurements have also suggested that the scattering is dominated by the low energy surface polar mode of the SiO2 substrate. Here, we show that the saturated velocity of graphene on SiO2 is relatively independent of the density and that the scattering is dominated by the high energy surface polar mode of the substrate.

COSMIC-RAY POSITRONS FROM MILLISECOND PULSARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Venter, C.; Kopp, A.; Büsching, I.

2015-07-10

Observations by the Fermi Large Area Telescope of γ-ray millisecond pulsar (MSP) light curves imply copious pair production in their magnetospheres, and not exclusively in those of younger pulsars. Such pair cascades may be a primary source of Galactic electrons and positrons, contributing to the observed enhancement in positron flux above ∼10 GeV. Fermi has also uncovered many new MSPs, impacting Galactic stellar population models. We investigate the contribution of Galactic MSPs to the flux of terrestrial cosmic-ray electrons and positrons. Our population synthesis code predicts the source properties of present-day MSPs. We simulate their pair spectra invoking an offset-dipolemore » magnetic field. We also consider positrons and electrons that have been further accelerated to energies of several TeV by strong intrabinary shocks in black widow (BW) and redback (RB) systems. Since MSPs are not surrounded by pulsar wind nebulae or supernova shells, we assume that the pairs freely escape and undergo losses only in the intergalactic medium. We compute the transported pair spectra at Earth, following their diffusion and energy loss through the Galaxy. The predicted particle flux increases for non-zero offsets of the magnetic polar caps. Pair cascades from the magnetospheres of MSPs are only modest contributors around a few tens of GeV to the lepton fluxes measured by the Alpha Magnetic Spectrometer, PAMELA, and Fermi, after which this component cuts off. The contribution by BWs and RBs may, however, reach levels of a few tens of percent at tens of TeV, depending on model parameters.« less

Spin-fluctuation-induced non-Fermi-liquid behavior with suppressed superconductivity in LiFe 1-xCo xAs

DOE PAGES

Y. M. Dai; Miao, H.; Xing, L. Y.; ...

2015-09-15

A series of LiFe 1–xCo xAs compounds with different Co concentrations by transport, optical spectroscopy, angle-resolved photoemission spectroscopy, and nuclear magnetic resonance. We observe a Fermi-liquid to non-Fermi-liquid to Fermi-liquid (FL-NFL-FL) crossover alongside a monotonic suppression of the superconductivity with increasing Co content. In parallel to the FL-NFL-FL crossover, we find that both the low-energy spin fluctuations and Fermi surface nesting are enhanced and then diminished, strongly suggesting that the NFL behavior in LiFe 1–xCo xAs is induced by low-energy spin fluctuations that are very likely tuned by Fermi surface nesting. Our study reveals a unique phase diagram of LiFemore » 1–xCo xAs where the region of NFL is moved to the boundary of the superconducting phase, implying that they are probably governed by different mechanisms.« less

Origin of Pressure-induced Superconducting Phase in K xFe 2-ySe 2 studied by Synchrotron X-ray Diffraction and Spectroscopy

DOE PAGES

Yamamoto, Yoshiya; Yamaoka, Hitoshi; Tanaka, Masashi; ...

2016-08-08

Pressure dependence of the electronic and crystal structures of K xFe 2–ySe 2, which has pressure-induced two superconducting domes of SC I and SC II, was investigated by x-ray emission spectroscopy and diffraction. X-ray diffraction data show that compressibility along the c-axis changes around 12 GPa, where a new superconducting phase of SC II appears. This suggests a possible tetragonal to collapsed tetragonal phase transition. X-ray emission spectroscopy data also shows the change in the electronic structure around 12 GPa. These results can be explained by the scenario that the two SC domes under pressure originate from the change ofmore » Fermi surface topology. Lastly, our results here show the pronounced increase of the density of states near the Fermi surface under pressure with a structural phase transition, which can help address our fundamental understanding for the appearance of the SC II phase.« less

Origin of Pressure-induced Superconducting Phase in KxFe2-ySe2 studied by Synchrotron X-ray Diffraction and Spectroscopy

NASA Astrophysics Data System (ADS)

Yamamoto, Yoshiya; Yamaoka, Hitoshi; Tanaka, Masashi; Okazaki, Hiroyuki; Ozaki, Toshinori; Takano, Yoshihiko; Lin, Jung-Fu; Fujita, Hidenori; Kagayama, Tomoko; Shimizu, Katsuya; Hiraoka, Nozomu; Ishii, Hirofumi; Liao, Yen-Fa; Tsuei, Ku-Ding; Mizuki, Jun'Ichiro

2016-08-01

Pressure dependence of the electronic and crystal structures of KxFe2-ySe2, which has pressure-induced two superconducting domes of SC I and SC II, was investigated by x-ray emission spectroscopy and diffraction. X-ray diffraction data show that compressibility along the c-axis changes around 12 GPa, where a new superconducting phase of SC II appears. This suggests a possible tetragonal to collapsed tetragonal phase transition. X-ray emission spectroscopy data also shows the change in the electronic structure around 12 GPa. These results can be explained by the scenario that the two SC domes under pressure originate from the change of Fermi surface topology. Our results here show the pronounced increase of the density of states near the Fermi surface under pressure with a structural phase transition, which can help address our fundamental understanding for the appearance of the SC II phase.

Electronic origin of high-temperature superconductivity in single-layer FeSe superconductor.

PubMed

Liu, Defa; Zhang, Wenhao; Mou, Daixiang; He, Junfeng; Ou, Yun-Bo; Wang, Qing-Yan; Li, Zhi; Wang, Lili; Zhao, Lin; He, Shaolong; Peng, Yingying; Liu, Xu; Chen, Chaoyu; Yu, Li; Liu, Guodong; Dong, Xiaoli; Zhang, Jun; Chen, Chuangtian; Xu, Zuyan; Hu, Jiangping; Chen, Xi; Ma, Xucun; Xue, Qikun; Zhou, X J

2012-07-03

The recent discovery of high-temperature superconductivity in iron-based compounds has attracted much attention. How to further increase the superconducting transition temperature (T(c)) and how to understand the superconductivity mechanism are two prominent issues facing the current study of iron-based superconductors. The latest report of high-T(c) superconductivity in a single-layer FeSe is therefore both surprising and significant. Here we present investigations of the electronic structure and superconducting gap of the single-layer FeSe superconductor. Its Fermi surface is distinct from other iron-based superconductors, consisting only of electron-like pockets near the zone corner without indication of any Fermi surface around the zone centre. Nearly isotropic superconducting gap is observed in this strictly two-dimensional system. The temperature dependence of the superconducting gap gives a transition temperature T(c)~ 55 K. These results have established a clear case that such a simple electronic structure is compatible with high-T(c) superconductivity in iron-based superconductors.

Theory of Kerr and Faraday rotations and linear dichroism in Topological Weyl Semimetals.

PubMed

Kargarian, Mehdi; Randeria, Mohit; Trivedi, Nandini

2015-08-03

We consider the electromagnetic response of a topological Weyl semimetal (TWS) with a pair of Weyl nodes in the bulk and corresponding Fermi arcs in the surface Brillouin zone. We compute the frequency-dependent complex conductivities σαβ(ω) and also take into account the modification of Maxwell equations by the topological θ-term to obtain the Kerr and Faraday rotations in a variety of geometries. For TWS films thinner than the wavelength, the Kerr and Faraday rotations, determined by the separation between Weyl nodes, are significantly larger than in topological insulators. In thicker films, the Kerr and Faraday angles can be enhanced by choice of film thickness and substrate refractive index. We show that, for radiation incident on a surface with Fermi arcs, there is no Kerr or Faraday rotation but the electric field develops a longitudinal component inside the TWS, and there is linear dichroism signal. Our results have implications for probing the TWS phase in various experimental systems.

a Positron Study of the Electronic Structure of Yttrium Barium Copper Oxide.

NASA Astrophysics Data System (ADS)

Haghighi, Hossein

The work described in this thesis is concerned with a study of the electronic structure of the high T _{c} superconductor YBa _2Cu_3O _7 using the technique of two dimensional angular correlation of annihilation radiation (2D-ACAR). We have studied this compound with a view to clarifying whether YBa_2Cu_3O _7 possess a Fermi surface. The numerous different theories that have been proposed to explain the superconductivity phase of these types of materials can be classified into two main groups. The theories in the first group assume the existence of a conventional Fermi fluid and Fermi surface. The alternative more exotic models do not require a Fermi surface but are based on the Mott-Hubbard model of strongly correlated charge and spin excitations. Prior to this work all 2D-ACAR studies of YBa _2Cu_3O _7 involved twinned crystals and modest statistics and little of significance was learned other than that, consistent with that of predictions of theory, the positron was preferentially annihilating on the copper-oxygen chains. The studies of untwinned crystals of YBa_2Cu _3O_7, herein described are of much higher statistics and resulted in one of the clearest imaginable manifestations of a Fermi surface in the form of an extended discontinuity in the measured momentum spectrum. This discontinuity is even more apparent in the LCW-folded spectrum with a form and profile in substantial agreement with the theoretical predictions of a Gamma-X electron ridge Fermi surface section arising from states in the Cu-O chains.

de Haas–van Alphen study of role of 4 f electrons in antiferromagnetic CeZn 11 as compared to its nonmagnetic analog LaZn 11

DOE Office of Scientific and Technical Information (OSTI.GOV)

Blake, S. F.; Hodovanets, H.; McCollam, A.

Here we present a de Haas–van Alphen study of the Fermi surface of the low-temperature antiferromagnet CeZn 11 and its nonmagnetic analog LaZn 11, measured by torque magnetometry up to fields of 33T and at temperatures down to 320 mK . Both systems possess similar de Haas–van Alphen frequencies, with three clear sets of features—ranging from 50 T to 4 kT —corresponding to three bands of a complex Fermi surface, with an expected fourth band also seen weakly in CeZn 11 . The effective masses of the charge carriers are very light (<1 m e) in LaZn 11 but amore » factor of 2–4 larger in CeZn 11, indicative of stronger electronic correlations. We perform detailed density functional theory (DFT) calculations for CeZn 11 and find that only DFT+ U calculations with U = 1.5 eV , which localize the 4 f states, provide a good match to the measured de Haas–van Alphen frequencies, once the presence of magnetic breakdown orbits is also considered. Finally, our study suggests that the Fermi surface of CeZn 11 is very close to that of LaZn 11 being dominated by Zn 3d , as the Ce 4 f states are localized and have little influence on its electronic structure, however, they are responsible for its magnetic order and contribute to enhance electronic correlations.« less

de Haas–van Alphen study of role of 4 f electrons in antiferromagnetic CeZn 11 as compared to its nonmagnetic analog LaZn 11

DOE PAGES

Blake, S. F.; Hodovanets, H.; McCollam, A.; ...

2016-12-02

Here we present a de Haas–van Alphen study of the Fermi surface of the low-temperature antiferromagnet CeZn 11 and its nonmagnetic analog LaZn 11, measured by torque magnetometry up to fields of 33T and at temperatures down to 320 mK . Both systems possess similar de Haas–van Alphen frequencies, with three clear sets of features—ranging from 50 T to 4 kT —corresponding to three bands of a complex Fermi surface, with an expected fourth band also seen weakly in CeZn 11 . The effective masses of the charge carriers are very light (<1 m e) in LaZn 11 but amore » factor of 2–4 larger in CeZn 11, indicative of stronger electronic correlations. We perform detailed density functional theory (DFT) calculations for CeZn 11 and find that only DFT+ U calculations with U = 1.5 eV , which localize the 4 f states, provide a good match to the measured de Haas–van Alphen frequencies, once the presence of magnetic breakdown orbits is also considered. Finally, our study suggests that the Fermi surface of CeZn 11 is very close to that of LaZn 11 being dominated by Zn 3d , as the Ce 4 f states are localized and have little influence on its electronic structure, however, they are responsible for its magnetic order and contribute to enhance electronic correlations.« less

Experimental observation of optical Weyl points and Fermi arcs

NASA Astrophysics Data System (ADS)

Rechtsman, Mikael

We directly observe the presence type-II Weyl points for optical photons in a three-dimensional dielectric structure comprising arrays of evanescently-coupled, single-mode, helical waveguides. We also observe the corresponding Fermi arc surface states emerging from Weyl points (despite the use of the `Fermi arc' terminology, we are referring to bosons rather than fermions). The Weyl points are manifested by the presence of conical diffraction at the Weyl frequency in the photonic band structure, and the Fermi arc states are manifested by the emergence of surface states as we scan in frequency past the Weyl point. We map the Weyl points to Dirac points of the isofrequency surface, and the Fermi arcs to chiral edge states of an anomalous Floquet insulator. In collaboration with: Jiho Noh, Sheng Huang, Daniel Leykam*, Y. D. Chong, Kevin Chen, and Mikael C. Rechtsman M.C.R. acknowledges the National Science Foundation under Award Number ECCS-1509546, the Penn State MRSEC, Center for Nanoscale Science, under Award Number NSF DMR-1420620, and the Alfred P. Sloan Foundation under fellowship number FG-2016-6418.

Optical Asymmetry and Nonlinear Light Scattering from Colloidal Gold Nanorods

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lien, Miao-Bin; Kim, Ji-Young; Han, Myung-Geun

A systematic study is presented of the intensity-dependent nonlinear light scattering spectra of gold nanorods under resonant excitation of the longitudinal surface plasmon resonance (SPR). The spectra exhibit features due to coherent second and third harmonic generation as well as a broadband feature that has been previously attributed to multiphoton photoluminescence arising primarily from interband optical transitions in the gold. A detailed study of the spectral dependence of the scaling of the scattered light with excitation intensity shows unexpected scaling behavior of the coherent signals, which is quantitatively accounted for by optically induced damping of the SPR mode through amore » Fermi liquid model of the electronic scattering. The broadband feature is shown to arise not from luminescence, but from scattering of the secondorder longitudinal SPR mode with the electron gas, where efficient excitation of the 2nd order mode arises from an optical asymmetry of the nanorod. The electronic-temperature-dependent plasmon damping and the Fermi-Dirac distribution together determine the intensity dependence of the broadband emission, and the structure-dependent absorption spectrum determines the spectral shape through the fluctuation-dissipation theorem. Hence a complete self-consistent picture of both coherent and incoherent light scattering is obtained with a single set of physical parameters.« less

Optical Asymmetry and Nonlinear Light Scattering from Colloidal Gold Nanorods

DOE PAGES

Lien, Miao-Bin; Kim, Ji-Young; Han, Myung-Geun; ...

2017-05-16

A systematic study is presented of the intensity-dependent nonlinear light scattering spectra of gold nanorods under resonant excitation of the longitudinal surface plasmon resonance (SPR). The spectra exhibit features due to coherent second and third harmonic generation as well as a broadband feature that has been previously attributed to multiphoton photoluminescence arising primarily from interband optical transitions in the gold. A detailed study of the spectral dependence of the scaling of the scattered light with excitation intensity shows unexpected scaling behavior of the coherent signals, which is quantitatively accounted for by optically induced damping of the SPR mode through amore » Fermi liquid model of the electronic scattering. The broadband feature is shown to arise not from luminescence, but from scattering of the secondorder longitudinal SPR mode with the electron gas, where efficient excitation of the 2nd order mode arises from an optical asymmetry of the nanorod. The electronic-temperature-dependent plasmon damping and the Fermi-Dirac distribution together determine the intensity dependence of the broadband emission, and the structure-dependent absorption spectrum determines the spectral shape through the fluctuation-dissipation theorem. Hence a complete self-consistent picture of both coherent and incoherent light scattering is obtained with a single set of physical parameters.« less

Detecting Fermi-level shifts by Auger electron spectroscopy in Si and GaAs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Debehets, J.; Homm, P.; Menghini, M.

In this study, changes in surface Fermi-level of Si and GaAs, caused by doping and cleaning, are investigated by Auger electron spectroscopy. Based on the Auger voltage contrast, we compared the Auger transition peak energy but with higher accuracy by using a more accurate analyzer and an improved peak position determination method. For silicon, a peak shift as large as 0.46 eV was detected when comparing a cleaned p-type and n-type wafer, which corresponds rather well with the theoretical difference in Fermi-levels. If no cleaning was applied, the peak position did not differ significantly for both wafer types, indicating Fermi-levelmore » pinning in the band gap. For GaAs, peak shifts were detected after cleaning with HF and (NH 4) 2S-solutions in an inert atmosphere (N 2-gas). Although the (NH 4) 2S-cleaning in N 2 is very efficient in removing the oxygen from the surface, the observed Ga- and As-peak shifts are smaller than those obtained after the HF-cleaning. It is shown that the magnitude of the shift is related to the surface composition. After Si-deposition on the (NH 4) 2S-cleaned surface, the Fermi-level shifts back to a similar position as observed for an as-received wafer, indicating that this combination is not successful in unpinning the Fermi-level of GaAs.« less

Detecting Fermi-level shifts by Auger electron spectroscopy in Si and GaAs

DOE PAGES

Debehets, J.; Homm, P.; Menghini, M.; ...

2018-01-12

In this study, changes in surface Fermi-level of Si and GaAs, caused by doping and cleaning, are investigated by Auger electron spectroscopy. Based on the Auger voltage contrast, we compared the Auger transition peak energy but with higher accuracy by using a more accurate analyzer and an improved peak position determination method. For silicon, a peak shift as large as 0.46 eV was detected when comparing a cleaned p-type and n-type wafer, which corresponds rather well with the theoretical difference in Fermi-levels. If no cleaning was applied, the peak position did not differ significantly for both wafer types, indicating Fermi-levelmore » pinning in the band gap. For GaAs, peak shifts were detected after cleaning with HF and (NH 4) 2S-solutions in an inert atmosphere (N 2-gas). Although the (NH 4) 2S-cleaning in N 2 is very efficient in removing the oxygen from the surface, the observed Ga- and As-peak shifts are smaller than those obtained after the HF-cleaning. It is shown that the magnitude of the shift is related to the surface composition. After Si-deposition on the (NH 4) 2S-cleaned surface, the Fermi-level shifts back to a similar position as observed for an as-received wafer, indicating that this combination is not successful in unpinning the Fermi-level of GaAs.« less

Communication: The electronic entropy of charged defect formation and its impact on thermochemical redox cycles

NASA Astrophysics Data System (ADS)

Lany, Stephan

2018-02-01

The ideal material for solar thermochemical water splitting, which has yet to be discovered, must satisfy stringent conditions for the free energy of reduction, including, in particular, a sufficiently large positive contribution from the solid-state entropy. By inverting the commonly used relationship between defect formation energy and defect concentration, it is shown here that charged defect formation causes a large electronic entropy contribution manifesting itself as the temperature dependence of the Fermi level. This result is a general feature of charged defect formation and motivates new materials design principles for solar thermochemical hydrogen production.

Communication: The electronic entropy of charged defect formation and its impact on thermochemical redox cycles

DOE PAGES

Lany, Stephan

2018-02-21

The ideal material for solar thermochemical water splitting, which has yet to be discovered, must satisfy stringent conditions for the free energy of reduction, including, in particular, a sufficiently large positive contribution from the solid-state entropy. By inverting the commonly used relationship between defect formation energy and defect concentration, it is shown here that charged defect formation causes a large electronic entropy contribution manifesting itself as the temperature dependence of the Fermi level. This result is a general feature of charged defect formation and motivates new materials design principles for solar thermochemical hydrogen production.

Communication: The electronic entropy of charged defect formation and its impact on thermochemical redox cycles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lany, Stephan

The ideal material for solar thermochemical water splitting, which has yet to be discovered, must satisfy stringent conditions for the free energy of reduction, including, in particular, a sufficiently large positive contribution from the solid-state entropy. By inverting the commonly used relationship between defect formation energy and defect concentration, it is shown here that charged defect formation causes a large electronic entropy contribution manifesting itself as the temperature dependence of the Fermi level. This result is a general feature of charged defect formation and motivates new materials design principles for solar thermochemical hydrogen production.

Anomalous transport phenomena in Weyl metal beyond the Drude model for Landau's Fermi liquids.

PubMed

Kim, Ki-Seok; Kim, Heon-Jung; Sasaki, M; Wang, J-F; Li, L

2014-12-01

Landau's Fermi-liquid theory is the standard model for metals, characterized by the existence of electron quasiparticles near a Fermi surface as long as Landau's interaction parameters lie below critical values for instabilities. Recently this fundamental paradigm has been challenged by the physics of strong spin-orbit coupling, although the concept of electron quasiparticles remains valid near the Fermi surface, where Landau's Fermi-liquid theory fails to describe the electromagnetic properties of this novel metallic state, referred to as Weyl metal. A novel ingredient is that such a Fermi surface encloses a Weyl point with definite chirality, referred to as a chiral Fermi surface, which can arise from breaking of either time reversal or inversion symmetry in systems with strong spin-orbit coupling, responsible for both the Berry curvature and the chiral anomaly. As a result, electromagnetic properties of the Weyl metallic state are described not by conventional Maxwell equations but by axion electrodynamics, where Maxwell equations are modified with a topological-in-origin spatially modulated [Formula: see text] term. This novel metallic state was realized recently in Bi[Formula: see text]Sb x around [Formula: see text] under magnetic fields, where the Dirac spectrum appears around the critical point between the normal semiconducting ([Formula: see text]) and topological semiconducting phases ([Formula: see text]) and the time reversal symmetry breaking perturbation causes the Dirac point to split into a pair of Weyl points along the direction of the applied magnetic field for a very strong spin-orbit coupled system. In this review article, we discuss how the topological structure of both the Berry curvature and the chiral anomaly (axion electrodynamics) gives rise to anomalous transport phenomena in [Formula: see text]Sb x around [Formula: see text] under magnetic fields, thus modifying the Drude model of Landau's Fermi liquids.

Spectral probes of the holographic Fermi ground state: Dialing between the electron star and AdS Dirac hair

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cubrovic, Mihailo; Liu Yan; Schalm, Koenraad

2011-10-15

We argue that the electron star and the anti-de Sitter (AdS) Dirac hair solution are two limits of the free charged Fermi gas in AdS. Spectral functions of holographic duals to probe fermions in the background of electron stars have a free parameter that quantifies the number of constituent fermions that make up the charge and energy density characterizing the electron star solution. The strict electron star limit takes this number to be infinite. The Dirac hair solution is the limit where this number is unity. This is evident in the behavior of the distribution of holographically dual Fermi surfaces.more » As we decrease the number of constituents in a fixed electron star background the number of Fermi surfaces also decreases. An improved holographic Fermi ground state should be a configuration that shares the qualitative properties of both limits.« less

Inhomogeneous Weyl and Dirac Semimetals: Transport in Axial Magnetic Fields and Fermi Arc Surface States from Pseudo-Landau Levels

NASA Astrophysics Data System (ADS)

Grushin, Adolfo G.; Venderbos, Jörn W. F.; Vishwanath, Ashvin; Ilan, Roni

2016-10-01

Topological Dirac and Weyl semimetals have an energy spectrum that hosts Weyl nodes appearing in pairs of opposite chirality. Topological stability is ensured when the nodes are separated in momentum space and unique spectral and transport properties follow. In this work, we study the effect of a space-dependent Weyl node separation, which we interpret as an emergent background axial-vector potential, on the electromagnetic response and the energy spectrum of Weyl and Dirac semimetals. This situation can arise in the solid state either from inhomogeneous strain or nonuniform magnetization and can also be engineered in cold atomic systems. Using a semiclassical approach, we show that the resulting axial magnetic field B5 is observable through an enhancement of the conductivity as σ ˜B52 due to an underlying chiral pseudomagnetic effect. We then use two lattice models to analyze the effect of B5 on the spectral properties of topological semimetals. We describe the emergent pseudo-Landau-level structure for different spatial profiles of B5, revealing that (i) the celebrated surface states of Weyl semimetals, the Fermi arcs, can be reinterpreted as n =0 pseudo-Landau levels resulting from a B5 confined to the surface, (ii) as a consequence of position-momentum locking, a bulk B5 creates pseudo-Landau levels interpolating in real space between Fermi arcs at opposite surfaces, and (iii) there are equilibrium bound currents proportional to B5 that average to zero over the sample, which are the analogs of bound currents in magnetic materials. We conclude by discussing how our findings can be probed experimentally.

Evidence of a 2D Fermi surface due to surface states in a p-type metallic Bi2Te3

NASA Astrophysics Data System (ADS)

Shrestha, K.; Marinova, V.; Lorenz, B.; Chu, C. W.

2018-05-01

We present a systematic quantum oscillations study on a metallic, p-type Bi2Te3 topological single crystal in magnetic fields up to B  =  7 T. The maxima/minima positions of oscillations measured at different tilt angles align to one another when plotted as a function of the normal component of magnetic field, confirming the presence of the 2D Fermi surface. Additionally, the Berry phase, β  =  0.4  ±  0.05 obtained from the Landau level fan plot, is very close to the theoretical value of 0.5 for the Dirac particles, confirming the presence of topological surface states in the Bi2Te3 single crystal. Using the Lifshitz–Kosevich analyses, the Fermi energy is estimated to be meV, which is lower than that of other bismuth-based topological systems. The detection of surface states in the Bi2Te3 crystal can be explained by our previous hypothesis of the lower position of the Fermi surface that cuts the ‘M’-shaped valence band maxima. As a result, the bulk state frequency is shifted to higher magnetic fields, which allows measurement of the surface states signal at low magnetic fields.

Skyrme interaction to second order in nuclear matter

NASA Astrophysics Data System (ADS)

Kaiser, N.

2015-09-01

Based on the phenomenological Skyrme interaction various density-dependent nuclear matter quantities are calculated up to second order in many-body perturbation theory. The spin-orbit term as well as two tensor terms contribute at second order to the energy per particle. The simultaneous calculation of the isotropic Fermi-liquid parameters provides a rigorous check through the validity of the Landau relations. It is found that published results for these second order contributions are incorrect in most cases. In particular, interference terms between s-wave and p-wave components of the interaction can contribute only to (isospin or spin) asymmetry energies. Even with nine adjustable parameters, one does not obtain a good description of the empirical nuclear matter saturation curve in the low density region 0\\lt ρ \\lt 2{ρ }0. The reason for this feature is the too strong density-dependence {ρ }8/3 of several second-order contributions. The inclusion of the density-dependent term \\frac{1}{6}{t}3{ρ }1/6 is therefore indispensable for a realistic description of nuclear matter in the Skyrme framework.

Metal insulator transition in nickel substituted FeSi

NASA Astrophysics Data System (ADS)

Krishnan, M.; Mishra, Ashish; Singh, Durgesh; Venkatesh, R.; Gangrade, Mohan; Ganesan, V.

2018-04-01

Resistivity of Fe1-xNixSi has been reported. Metal Insulator transition (MIT) is observed in Nickel (Ni) substituted FeSi for x in the range from 2 to 4 percentage. Two Band Model has been employed in order to calculate activation energy and to predict how band structure renormalized with substitution of nickel in FeSi. At sufficient level of nickel concentration an impurity band forms around Fermi level and contributes to the conduction heavily at low temperatures. Concentration around x = 0.04, displays metallic property below ˜ 70 K and is quantitatively similar to systems like Fe1-xTxSi (T = Co, Mn). Metallic component thus derived from Ni substituted FeSi seems to have an unconventional temperature dependence that may be attributed to the onset of departures from Fermi liquid picture.

Homogeneous Atomic Fermi Gases

NASA Astrophysics Data System (ADS)

Mukherjee, Biswaroop; Yan, Zhenjie; Patel, Parth B.; Hadzibabic, Zoran; Yefsah, Tarik; Struck, Julian; Zwierlein, Martin W.

2017-03-01

We report on the creation of homogeneous Fermi gases of ultracold atoms in a uniform potential. In the momentum distribution of a spin-polarized gas, we observe the emergence of the Fermi surface and the saturated occupation of one particle per momentum state: the striking consequence of Pauli blocking in momentum space for a degenerate gas. Cooling a spin-balanced Fermi gas at unitarity, we create homogeneous superfluids and observe spatially uniform pair condensates. For thermodynamic measurements, we introduce a hybrid potential that is harmonic in one dimension and uniform in the other two. The spatially resolved compressibility reveals the superfluid transition in a spin-balanced Fermi gas, saturation in a fully polarized Fermi gas, and strong attraction in the polaronic regime of a partially polarized Fermi gas.

Electronic Structure and Thermoelectric Properties of Transition Metal Monosilicides

NASA Astrophysics Data System (ADS)

Pshenay-Severin, D. A.; Ivanov, Yu. V.; Burkov, A. T.; Novikov, S. V.; Zaitsev, V. K.; Reith, H.

2018-06-01

We present theoretical and experimental results on electronic structure and thermoelectric properties of cobalt monosilicide (CoSi) and of Co1- x M x Si diluted alloys (M = Fe and Ni) at temperatures from 2 K to 800 K. CoSi crystallizes into a non-centrosymmetric cubic B20 structure, which suggests the possibility of a topologically non-trivial electronic structure. We show that the electronic structure of CoSi exhibits linear band crossings in close vicinity to Fermi energy, confirming the possibility of non-trivial topology. The proximity of the linear-dispersion bands to Fermi energy implies their important contribution to the electronic transport. Calculation of thermopower of CoSi, using ab initio band structure and the constant relaxation time approximation, is carried out. It reveals that many body corrections to the electronic spectrum are important in order to obtain qualitative agreement of theoretical and experimental temperature dependences of thermopower. Phonon dispersion and lattice thermal conductivity are calculated. The phonons give a major contribution to the thermal conductivity of the compound below room temperature.

Quantum dimer model for the pseudogap metal

PubMed Central

Punk, Matthias; Allais, Andrea; Sachdev, Subir

2015-01-01

We propose a quantum dimer model for the metallic state of the hole-doped cuprates at low hole density, p. The Hilbert space is spanned by spinless, neutral, bosonic dimers and spin S=1/2, charge +e fermionic dimers. The model realizes a “fractionalized Fermi liquid” with no symmetry breaking and small hole pocket Fermi surfaces enclosing a total area determined by p. Exact diagonalization, on lattices of sizes up to 8×8, shows anisotropic quasiparticle residue around the pocket Fermi surfaces. We discuss the relationship to experiments. PMID:26195771

Interplanar coupling-dependent magnetoresistivity in high-purity layered metals

DOE PAGES

Kikugawa, N.; Goswami, P.; Kiswandhi, A.; ...

2016-03-29

The magnetic field-induced changes in the conductivity of metals are the subject of intense interest, both for revealing new phenomena and as a valuable tool for determining their Fermi surface. Here we report a hitherto unobserved magnetoresistive effect in ultra-clean layered metals, namely a negative longitudinal magnetoresistance that is capable of overcoming their very pronounced orbital one. This effect is correlated with the interlayer coupling disappearing for fields applied along the so-called Yamaji angles where the interlayer coupling vanishes. Therefore, it is intrinsically associated with the Fermi points in the field-induced quasi-one-dimensional electronic dispersion, implying that it results from themore » axial anomaly among these Fermi points. In its original formulation, the anomaly is predicted to violate separate number conservation laws for left- and right-handed chiral (for example, Weyl) fermions. Furthermore, its observation in PdCoO 2, PtCoO 2 and Sr 2RuO 4 suggests that the anomaly affects the transport of clean conductors, in particular near the quantum limit.« less

Effects of impurity adsorption on topological surface states of Bi2Te3

NASA Astrophysics Data System (ADS)

Shati, Khaqan; Arshad Farhan, M.; Selva Chandrasekaran, S.; Shim, Ji Hoon; Lee, Geunsik

2017-08-01

Electronic structures of Bi2Te3 with adsorption of Rb, In, Ga and Au atoms are studied by using the first-principle method, focusing on the effect of non-magnetic impurities on the topologically protected surface states. Upon monolayer formation, the bulk conduction band is moved down to the Fermi level with a significant Rashba splitting due to n-doping behavior with band modification details depending on the adatom chemistry. Our study shows the robustness of the intrinsic spin-momentum coupled surface band and emergence of a new similar one, which could provide helpful insight for developing novel spintronic devices.

Effect of Fermi surface nesting on resonant spin excitations in Ba{<_1-x}K{<_x}Fe{<_2}As{<_2}.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Castellan, J.-P.; Rosenkranz, S.; Goremychkin, E.A.

2011-01-01

We report inelastic neutron scattering measurements of the resonant spin excitations in Ba{sub 1-x}K{sub x}Fe{sub 2}As{sub 2} over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s{sub {+-}}-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight, caused by the weakening of electron-electron correlations.

Electronegativity-dependent tin etching from thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pachecka, M., E-mail: m.pachecka@utwente.nl; Sturm, J. M.; Kruijs, R. W. E. van de

2016-07-15

The influence of a thin film substrate material on the etching of a thin layer of deposited tin (Sn) by hydrogen radicals was studied. The amount of remaining Sn was quantified for materials that cover a range of electronegativities. We show that, for metals, etching depends on the relative electronegativity of the surface material and Sn. Tin is chemically etched from surfaces with an electronegativity smaller than Sn, while incomplete Sn etching is observed for materials with an electronegativity larger than Sn. Furthermore, the amount of remaining Sn increases as the electronegativity of the surface material increases. We speculate, that,more » due to Fermi level differences in the material’s electronic structure, the energy of the two conduction bands shift such that the availability of electrons for binding with hydrogen is significantly reduced.« less

Model construction and superconductivity analysis of organic conductors β-(BDA-TTP)2MF6 (M = P, As, Sb and Ta) based on first-principles band calculation

NASA Astrophysics Data System (ADS)

Aizawa, H.; Kuroki, K.; Yasuzuka, S.; Yamada, J.

2012-11-01

We perform a first-principles band calculation for a group of quasi-two-dimensional organic conductors β-(BDA-TTP)2MF6 (M = P, As, Sb and Ta). The ab-initio calculation shows that the density of states is correlated with the bandwidth of the singly occupied (highest) molecular orbital, while it is not necessarily correlated with the unit-cell volume. The direction of the major axis of the cross section of the Fermi surface lies in the Γ-B-direction, which differs from that obtained by the extended Hückel calculation. Then, we construct a tight-binding model which accurately reproduces the ab-initio band structure. The obtained transfer energies give a smaller dimerization than in the extended Hückel band. As to the difference in the anisotropy of the Fermi surface, the transfer energies along the inter-stacking direction are smaller than those obtained in the extended Hückel calculation. Assuming spin-fluctuation-mediated superconductivity, we apply random phase approximation to a two-band Hubbard model. This two-band Hubbard model is composed of the tight-binding model derived from the first-principles band structure and an on-site (intra-molecule) repulsive interaction taken as a variable parameter. The obtained superconducting gap changes sign four times along the Fermi surface like in a d-wave gap, and the nodal direction is different from that obtained in the extended Hückel model. Anion dependence of Tc is qualitatively consistent with the experimental observation.

Constraints on cosmological dark matter annihilation from the Fermi-LAT isotropic diffuse gamma-ray measurement

DOE PAGES

Abdo, A. A.; Ackermann, M.; Ajello, M.; ...

2010-04-01

The first published Fermi large area telescope (Fermi-LAT) measurement of the isotropic diffuse gamma-ray emission is in good agreement with a single power law, and is not showing any signature of a dominant contribution from dark matter sources in the energy range from 20 to 100 GeV. Here, we use the absolute size and spectral shape of this measured flux to derive cross section limits on three types of generic dark matter candidates: annihilating into quarks, charged leptons and monochromatic photons. Predicted gamma-ray fluxes from annihilating dark matter are strongly affected by the underlying distribution of dark matter, and bymore » using different available results of matter structure formation we assess these uncertainties. We also quantify how the dark matter constraints depend on the assumed conventional backgrounds and on the Universe's transparency to high-energy gamma-rays. In reasonable background and dark matter structure scenarios (but not in all scenarios we consider) it is possible to exclude models proposed to explain the excess of electrons and positrons measured by the Fermi-LAT and PAMELA experiments. Derived limits also start to probe cross sections expected from thermally produced relics (e.g. in minimal supersymmetry models) annihilating predominantly into quarks. Finally, for the monochromatic gamma-ray signature, the current measurement constrains only dark matter scenarios with very strong signals.« less

Symmetry lowering of pentacene molecular states interacting with a Cu surface

NASA Astrophysics Data System (ADS)

Baldacchini, Chiara; Mariani, Carlo; Betti, Maria Grazia; Vobornik, Ivana; Fujii, Jun; Annese, Emilia; Rossi, Giorgio; Ferretti, Andrea; Calzolari, Arrigo; di Felice, Rosa; Ruini, Alice; Molinari, Elisa

2007-12-01

Pentacene adsorbed on the Cu(119) vicinal surface forms long-range ordered chain structures. Photoemission spectroscopy measurements and ab initio density functional theory simulations provide consistent evidences that pentacene molecular orbitals mix with the copper bands, giving rise to interaction states localized at the interface. Angular-resolved and polarization dependent photoemission spectroscopy shows that most of the pentacene derived intensity is strongly dichroic. The symmetry of the molecular states of the free pentacene molecules is reduced upon adsorption on Cu(119), as a consequence of the molecule-metal interaction. Theoretical results show a redistribution of the charge density in π molecular states close to the Fermi level, consistent with the photoemission intensities (density of states) and polarization dependence (orbital symmetry).

Anisotropic strain induced directional metallicity in highly epitaxial LaBaCo 2O 5.5+δ thin films on (110) NdGaO 3

DOE PAGES

Ma, Chunrui; Han, Dong; Liu, Ming; ...

2016-11-21

Highly directional-dependent metal-insulator transition is observed in epitaxial double perovskite LaBaCo 2O 5.5+δ films. The film exhibit metallic along [100], but remain semiconducting along [010] under application of a magnetic field parallel to the surface of the film. The physical origin for the properties is identified as in-plane tensile strain arising from oxygen vacancies. First-principle calculations suggested the tensile strain drastically alters the band gap, and the vanishing gap opens up [100] conduction channels for Fermi-surface electrons. Lastly, our observation of strain-induced highly directional-dependent metal-insulator transition may open up new dimension for multifunctional devices.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Koshelev, A. E.

Multiple-band electronic structure and proximity to antiferromagnetic (AF) instability are the key properties of iron-based superconductors. In this paper, we explore the influence of scattering by the AF spin fluctuations on transport of multiple-band metals above the magnetic transition. A salient feature of scattering on the AF fluctuations is that it is strongly enhanced at the Fermi surface locations where the nesting is perfect (“hot spots” or “hot lines”). We review derivation of the collision integral for the Boltzmann equation due to AF-fluctuations scattering. In the paramagnetic state, the enhanced scattering rate near the hot lines leads to anomalous behaviormore » of electronic transport in magnetic field. We explore this behavior by analytically solving the Boltzmann transport equation with approximate transition rates. This approach accounts for return scattering events and is more accurate than the relaxation-time approximation. The magnetic-field dependences are characterized by two very different field scales: the lower scale is set by the hot-spot width and the higher scale is set by the total scattering amplitude. A conventional magnetotransport behavior is limited to magnetic fields below the lower scale. In the wide range in-between these two scales, the longitudinal conductivity has linear dependence on the magnetic field and the Hall conductivity has quadratic dependence. The linear dependence of the diagonal component reflects growth of the Fermi-surface area affected by the hot spots proportional to the magnetic field. Finally, we discuss applicability of this theoretical framework for describing of anomalous magnetotransport properties in different iron pnictides and chalcogenides in the paramagnetic state.« less

Fermi Surface of Sr_{2}RuO_{4}: Spin-Orbit and Anisotropic Coulomb Interaction Effects.

PubMed

Zhang, Guoren; Gorelov, Evgeny; Sarvestani, Esmaeel; Pavarini, Eva

2016-03-11

The topology of the Fermi surface of Sr_{2}RuO_{4} is well described by local-density approximation calculations with spin-orbit interaction, but the relative size of its different sheets is not. By accounting for many-body effects via dynamical mean-field theory, we show that the standard isotropic Coulomb interaction alone worsens or does not correct this discrepancy. In order to reproduce experiments, it is essential to account for the Coulomb anisotropy. The latter is small but has strong effects; it competes with the Coulomb-enhanced spin-orbit coupling and the isotropic Coulomb term in determining the Fermi surface shape. Its effects are likely sizable in other correlated multiorbital systems. In addition, we find that the low-energy self-energy matrix-responsible for the reshaping of the Fermi surface-sizably differs from the static Hartree-Fock limit. Finally, we find a strong spin-orbital entanglement; this supports the view that the conventional description of Cooper pairs via factorized spin and orbital part might not apply to Sr_{2}RuO_{4}.

Carrier-dependent magnetic anisotropy of cobalt doped titanium dioxide

PubMed Central

Shao, Bin; Feng, Min; Zuo, Xu

2014-01-01

Using first-principles calculations, we predict that the magnetic anisotropy energy of Co-doped TiO2 sensitively depends on carrier accumulation. This magnetoelectric phenomenon provides a potential route to a direct manipulation of the magnetization direction in diluted magnetic semiconductor by external electric-fields. We calculate the band structures and reveal the origin of the carrier-dependent magnetic anisotropy energy in k-space. It is shown that the carrier accumulation shifts the Fermi energy, and consequently, regulates the competing contributions to the magnetic anisotropy energy. The calculations provide an insight to understanding this magnetoelectric phenomenon, and a straightforward way to search prospective materials for electrically controllable spin direction of carriers. PMID:25510846

Quasiparticle interference of Fermi arc states in the type-II Weyl semimetal candidate WT e2

NASA Astrophysics Data System (ADS)

Yuan, Yuan; Yang, Xing; Peng, Lang; Wang, Zhi-Jun; Li, Jian; Yi, Chang-Jiang; Xian, Jing-Jing; Shi, You-Guo; Fu, Ying-Shuang

2018-04-01

Weyl semimetals possess linear dispersions through pairs of Weyl nodes in three-dimensional momentum spaces, whose hallmark arclike surface states are connected to Weyl nodes with different chirality. WT e2 was recently predicted to be a new type of Weyl semimetal. Here, we study the quasiparticle interference (QPI) of its Fermi arc surface states by combined spectroscopic-imaging scanning tunneling spectroscopy and density functional theory calculations. We observed the electron scattering on two types of WT e2 surfaces unambiguously. Its scattering signal can be ascribed mainly to trivial surface states. We also address the QPI feature of nontrivial surface states from theoretical calculations. The experimental QPI patterns show some features that are likely related to the nontrivial Fermi arc states, whose existence is, however, not conclusive. Our study provides an indispensable clue for studying the Weyl semimetal phase in WT e2 .

Nonmonotonic pressure evolution of the upper critical field in superconducting FeSe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kaluarachchi, Udhara S.; Taufour, Valentin; Böhmer, Anna E.

2016-02-01

The pressure dependence of the upper critical field, H c2,c, of single crystalline FeSe was studied using measurements of the interplane resistivity, ρ c, in magnetic fields parallel to tetragonal c axis. H c2,c(T) curves obtained under hydrostatic pressures up to 1.56 GPa, the range over which the superconducting transition temperature, T c, of FeSe exhibits a nonmonotonic dependence with local maximum at p 1 ≈ 0.8 GPa and local minimum at p 2 ≈ 1.2 GPa. The slope of the upper critical field at T c,(dH c2,c/dT)T c, also exhibits a nonmonotonic pressure dependence with distinct changes at pmore » 1 and p 2. For p < p 1 the slope can be described within a multiband orbital model. For both p 1 < p < p 2 and p > p 2 the slope is in good semiquantitative agreement with a single band, orbital Helfand-Werthamer theory with Fermi velocities determined from Shubnikov–de Haas measurements. Lastly, this finding indicates that Fermi surface changes are responsible for the local minimum of T c(p) at p 2 ≈ 1.2 GPa.« less

Enhanced electron emission from coated metal targets: Effect of surface thickness on performance

NASA Astrophysics Data System (ADS)

Madas, Saibabu; Mishra, S. K.; Upadhyay Kahaly, Mousumi

2018-03-01

In this work, we establish an analytical formalism to address the temperature dependent electron emission from a metallic target with thin coating, operating at a finite temperature. Taking into account three dimensional parabolic energy dispersion for the target (base) material and suitable thickness dependent energy dispersion for the coating layer, Fermi Dirac statistics of electron energy distribution and Fowler's mechanism of the electron emission, we discuss the dependence of the emission flux on the physical properties such as the Fermi level, work function, thickness of the coating material, and operating temperature. Our systematic estimation of how the thickness of coating affects the emission current demonstrates superior emission characteristics for thin coating layer at high temperature (above 1000 K), whereas in low temperature regime, a better response is expected from thicker coating layer. This underlying fundamental behavior appears to be essentially identical for all configurations when work function of the coating layer is lower than that of the bulk target work function. The analysis and predictions could be useful in designing new coated materials with suitable thickness for applications in the field of thin film devices and field emitters.

Electron acceleration in a secondary magnetic island formed during magnetic reconnection with a guide field

NASA Astrophysics Data System (ADS)

Wang, Huanyu; Lu, Quanming; Huang, Can; Wang, Shui

2017-05-01

Secondary magnetic islands may be generated in the vicinity of an X line during magnetic reconnection. In this paper, by performing two-dimensional (2-D) particle-in-cell simulations, we investigate the role of a secondary magnetic island in electron acceleration during magnetic reconnection with a guide field. The electron motions are found to be adiabatic, and we analyze the contributions of the parallel electric field and Fermi and betatron mechanisms to electron acceleration in the secondary island during the evolution of magnetic reconnection. When the secondary island is formed, electrons are accelerated by the parallel electric field due to the existence of the reconnection electric field in the electron current sheet. Electrons can be accelerated by both the parallel electric field and Fermi mechanism when the secondary island begins to merge with the primary magnetic island, which is formed simultaneously with the appearance of X lines. With the increase in the guide field, the contributions of the Fermi mechanism to electron acceleration become less and less important. When the guide field is sufficiently large, the contribution of the Fermi mechanism is almost negligible.

PREFACE: Anisotropic and multiband pairing: from borides to multicomponent superconductivity Anisotropic and multiband pairing: from borides to multicomponent superconductivity

NASA Astrophysics Data System (ADS)

Annett, James; Kusmartsev, Feodor; Bianconi, Antonio

2009-01-01

In 2001, the discovery of superconductivity in MgB2 rapidly led to the understanding that its complex multi-sheeted Fermi surface had two distinct values of the gap parameter Δ, each with its own characteristic temperature dependence. While the theory of multigap superconductivity had been developed long ago, this was the first well studied example where multigap behaviour was observed clearly, and indeed is essential to understand the full superconducting properties of the material. Following this discovery, evidence for multigap behaviour has appeared in a number of materials, including cuprates, ruthenates, and most recently the iron pnictides. As well as multigap pairing on different Fermi-surface sheets, strong gap anisotropy in k-space and strong modulations of the gap in real space (e.g. stripes and phase separation models) are also important in cuprates. The aim of this special section is to present a selection of high-quality papers from experts in these diverse systems, showing the links and common physical issues arising from the existence of multi-component Cooper pairing. The papers collected together for the special section provide a snapshot of the current state of the understanding of multi-component superconductivity in a wide range of materials. In a model motivated by MgB2, Tanaka and Eschrig describe Abrikosov vortex lattice in a two-gap superconductor, examining how the vortex structure is modified by three-dimensionality or quasi two-dimensionality of the Fermi surface. The multi-sheeted Fermi surfaces of the nickel borocarbides are probed using angle-resolved positron annihilation spectroscopy, described by Dugdale et al, leading to a full three-dimensional picture of the complex Fermi surface in this superconducting material. Possible evidence for multigap superconductivity in the iron pnictides, obtained using Andreev point contact spectroscopy, is described by Samuely et al. The iron pnictides are also the subject of the article by Caivano et al, in which it is proposed that the Feschbach resonance mechanism operating near to a quantum critical point may lead to stripe-like fluctuations in these materials. A number of papers describe multigap-related effects in high-Tc superconductors. In particular, Atkinson shows how the existence of CuO chain states at the Fermi surface leads to a set of resonances in the induced gap in the chain layer, which have a pronounced effect on the vortex core shape. Kristoffel et al discuss the existence of the two coherence lengths in two-gap superconductors, and describe how this leads to spatially periodic fluctuations, with possible application to high-temperature superconductivity. Kugel et al describe a scenario for phase separation due to long-range Coulomb forces leading to microstrain and nanoscale inhomogeneities in high-Tc cuprates. Kusmartsev and Saarela also argue that charge over-screening may lead to 'Coulomb bubbles' in high-Tc superconductors. Finally, Wysokiński et al describe multigap effects in strontium ruthenate, in particular the effects on the NMR relaxation rate spectra, which are obtained for NMR on different nuclear species.

Fermi-surface reconstruction and the origin of high-temperature superconductivity.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Norman, M. R.; Materials Science Division

2010-01-01

In crystalline lattices, the conduction electrons form waves, known as Bloch states, characterized by a momentum vector k. The defining characteristic of metals is the surface in momentum space that separates occupied from unoccupied states. This 'Fermi' surface may seem like an abstract concept, but it can be measured and its shape can have profound consequences for the thermal, electronic, and magnetic properties of a material. In the presence of an external magnetic field B, electrons in a metal spiral around the field direction, and within a semiclassical momentum-space picture, orbit around the Fermi surface. Physical properties, such as themore » magnetization, involve a sum over these orbits, with extremal orbits on the Fermi surface, i.e., orbits with minimal or maximal area, dominating the sum [Fig. 1(a)]. Upon quantization, the resulting electron energy spectrum consists of Landau levels separated by the cyclotron energy, which is proportional to the magnetic field. As the magnetic field causes subsequent Landau levels to cross through the Fermi energy, physical quantities, such as the magnetization or resistivity, oscillate in response. It turns out that the period of these oscillations, when plotted as a function of 1/B, is proportional to the area of the extremal orbit in a plane perpendicular to the applied field [Fig. 1(b)]. The power of the quantum oscillation technique is obvious: By changing the field direction, one can map out the Fermi surface, much like a blind man feeling an elephant. The nature and topology of the Fermi surface in high-T{sub c} cuprates has been debated for many years. Soon after the materials were discovered by Bednorz and Mueller, it was realized that superconductivity was obtained by doping carriers into a parent insulating state. This insulating state appears to be due to strong electronic correlations, and is known as a Mott insulator. In the case of cuprates, the electronic interactions force the electrons on the copper ion lattice into a d{sup 9} configuration, with one localized hole in the 3d shell per copper site. Given the localized nature of this state, it was questioned whether a momentum-space picture was an appropriate description of the physics of the cuprates. In fact, this question relates to a long-standing debate in the physics community: Since the parent state is also an antiferromagnet, one can, in principle, map the Mott insulator to a band insulator with magnetic order. In this 'Slater' picture, Mott physics is less relevant than the magnetism itself. It is therefore unclear which of the two, magnetism or Mott physics, is more fundamentally tied to superconductivity in the cuprates. After twenty years of effort, definitive quantum oscillations that could be used to map the Fermi surface were finally observed in a high-temperature cuprate superconductor in 2007. This and subsequent studies reveal a profound rearrangement of the Fermi surface in underdoped cuprates. The cause of the reconstruction, and its implication for the origin of high-temperature superconductivity, is a subject of active debate.« less

Electronic properties of core-shell nanowire resonant tunneling diodes

PubMed Central

2014-01-01

The electronic sub-band structure of InAs/InP/InAs/InP/InAs core-shell nanowire resonant tunneling diodes has been investigated in the effective mass approximation by varying the core radius and the thickness of the InP barriers and InAs shells. A top-hat, double-barrier potential profile and optimal energy configuration are obtained for core radii and surface shells >10 nm, InAs middle shells <10 nm, and 5 nm InP barriers. In this case, two sub-bands exist above the Fermi level in the InAs middle shell which belongs to the m = 0 and m = 1 ladder of states that have similar wave functions and energies. On the other hand, the lowest m = 0 sub-band in the core falls below the Fermi level but the m = 1 states do not contribute to the current transport since they reside energetically well above the Fermi level. We compare the case of GaAs/AlGaAs/GaAs/AlGaAs/GaAs which may conduct current with smaller applied voltages due to the larger effective mass of electrons in GaAs and discuss the need for doping. PMID:25288912

Electronic properties of core-shell nanowire resonant tunneling diodes.

PubMed

Zervos, Matthew

2014-01-01

The electronic sub-band structure of InAs/InP/InAs/InP/InAs core-shell nanowire resonant tunneling diodes has been investigated in the effective mass approximation by varying the core radius and the thickness of the InP barriers and InAs shells. A top-hat, double-barrier potential profile and optimal energy configuration are obtained for core radii and surface shells >10 nm, InAs middle shells <10 nm, and 5 nm InP barriers. In this case, two sub-bands exist above the Fermi level in the InAs middle shell which belongs to the m = 0 and m = 1 ladder of states that have similar wave functions and energies. On the other hand, the lowest m = 0 sub-band in the core falls below the Fermi level but the m = 1 states do not contribute to the current transport since they reside energetically well above the Fermi level. We compare the case of GaAs/AlGaAs/GaAs/AlGaAs/GaAs which may conduct current with smaller applied voltages due to the larger effective mass of electrons in GaAs and discuss the need for doping.

Unifying ultrafast demagnetization and intrinsic Gilbert damping in Co/Ni bilayers with electronic relaxation near the Fermi surface

NASA Astrophysics Data System (ADS)

Zhang, Wei; He, Wei; Zhang, Xiang-Qun; Cheng, Zhao-Hua; Teng, Jiao; Fähnle, Manfred

2017-12-01

The ability to controllably manipulate the laser-induced ultrafast magnetic dynamics is a prerequisite for future high-speed spintronic devices. The optimization of devices requires the controllability of the ultrafast demagnetization time τM and intrinsic Gilbert damping αintr. In previous attempts to establish a relationship between τM and αintr, the rare-earth doping of a permalloy film with two different demagnetization mechanisms was not a suitable candidate. Here, we choose Co/Ni bilayers to investigate the relations between τM and αintr by means of the time-resolved magneto-optical Kerr effect (TR-MOKE) via adjusting the thickness of the Ni layers, and obtain an approximately proportional relation between these two parameters. The remarkable agreement between the TR-MOKE experiment and the prediction of a breathing Fermi-surface model confirms that a large Elliott-Yafet spin-mixing parameter b2 is relevant to the strong spin-orbital coupling at the Co/Ni interface. More importantly, a proportional relation between τM and αintr in such metallic films or heterostructures with electronic relaxation near the Fermi surface suggests the local spin-flip scattering dominates the mechanism of ultrafast demagnetization, otherwise the spin-current mechanism dominates. It is an effective method to distinguish the dominant contributions to ultrafast magnetic quenching in metallic heterostructures by simultaneously investigating both the ultrafast demagnetization time and Gilbert damping. Our work can open an avenue to manipulate the magnitude and efficiency of terahertz emission in metallic heterostructures such as perpendicular magnetic anisotropic Ta/Pt/Co/Ni/Pt/Ta multilayers, and then it has an immediate implication for the design of high-frequency spintronic devices.

Gyrotropic Magnetic Effect and the Magnetic Moment on the Fermi Surface.

PubMed

Zhong, Shudan; Moore, Joel E; Souza, Ivo

2016-02-19

The current density j^{B} induced in a clean metal by a slowly-varying magnetic field B is formulated as the low-frequency limit of natural optical activity, or natural gyrotropy. Working with a multiband Pauli Hamiltonian, we obtain from the Kubo formula a simple expression for α_{ij}^{GME}=j_{i}^{B}/B_{j} in terms of the intrinsic magnetic moment (orbital plus spin) of the Bloch electrons on the Fermi surface. An alternate semiclassical derivation provides an intuitive picture of the effect, and takes into account the influence of scattering processes in dirty metals. This "gyrotropic magnetic effect" is fundamentally different from the chiral magnetic effect driven by the chiral anomaly and governed by the Berry curvature on the Fermi surface, and the two effects are compared for a minimal model of a Weyl semimetal. Like the Berry curvature, the intrinsic magnetic moment should be regarded as a basic ingredient in the Fermi-liquid description of transport in broken-symmetry metals.

Shubnikov-de Haas quantum oscillations reveal a reconstructed Fermi surface near optimal doping in a thin film of the cuprate superconductor Pr 1.86 Ce 0.14 CuO 4 ± δ

DOE PAGES

Breznay, Nicholas P.; Hayes, Ian M.; Ramshaw, B. J.; ...

2016-09-16

In this work, we study magnetotransport properties of the electron-doped superconductor Pr 2-xCe xCuO 4±δ with x = 0.14 in magnetic fields up to 92 T, and observe Shubnikov-de Haas magnetic quantum oscillations. The oscillations display a single frequency F = 255 ± 10 T, indicating a small Fermi pocket that is ~1 % of the two-dimensional Brillouin zone and consistent with a Fermi surface reconstructed from the large holelike cylinder predicted for these layered materials. Despite the low nominal doping, all electronic properties including the effective mass and Hall effect are consistent with overdoped compounds. In conclusion, our studymore » demonstrates that the exceptional chemical control afforded by high quality thin films will enable Fermi surface studies deep into the overdoped cuprate phase diagram.« less

Gyrotropic Magnetic Effect and the Magnetic Moment on the Fermi Surface

NASA Astrophysics Data System (ADS)

Zhong, Shudan; Moore, Joel E.; Souza, Ivo

2016-02-01

The current density jB induced in a clean metal by a slowly-varying magnetic field B is formulated as the low-frequency limit of natural optical activity, or natural gyrotropy. Working with a multiband Pauli Hamiltonian, we obtain from the Kubo formula a simple expression for αij GME=jiB/Bj in terms of the intrinsic magnetic moment (orbital plus spin) of the Bloch electrons on the Fermi surface. An alternate semiclassical derivation provides an intuitive picture of the effect, and takes into account the influence of scattering processes in dirty metals. This "gyrotropic magnetic effect" is fundamentally different from the chiral magnetic effect driven by the chiral anomaly and governed by the Berry curvature on the Fermi surface, and the two effects are compared for a minimal model of a Weyl semimetal. Like the Berry curvature, the intrinsic magnetic moment should be regarded as a basic ingredient in the Fermi-liquid description of transport in broken-symmetry metals.

Enhanced Electromagnetic and Chemical/Biological Sensing. Properties of Atomic Cluster-Derived Materials

DTIC Science & Technology

2003-02-24

electron injection at interfaces, analysis of the voltage dependence of the electrostatic potential across molecules, the nature of binding at the...nanoscale titania into a metallic surface), analysis of the so-called band lineup between the molecular levels and the Fermi levels of the metal...observe the CNT’s in the electron microscope with the possibility to manipulate them externally and to apply potentials to them. These new

Converting topological insulators into topological metals within the tetradymite family

NASA Astrophysics Data System (ADS)

Chen, K.-W.; Aryal, N.; Dai, J.; Graf, D.; Zhang, S.; Das, S.; Le Fèvre, P.; Bertran, F.; Yukawa, R.; Horiba, K.; Kumigashira, H.; Frantzeskakis, E.; Fortuna, F.; Balicas, L.; Santander-Syro, A. F.; Manousakis, E.; Baumbach, R. E.

2018-04-01

We report the electronic band structures and concomitant Fermi surfaces for a family of exfoliable tetradymite compounds with the formula T2C h2P n , obtained as a modification to the well-known topological insulator binaries Bi2(Se,Te ) 3 by replacing one chalcogen (C h ) with a pnictogen (P n ) and Bi with the tetravalent transition metals T = Ti, Zr, or Hf. This imbalances the electron count and results in layered metals characterized by relatively high carrier mobilities and bulk two-dimensional Fermi surfaces whose topography is well-described by first-principles calculations. Intriguingly, slab electronic structure calculations predict Dirac-like surface states. In contrast to Bi2Se3 , where the surface Dirac bands are at the Γ point, for (Zr,Hf ) 2Te2 (P,As) there are Dirac cones of strong topological character around both the Γ ¯ and M ¯ points, which are above and below the Fermi energy, respectively. For Ti2Te2P , the surface state is predicted to exist only around the M ¯ point. In agreement with these predictions, the surface states that are located below the Fermi energy are observed by angle-resolved photoemission spectroscopy measurements, revealing that they coexist with the bulk metallic state. Thus this family of materials provides a foundation upon which to develop novel phenomena that exploit both the bulk and surface states (e.g., topological superconductivity).

Metal-insulator quantum critical point beneath the high Tc superconducting dome.

PubMed

Sebastian, Suchitra E; Harrison, N; Altarawneh, M M; Mielke, C H; Liang, Ruixing; Bonn, D A; Hardy, W N; Lonzarich, G G

2010-04-06

An enduring question in correlated systems concerns whether superconductivity is favored at a quantum critical point (QCP) characterized by a divergent quasiparticle effective mass. Despite such a scenario being widely postulated in high T(c) cuprates and invoked to explain non-Fermi liquid transport signatures, experimental evidence is lacking for a critical divergence under the superconducting dome. We use ultrastrong magnetic fields to measure quantum oscillations in underdoped YBa(2)Cu(3)O(6+x), revealing a dramatic doping-dependent upturn in quasiparticle effective mass at a critical metal-insulator transition beneath the superconducting dome. Given the location of this QCP under a plateau in T(c) in addition to a postulated QCP at optimal doping, we discuss the intriguing possibility of two intersecting superconducting subdomes, each centered at a critical Fermi surface instability.

Electronic structures of of PuX (X=S, Se, Te)

NASA Astrophysics Data System (ADS)

Maehira, Takahiro; Sakai, Eijiro; Tatetsu, Yasutomi

2013-08-01

We have calculated the energy band structures and the Fermi surfaces of PuS, PuSe, and PuTe by using a self-consistent relativistic linear augmented-plane-wave method with the exchange and correlation potential in the local density approximation. In general, the energy bands near the Fermi level are mainly caused by the hybridization between the Pu 5 f and the monochalcogenide p electrons. The obtained main Fermi surfaces consisted of two hole sheets and one electron sheet, which were constructed from the band having both the Pu 5 f state and the monochalcogenide p state.

Multiple charge density wave states at the surface of TbT e 3

DOE PAGES

Fu, Ling; Kraft, Aaron M.; Sharma, Bishnu; ...

2016-11-01

We studied TbTe 3 using scanning tunneling microscopy (STM) in the temperature range of 298–355 K. Our measurements detect a unidirectional charge density wave (CDW) state in the surface Te layer with a wave vector consistent with that of the bulk q CDW = 0.30 ± 0.01c*. However, unlike previous STM measurements, and differing from measurements probing the bulk, we detect two perpendicular orientations for the unidirectional CDW with no directional preference for the in-plane crystal axes (a or c axis) and no noticeable difference in wave vector magnitude. In addition, we find regions in which the bidirectional CDW statesmore » coexist. We propose that observation of two unidirectional CDW states indicates a decoupling of the surface Te layer from the rare-earth block layer below, and that strain variations in the Te surface layer drive the local CDW direction to the specific unidirectional or, in rare occurrences, bidirectional CDW orders observed. This indicates that similar driving mechanisms for CDW formation in the bulk, where anisotropic lattice strain energy is important, are at play at the surface. Furthermore, the wave vectors for the bidirectional order we observe differ from those theoretically predicted for checkerboard order competing with stripe order in a Fermi-surface nesting scenario, suggesting that factors beyond Fermi-surface nesting drive CDW order in TbTe 3. As a result, our temperature-dependent measurements provide evidence for localized CDW formation above the bulk transition temperature T CDW.« less

Quantum oscillations in nodal line systems

NASA Astrophysics Data System (ADS)

Yang, Hui; Moessner, Roderich; Lim, Lih-King

2018-04-01

We study signatures of magnetic quantum oscillations in three-dimensional nodal line semimetals at zero temperature. The extended nature of the degenerate bands can result in a Fermi surface geometry with topological genus one, as well as a Fermi surface of electron and hole pockets encapsulating the nodal line. Moreover, the underlying two-band model to describe a nodal line is not unique, in that there are two classes of Hamiltonian with distinct band topology giving rise to the same Fermi-surface geometry. After identifying the extremal cyclotron orbits in various magnetic field directions, we study their concomitant Landau levels and resulting quantum oscillation signatures. By Landau-fan-diagram analyses, we extract the nontrivial π Berry phase signature for extremal orbits linking the nodal line.

Strong correlation effects in theoretical STM studies of magnetic adatoms

NASA Astrophysics Data System (ADS)

Dang, Hung T.; dos Santos Dias, Manuel; Liebsch, Ansgar; Lounis, Samir

2016-03-01

We present a theoretical study for the scanning tunneling microscopy (STM) spectra of surface-supported magnetic nanostructures, incorporating strong correlation effects. As concrete examples, we study Co and Mn adatoms on the Cu(111) surface, which are expected to represent the opposite limits of Kondo physics and local moment behavior, using a combination of density functional theory and both quantum Monte Carlo and exact diagonalization impurity solvers. We examine in detail the effects of temperature T , correlation strength U , and impurity d electron occupancy Nd on the local density of states. We also study the effective coherence energy scale, i.e., the Kondo temperature TK, which can be extracted from the STM spectra. Theoretical STM spectra are computed as a function of STM tip position relative to each adatom. Because of the multiorbital nature of the adatoms, the STM spectra are shown to consist of a complicated superposition of orbital contributions, with different orbital symmetries, self-energies, and Kondo temperatures. For a Mn adatom, which is close to half-filling, the STM spectra are featureless near the Fermi level. On the other hand, the quasiparticle peak for a Co adatom gives rise to strongly position-dependent Fano line shapes.

Adsorption effect on the formation of conductive path in defective TiO2: ab initio calculations

NASA Astrophysics Data System (ADS)

Li, Lei; Li, Wenshi; Qin, Han; Yang, Jianfeng; Mao, Ling-Feng

2017-10-01

Although the metal/TiO2/metal junctions providing resistive switching properties have attracted lots of attention in recent decades, revealing the atomic-nature of conductive path in TiO2 active layer remains a critical challenge. Here the effects of metal adsorption on defective TiO2(1 1 0) surface are theoretically investigated via ab initio calculations. The dependence of the conductive path on the adsorption of Ti/Zr/Cu/Pt/O atoms above a lattice Ti-ion in (1 1 0) plane and at 〈1 1 0〉 direction of the defective TiO2(0 0 1) surface are compared. It is found that Ti adsorptions in both sites give larger contributions to the presence of conductive path with more stability and larger transport coefficients at Fermi level, whereas the O adsorptions at both sites fail to produce conductive path. Moreover, the adsorptions of Zr/Cu/Pt atoms reduce the existence possibility of conductive path, especially absorbed above the lattice Ti-ion at 〈1 1 0〉 direction. Thus, it is helpful to clarify the interaction of the metal electrode and oxide layer in resistive random access memory.

Origin of in-plane anisotropic resistivity in the antiferromagnetic phase of Fe1 +xTe

NASA Astrophysics Data System (ADS)

Kaneshita, Eiji; Tohyama, Takami

2016-07-01

Motivated by a recent experimental report on in-plane anisotropic resistivity in the double-striped antiferromagnetic phase of FeTe, we theoretically calculate in-plane resistivity by applying a memory function approach to the ordered phase. We find that the resistivity is larger along an antiferromagnetically ordered direction than along a ferromagnetically ordered one, consistent with experimental observation. The anisotropic results are mainly contributed from Drude weight, whose behavior is attributed to Fermi surface topology of the ordered phase.

Heavy fermion behavior in the quasi-one-dimensional Kondo lattice CeCo 2Ga 8

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Le; Fu, Zhaoming; Sun, Jianping

Dimensionality plays an essential role in determining the anomalous non-Fermi liquid properties in heavy fermion systems. So far most heavy fermion compounds are quasi-two-dimensional or three-dimensional. Here we report the synthesis and systematic investigations of the single crystals of the quasi-one-dimensional Kondo lattice CeCo 2Ga 8. Resistivity measurements at ambient pressure reveal the onset of coherence at T * ≈ 20 K and non-Fermi liquid behavior with linear temperature dependence over a decade in temperature from 2 to 0.1 K. The specific heat increases logarithmically with lowering temperature between 10 and 2 K and reaches 800 mJ/mol K 2 atmore » 1 K, suggesting that CeCo 2Ga 8 is a heavy fermion compound in the close vicinity of a quantum critical point. Resistivity measurements under pressure further confirm the non-Fermi liquid behavior in a large temperature–pressure range. The magnetic susceptibility is found to follow the typical behavior for a one-dimensional spin chain from 300 K down to T *, and first-principles calculations predict flat Fermi surfaces for the itinerant f-electron bands. These suggest that CeCo 2Ga 8 is a rare example of the quasi-one-dimensional Kondo lattice, but its non-Fermi liquid behaviors resemble those of the quasi-two-dimensional YbRh 2Si 2 family. The study of the quasi-one-dimensional CeCo 2Ga 8 family may therefore help us to understand the role of dimensionality on heavy fermion physics and quantum criticality.« less

Heavy fermion behavior in the quasi-one-dimensional Kondo lattice CeCo2Ga8

NASA Astrophysics Data System (ADS)

Wang, Le; Fu, Zhaoming; Sun, Jianping; Liu, Min; Yi, Wei; Yi, Changjiang; Luo, Yongkang; Dai, Yaomin; Liu, Guangtong; Matsushita, Yoshitaka; Yamaura, Kazunari; Lu, Li; Cheng, Jin-Guang; Yang, Yi-feng; Shi, Youguo; Luo, Jianlin

2017-07-01

Dimensionality plays an essential role in determining the anomalous non-Fermi liquid properties in heavy fermion systems. So far most heavy fermion compounds are quasi-two-dimensional or three-dimensional. Here we report the synthesis and systematic investigations of the single crystals of the quasi-one-dimensional Kondo lattice CeCo2Ga8. Resistivity measurements at ambient pressure reveal the onset of coherence at T * ≈ 20 K and non-Fermi liquid behavior with linear temperature dependence over a decade in temperature from 2 to 0.1 K. The specific heat increases logarithmically with lowering temperature between 10 and 2 K and reaches 800 mJ/mol K2 at 1 K, suggesting that CeCo2Ga8 is a heavy fermion compound in the close vicinity of a quantum critical point. Resistivity measurements under pressure further confirm the non-Fermi liquid behavior in a large temperature-pressure range. The magnetic susceptibility is found to follow the typical behavior for a one-dimensional spin chain from 300 K down to T *, and first-principles calculations predict flat Fermi surfaces for the itinerant f-electron bands. These suggest that CeCo2Ga8 is a rare example of the quasi-one-dimensional Kondo lattice, but its non-Fermi liquid behaviors resemble those of the quasi-two-dimensional YbRh2Si2 family. The study of the quasi-one-dimensional CeCo2Ga8 family may therefore help us to understand the role of dimensionality on heavy fermion physics and quantum criticality.

Heavy fermion behavior in the quasi-one-dimensional Kondo lattice CeCo 2Ga 8

DOE PAGES

Wang, Le; Fu, Zhaoming; Sun, Jianping; ...

2017-07-04

Dimensionality plays an essential role in determining the anomalous non-Fermi liquid properties in heavy fermion systems. So far most heavy fermion compounds are quasi-two-dimensional or three-dimensional. Here we report the synthesis and systematic investigations of the single crystals of the quasi-one-dimensional Kondo lattice CeCo 2Ga 8. Resistivity measurements at ambient pressure reveal the onset of coherence at T * ≈ 20 K and non-Fermi liquid behavior with linear temperature dependence over a decade in temperature from 2 to 0.1 K. The specific heat increases logarithmically with lowering temperature between 10 and 2 K and reaches 800 mJ/mol K 2 atmore » 1 K, suggesting that CeCo 2Ga 8 is a heavy fermion compound in the close vicinity of a quantum critical point. Resistivity measurements under pressure further confirm the non-Fermi liquid behavior in a large temperature–pressure range. The magnetic susceptibility is found to follow the typical behavior for a one-dimensional spin chain from 300 K down to T *, and first-principles calculations predict flat Fermi surfaces for the itinerant f-electron bands. These suggest that CeCo 2Ga 8 is a rare example of the quasi-one-dimensional Kondo lattice, but its non-Fermi liquid behaviors resemble those of the quasi-two-dimensional YbRh 2Si 2 family. The study of the quasi-one-dimensional CeCo 2Ga 8 family may therefore help us to understand the role of dimensionality on heavy fermion physics and quantum criticality.« less

Electronic properties of GeTe and Ag- or Sb-substituted GeTe studied by low-temperature Te 125 NMR

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cui, J.; Levin, E. M.; Lee, Y.

We have carried out 125Te nuclear magnetic resonance (NMR) in a wide temperature range of 1.5–300 K to investigate the electronic properties of Ge 50 Te 50, Ag 2 Ge 48Te 50 , and Sb 2 Ge 48 Te 50 from a microscopic point of view. From the temperature dependence of the NMR shift (K) and nuclear spin lattice relaxation rate (1/T 1), we found that two bands contribute to the physical properties of the materials. One band overlaps the Fermi level providing the metallic state where no strong electron correlations are revealed by Korringa analysis. The other band ismore » separated from the Fermi level by an energy gap of E g/k B ~67 K, which gives rise to semiconductorlike properties. First-principles calculation reveals that the metallic band originates from the Ge vacancy while the semiconductorlike band is related to the fine structure of the density of states near the Fermi level. We find low-temperature Te125 NMR data for the materials studied here clearly show that Ag substitution increases hole concentration while Sb substitution decreases it.« less

Electronic properties of GeTe and Ag- or Sb-substituted GeTe studied by low-temperature Te 125 NMR

DOE PAGES

Cui, J.; Levin, E. M.; Lee, Y.; ...

2016-08-18

We have carried out 125Te nuclear magnetic resonance (NMR) in a wide temperature range of 1.5–300 K to investigate the electronic properties of Ge 50 Te 50, Ag 2 Ge 48Te 50 , and Sb 2 Ge 48 Te 50 from a microscopic point of view. From the temperature dependence of the NMR shift (K) and nuclear spin lattice relaxation rate (1/T 1), we found that two bands contribute to the physical properties of the materials. One band overlaps the Fermi level providing the metallic state where no strong electron correlations are revealed by Korringa analysis. The other band ismore » separated from the Fermi level by an energy gap of E g/k B ~67 K, which gives rise to semiconductorlike properties. First-principles calculation reveals that the metallic band originates from the Ge vacancy while the semiconductorlike band is related to the fine structure of the density of states near the Fermi level. We find low-temperature Te125 NMR data for the materials studied here clearly show that Ag substitution increases hole concentration while Sb substitution decreases it.« less

The Luminosity Function of Fermi-detected Flat-Spectrum Radio Quasars

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ajello, M.; Shaw, M.S.; Romani, R.W.

2012-04-16

Fermi has provided the largest sample of {gamma}-ray selected blazars to date. In this work we use a complete sample of FSRQs detected during the first year of operation to determine the luminosity function (LF) and its evolution with cosmic time. The number density of FSRQs grows dramatically up to redshift {approx}0.5-2.0 and declines thereafter. The redshift of the peak in the density is luminosity dependent, with more luminous sources peaking at earlier times; thus the LF of {gamma}-ray FSRQs follows a luminosity-dependent density evolution similarly to that of radio-quiet AGN. Also using data from the Swift Burst Alert Telescopemore » we derive the average spectral energy distribution of FSRQs in the 10 keV-100GeV band and show that there is no correlation of the peak {gamma}-ray luminosity with {gamma}-ray peak frequency. The coupling of the SED and LF allows us to predict that the contribution of FSRQs to the Fermi isotropic {gamma}-ray background is 9.3{sub -1.0}{sup +1.6}% ({+-}3% systematic uncertainty) in the 0.1-100GeV band. Finally we determine the LF of unbeamed FSRQs, finding that FSRQs have an average Lorentz factor of {gamma} = 11.7{sub -2.2}{sup +3.3}, that most are seen within 5{sup o} of the jet axis, and that they represent only {approx}0.1% of the parent population.« less

Temperature Dependence of Quasiparticle Spectral Weight and Coherence in High Tc Superconductors

NASA Astrophysics Data System (ADS)

He, Yang; Zhang, Jessie; Hoffman, Jennifer; Hoffman Lab Team

2014-03-01

Superconductivity arises from the Cooper pairing of quasiparticles on the Fermi surface. Understanding the formation of Cooper pairs is an essential step towards unveiling the mechanism of high Tc superconductivity. We compare scanning tunneling microscope investigations of the temperature dependence of quasiparticle spectral weight and quasiparticle interference in several families of high Tc materials. We calculate the coherent spectral weight related to superconductivity, despite the coexistence of competing orders. The relation between pairing temperature and coherent spectral weight is discussed. We acknowledge support by the New York Community Trust-George Merck Fund.

Critical temperature of metallic hydrogen sulfide at 225-GPa pressure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kudryashov, N. A.; Kutukov, A. A.; Mazur, E. A., E-mail: EAMazur@mephi.ru

2017-01-15

The Eliashberg theory generalized for electron—phonon systems with a nonconstant density of electron states and with allowance made for the frequency behavior of the electron mass and chemical potential renormalizations is used to study T{sub c} in the SH{sub 3} phase of hydrogen sulfide under pressure. The phonon contribution to the anomalous electron Green’s function is considered. The pairing within the total width of the electron band and not only in a narrow layer near the Fermi surface is taken into account. The frequency and temperature dependences of the complex mass renormalization ReZ(ω), the density of states N(ε) renormalized bymore » the electron—phonon interactions, and the electron—phonon spectral function obtained computationally are used to calculate the anomalous electron Green’s function. A generalized Eliashberg equation with a variable density of electron states has been solved. The frequency dependence of the real and imaginary parts of the order parameter in the SH{sub 3} phase has been obtained. The value of T{sub c} ≈ 177 K in the SH{sub 3} phase of hydrogen sulfide at pressure P = 225 GPa has been determined by solving the system of Eliashberg equations.« less

Study of sulfur bonding on gallium arsenide (100) surfaces using supercritical fluid extraction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cabauy, P.; Darici, Y.; Furton, K.G.

1995-12-01

In the last decades Gallium Arsenide (GaAs) has been considered the semiconductor that will replace silicon because of its direct band gap and high electron mobility. Problems with GaAs Fermi level pinning has halted its widespread use in the electronics industry. The formation of oxides on GaAs results in a high density of surface states that effectively pin the surface Fermi level at the midgap. Studies on sulfur passivation have eliminated oxidation and virtually unpinned the Fermi level on the GaAs surface. This has given rise to interest in sulfur-GaAs bonds. In this presentation, we will discuss the types ofmore » sulfur bonds extracted from a sulfur passivated GaAs (100) using Supercritical Fluid (CO2) Extraction (SFE). SFE can be a valuable tool in the study of chemical speciations on semiconductor surfaces. The variables evaluated to effectively study the sulfur species from the GaAs surface include passivation techniques, supercritical fluid temperatures, densities, and extraction times.« less

Topological semimetals with Riemann surface states

NASA Astrophysics Data System (ADS)

Fang, Chen; Lu, Ling; Liu, Junwei; Fu, Liang

Topological semimetals have robust bulk band crossings between the conduction and the valence bands. Among them, Weyl semimetals are so far the only class having topologically protected signatures on the surface known as the ``Fermi arcs''. Here we theoretically find new classes of topological semimetals protected by nonsymmorphic glide reflection symmetries. On a symmetric surface, there are multiple Fermi arcs protected by nontrivial Z2 spectral flows between two high-symmetry lines (or two segments of one line) in the surface Brillouin zone. We observe that so far topological semimetals with protected Fermi arcs have surface dispersions that can be mapped to noncompact Riemann surfaces representing simple holomorphic functions. We propose perovskite superlattice [(SrIrO3)2m, (CaIrO3)2n] as a nonsymmorphic Dirac semimetal. C.F. and L.F. were supported by the S3TEC Solid State Solar Thermal Energy Conversion Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award No. DE-SC0001299/DE.

Anisotropic breakdown of Fermi liquid quasiparticle excitations in overdoped La₂-xSrxCuO₄.

PubMed

Chang, J; Månsson, M; Pailhès, S; Claesson, T; Lipscombe, O J; Hayden, S M; Patthey, L; Tjernberg, O; Mesot, J

2013-01-01

High-temperature superconductivity emerges from an un-conventional metallic state. This has stimulated strong efforts to understand exactly how Fermi liquids breakdown and evolve into an un-conventional metal. A fundamental question is how Fermi liquid quasiparticle excitations break down in momentum space. Here we show, using angle-resolved photoemission spectroscopy, that the Fermi liquid quasiparticle excitations of the overdoped superconducting cuprate La1.77Sr0.23CuO4 is highly anisotropic in momentum space. The quasiparticle scattering and residue behave differently along the Fermi surface and hence the Kadowaki-Wood's relation is not obeyed. This kind of Fermi liquid breakdown may apply to a wide range of strongly correlated metal systems where spin fluctuations are present.

Controlled calculation of the thermal conductivity for a spinon Fermi surface coupled to a U(1) gauge field

DOE Office of Scientific and Technical Information (OSTI.GOV)

Freire, Hermann, E-mail: hfreire@mit.edu

2014-10-15

Motivated by recent transport measurements on the candidate spin-liquid phase of the organic triangular lattice insulator EtMe{sub 3}Sb[Pd(dmit){sub 2}]{sub 2}, we perform a controlled calculation of the thermal conductivity at intermediate temperatures in a spin liquid system where a spinon Fermi surface is coupled to a U(1) gauge field. The present computation builds upon the double expansion approach developed by Mross et al. (2010) for small ϵ=z{sub b}−2 (where z{sub b} is the dynamical critical exponent of the gauge field) and large number of fermionic species N. Using the so-called memory matrix formalism that most crucially does not assume the existencemore » of well-defined quasiparticles at low energies in the system, we calculate the temperature dependence of the thermal conductivity κ of this model due to non-critical Umklapp scattering of the spinons for a finite N and small ϵ. Then we discuss the physical implications of such theoretical result in connection with the experimental data available in the literature.« less

The overdoped region of the high Tc superconducting Bi2212 revisited

NASA Astrophysics Data System (ADS)

Zaki, N.; Yang, H.-B.; Rameau, J. D.; Johnson, P. D.; Claus, H.; Hinks, D. G.

High-resolution angle-resolved photoemission (ARPES) is used to probe the temperature dependence of the gaps observed in the antinodal region of the Fermi surface (FS) in overdoped Bi2212. In particular we study samples with doping levels greater than 0.19, the latter having previously been determined to be the doping level associated with a Fermi surface reconstruction. Careful simulation of the measured ARPES spectra indicates that any gap observed in this region of the FS at these doping levels is a reflection of the range of superconducting gaps associated with inhomogeneities observed in STM studies of the same systems. With this observation we are able to reexamine the phase diagram associated with the Bi2212 system and discuss the origin of the pseudogap associated with the underdoped region. This work is supported in part by the Center for Emergent Superconductivity (CES), an EFRC funded by the U.S. DOE. The work is also supported in part by the U.S. DOE under Contract No. DE-AC02- 98CH10886 at BNL and Contract No. DE-AC02-06CH11357 at ANL.

Analysis of Deep and Shallow Traps in Semi-Insulating CdZnTe

DOE PAGES

Kim, Kihyun; Yoon, Yongsu; James, Ralph B.

2018-03-13

Trap levels which are deep or shallow play an important role in the electrical and the optical properties of a semiconductor; thus, a trap level analysis is very important in most semiconductor devices. Deep-level defects in CdZnTe are essential in Fermi level pinning at the middle of the bandgap and are responsible for incomplete charge collection and polarization effects. However, a deep level analysis in semi-insulating CdZnTe (CZT) is very difficult. Theoretical capacitance calculation for a metal/insulator/CZT (MIS) device with deep-level defects exhibits inflection points when the donor/acceptor level crosses the Fermi level in the surface-charge layer (SCL). Three CZTmore » samples with different resistivities, 2 × 10 4 (n-type), 2 × 10 6 (p-type), and 2 × 10 10 (p-type) Ω·cm, were used in fabricating the MIS devices. These devices showed several peaks in their capacitance measurements due to upward/downward band bending that depend on the surface potential. In conclusion, theoretical and experimental capacitance measurements were in agreement, except in the fully compensated case.« less

Classification of reflection-symmetry-protected topological semimetals and nodal superconductors

NASA Astrophysics Data System (ADS)

Chiu, Ching-Kai; Schnyder, Andreas P.

2014-11-01

While the topological classification of insulators, semimetals, and superconductors in terms of nonspatial symmetries is well understood, less is known about topological states protected by crystalline symmetries, such as mirror reflections and rotations. In this work, we systematically classify topological semimetals and nodal superconductors that are protected, not only by nonspatial (i.e., global) symmetries, but also by a crystal reflection symmetry. We find that the classification crucially depends on (i) the codimension of the Fermi surface (nodal line or point) of the semimetal (superconductor), (ii) whether the mirror symmetry commutes or anticommutes with the nonspatial symmetries, and (iii) how the Fermi surfaces (nodal lines or points) transform under the mirror reflection and nonspatial symmetries. The classification is derived by examining all possible symmetry-allowed mass terms that can be added to the Bloch or Bogoliubov-de Gennes Hamiltonian in a given symmetry class and by explicitly deriving topological invariants. We discuss several examples of reflection-symmetry-protected topological semimetals and nodal superconductors, including topological crystalline semimetals with mirror Z2 numbers and topological crystalline nodal superconductors with mirror winding numbers.

Analysis of Deep and Shallow Traps in Semi-Insulating CdZnTe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Kihyun; Yoon, Yongsu; James, Ralph B.

Trap levels which are deep or shallow play an important role in the electrical and the optical properties of a semiconductor; thus, a trap level analysis is very important in most semiconductor devices. Deep-level defects in CdZnTe are essential in Fermi level pinning at the middle of the bandgap and are responsible for incomplete charge collection and polarization effects. However, a deep level analysis in semi-insulating CdZnTe (CZT) is very difficult. Theoretical capacitance calculation for a metal/insulator/CZT (MIS) device with deep-level defects exhibits inflection points when the donor/acceptor level crosses the Fermi level in the surface-charge layer (SCL). Three CZTmore » samples with different resistivities, 2 × 10 4 (n-type), 2 × 10 6 (p-type), and 2 × 10 10 (p-type) Ω·cm, were used in fabricating the MIS devices. These devices showed several peaks in their capacitance measurements due to upward/downward band bending that depend on the surface potential. In conclusion, theoretical and experimental capacitance measurements were in agreement, except in the fully compensated case.« less

Superconducting symmetries and magnetic responses of uranium heavy-fermion systems UBe13 and UPd2Al3

NASA Astrophysics Data System (ADS)

Shimizu, Yusei; Kittaka, Shunichiro; Sakakibara, Toshiro; Aoki, Dai

2018-05-01

Low-temperature thermodynamic investigation for UBe13 and UPd2Al3 were performed in order to gain insight into their unusual ground states of 5 f electrons. Our heat-capacity data for the cubic UBe13 strongly suggest that nodal quasiparticles are absent and its superconducting (SC) gap is fully open over the Fermi surface. Moreover, two unusual thermodynamic anomalies are also observed in UBe13 at ∼ 3 T and ∼ 9 T; the lower-field anomaly is seen only in the SC mixed state by dc magnetization M (H) as well as heat-capacity C (H) , while the higher-field anomaly appears for C (H) in the normal phase above the upper critical field. On the other hand, field-orientation dependence of the heat capacity in the hexagonal UPd2Al3 shows a significantly anisotropic behavior of C (H) ∝H 1 / 2 , reflecting the nodal gap structure of this system. Our result strongly suggests the presence of a horizontal line node on the Fermi surface with heavy effective mass in UPd2Al3.

Fermi LAT Search for Dark Matter in Gamma-Ray Lines and the Inclusive Photon Spectrum

NASA Technical Reports Server (NTRS)

Ackermann, M.; Ajello, M.; Albert, A.; Baldini, L.; Barbiellini, G.; Bechtol, K.; Bellazzini, R.; Berenji, B.; Blandford, R. D.; Bloom, E. D.;

Fermi LAT search for dark matter in gamma-ray lines and the inclusive photon spectrum

DOE PAGES

Ackermann, M.

2012-07-05

Dark matter particle annihilation or decay can produce monochromatic gamma-ray lines and contribute to the diffuse gamma-ray background. Furthermore, we present the flux upper limits for gamma-ray spectral lines from 7 to 200 GeV and for the diffuse gamma-ray background from 4.8 GeV to 264 GeV obtained from two years of Fermi Large Area Telescope data integrated over most of the sky. Here, we give cross-section upper limits and decay lifetime lower limits for dark matter models that produce gamma-ray lines or contribute to the diffuse spectrum, including models proposed as explanations of the PAMELA and Fermi cosmic-ray data.

Spectroscopic determination of surface geometry: Ti(0001)-H(1×1)

NASA Astrophysics Data System (ADS)

Feibelman, Peter J.; Hamann, D. R.

1980-02-01

The electronic structure of a Ti(0001) film covered by a monolayer of H is shown to depend strongly on the location of the H atom in the surface unit cell. Best agreement with experiment is found with the H's in three-fold sites, 0.8 a.u. outside the outer Ti layer. In this geometry the H atoms "heal" the surface-the clean Ti(0001) surface state near the Fermi level is removed and the outer layer d-like local density of states (LDOS) is quite similar to that of the interior. Additionally, the calculated work function is 4.0 eV and an H-derived peak in the calculated LDOS appears 5 eV below EF, in agreement with photoemission measurements.

Separation of charge-order and magnetic QCPs in heavy fermions and high Tc cuprates

NASA Astrophysics Data System (ADS)

Harrison, Neil

2010-03-01

The Fermi surface topology of high temperature superconductors inferred from magnetic quantum oscillation measurements provides clues for the origin of unconventional pairing thus previously not accessed by other spectroscopy techniques. While the overdoped regime of the high Tc phase diagram has a large Fermi surface consistent with bandstructure calculations, the underdoped regime of YBa2Cu2O6+x is found to be composed of small pockets. There is considerable debate as to whether the small observed ``pocket'' is hole-like or electron-like- whether the Fermi surface is best described by a t-J model or a conventional band folding picture- whether or not a Fermi liquid description applies- or- whether bilayer coupling splits the degeneracy of the observed pockets. We (myself and collaborators) have now collected an extensive body of experimental data that brings this debate to rest, but raises new questions about the nature of itinerant magnetism in underdoped high Tc cuprates. Quantum oscillation measurements are performed on multiple samples in magnetic fields extending to 85 T, temperatures between 30 mK (dilution fridge in dc fields to 45 T) and 18 K, over a range of hole dopings and with samples rotated in-situ about multiple axes with respect to the magnetic field. We perform a topographical map of the Fermi surface, enabling the in-plane shape of one of the pockets to be determined- imposing stringent constraints on the origin of the Fermi surface. While quantum oscillations measurements are consistent with a topological Fermi surface change associated with magnetism near optimal doping, they also point to a secondary instability deep within the underdoped regime beneath a high Tc superconducting sub-dome. An steep upturn in the quasiparticle effective mass is observed on underdoping, suggestive of a quantum critical point near x= 0.46 separating the metallic regime (composed of small pockets) from a more underdoped insulating charge-ordered regime (earlier reported in neutron scattering measurements). Our findings suggest the importance of two critical instabilities affecting the Fermi surface beneath the high Tc superconducting dome(s). While one of these has been proposed to provide the likely origin of unconventional pairing in the cuprates, the other can be an important factor in boosting transition temperatures. [4pt] This work is supported by the DoE BES grant ``Science in 100 T''. The author would like to thank collaborators S. E. Sebastian, C. H. Mielke, P. A. Goddard, M. M. Altarawneh, R. Liang, D. A. Bonn, W. N. Hardy and G. G. Lonzarich, and supporting staff at the National High Magnetic Field Laboratory (NHMFL). Quantum oscillation experiments are performed at the NHMFL, which is funded by the NSF with support from the DoE and State of Florida.

Energy and Momentum Relaxation Times of 2D Electrons Due to Near Surface Deformation Potential Scattering

NASA Astrophysics Data System (ADS)

Pipa, Viktor; Vasko, Fedor; Mitin, Vladimir

1997-03-01

The low temperature energy and momentum relaxation rates of 2D electron gas placed near the free or clamped surface of a semi-infinit sample are calculated. To describe the electron-acoustic phonon interaction with allowance of the surface effect the method of elasticity theory Green functions was used. This method allows to take into account the reflection of acoustic waves from the surface and related mutual conversion of LA and TA waves. It is shown that the strength of the deformation potential scattering at low temperatures substantially depends on the mechanical conditions at the surface: relaxation rates are suppressed for the free surface while for the rigid one the rates are enhanced. The dependence of the conductivity on the distance between the 2D layer and the surface is discussed. The effect is most pronounced in the range of temperatures 2 sl pF < T < (2 hbar s_l)/d, where pF is the Fermi momentum, sl is the velocity of LA waves, d is the width of the quantum well.

Experimental observation of topological Fermi arcs in type-II Weyl semimetal MoTe2

NASA Astrophysics Data System (ADS)

Deng, Ke; Wan, Guoliang; Deng, Peng; Zhang, Kenan; Ding, Shijie; Wang, Eryin; Yan, Mingzhe; Huang, Huaqing; Zhang, Hongyun; Xu, Zhilin; Denlinger, Jonathan; Fedorov, Alexei; Yang, Haitao; Duan, Wenhui; Yao, Hong; Wu, Yang; Fan, Shoushan; Zhang, Haijun; Chen, Xi; Zhou, Shuyun

2016-12-01

Weyl semimetal is a new quantum state of matter hosting the condensed matter physics counterpart of the relativistic Weyl fermions originally introduced in high-energy physics. The Weyl semimetal phase realized in the TaAs class of materials features multiple Fermi arcs arising from topological surface states and exhibits novel quantum phenomena, such as a chiral anomaly-induced negative magnetoresistance and possibly emergent supersymmetry. Recently it was proposed theoretically that a new type (type-II) of Weyl fermion that arises due to the breaking of Lorentz invariance, which does not have a counterpart in high-energy physics, can emerge as topologically protected touching between electron and hole pockets. Here, we report direct experimental evidence of topological Fermi arcs in the predicted type-II Weyl semimetal MoTe2 (refs ,,). The topological surface states are confirmed by directly observing the surface states using bulk- and surface-sensitive angle-resolved photoemission spectroscopy, and the quasi-particle interference pattern between the putative topological Fermi arcs in scanning tunnelling microscopy. By establishing MoTe2 as an experimental realization of a type-II Weyl semimetal, our work opens up opportunities for probing the physical properties of this exciting new state.

Anisotropy of the Seebeck Coefficient in the Cuprate Superconductor YBa2 Cu3 Oy : Fermi-Surface Reconstruction by Bidirectional Charge Order

NASA Astrophysics Data System (ADS)

Cyr-Choinière, O.; Badoux, S.; Grissonnanche, G.; Michon, B.; Afshar, S. A. A.; Fortier, S.; LeBoeuf, D.; Graf, D.; Day, J.; Bonn, D. A.; Hardy, W. N.; Liang, R.; Doiron-Leyraud, N.; Taillefer, Louis

2017-07-01

The Seebeck coefficient S of the cuprate YBa2 Cu3 Oy is measured in magnetic fields large enough to suppress superconductivity, at hole dopings p =0.11 and p =0.12 , for heat currents along the a and b directions of the orthorhombic crystal structure. For both directions, S /T decreases and becomes negative at low temperature, a signature that the Fermi surface undergoes a reconstruction due to broken translational symmetry. Above a clear threshold field, a strong new feature appears in Sb, for conduction along the b axis only. We attribute this feature to the onset of 3D-coherent unidirectional charge-density-wave modulations seen by x-ray diffraction, also along the b axis only. Because these modulations have a sharp onset temperature well below the temperature where S /T starts to drop towards negative values, we infer that they are not the cause of Fermi-surface reconstruction. Instead, the reconstruction must be caused by the quasi-2D bidirectional modulations that develop at significantly higher temperature. The unidirectional order only confers an additional anisotropy to the already reconstructed Fermi surface, also manifest as an in-plane anisotropy of the resistivity.

Momentum peak shift and width of longitudinal momentum distribution of projectilelike fragments produced at E =290 MeV /nucleon

NASA Astrophysics Data System (ADS)

Momota, S.; Kanazawa, M.; Kitagawa, A.; Sato, S.

2018-04-01

Longitudinal momentum (PL) distributions of projectilelike fragments produced at E =290 MeV /nucleon are investigated. PL distributions of fragments produced by Ar and Kr beams with a wide variety of targets (C, Al, Nb, Tb, and Au) were measured using the fragment separator at HIMAC. PL distributions observed for fragments with a wide range of mass losses Δ A (1-30 for Ar beam and 1-64 for Kr beam), show a slightly, but definitely asymmetric nature. The peak shift and width were obtained from the observed PL distributions. No significant target dependence was found in either the peak shift or width. For the practical application, the variation in momentum peak shift with fragment mass (AF) was represented by a parabolic function. The width on the high-PL side (σHigh) is well reproduced by the Goldhaber formula, which is obtained from the contribution of the Fermi momentum. The behavior of the reduced width, σ0, obtained from σHigh via the Goldhaber formulation, is consistent with the mass-dependent Fermi momentum of a nucleon. The width on the low-PL side (σLow) is markedly larger than σHigh and exhibits a clear AF dependence.

Three-dimensional superconducting gap in FeSe from angle-resolved photoemission spectroscopy

NASA Astrophysics Data System (ADS)

Kushnirenko, Y. S.; Fedorov, A. V.; Haubold, E.; Thirupathaiah, S.; Wolf, T.; Aswartham, S.; Morozov, I.; Kim, T. K.; Büchner, B.; Borisenko, S. V.

2018-05-01

We present a systematic angle-resolved photoemission spectroscopy study of the superconducting gap in FeSe. The gap function is determined in a full Brillouin zone including all Fermi surfaces and kz dependence. We find significant anisotropy of the superconducting gap in all momentum directions. While the in-plane anisotropy can be explained by both nematicity-induced pairing anisotropy and orbital-selective pairing, the kz anisotropy requires an additional refinement of the theoretical approaches.

Andreev reflection without Fermi surface alignment in high- T c van der Waals heterostructures

DOE PAGES

Zareapour, Parisa; Hayat, Alex; Zhao, Shu Yang F.; ...

2017-04-05

We address the controversy over the proximity effect between topological materials and high-T c superconductors. Junctions are produced between Bi 2Sr 2CaCu 2Omore » $${}_{8+\\delta }$$ and materials with different Fermi surfaces (Bi 2Te 3 and graphite). Both cases reveal tunneling spectra that are consistent with Andreev reflection. This is confirmed by a magnetic field that shifts features via the Doppler effect. This is modeled with a single parameter that accounts for tunneling into a screening supercurrent. Thus the tunneling involves Cooper pairs crossing the heterostructure, showing that the Fermi surface mismatch does not hinder the ability to form transparent interfaces, which is accounted for by the extended Brillouin zone and different lattice symmetries.« less

Electronic structures of Plutonium compounds with the NaCl-type monochalcogenides structure

NASA Astrophysics Data System (ADS)

Maehira, Takahiro; Tatetsu, Yasutomi

2012-12-01

We calculate the energy band structure and the Fermi surface of PuS, PuSe and PuTe by using a self-consistent relativistic linear augmented-plane-wave method with the exchange and correlation potential in a local density approximation. It is found in common that the energy bands in the vicinity of the Fermi level are mainly due to the hybridization between Pu 5/ and monochalcogenide p electrons. The obtained main Fermi surfaces are composed of two hole sheets and one electron sheet, all of which are constructed from the band having the Pu 5/ state and the monochalcogenide p state.

Quasiparticle lifetime in a mixture of Bose and Fermi superfluids.

PubMed

Zheng, Wei; Zhai, Hui

2014-12-31

In this Letter, we study the effect of quasiparticle interactions in a Bose-Fermi superfluid mixture. We consider the lifetime of a quasiparticle of the Bose superfluid due to its interaction with quasiparticles in the Fermi superfluid. We find that this damping rate, i.e., the inverse of the lifetime, has quite a different threshold behavior at the BCS and the BEC side of the Fermi superfluid. The damping rate is a constant near the threshold momentum in the BCS side, while it increases rapidly in the BEC side. This is because, in the BCS side, the decay process is restricted by the constraint that the fermion quasiparticle is located near the Fermi surface, while such a restriction does not exist in the BEC side where the damping process is dominated by bosonic quasiparticles of the Fermi superfluid. Our results are related to the collective mode experiment in the recently realized Bose-Fermi superfluid mixture.

Electrostatic energy and screened charge interaction near the surface of metals with different Fermi surface shape

NASA Astrophysics Data System (ADS)

Gabovich, A. M.; Il'chenko, L. G.; Pashitskii, E. A.; Romanov, Yu. A.

1980-04-01

Using the Poisson equation Green function for a self-consistent field in a spatially inhomogeneous system, expressions for the electrostatic energy and screened charge interaction near the surface of a semi-infinite metal and a thin quantizing film are derived. It is shown that the decrease law and Friedel oscillation amplitude of adsorbed atom indirect interaction are determined by the electron spectrum character and the Fermi surface shape. The results obtained enable us to explain, in particular, the submonolayer adsorbed film structure on the W and Mo surfaces.

Magnetotransport of multiple-band nearly antiferromagnetic metals due to hot-spot scattering

DOE PAGES

Koshelev, A. E.

2016-09-30

Multiple-band electronic structure and proximity to antiferromagnetic (AF) instability are the key properties of iron-based superconductors. In this paper, we explore the influence of scattering by the AF spin fluctuations on transport of multiple-band metals above the magnetic transition. A salient feature of scattering on the AF fluctuations is that it is strongly enhanced at the Fermi surface locations where the nesting is perfect (“hot spots” or “hot lines”). We review derivation of the collision integral for the Boltzmann equation due to AF-fluctuations scattering. In the paramagnetic state, the enhanced scattering rate near the hot lines leads to anomalous behaviormore » of electronic transport in magnetic field. We explore this behavior by analytically solving the Boltzmann transport equation with approximate transition rates. This approach accounts for return scattering events and is more accurate than the relaxation-time approximation. The magnetic-field dependences are characterized by two very different field scales: the lower scale is set by the hot-spot width and the higher scale is set by the total scattering amplitude. A conventional magnetotransport behavior is limited to magnetic fields below the lower scale. In the wide range in-between these two scales, the longitudinal conductivity has linear dependence on the magnetic field and the Hall conductivity has quadratic dependence. The linear dependence of the diagonal component reflects growth of the Fermi-surface area affected by the hot spots proportional to the magnetic field. Finally, we discuss applicability of this theoretical framework for describing of anomalous magnetotransport properties in different iron pnictides and chalcogenides in the paramagnetic state.« less

Potential Antiferromagnetic Fluctuations in Hole-Doped Iron-Pnictide Superconductor Ba1-xKxFe2As2 Studied by 75As Nuclear Magnetic Resonance Measurement0.1143/JPSJ.81.054704

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hirano, Masanori; Yamada, Yuji; Saito, Taku

2012-04-12

We have performed 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on single-crystalline Ba1-xKxFe2As2 for x = 0.27–1. 75As nuclear quadruple resonance frequency (νQ) increases linearly with increasing x. The Knight shift K in the normal state shows Pauli paramagnetic behavior with a weak temperature T dependence. K increases gradually with increasing x. By contrast, the nuclear spin–lattice relaxation rate 1/T1 in the normal state has a strong T dependence, which indicates the existence of large antiferomagnetic (AF) spin fluctuations for all x's. The T dependence of 1/T1 shows a gaplike behavior below approximately 100 K formore » 0.6 < x < 0.9. This behaviors is well explained by the change in the band structure with the expansion of hole Fermi surfaces and the shrinkage and disappearance of electron Fermi surfaces at the Brillouin zone (BZ) with increasing x. The anisotropy of 1/T1, represented by the ratio of 1/T1ab to 1/T1c, is always larger than 1 for all x's, which indicates that stripe-type AF fluctuations are dominant in this system. The K in the superconducting (SC) state decreases, which corresponds to the appearance of spin-singlet superconductivity. The T dependence of 1/T1 in the SC state indicates a multiple-SC-gap feature. A simple two-gap model analysis shows that the larger superconducting gap gradually decreases with increasing x from 0.27 to 1 and a smaller gap decreases rapidly and nearly vanishes for x > 0.6 where electron pockets in BZ disappear.« less

Fields in laser-ablated plasmas generalized to degenerate electrons and to Fermi energy in nuclei with change to quark-gluon plasma

NASA Astrophysics Data System (ADS)

Hora, Heinrich; Miley, George H.; Osman, Frederick; Hammerling, Peter X.

2004-09-01

The studies of laser ablation have lead to a new theory of nuclei, endothermic nuclei generation and quark-gluon plasmas. The surface of ablated plasma expanding into vacuum after high power laser irradiation of targets, contains an electric double layer having the thickness of the Debye length. This led to the discovery of surface tension of plasmas and to the internal dynamic electric fields in all inhomogeneous plasmas. The surface causes stabilization by short length surface waves smoothing the expanding plasma plume. Generalizing this to the degenerate electrons in a metal with the Fermi energy instead of the temperature, resulted in the surface tension of metals in agreement with measurements. Taking then the Fermi energy in the Debye length for nucleons results in a theory of nuclei with stable confinement of protons and neutrons just at the well known nuclear density, and in the Debye length equal to Hofstadter's decay of the nuclear surface. Increasing the nuclear density by a factor of 6 leads to the change of the Fermi energy into its relativistic branch where no surface energy is possible and the particle mass is not defined, permitting the quark-gluon plasma. Expansion of this higher density at the big band or in a supernova results in nucleation and element generation. The Boltzmann equilibrium permits the synthesis of nuclei even in the endothermic range limited to about uranium.

Redox-dependent spatially resolved electrochemistry at graphene and graphite step edges.

PubMed

Güell, Aleix G; Cuharuc, Anatolii S; Kim, Yang-Rae; Zhang, Guohui; Tan, Sze-yin; Ebejer, Neil; Unwin, Patrick R

2015-04-28

The electrochemical (EC) behavior of mechanically exfoliated graphene and highly oriented pyrolytic graphite (HOPG) is studied at high spatial resolution in aqueous solutions using Ru(NH3)6(3+/2+) as a redox probe whose standard potential sits close to the intrinsic Fermi level of graphene and graphite. When scanning electrochemical cell microscopy (SECCM) data are coupled with that from complementary techniques (AFM, micro-Raman) applied to the same sample area, different time-dependent EC activity between the basal planes and step edges is revealed. In contrast, other redox couples (ferrocene derivatives) whose potential is further removed from the intrinsic Fermi level of graphene and graphite show uniform and high activity (close to diffusion-control). Macroscopic voltammetric measurements in different environments reveal that the time-dependent behavior after HOPG cleavage, peculiar to Ru(NH3)6(3+/2+), is not associated particularly with any surface contaminants but is reasonably attributed to the spontaneous delamination of the HOPG with time to create partially coupled graphene layers, further supported by conductive AFM measurements. This process has a major impact on the density of states of graphene and graphite edges, particularly at the intrinsic Fermi level to which Ru(NH3)6(3+/2+) is most sensitive. Through the use of an improved voltammetric mode of SECCM, we produce movies of potential-resolved and spatially resolved HOPG activity, revealing how enhanced activity at step edges is a subtle effect for Ru(NH3)6(3+/2+). These latter studies allow us to propose a microscopic model to interpret the EC response of graphene (basal plane and edges) and aged HOPG considering the nontrivial electronic band structure.

Superconductivity and charge density wave in ZrTe 3–xSe x

DOE PAGES

Zhu, Xiangde; Ning, Wei; Li, Lijun; ...

2016-06-02

Charge density wave (CDW), the periodic modulation of the electronic charge density, will open a gap on the Fermi surface that commonly leads to decreased or vanishing conductivity. On the other hand superconductivity, a commonly believed competing order, features a Fermi surface gap that results in infinite conductivity. Here we report that superconductivity emerges upon Se doping in CDW conductor ZrTe 3 when the long range CDW order is gradually suppressed. Superconducting critical temperature T c(x) in ZrTe 3–xSe x (0 ≤ x ≤ 0.1) increases up to 4 K plateau for 0.04 ≤ x ≤ 0.07. Further increase inmore » Se content results in diminishing T c and filametary superconductivity. The CDW modes from Raman spectra are observed in x = 0.04 and 0.1 crystals, where signature of ZrTe 3 CDW order in resistivity vanishes. As a result, the electronic-scattering for high T c crystals is dominated by local CDW fluctuations at high temperatures, the resistivity is linear up to highest measured T = 300 K and contributes to substantial in-plane anisotropy.« less

Tuning Surface Energy Landscapes in Metallic Quantum Films using Alkali Adsorbates

NASA Astrophysics Data System (ADS)

Khajetoorians, Alexander; Qin, Shengyong; Zhu, Wenguang; Eisele, Holger; Zhang, Zhenyu; Shih, Chih-Kang

2008-03-01

Quantum confinement shows a strong interplay with growth and kinetics in thin metal systems where the Fermi wavelength has a special relationship to the surface normal lattice constant. In the case of Pb/Si(111) systems, this relationship reveals an interesting thickness-dependent bilayer oscillation in the density of states and surface energy up to a phase. In this paper, we report on a novel effect: tuning of the energy landscape of a flat-top quantum Pb mesa using Cs adsorbates. Using STM/STS, we show that depositing Cs adsorbates on a thin Pb mesa promotes quantum stable Pb nanoislands on preferentially unstable thicknesses. Thickness-dependent nanoisland densities show a strong bilayer oscillation correlating with quantum stability. By modifying the Cs coverage on the mesa surface, we can tune the lateral size distribution of the nanoislands and the overall amplitude of the island density oscillation. Nanoisland formation is linked to a step decoration of Cs adatoms along the step edge of the nanoisland.

Signatures of Fermi Arcs in the Quasiparticle Interferences of the Weyl Semimetals TaAs and NbP.

PubMed

Chang, Guoqing; Xu, Su-Yang; Zheng, Hao; Lee, Chi-Cheng; Huang, Shin-Ming; Belopolski, Ilya; Sanchez, Daniel S; Bian, Guang; Alidoust, Nasser; Chang, Tay-Rong; Hsu, Chuang-Han; Jeng, Horng-Tay; Bansil, Arun; Lin, Hsin; Hasan, M Zahid

2016-02-12

The recent discovery of the first Weyl semimetal in TaAs provides the first observation of a Weyl fermion in nature. Such a topological semimetal features a novel type of anomalous surface state, the Fermi arc, which connects a pair of Weyl nodes through the boundary of the crystal. Here, we present theoretical calculations of the quasiparticle interference (QPI) patterns that arise from the surface states including the topological Fermi arcs in the Weyl semimetals TaAs and NbP. Most importantly, we discover that the QPI exhibits termination points that are fingerprints of the Weyl nodes in the interference pattern. Our results, for the first time, propose a universal interference signature of the topological Fermi arcs in TaAs, which is fundamental for scanning tunneling microscope (STM) measurements on this prototypical Weyl semimetal compound. More generally, our work provides critical guideline and methodology for STM studies on new Weyl semimetals. Further, the scattering channels revealed by our QPIs are broadly relevant to surface transport and device applications based on Weyl semimetals.

Hybrid nodal loop metal: Unconventional magnetoresponse and material realization

NASA Astrophysics Data System (ADS)

Zhang, Xiaoming; Yu, Zhi-Ming; Lu, Yunhao; Sheng, Xian-Lei; Yang, Hui Ying; Yang, Shengyuan A.

2018-03-01

A nodal loop is formed by a band crossing along a one-dimensional closed manifold, with each point on the loop a linear nodal point in the transverse dimensions, and can be classified as type I or type II depending on the band dispersion. Here, we propose a class of nodal loops composed of both type-I and type-II points, which are hence termed as hybrid nodal loops. Based on first-principles calculations, we predict the realization of such loops in the existing electride material Ca2As . For a hybrid loop, the Fermi surface consists of coexisting electron and hole pockets that touch at isolated points for an extended range of Fermi energies, without the need for fine-tuning. This leads to unconventional magnetic responses, including the zero-field magnetic breakdown and the momentum-space Klein tunneling observable in the magnetic quantum oscillations, as well as the peculiar anisotropy in the cyclotron resonance.

Temperature dependence of the vibrational spectra of acetanilide: Davydov solitons or Fermi coupling?

NASA Astrophysics Data System (ADS)

Johnston, Clifford T.; Swanson, Basil I.

1985-03-01

The unusual temperature dependence of the amide-I region in the IR spectrum of acetanilide (C 6H 5NHCOCH 3) has recently been attributed to a self-trapped Davydov-like soliton. The temperature dependence of the single-crystal Raman scattering, from acetanilide and its ND and 13CO substituted analogs in the phonon and internal mode regions has now been studied. The behavior of the amide-I region in the Raman spectra of the normal isotopic species is similar to that observed earlier in infrared studies. However, on the basis of results obtained from the ND and 13CO substituted species the unusual temperature dependence in the 1650 cm -1 region has been attributed to Fermi coupling of the amide-I fundamental and a combination band involving the in-plane NH deformation and a low-frequency torsional mode. As temperature is lowered, the strong blue-shift of the torsional mode results in a commensurate blue-shift in the combination level thereby increasing the Fermi coupling. Temperature tuning of the Fermi coupling results in the anomalous intensity changes observed in the IR and Raman spectra of the amide-I region for the normal isotopic species.

Study of certain features of the electronic structure of the ternary alloys Ni2(Mn, Fe) and Ni3(Mn, Co)

NASA Technical Reports Server (NTRS)

Zhukova, V. M.; Fadin, V. P.

1981-01-01

The changes in electronic structure related to transport processes occurring during the alloying of he alloy Ni3Mn with iron and cobalt, and the ordering of the ternary alloys thus formed are presented. The Hall effect, the absolute thermal emf, the internal saturation induction, the Nernst-Ettingshausen constant, and the electrical resistivity were measured. Results show a decrease in the contribution of hole sections of the Fermi surface to the transport process occurs together with a considerable increase in the contribution of electron sections. In this case, the mobility of 3 dimensional holes decreases and the mobility of 4s electrons increases considerably.

Trivial and topological Fermi arcs in the type-II Weyl semimetal candidate MoTe2

NASA Astrophysics Data System (ADS)

Tamai, Anna; Wu, Quansheng; Cucchi, Irene; Bruno, Flavio; Barreteau, Celine; Giannini, Enrico; Soluyanov, Alexey; Baumberger, Felix

Weyl semimetals are commonly identified by detecting their characteristic open surface state Fermi arcs in angle-resolved photoemission (ARPES) experiments. However, in type-II Weyl semimetals the Fermi arcs generally disappear in the bulk carrier pockets before reaching the Weyl points where they terminate - making it harder to unambiguously identify this new electronic state. Using laser-based ARPES, we have resolved multiple distinct Fermi arcs on the inequivalent top and bottom (001) surfaces of the candidate type-II Weyl semimetal MoTe2. By comparing our ARPES data with systematic electronic structure calculations simulating different Weyl point arrangements, we show that some of these arcs are false positives as they can be explained without Weyl points, while others are only reproduced in scenarios with at least eight Weyl points. Our results thus suggest that MoTe2 is the first experimental realisation of a type-II Weyl semimetal.

STATISTICS OF GAMMA-RAY POINT SOURCES BELOW THE FERMI DETECTION LIMIT

DOE Office of Scientific and Technical Information (OSTI.GOV)

Malyshev, Dmitry; Hogg, David W., E-mail: dm137@nyu.edu

2011-09-10

An analytic relation between the statistics of photons in pixels and the number counts of multi-photon point sources is used to constrain the distribution of gamma-ray point sources below the Fermi detection limit at energies above 1 GeV and at latitudes below and above 30 deg. The derived source-count distribution is consistent with the distribution found by the Fermi Collaboration based on the first Fermi point-source catalog. In particular, we find that the contribution of resolved and unresolved active galactic nuclei (AGNs) to the total gamma-ray flux is below 20%-25%. In the best-fit model, the AGN-like point-source fraction is 17%more » {+-} 2%. Using the fact that the Galactic emission varies across the sky while the extragalactic diffuse emission is isotropic, we put a lower limit of 51% on Galactic diffuse emission and an upper limit of 32% on the contribution from extragalactic weak sources, such as star-forming galaxies. Possible systematic uncertainties are discussed.« less

Large magnetoresistance and Fermi surface study of Sb2Se2Te single crystal

NASA Astrophysics Data System (ADS)

Shrestha, K.; Marinova, V.; Graf, D.; Lorenz, B.; Chu, C. W.

2017-09-01

We have studied the magnetotransport properties of a Sb2Se2Te single crystal. Magnetoresistance (MR) is maximum when the magnetic field is perpendicular to the sample surface and reaches a value of 1100% at B = 31 T with no sign of saturation. MR shows Shubnikov de Haas (SdH) oscillations above B = 15 T. The frequency spectrum of SdH oscillations consists of three distinct peaks at α = 32 T, β = 80 T, and γ = 117 T indicating the presence of three Fermi surface pockets. Among these frequencies, β is the prominent peak in the frequency spectrum of SdH oscillations measured at different tilt angles of the sample with respect to the magnetic field. From the angle dependence β and Berry phase calculations, we have confirmed the trivial topology of the β-pocket. The cyclotron masses of charge carriers, obtained by using the Lifshitz-Kosevich formula, are found to be mβ*=0.16mo and m γ*=0.63 mo for the β and γ bands, respectively. The Large MR of Sb2Se2Te is suitable for utilization in electronic instruments such as computer hard discs, high field magnetic sensors, and memory devices.

Correlation between the Knight shift of chemisorbed CO and the Fermi level local density of states at clean platinum catalyst surfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tong, Y.Y.; Rice, C.; Godbout, N.

1999-04-07

Due to its fundamental importance in heterogeneous catalysis, as well as in electrocatalysis, the chemisorption and reaction of CO on transition metal surfaces has been an important focus of modern surface science. Here, the NMR spectroscopy of {sup 13}CO adsorbed onto transition metal surfaces has been shown to be a very powerful probe of molecular structure and dynamics of CO itself, as well as a probe of the electronic properties of the transition metal surfaces onto which it is adsorbed. The authors have investigated the {sup 195}Pt and {sup 13}C nuclear magnetic resonance (NMR) spectroscopy of clean-surface platinum catalysts andmore » of CO chemisorbed onto Pt catalysts surfaces. They use Knight shift, relaxation, and J-coupling data to deduce information about the Fermi level local density of states (E{sub f}-LDOS) at catalyst surfaces. There is a linear correlation between the Knight shifts of chemisorbed CO and the clean surface E{sub f}-LDOS of platinum onto which the CO is bound, as determined by {sup 13}C and {sup 195}Pt NMR. The correlation amounts to {approximately} 12 ppm/Ry{sup {minus}1} {center_dot} atom{sup {minus}1}, the same as that which can be deduced for CO on palladium, as well as from the electrode potential dependence of {sup 13}C Knight shifts and infrared vibrational frequencies, {nu}{sub CO}, and the relationship between {nu}{sub CO} and the E{sub f}-LDOS at clean platinum surfaces. The ability to now directly relate meal and adsorbate electronic properties opens up new avenues for investigating metal-ligand interactions in heterogeneous catalysis and electrocatalysis.« less

Spatially Resolved Quantification of the Surface Reactivity of Solid Catalysts.

PubMed

Huang, Bing; Xiao, Li; Lu, Juntao; Zhuang, Lin

2016-05-17

A new property is reported that accurately quantifies and spatially describes the chemical reactivity of solid surfaces. The core idea is to create a reactivity weight function peaking at the Fermi level, thereby determining a weighted summation of the density of states of a solid surface. When such a weight function is defined as the derivative of the Fermi-Dirac distribution function at a certain non-zero temperature, the resulting property is the finite-temperature chemical softness, termed Fermi softness (SF ), which turns out to be an accurate descriptor of the surface reactivity. The spatial image of SF maps the reactive domain of a heterogeneous surface and even portrays morphological details of the reactive sites. SF analyses reveal that the reactive zones on a Pt3 Y(111) surface are the platinum sites rather than the seemingly active yttrium sites, and the reactivity of the S-dimer edge of MoS2 is spatially anisotropic. Our finding is of fundamental and technological significance to heterogeneous catalysis and industrial processes demanding rational design of solid catalysts. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Angular power spectrum of the diffuse gamma-ray emission as measured by the Fermi Large Area Telescope and constraints on its dark matter interpretation

DOE PAGES

Fornasa, Mattia; Cuoco, Alessandro; Zavala, Jesús; ...

2016-12-09

The isotropic gamma-ray background arises from the contribution of unresolved sources, including members of confirmed source classes and proposed gamma-ray emitters such as the radiation induced by dark matter annihilation and decay. Clues about the properties of the contributing sources are imprinted in the anisotropy characteristics of the gamma-ray background. We use 81 months of Pass 7 Reprocessed data from the Fermi Large Area Telescope to perform a measurement of the anisotropy angular power spectrum of the gamma-ray background. Here, we analyze energies between 0.5 and 500 GeV, extending the range considered in the previous measurement based on 22 monthsmore » of data. We also compute, for the first time, the cross-correlation angular power spectrum between different energy bins. The derived angular spectra are compatible with being Poissonian, i.e. constant in multipole. Furthermore, the energy dependence of the anisotropy suggests that the signal is due to two populations of sources, contributing, respectively, below and above ~ 2 GeV . Finally, using data from state-of-the-art numerical simulations to model the dark matter distribution, we constrain the contribution from dark matter annihilation and decay in Galactic and extra-Galactic structures to the measured anisotropy. These constraints are competitive with those that can be derived from the average intensity of the isotropic gamma-ray background.« less

Scattering of surface electrons by isolated steps versus periodic step arrays

NASA Astrophysics Data System (ADS)

Ortega, J. E.; Lobo-Checa, J.; Peschel, G.; Schirone, S.; Abd El-Fattah, Z. M.; Matena, M.; Schiller, F.; Borghetti, P.; Gambardella, P.; Mugarza, A.

2013-03-01

We investigate the scattering of electrons belonging to Shockley states of (111)-oriented noble metal surfaces using angle-resolved photoemission (ARPES) and scanning tunneling microscopy (STM). Both ARPES and STM indicate that monatomic steps on a noble metal surface may act either as strongly repulsive or highly transmissive barriers for surface electrons, depending on the coherence of the step lattice, and irrespectively of the average step spacing. By measuring curved crystal surfaces with terrace length ranging from 30 to 180 Å, we show that vicinal surfaces of Au and Ag with periodic step arrays exhibit a remarkable wave function coherence beyond 100 Å step spacings, well beyond the Fermi wavelength limit and independently of the projection of the bulk band gap on the vicinal plane. In contrast, the analysis of transmission resonances investigated by STM shows that a pair of isolated parallel steps defining a 58 Å wide terrace confines and decouples the surface state of the small terrace from that of the (111) surface. We conclude that the formation of laterally confined quantum well states in vicinal surfaces as opposed to propagating superlattice states depends on the loss of coherence driven by imperfection in the superlattice order.

Trigonal warping induced unusual spin texture and strong spin polarization in graphene with the Rashba effect

NASA Astrophysics Data System (ADS)

Ma, Da-Shuai; Yu, Zhi-Ming; Pan, Hui; Yao, Yugui

2018-02-01

We study the electronic and scattering properties of graphene with moderate Rashba spin-orbit coupling (SOC). The Rashba SOC in graphene tends to distort the band structure and gives rise to a trigonally warped Fermi surface. For electrons at a pronouncedly warped Fermi surface, the spin direction exhibits a staircase profile as a function of the momentum, making an unusual spin texture. We also study the spin-resolved scattering on a Rashba barrier and find that the trigonal warping is essential for producing spin polarization of the transmitted current. Particularly, both the direction and strength of the spin polarization can be controlled by kinds of electric methods. Our work unveils that not only SOC but also the geometry of the Fermi surface is important for generating spin polarization.

Fermi surface interconnectivity and topology in Weyl fermion semimetals TaAs, TaP, NbAs, and NbP

DOE PAGES

Lee, Chi-Cheng; Xu, Su-Yang; Huang, Shin-Ming; ...

2015-12-01

The family of binary compounds including TaAs, TaP, NbAs, and NbP was recently discovered as the first realization of Weyl semimetals. In order to develop a comprehensive description of the charge carriers in these Weyl semimetals, we performed detailed and systematic electronic band structure calculations which reveal the nature of Fermi surfaces and their complex interconnectivity in TaAs, TaP, NbAs, and NbP. In conclusion, our work reports a comparative and comprehensive study of Fermi surface topology and band structure details of all known members of the Weyl semimetal family and hence provides the fundamental knowledge for realizing the many predictedmore » exotic topological quantum physics of Weyl semimetals based on the TaAs class of materials.« less

Thermodynamics of the magnetic-field-induced "normal" state in an underdoped high Tc superconductor

NASA Astrophysics Data System (ADS)

Riggs, Scott Chandler

High magnetic fields are used to kill superconductivity and probe what happens to system when it cannot reach the ideal ground state, i.e. what is the normal-state ground state? Early work in High-Tc, where the application of magnetic field destroyed the zero resistance state and recovered a resistivity value that connected continuously with the zero field curve, lead people to believe this magnetic-field-induced-state had fully driven the system normal, revealing the true underlying ground state, without any vestige of superconductivity. Many experiments done in this region of phase space have results interpreted as coming from the low energy ground state excitations. With the emergence of ultra-clean crystals in a unique family of hole doped high-Tc superconductors, YBa2Cu3O 7-delta, YBCO, a new and highly unexpected phenomena of quantum oscillations were discovered, and they followed the standard Liftshitz-Kosevich (LK) theory for a normal metal. The results suddenly made the problem of high-T c appear to be analogous to superconductivity in the organics, which is brought about by a wave-vector nesting and Fermi surface reconstruction. The only problem, it appeared, that needed to be reconciled was with Angle Resolved Photo-Emission Spectroscopy (ARPES) and Scanning Tunneling Microscopy (STM) data that claimed to see no such Fermi surface, instead only "arcs", a set of disconnected segments in the Brillouin zone which quasiparticle peaks are observed at the Fermi energy, which in a mean field description does not allow for a continuous Fermi surface contour. These two discrepancies led to the "arc vs pocket" debate, which is still unresolved. The other kink in the quantum oscillation armor is that, to this date, quantum oscillations in the hole-doped cuprates have only been seen in YBCO, the only cuprate structure to have CuO chains, which conduct and are located in between two CuO2 superconducting planes in the unit cell. In an attempt to reconcile the "arc vs pocket" debate we measure specific heat on an ultra-clean de-twinned single crystal of underdoped YBCO 6.56 with a Tc = 60 K, up to fields twice irreversibility field, define as the onset of the resistive transition. The zero temperature extrapolation of the electronic contribution to the specific heat, gamma, is the total quasiparticle density of states. For a two-dimensional system with parabolic energy bands, gamma is simply the sum of each pocket multiplied by its effective mass. Therefore, by determining gamma at high fields and using previously determined values for the effective mass from quantum oscillation transport measurements we can simply play a counting game to determine the number of pockets in the Fermi surface. Furthermore, at low fields the response to the specific heat as a function of magnetic field in a d-wave superconductor is known to have a (H) dependence, and we can look for deviations from this (H) , which are expected to happen when the system is no longer in a superconducting state. Results from our specific heat experiment shed new light on the true nature of the magnetic field induced "normal" state, and should force reinterpretation of many experimental findings. The specific heat measurements foremost show a smooth evolution of gamma from low to high magnetic fields which follows a Ac (H) dependence, with the prefactor, Ac giving the correct magnitude for the anisotropy of the d-wave superconducting gap. This means with the application of magnetic fields strong enough to restore the resistive state, the superconducting gap still exits. Additionally, we see quantum oscillations that follow conventional LK formalism and can determine an effective mass uniquely, where no fitting parameters are required. Interestingly, these oscillations fit on top of the (H) finding. How can the (H) and quantum oscillation whose phenomena arise from very different physics be reconciled? Looking at our own zero field gamma value of 1.85 mJ mol-1 K-2, which is intrinsic for YBCO, allows the pocket counting game to begin. Coupling bandstructure calculations, angle dependent quantum oscillation measurements, which determine the shape of the pocket, with the zero field gamma value leads to the simplest interpretation; quantum oscillatory phenomena is a manifestation of the CuO chain and BaO insulating layer orbital hybridization band and is likely not relevant to high temperature superconductivity.

Nuclear relaxation rate in layered superconductors with unconventional pairing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Maleyev, S.V.; Yashenkin, A.G.; Aristov, D.N.

1994-11-01

The cubic temperature dependence of the nuclear relaxation rate (NRR) in layered superconductors with the order parameter having zeros at the Fermi surface (FS) is found to be universal under quite general conditions. The coefficient in the quasi-Korringa term for the NRR appearing at low temperatures due to impurity scattering is estimated. It is shown that an anisotropy of the gap function over the FS leads to the disappearance of the Hebel-Slichter coherence peak close to [ital T][sub [ital c

2D massless Dirac Fermi gas model of superconductivity in the surface state of a topological insulator at high magnetic fields

NASA Astrophysics Data System (ADS)

Zhuravlev, Vladimir; Duan, Wenye; Maniv, Tsofar

2017-10-01

The Nambu-Gorkov Green's function approach is applied to strongly type-II superconductivity in a 2D spin-momentum-locked (Weyl) Fermi gas model at high perpendicular magnetic fields. The resulting phase diagram can be mapped onto that derived for the standard, parabolic band-structure model, having the same Fermi surface parameters, E F and v, but with cyclotron effective mass m\\ast=EF/2v2 . Significant deviations from the predicted mapping are found only for very small E F , when the Landau-Level filling factors are smaller than unity, and E F shrinks below the cutoff energy.

Time-dependent Models for Blazar Emission with the Second-order Fermi Acceleration

NASA Astrophysics Data System (ADS)

Asano, Katsuaki; Takahara, Fumio; Kusunose, Masaaki; Toma, Kenji; Kakuwa, Jun

2014-01-01

The second-order Fermi acceleration (Fermi-II) driven by turbulence may be responsible for the electron acceleration in blazar jets. We test this model with time-dependent simulations. The hard electron spectrum predicted by the Fermi-II process agrees with the hard photon spectrum of 1ES 1101-232. For other blazars that show softer spectra, the Fermi-II model requires radial evolution of the electron injection rate and/or diffusion coefficient in the outflow. Such evolutions can yield a curved electron spectrum, which can reproduce the synchrotron spectrum of Mrk 421 from the radio to the X-ray regime. The photon spectrum in the GeV energy range of Mrk 421 is hard to fit with a synchrotron self-Compton model. However, if we introduce an external radio photon field with a luminosity of 4.9 × 1038 erg s-1, GeV photons are successfully produced via inverse Compton scattering. The temporal variability of the diffusion coefficient or injection rate causes flare emission. The observed synchronicity of X-ray and TeV flares implies a decrease of the magnetic field in the flaring source region.

THE LUMINOSITY FUNCTION OF FERMI-DETECTED FLAT-SPECTRUM RADIO QUASARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ajello, M.; Shaw, M. S.; Romani, R. W.

2012-06-01

Fermi has provided the largest sample of {gamma}-ray-selected blazars to date. In this work we use a complete sample of flat spectrum radio quasars (FSRQs) detected during the first year of operation to determine the luminosity function (LF) and its evolution with cosmic time. The number density of FSRQs grows dramatically up to redshift {approx}0.5-2.0 and declines thereafter. The redshift of the peak in the density is luminosity dependent, with more luminous sources peaking at earlier times; thus the LF of {gamma}-ray FSRQs follows a luminosity-dependent density evolution similar to that of radio-quiet active galactic nuclei. Also, using data frommore » the Swift Burst Alert Telescope we derive the average spectral energy distribution (SED) of FSRQs in the 10 keV-300 GeV band and show that there is no correlation between the luminosity at the peak of the {gamma}-ray emission component and its peak frequency. Using this luminosity-independent SED with the derived LF allows us to predict that the contribution of FSRQs to the Fermi isotropic {gamma}-ray background is 9.3{sup +1.6}{sub -1.0}% ({+-}3% systematic uncertainty) in the 0.1-100 GeV band. Finally we determine the LF of unbeamed FSRQs, finding that FSRQs have an average Lorentz factor of {gamma} = 11.7{sup +3.3}{sub -2.2}, that most are seen within 5 Degree-Sign of the jet axis, and that they represent only {approx}0.1% of the parent population.« less

Superconductivity in the two-dimensional Hubbard model

NASA Astrophysics Data System (ADS)

Beenen, J.; Edwards, D. M.

1995-11-01

Quasiparticle bands of the two-dimensional Hubbard model are calculated using the Roth two-pole approximation to the one-particle Green's function. Excellent agreement is obtained with recent Monte Carlo calculations, including an anomalous volume of the Fermi surface near half-filling, which can possibly be explained in terms of a breakdown of Fermi liquid theory. The calculated bands are very flat around the (π,0) points of the Brillouin zone in agreement with photoemission measurements of cuprate superconductors. With doping there is a shift in spectral weight from the upper band to the lower band. The Roth method is extended to deal with superconductivity within a four-pole approximation allowing electron-hole mixing. It is shown that triplet p-wave pairing never occurs. A self-consistent solution with singlet dx2-y2-wave pairing is found and optimal doping occurs when the van Hove singularity, corresponding to the flat band part, lies at the Fermi level. Nearest-neighbor antiferromagnetic correlations play an important role in flattening the bands near the Fermi level and in favoring superconductivity. However, the mechanism for superconductivity is a local one, in contrast to spin-fluctuation exchange models. For reasonable values of the hopping parameter the transition temperature Tc is in the range 10-100 K. The optimum doping δc lies between 0.14 and 0.25, depending on the ratio U/t. The gap equation has a BCS-like form and 2Δmax/kTc~=4.

Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS 2

DOE PAGES

Friedemann, S.; Chang, H.; Gamża, M. B.; ...

2016-05-12

One early triumph of quantum physics is the explanation why some materials are metallic whereas others are insulating. While a treatment based on single electron states is correct for most materials this approach can fail spectacularly, when the electrostatic repulsion between electrons causes strong correlations. Not only can these favor new and subtle forms of matter, such as magnetism or superconductivity, they can even cause the electrons in a half-filled energy band to lock into position, producing a correlated, or Mott insulator. The transition into the Mott insulating state raises important fundamental questions. Foremost among these is the fate ofmore » the electronic Fermi surface and the associated charge carrier mass, as the Mott transition is approached. We report the first direct observation of the Fermi surface on the metallic side of a Mott insulating transition by high pressure quantum oscillatory measurements in NiS 2. We find our results point at a large Fermi surface consistent with Luttinger's theorem and a strongly enhanced quasiparticle effective mass. These two findings are in line with central tenets of the Brinkman-Rice picture of the correlated metal near the Mott insulating state and rule out alternative scenarios in which the carrier concentration vanishes continuously at the metal-insulator transition.« less

Dimensional Crossover and Its Interplay with In-Plane Anisotropy of Upper Critical Field in β-(BDA-TTP)2SbF6

NASA Astrophysics Data System (ADS)

Yasuzuka, Syuma; Koga, Hiroaki; Yamamura, Yasuhisa; Saito, Kazuya; Uji, Shinya; Terashima, Taichi; Akutsu, Hiroki; Yamada, Jun-ichi

2017-08-01

Resistance measurements have been performed to investigate the dimensionality and the in-plane anisotropy of the upper critical field (Hc2) for β-(BDA-TTP)2SbF6 in fields H up to 15 T and at temperatures T from 1.5 to 7.5 K, where BDA-TTP stands for 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene. The upper critical fields parallel and perpendicular to the conduction layer are determined and dimensional crossover from anisotropic three-dimensional behavior to two-dimensional behavior is found at around 6 K. When the direction of H is varied within the conducting layer at 6.0 K, Hc2 shows twofold symmetry: Hc2 along the minimum Fermi wave vector (maximum Fermi velocity) is larger than that along the maximum Fermi wave vector (minimum Fermi velocity). The normal-state magnetoresistance has twofold symmetry similar to Hc2 and shows a maximum when the magnetic field is nearly parallel to the maximum Fermi wave vector. This tendency is consistent with the Fermi surface anisotropy. At 3.5 K, we found clear fourfold symmetry of Hc2 despite the fact that the normal-state magnetoresistance shows twofold symmetry arising from the Fermi surface anisotropy. The origin of the fourfold symmetry of Hc2 is discussed in terms of the superconducting gap structure in β-(BDA-TTP)2SbF6.

Relativistic extended Thomas-Fermi calculations with exchange term contributions

NASA Astrophysics Data System (ADS)

Haddad, S.; Weigel, M. K.

1994-10-01

In this investigation we present self-consistent relativistic extended Thomas-Fermi (ETF) and extended Thomas-Fermi-Fock (ETFF) approaches, derived from the semiclassical treatment of the relativistic nuclear Hartree-Fock problem. The approximations are used to describe the ground-state properties of finite nuclei. The resulting equations are solved numerically for several one-boson-exchange (OBE) lagrangians. The results are discussed and compared with the outcome of full quantal Hartree and Hartree-Fock calculations, other semiclassical treatments and experimental data.

Thermoelectric Transport Signatures of Dirac Composite Fermions in the Half-Filled Landau Level

NASA Astrophysics Data System (ADS)

Potter, Andrew C.; Serbyn, Maksym; Vishwanath, Ashvin

2016-07-01

The half-filled Landau level is expected to be approximately particle-hole symmetric, which requires an extension of the Halperin-Lee-Read (HLR) theory of the compressible state observed at this filling. Recent work indicates that, when particle-hole symmetry is preserved, the composite fermions experience a quantized π -Berry phase upon winding around the composite Fermi surface, analogous to Dirac fermions at the surface of a 3D topological insulator. In contrast, the effective low-energy theory of the composite fermion liquid originally proposed by HLR lacks particle-hole symmetry and has vanishing Berry phase. In this paper, we explain how thermoelectric transport measurements can be used to test the Dirac nature of the composite fermions by quantitatively extracting this Berry phase. First, we point out that longitudinal thermopower (Seebeck effect) is nonvanishing because of the unusual nature of particle-hole symmetry in this context and is not sensitive to the Berry phase. In contrast, we find that off-diagonal thermopower (Nernst effect) is directly related to the topological structure of the composite Fermi surface, vanishing for zero Berry phase and taking its maximal value for π Berry phase. In contrast, in purely electrical transport signatures, the Berry phase contributions appear as small corrections to a large background signal, making the Nernst effect a promising diagnostic of the Dirac nature of composite fermions.

Prominent metallic surface conduction and the singular magnetic response of topological Dirac fermion in three-dimensional topological insulator Bi1.5Sb0.5Te1.7Se1.3.

PubMed

Dutta, Prithwish; Pariari, Arnab; Mandal, Prabhat

2017-07-07

We report semiconductor to metal-like crossover in the temperature dependence of resistivity (ρ) due to the switching of charge transport from bulk to surface channel in three-dimensional topological insulator Bi 1.5 Sb 0.5 Te 1.7 Se 1.3 . Unlike earlier studies, a much sharper drop in ρ(T) is observed below the crossover temperature due to the dominant surface conduction. Remarkably, the resistivity of the conducting surface channel follows a rarely observable T 2 dependence at low temperature, as predicted theoretically for a two-dimensional Fermi liquid system. The field dependence of magnetization shows a cusp-like paramagnetic peak in the susceptibility (χ) at zero field over the diamagnetic background. The peak is found to be robust against temperature and χ decays linearly with the field from its zero-field value. This unique behavior of the χ is associated with the spin-momentum locked topological surface state in Bi 1.5 Sb 0.5 Te 1.7 Se 1.3 . The reconstruction of the surface state with time is clearly reflected through the reduction of the peak height with the age of the sample.

Engineering and Probing Topological Properties of Dirac Semimetal Films by Asymmetric Charge Transfer.

PubMed

Villanova, John W; Barnes, Edwin; Park, Kyungwha

2017-02-08

Dirac semimetals (DSMs) have topologically robust three-dimensional Dirac (doubled Weyl) nodes with Fermi-arc states. In heterostructures involving DSMs, charge transfer occurs at the interfaces, which can be used to probe and control their bulk and surface topological properties through surface-bulk connectivity. Here we demonstrate that despite a band gap in DSM films, asymmetric charge transfer at the surface enables one to accurately identify locations of the Dirac-node projections from gapless band crossings and to examine and engineer properties of the topological Fermi-arc surface states connecting the projections, by simulating adatom-adsorbed DSM films using a first-principles method with an effective model. The positions of the Dirac-node projections are insensitive to charge transfer amount or slab thickness except for extremely thin films. By varying the amount of charge transfer, unique spin textures near the projections and a separation between the Fermi-arc states change, which can be observed by gating without adatoms.

Magnetoresistance of a nanostep junction based on topological insulators

NASA Astrophysics Data System (ADS)

Hu, Wei; Hong, Jin-Bin; Zhai, Feng

2018-06-01

We investigate ballistic transport of helical electrons in a three-dimensional topological insulator traversing a nanostep junction. We find that a magnetic field perpendicular to its side surface shrinks the phase space for transmission, leading to magnetoresistance for the Fermi energy close to the Dirac point of the top surface. We also find transmission resonances and suppression of the Fano factor due to Landau-level-related quasibound states. The transmission blockade in the off-resonance case can result in a huge magnetoresistance for Fermi energy higher than the Dirac point of the side surface.

Engineering frequency-dependent superfluidity in Bose-Fermi mixtures

NASA Astrophysics Data System (ADS)

Arzamasovs, Maksims; Liu, Bo

2018-04-01

Unconventional superconductivity and superfluidity are among the most exciting and fascinating quantum phenomena in condensed-matter physics. Usually such states are characterized by nontrivial spin or spatial symmetry of the pairing order parameter, such as "spin triplet" or "p wave." However, besides spin and spatial dependence the order parameter may have unconventional frequency dependence which is also permitted by Fermi-Dirac statistics. Odd-frequency fermionic pairing is an exciting paradigm when discussing exotic superfluidity or superconductivity and is yet to be realized in experiments. In this paper we propose a symmetry-based method of controlling frequency dependence of the pairing order parameter via manipulating the inversion symmetry of the system. First, a toy model is introduced to illustrate that frequency dependence of the order parameter can be achieved through our proposed approach. Second, by taking advantage of recent rapid developments in producing spin-orbit-coupled dispersions in ultracold gases, we propose a Bose-Fermi mixture to realize such frequency-dependent superfluid. The key idea is introducing the frequency-dependent attraction between fermions mediated by Bogoliubov phonons with asymmetric dispersion. Our proposal should pave an alternative way for exploring frequency-dependent superfluids with cold atoms.

Effective Mass Calculations for Two-dimensional Gas of Dipolar Fermions

NASA Astrophysics Data System (ADS)

Seydi, I.; Abedinpour, S. H.; Tanatar, B.

2017-06-01

We consider a two-dimensional system of ultracold dipolar fermions with dipole moments aligned in the perpendicular direction. We use the static structure factor information from Fermi-Hypernetted-Chain calculations to obtain the effective many-body dipole-dipole interaction and calculate the many-body effective mass of the system within the G0W approximation to the self-energy. A large cancellation between different contributions to the self-energy results in a weak dependence of the effective mass on the interaction strength over a large range of coupling constants.

Applications of Fermi-Lowdin-Orbital Self-Interaction Correction Scheme to Organic Systems

NASA Astrophysics Data System (ADS)

Baruah, Tunna; Kao, Der-You; Yamamoto, Yoh

Recent progress in treating the self-interaction errors by means of local, Lowdin-orthogonalized Fermi Orbitals offers a promising route to study the effect of self-interaction errors in the electronic structure of molecules. The Fermi orbitals depend on the location of the electronic positions, called as Fermi orbital descriptors. One advantage of using the Fermi orbitals is that the corrected Hamiltonian is unitarily invariant. Minimization of the corrected energies leads to an optimized set of centroid positions. Here we discuss the applications of this method to various systems from constituent atoms to several medium size molecules such as Mg-porphyrin, C60, pentacene etc. The applications to the ionic systems will also be discussed. De-SC0002168, NSF-DMR 125302.

Optical Response of Sr2RuO4 Reveals Universal Fermi-Liquid Scaling and Quasiparticles Beyond Landau Theory

NASA Astrophysics Data System (ADS)

Stricker, D.; Mravlje, J.; Berthod, C.; Fittipaldi, R.; Vecchione, A.; Georges, A.; van der Marel, D.

2014-08-01

We report optical measurements demonstrating that the low-energy relaxation rate (1/τ) of the conduction electrons in Sr2RuO4 obeys scaling relations for its frequency (ω) and temperature (T) dependence in accordance with Fermi-liquid theory. In the thermal relaxation regime, 1/τ∝(ℏω)2+(pπkBT)2 with p=2, and ω/T scaling applies. Many-body electronic structure calculations using dynamical mean-field theory confirm the low-energy Fermi-liquid scaling and provide quantitative understanding of the deviations from Fermi-liquid behavior at higher energy and temperature. The excess optical spectral weight in this regime provides evidence for strongly dispersing "resilient" quasiparticle excitations above the Fermi energy.

Valley density-wave (VDW) and Superconductivity in Iron-Pnictides

NASA Astrophysics Data System (ADS)

Cvetkovic, Vladimir; Tesanovic, Zlatko

2009-03-01

One of the experimentally observed features of iron-pnictide superconductors is the structural transition and SDW ordering occurring at almost the same temperature. Starting from a tight-binding model [1], we construct an effective theory for iron-pnictides with the distinctive two hole and two electron Fermi surfaces. This theory is then mapped onto a negative-U Hubbard model with additional orbital and spin flavors [2]. We demonstrate that the superconducting instability of the attractive Hubbard model --- valley density-wave (VDW) --- corresponds to the observed structural and SDW orders. The deviations from perfect nesting between the hole and electron Fermi surfaces are mapped onto the Zeeman field which causes portions of Fermi surface to remain ungapped. The origin of pnictide superconductivity in this model, and its ties to the VDW are discussed. [1] V. Cvetkovic and Z. Tesanovic, http://arxiv.org/abs/0804.4678. [2] V. Cvetkovic and Z. Tesanovic, http://arxiv.org/abs/0808.3742.

Superconductivity in YTE2Ge2 compounds (TE = d-electron transition metal)

NASA Astrophysics Data System (ADS)

Chajewski, G.; Samsel-Czekała, M.; Hackemer, A.; Wiśniewski, P.; Pikul, A. P.; Kaczorowski, D.

2018-05-01

Polycrystalline samples of YTE2Ge2 with TE = Co, Ni, Ru, Rh, Pd and Pt were synthesized and characterized by means of X-ray powder diffraction and low-temperature electrical resistivity and specific heat measurements, supplemented by fully relativistic full-potential local-orbital band structure calculations. We confirm that most of the compounds studied crystallize in a body-centered tetragonal ThCr2S2 -type structure (space group I 4 / mmm) and have three-dimensional Fermi surfaces, while only one of them (YPt2Ge2) forms with a primitive tetragonal CaBe2Ge2 -type unit cell (space group P 4 / nmm) and possesses quasi-two-dimensional Fermi surface sheets with some nesting. Physical properties data show conventional superconductivity in the phases with TE = Co, Pd and Pt, i.e. independently of the structure type (and hence the dimensionality of the Fermi surface).

Non-Fermi surface nesting driven commensurate magnetic ordering in Fe-doped S r 2 Ru O 4

DOE PAGES

Zhu, M.; Shanavas, K. V.; Wang, Y.; ...

2017-02-10

Sr 2RuO 4, an unconventional superconductor, is known to possess an incommensurate spin-density wave instability driven by Fermi surface nesting. Here we report a static spin-density wave ordering with a commensurate propagation vector q c = (0.250.250) in Fe-doped Sr 2RuO 4, despite the magnetic fluctuations persisting at the incommensurate wave vectors q ic = (0.30.3L) as in the parent compound. The latter feature is corroborated by the first-principles calculations, which show that Fe substitution barely changes the nesting vector of the Fermi surface. Finally, these results suggest that in addition to the known incommensurate magnetic instability, Sr 2RuO 4more » is also in proximity to a commensurate magnetic tendency that can be stabilized via Fe doping.« less

High-resolution angle-resolved photoemission study of electronic structure and charge-density wave formation in HoTe3

NASA Astrophysics Data System (ADS)

Liu, Guodong; Wang, Chenlu; Zhang, Yan; Hu, Bingfeng; Mou, Daixiang; Yu, Li; Zhao, Lin; Zhou, Xingjiang; Wang, Nanlin; Chen, Chuangtian; Xu, Zuyan

We performed high-resolution angle-resolved photoemission spectroscopy (ARPES) measurement on high quality crystal of HoTe3, an intriguing quasi-two-dimensional rare-earth-element tritelluride charge-density-wave (CDW) compound. The main features of the electronic structure in this compound are established by employing a quasi-CW laser (7eV) and a helium discharging lamp (21.22 eV) as excitation light sources. It reveals many bands back folded according to the CDW periodicity and two incommensurate CDW gaps created by perpendicular Fermi surface (FS) nesting vectors. A large gap is found to open in well nested regions of the Fermi surface sheets, whereas other Fermi surface sections with poor nesting remain ungapped. In particular, some peculiar features are identified by using our ultra-high resolution and bulk sensitive laser-ARPES.

First-Principles Study of the Band Diagrams and Schottky-Type Barrier Heights of Aqueous Ta3N5 Interfaces.

PubMed

Watanabe, Eriko; Ushiyama, Hiroshi; Yamashita, Koichi

2017-03-22

The photo(electro)chemical production of hydrogen by water splitting is an efficient and sustainable method for the utilization of solar energy. To improve photo(electro)catalytic activity, a Schottky-type barrier is typically useful to separate excited charge carriers in semiconductor electrodes. Here, we focused on studying the band diagrams and the Schottky-type barrier heights of Ta 3 N 5 , which is one of the most promising materials as a photoanode for water splitting. The band alignments of the undoped and n-type Ta 3 N 5 with adsorbents in a vacuum were examined to determine how impurities and adsorbents affect the band positions and Fermi energies. The band edge positions as well as the density of surface states clearly depended on the density of O N impurities in the bulk and surface regions. Finally, the band diagrams of the n-type Ta 3 N 5 /water interfaces were calculated with an improved interfacial model to include the effect of electrode potential with explicit water molecules. We observed partial Fermi level pinning in our calculations at the Ta 3 N 5 /water interface, which affects the driving force for charge separation.

Theory of nodal s ±-wave pairing symmetry in the Pu-based 115 superconductor family

DOE PAGES

Das, Tanmoy; Zhu, Jian -Xin; Graf, Matthias J.

2015-02-27

The spin-fluctuation mechanism of superconductivity usually results in the presence of gapless or nodal quasiparticle states in the excitation spectrum. Nodal quasiparticle states are well established in copper-oxide, and heavy-fermion superconductors, but not in iron-based superconductors. Here, we study the pairing symmetry and mechanism of a new class of plutonium-based high-T c superconductors and predict the presence of a nodal s⁺⁻ wave pairing symmetry in this family. Starting from a density-functional theory (DFT) based electronic structure calculation we predict several three-dimensional (3D) Fermi surfaces in this 115 superconductor family. We identify the dominant Fermi surface “hot-spots” in the inter-band scatteringmore » channel, which are aligned along the wavevector Q = (π, π, π), where degeneracy could induce sign-reversal of the pairing symmetry. Our calculation demonstrates that the s⁺⁻ wave pairing strength is stronger than the previously thought d-wave pairing; and more importantly, this pairing state allows for the existence of nodal quasiparticles. Finally, we predict the shape of the momentum- and energy-dependent magnetic resonance spectrum for the identification of this pairing symmetry.« less

Probing the Fermi surface and magnetotransport properties of MoAs2

NASA Astrophysics Data System (ADS)

Singha, Ratnadwip; Pariari, Arnab; Gupta, Gaurav Kumar; Das, Tanmoy; Mandal, Prabhat

2018-04-01

Transition-metal dipnictides (TMDs) have recently been identified as possible candidates to host a topology-protected electronic band structure. These materials belong to an isostructural family and show several exotic transport properties. Especially, the large values of magnetoresistance (MR) and carrier mobility have drawn significant attention from the perspective of technological applications. In this paper, we investigate the magnetotransport and Fermi surface properties of single-crystalline MoAs2, another member of this group of compounds. A field-induced resistivity plateau and a large MR have been observed, which are comparable to those in several topological systems. Interestingly, in contrast to other isostructural materials, the carrier density in MoAs2 is quite high and shows single-band-dominated transport. The Fermi pockets, which have been identified from the quantum oscillation, are the largest among the members of this group and have significant anisotropy with crystallographic direction. Our first-principles calculations reveal a substantial difference between the band structures of MoAs2 and that of other TMDs. The calculated Fermi surface consists of one electron pocket and another "open-orbit" hole pocket, which has not been observed in TMDs so far.

On the important role of the anti-Jahn-Teller effect in underdoped cuprate superconductors

NASA Astrophysics Data System (ADS)

Kamimura, Hiroshi; Matsuno, Shunichi; Mizokawa, Takashi; Sasaoka, Kenji; Shiraishi, Kenji; Ushio, Hideki

2013-04-01

In this paper it is shown that the "anti-Jahn-Teller effect" plays an essential role in giving rise to a small Fermi surface of Fermi pockets above Tc and d-wave superconductivity below Tc in underdoped cuprates. In the first part of the present paper, we review the latest developments of the model proposed by Kamimura and Suwa, which bears important characteristics born from the interplay of Jahn-Teller Physics and Mott Physics. It is shown that the feature of Fermi surfaces in underdoped LSCO is the Fermi pockets in the nodal region constructed by doped holes under the coexistence of a metallic state and of the local antiferromagnetic order. In the antinodal region in the momentum space, there are no Fermi surfaces. Then it is discussed that the phonon-involved mechanism based on the Kamimura-Suwa model leads to the d-wave superconductivity. In particular, it is shown that the origin of strong electron-phonon interactions in cuprates is due to the anti-Jahn-Teller effect. In the second part a recent theoretical result on the energy distribution curves (EDCs) of angle-resolved photoemission spectroscopy (ARPES) below Tc is discussed. It is shown that the feature of ARPES profiles of underdoped cuprates consists of a coherent peak in the nodal region and the real transitions of photoexcited electrons from occupied states below the Fermi level to a free-electron state above the vacuum level in the antinodal region, where the latter transitions form a broad hump. From this feature, the origin of the two distinct gaps observed by ARPES is elucidated without introducing the concept of the pseudogap. Finally, a remark is made on the phase diagram of underdoped cuprates.

Realistic estimation for the detectability of dark matter subhalos using Fermi-LAT catalogs

NASA Astrophysics Data System (ADS)

Calore, Francesca; De Romeri, Valentina; Di Mauro, Mattia; Donato, Fiorenza; Marinacci, Federico

2017-09-01

Numerical simulations of structure formation have made remarkable progress in recent years, in particular due to the inclusion of baryonic physics evolving with the dark matter component. We generate Monte Carlo realizations of the dark matter subhalo population based on the results of the recent hydrodynamical simulation suite of Milky Way-sized galaxies [F. Marinacci, R. Pakmor, and V. Springel, Mon. Not. R. Astron. Soc. 437, 1750 (2014)., 10.1093/mnras/stt2003]. We then simulate the gamma-ray sky for both the setup of the 3FGL and 2FHL Fermi Large Area Telescope (LAT) catalogs, including the contribution from the annihilation of dark matter in the subhalos. We find that the flux sensitivity threshold strongly depends on the particle dark matter mass and, more mildly, also on its annihilation channel and the observation latitude. The results differ for the 3FGL and 2FHL catalogs, given their different energy thresholds. We also predict that the number of dark matter subhalos among the unassociated sources is very small. A null number of detectable subhalos in the Fermi-LAT 3FGL catalog would imply upper limits on the dark matter annihilation cross section into b b ¯ of 2 ×10-26(5 ×10-25) cm3 /s with MDM=50 (1000 ) GeV . We find less than one extended subhalo in the Fermi-LAT 3FGL catalog. As a matter of fact, the differences in the spatial and mass distribution of subhalos between hydrodynamic and dark matter-only runs do not have significant impact on the detectability of dark subhalos in gamma rays.

THE MECHANISMS OF ELECTRON ACCELERATION DURING MULTIPLE X LINE MAGNETIC RECONNECTION WITH A GUIDE FIELD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Huanyu; Lu, Quanming; Huang, Can

2016-04-20

The interactions between magnetic islands are considered to play an important role in electron acceleration during magnetic reconnection. In this paper, two-dimensional particle-in-cell simulations are performed to study electron acceleration during multiple X line reconnection with a guide field. Because the electrons remain almost magnetized, we can analyze the contributions of the parallel electric field, Fermi, and betatron mechanisms to electron acceleration during the evolution of magnetic reconnection through comparison with a guide-center theory. The results show that with the magnetic reconnection proceeding, two magnetic islands are formed in the simulation domain. Next, the electrons are accelerated by both themore » parallel electric field in the vicinity of the X lines and the Fermi mechanism due to the contraction of the two magnetic islands. Then, the two magnetic islands begin to merge into one, and, in such a process, the electrons can be accelerated by both the parallel electric field and betatron mechanisms. During the betatron acceleration, the electrons are locally accelerated in the regions where the magnetic field is piled up by the high-speed flow from the X line. At last, when the coalescence of the two islands into one big island finishes, the electrons can be further accelerated by the Fermi mechanism because of the contraction of the big island. With the increase of the guide field, the contributions of the Fermi and betatron mechanisms to electron acceleration become less and less important. When the guide field is sufficiently large, the contributions of the Fermi and betatron mechanisms are almost negligible.« less

Generalized susceptibilities and Landau parameters for anisotropic Fermi liquids

NASA Astrophysics Data System (ADS)

Rodríguez-Ponte, P.; Cabra, D.; Grandi, N.

2015-05-01

We study Fermi liquids (FLs) with a Fermi surface that lacks continuous rotational invariance and in the presence of an arbitrary quartic interaction. We obtain the expressions of the generalized static susceptibilities that measure the linear response of a generic order parameter to a perturbation of the Hamiltonian. We apply our formulae to the spin and charge susceptibilities. Based on the resulting expressions, we make a proposal for the definition of the Landau parameters in nonisotropic FL.

Quantum Size Effects in Transport Properties of Bi2Te3 Topological Insulator Thin Films

NASA Astrophysics Data System (ADS)

Rogacheva, E. I.; Budnik, A. V.; Nashchekina, O. N.; Meriuts, A. V.; Dresselhaus, M. S.

2017-07-01

Bi2Te3 compound and Bi2Te3-based solid solutions have attracted much attention as promising thermoelectric materials for refrigerating devices. The possibility of enhancing the thermoelectric efficiency in low-dimensional structures has stimulated studies of Bi2Te3 thin films. Now, interest in studying the transport properties of Bi2Te3 has grown sharply due to the observation of special properties characteristic of three-dimensional (3D) topological insulators in Bi2Te3. One of the possible manifestations of quantum size effects in two-dimensional structures is an oscillatory behavior of the dependences of transport properties on film thickness, d. The goal of this work is to summarize our earlier experimental results on the d-dependences of transport properties of Bi2Te3 thin films obtained by thermal evaporation in a vacuum on glass substrates, and to present our new results of theoretical calculations of the oscillations periods within the framework of the model of an infinitely deep potential well, which takes into account the dependence of the Fermi energy on d and the contribution of all energy subbands below the Fermi level to the conductivity. On the basis of the data obtained, some general regularities and specificity of the quantum size effects manifestation in 3D topological insulators are established.

Fermi edge singularities in the mesoscopic regime: Photoabsorption spectra

NASA Astrophysics Data System (ADS)

Hentschel, Martina; Ullmo, Denis; Baranger, Harold U.

2007-12-01

We study Fermi edge singularities in photoabsorption spectra of generic mesoscopic systems such as quantum dots or nanoparticles. We predict deviations from macroscopic-metallic behavior and propose experimental setups for the observation of these effects. The theory is based on the model of a localized, or rank one, perturbation caused by the (core) hole left behind after the photoexcitation of an electron into the conduction band. The photoabsorption spectra result from the competition between two many-body responses, Anderson’s orthogonality catastrophe and the Mahan-Nozières-DeDominicis contribution. Both mechanisms depend on the system size through the number of particles and, more importantly, fluctuations produced by the coherence characteristic of mesoscopic samples. The latter lead to a modification of the dipole matrix element and trigger one of our key results: a rounded K -edge typically found in metals will turn into a (slightly) peaked edge on average in the mesoscopic regime. We consider in detail the effect of the “bound state” produced by the core hole.

SmB6 electron-phonon coupling constant from time- and angle-resolved photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Sterzi, A.; Crepaldi, A.; Cilento, F.; Manzoni, G.; Frantzeskakis, E.; Zacchigna, M.; van Heumen, E.; Huang, Y. K.; Golden, M. S.; Parmigiani, F.

2016-08-01

SmB6 is a mixed valence Kondo system resulting from the hybridization between localized f electrons and delocalized d electrons. We have investigated its out-of-equilibrium electron dynamics by means of time- and angle-resolved photoelectron spectroscopy. The transient electronic population above the Fermi level can be described by a time-dependent Fermi-Dirac distribution. By solving a two-temperature model that well reproduces the relaxation dynamics of the effective electronic temperature, we estimate the electron-phonon coupling constant λ to range from 0.13 ±0.03 to 0.04 ±0.01 . These extremes are obtained assuming a coupling of the electrons with either a phonon mode at 10 or 19 meV. A realistic value of the average phonon energy will give an actual value of λ within this range. Our results provide an experimental report on the material electron-phonon coupling, contributing to both the electronic transport and the macroscopic thermodynamic properties of SmB6.

Electronic structure basis for the extraordinary magnetoresistance in WTe 2

DOE PAGES

Pletikosić, I.; Ali, Mazhar N.; Fedorov, A. V.; ...

2014-11-19

The electronic structure basis of the extremely large magnetoresistance in layered non-magnetic tungsten ditelluride has been investigated by angle-resolved photoelectron spectroscopy. Hole and electron pockets of approximately the same size were found at the Fermi level, suggesting that carrier compensation should be considered the primary source of the effect. The material exhibits a highly anisotropic, quasi one-dimensional Fermi surface from which the pronounced anisotropy of the magnetoresistance follows. As a result, a change in the Fermi surface with temperature was found and a high-density-of-states band that may take over conduction at higher temperatures and cause the observed turn-on behavior ofmore » the magnetoresistance in WTe₂ was identified.« less

Large Diamagnetic Susceptibility from Petit Fermi Surfaces in LaV2Al20

NASA Astrophysics Data System (ADS)

Hirose, Takahiro; Okamoto, Yoshihiko; Yamaura, Jun-ichi; Hiroi, Zenji

2015-11-01

The large diamagnetic susceptibility of LaV2Al20 is studied by magnetization and de Haas-van Alphen (dHvA) oscillation measurements on single crystals as well as by Ti-for-V substitution (hole doping) experiments. Its origin is ascribed to a tiny holelike Fermi surface (FS) with a low Fermi temperature of 140 K and a small dHvA frequency of 19 T. The FS has a characteristic anisotropy that is approximated by six spheroidal hole pockets elongated along the cubic <001> directions with a minimum effective mass of 0.067 times the free electron mass. This characteristic FS can generate an unusually large Landau-Peierls diamagnetic susceptibility as observed experimentally in LaV2Al20.

Thermoelasticity and anomalies in the pressure dependence of phonon velocities in niobium

NASA Astrophysics Data System (ADS)

Zou, Yongtao; Li, Ying; Chen, Haiyan; Welch, David; Zhao, Yusheng; Li, Baosheng

2018-01-01

Compressional and shear wave velocities of polycrystalline niobium have been measured at simultaneously high pressures and temperatures up to 5.8 GPa and 1073 K, respectively, using ultrasonic interferometry in conjunction with synchrotron x-ray techniques. An anomalous pressure-induced softening behavior in the phonon velocities, probably owing to the topological change in the Fermi surface, has been observed at ˜4.8 GPa during cold compression, which is supported by the elasticity data from our first-principles calculations. In contrast, both the bulk (BS) and shear (G) moduli increase with pressures but decrease with temperatures upon compression at extreme P-T up to 5.8 GPa and 1073 K. Using finite strain equation-of-state approaches, the elasticity of bulk and shear moduli and their pressure and temperature dependences are derived from the directly measured velocities and densities, yielding BS0 = 174.9(3.2) GPa, G0 = 37.1(3) GPa, ∂BS/∂P = 3.97(9), ∂G/∂P = 0.83(5), ∂BS/∂T = -0.064(7) GPa/K, and ∂G/∂T = -0.012(3) GPa/K. On the basis of the current thermoelasticity data, Debye temperature and the high-pressure melting curve of Nb are derived. The origin of the anomalies in shear behavior at high pressure might be attributed to the progressive s-d electron-transfer-induced topological changes of the Fermi surface upon compression.

Extended Thomas-Fermi density functional for the unitary Fermi gas

NASA Astrophysics Data System (ADS)

Salasnich, Luca; Toigo, Flavio

2008-11-01

We determine the energy density ξ(3/5)nɛF and the gradient correction λℏ2(∇n)2/(8mn) of the extended Thomas-Fermi (ETF) density functional, where n is the number density and ɛF is the Fermi energy, for a trapped two-component Fermi gas with infinite scattering length (unitary Fermi gas) on the basis of recent diffusion Monte Carlo (DMC) calculations [Phys. Rev. Lett. 99, 233201 (2007)]. In particular we find that ξ=0.455 and λ=0.13 give the best fit of the DMC data with an even number N of particles. We also study the odd-even splitting γN1/9ℏω of the ground-state energy for the unitary gas in a harmonic trap of frequency ω determining the constant γ . Finally we investigate the effect of the gradient term in the time-dependent ETF model by introducing generalized Galilei-invariant hydrodynamics equations.

Seebeck effect on a weak link between Fermi and non-Fermi liquids

NASA Astrophysics Data System (ADS)

Nguyen, T. K. T.; Kiselev, M. N.

2018-02-01

We propose a model describing Seebeck effect on a weak link between two quantum systems with fine-tunable ground states of Fermi and non-Fermi liquid origin. The experimental realization of the model can be achieved by utilizing the quantum devices operating in the integer quantum Hall regime [Z. Iftikhar et al., Nature (London) 526, 233 (2015), 10.1038/nature15384] designed for detection of macroscopic quantum charged states in multichannel Kondo systems. We present a theory of thermoelectric transport through hybrid quantum devices constructed from quantum-dot-quantum-point-contact building blocks. We discuss pronounced effects in the temperature and gate voltage dependence of thermoelectric power associated with a competition between Fermi and non-Fermi liquid behaviors. High controllability of the device allows to fine tune the system to different regimes described by multichannel and multi-impurity Kondo models.

Electronic characteristics of Tl 2Ba 2CuO 6. Fermi surface, positron wavefunction, electric field gradients, and transport parameters

NASA Astrophysics Data System (ADS)

Singh, David J.; Pickett, Warren E.

1992-12-01

A number of properties identifiable from the electronic bands and one-electron wavefunctions have been obtained from a well converged self-consistent calculation of the electronic structure of Tl 2Ba 2CuO 6. The Fermi surface is found to consist of two sheets: a two-dimensional barrel surface arising from the CuO 2 layer, and a three-dimensional spheroid arising from states with strong TlO character but actually extending throughout all layers of the structure. This feature has important implications for the transport properties, and especially for the degree of anisotropy. We compare with transport data on single crystals of Tl 2Ba 2CuO 6. The calculated Fermi surface of the spheroid is found to be in substantial agreement with the measured period of magnetization oscillations in the de Haas-van Alphen effect by Kido et al. The positron wavefunction engulfs the CuO 2 layers, making this material a promising case for mapping out with positron 2D-ACAR the layer-derived Fermi surface that is believed to be central to high-temperature superconductivity. The electric field gradients are predicted and compared with calculations for other cuprates. The Hall coefficient RHxyz (carrier motion on the a-b plane) is found to be positive and within a factor of 1.5 of that measured on ceramic samples, while the other non-vanishing component of the Hall tensor is predicted to be negative.

Effets Seebeck et Nernst dans les cuprates: Etude de la reconstruction de la surface de Fermi sous champ magnetique intense

NASA Astrophysics Data System (ADS)

Laliberte, Francis

2010-06-01

Ce memoire presente des mesures de transport thermoelectrique, les effets Seebeck et Nernst, dans une serie d'echantillons de supraconducteurs a haute temperature critique. Des resultats obtenus recemment au Laboratoire National des Champs Magnetiques Intenses a Grenoble sur La1.7Eu0.2Sr0.1 CuO4, La1.675Eu0.2Sr0.125CuO 4, La1.64Eu0.2Sr0.16CuO4, La1.74Eu0.1Sr0.16CuO4 et La 1.4Nd0.4Sr0.2CuO4 sont analyses. Une attention particuliere est accordee aux equations de la theorie semi-classique du transport et leur validite est verifiee. La procedure experimentale et les materiaux utilises pour concevoir les montages de mesures sont expliques en detail. Enfin, un chapitre est dedie a l'explication et l'interpretation des resultats de transport thermoelectrique sur YBa2Cu3O6+delta publies au cours de l'hiver 2010 dans les revues Nature et Physical Review Letters. Les donnees d'effet Seebeck dans les echantillons de La 1.8-x,Eu0.2SrxCuO 4, ou un changement de signe est observe, permettent de conclure a la presence d'une poche d'electrons dans la surface de Fermi qui domine le transport a basse temperature dans la region sous-dopee du diagramme de phase. Cette conclusion est similaire a celle obtenue par des mesures d'effet Hall dans YBa 2Cu3O6+delta et elle cadre bien dans un scenario de reconstruction de la surface de Fermi. Les donnees d'effet Nernst recueillies indiquent que la contribution des fluctuations supraconductrices est limitee a un modeste intervalle de temperature au-dessus de la temperature critique.

Stable Weyl points, trivial surface states, and particle-hole compensation in WP2

NASA Astrophysics Data System (ADS)

Razzoli, E.; Zwartsenberg, B.; Michiardi, M.; Boschini, F.; Day, R. P.; Elfimov, I. S.; Denlinger, J. D.; Süss, V.; Felser, C.; Damascelli, A.

2018-05-01

A possible connection between extremely large magnetoresistance and the presence of Weyl points has garnered much attention in the study of topological semimetals. Exploration of these concepts in transition-metal diphosphides WP2 has been complicated by conflicting experimental reports. Here we combine angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to disentangle surface and bulk contributions to the ARPES intensity, the superposition of which has plagued the determination of the band structure in WP2. Our results show that while the hole- and electronlike Fermi surface sheets originating from surface states have different areas, the bulk-band structure of WP2 is electron-hole compensated in agreement with DFT. Furthermore, the ARPES band structure is compatible with the presence of at least four temperature-independent Weyl points, confirming the topological nature of WP2 and its stability against lattice distortions.

Fermiology and Superconductivity of Topological Surface States in PdTe2

NASA Astrophysics Data System (ADS)

Clark, O. J.; Neat, M. J.; Okawa, K.; Bawden, L.; Marković, I.; Mazzola, F.; Feng, J.; Sunko, V.; Riley, J. M.; Meevasana, W.; Fujii, J.; Vobornik, I.; Kim, T. K.; Hoesch, M.; Sasagawa, T.; Wahl, P.; Bahramy, M. S.; King, P. D. C.

2018-04-01

We study the low-energy surface electronic structure of the transition-metal dichalcogenide superconductor PdTe2 by spin- and angle-resolved photoemission, scanning tunneling microscopy, and density-functional theory-based supercell calculations. Comparing PdTe2 with its sister compound PtSe2 , we demonstrate how enhanced interlayer hopping in the Te-based material drives a band inversion within the antibonding p -orbital manifold well above the Fermi level. We show how this mediates spin-polarized topological surface states which form rich multivalley Fermi surfaces with complex spin textures. Scanning tunneling spectroscopy reveals type-II superconductivity at the surface, and moreover shows no evidence for an unconventional component of its superconducting order parameter, despite the presence of topological surface states.

Electronic structures of U X3 (X =Al , Ga, and In) studied by photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Fujimori, Shin-ichi; Kobata, Masaaki; Takeda, Yukiharu; Okane, Tetsuo; Saitoh, Yuji; Fujimori, Atsushi; Yamagami, Hiroshi; Haga, Yoshinori; Yamamoto, Etsuji; Ōnuki, Yoshichika

2017-09-01

The electronic structures of U X3 (X =Al , Ga , and In ) were studied by photoelectron spectroscopy to understand the relationship between their electronic structures and magnetic properties. The band structures and Fermi surfaces of UAl3 and UGa3 were revealed experimentally by angle-resolved photoelectron spectroscopy (ARPES), and they were compared with the result of band-structure calculations. The topologies of the Fermi surfaces and the band structures of UAl3 and UGa3 were explained reasonably well by the calculation, although bands near the Fermi level (EF) were renormalized owing to the finite electron correlation effect. The topologies of the Fermi surfaces of UAl3 and UGa3 are very similar to each other, except for some minor differences. Such minor differences in their Fermi surface or electron correlation effect might take an essential role in their different magnetic properties. No significant changes were observed between the ARPES spectra of UGa3 in the paramagnetic and antiferromagnetic phases, suggesting that UGa3 is an itinerant weak antiferromagnet. The effect of chemical pressure on the electronic structures of U X3 compounds was also studied by utilizing the smaller lattice constants of UAl3 and UGa3 than that of UIn3. The valence band spectrum of UIn3 is accompanied by a satellitelike structure on the high-binding-energy side. The core-level spectrum of UIn3 is also qualitatively different from those of UAl3 and UGa3. These findings suggest that the U 5 f states in UIn3 are more localized than those in UAl3 and UGa3.

Spectroscopic views of high-Tc superconductors

NASA Astrophysics Data System (ADS)

Wendin, Göran

1989-01-01

Recent progress in the fields of photoelectron spectroscopy, electron energy loss spectroscopy, inverse photoemission, and infrared- and optical reflectivity applied to high-Tc superconductors is analyzed in terms of correlation effects, transport properties and Fermi liquid behaviour. For the CuO2 based materials, a picture emerges of localized holes in copper 3d levels and itinerant holes in oxygen 2p-like bands. A Fermi liquid picture and a superconducting gap is indicated by angle-resolved photo-emission, infrared absorption, and NMR. A Fermi surface is indicated by positron annihilation. Infrared absorption reveals strongly frequency and temperature dependent scattering and polaronic behaviour for frequencies below 0.1 eV. Infrared absorption indicates a maximum superconducting gap of 2Δ/kBTc = 8 and suggests that ordinary samples may show a range of gaps 2 < 2Δ/kBTc < 8 resulting in commonly measured average values of 2Δ/kBTc = 5. An interesting possibility in YBaCuO, suggested by infrared reflectivity and photoconductivity measurements, is that polarons in the CuO2 planes with 0.13 eV excitation energy mediate an attractive interaction between quasi-holes in O 2p-derived conduction bands. The polarons will involve important lattice distortions even if, as is frequently assumed, magnetic polaron effects may be the essential thing.

Ultrafast strong-field photoelectron emission from biased metal surfaces: exact solution to time-dependent Schrödinger Equation

PubMed Central

Zhang, Peng; Lau, Y. Y.

2016-01-01

Laser-driven ultrafast electron emission offers the possibility of manipulation and control of coherent electron motion in ultrashort spatiotemporal scales. Here, an analytical solution is constructed for the highly nonlinear electron emission from a dc biased metal surface illuminated by a single frequency laser, by solving the time-dependent Schrödinger equation exactly. The solution is valid for arbitrary combinations of dc electric field, laser electric field, laser frequency, metal work function and Fermi level. Various emission mechanisms, such as multiphoton absorption or emission, optical or dc field emission, are all included in this single formulation. The transition between different emission processes is analyzed in detail. The time-dependent emission current reveals that intense current modulation may be possible even with a low intensity laser, by merely increasing the applied dc bias. The results provide insights into the electron pulse generation and manipulation for many novel applications based on ultrafast laser-induced electron emission. PMID:26818710

Magnetotransport in Dirac metals: Chiral magnetic effect and quantum oscillations

DOE PAGES

Monteiro, Gustavo M.; Abanov, Alexander G.; Kharzeev, Dmitri E.

2015-10-08

Dirac metals are characterized by the linear dispersion of fermionic quasiparticles, with the Dirac point hidden inside a Fermi surface. We study the magnetotransport in these materials using chiral kinetic theory to describe within the same framework both the negative magnetoresistance caused by the chiral magnetic effect and quantum oscillations in the magnetoresistance due to the existence of the Fermi surface. Lastly, we discuss the relevance of obtained results to recent measurements on Cd 3As 2.

Topological surface Fermi arcs in the magnetic Weyl semimetal Co3Sn2S2

NASA Astrophysics Data System (ADS)

Xu, Qiunan; Liu, Enke; Shi, Wujun; Muechler, Lukas; Gayles, Jacob; Felser, Claudia; Sun, Yan

2018-06-01

Very recently, the half-metallic compound Co3Sn2S2 was proposed to be a magnetic Weyl semimetal (WSM) with Weyl points only 60 meV above the Fermi level EF. Owing to the low charge carrier density and large Berry curvature induced, Co3Sn2S2 possesses both a large anomalous Hall conductivity and a large anomalous Hall angle, which provide strong evidence for the existence of Weyl points in Co3Sn2S2 . In this work, we theoretically study the surface topological feature of Co3Sn2S2 and its counterpart Co3Sn2Se2 . By cleaving the sample at the weak Sn-S/Se bonds, one can achieve two different surfaces terminated with Sn and S/Se atoms, respectively. The resulting Fermi-arc-related states can range from the energy of the Weyl points to EF-0.1 eV in the Sn-terminated surface. Therefore, it should be possible to observe the Fermi arcs in angle-resolved photoemission spectroscopy (ARPES) measurements. Furthermore, in order to simulate quasiparticle interference in scanning tunneling microscopy (STM) measurements, we also calculate the joint density of states for both terminals. This work should be helpful for a comprehensive understanding of the topological properties of these two magnetic WSMs and further ARPES and STM measurements.

Time-dependent models for blazar emission with the second-order Fermi acceleration

DOE Office of Scientific and Technical Information (OSTI.GOV)

Asano, Katsuaki; Takahara, Fumio; Toma, Kenji

The second-order Fermi acceleration (Fermi-II) driven by turbulence may be responsible for the electron acceleration in blazar jets. We test this model with time-dependent simulations. The hard electron spectrum predicted by the Fermi-II process agrees with the hard photon spectrum of 1ES 1101–232. For other blazars that show softer spectra, the Fermi-II model requires radial evolution of the electron injection rate and/or diffusion coefficient in the outflow. Such evolutions can yield a curved electron spectrum, which can reproduce the synchrotron spectrum of Mrk 421 from the radio to the X-ray regime. The photon spectrum in the GeV energy range ofmore » Mrk 421 is hard to fit with a synchrotron self-Compton model. However, if we introduce an external radio photon field with a luminosity of 4.9 × 10{sup 38} erg s{sup –1}, GeV photons are successfully produced via inverse Compton scattering. The temporal variability of the diffusion coefficient or injection rate causes flare emission. The observed synchronicity of X-ray and TeV flares implies a decrease of the magnetic field in the flaring source region.« less

A position-dependent mass model for the Thomas–Fermi potential: Exact solvability and relation to δ-doped semiconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schulze-Halberg, Axel, E-mail: xbataxel@gmail.com; García-Ravelo, Jesús; Pacheco-García, Christian

We consider the Schrödinger equation in the Thomas–Fermi field, a model that has been used for describing electron systems in δ-doped semiconductors. It is shown that the problem becomes exactly-solvable if a particular effective (position-dependent) mass distribution is incorporated. Orthogonal sets of normalizable bound state solutions are constructed in explicit form, and the associated energies are determined. We compare our results with the corresponding findings on the constant-mass problem discussed by Ioriatti (1990) [13]. -- Highlights: ► We introduce an exactly solvable, position-dependent mass model for the Thomas–Fermi potential. ► Orthogonal sets of solutions to our model are constructed inmore » closed form. ► Relation to delta-doped semiconductors is discussed. ► Explicit subband bottom energies are calculated and compared to results obtained in a previous study.« less

Low-energy surface states in the normal state of α - PdBi 2 superconductor

DOE PAGES

Choi, Hongchul; Neupane, Madhab; Sasagawa, T.; ...

2017-08-25

Topological superconductors as characterized by Majorana surface states have been actively searched for their significance in fundamental science and technological implication. The large spin-orbit coupling in Bi-Pd binaries has stimulated extensive investigations on the topological surface states in these superconducting compounds. Here we report a study of normal-state electronic structure in a centrosymmetric α-PdBi 2 within density functional theory calculations. By investigating the electronic structure from the bulk to slab geometries in this system, we predict for the first time that α-PdBi 2 can host orbital-dependent and asymmetric Rashba surface states near the Fermi energy. This study suggests that α-PdBimore » 2 will be a good candidate to explore the relationship between superconductivity and topology in condensed matter physics.« less

Diffuse Cosmic Rays Shining in the Galactic Center: A Novel Interpretation of H.E.S.S. and Fermi-LAT γ-Ray Data.

PubMed

Gaggero, D; Grasso, D; Marinelli, A; Taoso, M; Urbano, A

2017-07-21

We present a novel interpretation of the γ-ray diffuse emission measured by Fermi-LAT and H.E.S.S. in the Galactic center (GC) region and the Galactic ridge (GR). In the first part we perform a data-driven analysis based on PASS8 Fermi-LAT data: We extend down to a few GeV the spectra measured by H.E.S.S. and infer the primary cosmic-ray (CR) radial distribution between 0.1 and 3 TeV. In the second part we adopt a CR transport model based on a position-dependent diffusion coefficient. Such behavior reproduces the radial dependence of the CR spectral index recently inferred from the Fermi-LAT observations. We find that the bulk of the GR emission can be naturally explained by the interaction of the diffuse steady-state Galactic CR sea with the gas present in the central molecular zone. Although we confirm the presence of a residual radial-dependent emission associated with a central source, the relevance of the large-scale diffuse component prevents to claim a solid evidence of GC pevatrons.

Quasiclassical Theory of Spin Dynamics in Superfluid ^3He: Kinetic Equations in the Bulk and Spin Response of Surface Majorana States

NASA Astrophysics Data System (ADS)

Silaev, M. A.

2018-06-01

We develop a theory based on the formalism of quasiclassical Green's functions to study the spin dynamics in superfluid ^3He. First, we derive kinetic equations for the spin-dependent distribution function in the bulk superfluid reproducing the results obtained earlier without quasiclassical approximation. Then, we consider spin dynamics near the surface of fully gapped ^3He-B-phase taking into account spin relaxation due to the transitions in the spectrum of localized fermionic states. The lifetimes of longitudinal and transverse spin waves are calculated taking into account the Fermi-liquid corrections which lead to a crucial modification of fermionic spectrum and spin responses.

Gold-induced nanowires on the Ge(100) surface yield a 2D and not a 1D electronic structure

NASA Astrophysics Data System (ADS)

de Jong, N.; Heimbuch, R.; Eliëns, S.; Smit, S.; Frantzeskakis, E.; Caux, J.-S.; Zandvliet, H. J. W.; Golden, M. S.

2016-06-01

Atomic nanowires on semiconductor surfaces induced by the adsorption of metallic atoms have attracted a lot of attention as possible hosts of the elusive, one-dimensional Tomonaga-Luttinger liquid. The Au/Ge(100) system in particular is the subject of controversy as to whether the Au-induced nanowires do indeed host exotic, 1D (one-dimensional) metallic states. In light of this debate, we report here a thorough study of the electronic properties of high quality nanowires formed at the Au/Ge(100) surface. The high-resolution ARPES data show the low-lying Au-induced electronic states to possess a dispersion relation that depends on two orthogonal directions in k space. Comparison of the E (kx,ky) surface measured using high-resolution ARPES to tight-binding calculations yields hopping parameters in the two different directions that differ by approximately factor of two. Additionally, by pinpointing the Au-induced surface states in the first, second, and third surface Brillouin zones and analyzing their periodicity in k||, the nanowire propagation direction seen clearly in STM can be imported into the ARPES data. We find that the larger of the two hopping parameters corresponds, in fact, to the direction perpendicular to the nanowires (tperp). This proves that the Au-induced electron pockets possess a two-dimensional, closed Fermi surface, and this firmly places the Au/Ge(100) nanowire system outside potential hosts of a Tomonaga-Luttinger liquid. We combine these ARPES data with scanning tunneling spectroscopic measurements of the spatially resolved electronic structure and find that the spatially straight—wirelike—conduction channels observed up to energies of order one electron volt below the Fermi level do not originate from the Au-induced states seen in the ARPES data. The former are rather more likely to be associated with bulk Ge states that are localized to the subsurface region. Despite our proof of the 2D (two-dimentional) nature of the Au-induced nanowire and subsurface Ge-related states, an anomalous suppression of the density of states at the Fermi level is observed in both the STS and ARPES data, and this phenomenon is discussed in the light of the effects of disorder.

Influence of metallic surface states on electron affinity of epitaxial AlN films

NASA Astrophysics Data System (ADS)

Mishra, Monu; Krishna, Shibin; Aggarwal, Neha; Gupta, Govind

2017-06-01

The present article investigates surface metallic states induced alteration in the electron affinity of epitaxial AlN films. AlN films grown by plasma-assisted molecular beam epitaxy system with (30% and 16%) and without metallic aluminium on the surface were probed via photoemission spectroscopic measurements. An in-depth analysis exploring the influence of metallic aluminium and native oxide on the electronic structure of the films is performed. It was observed that the metallic states pinned the Fermi Level (FL) near valence band edge and lead to the reduction of electron affinity (EA). These metallic states initiated charge transfer and induced changes in surface and interface dipoles strength. Therefore, the EA of the films varied between 0.6-1.0 eV due to the variation in contribution of metallic states and native oxide. However, the surface barrier height (SBH) increased (4.2-3.5 eV) adversely due to the availability of donor-like surface states in metallic aluminium rich films.

Cooperative Search of Autonomous Vehicles for Unknown Targets

NASA Astrophysics Data System (ADS)

Yang, Sheng Qing; Yu, Jian Qiao; Zhang, Si Yu

2013-01-01

We study the orbital-dependent superconducting pairing in a five-orbital t-J1-J2 model for iron pnictides. Depending on the orbital selectivity of electron correlations and the orbital characters along the Fermi surface, the superconducting gap in an A_{1g} pairing state may exhibit anisotropy. This anisotropy varies with the degree of J1-J2 magnetic frustration. We have also calculated the dynamical spin susceptibility in the superconducting state. The frequency dependence of the susceptibility at the antiferromagnetic wavevector (\\pi,0) shows a resonance, whose width is enhanced by the orbital dependence of the superconducting gap; when the latter is sufficiently strong, the resonance peak may be split into two. We discuss the implications of our results on the recent angle-resolved photoemission and neutron-scattering measurements in several superconducting iron pnictides.

A two-dimensional ACAR study of untwinned YBa2Cu3O(7-x)

NASA Astrophysics Data System (ADS)

Smedskjaer, L. C.; Bansil, A.

1991-12-01

We have carried out 2D-ACAR measurements on an untwinned single crystal of YBa2Cu3O(sub 7-x) as a function of temperature, for five temperatures ranging from 30K to 300K. We show that these temperature-dependent 2D-ACAR spectra can be described to a good approximation as a superposition of two temperature independent spectra with temperature-dependent weighting factors. We show further how the data can be used to correct for the 'background' in the experimental spectrum. Such a 'background corrected' spectrum is in remarkable accord with the corresponding band theory predictions, and displays, in particular, clear signatures of the electron ridge Fermi surface.

Pressure Dependence of the Charge-Density-Wave Gap in Rare-Earth Tri-Tellurides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sacchetti, A.; /Zurich, ETH; Arcangeletti, E.

2009-12-14

We investigate the pressure dependence of the optical properties of CeTe{sub 3}, which exhibits an incommensurate charge-density-wave (CDW) state already at 300 K. Our data are collected in the mid-infrared spectral range at room temperature and at pressures between 0 and 9 GPa. The energy for the single particle excitation across the CDW gap decreases upon increasing the applied pressure, similarly to the chemical pressure by rare-earth substitution. The broadening of the bands upon lattice compression removes the perfect nesting condition of the Fermi surface and therefore diminishes the impact of the CDW transition on the electronic properties of RTe{submore » 3}.« less

Self-consistent modelling of electrochemical strain microscopy in mixed ionic-electronic conductors: Nonlinear and dynamic regimes

DOE PAGES

Varenyk, O. V.; Silibin, M. V.; Kiselev, Dmitri A.; ...

2015-08-19

The frequency dependent Electrochemical Strain Microscopy (ESM) response of mixed ionic-electronic conductors is analyzed within the framework of Fermi-Dirac statistics and the Vegard law, accounting for steric effects from mobile donors. The emergence of dynamic charge waves and nonlinear deformation of the surface in response to bias applied to the tip-surface junction is numerically explored. The 2D maps of the strain and concentration distributions across the mixed ionic-electronic conductor and bias-induced surface displacements are calculated. Furthermore, the obtained numerical results can be applied to quantify the ESM response of Li-based solid electrolytes, materials with resistive switching, and electroactive ferroelectric polymers,more » which are of potential interest for flexible and high-density non-volatile memory devices.« less

Quantum transport in mesoscopic 3He films: experimental study of the interference of bulk and boundary scattering.

PubMed

Sharma, P; Córcoles, A; Bennett, R G; Parpia, J M; Cowan, B; Casey, A; Saunders, J

2011-11-04

We discuss the mass transport of a degenerate Fermi liquid ^{3}He film over a rough surface, and the film momentum relaxation time, in the framework of theoretical predictions. In the mesoscopic regime, the anomalous temperature dependence of the relaxation time is explained in terms of the interference between elastic boundary scattering and inelastic quasiparticle-quasiparticle scattering within the film. We exploit a quasiclassical treatment of quantum size effects in the film in which the surface roughness, whose power spectrum is experimentally determined, is mapped into an effective disorder potential within a film of uniform thickness. Confirmation is provided by the introduction of elastic scattering centers within the film. The improved understanding of surface roughness scattering may impact on enhancing the conductivity in thin metallic films.

Self-consistent modelling of electrochemical strain microscopy in mixed ionic-electronic conductors: Nonlinear and dynamic regimes

NASA Astrophysics Data System (ADS)

Varenyk, O. V.; Silibin, M. V.; Kiselev, D. A.; Eliseev, E. A.; Kalinin, S. V.; Morozovska, A. N.

2015-08-01

The frequency dependent Electrochemical Strain Microscopy (ESM) response of mixed ionic-electronic conductors is analyzed within the framework of Fermi-Dirac statistics and the Vegard law, accounting for steric effects from mobile donors. The emergence of dynamic charge waves and nonlinear deformation of the surface in response to bias applied to the tip-surface junction is numerically explored. The 2D maps of the strain and concentration distributions across the mixed ionic-electronic conductor and bias-induced surface displacements are calculated. The obtained numerical results can be applied to quantify the ESM response of Li-based solid electrolytes, materials with resistive switching, and electroactive ferroelectric polymers, which are of potential interest for flexible and high-density non-volatile memory devices.

Effect of lateral size and thickness on the electronic structure and optical properties of quasi two-dimensional CdSe and CdS nanoplatelets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bose, Sumanta; Fan, W. J., E-mail: ewjfan@ntu.edu.sg; Zhang, D. H.

2016-04-14

The effect of lateral size and vertical thickness of CdSe and CdS nanoplatelets (NPLs) on their electronic structure and optical properties are investigated using an effective-mass envelope function theory based on the 8-band k ⋅ p model with valence force field considerations. Volumetrically larger NPLs have lower photon emission energy due to limited quantum confinement, but a greater transition matrix element (TME) due to larger electron-hole wavefunction overlap. The optical gain characteristics depend on several factors such as TME, Fermi factor, carrier density, NPL dimensions, material composition, and dephasing rate. There is a red shift in the peak position, moremore » so with an increase in thickness than lateral size. For an increasing carrier density, the gain spectrum undergoes a slight blue shift due to band filling effect. For a fixed carrier density, the Fermi factor is higher for volumetrically larger NPLs and so is the difference between the quasi-Fermi level separation and the effective bandgap. The transparency injection carrier density (and thus input current density threshold) is dimension dependent and falls for volumetrically larger NPLs, as they can attain the requisite exciton count for transparency with a relatively lower density. Between CdSe and CdS, CdSe has lower emission energy due to smaller bandgap, but a higher TME due to lower effective mass. CdS, however, has a higher so hole contribution due to a lower spin-orbit splitting energy. Both CdSe and CdS NPLs are suitable candidates for short-wavelength LEDs and lasers in the visible spectrum, but CdSe is expected to exhibit better optical performance.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Breznay, Nicholas P.; Hayes, Ian M.; Ramshaw, B. J.

In this work, we study magnetotransport properties of the electron-doped superconductor Pr 2-xCe xCuO 4±δ with x = 0.14 in magnetic fields up to 92 T, and observe Shubnikov-de Haas magnetic quantum oscillations. The oscillations display a single frequency F = 255 ± 10 T, indicating a small Fermi pocket that is ~1 % of the two-dimensional Brillouin zone and consistent with a Fermi surface reconstructed from the large holelike cylinder predicted for these layered materials. Despite the low nominal doping, all electronic properties including the effective mass and Hall effect are consistent with overdoped compounds. In conclusion, our studymore » demonstrates that the exceptional chemical control afforded by high quality thin films will enable Fermi surface studies deep into the overdoped cuprate phase diagram.« less

High Dielectrics on High Carrier Mobility InGaAs Compound Semiconductors and GaN - Growth, Interfacial Structural Studies, and Surface Fermi Level Unpinning

DTIC Science & Technology

2010-02-19

UHV- deposited Al2O3(3nm)/ Ga2O3 (Gd2O3)(8.5nm) on n- and p-In0.2Ga0.8As/GaAs. The results exhibit very high-quality interface and free-moving Fermi...κ Ga2O3 (Gd2O3) [GGO] and Gd2O3 on InGaAs, without an interfacial layer. InxGa1−xAs MOSFETs have been successfully demonstrated with excellent device... Ga2O3 (Gd2O3)/In0.2Ga0.8As and high temperature (850°C) stability Scaling high κ oxides to nanometer range as well as unpinning surface Fermi level

Coverage dependent work function of graphene on a Cu(111) substrate with intercalated alkali metals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cook, Brandon G.; Russakoff, Arthur; Varga, Kalman

2015-05-26

Using first-principles calculations, it is shown that the work function of graphene on copper can be adjusted by varying the concentration of intercalated alkali metals. Using density functional theory, we calculate the modulation of work function when Li, Na, or K are intercalated between graphene and a Cu(111) surface. Furthermore, the physical origins of the change in work function are explained in terms of phenomenological models accounting for the formation and depolarization of interfacial dipoles and the shift in the Fermi-level induced via charge transfer.

Dirac points, spinons and spin liquid in twisted bilayer graphene

NASA Astrophysics Data System (ADS)

Irkhin, V. Yu.; Skryabin, Yu. N.

2018-05-01

Twisted bilayer graphene is an excellent example of highly correlated system demonstrating a nearly flat electron band, the Mott transition and probably a spin liquid state. Besides the one-electron picture, analysis of Dirac points is performed in terms of spinon Fermi surface in the limit of strong correlations. Application of gauge field theory to describe deconfined spin liquid phase is treated. Topological quantum transitions, including those from small to large Fermi surface in the presence of van Hove singularities, are discussed.

Time-Dependent Thomas-Fermi Approach for Electron Dynamics in Metal Clusters

NASA Astrophysics Data System (ADS)

Domps, A.; Reinhard, P.-G.; Suraud, E.

1998-06-01

We propose a time-dependent Thomas-Fermi approach to the (nonlinear) dynamics of many-fermion systems. The approach relies on a hydrodynamical picture describing the system in terms of collective flow. We investigate in particular an application to electron dynamics in metal clusters. We make extensive comparisons with fully fledged quantal dynamical calculations and find overall good agreement. The approach thus provides a reliable and inexpensive scheme to study the electronic response of large metal clusters.

Superconducting states of topological surface states in β-PdBi2 investigated by STM/STS

NASA Astrophysics Data System (ADS)

Iwaya, Katsuya; Okawa, Kenjiro; Hanaguri, Tetsuo; Kohsaka, Yuhki; Machida, Tadashi; Sasagawa, Takao

We investigate superconducting (SC) states of topological surface states in β-PdBi2 using very low temperature STM. Characteristic quasiparticle interference patterns strongly support the existence of the spin-polarized surface states at the Fermi level in the normal state. A fully-opened SC gap well described by the conventional BCS model is observed, indicating the SC gap opening at the spin-polarized Fermi surfaces. Considering a possible mixing of odd- and even parity orbital functions in C4v group symmetry lowered from D4h near the surface, we suggest that the SC gap consists of the mixture of s- and p-wave SC gap functions in the two-dimensional state.

Evidence for carrier localization in the pseudogap state of cuprate superconductors from coherent quench experiments

PubMed Central

Madan, I.; Kurosawa, T.; Toda, Y.; Oda, M.; Mertelj, T.; Mihailovic, D.

2015-01-01

A ‘pseudogap' was introduced by Mott to describe a state of matter that has a minimum in the density of states at the Fermi level, deep enough for states to become localized. It can arise either from Coulomb repulsion between electrons, and/or incipient charge or spin order. Here we employ ultrafast spectroscopy to study dynamical properties of the normal to pseudogap state transition in the prototype high-temperature superconductor Bi2Sr2CaCu2O8+δ. We perform a systematic temperature and doping dependence study of the pseudogap photodestruction and recovery in coherent quench experiments, revealing marked absence of critical behaviour of the elementary excitations, which implies an absence of collective electronic ordering beyond a few coherence lengths on short timescales. The data imply ultrafast carrier localization into a textured polaronic state arising from a competing Coulomb interaction and lattice strain, enhanced by a Fermi surface instability. PMID:25891310

Emptying Dirac valleys in bismuth using high magnetic fields

DOE PAGES

Zhu, Zengwei; Wang, Jinhua; Zuo, Huakun; ...

2017-05-19

The Fermi surface of elemental bismuth consists of three small rotationally equivalent electron pockets, offering a valley degree of freedom to charge carriers. A relatively small magnetic field can confine electrons to their lowest Landau level. This is the quantum limit attained in other dilute metals upon application of sufficiently strong magnetic field. Here in this paper we report on the observation of another threshold magnetic field never encountered before in any other solid. Above this field, B empty, one or two valleys become totally empty. Drying up a Fermi sea by magnetic field in the Brillouin zone leads tomore » a manyfold enhancement in electric conductance. We trace the origin of the large drop in magnetoresistance across B empty to transfer of carriers between valleys with highly anisotropic mobilities. The non-interacting picture of electrons with field-dependent mobility explains most results but the Coulomb interaction may play a role in shaping the fine details.« less

Anisotropies in the diffuse gamma-ray background measured by the Fermi LAT

DOE PAGES

Ackermann, M.; Ajello, M.; Albert, A.; ...

2012-04-23

The contribution of unresolved sources to the diffuse gamma-ray background could induce anisotropies in this emission on small angular scales. Here, we analyze the angular power spectrum of the diffuse emission measured by the Fermi Large Area Telescope at Galactic latitudes | b | > 30 ° in four energy bins spanning 1–50 GeV. At multipoles ℓ ≥ 155 , corresponding to angular scales ≲ 2 ° , angular power above the photon noise level is detected at > 99.99 % confidence level in the 1–2 GeV, 2–5 GeV, and 5–10 GeV energy bins, and at > 99 % confidencemore » level at 10–50 GeV. Within each energy bin the measured angular power takes approximately the same value at all multipoles ℓ ≥ 155 , suggesting that it originates from the contribution of one or more unclustered source populations. Furthermore, the amplitude of the angular power normalized to the mean intensity in each energy bin is consistent with a constant value at all energies, C P / < I > 2 = 9.05 ± 0.84 × 10 - 6 sr , while the energy dependence of C P is consistent with the anisotropy arising from one or more source populations with power-law photon spectra with spectral index Γ s = 2.40 ± 0.07 . We also discuss the implications of the measured angular power for gamma-ray source populations that may provide a contribution to the diffuse gamma-ray background.« less

Anisotropies in the Diffuse Gamma-Ray Background Measured by the Fermi LAT

NASA Technical Reports Server (NTRS)

Ferrara, E. C.; McEnery, J. E.; Troja, E.

2012-01-01

The contribution of unresolved sources to the diffuse gamma-ray background could induce anisotropies in this emission on small angular scales. We analyze the angular power spectrum of the diffuse emission measured by the Fermi LAT at Galactic latitudes absolute value of b > 30 deg in four energy bins spanning 1 to 50 GeV. At multipoles l >= 155, corresponding to angular scales approx < 2 deg, angular power above the photon noise level is detected at > 99.99% CL in the 1-2 GeV, 2- 5 GeV, and 5- 10 GeV energy bins, and at > 99% CL at 10-50 GeV. Within each energy bin the measured angular power takes approximately the same value at all multipoles l >= 155, suggesting that it originates from the contribution of one or more unclustered source populations. The amplitude of the angular power normalized to the mean intensity in each energy bin is consistent with a constant value at all energies, C(sub p) / (I)(exp 2) = 9.05 +/- 0.84 x 10(exp -6) sr, while the energy dependence of C(sub p) is consistent with the anisotropy arising from one or more source populations with power-law photon spectra with spectral index Gamma (sub s) = 2.40 +/- 0.07. We discuss the implications of the measured angular power for gamma-ray source populations that may provide a contribution to the diffuse gamma-ray background.

Polarization-dependent X-ray photoemission spectroscopy for High-Tc cuprate superconductors

NASA Astrophysics Data System (ADS)

Yamagami, Kohei; Kanai, Yuina; Naimen, Sho; Fujiwara, Hidenori; Kiss, Takayuki; Tanaka, Arata; Higashiya, Atsushi; Imada, Shin; Kadono, Toshiharu; Tamasaku, Kenji; Muro, Takayuki; Yabashi, Makina; Ishikawa, Tetsuya; Eisaki, Hiroshi; Miyasaka, Shigeki; Tajima, Setsuko; Sekiyama, Akira

2018-05-01

We have performed photon energy (hν) and linear polarization dependent X-ray photoemission for optimal doped Pb-Bi2Sr2CaCu2O8+δ (Bi2212) to investigate the ground Cu 3d orbital symmetry. We identified that the bulk Cu 3d components in valence-band spectra develop with decreasing hν from 7900 eV to 460 eV. Moreover, the photoelectron intensity ratio of the valence-band spectra measured at hν = 460 eV has shown that the Cu 3dx2-y2 orbital contributions are dominant near the Fermi level (EF). Meanwhile, we revealed that the bulk Cu 2p3/2 core-level spectra without the Bi 4s component is detected at hν = 1550 eV compared with hν and linear-polarization-dependent spectra.

First principles studies of the dependence of magnetism on the crystal phase in 4d and 5d late transition metals

NASA Astrophysics Data System (ADS)

Hüger, E.; Osuch, K.

2005-03-01

We investigate the possibility of inducing ferromagnetic order in 4d and 5d late transition metals through crystal symmetry change. First principles, self-consistent density functional theory calculations, with spin-orbit coupling included, performed at 0 K show that ferromagnetism occurs in the bulk of Rh and Pd at the optimum lattice constant if Rh is in the bcc and Pd in the hcp/dhcp phase. The ferromagnetic order originates in the d-band occupancy of Rh or Pd which locates the Fermi energy at the top of the highest peak of the respective (paramagnetic) density of states induced by the bcc or hcp/dhcp structure. This peak in the density of states is caused by flat bands which lie at the surface of the respective Brillouin zone. For a bcc crystal these flat bands have the eg character and are positioned at the surface of the bcc Brillouin zone along the N-P line. The origin of the flatness of the bands was found to be the translation symmetry of the cubic lattice which causes the bands with the eg character to be narrow along the k-lines whose k-vector directions are furthest off the directions to which the orbitals of the eg symmetry point. Due to the d-band occupancy of Rh these flat bands lie in the paramagnetic state at the Fermi energy, whereas in the ferromagnetic state they exhibit the largest energetic split. This indicates that a smaller degree of orbital overlap narrows electronic bands enhancing the tendency of the system for ferromagnetic band split. For the hcp/dhcp structure the states contributing to the high density of para-magnetic states at the Fermi level of Pd lie in the vicinity of the M-L line of the hcp Brillouin zone boundary, which possesses a high number of symmetry (M and L) points. Moreover, the M-L line is aligned with the stacking sequence direction ([0001]) which is furthest off the densest-packed atomic chain direction of an hcp-crystal and, consequently, the weakest-bond direction in the crystal. This makes the narrow bands along the M-L line flat. The instability of the bcc and the meta-stability of the hcp crystal phase modifications for metals with native close-packed crystal structures is subsequently analysed in order to find whether they can be grown as films on suitable substrates.

Covalent electron transfer chemistry of graphene with diazonium salts.

PubMed

Paulus, Geraldine L C; Wang, Qing Hua; Strano, Michael S

2013-01-15

Graphene is an atomically thin, two-dimensional allotrope of carbon with exceptionally high carrier mobilities, thermal conductivity, and mechanical strength. From a chemist's perspective, graphene can be regarded as a large polycyclic aromatic molecule and as a surface without a bulk contribution. Consequently, chemistries typically performed on organic molecules and surfaces have been used as starting points for the chemical functionalization of graphene. The motivations for chemical modification of graphene include changing its doping level, opening an electronic band gap, charge storage, chemical and biological sensing, making new composite materials, and the scale-up of solution-processable graphene. In this Account, we focus on graphene functionalization via electron transfer chemistries, in particular via reactions with aryl diazonium salts. Because electron transfer chemistries depend on the Fermi energy of graphene and the density of states of the reagents, the resulting reaction rate depends on the number of graphene layers, edge states, defects, atomic structure, and the electrostatic environment. We limit our Account to focus on pristine graphene over graphene oxide, because free electrons in the latter are already bound to oxygen-containing functionalities and the resulting chemistries are dominated by localized reactivity and defects. We describe the reaction mechanism of diazonium functionalization of graphene and show that the reaction conditions determine the relative degrees of chemisorption and physisorption, which allows for controlled modulation of the electronic properties of graphene. Finally we discuss different applications for graphene modified by this chemistry, including as an additive in polymer matrices, as biosensors when coupled with cells and biomolecules, and as catalysts when combined with nanoparticles.

A unified physical model of Seebeck coefficient in amorphous oxide semiconductor thin-film transistors

NASA Astrophysics Data System (ADS)

Lu, Nianduan; Li, Ling; Sun, Pengxiao; Banerjee, Writam; Liu, Ming

2014-09-01

A unified physical model for Seebeck coefficient was presented based on the multiple-trapping and release theory for amorphous oxide semiconductor thin-film transistors. According to the proposed model, the Seebeck coefficient is attributed to the Fermi-Dirac statistics combined with the energy dependent trap density of states and the gate-voltage dependence of the quasi-Fermi level. The simulation results show that the gate voltage, energy disorder, and temperature dependent Seebeck coefficient can be well described. The calculation also shows a good agreement with the experimental data in amorphous In-Ga-Zn-O thin-film transistor.

Direct, experimental evidence of the Fermi surface in YBa2Cu3O(7-x)

NASA Astrophysics Data System (ADS)

Haghighi, H.; Kaiser, J. H.; Rayner, S. L.; West, R. N.; Liu, J. Z.; Shelton, R.; Howell, R. H.; Sterne, P. A.; Solal, F. R.; Fluss, M. J.

1991-04-01

We report new measurements of the electron positron momentum spectra of YBa2Cu3O(7-x) performed with ultra-high statistical precision. These data differ from previous results in two significant respects: They show the D(sub 2) symmetry appropriate for untwinned crystals and, more importantly, they show unmistakable, statistically significant, discontinuities that are evidence of a major Fermi surface section. These results provide a partial answer to a question of special significance to the study of high temperature superconductors i.e., the distribution of the electrons in the material, the electronic structure. Special consideration has been given both experimentally and theoretically to the existence and shape of a Fermi surface in the materials and to the superconducting gap. There are only three experimental techniques that can provide details of the electronic structure at useful resolutions. They are angular correlation of positron annihilation radiation, ACAR, angle resolved photo emission, PE, and de Haas van Alphen measurements.

Fermi surfaces properties of AuAl2, AuGa2, and AuIn2 with the CaF2-type cubic structure

NASA Astrophysics Data System (ADS)

Nishimura, K.; Kakihana, M.; Suzuki, F.; Yara, T.; Hedo, M.; Nakama, T.; Ōnuki, Y.; Harima, H.

2018-05-01

We grew high-quality single crystals of AuAl2, AuGa2, and AuIn2 with the fluorite (CaF2)-type cubic structure and determined the Fermi surface properties by the de Haas-van Alphen (dHvA) experiments using full-potential LAPW bad calculations. The Fermi surface and optical properties for three compounds were once studied from an interest of colors because AuAl2 has a striking bright reddish-purple color, whereas AuGa2 and AuIn2 are, respectively, neutral and bluish. The detected dHvA frequencies in the present study are found to be in a wide range of (0.1-13)×107 Oe. The main dHvA branches for three compounds are in excellent agreement with the theoretical ones, but some dHvA branches with small dHvA frequencies are slightly deviated from the theoretical ones, especially in AuGa2 and AuIn2.

Observation of topological nodal fermion semimetal phase in ZrSiS

DOE PAGES

Neupane, Madhab; Belopolski, Ilya; Hosen, M. Mofazzel; ...

2016-05-11

We present that unveiling new topological phases of matter is one of the current objectives in condensed matter physics. Recent experimental discoveries of Dirac and Weyl semimetals prompt the search for other exotic phases of matter. Here we present a systematic angle-resolved photoemission spectroscopy study of ZrSiS, a prime topological nodal semimetal candidate. Our wider Brillouin zone (BZ) mapping shows multiple Fermi surface pockets such as the diamond-shaped Fermi surface, elliptical-shaped Fermi surface, and a small electron pocket encircling at the zone center (Γ) point, the M point, and the X point of the BZ, respectively. We experimentally establish themore » spinless nodal fermion semimetal phase in ZrSiS, which is supported by our first-principles calculations. Our findings evidence that the ZrSiS-type of material family is a new platform on which to explore exotic states of quantum matter; these materials are expected to provide an avenue for engineering two-dimensional topological insulator systems.« less

Bulk Fermi Surface of Charge-Neutral Excitations in SmB_{6} or Not: A Heat-Transport Study.

PubMed

Xu, Y; Cui, S; Dong, J K; Zhao, D; Wu, T; Chen, X H; Sun, Kai; Yao, Hong; Li, S Y

2016-06-17

Recently, there have been increasingly hot debates on whether a bulk Fermi surface of charge-neutral excitations exists in the topological Kondo insulator SmB_{6}. To unambiguously resolve this issue, we perform the low-temperature thermal conductivity measurements of a high-quality SmB_{6} single crystal down to 0.1 K and up to 14.5 T. Our experiments show that the residual linear term of thermal conductivity at the zero field is zero, within the experimental accuracy. Furthermore, the thermal conductivity is insensitive to the magnetic field up to 14.5 T. These results demonstrate the absence of fermionic charge-neutral excitations in bulk SmB_{6}, such as scalar Majorana fermions or spinons and, thus, exclude the existence of a bulk Fermi surface suggested by a recent quantum oscillation study of SmB_{6}. This puts a strong constraint on the explanation of the quantum oscillations observed in SmB_{6}.

Evolution of the electronic structure of La2-xSrxCuO4 with doping determined by positron-annihilation spectroscopy

NASA Astrophysics Data System (ADS)

Howell, R. H.; Sterne, P. A.; Fluss, M. J.; Kaiser, J. H.; Kitazawa, K.; Kojima, H.

1994-05-01

We have measured and calculated the electron-positron momentum distribution of La2-xSrxCuO4 samples for Sr concentrations of 0, 0.1, 0.13, and 0.2. Measured distributions were obtained at room temperature with high statistical precision, greater than 4×108 events, in the Lawrence Livermore National Laboratory positron-annihilation angular correlation spectrometer on single-crystal samples fabricated using the traveling solvent floating zone technique. Corresponding theoretical momentum-density calculations were performed using the linear muffin-tin-orbital method. The momentum distribution of all samples contained features derived from the overlap of the positron distribution with the valence electrons. In addition, discontinuities typical of a Fermi surface are seen in the doped samples. The form and position of these features are in general agreement with the Fermi surface and overall momentum distributions as predicted by band theory. However, the evolution of the Fermi surface with doping differed significantly from expectations based on single electron band theories.

Dark lump excitations in superfluid Fermi gases

NASA Astrophysics Data System (ADS)

Xu, Yan-Xia; Duan, Wen-Shan

2012-11-01

We study the linear and nonlinear properties of two-dimensional matter-wave pulses in disk-shaped superfluid Fermi gases. A Kadomtsev—Petviashvili I (KPI) solitary wave has been realized for superfluid Fermi gases in the limited cases of Bardeen—Cooper—Schrieffer (BCS) regime, Bose—Einstein condensate (BEC) regime, and unitarity regime. One-lump solution as well as one-line soliton solutions for the KPI equation are obtained, and two-line soliton solutions with the same amplitude are also studied in the limited cases. The dependence of the lump propagating velocity and the sound speed of two-dimensional superfluid Fermi gases on the interaction parameter are investigated for the limited cases of BEC and unitarity.

An Integrable Approximation for the Fermi Pasta Ulam Lattice

NASA Astrophysics Data System (ADS)

Rink, Bob

This contribution presents a review of results obtained from computations of approximate equations of motion for the Fermi-Pasta-Ulam lattice. These approximate equations are obtained as a finite-dimensional Birkhoff normal form. It turns out that in many cases, the Birkhoff normal form is suitable for application of the KAM theorem. In particular, this proves Nishida's 1971 conjecture stating that almost all low-energetic motions of the anharmonic Fermi-Pasta-Ulam lattice with fixed endpoints are quasi-periodic. The proof is based on the formal Birkhoff normal form computations of Nishida, the KAM theorem and discrete symmetry considerations.

Measurement of the Atomic Orbital Composition of the Near-Fermi-Level Electronic States in the Lanthanum Monopnictides LaBi and LaSb

NASA Astrophysics Data System (ADS)

Nummy, Thomas; Waugh, Justin; Parham, Stephen; Li, Haoxiang; Zhou, Xiaoqing; Plumb, Nick; Tafti, Fazel; Dessau, Daniel

Angle resolved photoemission spectroscopy (ARPES) is used to measure the electronic structure of the Extreme Magnetoresistance (XMR) topological semimetal candidates LaBi and LaSb. Using a wide range of photon energies the true bulk states are cleanly disentangled from the various types of surface states, which may exist due to surface projections of bulk states as well as for topological reasons. The orbital content of the near-EF states are extracted using varying photon polarizations. The measured bulk bands are somewhat lighter and are energy shifted compared to the results of Density Functional calculations, which is a minor effect in LaBi and a more serious effect in LaSb. This bulk band structure puts LaBi in the v = 1 class of Topological Insulators (or semimetals), consistent with the measured Dirac-like surface states. LaSb on the other hand is at the verge of a topological band inversion, with a less-clear case for any distinctly topological surface states. The low-dimensional cigar-shaped bulk Fermi surfaces for both compounds are separated out by orbital content, with a crossover from pnictide d orbitals to La p orbitals around the Fermi surface, which through strong spin-orbit coupling may be relevant for the Extreme Magnetoresistance. NSF GRFP.

Experimental discovery of a topological Weyl semimetal state in TaP

DOE PAGES

Xu, Su -Yang; Belopolski, Ilya; Sanchez, Daniel S.; ...

2015-11-13

Here, Weyl semimetals are expected to open up new horizons in physics and materials science because they provide the first realization of Weyl fermions and exhibit protected Fermi arc surface states. However, they had been found to be extremely rare in nature. Recently, a family of compounds, consisting of tantalum arsenide, tantalum phosphide (TaP), niobium arsenide, and niobium phosphide, was predicted as a Weyl semimetal candidates. We experimentally realize a Weyl semimetal state in TaP. Using photoemission spectroscopy, we directly observe the Weyl fermion cones and nodes in the bulk, and the Fermi arcs on the surface. Moreover, we findmore » that the surface states show an unexpectedly rich structure, including both topological Fermi arcs and several topologically trivial closed contours in the vicinity of the Weyl points, which provides a promising platform to study the interplay between topological and trivial surface states on a Weyl semimetal’s surface. We directly demonstrate the bulk-boundary correspondence and establish the topologically nontrivial nature of the Weyl semimetal state in TaP, by resolving the net number of chiral edge modes on a closed path that encloses the Weyl node. This also provides, for the first time, an experimentally practical approach to demonstrating a bulk Weyl fermion from a surface state dispersion measured in photoemission.« less

Conjugate-gradient optimization method for orbital-free density functional calculations.

PubMed

Jiang, Hong; Yang, Weitao

2004-08-01

Orbital-free density functional theory as an extension of traditional Thomas-Fermi theory has attracted a lot of interest in the past decade because of developments in both more accurate kinetic energy functionals and highly efficient numerical methodology. In this paper, we developed a conjugate-gradient method for the numerical solution of spin-dependent extended Thomas-Fermi equation by incorporating techniques previously used in Kohn-Sham calculations. The key ingredient of the method is an approximate line-search scheme and a collective treatment of two spin densities in the case of spin-dependent extended Thomas-Fermi problem. Test calculations for a quartic two-dimensional quantum dot system and a three-dimensional sodium cluster Na216 with a local pseudopotential demonstrate that the method is accurate and efficient. (c) 2004 American Institute of Physics.

Quantum oscillations in the type-II Dirac semi-metal candidate PtSe2

NASA Astrophysics Data System (ADS)

Yang, Hao; Schmidt, Marcus; Süss, Vicky; Chan, Mun; Balakirev, Fedor F.; McDonald, Ross D.; Parkin, Stuart S. P.; Felser, Claudia; Yan, Binghai; Moll, Philip J. W.

2018-04-01

Three-dimensional topological semi-metals carry quasiparticle states that mimic massless relativistic Dirac fermions, elusive particles that have never been observed in nature. As they appear in the solid body, they are not bound to the usual symmetries of space-time and thus new types of fermionic excitations that explicitly violate Lorentz-invariance have been proposed, the so-called type-II Dirac fermions. We investigate the electronic spectrum of the transition-metal dichalcogenide PtSe2 by means of quantum oscillation measurements in fields up to 65 T. The observed Fermi surfaces agree well with the expectations from band structure calculations, that recently predicted a type-II Dirac node to occur in this material. A hole- and an electron-like Fermi surface dominate the semi-metal at the Fermi level. The quasiparticle mass is significantly enhanced over the bare band mass value, likely by phonon renormalization. Our work is consistent with the existence of type-II Dirac nodes in PtSe2, yet the Dirac node is too far below the Fermi level to support free Dirac–fermion excitations.

Origin of the non-monotonic variance of Tc in the 1111 iron based superconductors with isovalent doping

PubMed Central

Usui, Hidetomo; Suzuki, Katsuhiro; Kuroki, Kazuhiko

2015-01-01

Motivated by recent experimental investigations of the isovalent doping iron-based superconductors LaFe(AsxP1-x)O1-yFy and NdFe(AsxP1-x)O1-yFy, we theoretically study the correlation between the local lattice structure, the Fermi surface, the spin fluctuation-mediated superconductivity, and the composition ratio. In the phosphides, the dXZ and dYZ orbitals barely hybridize around the Γ point to give rise to two intersecting ellipse shape Fermi surfaces. As the arsenic content increases and the Fe-As-Fe bond angle is reduced, the hybridization increases, so that the two bands are mixed to result in concentric inner and outer Fermi surfaces, and the orbital character gradually changes to dxz and dyz, where x–y axes are rotated by 45 degrees from X–Y. This makes the orbital matching between the electron and hole Fermi surfaces better and enhances the spin fluctuation within the dxz/yz orbitals. On the other hand, the hybridization splits the two bands, resulting in a more dispersive inner band. Hence, there is a trade-off between the density of states and the orbital matching, thereby locally maximizing the dxz/yz spin fluctuation and superconductivity in the intermediate regime of As/P ratio. The consistency with the experiment strongly indicate the importance of the spin fluctuation played in this series of superconductors. PMID:26073071

A cross-correlation study of the Fermi-LAT γ-ray diffuse extragalactic signal

DOE PAGES

Xia, Jun -Qing; Cuoco, Alessandro; Branchini, Enzo; ...

2011-09-12

In this work, starting from 21 months of data from the Fermi Large Area Telescope (LAT), we derive maps of the residual isotropic γ-ray emission, a relevant fraction of which is expected to be contributed by the extragalactic diffuse γ-ray background (EGB). We search for the auto-correlation signals in the above γ-ray maps and for the cross-correlation signal with the angular distribution of different classes of objects that trace the large-scale structure of the Universe. We compute the angular two-point auto-correlation function of the residual Fermi-LAT maps at energies E > 1 GeV, E > 3 GeV and E >more » 30 GeV well above the Galactic plane and find no significant correlation signal. This is, indeed, what is expected if the EGB were contributed by BL Lacertae (BLLacs), Flat Spectrum Radio Quasars (FSRQs) or star-forming galaxies, since, in this case, the predicted signal is very weak. Then, we search for the Integrated Sachs–Wolfe (ISW) signature by cross-correlating the Fermi-LAT maps with the 7-year Wilkinson Microwave Anisotropy Probe ( WMAP7) cosmic microwave background map. We find a cross-correlation consistent with zero, even though the expected signal is larger than that of the EGB auto-correlation. Lastly, in an attempt to constrain the nature of the γ-ray background, we cross-correlate the Fermi-LAT maps with the angular distributions of objects that may contribute to the EGB: quasi-stellar objects (QSOs) in the Sloan Digital Sky Survey Data Release 6 (SDSS-DR6) catalogue, NRAO VLA Sky Survey (NVSS) galaxies, Two Micron All Sky Survey (2MASS) galaxies and Luminous Red Galaxies (LRGs) in the SDSS catalogue. The cross-correlation is always consistent with zero, in agreement with theoretical expectations, but we find (with low statistical significance) some interesting features that may indicate that some specific classes of objects contribute to the EGB. A χ 2 analysis confirms that the correlation properties of the 21-month data do not provide strong constraints of the EGB origin. However, the results suggest that the situation will significantly improve with the 5- and 10-yr Fermi-LAT data. In future, the EGB analysis will then allow placing significant constraints on the nature of the EGB and might provide, in addition, a detection of the ISW signal.« less

Evidence against a charge density wave on Bi(111)

DOE PAGES

Kim, T. K.; Wells, J.; Kirkegaard, C.; ...

2005-08-18

The Bi(111) surface was studied by scanning tunneling microscopy (STM), transmission electron microscopy (TEM) and angle-resolved photoemission (ARPES) in order to verify the existence of a recently proposed surface charge density wave (CDW). The STM and TEM results to not support a CDW scenario at low temperatures. Thus the quasiparticle interference pattern observed in STM confirms the spin-orbit split character of the surface states which prevents the formation of a CDW, even in the case of good nesting. The dispersion of the electronic states observed with ARPES agrees well with earlier findings. In particular, the Fermi contour of the electronmore » pocket at the centre of the surface Brillouin zone is found to have a hexagonal shape. However, no gap opening or other signatures of a CDW phase transition can be found in the temperature-dependent data.« less

A high-efficiency spin polarizer based on edge and surface disordered silicene nanoribbons

NASA Astrophysics Data System (ADS)

Xu, Ning; Zhang, Haiyang; Wu, Xiuqiang; Chen, Qiao; Ding, Jianwen

2018-07-01

Using the tight-binding formalism, we explore the effect of weak disorder upon the conductance of zigzag edge silicene nanoribbons (SiNRs), in the limit of phase-coherent transport. We find that the fashion of the conductance varies with disorder, and depends strongly on the type of disorder. Conductance dips are observed at the Van Hove singularities, owing to quasilocalized states existing in surface disordered SiNRs. A conductance gap is observed around the Fermi energy for both edge and surface disordered SiNRs, because edge states are localized. The average conductance of the disordered SiNRs decreases exponentially with the increase of disorder, and finally tends to disappear. The near-perfect spin polarization can be realized in SiNRs with a weak edge or surface disorder, and also can be attained by both the local electric field and the exchange field.

Control of magnetism in dilute magnetic semiconductor (Ga,Mn)As films by surface decoration of molecules

NASA Astrophysics Data System (ADS)

Wang, Hailong; Wang, Xiaolei; Xiong, Peng; Zhao, Jianhua

2016-03-01

The responses of magnetic moments to external stimuli such as magnetic-field, heat, light and electric-field have been utilized to manipulate the magnetism in magnetic semiconductors, with many of the novel ideas applied even to ferromagnetic metals. Here, we review a new experimental development on the control of magnetism in (Ga,Mn)As thin films by surface decoration of organic molecules: Molecules deposited on the surface of (Ga,Mn)As thin films are shown to be capable of significantly modulating their saturation magnetization and Curie temperature. These phenomena are shown to originate from the carrier-mediated ferromagnetism in (Ga,Mn)As and the surface molecules acting as acceptors or donors depending on their highest occupied molecular orbitals, resembling the charge transfer mechanism in a pn junction in which the equilibrium state is reached on the alignment of Fermi levels.

Absence of Dirac states in BaZnBi 2 induced by spin-orbit coupling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ren, Weijun; Wang, Aifeng; Graf, D.

We report magnetotransport properties of BaZnBi 2 single crystals. Whereas electronic structure features Dirac states, such states are removed from the Fermi level by spin-orbit coupling (SOC) and consequently electronic transport is dominated by the small hole and electron pockets. Our results are consistent with not only three-dimensional, but also with quasi-two-dimensional portions of the Fermi surface. The SOC-induced gap in Dirac states is much larger when compared to isostructural SrMnBi 2. This suggests that not only long-range magnetic order, but also mass of the alkaline-earth atoms A in ABX 2 ( A = alkaline-earth, B = transition-metal, and Xmore » = Bi/Sb) are important for the presence of low-energy states obeying the relativistic Dirac equation at the Fermi surface.« less

Magnetothermoelectric properties of Bi2Se3

NASA Astrophysics Data System (ADS)

Fauqué, Benoît; Butch, Nicholas P.; Syers, Paul; Paglione, Johnpierre; Wiedmann, Steffen; Collaudin, Aurélie; Grena, Benjamin; Zeitler, Uli; Behnia, Kamran

2013-01-01

We present a study of entropy transport in Bi2Se3 at low temperatures and high magnetic fields. In the zero-temperature limit, the magnitude of the Seebeck coefficient quantitatively tracks the Fermi temperature of the three-dimensional Fermi surface at the Γ point as the carrier concentration changes by two orders of magnitude (1017 to 1019 cm-3). In high magnetic fields, the Nernst response displays giant quantum oscillations indicating that this feature is not exclusive to compensated semimetals. A comprehensive analysis of the Landau level spectrum firmly establishes a large g factor in this material and a substantial decrease of the Fermi energy with increasing magnetic field across the quantum limit. Thus, the presence of bulk carriers significantly affects the spectrum of the intensively debated surface states in Bi2Se3 and related materials.

Absence of Dirac states in BaZnBi 2 induced by spin-orbit coupling

DOE PAGES

Ren, Weijun; Wang, Aifeng; Graf, D.; ...

2018-01-22

We report magnetotransport properties of BaZnBi 2 single crystals. Whereas electronic structure features Dirac states, such states are removed from the Fermi level by spin-orbit coupling (SOC) and consequently electronic transport is dominated by the small hole and electron pockets. Our results are consistent with not only three-dimensional, but also with quasi-two-dimensional portions of the Fermi surface. The SOC-induced gap in Dirac states is much larger when compared to isostructural SrMnBi 2. This suggests that not only long-range magnetic order, but also mass of the alkaline-earth atoms A in ABX 2 ( A = alkaline-earth, B = transition-metal, and Xmore » = Bi/Sb) are important for the presence of low-energy states obeying the relativistic Dirac equation at the Fermi surface.« less

Multiparticle instability in a spin-imbalanced Fermi gas

NASA Astrophysics Data System (ADS)

Whitehead, T. M.; Conduit, G. J.

2018-01-01

Weak attractive interactions in a spin-imbalanced Fermi gas induce a multiparticle instability, binding multiple fermions together. The maximum binding energy per particle is achieved when the ratio of the number of up- and down-spin particles in the instability is equal to the ratio of the up- and down-spin densities of states in momentum at the Fermi surfaces, to utilize the variational freedom of all available momentum states. We derive this result using an analytical approach, and verify it using exact diagonalization. The multiparticle instability extends the Cooper pairing instability of balanced Fermi gases to the imbalanced case, and could form the basis of a many-body state, analogously to the construction of the Bardeen-Cooper-Schrieffer theory of superconductivity out of Cooper pairs.

Enhanced superconductivity in the high pressure phase of SnAs studied from first principles

NASA Astrophysics Data System (ADS)

Sreenivasa Reddy, P. V.; Kanchana, V.; Millichamp, T. E.; Vaitheeswaran, G.; Dugdale, S. B.

2017-01-01

First principles calculations are performed using density functional theory and density functional perturbation theory for SnAs. Total energy calculations show the first order phase transition from an NaCl structure to a CsCl one at around 37 GPa, which is also confirmed from enthalpy calculations and agrees well with experimental work. Calculations of the phonon structure and hence the electron-phonon coupling, λep, and superconducting transition temperature, Tc, across the phase diagram are performed. These calculations give an ambient pressure Tc, in the NaCl structure, of 3.08 K, in good agreement with experiment whilst at the transition pressure, in the CsCl structure, a drastically increased value of Tc = 12.2 K is found. Calculations also show a dramatic increase in the electronic density of states at this pressure. The lowest energy acoustic phonon branch in each structure also demonstrates some softening effects. Electronic structure calculations of the Fermi surface in both phases are presented for the first time as well as further calculations of the generalised susceptibility with the inclusion of matrix elements. These calculations indicate that the softening is not derived from Fermi surface nesting and it is concluded to be due to a wavevector-dependent enhancement of the electron-phonon coupling.

Magnetotransport study of Dirac fermions in YbMnBi2 and CaMnBi2

NASA Astrophysics Data System (ADS)

Wang, Aifeng; Zaliznyak, Igor; Graf, David; Ren, Weijun; Wang, Kefeng; Wu, Lijun; Garlea, Ovidiu; Warren, John; Bozin, Emil; Zhu, Yimei; Petrovic, Cedomir

It is well known that AMnBi2 (A = alkaline earth) with two dimensional (2D) bismuth layer host quasi-2D Dirac states similar to graphene and topological insulators. The Dirac state is significantly affected by the alkaline earth in the block layer. Angle-resolved photoemission spectroscopy (ARPES) indicates that YbMnBi2 could be the first Weyl semimetal with time-reversal symmetry breaking, whereas the anisotropic Dirac state in SrMnBi2 can host a valley-polarized interlayer current through magnetic valley control. Here, we study in-plane magnetotransport in YbMnBi2, and interlayer magnetotransport in CaMnBi2. The angular-dependent magnetoresistance, nonzero Berry phase, and small cyclotron mass confirm the presence of Dirac fermion and quasi-2D fermi surface in YbMnBi2. The interlayer electronic transport in CaMnBi2 suggest valley polarized conduction and a Dirac state on the side wall of the warped cylindrical Fermi surface of CaMnBi2. Work at BNL was supported by the U.S. Department of Energy-BES, Division of Materials Science and Engineering, under Contract No. DE-SC0012704. Work at the National High Magnetic Field Laboratory is supported by the NSF Cooperative Agreement No. DMR-06541.

Sensitivity of Fermi level position at Ga-polar, N-polar, and nonpolar m-plane GaN surfaces to vacuum and air ambient

NASA Astrophysics Data System (ADS)

Janicki, Łukasz; Ramírez-López, Manolo; Misiewicz, Jan; Cywiński, Grzegorz; Boćkowski, Michał; Muzioł, Grzegorz; Chèze, Caroline; Sawicka, Marta; Skierbiszewski, Czesław; Kudrawiec, Robert

2016-05-01

Ga-polar, N-polar, and nonpolar m-plane GaN UN+ structures have been examined in air and vacuum ambient by contactless electroreflectance (CER). This technique is very sensitive to the surface electric field that varies with the Fermi level position at the surface. For UN+ GaN structures [i.e., GaN (undoped)/GaN (n-type)/substrate], a homogeneous built-in electric field is expected in the undoped GaN layer that is manifested by Franz-Keldysh oscillation (FKO) in CER spectra. A clear change in FKO has been observed in CER spectra for N-polar and nonpolar m-plane structures when changing from air to vacuum ambient. This means that those surfaces are very sensitive to ambient atmosphere. In contrast to that, only a small change in FKO can be seen in the Ga-polar structure. This clearly shows that the ambient sensitivity of the Fermi level position at the GaN surface varies with the crystallographic orientation and is very high for N-polar and nonpolar m-plane surfaces. This feature of the N-polar and nonpolar m-plane surfaces can be very important for GaN-based devices grown on these crystallographic orientations and can be utilized in some of the devices, e.g., sensors.

Universal functions of nuclear proximity potential for Skyrme nucleus-nucleus interaction in a semiclassical approach

NASA Astrophysics Data System (ADS)

Gupta, Raj K.; Singh, Dalip; Kumar, Raj; Greiner, Walter

2009-07-01

The universal function of the nuclear proximity potential is obtained for the Skyrme nucleus-nucleus interaction in the semiclassical extended Thomas-Fermi (ETF) approach. This is obtained as a sum of the spin-orbit-density-independent and spin-orbit-density-dependent parts of the Hamiltonian density, since the two terms behave differently, the spin-orbit-density-independent part mainly attractive and the spin-orbit-density-dependent part mainly repulsive. The semiclassical expansions of kinetic energy density and spin-orbit density are allowed up to second order, and the two-parameter Fermi density, with its parameters fitted to experiments, is used for the nuclear density. The universal functions or the resulting nuclear proximity potential reproduce the 'exact' Skyrme nucleus-nucleus interaction potential in the semiclassical approach, within less than ~1 MeV of difference, both at the maximum attraction and in the surface region. An application of the resulting interaction potential to fusion excitation functions shows clearly that the parameterized universal functions of nuclear proximity potential substitute completely the 'exact' potential in the Skyrme energy density formalism based on the semiclassical ETF method, including also the modifications of interaction barriers at sub-barrier energies in terms of modifying the constants of the universal functions.

Interplay between structure, stoichiometry, and electron transfer dynamics in SILAR-based quantum dot-sensitized oxides.

PubMed

Wang, Hai; Barceló, Irene; Lana-Villarreal, Teresa; Gómez, Roberto; Bonn, Mischa; Cánovas, Enrique

2014-10-08

We quantify the rate and efficiency of picosecond electron transfer (ET) from PbS nanocrystals, grown by successive ionic layer adsorption and reaction (SILAR), into a mesoporous SnO2 support. Successive SILAR deposition steps allow for stoichiometry- and size-variation of the QDs, characterized using transmission electron microscopy. Whereas for sulfur-rich (p-type) QD surfaces substantial electron trapping at the QD surface occurs, for lead-rich (n-type) QD surfaces, the QD trapping channel is suppressed and the ET efficiency is boosted. The ET efficiency increase achieved by lead-rich QD surfaces is found to be QD-size dependent, increasing linearly with QD surface area. On the other hand, ET rates are found to be independent of both QD size and surface stoichiometry, suggesting that the donor-acceptor energetics (constituting the driving force for ET) are fixed due to Fermi level pinning at the QD/oxide interface. Implications of our results for QD-sensitized solar cell design are discussed.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Koryazhkina, M. N., E-mail: mahavenok@mail.ru; Tikhov, S. V.; Gorshkov, O. N.

It is shown that the formation of Au nanoparticles at the insulator–silicon interface in structures with a high density of surface states results in a shift of the Fermi-level pinning energy at this interface towards the valence-band ceiling in silicon and in increasing the surface-state density at energies close to the Fermi level. In this case, a band with a peak at 0.85 eV arises on the photosensivity curves of the capacitor photovoltage, which is explained by the photoemission of electrons from the formed Au-nanoparticle electron states near the valence-band ceiling in silicon.

Fermi surface measurements in YBa2Cu3O(7-x) and La(1.874)Sr(126)CuO4

NASA Astrophysics Data System (ADS)

Howell, R. H.; Sterne, P. A.; Solal, F.; Fluss, M. J.; Haghighi, H.; Kaiser, J. H.; Rayner, S. L.; West, R. N.; Liu, J. Z.; Shelton, R.

1991-06-01

We report new, ultra high precision measurements of the electron-positron momentum spectra of YBa2Cu3O(7-x) and La(1.874)Sr(126)CuO4. The YBCO experiments were performed on twin free, single crystals and show discontinuities with the symmetry of the Fermi surface of the CuO chain bands. Conduction band and underlying features in LSCO share the same symmetry and can only be separated with the aid of LDA calculations.

Fermi surface measurements in YBa 2Cu 3O 7- x and La 1.874Sr .126CuO 4

NASA Astrophysics Data System (ADS)

Howell, R. H.; Sterne, P. A.; Solal, F.; Fluss, M. J.; Haghight, H.; Kaiser, J. H.; Rayner, S. L.; West, R. N.; Liu, J. Z.; Shelton, R.; Kojima, H.; Kitazawa, K.

1991-12-01

We report new, ultra high precision measurements of the electron-positron momentum spectra of YBa 2Cu 3O 7- x and La 1.874Sr .126CuO 4. The YBCO experiments were performed on twin free, single crystals and show discontinuities with the symmetry of the Fermi surface of the CuO chain bands. Conduction band and underlying features in LSCO share the same symmetry and can only be separated with the aid of LDA calculations.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pederson, Mark R.; Baruah, Tunna; Basurto, Luis

We have applied a recently developed method to incorporate the self-interaction correction through Fermi orbitals to Mg-porphyrin, C{sub 60}, and pentacene molecules. The Fermi-Löwdin orbitals are localized and unitarily invariant to the Kohn-Sham orbitals from which they are constructed. The self-interaction-corrected energy is obtained variationally leading to an optimum set of Fermi-Löwdin orbitals (orthonormalized Fermi orbitals) that gives the minimum energy. A Fermi orbital, by definition, is dependent on a certain point which is referred to as the descriptor position. The degree to which the initial choice of descriptor positions influences the variational approach to the minimum and the complexitymore » of the energy landscape as a function of Fermi-orbital descriptors is examined in detail for Mg-porphyrin. The applications presented here also demonstrate that the method can be applied to larger molecular systems containing a few hundred electrons. The atomization energy of the C{sub 60} molecule within the Fermi-Löwdin-orbital self-interaction-correction approach is significantly improved compared to local density approximation in the Perdew-Wang 92 functional and generalized gradient approximation of Perdew-Burke-Ernzerhof functionals. The eigenvalues of the highest occupied molecular orbitals show qualitative improvement.« less

Self-interaction corrections applied to Mg-porphyrin, C60, and pentacene molecules

NASA Astrophysics Data System (ADS)

Pederson, Mark R.; Baruah, Tunna; Kao, Der-you; Basurto, Luis

2016-04-01

We have applied a recently developed method to incorporate the self-interaction correction through Fermi orbitals to Mg-porphyrin, C60, and pentacene molecules. The Fermi-Löwdin orbitals are localized and unitarily invariant to the Kohn-Sham orbitals from which they are constructed. The self-interaction-corrected energy is obtained variationally leading to an optimum set of Fermi-Löwdin orbitals (orthonormalized Fermi orbitals) that gives the minimum energy. A Fermi orbital, by definition, is dependent on a certain point which is referred to as the descriptor position. The degree to which the initial choice of descriptor positions influences the variational approach to the minimum and the complexity of the energy landscape as a function of Fermi-orbital descriptors is examined in detail for Mg-porphyrin. The applications presented here also demonstrate that the method can be applied to larger molecular systems containing a few hundred electrons. The atomization energy of the C60 molecule within the Fermi-Löwdin-orbital self-interaction-correction approach is significantly improved compared to local density approximation in the Perdew-Wang 92 functional and generalized gradient approximation of Perdew-Burke-Ernzerhof functionals. The eigenvalues of the highest occupied molecular orbitals show qualitative improvement.

Localization of massless Dirac particles via spatial modulations of the Fermi velocity

NASA Astrophysics Data System (ADS)

Downing, C. A.; Portnoi, M. E.

2017-08-01

The electrons found in Dirac materials are notorious for being difficult to manipulate due to the Klein phenomenon and absence of backscattering. Here we investigate how spatial modulations of the Fermi velocity in two-dimensional Dirac materials can give rise to localization effects, with either full (zero-dimensional) confinement or partial (one-dimensional) confinement possible depending on the geometry of the velocity modulation. We present several exactly solvable models illustrating the nature of the bound states which arise, revealing how the gradient of the Fermi velocity is crucial for determining fundamental properties of the bound states such as the zero-point energy. We discuss the implications for guiding electronic waves in few-mode waveguides formed by Fermi velocity modulation.

The MARS15-based FermiCORD code system for calculation of the accelerator-induced residual dose

NASA Astrophysics Data System (ADS)

Grebe, A.; Leveling, A.; Lu, T.; Mokhov, N.; Pronskikh, V.

2018-01-01

The FermiCORD code system, a set of codes based on MARS15 that calculates the accelerator-induced residual doses at experimental facilities of arbitrary configurations, has been developed. FermiCORD is written in C++ as an add-on to Fortran-based MARS15. The FermiCORD algorithm consists of two stages: 1) simulation of residual doses on contact with the surfaces surrounding the studied location and of radionuclide inventories in the structures surrounding those locations using MARS15, and 2) simulation of the emission of the nuclear decay γ-quanta by the residuals in the activated structures and scoring the prompt doses of these γ-quanta at arbitrary distances from those structures. The FermiCORD code system has been benchmarked against similar algorithms based on other code systems and against experimental data from the CERF facility at CERN, and FermiCORD showed reasonable agreement with these. The code system has been applied for calculation of the residual dose of the target station for the Mu2e experiment and the results have been compared to approximate dosimetric approaches.

Field-orientation dependence of low-energy quasiparticle excitations in the heavy-electron superconductor UBe(13).

PubMed

Shimizu, Yusei; Kittaka, Shunichiro; Sakakibara, Toshiro; Haga, Yoshinori; Yamamoto, Etsuji; Amitsuka, Hiroshi; Tsutsumi, Yasumasa; Machida, Kazushige

2015-04-10

Low-energy quasiparticle excitations in the superconducting (SC) state of UBe_{13} were studied by means of specific-heat (C) measurements in a rotating field. Quite unexpectedly, the magnetic-field dependence of C(H) is linear in H with no angular dependence at low fields in the SC state, implying that the gap is fully open over the Fermi surfaces, in stark contrast to previous expectations. In addition, a characteristic cubic anisotropy of C(H) was observed above 2 T with a maximum (minimum) for H∥[001] ([111]) within the (11[over ¯]0) plane, in the normal as well as in the SC states. This oscillation possibly originates from the anisotropic response of the heavy quasiparticle bands, and might be a key to understand the unusual properties of UBe_{13}.

Anomalous Nernst effect in type-II Weyl semimetals

NASA Astrophysics Data System (ADS)

Saha, Subhodip; Tewari, Sumanta

2018-01-01

Topological Weyl semimetals (WSM), a new state of quantum matter with gapless nodal bulk spectrum and open Fermi arc surface states, have recently sparked enormous interest in condensed matter physics. Based on the symmetry and fermiology, it has been proposed that WSMs can be broadly classified into two types, type-I and type-II Weyl semimetals. While the undoped, conventional, type-I WSMs have point like Fermi surface and vanishing density of states (DOS) at the Fermi energy, the type-II Weyl semimetals break Lorentz symmetry explicitly and have tilted conical spectra with electron and hole pockets producing finite DOS at the Fermi level. The tilted conical spectrum and finite DOS at Fermi level in type-II WSMs have recently been shown to produce interesting effects such as a chiral anomaly induced longitudinal magnetoresistance that is strongly anisotropic in direction and a novel anomalous Hall effect. In this work, we consider the anomalous Nernst effect in type-II WSMs in the absence of an external magnetic field using the framework of semi-classical Boltzmann theory. Based on both a linearized model of time-reversal breaking WSM with a higher energy cut-off and a more realistic lattice model, we show that the anomalous Nernst response in these systems is strongly anisotropic in space, and can serve as a reliable signature of type-II Weyl semimetals in a host of magnetic systems with spontaneously broken time reversal symmetry.

Discussion on the energy content of the galactic dark matter Bose-Einstein condensate halo in the Thomas-Fermi approximation

DOE Office of Scientific and Technical Information (OSTI.GOV)

De Souza, J.C.C.; Pires, M.O.C., E-mail: jose.souza@ufabc.edu.br, E-mail: marcelo.pires@ufabc.edu.br

We show that the galactic dark matter halo, considered composed of an axionlike particles Bose-Einstein condensate [6] trapped by a self-graviting potential [5], may be stable in the Thomas-Fermi approximation since appropriate choices for the dark matter particle mass and scattering length are made. The demonstration is performed by means of the calculation of the potential, kinetic and self-interaction energy terms of a galactic halo described by a Boehmer-Harko density profile. We discuss the validity of the Thomas-Fermi approximation for the halo system, and show that the kinetic energy contribution is indeed negligible.

New Results from Fermi-LAT and Their Implications for the Nature of Dark Matter and the Origin of Cosmic Rays

NASA Technical Reports Server (NTRS)

Moiseev, Alexander

2009-01-01

The measured spectrum is compatible with a power law within our current systematic errors. The spectral index (-3.04) is harder than expected from previous experiments and simple theoretical considerations. "Pre-Fermi" diffusive model requires a harder electron injection spectrum (by 0.12) to fit the Fermi data, but inconsistent with positron excess reported by Pamela if it extends to higher energy. Additional component of electron flux from local source(s) may solve the problem; its origin, astrophysical or exotic, is still unclear. Valuable contribution to the calculation of IC component of diffuse gamma radiation.

Fluctuations Magnetiques des Gaz D'electrons Bidimensionnels: Application AU Compose Supraconducteur LANTHANE(2-X) Strontium(x) Cuivre OXYGENE(4)

NASA Astrophysics Data System (ADS)

Benard, Pierre

Nous presentons une etude des fluctuations magnetiques de la phase normale de l'oxyde de cuivre supraconducteur La_{2-x}Sr _{x}CuO_4 . Le compose est modelise par le Hamiltonien de Hubbard bidimensionnel avec un terme de saut vers les deuxiemes voisins (modele tt'U). Le modele est etudie en utilisant l'approximation de la GRPA (Generalized Random Phase Approximation) et en incluant les effets de la renormalisation de l'interaction de Hubbard par les diagrammes de Brueckner-Kanamori. Dans l'approche presentee dans ce travail, les maximums du facteur de structure magnetique observes par les experiences de diffusion de neutrons sont associes aux anomalies 2k _{F} de reseau du facteur de structure des gaz d'electrons bidimensionnels sans interaction. Ces anomalies proviennent de la diffusion entre particules situees a des points de la surface de Fermi ou les vitesses de Fermi sont tangentes, et conduisent a des divergences dont la nature depend de la geometrie de la surface de Fermi au voisinage de ces points. Ces resultats sont ensuite appliques au modele tt'U, dont le modele de Hubbard usuel tU est un cas particulier. Dans la majorite des cas, les interactions ne determinent pas la position des maximums du facteur de structure. Le role de l'interaction est d'augmenter l'intensite des structures du facteur de structure magnetique associees a l'instabilite magnetique du systeme. Ces structures sont souvent deja presentes dans la partie imaginaire de la susceptibilite sans interaction. Le rapport d'intensite entre les maximums absolus et les autres structures du facteur de structure magnetique permet de determiner le rapport U_ {rn}/U_{c} qui mesure la proximite d'une instabilite magnetique. Le diagramme de phase est ensuite etudie afin de delimiter la plage de validite de l'approximation. Apres avoir discute des modes collectifs et de l'effet d'une partie imaginaire non-nulle de la self-energie, l'origine de l'echelle d'energie des fluctuations magnetiques est examinee. Il est ensuite demontre que le modele a trois bandes predit les memes resultats pour la position des structures du facteur de structure magnetique que le modele a une bande, dans la limite ou l'hybridation des orbitales des atomes d'oxygene des plans Cu-O_2 et l'amplitude de sauts vers les seconds voisins sont nulles. Il est de plus constate que l'effet de l'hybridation des orbitales des atomes d'oxygene est bien modelise par le terme de saut vers les seconds voisins. Meme si ils decrivent correctement le comportement qualitatif des maximums du facteur de structure magnetique, les modeles a trois bandes et a une bande ne permettent pas d'obtenir une position de ces structures conforme avec les mesures experimentales, si on suppose que la bande est rigide, c'est-a-dire que les parametres du Hamiltonien sont independants de la concentration de strontium. Ceci peut etre cause par la dependance des parametres du Hamiltonien sur la concentration de strontium. Finalement, les resultats sont compares avec les experiences de diffusion de neutrons et les autres theories, en particulier celles de Littlewood et al. (1993) et de Q. Si et al. (1993). La comparaison avec les resultats experimentaux pour le compose de lanthane suggere que le liquide de Fermi possede une surface de Fermi disjointe, et qu'il est situe pres d'une instabilite magnetique incommensurable.

Lifshitz Transitions, Type-II Dirac and Weyl Fermions, Event Horizon and All That

NASA Astrophysics Data System (ADS)

Volovik, G. E.; Zhang, K.

2017-12-01

The type-II Weyl and type-II Dirac points emerge in semimetals and also in relativistic systems. In particular, the type-II Weyl fermions may emerge behind the event horizon of black holes. In this case the horizon with Painlevé-Gullstrand metric serves as the surface of the Lifshitz transition. This relativistic analogy allows us to simulate the black hole horizon and Hawking radiation using the fermionic superfluid with supercritical velocity, and the Dirac and Weyl semimetals with the interface separating the type-I and type-II states. The difference between such type of the artificial event horizon and that which arises in acoustic metric is discussed. At the Lifshitz transition between type-I and type-II fermions the Dirac lines may also emerge, which are supported by the combined action of topology and symmetry. The type-II Weyl and Dirac points also emerge as the intermediate states of the topological Lifshitz transitions. Different configurations of the Fermi surfaces, involved in such Lifshitz transition, are discussed. In one case the type-II Weyl point connects the Fermi pockets and the Lifshitz transition corresponds to the transfer of the Berry flux between the Fermi pockets. In the other case the type-II Weyl point connects the outer and inner Fermi surfaces. At the Lifshitz transition the Weyl point is released from both Fermi surfaces. They loose their Berry flux, which guarantees the global stability, and without the topological support the inner surface disappears after shrinking to a point at the second Lifshitz transition. These examples reveal the complexity and universality of topological Lifshitz transitions, which originate from the ubiquitous interplay of a variety of topological characters of the momentum-space manifolds. For the interacting electrons, the Lifshitz transitions may lead to the formation of the dispersionless (flat) band with zero energy and singular density of states, which opens the route to room-temperature superconductivity. Originally, the idea of the enhancement of T_c due to flat band has been put forward by the nuclear physics community, and this also demonstrates the close connections between different areas of physics.

Theoretical Interpretation of Pass 8 Fermi -LAT e + + e - Data

DOE PAGES

Di Mauro, M.; Manconi, S.; Vittino, A.; ...

2017-08-17

The flux of positrons and electrons (e + + e -) has been measured by the Fermi Large Area Telescope (LAT) in the energy range between 7 GeV and 2 TeV. Here, we discuss a number of interpretations of Pass 8 Fermi-LAT e + + e - spectrum, combining electron and positron emission from supernova remnants (SNRs) and pulsar wind nebulae (PWNe), or produced by the collision of cosmic rays (CRs) with the interstellar medium. We also found that the Fermi-LAT spectrum is compatible with the sum of electrons from a smooth SNR population, positrons from cataloged PWNe, and amore » secondary component. If we include in our analysis constraints from the AMS-02 positron spectrum, we obtain a slightly worse fit to the e + + e - Fermi-LAT spectrum, depending on the propagation model. As an additional scenario, we replace the smooth SNR component within 0.7 kpc with the individual sources found in Green's catalog of Galactic SNRs. We find that separate consideration of far and near sources helps to reproduce the e + + e - Fermi-LAT spectrum. However, we show that the fit degrades when the radio constraints on the positron emission from Vela SNR (which is the main contributor at high energies) are taken into account. We find that a break in the power-law injection spectrum at about 100 GeV can also reproduce the measured e + + e -spectrum and, among the CR propagation models that we consider, no reasonable break of the power-law dependence of the diffusion coefficient can modify the electron flux enough to reproduce the observed shape.« less

Theoretical Interpretation of Pass 8 Fermi -LAT e + + e - Data

DOE Office of Scientific and Technical Information (OSTI.GOV)

Di Mauro, M.; Manconi, S.; Vittino, A.

The flux of positrons and electrons (e + + e -) has been measured by the Fermi Large Area Telescope (LAT) in the energy range between 7 GeV and 2 TeV. Here, we discuss a number of interpretations of Pass 8 Fermi-LAT e + + e - spectrum, combining electron and positron emission from supernova remnants (SNRs) and pulsar wind nebulae (PWNe), or produced by the collision of cosmic rays (CRs) with the interstellar medium. We also found that the Fermi-LAT spectrum is compatible with the sum of electrons from a smooth SNR population, positrons from cataloged PWNe, and amore » secondary component. If we include in our analysis constraints from the AMS-02 positron spectrum, we obtain a slightly worse fit to the e + + e - Fermi-LAT spectrum, depending on the propagation model. As an additional scenario, we replace the smooth SNR component within 0.7 kpc with the individual sources found in Green's catalog of Galactic SNRs. We find that separate consideration of far and near sources helps to reproduce the e + + e - Fermi-LAT spectrum. However, we show that the fit degrades when the radio constraints on the positron emission from Vela SNR (which is the main contributor at high energies) are taken into account. We find that a break in the power-law injection spectrum at about 100 GeV can also reproduce the measured e + + e -spectrum and, among the CR propagation models that we consider, no reasonable break of the power-law dependence of the diffusion coefficient can modify the electron flux enough to reproduce the observed shape.« less

Positron Annihilation in Superconducting 123 Compounds

NASA Astrophysics Data System (ADS)

Peter, M.; Manuel, A. A.; Erb, A.

After a brief review of the theory of angular correlation of positron annihilation radiation (ACAR), we illustrate experimental principles and give examples of successful determination of electron momentum density (EMD) and of positron lifetime in solids. The central question which we try to answer concerns the contribution of positron spectroscopy to the knowledge and understanding of the new high temperature superconducting oxides. We find that in these oxides also, partially filled bands exist and we can observe parts of their Fermi surface and measure lifetimes in accordance with band theoretical calculations. There are characteristic differences, however. The intensity of the anisotropy of the ACAR signal is below theoretical expectation and signals depend on sample preparation. Recent studies by the Geneva group have concerned dependence of the signals on impurities, on oxygen content and on the thermal history of preparation. Of particular interest are correlations between the variations of these signals and between the variations of structural and transport properties in these substances. Besides deliberate additions of impurities, the Geneva group also reports progress in the preparations of samples of highest purity (barium zirconate crucibles). The alloy series PrxY1-xBa2Cu3O7-δ is of special interest because of exceptional transport properties. The recent positron results on these materials will be presented and commented in the light of theoretical models and in the light of the reported superconductivity of the Pr-compound.

Valley polarization in bismuth

NASA Astrophysics Data System (ADS)

Fauque, Benoit

2013-03-01

The electronic structure of certain crystal lattices can contain multiple degenerate valleys for their charge carriers to occupy. The principal challenge in the development of valleytronics is to lift the valley degeneracy of charge carriers in a controlled way. In bulk semi-metallic bismuth, the Fermi surface includes three cigar-shaped electron valleys lying almost perpendicular to the high symmetry axis known as the trigonal axis. The in-plane mass anisotropy of each valley exceeds 200 as a consequence of Dirac dispersion, which drastically reduces the effective mass along two out of the three orientations. According to our recent study of angle-dependent magnetoresistance in bismuth, a flow of Dirac electrons along the trigonal axis is extremely sensitive to the orientation of in-plane magnetic field. Thus, a rotatable magnetic field can be used as a valley valve to tune the contribution of each valley to the total conductivity. As a consequence of a unique combination of high mobility and extreme mass anisotropy in bismuth, the effect is visible even at room temperature in a magnetic field of 1 T. Thus, a modest magnetic field can be used as a valley valve in bismuth. The results of our recent investigation of angle-dependent magnetoresistance in other semi-metals and doped semiconductors suggest that a rotating magnetic field can behave as a valley valve in a multi-valley system with sizeable mass anisotropy.

Critical temperature for shape transition in hot nuclei within covariant density functional theory

NASA Astrophysics Data System (ADS)

Zhang, W.; Niu, Y. F.

2018-05-01

Prompted by the simple proportional relation between critical temperature for pairing transition and pairing gap at zero temperature, we investigate the relation between critical temperature for shape transition and ground-state deformation by taking even-even Cm-304286 isotopes as examples. The finite-temperature axially deformed covariant density functional theory with BCS pairing correlation is used. Since the Cm isotopes are the newly proposed nuclei with octupole correlations, we studied in detail the free energy surface, the Nilsson single-particle (s.p.) levels, and the components of s.p. levels near the Fermi level in 292Cm. Through this study, the formation of octupole equilibrium is understood by the contribution coming from the octupole driving pairs with Ω [N ,nz,ml] and Ω [N +1 ,nz±3 ,ml] for single-particle levels near the Fermi surfaces as it provides a good manifestation of the octupole correlation. Furthermore, the systematics of deformations, pairing gaps, and the specific heat as functions of temperature for even-even Cm-304286 isotopes are discussed. Similar to the relation between the critical pairing transition temperature and the pairing gap at zero temperature Tc=0.6 Δ (0 ) , a proportional relation between the critical shape transition temperature and the deformation at zero temperature Tc=6.6 β (0 ) is found for both octupole shape transition and quadrupole shape transition for the isotopes considered.

A comparative study of different methods for calculating electronic transition rates

NASA Astrophysics Data System (ADS)

Kananenka, Alexei A.; Sun, Xiang; Schubert, Alexander; Dunietz, Barry D.; Geva, Eitan

2018-03-01

We present a comprehensive comparison of the following mixed quantum-classical methods for calculating electronic transition rates: (1) nonequilibrium Fermi's golden rule, (2) mixed quantum-classical Liouville method, (3) mean-field (Ehrenfest) mixed quantum-classical method, and (4) fewest switches surface-hopping method (in diabatic and adiabatic representations). The comparison is performed on the Garg-Onuchic-Ambegaokar benchmark charge-transfer model, over a broad range of temperatures and electronic coupling strengths, with different nonequilibrium initial states, in the normal and inverted regimes. Under weak to moderate electronic coupling, the nonequilibrium Fermi's golden rule rates are found to be in good agreement with the rates obtained via the mixed quantum-classical Liouville method that coincides with the fully quantum-mechanically exact results for the model system under study. Our results suggest that the nonequilibrium Fermi's golden rule can serve as an inexpensive yet accurate alternative to Ehrenfest and the fewest switches surface-hopping methods.

Nicholas Metropolis Award for Outstanding Doctoral Thesis Work in Computational Physics Talk: Equation of State of the Dilute Fermi Gases

NASA Astrophysics Data System (ADS)

Chang, Soon Yong

2008-04-01

In the recent years, dilute Fermi gases have played the center stage role in the many-body physics. The gas of neutral alkali atoms such as Lithium-6 and Potassium-40 can be trapped at temperatures below the Fermi degeneracy. The most relevant feature of these gases is that the interaction is tunable and strongly interacting superfluid can be artificially created. I will discuss the recent progress in understanding the ground state properties of the dilute Fermi gases at different interaction regimes. First, I will present the case of the spin symmetric systems where the Fermi gas can smoothly crossover from the BCS regime to the BEC regime. Then, I will discuss the case of the spin polarized systems, where different quantum phases can occur as a function of the polarization. In the laboratory, the trapped Fermi gas shows spatial dependence of the different quantum phases. This can be understood in the context of the local variation of the chemical potential. I will present the most accurate quantum ab initio results and the relevant experiments.

Fractionalized Fermi liquid in a Kondo-Heisenberg model

DOE PAGES

Tsvelik, A. M.

2016-10-10

The Kondo-Heisenberg model is used as a controllable tool to demonstrate the existence of a peculiar metallic state with unbroken translational symmetry where the Fermi surface volume is not controlled by the total electron density. Here, I use a nonperturbative approach where the strongest interactions are taken into account by means of exact solution, and corrections are controllable. The resulting metallic state represents a fractionalized Fermi liquid where well defined quasiparticles coexist with gapped fractionalized collective excitations, in agreement with the general requirements formulated by T. Senthil et al. [Phys. Rev. Lett. 90, 216403 (2003)]. Furthermore, the system undergoes amore » phase transition to an ordered phase (charge density wave or superconducting), at the transition temperature which is parametrically small in comparison to the quasiparticle Fermi energy.« less

Surface modification effects of fluorine-doped tin dioxide by oxygen plasma ion implantation

NASA Astrophysics Data System (ADS)

Tang, Peng; Liu, Cai; Zhang, Jingquan; Wu, Lili; Li, Wei; Feng, Lianghuan; Zeng, Guanggen; Wang, Wenwu

2018-04-01

SnO2:F (FTO), as a kind of transparent conductive oxide (TCO), exhibits excellent transmittance and conductivity and is widely used as transparency electrodes in solar cells. It's very important to modifying the surface of FTO for it plays a critical role in CdTe solar cells. In this study, modifying effects of oxygen plasma on FTO was investigated systematically. Oxygen plasma treatment on FTO surface with ion accelerating voltage ranged from 0.4 kV to 1.6 kV has been processed. The O proportion of surface was increased after ion implantation. The Fermi level of surface measurement by XPS valance band spectra was lowered as the ion accelerating voltage increased to 1.2 kV and then raised as accelerating voltage was elevated to 1.6 kV. The work function measured by Kelvin probe force microscopy increased after ion implanting, and it was consistent with the variation of Fermi level. The change of energy band structure of FTO surface mainly originated from the surface composition variation. As FTO conduction was primarily due to oxyanion hole, the carrier was electron and its concentration was reduced while O proportion was elevated at the surface of FTO, as a result, the Fermi level lowered and the work function was enlarged. It was proved that oxygen plasma treatment is an effective method to modulate the energy band structure of the surface as well as other properties of FTO, which provides much more space for interface and surface modification and then photoelectric device performance promotion.

Dimensional crossover and thermoelectric properties in CeTe2-xSbx single crystals

NASA Astrophysics Data System (ADS)

Rhyee, Jong-Soo; Lee, Kyung Eun; Nyeong Kim, Jae; Shim, Ji Hoon; Min, Byeong Hun; Kwon, Yong Seung

2013-03-01

Several years before, we proposed that the charge density wave is a new pathway for high thermoelectric performance in In4Se3-x bulk crystalline materials. (Nature v.459, p. 965, 2009) Recently, from the increase of the chemical potential by halogen doped In4Se3-xH0.03 (H =Halogen elements) crystals, we achieved high ZT (maximum ZT 1.53) over a wide temperature range. (Adv. Mater. v.23, p.2191, 2011) Here we demonstrate the low dimensionality increases power factor in CeTe2-xSbx single crystals. The band structures of CeTe2 show the 2-dimensional (2D) Fermi surface nesting behavior as well as a 3-dimensional (3D) electron Fermi surface hindering the perfect charge density wave (CDW) gap opening. By hole doping with the substitution of Sb at the Te-site, the 3D-like Fermi surface disappears and the 2D perfect CDW gap opening enhances the power factor up to x = 0.1. With further hole doping, the Fermi surfaces become 3-dimensional structure with heavy hole bands. The enhancement of the power factor is observed near the dimensional crossover of CDW, at x = 0.1, where the CDW gap is maximized. This research was supported by Basic Science Research Program (2011-0021335), Mid-career Research Program (Strategy) (No. 2012R1A2A1A03005174) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, and TJ Park Junior Faculty Fellowship funded by the POSCO TJ Park Foundation.

Reinvestigating the surface and bulk electronic properties of Cd3As2

NASA Astrophysics Data System (ADS)

Roth, S.; Lee, H.; Sterzi, A.; Zacchigna, M.; Politano, A.; Sankar, R.; Chou, F. C.; Di Santo, G.; Petaccia, L.; Yazyev, O. V.; Crepaldi, A.

2018-04-01

Cd3As2 is widely considered among the few materials realizing the three-dimensional (3D) Dirac semimetal phase. Linearly dispersing states, responsible for the ultrahigh charge mobility, have been reported by several angle-resolved photoelectron spectroscopy (ARPES) investigations. However, in spite of the general agreement between these studies, some details are at odds. From scanning tunneling microscopy and optical experiments under magnetic field, a puzzling scenario emerges in which multiple states show linear dispersion at different energy scales. Here, we solve this apparent controversy by reinvestigating the electronic properties of the (112) surface of Cd3As2 by combining ARPES and theoretical calculations. We disentangle the presence of massive and massless metallic bulk and surface states, characterized by different symmetries. Our systematic experimental and theoretical study clarifies the complex band dispersion of Cd3As2 by extending the simplistic 3D Dirac semimetal model to account for multiple bulk and surface states crossing the Fermi level, and thus contributing to the unique material transport properties.

Calculation of the Schottky barrier and current–voltage characteristics of metal–alloy structures based on silicon carbide

DOE Office of Scientific and Technical Information (OSTI.GOV)

Altuhov, V. I., E-mail: altukhovv@mail.ru; Kasyanenko, I. S.; Sankin, A. V.

2016-09-15

A simple but nonlinear model of the defect density at a metal–semiconductor interface, when a Schottky barrier is formed by surface defects states localized at the interface, is developed. It is shown that taking the nonlinear dependence of the Fermi level on the defect density into account leads to a Schottky barrier increase by 15–25%. The calculated barrier heights are used to analyze the current–voltage characteristics of n-M/p-(SiC){sub 1–x}(AlN){sub x} structures. The results of calculations are compared to experimental data.

Comparison of electronic structure between monolayer silicenes on Ag (111)

NASA Astrophysics Data System (ADS)

Chun-Liang, Lin; Ryuichi, Arafune; Maki, Kawai; Noriaki, Takagi

2015-08-01

The electronic structures of monolayer silicenes (4 × 4 and ) grown on Ag (111) surface are studied by scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. While both phases have similar electronic structures around the Fermi level, significant differences are observed in the higher energy unoccupied states. The DFT calculations show that the contributions of Si 3pz orbitals to the unoccupied states are different because of their different buckled configurations. Project supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) through Grants-in-Aid for Scientific Research (Grant Nos. 24241040 and 25110008) and the World Premier International Research Center Initiative (WPI), MEXT, Japan.

Electronic and Magnetic Properties of Ni-Doped Zinc-Blende ZnO: A First-Principles Study.

PubMed

Xue, Suqin; Zhang, Fuchun; Zhang, Shuili; Wang, Xiaoyang; Shao, Tingting

2018-04-26

The electronic structure, band structure, density of state, and magnetic properties of Ni-doped zinc-blende (ZB) ZnO are studied by using the first-principles method based on the spin-polarized density-functional theory. The calculated results show that Ni atoms can induce a stable ferromagnetic (FM) ground state in Ni-doped ZB ZnO. The magnetic moments mainly originate from the unpaired Ni 3 d orbitals, and the O 2 p orbitals contribute a little to the magnetic moments. The magnetic moment of a supercell including a single Ni atom is 0.79 μ B . The electronic structure shows that Ni-doped ZB ZnO is a half-metallic FM material. The strong spin-orbit coupling appears near the Fermi level and shows obvious asymmetry for spin-up and spin-down density of state, which indicates a significant hybrid effects from the Ni 3 d and O 2 p states. However, the coupling of the anti-ferromagnetic (AFM) state show metallic characteristic, the spin-up and spin-down energy levels pass through the Fermi surface. The magnetic moment of a single Ni atom is 0.74 μ B . Moreover, the results show that the Ni 3 d and O 2 p states have a strong p - d hybridization effect near the Fermi level and obtain a high stability. The above theoretical results demonstrate that Ni-doped zinc blende ZnO can be considered as a potential half-metal FM material and dilute magnetic semiconductors.

Nematic order on the surface of a three-dimensional topological insulator

NASA Astrophysics Data System (ADS)

Lundgren, Rex; Yerzhakov, Hennadii; Maciejko, Joseph

2017-12-01

We study the spontaneous breaking of rotational symmetry in the helical surface state of three-dimensional topological insulators due to strong electron-electron interactions, focusing on time-reversal invariant nematic order. Owing to the strongly spin-orbit coupled nature of the surface state, the nematic order parameter is linear in the electron momentum and necessarily involves the electron spin, in contrast with spin-degenerate nematic Fermi liquids. For a chemical potential at the Dirac point (zero doping), we find a first-order phase transition at zero temperature between isotropic and nematic Dirac semimetals. This extends to a thermal phase transition that changes from first to second order at a finite-temperature tricritical point. At finite doping, we find a transition between isotropic and nematic helical Fermi liquids that is second order even at zero temperature. Focusing on finite doping, we discuss various observable consequences of nematic order, such as anisotropies in transport and the spin susceptibility, the partial breakdown of spin-momentum locking, collective modes and induced spin fluctuations, and non-Fermi-liquid behavior at the quantum critical point and in the nematic phase.

Fermi-Surface Topological Phase Transition and Horizontal Order-Parameter Nodes in CaFe2As2 Under Pressure

PubMed Central

Gonnelli, R. S.; Daghero, D.; Tortello, M.; Ummarino, G. A.; Bukowski, Z.; Karpinski, J.; Reuvekamp, P. G.; Kremer, R. K.; Profeta, G.; Suzuki, K.; Kuroki, K.

2016-01-01

Iron-based compounds (IBS) display a surprising variety of superconducting properties that seems to arise from the strong sensitivity of these systems to tiny details of the lattice structure. In this respect, systems that become superconducting under pressure, like CaFe2As2, are of particular interest. Here we report on the first directional point-contact Andreev-reflection spectroscopy (PCARS) measurements on CaFe2As2 crystals under quasi-hydrostatic pressure, and on the interpretation of the results using a 3D model for Andreev reflection combined with ab-initio calculations of the Fermi surface (within the density functional theory) and of the order parameter symmetry (within a random-phase-approximation approach in a ten-orbital model). The almost perfect agreement between PCARS results at different pressures and theoretical predictions highlights the intimate connection between the changes in the lattice structure, a topological transition in the holelike Fermi surface sheet, and the emergence on the same sheet of an order parameter with a horizontal node line. PMID:27216477

Fragile charge order in the nonsuperconducting ground state of the underdoped high-temperature superconductors.

PubMed

Tan, B S; Harrison, N; Zhu, Z; Balakirev, F; Ramshaw, B J; Srivastava, A; Sabok-Sayr, S A; Sabok, S A; Dabrowski, B; Lonzarich, G G; Sebastian, Suchitra E

2015-08-04

The normal state in the hole underdoped copper oxide superconductors has proven to be a source of mystery for decades. The measurement of a small Fermi surface by quantum oscillations on suppression of superconductivity by high applied magnetic fields, together with complementary spectroscopic measurements in the hole underdoped copper oxide superconductors, point to a nodal electron pocket from charge order in YBa2Cu3(6+δ). Here, we report quantum oscillation measurements in the closely related stoichiometric material YBa2Cu4O8, which reveals similar Fermi surface properties to YBa2Cu3(6+δ), despite the nonobservation of charge order signatures in the same spectroscopic techniques, such as X-ray diffraction, that revealed signatures of charge order in YBa2Cu3(6+δ). Fermi surface reconstruction in YBa2Cu4O8 is suggested to occur from magnetic field enhancement of charge order that is rendered fragile in zero magnetic fields because of its potential unconventional nature and/or its occurrence as a subsidiary to more robust underlying electronic correlations.

Fermi-Surface Topological Phase Transition and Horizontal Order-Parameter Nodes in CaFe2As2 Under Pressure

NASA Astrophysics Data System (ADS)

Gonnelli, R. S.; Daghero, D.; Tortello, M.; Ummarino, G. A.; Bukowski, Z.; Karpinski, J.; Reuvekamp, P. G.; Kremer, R. K.; Profeta, G.; Suzuki, K.; Kuroki, K.

2016-05-01

Iron-based compounds (IBS) display a surprising variety of superconducting properties that seems to arise from the strong sensitivity of these systems to tiny details of the lattice structure. In this respect, systems that become superconducting under pressure, like CaFe2As2, are of particular interest. Here we report on the first directional point-contact Andreev-reflection spectroscopy (PCARS) measurements on CaFe2As2 crystals under quasi-hydrostatic pressure, and on the interpretation of the results using a 3D model for Andreev reflection combined with ab-initio calculations of the Fermi surface (within the density functional theory) and of the order parameter symmetry (within a random-phase-approximation approach in a ten-orbital model). The almost perfect agreement between PCARS results at different pressures and theoretical predictions highlights the intimate connection between the changes in the lattice structure, a topological transition in the holelike Fermi surface sheet, and the emergence on the same sheet of an order parameter with a horizontal node line.

Nodal to nodeless superconducting energy-gap structure change concomitant with Fermi-surface reconstruction in the heavy-fermion compound CeCoIn 5

DOE PAGES

Kim, Hyunsoo; Tanatar, M. A.; Flint, R.; ...

2015-01-15

The London penetration depth, λ(T), was measured in single crystals of Ce 1-xR xCoIn 5, R=La, Nd and Yb down to T min ≈ 50 mK (T c/T min ~50) using a tunnel-diode resonator. In the cleanest samples Δλ(T) is best described by the power law, Δλ(T) ∝ T n, with n ~ 1, consistent with line nodes. Substitutions of Ce with La, Nd and Yb lead to similar monotonic suppressions of T c, however the effects on Δλ(T) differ. While La and Nd dopings lead to increase of the exponent n and saturation at n ~ 2, as expectedmore » for a dirty nodal superconductor, Yb doping leads to n > 3, suggesting a change from nodal to nodeless superconductivity. As a result, this superconducting gap structure change happen in the same doping range where changes of the Fermi surface topology were reported, implying that the nodal structure and Fermi surface topology are closely linked.« less

Single reconstructed Fermi surface pocket in an underdoped single-layer cuprate superconductor

DOE PAGES

Chan, Mun Keat; Harrison, Neil; Mcdonald, Ross David; ...

2016-07-22

The observation of a reconstructed Fermi surface via quantum oscillations in hole-doped cuprates opened a path towards identifying broken symmetry states in the pseudogap regime. However, such an identification has remained inconclusive due to the multi-frequency quantum oscillation spectra and complications accounting for bilayer effects in most studies. We overcome these impediments with high-resolution measurements on the structurally simpler cuprate HgBa2CuO4+δ (Hg1201), which features one CuO2 plane per primitive unit cell. We find only a single oscillatory component with no signatures of magnetic breakdown tunnelling to additional orbits. Therefore, the Fermi surface comprises a single quasi-two-dimensional pocket. Quantitative modelling ofmore » these results indicates that a biaxial charge density wave within each CuO2 plane is responsible for the reconstruction and rules out criss-crossed charge stripes between layers as a viable alternative in Hg1201. Lastly, we determine that the characteristic gap between reconstructed pockets is a significant fraction of the pseudogap energy« less

Split Fermi Surfaces of the Spin-Orbit-Coupled Metal Cd2Re2O7 Probed by de Haas-van Alphen Effect

NASA Astrophysics Data System (ADS)

Matsubayashi, Yasuhito; Sugii, Kaori; Hirose, Hishiro T.; Hirai, Daigorou; Sugiura, Shiori; Terashima, Taichi; Uji, Shinya; Hiroi, Zenji

2018-05-01

The superconducting pyrochlore oxide Cd2Re2O7 shows a structural transition with inversion symmetry breaking (ISB) at Ts1 = 200 K. A recent theory [L. Fu, Phys. Rev. Lett. 115, 026401 (2015)] suggests that the origin is an electronic instability that leads to a multipolar order in the spin-orbit-coupled metal. To observe the Fermi surface of the low-temperature phase of Cd2Re2O7, we perform de Haas-van Alphen effect measurements by means of magnetic torque. In reference to a calculated band structure, the spin-split Fermi surfaces with large cyclotron masses of 5-9m0 are revealed. The splitting is suggested to be due to an antisymmetric spin-orbit coupling induced by ISB, the strength of which is estimated to be approximately 67 K, which is rather smaller than those of typical non-centrosymmetric metals.

Fermi-level tuning of the Dirac surface state in (Bi1-x Sb x )2Se3 thin films

NASA Astrophysics Data System (ADS)

Satake, Yosuke; Shiogai, Junichi; Takane, Daichi; Yamada, Keiko; Fujiwara, Kohei; Souma, Seigo; Sato, Takafumi; Takahashi, Takashi; Tsukazaki, Atsushi

2018-02-01

We report on the electronic states and the transport properties of three-dimensional topological insulator (Bi1-x Sb x )2Se3 ternary alloy thin films grown on an isostructural Bi2Se3 buffer layer on InP substrates. By angle-resolved photoemission spectroscopy, we clearly detected Dirac surface states with a large bulk band gap of 0.2-0.3 eV in the (Bi1-x Sb x )2Se3 film with x  =  0.70. In addition, we observed by Hall effect measurements that the dominant charge carrier converts from electron (n-type) to hole (p-type) at around x  =  0.7, indicating that the Fermi level can be controlled across the Dirac point. Indeed, the carrier transport was shown to be governed by Dirac surface state in 0.63  ⩽  x  ⩽  0.75. These features suggest that Fermi-level tunable (Bi1-x Sb x )2Se3-based heterostructures provide a platform for extracting exotic topological phenomena.

Response of the Shockley surface state to an external electrical field: A density-functional theory study of Cu(111)

NASA Astrophysics Data System (ADS)

Berland, K.; Einstein, T. L.; Hyldgaard, P.

2012-01-01

The response of the Cu(111) Shockley surface state to an external electrical field is characterized by combining a density-functional theory calculation for a slab geometry with an analysis of the Kohn-Sham wave functions. Our analysis is facilitated by a decoupling of the Kohn-Sham states via a rotation in Hilbert space. We find that the surface state displays isotropic dispersion, quadratic until the Fermi wave vector but with a significant quartic contribution beyond. We calculate the shift in energetic position and effective mass of the surface state for an electrical field perpendicular to the Cu(111) surface; the response is linear over a broad range of field strengths. We find that charge transfer occurs beyond the outermost copper atoms and that accumulation of electrons is responsible for a quarter of the screening of the electrical field. This allows us to provide well converged determinations of the field-induced changes in the surface state for a moderate number of layers in the slab geometry.

Electronic structure Fermi liquid theory of high T(sub c) superconductors: Comparison with experiments

NASA Astrophysics Data System (ADS)

Freeman, A. J.; Yu, Jaejun

1990-04-01

For years, there has been controversy on whether the normal state of the Cu-oxide superconductors is a Fermi liquid or some other exotic ground state. However, some experimentalists are clarifying the nature of the normal state of the high T(sub c) superconductors by surmounting the experimental difficulties in producing clean, well characterized surfaces so as to obtain meaningful high resolved photoemission data, which agrees with earlier positron-annihilation experiments. The experimental work on high resolution angle resolved photoemission by Campuzano et al. and positron-annihilation studies by Smedskjaer et al. has verified the calculated Fermi surfaces in YBa2Cu3O7 superconductors and has provided evidence for the validity of the energy band approach. Similar good agreement was found for Bi2Sr2CaCu2O8 by Olson et al. As a Fermi liquid (metallic) nature of the normal state of the high T(sub c) superconductors becomes evident, these experimental observations have served to confirm the predictions of the local density functional calculations and hence the energy band approach as a valid natural starting point for further studies of their superconductivity.

Electronic structure Fermi liquid theory of high T(sub c) superconductors: Comparison with experiments

NASA Technical Reports Server (NTRS)

Freeman, A. J.; Yu, Jaejun

1990-01-01

For years, there has been controversy on whether the normal state of the Cu-oxide superconductors is a Fermi liquid or some other exotic ground state. However, some experimentalists are clarifying the nature of the normal state of the high T(sub c) superconductors by surmounting the experimental difficulties in producing clean, well characterized surfaces so as to obtain meaningful high resolved photoemission data, which agrees with earlier positron-annihilation experiments. The experimental work on high resolution angle resolved photoemission by Campuzano et al. and positron-annihilation studies by Smedskjaer et al. has verified the calculated Fermi surfaces in YBa2Cu3O7 superconductors and has provided evidence for the validity of the energy band approach. Similar good agreement was found for Bi2Sr2CaCu2O8 by Olson et al. As a Fermi liquid (metallic) nature of the normal state of the high T(sub c) superconductors becomes evident, these experimental observations have served to confirm the predictions of the local density functional calculations and hence the energy band approach as a valid natural starting point for further studies of their superconductivity.

Fermi surfaces and electronic topological transitions in metallic solid solutions

NASA Astrophysics Data System (ADS)

Bruno, E.; Ginatempo, B.; Guiliano, E. S.; Ruban, A. V.; Vekilov, Yu. Kh.

1994-12-01

Notwithstanding the substitutional disorder, the Fermi surface of metallic alloys can be measured and computed. We show that, from the theoretical point of view, it is defined as the locus of the peaks of the Bloch Spectral Function (BSF). Such Fermi surfaces, on varying the atomic concentrations, may undergo changes of their topology, known as Electronic Topological Transitions (ETT). Thus, for instance, pockets of electrons or holes may appear or disappear, necks may open or close. ETTs cause anomalous behaviours of thermodynamic, transport and elastic properties of metals and constitute a fascinating field in the study of Fermi liquid systems. Although ETTs could be studied on pure systems as a function of the thermodynamic variables, nevertheless such a study would often require extreme conditions, and would lead to experimental difficulties. On the other hand, it is possible to explore the variations of atomic concentration, i.e. the valence electron per atom ratio, in metallic solid solutions with a relative experimental ease. In this paper we review the theoretical techniques for the determination of Fermi surfaces in metallic solid solutions and discuss some examples of ETTs, namely LiMg, ZrNb, NbMo, MoRe, AgPd, CdMg, NiW and NiTi alloys, also in connection with experimental data as thermoelectric power, resistivity, elastic constants and electron-phonon coupling and with the determinations of the electron momentum distribution function from Compton scattering and positron annihilation experiments. We show that the ab initio calculations of the electronic structure for the quoted systems, together with a careful determination of the BSF, are able to predict quantitatively ETTs at those concentrations where physical quantities display anomalies, so confirming directly ETT theory. Although it is not the purpose of the present review to give a full account of electronic structure calculation schemes, however, we briefly discuss the ideas and the main physical approximations underlying theories of substitutional disorder in alloys. We shall pay some more attention to the Coherent Potential Approximation (CPA) in the Korringa-Kohn-Rostoker (KKR) multiple scattering framework and the Hohenberg and Kohn Density Functional Theory in the Local Density Approximation (LDA) for the exchange-correlation potential. The above choice is supported by the numerical versatility of the LDAKKRCPA theory, and, more important, by the a fortiori evidence that essentially equivalent results are obtained from different theoretical frameworks, provided the same basic physical approximations are used. Accordingly, when convenient, we present new LDAKKRCPA determinations of the Fermi surfaces, as for the ZrNbMoRe series.

Ab initio study of magnetocrystalline anisotropy, magnetostriction, and Fermi surface of L10 FeNi (tetrataenite)

NASA Astrophysics Data System (ADS)

Werwiński, Mirosław; Marciniak, Wojciech

2017-12-01

We present results of ab initio calculations of several L10 FeNi characteristics, such as the summary of the magnetocrystalline anisotropy energies (MAEs), the full potential calculations of the anisotropy constant K 3, and the combined analysis of the Fermi surface and 3D {k} -resolved MAE. Other calculated parameters are the spin and orbital magnetic moments, the magnetostrictive coefficient λ0 0 1 , the bulk modulus B 0, and the lattice parameters. The MAEs summary shows rather big discrepancies among the experimental MAEs from the literature and also among the calculated MAE’s. The MAEs calculated in this work with the full potential and generalized gradient approximation (GGA) are equal to 0.47 MJ m-3 from WIEN2k, 0.34 MJ m-3 from FPLO, and 0.23 MJ m-3 from FP-SPR-KKR code. These results suggest that the MAE in GGA is below 0.5 MJ m-3 . It is expected that due to the limitations of the GGA, this value is underestimated. The L10 FeNi has further potential to improve its MAE by modifications, like e.g. tetragonal strain or alloying. The presented 3D {k} -resolved map of the MAE combined with the Fermi surface gives a complete picture of the MAE contributions in the Brillouin zone. The obtained, from the full potential FP-SPR-KKR method, magnetocrystalline anisotropy constants K 2 and K 3 are several orders of magnitude smaller than the MAE/K 1 and equal to -2.0 kJ m-3 and 110 J m-3 , respectively. The calculated spin and orbital magnetic moments of the L10 FeNi are equal to 2.72 and 0.054 μB for Fe and 0.53 and 0.039 μB for Ni atoms, respectively. The calculations of geometry optimization lead to a c/a ratio equal to 1.0036, B 0 equal to 194 GPa, and λ0 0 1 equal to 9.4  ×  10-6.

Detecting Fermi-level shifts by Auger electron spectroscopy in Si and GaAs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Debehets, J.; Homm, P.; Menghini, M.

In this paper, changes in surface Fermi-level of Si and GaAs, caused by doping and cleaning, are investigated by Auger electron spectroscopy. Based on the Auger voltage contrast, we compared the Auger transition peak energy but with higher accuracy by using a more accurate detector and an improved peak position determination method. For silicon, a peak shift as large as 0.46 eV was detected when comparing a cleaned p-type and n-type wafer, which corresponds rather well with the theoretical difference in Fermi-level. If no cleaning was applied, the peak position did not differ significantly for both wafer types, indicating Fermi-levelmore » pinning in the band gap. For GaAs, peak shifts were detected after cleaning with HF and (NH4)2S-solutions in an inert atmosphere (N2-gas). Although the (NH4)2S-cleaning in N2 is very efficient in removing the oxygen from the surface, the observed Ga- and As-peak shifts are smaller than those obtained after the HF-cleaning. It is shown that the magnitude of the shift is related to the surface composition. After Si-deposition on the (NH4)2S-cleaned surface, the Fermi-level shifts back to a similar position as observed for an as-received wafer, indicating that this combination is not successful in unpinning the Fermi-level of GaAs. This work has been funded by J.D.'s PhD fellowship of the Fund of Scientific Research-Flanders (FWO-V) (Dossier No. 11U4516N). P.H. acknowledges support from Becas Chile-CONICYT. This research was also supported by the FWO Odysseus Program, the Belgian Hercules Stichting with the Project No. Her/08/25 and AKUL/13/19 and the KU Leuven project GOA "Fundamental challenges in Semiconductor Research". The authors would also like to thank Bastiaan Opperdoes and Ludwig Henderix for technical support. The work was supported by the U.S. Department of Energy (USDOE), Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, and performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). Battelle operates PNNL for the USDOE under contract DE-AC05-76RL01830.« less

Second-Order Fermi Acceleration and Emission in Blazar Jets

NASA Astrophysics Data System (ADS)

Asano, Katsuaki; Takahara, Fumio; Toma, Kenji; Kusunose, Masaaki; Kakuwa, Jun

The second-order Fermi acceleration (Fermi-II) driven by turbulence may be responsible for the electron acceleration in blazar jets. We test this model with time-dependent simulations, adopt it for 1ES 1101-232, and Mrk 421. The Fermi-II model with radial evolution of the electron injection rate and/or diffusion coefficient can reproduce the spectra from the radio to the gamma-ray regime. For Mrk 421, an external radio photon field with a luminosity of 4.9 begin{math} {times} 10 (38) erg s (-1) is required to agree with the observed GeV flux. The temporal variability of the diffusion coefficient or injection rate causes flare emission. The observed synchronicity of X-ray and TeV flares implies a decrease of the magnetic field in the flaring source region.

Collective modes of a two-dimensional Fermi gas at finite temperature

NASA Astrophysics Data System (ADS)

Mulkerin, Brendan C.; Liu, Xia-Ji; Hu, Hui

2018-05-01

We examine the breathing mode of a strongly interacting two-dimensional Fermi gas and the role of temperature on the anomalous breaking of scale invariance. By calculating the equation of state with different many-body T -matrix theories and the virial expansion, we obtain a hydrodynamic equation of the harmonically trapped Fermi gas (with trapping frequency ω0) through the local density approximation. By solving the hydrodynamic equations, we determine the breathing mode frequencies as a function of interaction strength and temperature. We find that the breathing mode anomaly depends sensitively on both interaction strength and temperature. In particular, in the strongly interacting regime, we predict a significant downshift of the breathing mode frequency, below the scale invariant value of 2 ω0 , for temperatures of the order of the Fermi temperature.

Acoustics of tachyon Fermi gas

DOE Office of Scientific and Technical Information (OSTI.GOV)

Trojan, Ernst; Vlasov, George V.

2011-06-15

We consider a Fermi gas of free tachyons as a continuous medium and find whether it satisfies the causality condition. There is no stable tachyon matter with the particle density below critical value n{sub T} and the Fermi momentum k{sub F}<{radical}((3/2))m that depends on the tachyon mass m. The pressure P and energy density E cannot be arbitrary small, but the situation P>E is not forbidden. Existence of shock waves in tachyon gas is also discussed. At low density n{sub T}

Analog Landau-He-McKellar-Wilkens quantization due to noninertial effects of the Fermi-Walker reference frame

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bakke, Knut

2010-05-15

We will show that when a neutral particle with permanent electric dipole moment interacts with a specific field configuration when the local reference frames of the observers are Fermi-Walker transported, the Landau quantization analog to the He-McKellar-Wilkens setup arises in the nonrelativistic quantum dynamics of the neutral particle due the noninertial effects of the Fermi-Walker reference frame. We show that the noninertial effects do not break the infinity degeneracy of the energy levels, but in this case, the cyclotron frequency depends on the angular velocity.

VizieR Online Data Catalog: First Fermi-LAT Inner Galaxy point source catalog (Ajello+, 2016)

NASA Astrophysics Data System (ADS)

Ajello, M.; Albert, A.; Atwood, W. B.; Barbiellini, G.; Bastieri, D.; Bechtol, K.; Bellazzini, R.; Bissaldi, E.; Blandford, R. D.; Bloom, E. D.; Bonino, R.; Bottacini, E.; Brandt, T. J.; Bregeon, J.; Bruel, P.; Buehler, R.; Buson, S.; Caliandro, G. A.; Cameron, R. A.; Caputo, R.; Caragiulo, M.; Caraveo, P. A.; Cecchi, C.; Chekhtman, A.; Chiang, J.; Chiaro, G.; Ciprini, S.; Cohen-Tanugi, J.; Cominsky, L. R.; Conrad, J.; Cutini, S.; D'Ammando, F.; de Angelis, A.; de Palma, F.; Desiante, R.; di Venere, L.; Drell, P. S.; Favuzzi, C.; Ferrara, E. C.; Fusco, P.; Gargano, F.; Gasparrini, D.; Giglietto, N.; Giommi, P.; Giordano, F.; Giroletti, M.; Glanzman, T.; Godfrey, G.; Gomez-Vargas, G. A.; Grenier, I. A.; Guiriec, S.; Gustafsson, M.; Harding, A. K.; Hewitt, J. W.; Hill, A. B.; Horan, D.; Jogler, T.; Johannesson, G.; Johnson, A. S.; Kamae, T.; Karwin, C.; Knodlseder, J.; Kuss, M.; Larsson, S.; Latronico, L.; Li, J.; Li, L.; Longo, F.; Loparco, F.; Lovellette, M. N.; Lubrano, P.; Magill, J.; Maldera, S.; Malyshev, D.; Manfreda, A.; Mayer, M.; Mazziotta, M. N.; Michelson, P. F.; Mitthumsiri, W.; Mizuno, T.; Moiseev, A. A.; Monzani, M. E.; Morselli, A.; Moskalenko, I. V.; Murgia, S.; Nuss, E.; Ohno, M.; Ohsugi, T.; Omodei, N.; Orlando, E.; Ormes, J. F.; Paneque, D.; Pesce-Rollins, M.; Piron, F.; Pivato, G.; Porter, T. A.; Raino, S.; Rando, R.; Razzano, M.; Reimer, A.; Reimer, O.; Ritz, S.; Sanchez-Conde, M.; Parkinson, P. M. S.; Sgro, C.; Siskind, E. J.; Smith, D. A.; Spada, F.; Spandre, G.; Spinelli, P.; Suson, D. J.; Tajima, H.; Takahashi, H.; Thayer, J. B.; Torres, D. F.; Tosti, G.; Troja, E.; Uchiyama, Y.; Vianello, G.; Winer, B. L.; Wood, K. S.; Zaharijas, G.; Zimmer, S.

2018-01-01

The Fermi Large Area Telescope (LAT) has provided the most detailed view to date of the emission toward the Galactic center (GC) in high-energy γ-rays. This paper describes the analysis of data taken during the first 62 months of the mission in the energy range 1-100GeV from a 15°x15° region about the direction of the GC. Specialized interstellar emission models (IEMs) are constructed to enable the separation of the γ-ray emissions produced by cosmic ray particles interacting with the interstellar gas and radiation fields in the Milky Way into that from the inner ~1kpc surrounding the GC, and that from the rest of the Galaxy. A catalog of point sources for the 15°x15° region is self-consistently constructed using these IEMs: the First Fermi-LAT Inner Galaxy Point Source Catalog (1FIG). The spatial locations, fluxes, and spectral properties of the 1FIG sources are presented, and compared with γ-ray point sources over the same region taken from existing catalogs. After subtracting the interstellar emission and point-source contributions a residual is found. If templates that peak toward the GC are used to model the positive residual the agreement with the data improves, but none of the additional templates tried account for all of its spatial structure. The spectrum of the positive residual modeled with these templates has a strong dependence on the choice of IEM. (2 data files).

Fermi-Lat Observations of High-Energy Gamma-Ray Emission Toward the Galactic Center

NASA Technical Reports Server (NTRS)

Ajello, M.; Albert, A.; Atwood, W.B.; Barbiellini, G.; Bastieri, D.; Bechtol, K.; Bellazzini, R.; Bissaldi, E.; Blandford, R. D.; Brandt, T. J.;

Electronic structure and the van Hove singularity scenario in high-T(sub c)H(g)Ba2CuO(4+delta) superconductors

NASA Technical Reports Server (NTRS)

Agrawal, Bal K.; Agrawal, Savitri

1995-01-01

The electronic structure and the hole concentrations in the high Tc superconductor HgBa2CuO(4+delta) (delta = O, 1) has been investigated by employing a first principles full potential self-consistent LMTO method with the local density functional theory. The scalar relativistic effects have been considered. The hole concentrations of the Cu-d and O-p(x,y) orbitals are seen to be larger for the HgBaCuO5 system than those of the HgBaCuO4 solid. However, the van Hove singularity (vHs) induced Cu-d and O-p peak which is seen to lie comparatively away and above the Fermi level in the delta = 1 system shifts towards the Fermi level in the delta = 0 system. Thus, the superconducting behavior appears to originate from the occurrence of the vHs peak at the Fermi level. The Fermi surface nesting area in the delta = 0 compound is seen to be larger than in the delta = 1 compound. The calculation reveals that the increase in pressure on the crystal enhances the hole concentrations but without showing any optimum value, On the other hand, the vHs peak approaches to-wards the Fermi level with pressure and crosses the Fermi surface near V/Vo approximately equals 0.625 (V and Vo are the crystal volumes at high and normal pressures, respectively). Our calculated value of the bulk modulus equal to 0.626 Mbar predicts the occurrence of this crossover at about 24 GPa which is in complete agreement with the experimental value. At this pressure the compound has maximum nesting area and self-doped behavior.

An analytical investigation: Effect of solar wind on lunar photoelectron sheath

NASA Astrophysics Data System (ADS)

Mishra, S. K.; Misra, Shikha

2018-02-01

The formation of a photoelectron sheath over the lunar surface and subsequent dust levitation, under the influence of solar wind plasma and continuous solar radiation, has been analytically investigated. The photoelectron sheath characteristics have been evaluated using the Poisson equation configured with population density contributions from half Fermi-Dirac distribution of the photoemitted electrons and simplified Maxwellian statistics of solar wind plasma; as a consequence, altitude profiles for electric potential, electric field, and population density within the photoelectron sheath have been derived. The expression for the accretion rate of sheath electrons over the levitated spherical particles using anisotropic photoelectron flux has been derived, which has been further utilized to characterize the charging of levitating fine particles in the lunar sheath along with other constituent photoemission and solar wind fluxes. This estimate of particle charge has been further manifested with lunar sheath characteristics to evaluate the altitude profile of the particle size exhibiting levitation. The inclusion of solar wind flux into analysis is noticed to reduce the sheath span and altitude of the particle levitation; the dependence of the sheath structure and particle levitation on the solar wind plasma parameters has been discussed and graphically presented.

Electron theory of fast and ultrafast dissipative magnetization dynamics.

PubMed

Fähnle, M; Illg, C

2011-12-14

For metallic magnets we review the experimental and electron-theoretical investigations of fast magnetization dynamics (on a timescale of ns to 100 ps) and of laser-pulse-induced ultrafast dynamics (few hundred fs). It is argued that for both situations the dominant contributions to the dissipative part of the dynamics arise from the excitation of electron-hole pairs and from the subsequent relaxation of these pairs by spin-dependent scattering processes, which transfer angular momentum to the lattice. By effective field theories (generalized breathing and bubbling Fermi-surface models) it is shown that the Gilbert equation of motion, which is often used to describe the fast dissipative magnetization dynamics, must be extended in several aspects. The basic assumptions of the Elliott-Yafet theory, which is often used to describe the ultrafast spin relaxation after laser-pulse irradiation, are discussed very critically. However, it is shown that for Ni this theory probably yields a value for the spin-relaxation time T(1) in good agreement with the experimental value. A relation between the quantity α characterizing the damping of the fast dynamics in simple situations and the time T(1) is derived. © 2011 IOP Publishing Ltd

Stoichiometric and Oxygen-Deficient VO2 as Versatile Hole Injection Electrode for Organic Semiconductors.

PubMed

Fu, Keke; Wang, Rongbin; Katase, Takayoshi; Ohta, Hiromichi; Koch, Norbert; Duhm, Steffen

2018-03-28

Using photoemission spectroscopy, we show that the surface electronic structure of VO 2 is determined by the temperature-dependent metal-insulator phase transition and the density of oxygen vacancies, which depends on the temperature and ultrahigh vacuum (UHV) conditions. The atomically clean and stoichiometric VO 2 surface is insulating at room temperature and features an ultrahigh work function of up to 6.7 eV. Heating in UHV just above the phase transition temperature induces the expected metallic phase, which goes in hand with the formation of oxygen defects (up to 6% in this study), but a high work function >6 eV is maintained. To demonstrate the suitability of VO 2 as hole injection contact for organic semiconductors, we investigated the energy-level alignment with the prototypical organic hole transport material N, N'-di(1-naphthyl)- N, N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB). Evidence for strong Fermi-level pinning and the associated energy-level bending in NPB is found, rendering an Ohmic contact for holes.

Fermi surface ridge at second and third Umklapp positron annihilations in Y Ba2Cu3O(7-delta)

NASA Astrophysics Data System (ADS)

Adam, G.; Adam, S.; Barbiellini, B.; Hoffmann, L.; Manuel, A. A.; Massidda, S.; Peter, M.

1993-06-01

Results of statistical noise smoothing of the electron momentum distribution obtained by two-dimensional angular correlation of the electron-positron annihilation radiation technique on untwinned YBa2Cu3O(7-delta) single crystals are reported. Two distinct signatures of the sheet of Fermi surface related to the CuO chains (the ridge) are resolved. The first occurs at second Umklapp processes, in agreement with previous evidence. The second one, identified for the first time, occurs at third Umklapp processes. Comparison with FLAPW calculations confirms this result.

Fermi surface ridge at second and third UMKLAPP positron annihilations in YBa 2Cu 3O 7- δ

NASA Astrophysics Data System (ADS)

Adam, Gh.; Adam, S.; Barbiellini, B.; Hoffmann, L.; Manuel, A. A.; Peter, M.; Massida, S.

1993-12-01

Results of statistical noise smoothing of the electron momentum distribution got by two-dimensional angular correlation of the electron-positron annihilation radiation technique on untwinned YBa 2Cu 3O 7- δ single crystals are reported. Two distinct signatures of the sheet of Fermi surface related to the CuO chains (the ridge) are resolved. The first occurs at second Umklapp processes, in agreement with previous evidence. The second one, identified for the first time, occurs at third Umklapp processes. Comparison with FLAPW calculations confirms this result.

Observation of a two-dimensional Fermi surface and Dirac dispersion in YbMnSb2

NASA Astrophysics Data System (ADS)

Kealhofer, Robert; Jang, Sooyoung; Griffin, Sinéad M.; John, Caolan; Benavides, Katherine A.; Doyle, Spencer; Helm, T.; Moll, Philip J. W.; Neaton, Jeffrey B.; Chan, Julia Y.; Denlinger, J. D.; Analytis, James G.

2018-01-01

We present the crystal structure, electronic structure, and transport properties of the material YbMnSb2, a candidate system for the investigation of Dirac physics in the presence of magnetic order. Our measurements reveal that this system is a low-carrier-density semimetal with a two-dimensional Fermi surface arising from a Dirac dispersion, consistent with the predictions of density-functional-theory calculations of the antiferromagnetic system. The low temperature resistivity is very large, suggesting that scattering in this system is highly efficient at dissipating momentum despite its Dirac-like nature.