Recovering the Fermi surface with 2D-ACAR spectroscopy in samples with defects
NASA Astrophysics Data System (ADS)
Dugdale, S. B.; Laverock, J.
2014-04-01
When two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) experiments are performed in metals containing defects, conventional analysis in which the measured momentum distribution is folded back into the first Brillouin zone is rendered ineffective due to the contribution from positrons annihilating from the defect. However, by working with the radial anisotropy of the spectrum, it is shown that an image of the Fermi surface can be recovered since the defect contribution is essentially isotropic.
Spin-dependent momentum density distribution and Fermi surface of Ho via 2D-ACAR measurements
NASA Astrophysics Data System (ADS)
Hamid, A. S.; Uedono, A.
2004-03-01
The first direct measurements of the spin-dependent positron-electron momentum density and Fermi surface of Ho are presented. The measurements were performed using two-dimensional angular correlation of annihilation radiation (ACAR) experiments with reversal magnetic field directions parallel and anti-parallel to the polarization direction of the positron. The analysis confirmed that two hybrid bands influence the Fermi surface of Ho. They are 5d-6s conduction hybrid bands and partial hybridization of 4f-5d bands. In fact, the measured Fermi surface revealed the behavior of the magnetic electrons. Further, the reciprocal lattice points revealed the electronic spin density distribution behavior. The general layout of the Fermi surface of Ho showed a multiply connected surface as an open hole running along the A axis with minority spin distribution and two electron surfaces centered on K and H points, respectively. Furthermore, this Fermi surface showed anti-ferromagnetic character. The measured Fermi surface of Ho showed agreement with the results of a previous band structure calculation method. (
NASA Astrophysics Data System (ADS)
Hamid, A. S.
We measured the angular momentum density distribution of YNi2B2C to acquire information about its electronic structure. The measurements were performed using the full-scale utility of the two-dimensional angular correlation of annihilation radiation (2D-ACAR). The measured spectra clarified that Ni (3d) like state, predominantly, affected the Fermi surface of YNi2B2C. Further, s- and p-like-states enhanced its superconducting properties. The Fermi surface of YNi2B2C. was reconstructed using Fourier transformation followed by the LCW (Loucks, Crisp and West) folding procedure. It showed a large and complex surface similar to that of the high temperature superconductors HTS, with anisotropic properties. It also disclosed the effect of d-like state. Nevertheless, the current Fermi surface could deliver the needed topological information to isolate its features. The general layouts of this Fermi surface are; two large electron surfaces running along Γ-Z direction; as well as an additional large electron surface centered on X point; beside one hole surface centered on 100 point. This Fermi surface was interpreted in view of the earlier results.
From weakly to strongly interacting 2D Fermi gases
NASA Astrophysics Data System (ADS)
Dyke, Paul; Fenech, Kristian; Lingham, Marcus; Peppler, Tyson; Hoinka, Sascha; Vale, Chris
2014-05-01
We study ultracold 2D Fermi gases of 6Li formed in a highly oblate trapping potential. The potential is generated by a cylindrically focused, blue detuned TEM01 mode laser beam. Weak magnetic field curvature provides highly harmonic confinement in the radial direction and we can readily produce single clouds with an aspect ratio of 230. Our experiments investigate the dimensional crossover from 3D to 2D for a two component Fermi gas in the Bose-Einstein Condensate to Bardeen Cooper Schrieffer crossover. Observation of an elbow in measurements of the cloud width vs. atom number is consistent with populating only the lowest transverse harmonic oscillator state for weak attractive interactions. This measurement is extended to the strongly interacting region using the broad Feshbach resonance at 832 G. We also report our progress towards measurement of the 2D equation of state for an interacting 2D Fermi gas via in-situ absorption imaging.
The 2d MIT: The Pseudogap and Fermi Liquid Theory
NASA Astrophysics Data System (ADS)
Castner, T. G.
2005-06-01
Fermi liquid theory for the 2d metal-insulator transition is extended to include the correlation gap in the density-of-states. The results are consistent with the scaling form g=gce[on(To/T)] at T larger than a characteristic T* ∝ xTc (Tc=Ec= mobility edge). The two-component model n1+nloc=n=nc(1+x) for n>nc is required and the theory explains the T-dependence of the data of Hanein et al. for p-type GaAs.
NASA Astrophysics Data System (ADS)
G, A., Major; Fretwell, H. M.; Dugdale, S. B.; Alam, M. A.
1998-11-01
A novel method for reconstructing the Fermi surface from experimental two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) projections is proposed. In this algorithm, the 3D electron momentum-density distribution is expanded in terms of a basis of wavelet-like functions. The parameters of the model, the wavelet coefficients, are determined by maximizing the likelihood function corresponding to the experimental data and the projections calculated from the model. In contrast to other expansions, in the case of that in terms of wavelets a relatively small number of model parameters are sufficient for representing the relevant parts of the 3D distribution, thus keeping computation times reasonably short. Unlike other reconstruction methods, this algorithm takes full account of the statistical information content of the data and therefore may help to reduce the amount of time needed for data acquisition. An additional advantage of wavelet expansion may be the possibility of retrieving the Fermi surface directly from the wavelet coefficients rather than indirectly using the reconstructed 3D distribution.
NASA Astrophysics Data System (ADS)
Cheng, Chingyun; Kangara, Jayampathi; Arakelyan, Ilya; Thomas, John
2016-05-01
We tune the dimensionality of a strongly interacting degenerate 6 Li Fermi gas from 2D to quasi-2D, by adjusting the radial confinement of pancake-shaped clouds to control the radial chemical potential. In the 2D regime with weak radial confinement, the measured pair binding energies are in agreement with 2D-BCS mean field theory, which predicts dimer pairing energies in the many-body regime. In the qausi-2D regime obtained with increased radial confinement, the measured pairing energy deviates significantly from 2D-BCS theory. In contrast to the pairing energy, the measured radii of the cloud profiles are not fit by 2D-BCS theory in either the 2D or quasi-2D regimes, but are fit in both regimes by a beyond mean field polaron-model of the free energy. Supported by DOE, ARO, NSF, and AFOSR.
Crossover from 2D to 3D in a Weakly Interacting Fermi Gas
Dyke, P.; Kuhnle, E. D.; Hu, H.; Mark, M.; Hoinka, S.; Lingham, M.; Hannaford, P.; Vale, C. J.; Whitlock, S.
2011-03-11
We have studied the transition from two to three dimensions in a low temperature weakly interacting {sup 6}Li Fermi gas. Below a critical atom number N{sub 2D} only the lowest transverse vibrational state of a highly anisotropic oblate trapping potential is occupied and the gas is two dimensional. Above N{sub 2D} the Fermi gas enters the quasi-2D regime where shell structure associated with the filling of individual transverse oscillator states is apparent. This dimensional crossover is demonstrated through measurements of the cloud size and aspect ratio versus atom number.
Stability of Fermi surfaces and K theory.
Horava, Petr
2005-07-01
Nonrelativistic Fermi liquids in d+1 dimensions exhibit generalized Fermi surfaces: (d-p)-dimensional submanifolds in the (k,omega)-space supporting gapless excitations. We show that the universality classes of stable Fermi surfaces are classified by K theory, with the pattern of stability determined by Bott periodicity. The Atiyah-Bott-Shapiro construction implies that the low-energy modes near a Fermi surface exhibit relativistic invariance in the transverse p+1 dimensions. This suggests an intriguing parallel between nonrelativistic Fermi liquids and D-branes of string theory. PMID:16090638
FFLO Superfluids in 2D Spin-Orbit Coupled Fermi Gases
Zheng, Zhen; Gong, Ming; Zhang, Yichao; Zou, Xubo; Zhang, Chuanwei; Guo, Guangcan
2014-01-01
We show that the combination of spin-orbit coupling and in-plane Zeeman field in a two-dimensional degenerate Fermi gas can lead to a larger parameter region for Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phases than that using spin-imbalanced Fermi gases. The resulting FFLO superfluids are also more stable due to the enhanced energy difference between FFLO and conventional Bardeen-Cooper-Schrieffer (BCS) excited states. We clarify the crucial role of the symmetry of Fermi surface on the formation of finite momentum pairing. The phase diagram for FFLO superfluids is obtained in the BCS-BEC crossover region and possible experimental observations of FFLO phases are discussed. PMID:25288379
Fermi Surface and Magnetic Structure of TmGa3
NASA Astrophysics Data System (ADS)
Biasini, M.; Kontrym-Sznajd, G.; Monge, M. A.; Gemmi, M.; Czopnik, A.; Jura, A.
2001-05-01
We carry out measurements of the two-dimensional angular correlation of the positron annihilation radiation (2D-ACAR) to reconstruct the complex multisheet Fermi surface (FS) of the cubic rare-earth (RE) compound TmGa3. We discover a correlation between the antiferromagnetic structures and the nesting of the FS along the [110] directions. Moreover, we propose methods to estimate the density of states at the Fermi energy ( EF) and the electronic contribution to the specific heat [we obtain N\\(EF\\) = 13.6 states/Ryd cell and γ = 2.4 mJ/mole K2].
Phase Separation and Pair Condensation in a Spin-Imbalanced 2D Fermi Gas.
Mitra, Debayan; Brown, Peter T; Schauß, Peter; Kondov, Stanimir S; Bakr, Waseem S
2016-08-26
We study a two-component quasi-two-dimensional Fermi gas with imbalanced spin populations. We probe the gas at different interaction strengths and polarizations by measuring the density of each spin component in the trap and the pair momentum distribution after time of flight. For a wide range of experimental parameters, we observe in-trap phase separation characterized by the appearance of a spin-balanced core surrounded by a polarized gas. Our momentum space measurements indicate pair condensation in the imbalanced gas even for large polarizations where phase separation vanishes, pointing to the presence of a polarized pair condensate. Our observation of zero momentum pair condensates in 2D spin-imbalanced gases opens the way to explorations of more exotic superfluid phases that occupy a large part of the phase diagram in lower dimensions. PMID:27610853
Deviations from Fermi-Liquid Behavior above Tc in 2D Short Coherence Length Superconductors
NASA Astrophysics Data System (ADS)
Trivedi, Nandini; Randeria, Mohit
1995-07-01
We show that there are qualitative differences between the temperature dependence of the spin and charge correlations in the normal state of the 2D attractive Hubbard model using quantum Monte Carlo simulations. The one-particle density of states shows a pseudogap above Tc with a depleted N0 with decreasing T. The susceptibility χs and the low frequency spin spectral weight track N0, which explains the spin-gap scaling: 1/T1T~χsT. However, collective excitations contribute to the charge channel, and the compressibility dn/dμ is T independent. This anomalous ``spin-charge separation'' is shown to exist even at intermediate \\|U\\| where the momentum distribution nk gives evidence for a degenerate Fermi system.
Entanglement Entropy and the Fermi Surface
NASA Astrophysics Data System (ADS)
Swingle, Brian
2010-07-01
Free fermions with a finite Fermi surface are known to exhibit an anomalously large entanglement entropy. The leading contribution to the entanglement entropy of a region of linear size L in d spatial dimensions is S˜Ld-1logL, a result that should be contrasted with the usual boundary law S˜Ld-1. This term depends only on the geometry of the Fermi surface and on the boundary of the region in question. I give an intuitive account of this anomalous scaling based on a low energy description of the Fermi surface as a collection of one-dimensional gapless modes. Using this picture, I predict a violation of the boundary law in a number of other strongly correlated systems.
Fermi surface anisotropy in the cuprates
NASA Astrophysics Data System (ADS)
Ramshaw, Brad
Broken rotational (C4) symmetry is a distinguishing feature for a number of experiments in the underdoped high-Tc cuprates, including electrical resistivity, neutron scattering, Nernst coefficient, and scanning tunneling microscopy. This broken symmetry has not been observed on the Fermi surface, however, with or without the presence of an applied magnetic field. We measure the angle-dependent magnetoresistance-a quantity known to be extremely sensitive to the geometry and symmetry of the Fermi surface-of YBa2Cu3O6.58, and find that the Fermi surface has a clear two-fold symmetry, breaking the C4 symmetry of the copper-oxide plane. We discuss the implications of this finding, including how it fits with recent X-ray measurements in high magnetic fields.
Switchable Fermi surface sheets in greigite
NASA Astrophysics Data System (ADS)
Zhang, B.; de Wijs, G. A.; de Groot, R. A.
2012-07-01
Greigite (Fe3S4) and magnetite (Fe3O4) are isostructural and isoelectronic ferrimagnets with quite distinct properties. Electronic structure calculations reveal greigite is a normal metal in contrast to half-metallic magnetite. Greigite shows a complex Fermi surface with a unique influence of relativistic effects: The existence of sheets of the Fermi surface depends on the direction of the magnetization. This enables spinorbitronics, spintronics on the level of a single compound rather than a device. Due to its relativistic origin, spin contamination is irrelevant in spinorbitronics and the entire periodic table is available for optimizations.
Programmable variable stiffness 2D surface design
NASA Astrophysics Data System (ADS)
Trabia, Sarah; Hwang, Taeseon; Yim, Woosoon
2014-03-01
Variable stiffness features can contribute to many engineering applications ranging from robotic joints to shock and vibration mitigation. In addition, variable stiffness can be used in the tactile feedback to provide the sense of touch to the user. A key component in the proposed device is the Biased Magnetorheological Elastomer (B-MRE) where iron particles within the elastomer compound develop a dipole interaction energy. A novel feature of this device is to introduce a field induced shear modulus bias via a permanent magnet which provides an offset with a current input to the electromagnetic control coil to change the compliance or modulus of a base elastomer in both directions (softer or harder). The B-MRE units can lead to the design of a variable stiffness surface. In this preliminary work, both computational and experimental results of the B-MRE are presented along with a preliminary design of the programmable variable stiffness surface design.
Unconventional Fermi surface in an insulating state
Harrison, Neil; Tan, B. S.; Hsu, Y. -T.; Zeng, B.; Hatnean, M. Ciomaga; Zhu, Z.; Hartstein, M.; Kiourlappou, M.; Srivastava, A.; Johannes, M. D.; Murphy, T. P.; Park, J. -H.; Balicas, L.; Lonzarich, G. G.; Balakrishnan, G.; Sebastian, Suchitra E.
2015-07-17
Insulators occur in more than one guise; a recent finding was a class of topological insulators, which host a conducting surface juxtaposed with an insulating bulk. Here, we report the observation of an unusual insulating state with an electrically insulating bulk that simultaneously yields bulk quantum oscillations with characteristics of an unconventional Fermi liquid. We present quantum oscillation measurements of magnetic torque in high-purity single crystals of the Kondo insulator SmB_{6}, which reveal quantum oscillation frequencies characteristic of a large three-dimensional conduction electron Fermi surface similar to the metallic rare earth hexaborides such as PrB_{6} and LaB_{6}. As a result, the quantum oscillation amplitude strongly increases at low temperatures, appearing strikingly at variance with conventional metallic behavior.
Signature of Fermi surface jumps in positron spectroscopy data
NASA Astrophysics Data System (ADS)
Adam, Gh.; Adam, S.
1999-08-01
A subtractionless method for solving Fermi surface sheets (FSS), from measured n-axis-projected momentum distribution histograms by two-dimensional angular correlation of the positron—electron annihilation radiation (2D-ACAR) technique, is discussed. The window least squares statistical noise smoothing filter described by Adam et al. [Nucl.Instr. & Meth. A 337 (1993) 188] is first refined such that the window free radial parameters (WRP) are optimally adapted to the data. In an ideal single crystal, the specific jumps induced in the WRP distribution by the existing Fermi surface jumps yield straightforward information on the resolved FSS. In a real crystal, the smearing of the derived WRP optimal values, which originates from positron annihilations with electrons at crystal imperfections, is ruled out by median smoothing of the obtained distribution, over symmetry defined stars of bins. The analysis of a gigacount 2D-ACAR spectrum, measured on the archetypal high- T c compound YBa 2Cu 3O 7- δ at room temperature, illustrates the method. Both electronic FSS, the ridge along ΓX direction and the pillbox centered at the S point of the first Brillouin zone, are resolved.
Focusing surface wave imaging with flexible 2D array
NASA Astrophysics Data System (ADS)
Zhou, Shiyuan; Fu, Junqiang; Li, Zhe; Xu, Chunguang; Xiao, Dingguo; Wang, Shaohan
2016-04-01
Curved surface is widely exist in key parts of energy and power equipment, such as, turbine blade cylinder block and so on. Cycling loading and harsh working condition of enable fatigue cracks appear on the surface. The crack should be found in time to avoid catastrophic damage to the equipment. A flexible 2D array transducer was developed. 2D Phased Array focusing method (2DPA), Mode-Spatial Double Phased focusing method (MSDPF) and the imaging method using the flexible 2D array probe are studied. Experiments using these focusing and imaging method are carried out. Surface crack image is obtained with both 2DPA and MSDPF focusing method. It have been proved that MSDPF can be more adaptable for curved surface and more calculate efficient than 2DPA.
Three-body bound states of two bosonic impurities immersed in a Fermi sea in 2D
NASA Astrophysics Data System (ADS)
Bellotti, F. F.; Frederico, T.; Yamashita, M. T.; Fedorov, D. V.; Jensen, A. S.; Zinner, N. T.
2016-04-01
We consider two identical impurities immersed in a Fermi sea for a broad range of masses and for both interacting and non-interacting impurities. The interaction between the particles is described through attractive zero-range potentials and the problem is solved in momentum space. The two impurities can attach to a fermion from the sea and form three-body bound states. The energy of these states increase as function of the Fermi momentum k F, leading to three-body bound states below the Fermi energy. The fate of the states depends highly on two- and three-body thresholds and we find evidence of medium-induced Borromean-like states in 2D. The corrections due to particle-hole fluctuations in the Fermi sea are considered in the three-body calculations and we show that in spite of the fact that they strongly affect both the two- and three-body systems, the correction to the point at which the three-body states cease to exist is small.
NASA Astrophysics Data System (ADS)
Hamid, A. S.; Uedono, A.
2004-11-01
The cover picture of this issue, taken from [1], shows a cross section of the Fermi surface in the basal plane of nickel. The measurements were carried out using 2D angular correlation of annihilation radiation (ACAR) experiments. The intersecting plane is normal to the c-axis, passing through the and X points. The light regions correspond to a high electron momentum density. The Fermi surface is presented as two hole surfaces around the point and two electron surfaces around the X point.
Observation of the Leggett-Rice effect in an ensemble of 2D Fermi gases
NASA Astrophysics Data System (ADS)
Luciuk, Christopher; Smale, Scott; Trotzky, Stefan; Sharum, Haille; Enss, Tilman; Thywissen, Joseph
2016-05-01
Transport properties of unitary Fermi gases have been studied extensively in the past few years. Because of strong interparticle scattering at unitarity, many transport phenomenon, in particular the spin diffusivity, are observed to be bounded. However, anomalously slow spin diffusion has been observed in two dimensions and remains to be understood. Here we study the spin currents that arise as a result of a non-equilibrium magnetization in an ensemble of two dimensional Fermi gases. Spin currents possess both a dissipative and reactive component. The dissipative component - parameterized by the spin diffusivity - is a measure of the scattering rate. The reactive component describes a part of the spin current that precesses around the local magnetization known as the Leggett-Rice effect. Using a spin-echo sequence we measure both the amplitude and phase of magnetization dynamics to extract these two transport parameters at a range of interaction strengths near a Feshbach resonance.
Fermi surfaces of surface states on Si(111)-Ag, Au
NASA Astrophysics Data System (ADS)
Crain, J. N.; Altmann, K. N.; Bromberger, C.; Himpsel, F. J.
2002-11-01
Metallic surface states on semiconducting substrates provide an opportunity to study low-dimensional electrons decoupled from the bulk. Angle resolved photoemission is used to determine the Fermi surface, group velocity, and effective mass for surface states on Si(111)(3)×(3)-Ag, Si(111)(3)×(3)-Au, and Si(111)(21)×(21)-(Ag+Au). For Si(111)(3)×(3)-Ag the Fermi surface consists of small electron pockets populated by electrons from a few % excess Ag. For Si(111)(21)×(21)-(Ag+Au) the pockets increase their size corresponding to a filling by three electrons per unit cell. The (21)×(21) superlattice leads to an intricate surface umklapp pattern and to minigaps of 110 meV, giving an interaction potential of 55 meV for the (21)×(21) superlattice.
Entanglement rules for holographic Fermi surfaces
NASA Astrophysics Data System (ADS)
Roychowdhury, Dibakar
2016-08-01
In this paper, based on the notion of Gauge/Gravity duality, we explore the laws of entanglement thermodynamics for most generic classes of Quantum Field Theories with hyperscaling violation. In our analysis, we note that for Quantum Field Theories with compressible quark like excitation, the first law of entanglement thermodynamics gets modified due to the presence of an additional term that could be identified as the entanglement chemical potential associated with hidden Fermi surfaces of the boundary theory. Most notably, we find that the so called entanglement chemical potential does not depend on the size of the entangling region and is purely determined by the quark d.o.f. encoded within the entangling region.
Evolution of electron Fermi surface with doping in cobaltates.
Ma, Xixiao; Lan, Yu; Qin, Ling; Kuang, Lülin; Feng, Shiping
2016-08-24
The notion of the electron Fermi surface is one of the characteristic concepts in the field of condensed matter physics, and it plays a crucial role in the understanding of the physical properties of doped Mott insulators. Based on the t-J model, we study the nature of the electron Fermi surface in the cobaltates, and qualitatively reproduce the essential feature of the evolution of the electron Fermi surface with doping. It is shown that the underlying hexagonal electron Fermi surface obeys Luttinger's theorem. The theory also predicts a Fermi-arc phenomenon at the low-doped regime, where the region of the hexagonal electron Fermi surface along the [Formula: see text]-K direction is suppressed by the electron self-energy, and then six disconnected Fermi arcs located at the region of the hexagonal electron Fermi surface along the [Formula: see text]-M direction emerge. However, this Fermi-arc phenomenon at the low-doped regime weakens with the increase of doping. PMID:27351111
Evolution of electron Fermi surface with doping in cobaltates
NASA Astrophysics Data System (ADS)
Ma, Xixiao; Lan, Yu; Qin, Ling; Kuang, Lülin; Feng, Shiping
2016-08-01
The notion of the electron Fermi surface is one of the characteristic concepts in the field of condensed matter physics, and it plays a crucial role in the understanding of the physical properties of doped Mott insulators. Based on the t-J model, we study the nature of the electron Fermi surface in the cobaltates, and qualitatively reproduce the essential feature of the evolution of the electron Fermi surface with doping. It is shown that the underlying hexagonal electron Fermi surface obeys Luttinger’s theorem. The theory also predicts a Fermi-arc phenomenon at the low-doped regime, where the region of the hexagonal electron Fermi surface along the Γ -K direction is suppressed by the electron self-energy, and then six disconnected Fermi arcs located at the region of the hexagonal electron Fermi surface along the Γ -M direction emerge. However, this Fermi-arc phenomenon at the low-doped regime weakens with the increase of doping.
Corrections to Fermi Liquid theory in 2D in a magnetic field
NASA Astrophysics Data System (ADS)
Chubukov, Andrey; Betouras, Joseph; Efremov, Dmitri
2005-03-01
In this work, we consider a Fermi liquid in two dimensions in a magnetic field, and study the effects of the Zeeman splitting on thermodynamics. We derive the temperature dependence of the spin susceptibility χs(T) from the thermodynamic potential, and show explicitly how 2pF scattering gives rise to a non- analytic temperature dependence of the susceptibility. We explain why small momentum scattering does not give rise to non-analytic χs(T). We discuss experimental implications of this result.
Non-equilibrium dynamics of an ensemble of 2D Fermi gases
NASA Astrophysics Data System (ADS)
Sharum, Haille; Smale, Scott; Luciuk, Christopher; Trotzky, Stefan; Enss, Tilman; Yu, Zhenhua; Zhang, Shizhong; Thywissen, Joseph
2016-05-01
We study the dynamics of an ensemble of two dimensional Fermi gases near Feshbach scattering resonances. We begin our experiments with a weakly interacting or non-interacting gas and initiate strong interactions on a timescale that is fast compared to equilibration. We probe the evolution of the short-ranged part of the many-body wavefunction via radio frequency spectroscopy. Alternatively, we perform spin echo measurements to reveal the dissipative (spin diffusion) and reactive (Leggett-Rice effect) components of transverse spin currents.
Pair condensation in a spin-imbalanced 2D Fermi gas
NASA Astrophysics Data System (ADS)
Mitra, Debayan; Brown, Peter; Schauss, Peter; Kondov, Stanimir; Bakr, Waseem
2016-05-01
We study the phase diagram of the strongly-interacting spin-imbalanced Fermi gas in two dimensions, where the low dimensionality enhances correlations and phase fluctuations. Our interest is motivated by the connection of this system with superconductivity in the presence of a large Zeeman field. We observe pair condensation for a range of spin imbalance and interaction strengths. The measurement of the phase diagram opens the door for a detailed investigation of exotic phases such as the Sarma/broken pair phase and the elusive FFLO phase.
Design Application Translates 2-D Graphics to 3-D Surfaces
NASA Technical Reports Server (NTRS)
2007-01-01
Fabric Images Inc., specializing in the printing and manufacturing of fabric tension architecture for the retail, museum, and exhibit/tradeshow communities, designed software to translate 2-D graphics for 3-D surfaces prior to print production. Fabric Images' fabric-flattening design process models a 3-D surface based on computer-aided design (CAD) specifications. The surface geometry of the model is used to form a 2-D template, similar to a flattening process developed by NASA's Glenn Research Center. This template or pattern is then applied in the development of a 2-D graphic layout. Benefits of this process include 11.5 percent time savings per project, less material wasted, and the ability to improve upon graphic techniques and offer new design services. Partners include Exhibitgroup/Giltspur (end-user client: TAC Air, a division of Truman Arnold Companies Inc.), Jack Morton Worldwide (end-user client: Nickelodeon), as well as 3D Exhibits Inc., and MG Design Associates Corp.
Fermi surfaces and energy gaps of high-temperature superconductors
Shen, Z.X.; Dessau, D.S.
1994-12-31
In this short paper, the authors describe their recent experimental results from high-temperature superconductors. In the normal state, the data reveals interesting features of the Fermi surfaces and low energy excitations near the Fermi level. In the superconducting state, the data shows a very strong anisotropy in the superconducting gap.
NASA Astrophysics Data System (ADS)
Hamid, A. S.; Uedono, A.
2004-11-01
The spin-dependent momentum density and Fermi surface of ferromagnetic Ni have been obtained through positron annihilation experiments. The measurements were carried out through 2D angular correlation of annihilation radiation (ACAR) using longitudinally polarized positrons. The magnetic field direction was reversed in order to study the effect of the spin-dependent positron-electron momentum space density on the Fermi surface of Ni. The results showed that ferromagnetic Ni had different Fermi surfaces for the majority-spin and minority-spin states. The differences due to the spin-states were studied in the momentum space and in the wave vector space. In general, the experimental results showed good agreement with previous theoretical calculations.
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
Manipulating superconductivity in ruthenates through Fermi surface engineering
NASA Astrophysics Data System (ADS)
Hsu, Yi-Ting; Cho, Weejee; Rebola, Alejandro Federico; Burganov, Bulat; Adamo, Carolina; Shen, Kyle M.; Schlom, Darrell G.; Fennie, Craig J.; Kim, Eun-Ah
2016-07-01
The key challenge in superconductivity research is to go beyond the historical mode of discovery-driven research. We put forth a new strategy, which is to combine theoretical developments in the weak-coupling renormalization-group approach with the experimental developments in lattice-strain-driven Fermi surface engineering. For concreteness we theoretically investigate how superconducting tendencies will be affected by strain engineering of ruthenates' Fermi surface. We first demonstrate that our approach qualitatively reproduces recent experiments under uniaxial strain. We then note that the order of a few percent strain, readily accessible to epitaxial thin films, can bring the Fermi surface close to van Hove singularity. Using the experimental observation of the change in the Fermi surface under biaxial epitaxial strain and ab initio calculations, we predict Tc for triplet pairing to be maximized by getting close to the van Hove singularities without tuning on to the singularity.
3D surface configuration modulates 2D symmetry detection.
Chen, Chien-Chung; Sio, Lok-Teng
2015-02-01
We investigated whether three-dimensional (3D) information in a scene can affect symmetry detection. The stimuli were random dot patterns with 15% dot density. We measured the coherence threshold, or the proportion of dots that were the mirror reflection of the other dots in the other half of the image about a central vertical axis, at 75% accuracy with a 2AFC paradigm under various 3D configurations produced by the disparity between the left and right eye images. The results showed that symmetry detection was difficult when the corresponding dots across the symmetry axis were on different frontoparallel or inclined planes. However, this effect was not due to a difference in distance, as the observers could detect symmetry on a slanted surface, where the depth of the two sides of the symmetric axis was different. The threshold was reduced for a hinge configuration where the join of two slanted surfaces coincided with the axis of symmetry. Our result suggests that the detection of two-dimensional (2D) symmetry patterns is subject to the 3D configuration of the scene; and that coplanarity across the symmetry axis and consistency between the 2D pattern and 3D structure are important factors for symmetry detection. PMID:25536469
2D Gridded Surface Data Value-Added Product
Tang, Q; Xie, S
2015-08-30
This report describes the Atmospheric Radiation Measurement (ARM) Best Estimate (ARMBE) 2-dimensional (2D) gridded surface data (ARMBE2DGRID) value-added product. Spatial variability is critically important to many scientific studies, especially those that involve processes of great spatial variations at high temporal frequency (e.g., precipitation, clouds, radiation, etc.). High-density ARM sites deployed at the Southern Great Plains (SGP) allow us to observe the spatial patterns of variables of scientific interests. The upcoming megasite at SGP with its enhanced spatial density will facilitate the studies at even finer scales. Currently, however, data are reported only at individual site locations at different time resolutions for different datastreams. It is difficult for users to locate all the data they need and requires extra effort to synchronize the data. To address these problems, the ARMBE2DGRID value-added product merges key surface measurements at the ARM SGP sites and interpolates the data to a regular 2D grid to facilitate the data application.
Fermi Surface of Nanocrystalline Embedded Particles in Materials: bcc Cu in Fe
NASA Astrophysics Data System (ADS)
Nagai, Y.; Chiba, T.; Tang, Z.; Akahane, T.; Kanai, T.; Hasegawa, M.; Takenaka, M.; Kuramoto, E.
2001-10-01
We report that a positron can act as a probe to directly reveal electronic structures of nanocrystalline embedded particles in materials. The Fermi surface (FS) of ``bcc'' Cu nanoparticles in an Fe matrix is observed as the first example. A two-dimensional angular correlation of the positron annihilation radiation (2D-ACAR) method is used to measure the momentum distribution which reflects the FS topology. The obtained 2D-ACAR spectra show strong and characteristic anisotropy associated with the necks of the FS around the \\{110\\} Brillouin zone boundaries of the bcc Cu, which are well reproduced by full-potential linearized argumented plane-wave calculations.
Shear viscosity of quasi-2D dipolar Bose-Fermi mixtures with long-range 1/r interactions
NASA Astrophysics Data System (ADS)
Darsheshdar, E.; Yavari, H.; Moniri, S. M.
2016-05-01
Low-temperature shear viscosity of a spin polarized two-component quasi-2D dipolar Fermi gas with long-range 1/ r interaction in the Bose-Einstein condensation (BEC) limit, where the system can be considered as dimers and the unpaired fermions, is calculated by means of the Kubo formalism. By taking into account the dimer-atom, dimer-dimer, and atom-atom interactions in the self-energies the viscous relaxation time (τ_{η}= (τ_{DA}^{-1}+τ_{DD}^{-1}+ τ_{AA}^{-1})^{-1}) is determined. Since the relaxation rates due to these interactions τ_{DA}^{-1} , τ_{DD}^{ -1} and τ_{AA}^{-1} varies, respectively, as T , T2 , and T in the low-temperature limit T→0 , the dimer-atom and atom-atom interactions play the dominant role to the shear viscosity and the shear viscosity varies as T^{-1} . For small polarization the effect of dimer-dimer interaction is important (τ_{DA},τ_{AA}≫τ_{DD}) , and the shear viscosity changes as the standard T^{-2} behviour. In this case, the temperature behavior of the dimer relaxation rate unaffected by 1/ r interaction and the contact, dipole-dipole, and 1/ r interactions play the same role in the temperature dependence of the shear viscosity. Our results have important consequences for developing experiments and theoretical researches on the transport properties of ultracold gases with repulsive or attractive long range 1/ r interaction.
Observation of Fermi arc surface states in a topological metal.
Xu, Su-Yang; Liu, Chang; Kushwaha, Satya K; Sankar, Raman; Krizan, Jason W; Belopolski, Ilya; Neupane, Madhab; Bian, Guang; Alidoust, Nasser; Chang, Tay-Rong; Jeng, Horng-Tay; Huang, Cheng-Yi; Tsai, Wei-Feng; Lin, Hsin; Shibayev, Pavel P; Chou, Fang-Cheng; Cava, Robert J; Hasan, M Zahid
2015-01-16
The topology of the electronic structure of a crystal is manifested in its surface states. Recently, a distinct topological state has been proposed in metals or semimetals whose spin-orbit band structure features three-dimensional Dirac quasiparticles. We used angle-resolved photoemission spectroscopy to experimentally observe a pair of spin-polarized Fermi arc surface states on the surface of the Dirac semimetal Na3Bi at its native chemical potential. Our systematic results collectively identify a topological phase in a gapless material. The observed Fermi arc surface states open research frontiers in fundamental physics and possibly in spintronics. PMID:25593189
Fermi Surfaces of Surface States on Si(111)
NASA Astrophysics Data System (ADS)
Crain, J. N.; Altmann, K. N.; Himpsel, F. J.; Bromberger, C.
2002-03-01
Metallic surface states on semi-conducting surfaces provide a unique opportunity to study low-dimensional bands that are decoupled from the bulk. Two such systems that have received much attention for their metallic surface states are Si(111)surd 3× surd 3 - Ag and Si(111) surd 3× surd 3 - Au. We present angle resolved photoemission data mapping the Fermi-surfaces for surd 3× surd 3 - Ag and surd 3× surd 3 - Au, and study the effects of doping the surface with additional Au atoms.[1] For surd 3× surd 3 - Au, an increase in the Au coverage is linked to an increase in the occupancy of the metallic surface state. In the case of surd 3× surd 3 - Ag, the addition of Au forms a new metallic band and a surd 21× surd 21 superlattice that are observed in photoemission. Reference: [1] J N Crain, K N Altmann, C Bromberger, F J Himpsel, submitted to Physics Review B.
Pressure dependence of the Fermi surface of hcp Yb
NASA Astrophysics Data System (ADS)
Schirber, J. E.; Beaudry, B. J.; Jepsen, O.
1981-06-01
The pressure dependence of Fermi-surface cross sections for principal symmetry directions has been investigated using solid He pressure generation techniques. Careful searches for de Haas-van Alphen signals were conducted from 2 to 9 kbar in both virgin fcc crystals and samples transformed from hcp to fcc. No sign of the frequency reported by Ribault was detected. Results are discussed in terms of theoretically calculated pressure-induced changes in the band structure and Fermi surface of the hcp phase of Yb.
Fermi surface, magnetic, and superconducting properties in actinide compounds
NASA Astrophysics Data System (ADS)
Ōnuki, Yoshichika; Settai, Rikio; Haga, Yoshinori; Machida, Yo; Izawa, Koichi; Honda, Fuminori; Aoki, Dai
2014-08-01
The de Haas-van Alphen effect, which is a powerful method to explore Fermi surface properties, has been observed in cerium, uranium, and nowadays even in neptunium and plutonium compounds. Here, we present the results of several studies concerning the Fermi surface properties of the heavy fermion superconductors UPt3 and NpPd5Al2, and of the ferromagnetic pressure-induced superconductor UGe2, together with those of some related compounds for which fascinating anisotropic superconductivity, magnetism, and heavy fermion behavior has been observed. xml:lang="fr"
Exotic Paired States with Anisotropic Spin-Dependent Fermi Surfaces
Feiguin, Adrian E.; Fisher, Matthew P. A.
2009-07-10
We propose a model for realizing exotic paired states in cold Fermi gases by using a spin-dependent optical lattice to engineer mismatched Fermi surfaces for each hyperfine species. The BCS phase diagram shows a stable paired superfluid state with coexisting pockets of momentum space with gapless unpaired carriers, similar to the Sarma state in polarized mixtures, but in our case the system is unpolarized. We propose the possible existence of an exotic 'Cooper-pair Bose-metal' phase, which has a gap for single fermion excitations but gapless and uncondensed 'Cooper-pair' excitations residing on a 'Bose surface' in momentum space.
Massively Parallel Computation of Soil Surface Roughness Parameters on A Fermi GPU
NASA Astrophysics Data System (ADS)
Li, Xiaojie; Song, Changhe
2016-06-01
Surface roughness is description of the surface micro topography of randomness or irregular. The standard deviation of surface height and the surface correlation length describe the statistical variation for the random component of a surface height relative to a reference surface. When the number of data points is large, calculation of surface roughness parameters is time-consuming. With the advent of Graphics Processing Unit (GPU) architectures, inherently parallel problem can be effectively solved using GPUs. In this paper we propose a GPU-based massively parallel computing method for 2D bare soil surface roughness estimation. This method was applied to the data collected by the surface roughness tester based on the laser triangulation principle during the field experiment in April 2012. The total number of data points was 52,040. It took 47 seconds on a Fermi GTX 590 GPU whereas its serial CPU version took 5422 seconds, leading to a significant 115x speedup.
Fermi surface behavior in the ABJM M2-brane theory
NASA Astrophysics Data System (ADS)
DeWolfe, Oliver; Henriksson, Oscar; Rosen, Christopher
2015-06-01
We calculate fermionic Green's functions for states of the three-dimensional Aharony-Bergman-Jafferis-Maldacena M2-brane theory at large N using the gauge-gravity correspondence. We embed extremal black brane solutions in four-dimensional maximally supersymmetric gauged supergravity, obtain the linearized Dirac equations for each spin-1 /2 mode that cannot mix with a gravitino, and solve these equations with infalling boundary conditions to calculate retarded Green's functions. For generic values of the chemical potentials, we find Fermi surfaces with universally non-Fermi liquid behavior, matching the situation for four-dimensional N =4 super-Yang-Mills. Fermi surface singularities appear and disappear discontinuously at the point where all chemical potentials are equal, reminiscent of a quantum critical point. One limit of parameter space has zero entropy at zero temperature, and fermionic fluctuations are perfectly stable inside an energy region around the Fermi surface. An ambiguity in the quantization of the fermions is resolved by supersymmetry.
Are the surface Fermi arcs in Dirac semimetals topologically protected?
Kargarian, Mehdi; Randeria, Mohit; Lu, Yuan-Ming
2016-08-01
Motivated by recent experiments probing anomalous surface states of Dirac semimetals (DSMs) Na3Bi and Cd3As2, we raise the question posed in the title. We find that, in marked contrast to Weyl semimetals, the gapless surface states of DSMs are not topologically protected in general, except on time-reversal-invariant planes of surface Brillouin zone. We first demonstrate this finding in a minimal four-band model with a pair of Dirac nodes at [Formula: see text] where gapless states on the side surfaces are protected only near [Formula: see text] We then validate our conclusions about the absence of a topological invariant protecting double Fermi arcs in DSMs, using a K-theory analysis for space groups of Na3Bi and Cd3As2 Generically, the arcs deform into a Fermi pocket, similar to the surface states of a topological insulator, and this pocket can merge into the projection of bulk Dirac Fermi surfaces as the chemical potential is varied. We make sharp predictions for the doping dependence of the surface states of a DSM that can be tested by angle-resolved photoemission spectroscopy and quantum oscillation experiments. PMID:27436895
Study of the Fermi surface of molybdenum and chromium via positron annihilation experiments
NASA Astrophysics Data System (ADS)
Biasini, Maurizio
2000-02-01
A quantitative mapping of the Fermi surface (FS) of molybdenum and chromium was sought by modelling the three-dimensional k-space occupancy with a small number of parameters which were determined by a least-squares fit to the two-dimensional angular correlation of the electron-positron annihilation radiation (2D-ACAR) data subjected to a Lock-Crisp-West (LCW) transformation. The resulting FS topology of molybdenum, unlike what was assumed in previous 2D-ACAR studies, does not support the nesting of its two main FS sheets. In the case of chromium, although the overall discrepancy with the FS expected from the theory is larger, the difference in shape between the same two FS sheets is of lesser extent. According to this analysis the ratio of the electron Fermi volume to the hole Fermi volume is found to deviate from unity, the value expected for compensated metals, for both materials. We suggest that these discrepancies might be due to positron wave function and/or electron-positron many-body distortions not predicted by the theory.
Life on the edge: a beginner’s guide to the Fermi surface
NASA Astrophysics Data System (ADS)
Dugdale, S. B.
2016-05-01
The concept of the Fermi surface is at the very heart of our understanding of the metallic state. Displaying intricate and often complicated shapes, the Fermi surfaces of real metals are both aesthetically beautiful and subtly powerful. A range of examples is presented of the startling array of physical phenomena whose origin can be traced to the shape of the Fermi surface, together with experimental observations of the particular Fermi surface features.
Spatial Variations in the Fermi Surface of Bi-2212
NASA Astrophysics Data System (ADS)
Main, Elizabeth; Pivonka, A. E.; Zeljkovic, I.; Gu, G.; Hudson, E. W.; Hoffman, J. E.
2011-03-01
In cuprate superconductors, scanning tunneling microscopy can be used to see variations in the Fermi surface on a nanometer length scale caused by doping inhomogeneity. Prior STM studies show that the local wavelength of the checkerboard, a weak charge modulation ascribed to antinodal Fermi surface nesting, varies with the size of the pseudogap in Bi 2 Sr 2 Cu O6 + δ (Bi-2201). Here we report similar STM measurements in Bi-2212. We therefore confirm the local relationship between pseudogap energy and charge ordering wavevector in a second high-Tc superconductor. We acknowledge support from AFOSR PECASE grant FA9550-06-1-0531, AFOSR DURIP grant FA9550-06-1-0359, NSF Career grant DMR-0847433 and NSF grant DMR-0904400.
Heavy fermions. Unconventional Fermi surface in an insulating state.
Tan, B S; Hsu, Y-T; Zeng, B; Hatnean, M Ciomaga; Harrison, N; Zhu, Z; Hartstein, M; Kiourlappou, M; Srivastava, A; Johannes, M D; Murphy, T P; Park, J-H; Balicas, L; Lonzarich, G G; Balakrishnan, G; Sebastian, Suchitra E
2015-07-17
Insulators occur in more than one guise; a recent finding was a class of topological insulators, which host a conducting surface juxtaposed with an insulating bulk. Here, we report the observation of an unusual insulating state with an electrically insulating bulk that simultaneously yields bulk quantum oscillations with characteristics of an unconventional Fermi liquid. We present quantum oscillation measurements of magnetic torque in high-purity single crystals of the Kondo insulator SmB6, which reveal quantum oscillation frequencies characteristic of a large three-dimensional conduction electron Fermi surface similar to the metallic rare earth hexaborides such as PrB6 and LaB6. The quantum oscillation amplitude strongly increases at low temperatures, appearing strikingly at variance with conventional metallic behavior. PMID:26138105
Spin fluctuation and Fermi surface instability in ferromagnetic superconductors
NASA Astrophysics Data System (ADS)
Aoki, Dai; Gourgout, Adrien; Pourret, Alexandre; Bastien, Gaël; Knebel, Georg; Flouquet, Jacques
2014-08-01
We review the ferromagnetic superconductivity observed in the uranium based compounds, namely UGe2, URhGe and UCoGe, where the spin-triplet state is most likely realized. An unusual upper critical field Hc2, which is enhanced under a magnetic field in a certain field direction, is discussed in terms of spin fluctuations and of Fermi surface instabilities. xml:lang="fr"
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.
Fermi Surface Instabilities in Ferromagnetic Superconductor URhGe
NASA Astrophysics Data System (ADS)
Aoki, Dai; Knebel, Georg; Flouquet, Jacques
2014-09-01
The field-reentrant (field-reinforced) superconductivity on ferromagnetic superconductors is one of the most interesting topics in unconventional superconductivity. The enhancement of effective mass and the induced ferromagnetic fluctuations play key roles for reentrant superconductivity. However, the associated change of the Fermi surface, which is often observed at (pseudo-) metamagnetic transition, can also be a key ingredient. In order to study the Fermi surface instability, we performed Hall effect measurements in the ferromagnetic superconductor URhGe. The Hall effect of URhGe is well explained by two contributions, namely by the normal Hall effect and by the large anomalous Hall effect due to skew scattering. The large change in the Hall coefficient is observed at low fields between the paramagnetic and ferromagnetic states for H || c-axis (easy-magnetization axis) in the orthorhombic structure, indicating that the Fermi surface is reconstructed in the ferromagnetic state below the Curie temperature (TCurie = 9.5 K). At low temperatures (T ll Ttext{Curie}), when the field is applied along the b-axis, the reentrant superconductivity was observed in both the Hall resistivity and the magnetoresistance below 0.4 K. Above 0.4 K, a large jump with the first-order nature was detected in the Hall resistivity at a spin-reorientation field HR ˜ 12.5 T, demonstrating that the marked change of the Fermi surface occurs between the ferromagnetic state and the polarized state above HR. The results can be understood by the Lifshitz-type transition, induced by the magnetic field or by the change of the effective magnetic field.
Are the surface Fermi arcs in Dirac semimetals topologically protected?
NASA Astrophysics Data System (ADS)
Lu, Yuan Ming; Kargarian, Mehdi; Randeria, Mohit
Motivated by recent experiments probing double Fermi arcs on the surface of Dirac semimetals (DSMs) Na3Bi and Cd3As2, we raise the question posed in the title. We find that, in marked contrast to Weyl semimetals, the Fermi arcs of DSMs are not topologically protected in general, except at certain time-reversal invariant momenta. For a simple 4-band model with a pair of Dirac nodes at k = (0, 0, +/-Q) gapless surface states are protected only at kz = 0. We identify symmetry allowed bulk perturbations that destroy Fermi arcs, but show that they are necessarily ``small'', i.e., higher order than terms kept in usual k . p theory. We validate our conclusions about the absence of a topological invariant protecting the surface states in DSMs using a K-theory analysis for the space groups of Na3Bi and Cd3As2 The authors acknowledge the support of the CEM, an NSF MRSEC, under Grant DMR-1420451.
Fermi surface reconstruction in FeSe under high pressure
NASA Astrophysics Data System (ADS)
Terashima, Taichi; Kikugawa, Naoki; Kiswandhi, Andhika; Graf, David; Choi, Eun-Sang; Brooks, James S.; Kasahara, Shigeru; Watashige, Tatsuya; Matsuda, Yuji; Shibauchi, Takasada; Wolf, Thomas; Böhmer, Anna E.; Hardy, Frédéric; Meingast, Christoph; Löhneysen, Hilbert v.; Uji, Shinya
2016-03-01
We report Shubnikov-de Haas (SdH) oscillation measurements on FeSe under high pressure up to P =16.1 kbar. We find a sudden change in SdH oscillations at the onset of the pressure-induced antiferromagnetism at P ˜8 kbar. We argue that this change can be attributed to a reconstruction of the Fermi surface by the antiferromagnetic order. The negative d Tc /d P observed in a range between P ˜8 and 12 kbar may be explained by the reduction in the density of states due to the reconstruction. The ratio of the transition temperature to the effective Fermi energy remains high under high pressure: kBTc/EF˜0.1 even at P =16.1 kbar.
Spin-orbit coupling at surfaces and 2D materials.
Krasovskii, E E
2015-12-16
Spin-orbit interaction gives rise to a splitting of surface states via the Rashba effect, and in topological insulators it leads to the existence of topological surface states. The resulting k(//) momentum separation between states with the opposite spin underlies a wide range of new phenomena at surfaces and interfaces, such as spin transfer, spin accumulation, spin-to-charge current conversion, which are interesting for fundamental science and may become the basis for a breakthrough in the spintronic technology. The present review summarizes recent theoretical and experimental efforts to reveal the microscopic structure and mechanisms of spin-orbit driven phenomena with the focus on angle and spin-resolved photoemission and scanning tunneling microscopy. PMID:26580290
Fermi Surface Topology and the ARPES Spectra of BISCO
NASA Astrophysics Data System (ADS)
Bansil, A.; Lindroos, M.
2001-03-01
The Fermi surface (FS) of BISCO has generated considerable recent controversy with attention focused on whether or not the FS is electron- or hole-like in the vicinity of the M(π,0) symmetry point. Given strong matrix element effects in BISCO[1], care is needed in ascertaining delicate FS features in terms of the the ARPES spectra. With this motivation, we have carried out extensive first-principles simulations of photointensity in the entire (k_x,k_y) plane for emission from the Fermi energy at a photon energy of 21.2 eV. The presence of electron or hole sheets in the underlying spectrum is simulated by varying the Fermi energy appropriately. The simulated ARPES spectra are analyzed using a variety of methods that have been invoked in the recent literature for the purpose of deducing FS topology in BISCO from the ARPES data. While different methods indeed help ameliorate matrix element effects to varying degrees, our study also reveals their limitations. The "renormalization" of the spectrum over a large energy window ( ~600 meV) tends to artificially introduce hole-like features, while the gradient of the total spectral weight has a tendancy to induce spurious electron-like features. These and related issues are discussed. Work supported in part by the U.S.D.O.E. [1] A. Bansil and M. Lindroos, Phys. Rev. Letters 83,5154(1999).
Fermi Surface of Donor and Acceptor Graphite Intercalation Compounds.
NASA Astrophysics Data System (ADS)
Wang, Guonan
The Fermi surfaces and the electronic properties of the donor-type stage-1 C_8K and stage-2 C_{24}K, as well as the acceptor-type stage-2 BiCl_3, stage-3 HgCl_2 and stage-3 SbF _5 graphite intercalation compounds were investigated by means of the de Haas-van Alphen effect. The dHvA spectra of the stage-1 C_8 K exhibit two dHvA frequencies, 3126 T and 4250 T. The corresponding effective masses were 0.86 m _0 and 0.92 m_0, respectively. The angular dependence of the dHvA frequencies for a direction within +/-18^circ of the c-axis showed that there are both three-dimensional and two dimensional parts of the Fermi surfaces in C _8K. The three-dimensional Fermi surface has a cross-sectional area corresponding to the dHvA frequency of 3126 T. The charge transfer per potassium atom measured from the dHvA effect is 0.97. This implies that the potassium is ionized completely. These dHvA experimental results support both the Tatar and Rabii model and the revised Ohno, Nakao and Kamimura model for C_8K. Two dominant dHvA frequencies were obtained in stage-2 C_{24}K. They are 286 T and 2570 T, respectively. The predictions of Blinowski's model are in agreement with the experimental data. The charge transfer per potassium is found to be 0.88. This suggests that the potassium s-band is above the Fermi level in C_{24}K. The dHvA measurements for the acceptor compounds show that the stage-2 BiCl_3 GIC had two dHvA frequencies, 327T and 1012T, and each stage -3 compound had three dominant frequencies. They are 121T, 523T and 664T for HgCl_2, and 172T, 656T and 852T for SbF_5. The cyclotron masses corresponding to the dHvA frequencies for these compounds were measured from the temperature dependence of the dHvA amplitudes. The theoretical predictions of the dHvA frequencies and the cyclotron masses from the Blinowski's band models for stage-2 and stage-3 compounds are in agreement with the experimental results. The angular dependence of the dHvA frequencies show that the Fermi
Fermi surface and magnetic structure of rare-earth-Ga3 compounds
NASA Astrophysics Data System (ADS)
Biasini, Maurizio; Kontrym-Sznajd, Grazyna; Ferro, Gianclaudio; Czopnik, Andrzej
2002-03-01
The measurement of the 2-dimensional angular correlation of the positron annihilation radiation (2D-ACAR), providing a 2D projection of the two-photon electron-positron momentum density, ρ(p), is a powerful tool to investigate the electronic structure of intermetallic compounds. Utilising tomographic reconstruction techniques (G Kontrym-Sznajd et al Mat. Scie. Forum 255-257) 754 (1997) and references therein., the experiment has the unique ability to sample the Brillouin Zone of truly 3-dimensional systems in a cartesian mesh, thus determining their Fermi surface (FS). Our studies have addressed the commensurate and incommensurate antiferromagnetic structures of TmGa3 and ErGa_3, respectively. For both compounds the FSs resulting from the 2D-ACAR experiments are in fair agreement with de Haas van Alphen measurements and with band structure calculations which constrain the 4f electrons to retain a local atomic character (M Biasini at al Phys. Rev. Lett 86), 4616, (2001).. Nevertheless, we discover different nesting features along the [110] directions which can account for the magnetic structures of the two compounds. Moreover, we propose methods to estimate the density of states at the Fermi energy (E_F) and the electronic contribution to the specific heat, γ. We obtain N(E_F)=17 states/ (Ryd cell), γ=2.8 (mJ/mole K^2) and N(E_F)=16 states/ (Ryd cell), γ=2.7 (mJ/mole K^2) for TmGa3 and ErGa_3, respectively.
Fermi surface evolution and luttinger theorem in naxcoo2: asystematic photoemission study
Yang H.-B.; Pan, Z.-H.; Sekharan, A.K.P.; Sato, T.; Souma, S.; Takahashi, T.; Jin, R.; Sales, B.C.; Mandrus, D.; Fedorov,A.V.; Wang,Z.; Ding, H.
2005-01-17
We report a systematic angle-resolved photoemission study on NaxCoO2 for a wide range of Na concentrations (0.3x0.72). In all the metallic samples at different x, we observed (i) only a single holelike Fermi surface centered around and (ii) its area changes with x according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The e band and the associated small Fermi surface pockets near the K points predicted by band calculations are found to sink below the Fermi energy in a manner almost independent of the doping and temperature.
NASA Astrophysics Data System (ADS)
Rodriguez, Jose P.; Araujo, Miguel A. N.; Sacramento, Pedro D.
2014-07-01
We uncover the low-energy spectrum of a t-J model for electrons on a square lattice of spin-1 iron atoms with 3dxz and 3dyz orbital character by applying Schwinger-boson-slave-fermion mean-field theory and by exact diagonalization of one hole roaming over a 4 × 4 × 2 lattice. Hopping matrix elements are set to produce hole bands centered at zero two-dimensional (2D) momentum in the free-electron limit. Holes can propagate coherently in the t-J model below a threshold Hund coupling when long-range antiferromagnetic order across the d + = 3d(x + iy)z and d - = 3d(x - iy)z orbitals is established by magnetic frustration that is off-diagonal in the orbital indices. This leads to two hole-pocket Fermi surfaces centered at zero 2D momentum. Proximity to a commensurate spin-density wave (cSDW) that exists above the threshold Hund coupling results in emergent Fermi surface pockets about cSDW momenta at a quantum critical point (QCP). This motivates the introduction of a new Gutzwiller wavefunction for a cSDW metal state. Study of the spin-fluctuation spectrum at cSDW momenta indicates that the dispersion of the nested band of one-particle states that emerges is electron-type. Increasing Hund coupling past the QCP can push the hole-pocket Fermi surfaces centered at zero 2D momentum below the Fermi energy level, in agreement with recent determinations of the electronic structure of mono-layer iron-selenide superconductors.
Shen, Jianfeng; Wu, Jingjie; Wang, Man; Dong, Pei; Xu, Jingxuan; Li, Xiaoguang; Zhang, Xiang; Yuan, Junhua; Wang, Xifan; Ye, Mingxin; Vajtai, Robert; Lou, Jun; Ajayan, Pulickel M
2016-05-01
A proper design of direct liquid phase exfoliation (LPE) for 2D materials as graphene, MoS2 , WS2 , h-BN, Bi2 Se3 , MoSe2 , SnS2 , and TaS2 with common cosolvents is carried out based on considering the polar and dispersive components of surface tensions of various cosolvents and 2D materials. It has been found that the exfoliation efficiency is enhanced by matching the ratio of surface tension components of cosolvents to that of the targeted 2D materials, based on which common cosolvents composed of IPA/water, THF/water, and acetone/water can be designed for sufficient LPE process. In this context, the library of low-toxic and low-cost solvents with low boiling points for LPE is infinitely enlarged when extending to common cosolvents. Polymer-based composites reinforced with a series of different 2D materials are compared with each other. It is demonstrated that the incorporation of cosolvents-exfoliated 2D materials can substantially improve the mechanical and thermal properties of polymer matrices. Typically, with the addition of 0.5 wt% of such 2D material as MoS2 nanosheets, the tensile strength and Young's modulus increased up to 74.85% and 136.97%, respectively. The different enhancement effect of 2D materials is corresponded to the intrinsic properties and LPE capacity of 2D materials. PMID:27059403
Contact Potentials, Fermi Level Equilibration, and Surface Charging.
Peljo, Pekka; Manzanares, José A; Girault, Hubert H
2016-06-14
This article focuses on contact electrification from thermodynamic equilibration of the electrochemical potential of the electrons of two conductors upon contact. The contact potential difference generated in bimetallic macro- and nanosystems, the Fermi level after the contact, and the amount and location of the charge transferred from one metal to the other are discussed. The three geometries considered are spheres in contact, Janus particles, and core-shell particles. In addition, the force between the two spheres in contact with each other is calculated and is found to be attractive. A simple electrostatic model for calculating charge distribution and potential profiles in both vacuum and an aqueous electrolyte solution is described. Immersion of these bimetallic systems into an electrolyte solution leads to the formation of an electric double layer at the metal-electrolyte interface. This Fermi level equilibration and the associated charge transfer can at least partly explain experimentally observed different electrocatalytic, catalytic, and optical properties of multimetallic nanosystems in comparison to systems composed of pure metals. For example, the shifts in the surface plasmon resonance peaks in bimetallic core-shell particles seem to result at least partly from contact charging. PMID:27176729
NASA Astrophysics Data System (ADS)
Al-Sawai, W.; Barbiellini, B.; Sakurai, Y.; Itou, M.; Mijnarends, P. E.; Markiewicz, R. S.; Kaprzyk, S.; Wakimoto, S.; Fujita, M.; Basak, S.; Lin, H.; Wang, Yung Jui; Eijt, S. W. H.; Schut, H.; Yamada, K.; Bansil, A.
2012-03-01
We have observed the bulk Fermi surface (FS) in an overdoped (x=0.3) single crystal of La2-xSrxCuO4 by using Compton scattering. A two-dimensional (2D) momentum density reconstruction from measured Compton profiles yields a clear FS signature in the third Brillouin zone along [100]. The quantitative agreement between density functional theory (DFT) calculations and momentum density experiment suggests that Fermi-liquid physics is restored in the overdoped regime. In particular the predicted FS topology is found to be in good accord with the corresponding experimental data. We find similar quantitative agreement between the measured 2D angular correlation of positron annihilation radiation (2D-ACAR) spectra and the DFT-based computations. However, 2D-ACAR does not give such a clear signature of the FS in the extended momentum space in either the theory or the experiment.
Fermi-surface reconstruction by stripe order in cuprate superconductors
Laliberté, F.; Chang, J.; Doiron-Leyraud, N.; Hassinger, E.; Daou, R.; Rondeau, M.; Ramshaw, B.J.; Liang, R.; Bonn, D.A.; Hardy, W.N.; Pyon, S.; Takayama, T.; Takagi, H.; Sheikin, I.; Malone, L.; Proust, C.; Behnia, K.; Taillefer, Louis
2011-01-01
The origin of pairing in a superconductor resides in the underlying normal state. In the cuprate high-temperature superconductor YBa2Cu3Oy (YBCO), application of a magnetic field to suppress superconductivity reveals a ground state that appears to break the translational symmetry of the lattice, pointing to some density-wave order. Here we use a comparative study of thermoelectric transport in the cuprates YBCO and La1.8−xEu0.2SrxCuO4 (Eu-LSCO) to show that the two materials exhibit the same process of Fermi-surface reconstruction as a function of temperature and doping. The fact that in Eu-LSCO this reconstruction coexists with spin and charge modulations that break translational symmetry shows that stripe order is the generic non-superconducting ground state of hole-doped cuprates. PMID:21847106
Ultrasonic probe of the AuZn Fermi surface.
Svitelskiy, O.; Suslov, A. V.; Singleton, J. M.; Lashley, J. C.
2005-01-01
We, for the first time, apply the ultrasonic pulse-echo technique to explore the Fermi surface of the martensite phase of the single crystalline AuZn shape memory alloy. The ultrasonic measurements were performed in the magnetic fields of up to 45 T in the temperature range of 0.07 < T < 300 K. In the martensite phase (T < 64 K), the oscillations of the speed of the longitudinal sound wave propagating in the (110) direction indicated a strong acoustic de Haas - van Alphen effect. In addition to the earlier described oscillations with frequencies of 1140 and 4720 Tesla, we observed a new frequency of 120 Tesla, which was predicted theoretically. Corresponding effective masses were in favorable agreement with those expected from band structure calculations.
Fermi-surface reconstruction by stripe order in cuprate superconductors.
Laliberté, F; Chang, J; Doiron-Leyraud, N; Hassinger, E; Daou, R; Rondeau, M; Ramshaw, B J; Liang, R; Bonn, D A; Hardy, W N; Pyon, S; Takayama, T; Takagi, H; Sheikin, I; Malone, L; Proust, C; Behnia, K; Taillefer, Louis
2011-01-01
The origin of pairing in a superconductor resides in the underlying normal state. In the cuprate high-temperature superconductor YBa(2)Cu(3)O(y) (YBCO), application of a magnetic field to suppress superconductivity reveals a ground state that appears to break the translational symmetry of the lattice, pointing to some density-wave order. Here we use a comparative study of thermoelectric transport in the cuprates YBCO and La(1.8-x)Eu(0.2)Sr(x)CuO(4) (Eu-LSCO) to show that the two materials exhibit the same process of Fermi-surface reconstruction as a function of temperature and doping. The fact that in Eu-LSCO this reconstruction coexists with spin and charge modulations that break translational symmetry shows that stripe order is the generic non-superconducting ground state of hole-doped cuprates. PMID:21847106
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
Direct observation of bulk Fermi surface at higher Brillouin zones in a heavily hole-doped cuprate
NASA Astrophysics Data System (ADS)
Al-Sawai, W.; Sakurai, Y.; Itou, M.; Barbiellini, B.; Mijnarends, P. E.; Markiewicz, R. S.; Kaprzyk, S.; Gillet, J.-M.; Wakimoto, S.; Fujita, M.; Basak, S.; Lin, H.; Bansil, A.; Yamada, K.
2010-03-01
We have observed the bulk Fermi surface (FS) in an overdoped (x=0.3) single crystal of La2-xSrxCuO4 by using Compton scattering. A 2-D momentum density reconstruction [1] from measured Compton profiles, yields a clear FS signature in a higher Brillouin zone centered at p=(1.5,1.5) a.u. The quantitative agreement with density functional theory (DFT) calculations [2] and momentum density experiment suggests that Fermi-liquid physics is restored in the overdoped regime. We have also measured the 2-D angular correlation of positron annihilation radiation (2D-ACAR) [3] and noticed a similar quantitative agreement with the DFT simulations. However, 2D-ACAR does not give a clear signature of the FS in the extended momentum space in both theory and experiment. Work supported in part by the US DOE.[1] Y. Tanaka et al., Phys. Rev. B 63, 045120 (2001).[2] S. Sahrakorpi et al., Phys. Rev. Lett. 95, 157601 (2005).[3] L. C. Smedskjaer et al., J. Phys. Chem. Solids 52, 1541 (1991).
Simulation of surface tension in 2D and 3D with smoothed particle hydrodynamics method
NASA Astrophysics Data System (ADS)
Zhang, Mingyu
2010-09-01
The methods for simulating surface tension with smoothed particle hydrodynamics (SPH) method in two dimensions and three dimensions are developed. In 2D surface tension model, the SPH particle on the boundary in 2D is detected dynamically according to the algorithm developed by Dilts [G.A. Dilts, Moving least-squares particle hydrodynamics II: conservation and boundaries, International Journal for Numerical Methods in Engineering 48 (2000) 1503-1524]. The boundary curve in 2D is reconstructed locally with Lagrangian interpolation polynomial. In 3D surface tension model, the SPH particle on the boundary in 3D is detected dynamically according to the algorithm developed by Haque and Dilts [A. Haque, G.A. Dilts, Three-dimensional boundary detection for particle methods, Journal of Computational Physics 226 (2007) 1710-1730]. The boundary surface in 3D is reconstructed locally with moving least squares (MLS) method. By transforming the coordinate system, it is guaranteed that the interface function is one-valued in the local coordinate system. The normal vector and curvature of the boundary surface are calculated according to the reconstructed boundary surface and then surface tension force can be calculated. Surface tension force acts only on the boundary particle. Density correction is applied to the boundary particle in order to remove the boundary inconsistency. The surface tension models in 2D and 3D have been applied to benchmark tests for surface tension. The ability of the current method applying to the simulation of surface tension in 2D and 3D is proved.
Unusual dimensionality effects and surface charge density in 2D Mg(OH)2
NASA Astrophysics Data System (ADS)
Suslu, Aslihan; Wu, Kedi; Sahin, Hasan; Chen, Bin; Yang, Sijie; Cai, Hui; Aoki, Toshihiro; Horzum, Seyda; Kang, Jun; Peeters, Francois M.; Tongay, Sefaattin
2016-02-01
We present two-dimensional Mg(OH)2 sheets and their vertical heterojunctions with CVD-MoS2 for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)2 sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)2 have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)2 is a 6 eV direct-gap insulator in 2D, and its optical band gap displays strong band renormalization effects from monolayer to bulk, marking the first experimental confirmation of confinement effects in 2D insulators. Interestingly, 2D-Mg(OH)2 sheets possess rather strong surface polarization (charge) effects which is in contrast to electrically neutral h-BN materials. Using 2D-Mg(OH)2 sheets together with CVD-MoS2 in the vertical stacking shows that a strong change transfer occurs from n-doped CVD-MoS2 sheets to Mg(OH)2, naturally depleting the semiconductor, pushing towards intrinsic doping limit and enhancing overall optical performance of 2D semiconductors. Results not only establish unusual confinement effects in 2D-Mg(OH)2, but also offer novel 2D-insulating material with unique physical, vibrational, and chemical properties for potential applications in flexible optoelectronics.
Unusual dimensionality effects and surface charge density in 2D Mg(OH)2
Suslu, Aslihan; Wu, Kedi; Sahin, Hasan; Chen, Bin; Yang, Sijie; Cai, Hui; Aoki, Toshihiro; Horzum, Seyda; Kang, Jun; Peeters, Francois M.; Tongay, Sefaattin
2016-01-01
We present two-dimensional Mg(OH)2 sheets and their vertical heterojunctions with CVD-MoS2 for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)2 sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)2 have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)2 is a 6 eV direct-gap insulator in 2D, and its optical band gap displays strong band renormalization effects from monolayer to bulk, marking the first experimental confirmation of confinement effects in 2D insulators. Interestingly, 2D-Mg(OH)2 sheets possess rather strong surface polarization (charge) effects which is in contrast to electrically neutral h-BN materials. Using 2D-Mg(OH)2 sheets together with CVD-MoS2 in the vertical stacking shows that a strong change transfer occurs from n-doped CVD-MoS2 sheets to Mg(OH)2, naturally depleting the semiconductor, pushing towards intrinsic doping limit and enhancing overall optical performance of 2D semiconductors. Results not only establish unusual confinement effects in 2D-Mg(OH)2, but also offer novel 2D-insulating material with unique physical, vibrational, and chemical properties for potential applications in flexible optoelectronics. PMID:26846617
Unusual dimensionality effects and surface charge density in 2D Mg(OH)2.
Suslu, Aslihan; Wu, Kedi; Sahin, Hasan; Chen, Bin; Yang, Sijie; Cai, Hui; Aoki, Toshihiro; Horzum, Seyda; Kang, Jun; Peeters, Francois M; Tongay, Sefaattin
2016-01-01
We present two-dimensional Mg(OH)2 sheets and their vertical heterojunctions with CVD-MoS2 for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)2 sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)2 have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)2 is a 6 eV direct-gap insulator in 2D, and its optical band gap displays strong band renormalization effects from monolayer to bulk, marking the first experimental confirmation of confinement effects in 2D insulators. Interestingly, 2D-Mg(OH)2 sheets possess rather strong surface polarization (charge) effects which is in contrast to electrically neutral h-BN materials. Using 2D-Mg(OH)2 sheets together with CVD-MoS2 in the vertical stacking shows that a strong change transfer occurs from n-doped CVD-MoS2 sheets to Mg(OH)2, naturally depleting the semiconductor, pushing towards intrinsic doping limit and enhancing overall optical performance of 2D semiconductors. Results not only establish unusual confinement effects in 2D-Mg(OH)2, but also offer novel 2D-insulating material with unique physical, vibrational, and chemical properties for potential applications in flexible optoelectronics. PMID:26846617
Two-dimensional Fermi surfaces in Kondo insulator SmB₆.
Li, G; Xiang, Z; Yu, F; Asaba, T; Lawson, B; Cai, P; Tinsman, C; Berkley, A; Wolgast, S; Eo, Y S; Kim, Dae-Jeong; Kurdak, C; Allen, J W; Sun, K; Chen, X H; Wang, Y Y; Fisk, Z; Li, Lu
2014-12-01
In the Kondo insulator samarium hexaboride (SmB6), strong correlation and band hybridization lead to an insulating gap and a diverging resistance at low temperature. The resistance divergence ends at about 3 kelvin, a behavior that may arise from surface conductance. We used torque magnetometry to resolve the Fermi surface topology in this material. The observed oscillation patterns reveal two Fermi surfaces on the (100) surface plane and one Fermi surface on the (101) surface plane. The measured Fermi surface cross sections scale as the inverse cosine function of the magnetic field tilt angles, which demonstrates the two-dimensional nature of the conducting electronic states of SmB6. PMID:25477456
The potential energy surface and chaos in 2D Hamiltonian systems
NASA Astrophysics Data System (ADS)
Li, Jiangdan; Zhang, Suying
2011-02-01
We provide a new insight into the relationship between the geometric property of the potential energy surface and chaotic behavior of 2D Hamiltonian dynamical systems, and give an indicator of chaos based on the geometric property of the potential energy surface by defining Mean Convex Index (MCI). We also discuss a model of unstable Hamiltonian in detail, and show our results in good agreement with HBLSL's (Horwitz, Ben Zion, Lewkowicz, Schiffer and Levitan) new Riemannian geometric criterion.
Surface wave phase velocities from 2-D surface wave tomography studies in the Anatolian plate
NASA Astrophysics Data System (ADS)
Arif Kutlu, Yusuf; Erduran, Murat; Çakır, Özcan; Vinnik, Lev; Kosarev, Grigoriy; Oreshin, Sergey
2014-05-01
We study the Rayleigh and Love surface wave fundamental mode propagation beneath the Anatolian plate. To examine the inter-station phase velocities a two-station method is used along with the Multiple Filter Technique (MFT) in the Computer Programs in Seismology (Herrmann and Ammon, 2004). The near-station waveform is deconvolved from the far-station waveform removing the propagation effects between the source and the station. This method requires that the near and far stations are aligned with the epicentre on a great circle path. The azimuthal difference of the earthquake to the two-stations and the azimuthal difference between the earthquake and the station are restricted to be smaller than 5o. We selected 3378 teleseismic events (Mw >= 5.7) recorded by 394 broadband local stations with high signal-to-noise ratio within the years 1999-2013. Corrected for the instrument response suitable seismogram pairs are analyzed with the two-station method yielding a collection of phase velocity curves in various period ranges (mainly in the range 25-185 sec). Diffraction from lateral heterogeneities, multipathing, interference of Rayleigh and Love waves can alter the dispersion measurements. In order to obtain quality measurements, we select only smooth portions of the phase velocity curves, remove outliers and average over many measurements. We discard these average phase velocity curves suspected of suffering from phase wrapping errors by comparing them with a reference Earth model (IASP91 by Kennett and Engdahl, 1991). The outlined analysis procedure yields 3035 Rayleigh and 1637 Love individual phase velocity curves. To obtain Rayleigh and Love wave travel times for a given region we performed 2-D tomographic inversion for which the Fast Marching Surface Tomography (FMST) code developed by N. Rawlinson at the Australian National University was utilized. This software package is based on the multistage fast marching method by Rawlinson and Sambridge (2004a, 2004b). The
Turbulent flow over a surface-mounted 2-D block in thermally-stratified boundary layers
NASA Astrophysics Data System (ADS)
Zhang, W.; Markfort, C. D.; Porte-Agel, F.
2013-12-01
Turbulent boundary-layer flows over complex topography have been of great interest in the atmospheric sciences and wind engineering communities. The geometry of the topography, surface characteristics and atmospheric thermal stability play important roles in determining momentum and scalar flux distribution. Studies of turbulent flow over simplified topography, such as 2-D or 3-D blocks and 2-D or 3-D sinusoidal hills, conducted under neutrally stratified boundary-layer conditions have provided insightful information of fluid dynamics. However, atmospheric thermal stability has rarely been incorporated into laboratory simulations, in particular, wind-tunnel experiments. Extension of such studies in thermally-stratified wind tunnels will fill this gap and advance our understanding of the underlying physics of flow over complex topography. Additionally, experimental data are useful for the development of new parameterizations for surface fluxes and validation of numerical models such as Large-Eddy Simulation (LES). A series of experiments involving neutral and thermally-stratified boundary-layer flows over a surface-mounted 2-D block, conducted at the Saint Anthony Falls Laboratory boundary-layer wind tunnel, will be presented. The 2-D block, with a width to height ratio of 2:1, occupied the lowest 25% of the turbulent boundary layer. Thermal stratification of the boundary layer was achieved by independently controlling the temperature of both the airflow, the test section floor and block surfaces. Measurements using high-resolution PIV, x-wire/cold-wire anemometry, thermal-couples and surface heat flux sensors were made to identify and quantify the turbulent flow properties, including the size of the recirculation zone, coherent vortex structures and the subsequent boundary layer recovery. Emphasis will be put on addressing thermal stability effects on momentum and scalar flux distribution.
CoPc 2D and 1D Arrangement on a Ferromagnetic Surface.
Annese, Emilia; ViolBarbosa, Carlos E; Rossi, Giorgio; Fujii, Jun
2016-05-31
We investigated the growth and electronic properties of Co-phthalocyanine (CoPc) molecule deposited on iron film with different structures (pseudomorph-fcc and bcc) and on iron nanowires by scanning tunnelling microscopy and X-ray absorption spectroscopy (XAS). CoPc molecules self-assemble in a two-dimensional (2D) arrangement with the molecular plane parallel to the iron surfaces, and the local order is lost after the first layer. The molecule-ferromagnet interaction causes the broadening of Co and N unoccupied molecular states as well as different electronic distribution of N states as a function of the atomic structure of iron surface. The ferromagnetic coupling between the molecule and the iron film is dominated by the electronic interaction between Co and the first Fe layer. CoPc 2D arrangement turns into 1D by using as a template the iron nanowire grown on a facet surface of oxidized Cu(332) surface. CoPc molecules interact weakly with the iron nanowires manifesting a substantial Co 3dz spectral feature in XAS spectrum and the possibility of a magnetic interaction between Co moment and iron nanowires. Both CoPc 2D and 1D arrangements can open up new interesting scenarios to tune the magnetic properties of hybrid interfaces involving metallorganic molecules. PMID:27191039
Indirect measurements of Fermi surface parameters of some chevrel phase materials
NASA Technical Reports Server (NTRS)
Woollam, J. A.; Alterovitz, S. A.
1979-01-01
A series of measurements of normal state and superconducting properties were made in zero and in high magnetic fields. When these results are combined with a complete set of theoretical expressions, a number of Fermi surface parameters are found.
Hall effect and Fermi surface reconstruction via electron pockets in the high-Tc cuprates
NASA Astrophysics Data System (ADS)
Storey, J. G.
2016-01-01
The mechanism by which the Fermi surface of high-T c cuprates undergoes a dramatic change from a large hole-like barrel to small arcs or pockets on entering the pseudogap phase remains a question of fundamental importance. Here we calculate the normal-state Hall coefficient from the resonating-valence-bond spin-liquid model developed by Yang, Rice and Zhang. In this model, reconstruction of the Fermi surface occurs via an intermediate regime where the Fermi surface consists of both hole- and electron-like pockets. We find that the doping (x) dependence of the Hall number transitions from 1+x to (x) over this narrow doping range. At low temperatures, a switch from a downturn to an upturn in the Hall coefficient signals the departure of the electron-like pockets from the Fermi surface.
Fermi-Surface Reconstruction and Complex Phase Equilibria in CaFe2As2
NASA Astrophysics Data System (ADS)
Gofryk, K.; Saparov, B.; Durakiewicz, T.; Chikina, A.; Danzenbächer, S.; Vyalikh, D. V.; Graf, M. J.; Sefat, A. S.
2014-05-01
Fermi-surface topology governs the relationship between magnetism and superconductivity in iron-based materials. Using low-temperature transport, angle-resolved photoemission, and x-ray diffraction, we show unambiguous evidence of large Fermi-surface reconstruction in CaFe2As2 at magnetic spin-density-wave and nonmagnetic collapsed-tetragonal (cT) transitions. For the cT transition, the change in the Fermi-surface topology has a different character with no contribution from the hole part of the Fermi surface. In addition, the results suggest that the pressure effect in CaFe2As2 is mainly leading to a rigid-band-like change of the valence electronic structure. We discuss these results and their implications for magnetism and superconductivity in this material.
NASA Astrophysics Data System (ADS)
Gweon, Gey-Hong
Using angle resolved photoemission spectroscopy (ARPES) as the main experimental tool and the single particle Green's function as the main theoretical tool, materials of various degrees of low dimensionality and different ground states are studied. The underlying theme of this thesis is that of one dimensional physics, which includes charge density waves (CDW's) and the Luttinger liquid (LL). The LL is the prime example of a lattice non-Fermi liquid (non-FL) and CDW fluctuations also give non-FL behaviors. Non-FL physics is an emerging paradigm of condensed matter physics. It is thought by some researchers that one dimensional LL behavior is a key element in solving the high temperature superconductivity problem. TiTe2 is a quasi-2 dimensional (quasi-2D) Fermi liquid (FL) material very well suited for ARPES lineshape studies. I report ARPES spectra at 300 K which show an unusual behavior of a peak moving through the Fermi energy (EF). I also report a good fit of the ARPES spectra at 25 K obtained by using a causal Green's function proposed by K. Matho. SmTe3 is a quasi-2D CDW material. The near EF ARPES spectra and intensity map reveal rich details of an anisotropic gap and imperfectly nested Fermi surface (FS) for a high temperature CDW. A simple model of imperfect nesting can be constructed from these data and predicts a CDW wavevector in very good agreement with the value known from electron diffraction. NaMo6O17 and KMo 6O17 are also quasi-2D CDW materials. The "hidden nesting" or "hidden 1 dimensionality" picture for the CDW is confirmed very well by our direct image of the FS. K0.3MoO3, the so-called "blue bronze," is a quasi-1 dimensional (quasi-1D) CDW material. Even in its metallic phase above the CDW transition temperature, its photoemission spectra show an anomalously weak intensity at EF and no clear metallic Fermi edge. I compare predictions of an LL model and a CDW fluctuation model regarding these aspects, and find that the LL scenario explains them
Wall surface temperature calculation in the SolEdge2D-EIRENE transport code
NASA Astrophysics Data System (ADS)
Denis, J.; Pégourié, B.; Bucalossi, J.; Bufferand, H.; Ciraolo, G.; Gardarein, J.-L.; Gaspar, J.; Grisolia, C.; Hodille, E.; Missirlian, M.; Serre, E.; Tamain, P.
2016-02-01
A thermal wall model is developed for the SolEdge2D-EIRENE edge transport code for calculating the surface temperature of the actively-cooled vessel components in interaction with the plasma. This is a first step towards a self-consistent evaluation of the recycling of particles, which depends on the wall surface temperature. The proposed thermal model is built to match both steady-state temperature and time constant of actively-cooled plasma facing components. A benchmark between this model and the Finite Element Modelling code CAST3M is performed in the case of an ITER-like monoblock. An example of application is presented for a SolEdge2D-EIRENE simulation of a medium-power discharge in the WEST tokamak, showing the steady-state wall temperature distribution and the temperature cycling due to an imposed Edge Localised Mode-like event.
Nonrigid point registration for 2D curves and 3D surfaces and its various applications
NASA Astrophysics Data System (ADS)
Wang, Hesheng; Fei, Baowei
2013-06-01
A nonrigid B-spline-based point-matching (BPM) method is proposed to match dense surface points. The method solves both the point correspondence and nonrigid transformation without features extraction. The registration method integrates a motion model, which combines a global transformation and a B-spline-based local deformation, into a robust point-matching framework. The point correspondence and deformable transformation are estimated simultaneously by fuzzy correspondence and by a deterministic annealing technique. Prior information about global translation, rotation and scaling is incorporated into the optimization. A local B-spline motion model decreases the degrees of freedom for optimization and thus enables the registration of a larger number of feature points. The performance of the BPM method has been demonstrated and validated using synthesized 2D and 3D data, mouse MRI and micro-CT images. The proposed BPM method can be used to register feature point sets, 2D curves, 3D surfaces and various image data.
Formation and properties of a terpyridine-based 2D MOF on the surface of water
NASA Astrophysics Data System (ADS)
Koitz, Ralph; Hutter, Jürg; Iannuzzi, Marcella
2016-06-01
Two-dimensional networks inspired by graphene are of prime importance in nanoscience. We present a computational study of an infinite molecular sheet confined on a water surface to assess its properties and formation mechanism. Terpyridine-based ligand molecules are interlinked by Zn ions to form an extended 2D metal-organic framework. We show that the network is stable on the water surface, and that the substrate affects the dynamic properties of the sheet, exhibiting a confining effect and flattening the sheet by 30%. We use metadynamics to characterize the process of network formation and breaking and determine an intra-network binding energy of 143 kJ mol‑1. Based on this mechanistic insight we propose that the 2D network strength can be tuned by varying the rigidity of the ligand through its chemical structure.
Strong phonon anomalies and Fermi surface nesting of simple cubic Polonium
NASA Astrophysics Data System (ADS)
Belabbes, A.; Zaoui, A.; Ferhat, M.
2010-12-01
The unknown lattice dynamics of simple cubic Polonium is calculated using first-principles density-functional perturbation theory with pseudopotentials and a plane-wave basis set. We notice several phonon anomalies, in particular along major symmetry directions namely M-R, R-Γ, Γ-M, M-X, and X-Γ. The analysis of the Fermi surface strongly suggests that the observed phonon anomalies are Kohn anomalies arising from strong Fermi surface nesting.
Quantum oscillations and the Fermi surface topology of the Weyl semimetal NbP
NASA Astrophysics Data System (ADS)
Klotz, J.; Wu, Shu-Chun; Shekhar, Chandra; Sun, Yan; Schmidt, Marcus; Nicklas, Michael; Baenitz, Michael; Uhlarz, M.; Wosnitza, J.; Felser, Claudia; Yan, Binghai
2016-03-01
The Weyl semimetal NbP was found to exhibit topological Fermi arcs and exotic magnetotransport properties. Here, we report on magnetic quantum-oscillation measurements on NbP and construct the three-dimensional Fermi surface with the help of band-structure calculations. We reveal a pair of spin-orbit-split electron pockets at the Fermi energy and a similar pair of hole pockets, all of which are strongly anisotropic. The Weyl points that are located in the kz≈π /c plane are found to exist 5 meV above the Fermi energy. Therefore, we predict that the chiral anomaly effect can be realized in NbP by electron doping to drive the Fermi energy to the Weyl points.
Oscillations of light absorption in 2D macroporous silicon structures with surface nanocoatings
NASA Astrophysics Data System (ADS)
Karachevtseva, L.; Kuchmii, S.; Lytvynenko, O.; Sizov, F.; Stronska, O.; Stroyuk, A.
2011-02-01
We investigated the near-IR light absorption oscillations in 2D macroporous silicon structures with microporous silicon layers and CdTe, ZnO surface nanocrystals. The electro-optical effect was taken into account within the strong electric field approximation. Well-separated oscillations were observed in the spectral ranges of the surface bonds of macroporous silicon structures with surface nanocrystals. The model of the resonant electron scattering on impurity states in electric field of heterojunction “silicon-nanocoating” on macropore surface as well as realization of Wannier-Stark effect on the randomly distributed surface bonds were considered. The Wannier-Stark ladders are not broken by impurities because of the longer scattering lifetime as compared with the period of electron oscillations in an external electric field, in all spectral regions considered for macroporous silicon structures with CdTe and ZnO surface nanocrystals.
Defining an optimal surface chemistry for pluripotent stem cell culture in 2D and 3D
NASA Astrophysics Data System (ADS)
Zonca, Michael R., Jr.
Surface chemistry is critical for growing pluripotent stem cells in an undifferentiated state. There is great potential to engineer the surface chemistry at the nanoscale level to regulate stem cell adhesion. However, the challenge is to identify the optimal surface chemistry of the substrata for ES cell attachment and maintenance. Using a high-throughput polymerization and screening platform, a chemically defined, synthetic polymer grafted coating that supports strong attachment and high expansion capacity of pluripotent stem cells has been discovered using mouse embryonic stem (ES) cells as a model system. This optimal substrate, N-[3-(Dimethylamino)propyl] methacrylamide (DMAPMA) that is grafted on 2D synthetic poly(ether sulfone) (PES) membrane, sustains the self-renewal of ES cells (up to 7 passages). DMAPMA supports cell attachment of ES cells through integrin beta1 in a RGD-independent manner and is similar to another recently reported polymer surface. Next, DMAPMA has been able to be transferred to 3D by grafting to synthetic, polymeric, PES fibrous matrices through both photo-induced and plasma-induced polymerization. These 3D modified fibers exhibited higher cell proliferation and greater expression of pluripotency markers of mouse ES cells than 2D PES membranes. Our results indicated that desirable surfaces in 2D can be scaled to 3D and that both surface chemistry and structural dimension strongly influence the growth and differentiation of pluripotent stem cells. Lastly, the feasibility of incorporating DMAPMA into a widely used natural polymer, alginate, has been tested. Novel adhesive alginate hydrogels have been successfully synthesized by either direct polymerization of DMAPMA and methacrylic acid blended with alginate, or photo-induced DMAPMA polymerization on alginate nanofibrous hydrogels. In particular, DMAPMA-coated alginate hydrogels support strong ES cell attachment, exhibiting a concentration dependency of DMAPMA. This research provides a
Mirror effects and optical meta-surfaces in 2d atomic arrays
NASA Astrophysics Data System (ADS)
Shahmoon, Ephraim; Wild, Dominik; Lukin, Mikhail; Yelin, Susanne
2016-05-01
Strong optical response of natural and artificial (meta-) materials typically relies on the fact that the lattice constant that separates their constituent particles (atoms or electromagnetic resonators, respectively) is much smaller than the optical wavelength. Here we consider a single layer of a 2d atom array with a lattice constant on the order of an optical wavelength, which can be thought of as a highly dilute 2d metamaterial (meta-surface). Our theoretical analysis shows how strong scattering of resonant incoming light off the array can be controlled by choosing its lattice constant, e.g. allowing the array to operate as a perfect mirror or a retro-reflector for most incident angles of the incoming light. We discuss the prospects for quantum metasurfaces, i.e. the ability to shape the output quantum state of light by controlling the atomic states, and the possible generality of our results as a universal wave phenomena.
Visualizing weakly bound surface Fermi arcs and their correspondence to bulk Weyl fermions
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
Visualizing weakly bound surface Fermi arcs and their correspondence to bulk Weyl fermions.
Batabyal, Rajib; Morali, Noam; Avraham, Nurit; Sun, Yan; Schmidt, Marcus; Felser, Claudia; Stern, Ady; Yan, Binghai; Beidenkopf, Haim
2016-08-01
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
Investigation on the GPS single scattering from a 2-D largescale sea surface
NASA Astrophysics Data System (ADS)
Wei, Yiwen; Guo, Lixin
2014-05-01
Global positioning system (GPS) signals reflected from the ocean surface can be used for various remote sensing purposes. In this paper, we develop a facet model to simulate the received GPS single from a 2-D largescale sea surface. In this model, the sea surface is envisaged as a two-scale profile on which the long waves are locally approximated by planar facets. The microscopic profile within a facet is assumed to be represented by a set of sinusoidal ripple patches. The complex reflective function of each modified facet is evaluated by a modified formula of the original Bass and Fuks' two-scale model, in which the phase factor of each facet is with the capillary wave modification. The scattering field and the bistatic scattering coefficient of facet model is derived in detail. With received GPS single, we give a detail analysis of the polarization property, the scattering property of GPS scattering signal over the sea surface.
Confinement properties of 2D porous molecular networks on metal surfaces
NASA Astrophysics Data System (ADS)
Müller, Kathrin; Enache, Mihaela; Stöhr, Meike
2016-04-01
Quantum effects that arise from confinement of electronic states have been extensively studied for the surface states of noble metals. Utilizing small artificial structures for confinement allows tailoring of the surface properties and offers unique opportunities for applications. So far, examples of surface state confinement include thin films, artificial nanoscale structures, vacancy and adatom islands, self-assembled 1D chains, vicinal surfaces, quantum dots and quantum corrals. In this review we summarize recent achievements in changing the electronic structure of surfaces by adsorption of nanoporous networks whose design principles are based on the concepts of supramolecular chemistry. Already in 1993, it was shown that quantum corrals made from Fe atoms on a Cu(1 1 1) surface using single atom manipulation with a scanning tunnelling microscope confine the Shockley surface state. However, since the atom manipulation technique for the construction of corral structures is a relatively time consuming process, the fabrication of periodic two-dimensional (2D) corral structures is practically impossible. On the other side, by using molecular self-assembly extended 2D porous structures can be achieved in a parallel process, i.e. all pores are formed at the same time. The molecular building blocks are usually held together by non-covalent interactions like hydrogen bonding, metal coordination or dipolar coupling. Due to the reversibility of the bond formation defect-free and long-range ordered networks can be achieved. However, recently also examples of porous networks formed by covalent coupling on the surface have been reported. By the choice of the molecular building blocks, the dimensions of the network (pore size and pore to pore distance) can be controlled. In this way, the confinement properties of the individual pores can be tuned. In addition, the effect of the confined state on the hosting properties of the pores will be discussed in this review article.
Confinement properties of 2D porous molecular networks on metal surfaces.
Müller, Kathrin; Enache, Mihaela; Stöhr, Meike
2016-04-20
Quantum effects that arise from confinement of electronic states have been extensively studied for the surface states of noble metals. Utilizing small artificial structures for confinement allows tailoring of the surface properties and offers unique opportunities for applications. So far, examples of surface state confinement include thin films, artificial nanoscale structures, vacancy and adatom islands, self-assembled 1D chains, vicinal surfaces, quantum dots and quantum corrals. In this review we summarize recent achievements in changing the electronic structure of surfaces by adsorption of nanoporous networks whose design principles are based on the concepts of supramolecular chemistry. Already in 1993, it was shown that quantum corrals made from Fe atoms on a Cu(1 1 1) surface using single atom manipulation with a scanning tunnelling microscope confine the Shockley surface state. However, since the atom manipulation technique for the construction of corral structures is a relatively time consuming process, the fabrication of periodic two-dimensional (2D) corral structures is practically impossible. On the other side, by using molecular self-assembly extended 2D porous structures can be achieved in a parallel process, i.e. all pores are formed at the same time. The molecular building blocks are usually held together by non-covalent interactions like hydrogen bonding, metal coordination or dipolar coupling. Due to the reversibility of the bond formation defect-free and long-range ordered networks can be achieved. However, recently also examples of porous networks formed by covalent coupling on the surface have been reported. By the choice of the molecular building blocks, the dimensions of the network (pore size and pore to pore distance) can be controlled. In this way, the confinement properties of the individual pores can be tuned. In addition, the effect of the confined state on the hosting properties of the pores will be discussed in this review article
Acoustic Receptivity of a Blasius Boundary Layer with 2-D and Oblique Surface Waviness
NASA Technical Reports Server (NTRS)
King, Rudolph A.; Breuer, Kenneth S.
2000-01-01
An experimental investigation was conducted to examine acoustic receptivity and subsequent boundary-layer instability evolution for a Blasius boundary layer formed on a flat plate in the presence of two-dimensional (2-D) and oblique (3-D) surface waviness. The effect of the non-localized surface roughness geometry and acoustic wave amplitude on the receptivity process was explored. The surface roughness had a well defined wavenumber spectrum with fundamental wavenumber k (sub w). A planar downstream traveling acoustic wave was created to temporally excite the flow near the resonance frequency of an unstable eigenmode corresponding to k (sub ts) = k (sub w). The range of acoustic forcing levels, epsilon, and roughness heights, DELTA h, examined resulted in a linear dependence of receptivity coefficients; however, the larger values of the forcing combination epsilon dot DELTA h resulted in subsequent nonlinear development of the Tollmien-Schlichting (T-S) wave. This study provided the first experimental evidence of a marked increase in the receptivity coefficient with increasing obliqueness of the surface waviness in excellent agreement with theory. Detuning of the 2-D and oblique disturbances was investigated by varying the streamwise wall-roughness wavenumber a,, and measuring the T-S response. For the configuration where laminar-to-turbulent breakdown occurred, the breakdown process was found to be dominated by energy at the fundamental and harmonic frequencies, indicative of K-type breakdown.
NASA Astrophysics Data System (ADS)
Randeria, Mohit; Banerjee, Sumilan; Rowland, James
2015-09-01
Most theoretical studies of chiral magnetism, and the resulting spin textures, have focused on 3D systems with broken bulk inversion symmetry, where skyrmions are stabilized by easy-axis anisotropy. In this talk I will describe our results on 2D and quasi-2D systems with broken surface inversion, where we find [1] that skyrmion crystals are much more stable than in 3D, especially for the case of easy-plane anisotropy. These results are of particular interest for thin films, surfaces, and oxide interfaces [2], where broken surface-inversion symmetry and Rashba spin-orbit coupling naturally lead to both the chiral Dzyaloshinskii-Moriya (DM) interaction and to easy-plane compass anisotropy. I will then turn to systems that break both bulk and surface inversion, resulting in two distinct DM terms arising from Dresselhaus and Rashba spin-orbit coupling. I will describe [3] the evolution of the skyrmion structure and of the phase diagram as a function of the ratio of Dresselhaus and Rashba terms, which can be tuned by varying film thickness and strain. [1] S. Banerjee, J. Rowland, O. Erten, and M. Randeria, PRX 4, 031045 (2014). [2] S. Banerjee, O. Erten, and M. Randeria, Nature Phys. 9, 626 (2013). [3] J. Rowland, S. Banerjee and M. Randeria, (unpublished).
Nested 1D-2D approach for urban surface flood modeling
NASA Astrophysics Data System (ADS)
Murla, Damian; Willems, Patrick
2015-04-01
Floods in urban areas as a consequence of sewer capacity exceedance receive increased attention because of trends in urbanization (increased population density and impermeability of the surface) and climate change. Despite the strong recent developments in numerical modeling of water systems, urban surface flood modeling is still a major challenge. Whereas very advanced and accurate flood modeling systems are in place and operation by many river authorities in support of flood management along rivers, this is not yet the case in urban water management. Reasons include the small scale of the urban inundation processes, the need to have very high resolution topographical information available, and the huge computational demands. Urban drainage related inundation modeling requires a 1D full hydrodynamic model of the sewer network to be coupled with a 2D surface flood model. To reduce the computational times, 0D (flood cones), 1D/quasi-2D surface flood modeling approaches have been developed and applied in some case studies. In this research, a nested 1D/2D hydraulic model has been developed for an urban catchment at the city of Gent (Belgium), linking the underground sewer (minor system) with the overland surface (major system). For the overland surface flood modelling, comparison was made of 0D, 1D/quasi-2D and full 2D approaches. The approaches are advanced by considering nested 1D-2D approaches, including infiltration in the green city areas, and allowing the effects of surface storm water storage to be simulated. An optimal nested combination of three different mesh resolutions was identified; based on a compromise between precision and simulation time for further real-time flood forecasting, warning and control applications. Main streets as mesh zones together with buildings as void regions constitute one of these mesh resolution (3.75m2 - 15m2); they have been included since they channel most of the flood water from the manholes and they improve the accuracy of
Tomographic investigation of fermi level pinning at focused ion beam milled semiconductor surfaces
NASA Astrophysics Data System (ADS)
Wolf, D.; Lubk, A.; Lenk, A.; Sturm, S.; Lichte, H.
2013-12-01
Electron holography in the transmission electron microscope (TEM) offers the spatial and signal resolution for studying effects like Fermi level pinning or dopant concentration variations important for the design of modern electronic devices. To overcome the loss of information along the projection direction, surface effects, and surface damage due to TEM specimen preparation, we apply electron holographic tomography to analyze the 3D potential distribution of semiconductor samples prepared by focused-ion-beam. We observe mid-band gap pinning of the Fermi level at Si surfaces but valence band pinning at Ge surfaces. The pinning extends over tens of nanometers into the bulk.
Infinitesimal-area 2D radiative analysis using parametric surface representation, through NURBS
Daun, K.J.; Hollands, K.G.T.
1999-07-01
The use of form factors in the treatment of radiant enclosures requires that the radiosity and surface properties be treated as uniform over finite areas. This restriction can be relaxed by applying an infinitesimal-area analysis, where the radiant exchange is taken to be between infinitesimal areas, rather than finite areas. This paper presents a generic infinitesimal-area formulation that can be applied to two-dimensional enclosure problems. (Previous infinitesimal-area analyses have largely been restricted to specific, one-dimensional problems.) Specifically, the paper shows how the analytical expression for the kernel of the integral equation can be obtained without human intervention, once the enclosure surface has been defined parametrically. This can be accomplished by using a computer algebra package or by using NURBS algorithms, which are the industry standard for the geometrical representations used in CAD-CAM codes. Once the kernel has been obtained by this formalism, the 2D integral equation can be set up and solved numerically. The result is a single general-purpose infinitesimal-area analysis code that can proceed from surface specification to solution. The authors have implemented this 2D code and tested it on 1D problems, whose solutions have been given in the literature, obtaining agreement commensurate with the accuracy of the published solutions.
Impact of Surface Roughness on Capillary Trapping Using 2D-Micromodel Visualization Experiments
NASA Astrophysics Data System (ADS)
Geistlinger, Helmut; Attaei-Dadavi, Iman; Vogel, Hans-Jörg
2016-04-01
According to experimental observations, capillary trapping is strongly dependent on the roughness of the pore-solid interface. We performed imbibition experiments in the range of capillary numbers (Ca) from 10^-6 to 5x10^-5 using 2D-micromodels, which exhibit a rough surface. The microstructure comprises a double-porosity structure with pronounced macropores. The dynamics of precursor thin-film flow and its importance for capillary trapping is studied. For the first time Thin-Film Dynamics and the Complex Interplay of Thin Film- and Corner Flow for Snap-off Trapping is visualized using fluorescence microscopy. The experimental data for thin-film flow advancement show a square-root time dependence. Contrary to smooth surfaces, we prove by strict thermodynamical arguments that complete wetting is possible in a broad range of contact angles (0 - 90°). We develop a pore-scale model, which describes the front dynamics of thin-film flow on rough surfaces. Furthermore, contact angle hysteresis is considered for rough surfaces. We conduct a comprehensive cluster analysis, studying the influence of viscous forces (capillary number) and buoyancy forces (bond number) on cluster size distribution and comparing the results with predictions from percolation theory. We found that our experimental results agree with theoretical results of percolation theory for Ca = 10^-6: (i) a universal power-like cluster size distribution, (ii) the linear surface-volume relationship of trapped clusters, and (iii) the existence of the cut-off correlation length for the maximal cluster height. The good agreement is a strong argument that the experimental cluster size distribution is caused by a percolation-like trapping process (Ordinary Percolation). [1] H. Geistlinger, I. Ataei-Dadavi, S. Mohammadian, and H.-J. Vogel (2015) The Impact of Pore structure and Surface Roughness on Capillary Trapping for 2D- and 3D-porous media: Comparison with Percolation theory. Special issue: Applications of
Skyrmions in quasi-2D chiral magnets with broken bulk and surface inversion symmetry
NASA Astrophysics Data System (ADS)
Rowland, James; Banerjee, Sumilan; Randeria, Mohit
2015-03-01
Most theoretical studies of skyrmions have focused on chiral magnets with broken bulk inversion symmetry, stabilized by easy-axis anisotropy. Recently, we considered 2D systems with broken surface inversion and showed that skyrmion crystals are more stable than in 3D, pointing out the importance of easy-plane anisotropy. In the present work we investigate quasi-2D systems which break both bulk and surface inversion symmetry. The Landau-Ginzburg free energy functional thus contains two Dzyloshinskii-Moriya terms of strength DD and DR arising from Dresselhaus and Rashba spin-orbit coupling respectively. We trace the evolution of the phase diagram as DD /DR is varied, and find that skyrmions are increasingly destabilized with respect to the cone phase as DD increases relative to DR. We find an evolution from vortex-like skyrmions in the pure Dresselhaus limit to hedgehog-like skyrmions in the pure Rashba limit. We discuss the relevance of these results to existing experiments and the prospects of tuning the ratio of Dresselhaus and Rashba spin-orbit coupling via film thickness and strain. Supported by NSF DMR-1410364 (J.R. and M.R.) and DOE-BES DE-SC0005035 (S.B.)
NASA Astrophysics Data System (ADS)
Abdelmalak, M.; Mourgues, R.; Bureau, D.
2012-04-01
The analysis of surface deformation in response to approaching intrusion is important for assessing volcanic hazards. In this paper, we present results from 2D scaled models of magma intrusion, in which we discuss the propagation mode and related surface deformation during dyke growth. Our experiments consist in the injection of analogue magma (Golden syrup) into cohesive fine-grained silica powder, simulating the brittle upper crust. Using an optical image correlation technique (Particle Imaging Velocimetry), we were able to follow the surface deformation, the displacements within the country rock and to calculate strains induced by the magma emplacement. We identified two kinds of intrusion morphologies resulting from different interactions between the dyke and plastic deformations occurring in the country rock near the surface. In both morphologies, the dyke is vertical at depth. Our analysis demonstrates that both hydraulic tensile opening and shear-related propagation operate during this first stage of vertical growth. At the same time, the surface lifted up and formed a smooth symmetrical dome. Both types of morphologies differ in the upper part. During a second stage of evolution, the first type of intrusion inclined at a dip between 45 to 65°. This inclination is not caused by shear deformations and is attributed to stress rotation near the tip. Closer to the surface, the growth of the inclined sheet creates shear bands which conduct the fluid toward the surface. The surface uplift becomes asymmetric. The second type of intrusion does not rotate at depth and continues its vertical propagation by catching vertical tensile cracks. The intrusion of magma in these cracks creates horizontal stresses which are responsible for the closure of fractures and the formation of reverse faults. At the surface the dome remains symmetrical. For both intrusions, the surface uplift accelerates during the second stage and it is strongly influenced by the presence or the
Probing Critical Surfaces in Momentum Space Using Real-Space Entanglement Entropy: Bose versus Fermi
NASA Astrophysics Data System (ADS)
Yang, Kun; Lai, Hsin-Hua
A co-dimension one critical surface in the momentum space can be either a familiar Fermi surface, which separates occupied states from empty ones in the non-interacting fermion case, or a novel Bose surface, where gapless bosonic excitations are anchored. Their presence gives rise to logarithmic violation of entanglement entropy area law. When they are convex, we show that the shape of these critical surfaces can be determined by inspecting the leading logarithmic term of real space entanglement entropy. The fundamental difference between a Fermi surface and a Bose surface is revealed by the fact that the logarithmic terms in entanglement entropies differ by a factor of two: SlogBose = 2SlogFermi , even when they have identical geometry. Our method has remarkable similarity with determining Fermi surface shape using quantum oscillation. We also discuss possible probes of concave critical surfaces in momentum space. HHL and KY acknowledge the National Science Foundation through Grants No. DMR-1004545, DMR-1157490, No. DMR-1442366, and State of Florida. HHL is also partially supported by NSF Grant No. DMR-1309531, and the Smalley Postdoctoral Fellowship in Quantum Ma.
The effect of polarity and surface states on the Fermi level at III-nitride surfaces
Reddy, P; Bryan, I; Bryan, Z; Guo, W; Hussey, L; Collazo, R; Sitar, Z
2014-09-28
Surface states and their influence on the Fermi level at the surface of GaN and AlN are studied using x-ray photoelectron spectroscopy (XPS). The effect of polarity on surface electronic properties was studied. Accurate modeling of the valence band edge and comparison with XPS data revealed the presence of donor surface states at 1.4 eV and acceptor states at energies > 2.7 eV from the valence band in GaN. Al polar AlN showed acceptor states at energies > 3.3 eV. Density of acceptor surface states was estimated to be between 10(13) and 10(14) eV(-1) cm(-2) in both GaN and AlN. The shift in charge neutrality levels and barrier heights due to polarity and the density of surface states on AlN and GaN were estimated from XPS measurements. Theoretical modeling and comparison with XPS data implied full compensation of spontaneous polarization charge by charged surface states. Barrier height measurements also reveal a dependence on polarity with phi(metal-polar)>phi(non-polar)>phi(nitrogen-polar) suggesting that the N-polar surface is the most suitable for Ohmic contacts. (C) 2014 AIP Publishing LLC.
Anomalous Fermi-Surface Dependent Pairing in a Self-Doped High-T(c) Superconductor
Chen, Yulin; Iyo, Akira; Yang, Wanli; Zhou, Xingjiang; Lu, Donghui; Eisaki, Hiroshi; Devereaux, Thomas P.; Hussain, Zahid; Shen, Z.-X.; /Stanford U., Phys. Dept. /SLAC, SSRL /AIST, Tsukuba /Waterloo U. /LBNL, ALS
2007-02-12
We report the discovery of a self-doped multilayer high T{sub c} superconductor Ba{sub 2}Ca{sub 3}Cu{sub 4}O{sub 8}F{sub 2} (F0234) which contains distinctly different superconducting gap magnitudes along its two Fermi-surface sheets. While formal valence counting would imply this material to be an undoped insulator, it is a self-doped superconductor with a T{sub c} of 60 K, possessing simultaneously both electron- and hole-doped Fermi-surface sheets. Intriguingly, the Fermi-surface sheet characterized by the much larger gap is the electron-doped one, which has a shape disfavoring two electronic features considered to be important for the pairing mechanism: the van Hove singularity and the antiferromagnetic ({pi}/{alpha}, {pi}/{alpha}) scattering.
Quantum Oscillation Studies of the Fermi Surface of LaFePO
Carrington, A.
2010-05-26
We review recent experimental measurements of the Fermi surface of the iron-pnictide superconductor LaFePO using quantum oscillation techniques. These studies show that the Fermi surface topology is close to that predicted by first principles density functional theory calculations, consisting of quasi-twodimensional electron-like and hole-like sheets. The total volume of the two hole sheets is almost equal to that of the two electron sheets, and the hole and electron Fermi surface sheets are close to a nesting condition. No evidence for the predicted three dimensional pocket arising from the Fe d{sub z}{sup 2} band is found. Measurements of the effective mass suggest a renormalisation of around two, close to the value for the overall band renormalisation found in recent angle resolved photoemission measurements.
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.
Pressure tuning the Fermi surface topology of the Weyl semimetal NbP
NASA Astrophysics Data System (ADS)
dos Reis, R. D.; Wu, S. C.; Sun, Y.; Ajeesh, M. O.; Shekhar, C.; Schmidt, M.; Felser, C.; Yan, B.; Nicklas, M.
2016-05-01
We report on the pressure evolution of the Fermi surface topology of the Weyl semimetal NbP, probed by Shubnikov-de Haas oscillations in the magnetoresistance combined with ab initio calculations of the band structure. Although we observe a drastic effect on the amplitudes of the quantum oscillations, the frequencies only exhibit a weak pressure dependence up to 2.8 GPa. The pressure-induced variations in the oscillation frequencies are consistent with our band-structure calculations. Furthermore, we can relate the changes in the amplitudes to small modifications in the shape of the Fermi surface. Our findings show evidence of the stability of the electronic band structure of NbP and demonstrate the power of combining quantum-oscillation studies and band-structure calculations to investigate pressure effects on the Fermi surface topology in Weyl semimetals.
Origin of Fermi-level pinning at GaAs surfaces and interfaces
NASA Astrophysics Data System (ADS)
Colleoni, Davide; Miceli, Giacomo; Pasquarello, Alfredo
2014-12-01
Through first-principles simulation methods, we assign the origin of Fermi-level pinning at GaAs surfaces and interfaces to the bistability between the As-As dimer and two As dangling bonds, which transform into each other upon charge trapping. This defect is shown to be naturally formed both at GaAs surfaces upon oxygen deposition and in the near-interface substoichiometric oxide. Using electron-counting arguments, we infer that the identified defect occurs in opposite charge states. The Fermi-level pinning then results from the amphoteric nature of this defect which drives the Fermi level to its defect level. These results account for the experimental characterization at both GaAs surfaces and interfaces within a unified picture, wherein the role of As antisites is elucidated.
Origin of Fermi-level pinning at GaAs surfaces and interfaces.
Colleoni, Davide; Miceli, Giacomo; Pasquarello, Alfredo
2014-12-10
Through first-principles simulation methods, we assign the origin of Fermi-level pinning at GaAs surfaces and interfaces to the bistability between the As-As dimer and two As dangling bonds, which transform into each other upon charge trapping. This defect is shown to be naturally formed both at GaAs surfaces upon oxygen deposition and in the near-interface substoichiometric oxide. Using electron-counting arguments, we infer that the identified defect occurs in opposite charge states. The Fermi-level pinning then results from the amphoteric nature of this defect which drives the Fermi level to its defect level. These results account for the experimental characterization at both GaAs surfaces and interfaces within a unified picture, wherein the role of As antisites is elucidated. PMID:25372411
Integrated Coupling of Surface and Subsurface Flow with HYDRUS-2D
NASA Astrophysics Data System (ADS)
Hartmann, Anne; Šimůnek, Jirka; Wöhling, Thomas; Schütze, Niels
2016-04-01
Describing interactions between surface and subsurface flow processes is important to adequately define water flow in natural systems. Since overland flow generation is highly influenced by rainfall and infiltration, both highly spatially heterogeneous processes, overland flow is unsteady and varies spatially. The prediction of overland flow needs to include an appropriate description of the interactions between the surface and subsurface flow. Coupling surface and subsurface water flow is a challenging task. Different approaches have been developed during the last few years, each having its own advantages and disadvantages. A new approach by Weill et al. (2009) to couple overland flow and subsurface flow based on a generalized Richards equation was implemented into the well-known subsurface flow model HYDRUS-2D (Šimůnek et al., 2011). This approach utilizes the one-dimensional diffusion wave equation to model overland flow. The diffusion wave model is integrated in HYDRUS-2D by replacing the terms of the Richards equation in a pre-defined runoff layer by terms defining the diffusion wave equation. Using this approach, pressure and flux continuity along the interface between both flow domains is provided. This direct coupling approach provides a strong coupling of both systems based on the definition of a single global system matrix to numerically solve the coupled flow problem. The advantage of the direct coupling approach, compared to the loosely coupled approach, is supposed to be a higher robustness, when many convergence problems can be avoided (Takizawa et al., 2014). The HYDRUS-2D implementation was verified using a) different test cases, including a direct comparison with the results of Weill et al. (2009), b) an analytical solution of the kinematic wave equation, and c) the results of a benchmark test of Maxwell et al. (2014), that included several known coupled surface subsurface flow models. Additionally, a sensitivity analysis evaluating the effects
Surface charge effects on the 2D conformation of supercoiled DNA.
Schmatko, Tatiana; Muller, Pierre; Maaloum, Mounir
2014-04-21
We have adsorbed plasmid pUc19 DNA on a supported bilayer. By varying the fraction of cationic lipids in the membrane, we have tuned the surface charge. Plasmid conformations were imaged by Atomic Force Microscopy (AFM). We performed two sets of experiments: deposition from salt free solution on charged bilayers and deposition from salty solutions on neutral bilayers. Both sets show similar trends: at low surface charge density or low bulk salt concentration, the internal electrostatic repulsion forces plasmids to adopt completely opened structures, while at high surface charge density or higher bulk salt concentration, usual supercoiled plectonemes are observed. We experimentally demonstrate the equivalence of surface screening by mobile interfacial charges and bulk screening from salt ions. At low to medium screening, the electrostatic repulsion at plasmid crossings is predominant, leading to a number of crossovers decreasing linearly with the characteristic screening length. We compare our data with an analytical 2D-equilibrated model developed recently for the system and extract the DNA effective charge density when strands are adsorbed at the surface. PMID:24647451
NASA Astrophysics Data System (ADS)
Charnukha, Aliaksei
2015-03-01
In the family of iron-based superconductors, 1111-type materials exhibit superconductivity with the highest transition temperature Tc=55K. Early theoretical predictions of their electronic structure revealed multiple large circular sheets of the Fermi surface. Here we use ARPES to show that two prototypical compounds of the 1111 type are at odds with this description. Their low-energy band structure is formed by the edges of several bands, which are pulled to the Fermi level from the depths of the theoretically predicted band structure by strong electronic interactions. We further demonstrate that although their low-energy electronic looks remarkably similar, the Tc differs by a factor of 2. Upon closer examination we uncover that one of the bands in the higher-Tc compound sinks to 2.3meV below the Fermi level and thus does not produce a Fermi surface. And yet we find that it hosts a superconducting energy gap 10x larger than the same band in the lower-Tc sister compound. Our results show that the Fermi-surface singularities in the iron-oxypnictides dramatically affect their low-energy electronic properties, including superconductivity, and must therefore be explicitly taken into account in any attempt to understand the pairing mechanism.
Quantum phase transitions, frustration, and the Fermi surface in the Kondo lattice model
NASA Astrophysics Data System (ADS)
Eidelstein, Eitan; Moukouri, S.; Schiller, Avraham
2011-07-01
The quantum phase transition from a spin-Peierls phase with a small Fermi surface to a paramagnetic Luttinger-liquid phase with a large Fermi surface is studied in the framework of a one-dimensional Kondo-Heisenberg model that consists of an electron gas away from half filling, coupled to a spin-1/2 chain by Kondo interactions. The Kondo spins are further coupled to each other with isotropic nearest-neighbor and next-nearest-neighbor antiferromagnetic Heisenberg interactions which are tuned to the Majumdar-Ghosh point. Focusing on three-eighths filling and using the density-matrix renormalization-group (DMRG) method, we show that the zero-temperature transition between the phases with small and large Fermi momenta appears continuous, and involves a new intermediate phase where the Fermi surface is not well defined. The intermediate phase is spin gapped and has Kondo-spin correlations that show incommensurate modulations. Our results appear incompatible with the local picture for the quantum phase transition in heavy fermion compounds, which predicts an abrupt change in the size of the Fermi momentum.
Fermi Surface of Superconducting LaFePO Determined by Quantum Oscillations
Coldea, A.I.; Fletcher, J.D.; Carrington, A.; Analytis, J.G.; Bangura, A.F.; Chu, J.-H.; Erickson, A.S.; Fisher, I.R.; Hussey, N.E.; McDonald, R.D.; /Los Alamos
2010-01-11
We report extensive measurements of quantum oscillations in the normal state of the Fe-based superconductor LaFePO, (T{sub c} {approx} 6 K) using low temperature torque magnetometry and transport in high static magnetic fields (45 T). We find that the Fermi surface is in broad agreement with the band-structure calculations with the quasiparticle mass enhanced by a factor {approx}2. The quasi-two dimensional Fermi surface consist of nearly-nested electron and hole pockets, suggesting proximity to a spin/charge density wave instability.
Zhang, Xiaoyu; Li, Lingling
2016-03-21
Net surface shortwave radiation (NSSR) significantly affects regional and global climate change, and is an important aspect of research on surface radiation budget balance. Many previous studies have proposed methods for estimating NSSR. This study proposes a method to calculate NSSR using FY-2D short-wave channel data. Firstly, a linear regression model is established between the top-of-atmosphere (TOA) broadband albedo (r) and the narrowband reflectivity (ρ_{1}), based on data simulated with MODTRAN 4.2. Secondly, the relationship between surface absorption coefficient (as) and broadband albedo (r) is determined by dividing the surface type into land, sea, or snow&ice, and NSSR can then be calculated. Thirdly, sensitivity analysis is performed for errors associated with sensor noise, vertically integrated atmospheric water content, view zenith angle and solar zenith angle. Finally, validation using ground measurements is performed. Results show that the root mean square error (RMSE) between the estimated and actual r is less than 0.011 for all conditions, and the RMSEs between estimated and real NSSR are 26.60 W/m^{2}, 9.99 W/m^{2}, and 23.40 W/m^{2}, using simulated data for land, sea, and snow&ice surfaces, respectively. This indicates that the proposed method can be used to adequately estimate NSSR. Additionally, we compare field measurements from TaiYuan and ChangWu ecological stations with estimates using corresponding FY-2D data acquired from January to April 2012, on cloud-free days. Results show that the RMSE between the estimated and actual NSSR is 48.56W/m^{2}, with a mean error of -2.23W/m^{2}. Causes of errors also include measurement accuracy and estimations of atmospheric water vertical contents. This method is only suitable for cloudless conditions. PMID:27136868
2D surface temperature measurement of plasma facing components with modulated active pyrometry
Amiel, S.; Loarer, T.; Pocheau, C.; Roche, H.; Gauthier, E.; Aumeunier, M.-H.; Courtois, X.; Jouve, M.; Balorin, C.; Moncada, V.; Le Niliot, C.; Rigollet, F.
2014-10-01
In nuclear fusion devices, such as Tore Supra, the plasma facing components (PFC) are in carbon. Such components are exposed to very high heat flux and the surface temperature measurement is mandatory for the safety of the device and also for efficient plasma scenario development. Besides this measurement is essential to evaluate these heat fluxes for a better knowledge of the physics of plasma-wall interaction, it is also required to monitor the fatigue of PFCs. Infrared system (IR) is used to manage to measure surface temperature in real time. For carbon PFCs, the emissivity is high and known (ε ~ 0.8), therefore the contribution of the reflected flux from environment and collected by the IR cameras can be neglected. However, the future tokamaks such as WEST and ITER will be equipped with PFCs in metal (W and Be/W, respectively) with low and variable emissivities (ε ~ 0.1–0.4). Consequently, the reflected flux will contribute significantly in the collected flux by IR camera. The modulated active pyrometry, using a bicolor camera, proposed in this paper allows a 2D surface temperature measurement independently of the reflected fluxes and the emissivity. Experimental results with Tungsten sample are reported and compared with simultaneous measurement performed with classical pyrometry (monochromatic and bichromatic) with and without reflective flux demonstrating the efficiency of this method for surface temperature measurement independently of the reflected flux and the emissivity.
2D surface temperature measurement of plasma facing components with modulated active pyrometry
NASA Astrophysics Data System (ADS)
Amiel, S.; Loarer, T.; Pocheau, C.; Roche, H.; Gauthier, E.; Aumeunier, M.-H.; Le Niliot, C.; Rigollet, F.; Courtois, X.; Jouve, M.; Balorin, C.; Moncada, V.
2014-10-01
In nuclear fusion devices, such as Tore Supra, the plasma facing components (PFC) are in carbon. Such components are exposed to very high heat flux and the surface temperature measurement is mandatory for the safety of the device and also for efficient plasma scenario development. Besides this measurement is essential to evaluate these heat fluxes for a better knowledge of the physics of plasma-wall interaction, it is also required to monitor the fatigue of PFCs. Infrared system (IR) is used to manage to measure surface temperature in real time. For carbon PFCs, the emissivity is high and known (ɛ ˜ 0.8), therefore the contribution of the reflected flux from environment and collected by the IR cameras can be neglected. However, the future tokamaks such as WEST and ITER will be equipped with PFCs in metal (W and Be/W, respectively) with low and variable emissivities (ɛ ˜ 0.1-0.4). Consequently, the reflected flux will contribute significantly in the collected flux by IR camera. The modulated active pyrometry, using a bicolor camera, proposed in this paper allows a 2D surface temperature measurement independently of the reflected fluxes and the emissivity. Experimental results with Tungsten sample are reported and compared with simultaneous measurement performed with classical pyrometry (monochromatic and bichromatic) with and without reflective flux demonstrating the efficiency of this method for surface temperature measurement independently of the reflected flux and the emissivity.
Fermi-surface-free superconductivity in underdoped (Bi,Pb)(Sr,La)2CuO6+δ (Bi2201)
Mistark, Peter; Hafiz, Hasnain; Markiewicz, Robert S.; Bansil, Arun
2015-01-01
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. 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. 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. PMID:26084605
Coherent quasiparticles with a small fermi surface in lightly doped Sr(3)Ir(2)O(7).
de la Torre, A; Hunter, E C; Subedi, A; McKeown Walker, S; Tamai, A; Kim, T K; Hoesch, M; Perry, R S; Georges, A; Baumberger, F
2014-12-19
We characterize the electron doping evolution of (Sr_{1-x}La_{x})_{3}Ir_{2}O_{7} by means of angle-resolved photoemission. Concomitant with the metal insulator transition around x≈0.05 we find the emergence of coherent quasiparticle states forming a closed small Fermi surface of volume 3x/2, where x is the independently measured La concentration. The quasiparticle weight Z remains large along the entire Fermi surface, consistent with the moderate renormalization of the low-energy dispersion, and no pseudogap is observed. This indicates a conventional, weakly correlated Fermi liquid state with a momentum independent residue Z≈0.5 in lightly doped Sr_{3}Ir_{2}O_{7}. PMID:25554897
Coherent quasiparticles with a small Fermi Surface in lightly doped Sr3Ir2O7
NASA Astrophysics Data System (ADS)
de la Torre, Alberto; McKeown Walker, Siobhan; Tamai, Anna; Hunter, Emily; Subedi, Alaska; Kim, Timur; Hoesch, Moritz; Perry, Robin; Georges, Antoine; Baumberger, Felix
2015-03-01
We characterize the electron doping evolution of (Sr1-xLax)Ir2O7 by means of angle-resolved photoemission. Concomitant with the metal insulator transition around x ~ 0 . 05 we find the emergence of coherent quasiparticle states forming a closed small Fermi surface of volume 3 x / 2 , where x is the independently measured La concentration. The quasiparticle weight Z remains large along the entire Fermi surface, consistent with the moderate renormalization of the low-energy dispersion and no pseudogap is observed. This indicates a conventional, weakly correlated Fermi liquid state with a momentum independent residue Z ~ 0 . 5 in lightly doped Sr3Ir2O7, in stark contrast with underdoped cuprates.
Coherent Quasiparticles with a Small Fermi Surface in Lightly Doped Sr3Ir2O7
NASA Astrophysics Data System (ADS)
de la Torre, A.; Hunter, E. C.; Subedi, A.; McKeown Walker, S.; Tamai, A.; Kim, T. K.; Hoesch, M.; Perry, R. S.; Georges, A.; Baumberger, F.
2014-12-01
We characterize the electron doping evolution of (Sr1 -xLax)3Ir2O7 by means of angle-resolved photoemission. Concomitant with the metal insulator transition around x ≈0.05 we find the emergence of coherent quasiparticle states forming a closed small Fermi surface of volume 3 x /2 , where x is the independently measured La concentration. The quasiparticle weight Z remains large along the entire Fermi surface, consistent with the moderate renormalization of the low-energy dispersion, and no pseudogap is observed. This indicates a conventional, weakly correlated Fermi liquid state with a momentum independent residue Z ≈0.5 in lightly doped Sr3Ir2O7 .
Surface delta interaction in the g7/2 - d5/2 model space
NASA Astrophysics Data System (ADS)
Yu, Xiaofei; Zamick, Larry
2016-05-01
Using an attractive surface delta interaction we obtain wave functions for 2 neutrons (or neutron holes) in the g7/2 -d5/2 model space. If we take the single particle energies to be degenerate we find that the g factors for I = 2 , 4 and 6 are all the same G (J) =gl, the orbital g factor of the nucleon. For a free neutron gl = 0, so in this case all 2 particles or 2 holes' g factors are equal to zero. Only the orbital part of the g-factors contributes - the spin part cancels out. We then consider the effects of introducing a single energy splitting between the 2 orbits. We make a linear approximation for all other n values.
Surface-Supported Robust 2D Lanthanide-Carboxylate Coordination Networks.
Urgel, José I; Cirera, Borja; Wang, Yang; Auwärter, Willi; Otero, Roberto; Gallego, José M; Alcamí, Manuel; Klyatskaya, Svetlana; Ruben, Mario; Martín, Fernando; Miranda, Rodolfo; Ecija, David; Barth, Johannes V
2015-12-16
Lanthanide-based metal-organic compounds and architectures are promising systems for sensing, heterogeneous catalysis, photoluminescence, and magnetism. Herein, the fabrication of interfacial 2D lanthanide-carboxylate networks is introduced. This study combines low- and variable-temperature scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) experiments, and density functional theory (DFT) calculations addressing their design and electronic properties. The bonding of ditopic linear linkers to Gd centers on a Cu(111) surface gives rise to extended nanoporous grids, comprising mononuclear nodes featuring eightfold lateral coordination. XPS and DFT elucidate the nature of the bond, indicating ionic characteristics, which is also manifest in appreciable thermal stability. This study introduces a new generation of robust low-dimensional metallosupramolecular systems incorporating the functionalities of the f-block elements. PMID:26524215
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
Quasiparticle interference of the Fermi arcs and surface-bulk connectivity of a Weyl semimetal.
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. PMID:26965625
Full 2D observation of water surface elevation from SWOT under different flow conditions
NASA Astrophysics Data System (ADS)
Domeneghetti, Alessio; Schumann, Guy; Rui, Wei; Durand, Michael; Pavelsky, Tamlin
2016-04-01
The upcoming Surface Water and Ocean Topography (SWOT) satellite mission is a joint project of NASA, Centre National d'Etudes Spatiales (CNES, France), the Canadian Space Agency, and the Space Agency of the UK that will provide a first global, high-resolution observation of ocean and terrestrial water surface heights. Characterized by an observation swath of 120 km and an orbit repeat interval of about 21 days, SWOT will provide unprecedented bi-dimensional observations of rivers wider than 50-100 m. Despite many research activities that have investigated potential uses of remotely sensed data from SWOT, potentials and limitations of the spatial observations provided by the satellite mission for flood modeling still remain poorly understood and investigated. In this study we present a first analysis of the spatial observation of water surface elevation that is expected from SWOT for a 140 km reach of the middle-lower portion of the Po River, in Northern Italy. The river stretch is characterized by a main channel varying from 200-500 m in width and a floodplain that can be as wide as 5 km and that is delimited by a system of major embankments. The reconstruction of the hydraulic behavior of the Po River is performed by means of a quasi-2d model built with detailed topographic and bathymetric information (LiDAR, 2 m resolution), while the simulation of the spatial observation sensed by SWOT is performed with a SWOT simulator that mimics the satellite sensor characteristics. Referring to water surface elevations associated with different flow conditions (maximum, minimum and average flow reproduced by means of the quasi-2d numerical model) this work provides a first characterization of the spatial observations provided by SWOT and highlights the strengths and limitations of the expected products. By referring to a real river reach the analysis provides a credible example of the type of spatial observations that will be available after launch of SWOT and offers a first
Diverse 2D structures obtained by adsorption of charged ABA triblock copolymer on different surfaces
NASA Astrophysics Data System (ADS)
Kontturi, Katri S.; Vesterinen, Arja-Helena; Seppälä, Jukka; Laine, Janne
2012-11-01
In the larger context of 2D polymeric structures, the morphologies obtained by adsorption and subsequent drying of charged, ABA type amphiphilic triblock copolymer of poly[2-(dimethylamino)ethyl metacrylate] (PDMAEMA) and poly(propylene oxide) (PPO) were investigated with atomic force microscopy and X-ray photoelectron spectroscopy as well as in situ adsorption analysis with quartz crystal microbalance with dissipation monitoring. Hydrophilic silica and hydrophobic polystyrene (PS) were used as substrates for adsorption. The structures emerging from the self-assembly of adsorbing polymer were profoundly influenced by composition of the aqueous solution and the choice of substrate. When adsorbed from dilute polymer solution where the concentration is so low that the polymer does not yet show surface-active behavior, the triblock copolymer unimers associated on hydrophilic silica surface forming large, irregular clustered aggregates, with sizes increasing with electrolyte concentration of the solution. On a hydrophobic PS substrate, on the other hand, unimers spread much more evenly, forming clear surface patterns. The roughness of these patterned structures was tuned with the electrolyte concentration of the solution. Adsorption from a more concentrated polymer solution, where the surface-activity of the polymer is perceptible, resulted in the formation of a smooth film with complete coverage over the hydrophilic silica substrate when the electrolyte concentration was high. On PS, on the other hand, nucleation of evenly scattered globular, disk-like micelles was induced. Besides the dry film morphology, the even distribution of the irreversibly adsorbed polymer over the PS surface was likely to serve as an optimal platform for the build-up of reversible hydrophobically bound multilayers at high electrolyte concentration. The multilayer formation was reversible because a decrease in the electrolyte concentration of the solution re-introduces strong electrostatic
Study on the electronic structure and Fermi surface of 3d-transition-metal disilisides CoSi2
NASA Astrophysics Data System (ADS)
Hamid, A. S.
2012-09-01
We have investigated the electronic structure, the momentum density distribution ρ( p), and the Fermi surface FS of single crystals of the Pyrite-type 3d-transition-metal disilisides CoSi2. The band structure calculations, the density of states DOS, and the FS, in vicinity of Fermi level, have been carried out using the full-potential linearized augmented plane wave FP-LAPW method within generalized gradient approximation GGA for exchange and correlation potential. The measurements have been performed via the 2D angular correlation of annihilation radiation ACAR experiments. ρ( p) has been reconstructed by using the Fourier transformation technique. The FS has been reconstructed within the first Brillion zone BZ through the Locks, Crisp, and West LCW folding procedures. The analysis confirmed that Si 3 sp states hybrid with both Co 3 d- t 2 g and Co 3 d- e g states around Γ and X points, respectively. The dimensions of the FS of CoSi2 have been compared to the present calculations as well as to the earlier results.
Fermi surface of superconducting LaFePO determined by quantum oscillations
Mcdonald, Ross D; Coldea, A I; Fletcher, J D; Carrington, A; Bangura, A F; Hussey, N E; Analytis, J G; Chu, J-h; Erickson, A S; Fisher, I R
2008-01-01
The recent discovery of superconductivity in ferrooxypnictides, which have a maximum transition temperature intermediate between the two other known high temperature superconductors MgB{sub 2} and the cuprate family, has generated huge interest and excitement. The most critical issue is the origin of the pairing mechanism. Whereas superconductivity in MgB{sub 2} has been shown to arise from strong electron-phonon coupling, the pairing glue in cuprate superconductors is thought by many to have a magnetic origin. The oxypnictides are highly susceptible to magnetic instabilities, prompting analogies with cuprate superconductivity. Progress on formulating the correct theory of superconductivity in these materials will be greatly aided by a detailed knowledge of the Fermi surface parameters. Here we report for the first time extensive measurements of quantum oscillations in a Fe-based superconductor, LaFePO, that provide a precise calliper of the size and shape of the Fermi surface and the effective masses of the relevant charge carriers. Our results show that the Fermi surface is composed of nearly-nested electron and hole pockets in broad agreement with the band-structure predictions but with significant enhancement of the quasiparticle masses. The correspondence in the electron and hole Fermi surface areas provides firm experimental evidence that LaFePO, whilst unreconstructed, lies extremely close to a spin-density-wave instability, thus favoring models that invoke such a magnetic origin for high-temperature superconductivity in oxypnictides.
The low temperature Fermi surface of IrTe2 probed by quantum oscillations.
NASA Astrophysics Data System (ADS)
Blake, Samuel; Coldea, Amalia; Watson, Matthew; Narayanan, Arjun; McCollam, Alix; Kasahara, Shigeru; Yamashita, Takuya; Watanabe, Daiki; Shibauchi, Takasada; Matsuda, Yuju; Schoonmaker, Robert
2014-03-01
The transition metal dichalcogenide IrTe2 undergoes a structural transition at 280K; doping on the Ir site suppresses this transition and induces superconductivity with Tc of about 3K. The nature of the structural transition is possibly driven by charge disproportionation and the effect this has on the electronic structure of the superconducting state is not fully understood. We report a low temperature investigation of the Fermi surface of IrTe2 from quantum oscillations, using torque measurements performed in magnetic fields up to 33T and temperatures down to 0.3K. The observed extremal areas of the Fermi surface likely correspond to frequencies of a reconstructed Fermi surface, with light effective masses below 0.8me. The angular dependence of these frequencies across multiple crystals of IrTe2 suggests these materials are prone to domain formation upon cooling. We compare our measured Fermi surface with those predicted by electronic structure calculations, based upon the existing structural models, for both above and below the structural transition. This work was supported by EPSRC (UK) and partly by EuroMagnet (EU contract number 228043).
Electron-hole doping asymmetry of Fermi surface reconstructed in a simple Mott insulator
NASA Astrophysics Data System (ADS)
Kawasugi, Yoshitaka; Seki, Kazuhiro; Edagawa, Yusuke; Sato, Yoshiaki; Pu, Jiang; Takenobu, Taishi; Yunoki, Seiji; Yamamoto, Hiroshi M.; Kato, Reizo
2016-08-01
It is widely recognized that the effect of doping into a Mott insulator is complicated and unpredictable, as can be seen by examining the Hall coefficient in high Tc cuprates. The doping effect, including the electron-hole doping asymmetry, may be more straightforward in doped organic Mott insulators owing to their simple electronic structures. Here we investigate the doping asymmetry of an organic Mott insulator by carrying out electric-double-layer transistor measurements and using cluster perturbation theory. The calculations predict that strongly anisotropic suppression of the spectral weight results in the Fermi arc state under hole doping, while a relatively uniform spectral weight results in the emergence of a non-interacting-like Fermi surface (FS) in the electron-doped state. In accordance with the calculations, the experimentally observed Hall coefficients and resistivity anisotropy correspond to the pocket formed by the Fermi arcs under hole doping and to the non-interacting FS under electron doping.
Quantum Oscillations, Thermoelectric Coefficients, and the Fermi Surface of Semimetallic WTe2.
Zhu, Zengwei; Lin, Xiao; Liu, Juan; Fauqué, Benoît; Tao, Qian; Yang, Chongli; Shi, Youguo; Behnia, Kamran
2015-05-01
We present a study of angle-resolved quantum oscillations of electric and thermoelectric transport coefficients in semimetallic WTe2, which has the particularity of displaying a large B(2) magnetoresistance. The Fermi surface consists of two pairs of electronlike and holelike pockets of equal volumes in a "Russian doll" structure. The carrier density, Fermi energy, mobility, and the mean-free path of the system are quantified. An additional frequency is observed above a threshold field and attributed to the magnetic breakdown across two orbits. In contrast to all other dilute metals, the Nernst signal remains linear in the magnetic field even in the high-field (ωcτ≫1) regime. Surprisingly, none of the pockets extend across the c axis of the first Brillouin zone, making the system a three-dimensional metal with moderate anisotropy in Fermi velocity, yet a large anisotropy in the mean-free path. PMID:25978245
Quantum Oscillations, Thermoelectric Coefficients, and the Fermi Surface of Semimetallic WTe2
NASA Astrophysics Data System (ADS)
Zhu, Zengwei; Lin, Xiao; Liu, Juan; Fauqué, Benoît; Tao, Qian; Yang, Chongli; Shi, Youguo; Behnia, Kamran
2015-05-01
We present a study of angle-resolved quantum oscillations of electric and thermoelectric transport coefficients in semimetallic WTe2, which has the particularity of displaying a large B2 magnetoresistance. The Fermi surface consists of two pairs of electronlike and holelike pockets of equal volumes in a "Russian doll" structure. The carrier density, Fermi energy, mobility, and the mean-free path of the system are quantified. An additional frequency is observed above a threshold field and attributed to the magnetic breakdown across two orbits. In contrast to all other dilute metals, the Nernst signal remains linear in the magnetic field even in the high-field (ωcτ ≫1 ) regime. Surprisingly, none of the pockets extend across the c axis of the first Brillouin zone, making the system a three-dimensional metal with moderate anisotropy in Fermi velocity, yet a large anisotropy in the mean-free path.
Electron-hole doping asymmetry of Fermi surface reconstructed in a simple Mott insulator.
Kawasugi, Yoshitaka; Seki, Kazuhiro; Edagawa, Yusuke; Sato, Yoshiaki; Pu, Jiang; Takenobu, Taishi; Yunoki, Seiji; Yamamoto, Hiroshi M; Kato, Reizo
2016-01-01
It is widely recognized that the effect of doping into a Mott insulator is complicated and unpredictable, as can be seen by examining the Hall coefficient in high Tc cuprates. The doping effect, including the electron-hole doping asymmetry, may be more straightforward in doped organic Mott insulators owing to their simple electronic structures. Here we investigate the doping asymmetry of an organic Mott insulator by carrying out electric-double-layer transistor measurements and using cluster perturbation theory. The calculations predict that strongly anisotropic suppression of the spectral weight results in the Fermi arc state under hole doping, while a relatively uniform spectral weight results in the emergence of a non-interacting-like Fermi surface (FS) in the electron-doped state. In accordance with the calculations, the experimentally observed Hall coefficients and resistivity anisotropy correspond to the pocket formed by the Fermi arcs under hole doping and to the non-interacting FS under electron doping. PMID:27492864
Fermi Surface and Van Hove Singularities in the Itinerant Metamagnet Sr(3)Ru(2)O(7)
Tamai, A.; Allan, M.P.; Mercure, J.F.; Meevasana, W.; Dunkel, R.; Lu, D.H.; Perry, R.S.; Mackenzie, A.P.; Singh, D.J.; Shen, Z.-X.; Baumberger, F.; /Scottish U. Research Reactor Ctr. /St. Andrews U.
2011-01-04
The low-energy electronic structure of the itinerant metamagnet Sr{sub 3}Ru{sub 2}O{sub 7} is investigated by angle resolved photoemission and density functional calculations. We find well-defined quasiparticle bands with resolution limited line widths and Fermi velocities up to an order of magnitude lower than in single layer Sr{sub 2}RuO{sub 4}. The complete topography, the cyclotron masses and the orbital character of the Fermi surface are determined, in agreement with bulk sensitive de Haas - van Alphen measurements. An analysis of the dxy band dispersion reveals a complex density of states with van Hove singularities (vHs) near the Fermi level; a situation which is favorable for magnetic instabilities.
Geometric Neural Computing for 2D Contour and 3D Surface Reconstruction
NASA Astrophysics Data System (ADS)
Rivera-Rovelo, Jorge; Bayro-Corrochano, Eduardo; Dillmann, Ruediger
In this work we present an algorithm to approximate the surface of 2D or 3D objects combining concepts from geometric algebra and artificial neural networks. Our approach is based on the self-organized neural network called Growing Neural Gas (GNG), incorporating versors of the geometric algebra in its neural units; such versors are the transformations that will be determined during the training stage and then applied to a point to approximate the surface of the object. We also incorporate the information given by the generalized gradient vector flow to select automatically the input patterns, and also in the learning stage in order to improve the performance of the net. Several examples using medical images are presented, as well as images of automatic visual inspection. We compared the results obtained using snakes against the GSOM incorporating the gradient information and using versors. Such results confirm that our approach is very promising. As a second application, a kind of morphing or registration procedure is shown; namely the algorithm can be used when transforming one model at time t 1 into another at time t 2. We include also examples applying the same procedure, now extended to models based on spheres.
Theoretical study of surface plasmons coupling in transition metallic alloy 2D binary grating
NASA Astrophysics Data System (ADS)
Dhibi, Abdelhak; Khemiri, Mehdi; Oumezzine, Mohamed
2016-05-01
The excitation of a surface plasmon polariton (SPP) wave on a metal-air interface by a 2D diffraction grating is numerically investigated. The grating consists of homogeneous alloys of two metals of a formula AxB1-x, or three metals of a formula AxByCz, where A, B and C could be silver (Ag), copper (Cu), gold (Au) or aluminum (Al). It is observed that all the alloys of two metals present a very small change of surface plasmon resonance (SPR) irrespective of composition x. Moreover, the addition of 25% of Al to two metals alloy is insufficient to change the SPR curves. The influence of the different grating parameters is discussed in details using rigorous coupled-wave analysis (RCWA) method. Furthermore, the SPR is highly dependent on grating periods (dx and dy) and the height of the grating h. The results reveal that dx= dy= 700 nm, h=40 nm and duty cycle w=0.5 are the optimal parameters for exciting SPP.
Evidence for a small hole pocket in the Fermi surface of underdoped YBa2Cu3Oy.
Doiron-Leyraud, N; Badoux, S; René de Cotret, S; Lepault, S; LeBoeuf, D; Laliberté, F; Hassinger, E; Ramshaw, B J; Bonn, D A; Hardy, W N; Liang, R; Park, J-H; Vignolles, D; Vignolle, B; Taillefer, L; Proust, C
2015-01-01
In underdoped cuprate superconductors, the Fermi surface undergoes a reconstruction that produces a small electron pocket, but whether there is another, as yet, undetected portion to the Fermi surface is unknown. Establishing the complete topology of the Fermi surface is key to identifying the mechanism responsible for its reconstruction. Here we report evidence for a second Fermi pocket in underdoped YBa2Cu3Oy, detected as a small quantum oscillation frequency in the thermoelectric response and in the c-axis resistance. The field-angle dependence of the frequency shows that it is a distinct Fermi surface, and the normal-state thermopower requires it to be a hole pocket. A Fermi surface consisting of one electron pocket and two hole pockets with the measured areas and masses is consistent with a Fermi-surface reconstruction by the charge-density-wave order observed in YBa2Cu3Oy, provided other parts of the reconstructed Fermi surface are removed by a separate mechanism, possibly the pseudogap. PMID:25616011
Evidence for a small hole pocket in the Fermi surface of underdoped YBa2Cu3Oy
Doiron-Leyraud, N.; Badoux, S.; René de Cotret, S.; Lepault, S.; LeBoeuf, D.; Laliberté, F.; Hassinger, E.; Ramshaw, B. J.; Bonn, D. A.; Hardy, W. N.; Liang, R.; Park, J.-H..; Vignolles, D.; Vignolle, B.; Taillefer, L.; Proust, C.
2015-01-01
In underdoped cuprate superconductors, the Fermi surface undergoes a reconstruction that produces a small electron pocket, but whether there is another, as yet, undetected portion to the Fermi surface is unknown. Establishing the complete topology of the Fermi surface is key to identifying the mechanism responsible for its reconstruction. Here we report evidence for a second Fermi pocket in underdoped YBa2Cu3Oy, detected as a small quantum oscillation frequency in the thermoelectric response and in the c-axis resistance. The field-angle dependence of the frequency shows that it is a distinct Fermi surface, and the normal-state thermopower requires it to be a hole pocket. A Fermi surface consisting of one electron pocket and two hole pockets with the measured areas and masses is consistent with a Fermi-surface reconstruction by the charge–density–wave order observed in YBa2Cu3Oy, provided other parts of the reconstructed Fermi surface are removed by a separate mechanism, possibly the pseudogap. PMID:25616011
Fermi surface instabilities in CeRh2Si2 at high magnetic field and pressure
NASA Astrophysics Data System (ADS)
Palacio Morales, A.; Pourret, A.; Seyfarth, G.; Suzuki, M.-T.; Braithwaite, D.; Knebel, G.; Aoki, D.; Flouquet, J.
2015-06-01
We present thermoelectric power (TEP) studies under pressure and high magnetic field in the antiferromagnet CeRh2Si2 at low temperature. Under a magnetic field, large quantum oscillations are observed in the TEP, S (H ) , in the antiferromagnetic phase. They suddenly disappear when entering in the polarized paramagnetic state at Hc, pointing out an important reconstruction of the Fermi surface. Under pressure, S /T increases strongly at low temperature near the critical pressure Pc, where the antiferromagnetic (AF) order is suppressed, implying the interplay of a Fermi surface change and low-energy excitations driven by spin and valence fluctuations. The difference between the TEP signal in the polarized paramagnetic state above Hc at ambient pressure and in the pressure-induced paramagnetic state above Pc can be explained by different Fermi surfaces. Band-structure calculations at P =0 stress that in the AF phase the 4 f contribution at the Fermi level (EF) is weak, while it is the main contribution in the paramagnetic domain. In the polarized paramagnetic phase the 4 f contribution at EF drops. Large quantum oscillations are observed in the antiferromagnetic state while these disappear in the polarized state above Hc. Comparison is made to the CeRu2Si2 series highly studied for its (H ,T ) phase diagram.
Interaction-induced singular Fermi surface in a high-temperature oxypnictide superconductor
NASA Astrophysics Data System (ADS)
Charnukha, A.; Thirupathaiah, S.; Zabolotnyy, V. B.; Büchner, B.; Zhigadlo, N. D.; Batlogg, B.; Yaresko, A. N.; Borisenko, S. V.
2015-05-01
In the family of iron-based superconductors, LaFeAsO-type materials possess the simplest electronic structure due to their pronounced two-dimensionality. And yet they host superconductivity with the highest transition temperature Tc ≈ 55K. Early theoretical predictions of their electronic structure revealed multiple large circular portions of the Fermi surface with a very good geometrical overlap (nesting), believed to enhance the pairing interaction and thus superconductivity. The prevalence of such large circular features in the Fermi surface has since been associated with many other iron-based compounds and has grown to be generally accepted in the field. In this work we show that a prototypical compound of the 1111-type, SmFe0.92Co0.08AsO , is at odds with this description and possesses a distinctly different Fermi surface, which consists of two singular constructs formed by the edges of several bands, pulled to the Fermi level from the depths of the theoretically predicted band structure by strong electronic interactions. Such singularities dramatically affect the low-energy electronic properties of the material, including superconductivity. We further argue that occurrence of these singularities correlates with the maximum superconducting transition temperature attainable in each material class over the entire family of iron-based superconductors.
Interaction-induced singular Fermi surface in a high-temperature oxypnictide superconductor
Charnukha, A.; Thirupathaiah, S.; Zabolotnyy, V. B.; Büchner, B.; Zhigadlo, N. D.; Batlogg, B.; Yaresko, A. N.; Borisenko, S. V.
2015-01-01
In the family of iron-based superconductors, LaFeAsO-type materials possess the simplest electronic structure due to their pronounced two-dimensionality. And yet they host superconductivity with the highest transition temperature Tc ≈ 55K. Early theoretical predictions of their electronic structure revealed multiple large circular portions of the Fermi surface with a very good geometrical overlap (nesting), believed to enhance the pairing interaction and thus superconductivity. The prevalence of such large circular features in the Fermi surface has since been associated with many other iron-based compounds and has grown to be generally accepted in the field. In this work we show that a prototypical compound of the 1111-type, SmFe0.92Co0.08AsO , is at odds with this description and possesses a distinctly different Fermi surface, which consists of two singular constructs formed by the edges of several bands, pulled to the Fermi level from the depths of the theoretically predicted band structure by strong electronic interactions. Such singularities dramatically affect the low-energy electronic properties of the material, including superconductivity. We further argue that occurrence of these singularities correlates with the maximum superconducting transition temperature attainable in each material class over the entire family of iron-based superconductors. PMID:25997611
Interaction-induced singular Fermi surface in a high-temperature oxypnictide superconductor.
Charnukha, A; Thirupathaiah, S; Zabolotnyy, V B; Büchner, B; Zhigadlo, N D; Batlogg, B; Yaresko, A N; Borisenko, S V
2015-01-01
In the family of iron-based superconductors, LaFeAsO-type materials possess the simplest electronic structure due to their pronounced two-dimensionality. And yet they host superconductivity with the highest transition temperature Tc ≈ 55K. Early theoretical predictions of their electronic structure revealed multiple large circular portions of the Fermi surface with a very good geometrical overlap (nesting), believed to enhance the pairing interaction and thus superconductivity. The prevalence of such large circular features in the Fermi surface has since been associated with many other iron-based compounds and has grown to be generally accepted in the field. In this work we show that a prototypical compound of the 1111-type, SmFe(0.92)Co(0.08)AsO , is at odds with this description and possesses a distinctly different Fermi surface, which consists of two singular constructs formed by the edges of several bands, pulled to the Fermi level from the depths of the theoretically predicted band structure by strong electronic interactions. Such singularities dramatically affect the low-energy electronic properties of the material, including superconductivity. We further argue that occurrence of these singularities correlates with the maximum superconducting transition temperature attainable in each material class over the entire family of iron-based superconductors. PMID:25997611
2D instabilities of surface gravity waves on a linear shear current
NASA Astrophysics Data System (ADS)
Francius, Marc; Kharif, Christian
2016-04-01
Periodic 2D surface water waves propagating steadily on a rotational current have been studied by many authors (see [1] and references therein). Although the recent important theoretical developments have confirmed that periodic waves can exist over flows with arbitrary vorticity, their stability and their nonlinear evolution have not been much studied extensively so far. In fact, even in the rather simple case of uniform vorticity (linear shear), few papers have been published on the effect of a vertical shear current on the side-band instability of a uniform wave train over finite depth. In most of these studies [2-5], asymptotic expansions and multiple scales method have been used to obtain envelope evolution equations, which allow eventually to formulate a condition of (linear) instability to long modulational perturbations. It is noted here that this instability is often referred in the literature as the Benjamin-Feir or modulational instability. In the present study, we consider the linear stability of finite amplitude two-dimensional, periodic water waves propagating steadily on the free surface of a fluid with constant vorticity and finite depth. First, the steadily propagating surface waves are computed with steepness up to very close to the highest, using a Fourier series expansions and a collocation method, which constitutes a simple extension of Fenton's method [6] to the cases with a linear shear current. Then, the linear stability of these permanent waves to infinitesimal 2D perturbations is developed from the fully nonlinear equations in the framework of normal modes analysis. This linear stability analysis is an extension of [7] to the case of waves in the presence of a linear shear current and permits the determination of the dominant instability as a function of depth and vorticity for a given steepness. The numerical results are used to assess the accuracy of the vor-NLS equation derived in [5] for the characteristics of modulational
Fermi surface reconstruction in hole-doped t-J models without long-range antiferromagnetic order
NASA Astrophysics Data System (ADS)
Punk, Matthias; Sachdev, Subir
2012-05-01
We calculate the Fermi surface of electrons in hole-doped, extended t-J models on a square lattice in a regime where no long-range antiferromagnetic order is present, and no symmetries are broken. Using the “spinon-dopon” formalism of Ribeiro and Wen, we show that short-range antiferromagnetic correlations lead to a reconstruction of the Fermi surface into hole pockets which are not necessarily centered at the antiferromagnetic Brillouin zone boundary. The Brillouin zone area enclosed by the Fermi surface is proportional to the density of dopants away from half-filling, in contrast to the conventional Luttinger theorem, which counts the total electron density. This state realizes a “fractionalized Fermi liquid” (FL*), which has been proposed as a possible ground state of the underdoped cuprates; we note connections to recent experiments. We also discuss the quantum phase transition from the FL* state to the Fermi liquid state with long-range antiferromagnetic order.
Ding, Yi; John, Nigel W; Smith, Lyndon; Sun, Jiuai; Smith, Melvyn
2015-10-01
Two-dimensional asymmetry, border irregularity, colour variegation and diameter (ABCD) features are important indicators currently used for computer-assisted diagnosis of malignant melanoma (MM); however, they often prove to be insufficient to make a convincing diagnosis. Previous work has demonstrated that 3D skin surface normal features in the form of tilt and slant pattern disruptions are promising new features independent from the existing 2D ABCD features. This work investigates that whether improved lesion classification can be achieved by combining the 3D features with the 2D ABCD features. Experiments using a nonlinear support vector machine classifier show that many combinations of the 2D ABCD features and the 3D features can give substantially better classification accuracy than using (1) single features and (2) many combinations of the 2D ABCD features. The best 2D and 3D feature combination includes the overall 3D skin surface disruption, the asymmetry and all the three colour channel features. It gives an overall 87.8 % successful classification, which is better than the best single feature with 78.0 % and the best 2D feature combination with 83.1 %. These demonstrate that (1) the 3D features have additive values to improve the existing lesion classification and (2) combining the 3D feature with all the 2D features does not lead to the best lesion classification. The two ABCD features not selected by the best 2D and 3D combination, namely (1) the border feature and (2) the diameter feature, were also studied in separate experiments. It found that inclusion of either feature in the 2D and 3D combination can successfully classify 3 out of 4 lesion groups. The only one group not accurately classified by either feature can be classified satisfactorily by the other. In both cases, they have shown better classification performances than those without the 3D feature in the combinations. This further demonstrates that (1) the 3D feature can be used to
Magnetic breakdown and Landau level spectra of a tunable double-quantum-well Fermi surface
Simmons, J.A.; Harff, N.E.; Lyo, S.K.; Klem, J.F.; Boebinger, G.S.; Pfeiffer, L.N.; West, K.W.
1997-12-31
By measuring longitudinal resistance, the authors map the Landau level spectra of double quantum wells as a function of both parallel (B{sub {parallel}}) and perpendicular (B{sub {perpendicular}}) magnetic fields. In this continuously tunable highly non-parabolic system, the cyclotron masses of the two Fermi surface orbits change in opposite directions with B{sub {parallel}}. This causes the two corresponding ladders of Landau levels formed at finite B{sub {perpendicular}} to exhibit multiple crossings. They also observe a third set of landau levels, independent of B{sub {parallel}}, which arise from magnetic breakdown of the Fermi surface. Both semiclassical and full quantum mechanical calculations show good agreement with the data.
Fermi-surface reconstruction and the origin of high-temperature superconductivity.
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 the 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
Cho, Dai-Ning; Brink, Jeroen van den; Fehske, Holger; Becker, Klaus W.; Sykora, Steffen
2016-01-01
We study the competition between unconventional superconducting pairing and charge density wave (CDW) formation for the two-dimensional Edwards Hamiltonian at half filling, a very general two-dimensional transport model in which fermionic charge carriers couple to a correlated background medium. Using the projective renormalization method we find that a strong renormalization of the original fermionic band causes a new hole-like Fermi surface to emerge near the center of the Brillouin zone, before it eventually gives rise to the formation of a charge density wave. On the new, disconnected parts of the Fermi surface superconductivity is induced with a sign-changing order parameter. We discuss these findings in the light of recent experiments on iron-based oxypnictide superconductors. PMID:26935887
NASA Astrophysics Data System (ADS)
Cho, Dai-Ning; Brink, Jeroen Van Den; Fehske, Holger; Becker, Klaus W.; Sykora, Steffen
2016-03-01
We study the competition between unconventional superconducting pairing and charge density wave (CDW) formation for the two-dimensional Edwards Hamiltonian at half filling, a very general two-dimensional transport model in which fermionic charge carriers couple to a correlated background medium. Using the projective renormalization method we find that a strong renormalization of the original fermionic band causes a new hole-like Fermi surface to emerge near the center of the Brillouin zone, before it eventually gives rise to the formation of a charge density wave. On the new, disconnected parts of the Fermi surface superconductivity is induced with a sign-changing order parameter. We discuss these findings in the light of recent experiments on iron-based oxypnictide superconductors.
Cho, Dai-Ning; Brink, Jeroen van den; Fehske, Holger; Becker, Klaus W; Sykora, Steffen
2016-01-01
We study the competition between unconventional superconducting pairing and charge density wave (CDW) formation for the two-dimensional Edwards Hamiltonian at half filling, a very general two-dimensional transport model in which fermionic charge carriers couple to a correlated background medium. Using the projective renormalization method we find that a strong renormalization of the original fermionic band causes a new hole-like Fermi surface to emerge near the center of the Brillouin zone, before it eventually gives rise to the formation of a charge density wave. On the new, disconnected parts of the Fermi surface superconductivity is induced with a sign-changing order parameter. We discuss these findings in the light of recent experiments on iron-based oxypnictide superconductors. PMID:26935887
Surface hole accumulation and Fermi level stabilization energy in SnTe
NASA Astrophysics Data System (ADS)
Nishitani, Junichi; Detert, Douglas; Beeman, Jeffrey; Yu, Kin Man; Walukiewicz, Wladek
2014-09-01
SnTe films were deposited by RF magnetron sputtering. The thickness dependence of the sheet hole concentration indicated the presence of a high hole density surface accumulation layer. Irradiation of SnTe by Ne+ ions led to the saturation of the hole concentration corresponding to a Fermi energy that is 0.5 eV below the valence band edge. The stabilized Fermi energy on the surface and in the heavily damaged bulk is in agreement with the amphoteric native defect model. These results show that SnTe is a unique semiconductor with an extremely high valence band edge located at 4.4 eV below the vacuum level.
Nodal Fermi surface pocket approaching an optimal quantum critical point in YBCO
NASA Astrophysics Data System (ADS)
Sebastian, Suchitra; Tan, Beng; Lonzarich, Gilbert; Ramshaw, Brad; Harrison, Neil; Balakirev, Fedor; Mielke, Chuck; Sabok, S.; Dabrowski, B.; Liang, Ruixing; Bonn, Doug; Hardy, Walter
2014-03-01
I present new quantum oscillation measurements over the entire underdoped regime in YBa2Cu3O6+x and YBa2Cu4O8 using ultra-high magnetic fields to destroy superconductivity and access the normal ground state. A robust small nodal Fermi surface created by charge order is found to extend over the entire underdoped range, exhibiting quantum critical signatures approaching optimal doping.
Electron pockets in the Fermi surface of hole-doped high-Tc superconductors.
LeBoeuf, David; Doiron-Leyraud, Nicolas; Levallois, Julien; Daou, R; Bonnemaison, J-B; Hussey, N E; Balicas, L; Ramshaw, B J; Liang, Ruixing; Bonn, D A; Hardy, W N; Adachi, S; Proust, Cyril; Taillefer, Louis
2007-11-22
High-temperature superconductivity in copper oxides occurs when the materials are chemically tuned to have a carrier concentration intermediate between their metallic state at high doping and their insulating state at zero doping. The underlying evolution of the electron system in the absence of superconductivity is still unclear, and a question of central importance is whether it involves any intermediate phase with broken symmetry. The Fermi surface of the electronic states in the underdoped 'YBCO' materials YBa2Cu3O(y) and YBa2Cu4O8 was recently shown to include small pockets, in contrast with the large cylinder that characterizes the overdoped regime, pointing to a topological change in the Fermi surface. Here we report the observation of a negative Hall resistance in the magnetic-field-induced normal state of YBa2Cu3O(y) and YBa2Cu4O8, which reveals that these pockets are electron-like rather than hole-like. We propose that these electron pockets most probably arise from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped copper oxides near the onset of antiferromagnetic order. Comparison with materials of the La2CuO4 family that exhibit spin/charge density-wave order suggests that a Fermi surface reconstruction also occurs in those materials, pointing to a generic property of high-transition-temperature (T(c)) superconductors. PMID:18033293
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.''
Quantum oscillations from generic surface Fermi arcs and bulk chiral modes in Weyl semimetals.
Zhang, Yi; Bulmash, Daniel; Hosur, Pavan; Potter, Andrew C; Vishwanath, Ashvin
2016-01-01
We re-examine the question of quantum oscillations from surface Fermi arcs and chiral modes in Weyl semimetals. By introducing two tools - semiclassical phase-space quantization and a numerical implementation of a layered construction of Weyl semimetals - we discover several important generalizations to previous conclusions that were implicitly tailored to the special case of identical Fermi arcs on top and bottom surfaces. We show that the phase-space quantization picture fixes an ambiguity in the previously utilized energy-time quantization approach and correctly reproduces the numerically calculated quantum oscillations for generic Weyl semimetals with distinctly curved Fermi arcs on the two surfaces. Based on these methods, we identify a 'magic' magnetic-field angle where quantum oscillations become independent of sample thickness, with striking experimental implications. We also analyze the stability of these quantum oscillations to disorder, and show that the high-field oscillations are expected to persist in samples whose thickness parametrically exceeds the quantum mean free path. PMID:27033563
Quantum oscillations from generic surface Fermi arcs and bulk chiral modes in Weyl semimetals
Zhang, Yi; Bulmash, Daniel; Hosur, Pavan; Potter, Andrew C.; Vishwanath, Ashvin
2016-01-01
We re-examine the question of quantum oscillations from surface Fermi arcs and chiral modes in Weyl semimetals. By introducing two tools - semiclassical phase-space quantization and a numerical implementation of a layered construction of Weyl semimetals - we discover several important generalizations to previous conclusions that were implicitly tailored to the special case of identical Fermi arcs on top and bottom surfaces. We show that the phase-space quantization picture fixes an ambiguity in the previously utilized energy-time quantization approach and correctly reproduces the numerically calculated quantum oscillations for generic Weyl semimetals with distinctly curved Fermi arcs on the two surfaces. Based on these methods, we identify a ‘magic’ magnetic-field angle where quantum oscillations become independent of sample thickness, with striking experimental implications. We also analyze the stability of these quantum oscillations to disorder, and show that the high-field oscillations are expected to persist in samples whose thickness parametrically exceeds the quantum mean free path. PMID:27033563
Multi-method determination of continuous 2D velocity profiles from the surface to 1 km
NASA Astrophysics Data System (ADS)
Peterie, S.; Miller, R. D.; Ivanov, J.; Schwenk, J.; Bailey, B. L.; Schwarzer, J.; Markiewicz, R.
2012-12-01
Compressional and shear reflection data provide critical measurements of velocity and attenuation that are necessary for numerical simulations of site response from earthquake energy and seismic investigations to lithologic and pore characterizations. Imperative for accurate site response models is a seismic velocity model extending from the surface to the depth of interest that is representative of the true subsurface. In general, no seismic method can be used to characterize the shallowest (< 30 m) and deepest (30 m to 1 km) portions of the subsurface in a single pass with a consistent set of equipment and acquisition parameters. With four unique seismic surveys targeting different portions of the subsurface and different components of the seismic wavefield, we were able to build a comprehensive dataset that facilitated continuous 2D velocity profiles. The upper kilometer underlying our study site consists of Lake Bonneville lucustrine sediments and post-Bonneville alluvium and colluvium from the nearby Wasatch Front in north central Utah (Eardley, 1938; Hintze, 2005). Four unique seismic surveys were acquired along each of two 1.5 km lines located approximately 3 km apart. Data for tomography and multi-channel analysis of surface waves (MASW) were acquired with a bungee accelerated weight drop and 4.5 Hz compressional geophones. P-wave and S-wave reflection data were acquired with an IVI minivib 1 and 28 Hz compressional and 14 Hz SH geophones, respectively. P-wave and S-wave velocities from the surface to 30 m were determined using tomography and MASW, respectively. Stacking velocities of reflections on common midpoint gathers from the vibroseis data were used to determine Vp and Vs from approximately 30 m to nearly 1 km below ground surface. Each Vp and Vs dataset were merged to generate continuous interval and average velocity profiles. The sutured velocity cross-sections were produced for both P- and S-waves in a fashion not previously described in the
Electronic structure, Fermi surface and dHvA effect in YIn3, LuIn3, and YbIn3
NASA Astrophysics Data System (ADS)
Antonov, V. N.
2014-04-01
The electronic structure, Fermi surface, angle dependence of the cyclotron masses and extremal cross sections of the Fermi surface of RIn3 (R = Y, Lu, and Yb) compounds were investigated from first principles using the fully relativistic Dirac linear muffin-tin orbital method. The effect of the spin-orbit (SO) interaction and Coulomb repulsion U in a frame of the LDA + SO + U method on the Fermi surface, orbital dependence of the cyclotron masses, and extremal cross sections of the Fermi surface are examined in details. A good agreement with experimental data of cyclotron masses and extremal cross sections of the Fermi surface was achieved.
Dynamic Linkages Between the Transition Zone & Surface Plate Motions in 2D Models of Subduction
NASA Astrophysics Data System (ADS)
Arredondo, K.; Billen, M. I.
2012-12-01
Descending subducted slabs affect both plate tectonics at the surface and overall mantle flow (e.g. Conrad and Lithgow-Bertelloni, 2002). For time-dependent numerical models, the potential evolution of these slabs, ranging from immediate penetration into the lower mantle to prior buckling and stagnation, are affected by parameters such as the plate age, the viscosity jump into the lower mantle, the presence of phase transitions, trench motion and the chosen governing equation approximation (e.g. Billen and Hirth, 2007). Similarly, the overall deviatoric stress within the slab, especially where modified by the phase transitions, may explain the uneven distribution of deep earthquakes with depth (e.g. Bina, 1997). Better understanding of these processes may arise from a more realistic 2-D model that is fully-dynamic, with an overriding plate, freely-moving trench, compositionally-layered slab and seven major phase transitions, in addition to using the compressible (TALA) form of the governing equations. Though the thermodynamic parameters of certain phase transitions may be uncertain, this study aims to test the latest data and encourage further mineralogical research. We will present fully-dynamic models, which explore the importance of the phase transitions, especially those that have been previously excluded such as the wadsleyite to ringwoodite and the pyroxene and garnet phase transitions. These phase transitions, coupled with the modeled compositionally distinct crust, harzburgite, and pyrolite lithosphere layers, may produce new large-scale dynamic behavior not seen in past numerical models, as well as stress variations within the slab related to deep slab seismicity. Feedback from the compositionally complex slab to the dynamic trench may provide further insight on the mechanics of slab stagnation and behavior in the upper and lower mantle. Billen, M. I., and G. Hirth, Rheologic controls on slab dynamics, Geochemistry, Geophysics and Geosystems, 8 (Q08012
NASA Astrophysics Data System (ADS)
Arakawa, Naoya
2016-06-01
Anomalous Hall effect (AHE) and spin Hall effect (SHE) are fundamental phenomena, and their potential for application is great. However, we understand the interaction effects unsatisfactorily, and should have clarified issues about the roles of the Fermi sea term and Fermi surface term of the conductivity of the intrinsic AHE or SHE of an interacting multiorbital metal and about the effects of spin-Coulomb drag on the intrinsic SHE. Here, we resolve the first issue and provide the first step about the second issue by developing a general formalism in the linear response theory with appropriate approximations and using analytic arguments. The most striking result is that even without impurities, the Fermi surface term, a non-Berry-curvature term, plays dominant roles at high or slightly low temperatures. In particular, this Fermi surface term causes the temperature dependence of the dc anomalous Hall or spin Hall conductivity due to the interaction-induced quasiparticle damping and the correction of the dc spin Hall conductivity due to the spin-Coulomb drag. Those results revise our understanding of the intrinsic AHE and SHE. We also find that the differences between the dc anomalous Hall and longitudinal conductivities arise from the difference in the dominant multiband excitations. This not only explains why the Fermi sea term such as the Berry-curvature term becomes important in clean and low-temperature case only for interband transports, but also provides the useful principles on treating the electron-electron interaction in an interacting multiorbital metal for general formalism of transport coefficients. Several correspondences between our results and experiments are finally discussed.
Observation of strong electron pairing on bands without Fermi surfaces in LiFe1-xCoxAs
NASA Astrophysics Data System (ADS)
Miao, H.; Qian, T.; Shi, X.; Richard, P.; Kim, T. K.; Hoesch, M.; Xing, L. Y.; Wang, X.-C.; Jin, C.-Q.; Hu, J.-P.; Ding, H.
2015-01-01
In conventional BCS superconductors, the quantum condensation of superconducting electron pairs is understood as a Fermi surface instability, in which the low-energy electrons are paired by attractive interactions. Whether this explanation is still valid in high-Tc superconductors such as cuprates and iron-based superconductors remains an open question. In particular, a fundamentally different picture of the electron pairs, which are believed to be formed locally by repulsive interactions, may prevail. Here we report a high-resolution angle-resolved photoemission spectroscopy study on LiFe1-xCoxAs. We reveal a large and robust superconducting gap on a band sinking below the Fermi level on Co substitution. The observed Fermi-surface-free superconducting order is also the largest over the momentum space, which rules out a proximity effect origin and indicates that the order parameter is not tied to the Fermi surface as a result of a surface instability.
Study of colloidal quantum dot surfaces using an innovative thin-film positron 2D-ACAR method
NASA Astrophysics Data System (ADS)
Barbiellini, B.; Bansil, A.; Eijt, S. W. H.; Schut, H.; Mijnarends, P. E.; Denison, A. B.
2006-03-01
Despite a wealth of information, many fundamental questions regarding the nature of the surface of nanosized inorganic particles and its relationship with the electronic structure remain unsolved. We have investigated the electron momentum density (EMD) of colloidal CdSe quantum-dots via depth-resolved positron 2D angular correlation of annihilation (2D-ACAR) spectroscopy at the Delft intense variable-energy positron beam. This method, in combination with first-principles calculations of the EMD, shows that implanted positrons are trapped at the surface of CdSe nanocrystals. They annihilate mostly with the Se electrons and monitor changes in composition and structure of the surface while hardly sensing the ligand molecules. We thus unambiguously confirm [1] the strong surface relaxation predicted by first-principles calculations [2]. Work supported by the USDOE.[1] S.W.H. Eijt et al., Nature Materials (in press).[2] A. Puzder, et al., Phys. Rev. Lett. 92, 217401 (2004).
Gyrotropic Magnetic Effect and the Magnetic Moment on the Fermi Surface.
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. PMID:26943554
Quantum Oscillations without a Fermi Surface and the Anomalous de Haas-van Alphen Effect
NASA Astrophysics Data System (ADS)
Knolle, Johannes; Cooper, Nigel R.
2015-10-01
The de Haas-van Alphen effect (dHvAE), describing oscillations of the magnetization as a function of magnetic field, is commonly assumed to be a definite sign for the presence of a Fermi surface (FS). Indeed, the effect forms the basis of a well-established experimental procedure for accurately measuring FS topology and geometry of metallic systems, with parameters commonly extracted by fitting to the Lifshitz-Kosevich (LK) theory based on Fermi liquid theory. Here we show that, in contrast to this canonical situation, there can be quantum oscillations even for band insulators of certain types. We provide simple analytic formulas describing the temperature dependence of the quantum oscillations in this setting, showing strong deviations from LK theory. We draw connections to recent experiments and discuss how our results can be used in future experiments to accurately determine, e.g., hybridization gaps in heavy-fermion systems.
Strong interaction effects at a Fermi surface in a model for voltage-biased bilayer graphene
NASA Astrophysics Data System (ADS)
Armour, Wes; Hands, Simon; Strouthos, Costas
2015-12-01
Monte Carlo simulation of a 2+1 dimensional model of voltage-biased bilayer graphene, consisting of relativistic fermions with chemical potential μ coupled to charged excitations with opposite sign on each layer, has exposed noncanonical scaling of bulk observables near a quantum critical point found at strong coupling. We present a calculation of the quasiparticle dispersion relation E (k ) as a function of exciton source j in the same system, employing partially twisted boundary conditions to boost the number of available momentum modes. The Fermi momentum kF and superfluid gap Δ are extracted in the j →0 limit for three different values of μ , and support a strongly interacting scenario at the Fermi surface with Δ ˜O (μ ) . We propose an explanation for the observation μ
Correlation-Driven Topological Fermi Surface Transition in FeSe.
Leonov, I; Skornyakov, S L; Anisimov, V I; Vollhardt, D
2015-09-01
The electronic structure and phase stability of paramagnetic FeSe is computed by using a combination of ab initio methods for calculating band structure and dynamical mean-field theory. Our results reveal a topological change (Lifshitz transition) of the Fermi surface upon a moderate expansion of the lattice. The Lifshitz transition is accompanied with a sharp increase of the local moments and results in an entire reconstruction of magnetic correlations from the in-plane magnetic wave vector, (π,π) to (π,0). We attribute this behavior to a correlation-induced shift of the van Hove singularity originating from the d(xy) and d(xz)/d(yz) bands at the M point across the Fermi level. We propose that superconductivity is strongly influenced, or even induced, by a van Hove singularity. PMID:26382687
Fermi-surface induced modulation in an optimally doped YBCO superconductor.
Liu, X.; Islam, Z.; Sinha, S. K.; Moss, S. C.; McQueeney, R. J.; Lang, J. C.; Welp, U.; Univ. of California at San Diego; Univ. of Houston; Iowa State Univ.
2008-01-01
We have observed a Fermi-surface (FS) induced lattice modulation in a YBa{sub 2}Cu{sub 3}O{sub 7-x} superconductor with a wave vector along CuO chains; i.e., q{sub 1} = (0,{delta},0). The value of {delta} {approx} 0.21 is twice the Fermi wave vector (2k{sub F}) along b* connecting nearly nested FS 'ridges'. The q{sub 1} modulation exists only within O-vacancy-ordered islands [characterized by q{sub 0} = (1/4,0,0)] and persists well above and below T{sub c}. Our results are consistent with the presence of a FS-induced charge-density wave.
NASA Astrophysics Data System (ADS)
Kakihana, Masashi; Nakamura, Ai; Teruya, Atsushi; Harima, Hisatomo; Haga, Yoshinori; Hedo, Masato; Nakama, Takao; Ōnuki, Yoshichika
2015-03-01
We grew single crystals of PdBiSe with the ullmannite-type cubic chiral structure and carried out de Haas-van Alphen (dHvA) experiments to clarify the Fermi surface properties. The Fermi surfaces are found to split into two different Fermi surfaces, reflecting the non-centrosymmetric crystal structure. A splitting energy between two nearly spherical Fermi surfaces named α and α' is determined as 1050-1260 K. These Fermi surfaces are identified to be due the band-149 and -150 electron Fermi surfaces centered at the Γ point from the results of full-potential linearized augmented plane wave (FLAPW) energy band calculations under consideration of a mass correction in the spin-orbit interaction for Bi-6p electrons based on the relativistic effect. The theoretical splitting energy between these Fermi surfaces is 1080-1150 K, which is in good agreement with the experimental value.
2D ACAR momentum density study of the nature of the positron surface state on Al(100)
Berko, S.; Canter, K.F.; Lynn, K.G.; Mills, A.P.; Roellig, L.O.; West, R.N.
1985-01-01
The two-dimensional angular correlation of the 2..gamma.. annihilation radiation (2D ACAR) has been measured from an Al(100) surface bombarded by 200-eV positrons. After removing the contribution of fast para-positronium annihilation, the spectrum from positrons annihilating at the surface exhibits a nearly isotropic conical shape with a (7.1 +- 0.5) mrad FWHM. 5 refs., 6 figs.
Shi, Ke Ji; Yuan, Ding Wang; Wang, Cheng Xin; Shu, Chen Hui; Li, Deng Yuan; Shi, Zi Liang; Wu, Xin Yan; Liu, Pei Nian
2016-03-18
On-surface Ullmann coupling reaction of aryl chlorides has been achieved on Cu(111), Ag(111), and Au(111), and the mechanism has been investigated on the single molecule level using scanning tunneling microscopy and density functional theory. The different reactivity of the aryl halides was utilized to design a stepwise on-surface synthesis, which affords a zigzag template and then converts to 2D porous networks. PMID:26938859
Surface corrections to the moment of inertia and shell structure in finite Fermi systems
NASA Astrophysics Data System (ADS)
Gorpinchenko, D. V.; Magner, A. G.; Bartel, J.; Blocki, J. P.
2016-02-01
The moment of inertia for nuclear collective rotations is derived within a semiclassical approach based on the Inglis cranking and Strutinsky shell-correction methods, improved by surface corrections within the nonperturbative periodic-orbit theory. For adiabatic (statistical-equilibrium) rotations it was approximated by the generalized rigid-body moment of inertia accounting for the shell corrections of the particle density. An improved phase-space trace formula allows to express the shell components of the moment of inertia more accurately in terms of the free-energy shell correction. Evaluating their ratio within the extended Thomas-Fermi effective-surface approximation, one finds good agreement with the quantum calculations.
NASA Astrophysics Data System (ADS)
Cumbrera, Ramiro; Millán, Humberto; Martín-Sotoca, Juan Jose; Pérez Soto, Luis; Sanchez, Maria Elena; Tarquis, Ana Maria
2016-04-01
methods for mapping geochemical anomalies caused by buried sources and for predicting undiscovered mineral deposits in covered areas. Journal of Geochemical Exploration, 122, 55-70. Cumbrera, R., Ana M. Tarquis, Gabriel Gascó, Humberto Millán (2012) Fractal scaling of apparent soil moisture estimated from vertical planes of Vertisol pit images. Journal of Hydrology (452-453), 205-212. Martin Sotoca; J.J. Antonio Saa-Requejo, Juan Grau and Ana M. Tarquis (2016). Segmentation of singularity maps in the context of soil porosity. Geophysical Research Abstracts, 18, EGU2016-11402. Millán, H., Cumbrera, R. and Ana M. Tarquis (2016) Multifractal and Levy-stable statistics of soil surface moisture distribution derived from 2D image analysis. Applied Mathematical Modelling, 40(3), 2384-2395.
Charnukha, A.; Evtushinsky, D. V.; Matt, C. E.; Xu, N.; Shi, M.; Büchner, B.; Zhigadlo, N. D.; Batlogg, B.; Borisenko, S. V.
2015-01-01
In the family of the iron-based superconductors, the REFeAsO-type compounds (with RE being a rare-earth metal) exhibit the highest bulk superconducting transition temperatures (Tc) up to 55 K and thus hold the key to the elusive pairing mechanism. Recently, it has been demonstrated that the intrinsic electronic structure of SmFe0.92Co0.08AsO (Tc = 18 K) is highly nontrivial and consists of multiple band-edge singularities in close proximity to the Fermi level. However, it remains unclear whether these singularities are generic to the REFeAsO-type materials and if so, whether their exact topology is responsible for the aforementioned record Tc. In this work, we use angle-resolved photoemission spectroscopy (ARPES) to investigate the inherent electronic structure of the NdFeAsO0.6F0.4 compound with a twice higher Tc = 38 K. We find a similarly singular Fermi surface and further demonstrate that the dramatic enhancement of superconductivity in this compound correlates closely with the fine-tuning of one of the band-edge singularities to within a fraction of the superconducting energy gap Δ below the Fermi level. Our results provide compelling evidence that the band-structure singularities near the Fermi level in the iron-based superconductors must be explicitly accounted for in any attempt to understand the mechanism of superconducting pairing in these materials. PMID:26678565
Orbital dependent Rashba splitting and electron-phonon coupling of 2D Bi phase on Cu(100) surface
Gargiani, Pierluigi; Lisi, Simone; Betti, Maria Grazia; Ibrahimi, Amina Taleb; Bertran, François; Le Fèvre, Patrick; Chiodo, Letizia
2013-11-14
A monolayer of bismuth deposited on the Cu(100) surface forms a highly ordered c(2×2) reconstructed phase. The low energy single particle excitations of the c(2×2) Bi/Cu(100) present Bi-induced states with a parabolic dispersion in the energy region close to the Fermi level, as observed by angle-resolved photoemission spectroscopy. The electronic state dispersion, the charge density localization, and the spin-orbit coupling have been investigated combining photoemission spectroscopy and density functional theory, unraveling a two-dimensional Bi phase with charge density well localized at the interface. The Bi-induced states present a Rashba splitting, when the charge density is strongly localized in the Bi plane. Furthermore, the temperature dependence of the spectral density close to the Fermi level has been evaluated. Dispersive electronic states offer a large number of decay channels for transitions coupled to phonons and the strength of the electron-phonon coupling for the Bi/Cu(100) system is shown to be stronger than for Bi surfaces and to depend on the electronic state symmetry and localization.
NASA Astrophysics Data System (ADS)
Torgoev, Almaz; Havenith, Hans-Balder
2016-01-01
A 2D elasto-dynamic modelling of the pure topographic seismic response is performed for six models with a total length of around 23.0 km. These models are reconstructed from the real topographic settings of the landslide-prone slopes situated in the Mailuu-Suu River Valley, Southern Kyrgyzstan. The main studied parameter is the Arias Intensity (Ia, m/sec), which is applied in the GIS-based Newmark method to regionally map the seismically-induced landslide susceptibility. This method maps the Ia values via empirical attenuation laws and our studies investigate a potential to include topographic input into them. Numerical studies analyse several signals with varying shape and changing central frequency values. All tests demonstrate that the spectral amplification patterns directly affect the amplification of the Ia values. These results let to link the 2D distribution of the topographically amplified Ia values with the parameter called as smoothed curvature. The amplification values for the low-frequency signals are better correlated with the curvature smoothed over larger spatial extent, while those values for the high-frequency signals are more linked to the curvature with smaller smoothing extent. The best predictions are provided by the curvature smoothed over the extent calculated according to Geli's law. The sample equations predicting the Ia amplification based on the smoothed curvature are presented for the sinusoid-shape input signals. These laws cannot be directly implemented in the regional Newmark method, as 3D amplification of the Ia values addresses more problem complexities which are not studied here. Nevertheless, our 2D results prepare the theoretical framework which can potentially be applied to the 3D domain and, therefore, represent a robust basis for these future research targets.
NASA Astrophysics Data System (ADS)
Torgoev, Almaz; Havenith, Hans-Balder
2016-07-01
A 2D elasto-dynamic modelling of the pure topographic seismic response is performed for six models with a total length of around 23.0 km. These models are reconstructed from the real topographic settings of the landslide-prone slopes situated in the Mailuu-Suu River Valley, Southern Kyrgyzstan. The main studied parameter is the Arias Intensity (Ia, m/sec), which is applied in the GIS-based Newmark method to regionally map the seismically-induced landslide susceptibility. This method maps the Ia values via empirical attenuation laws and our studies investigate a potential to include topographic input into them. Numerical studies analyse several signals with varying shape and changing central frequency values. All tests demonstrate that the spectral amplification patterns directly affect the amplification of the Ia values. These results let to link the 2D distribution of the topographically amplified Ia values with the parameter called as smoothed curvature. The amplification values for the low-frequency signals are better correlated with the curvature smoothed over larger spatial extent, while those values for the high-frequency signals are more linked to the curvature with smaller smoothing extent. The best predictions are provided by the curvature smoothed over the extent calculated according to Geli's law. The sample equations predicting the Ia amplification based on the smoothed curvature are presented for the sinusoid-shape input signals. These laws cannot be directly implemented in the regional Newmark method, as 3D amplification of the Ia values addresses more problem complexities which are not studied here. Nevertheless, our 2D results prepare the theoretical framework which can potentially be applied to the 3D domain and, therefore, represent a robust basis for these future research targets.
Imai, Kazue; Hayashi, Tomonori; Yamaoka, Mika; Kajimura, Junko; Yoshida, Kengo; Kusunoki, Yoichiro; Nakachi, Kei
2012-06-01
NKG2D is a primary activating receptor that triggers cell-mediated cytotoxicity in NK cells against tumor and virus-infected cells. We previously identified the NKG2D haplotypes in the natural killer gene complex region on chromosome 12p. Two major haplotype alleles, LNK1 and HNK1, were closely related to low and high natural cytotoxic activity phenotypes, respectively. Furthermore, the haplotype of HNK1/HNK1 has revealed a decreased risk of cancer compared with LNK1/LNK1. In the present study, using flow cytometry, we evaluated the functional effects of NKG2D haplotypes and five htSNPs in terms of the cell-surface expression of NKG2D protein on NK and CD8 T cells of peripheral blood among 732 atomic-bomb survivors. NKG2D expression on NK cells showed significant increases, in the order of LNK1/LNK1, LNK1/HNK1 and HNK1/HNK1 haplotypes (p for trend=0.003), or with major homozygous, heterozygous, and minor homozygous genotypes for individual htSNPs (p for trend=0.02-0.003). The same trend was observed for NKG2D expression on CD8 T cells. Our findings indicate that the NKG2D haplotypes are associated with the expression levels of NKG2D protein on NK and CD8 T cells, resulting in inter-individual variations in human cytotoxic response. PMID:22507622
Nonlinear optical conductivity of U (1 ) spin liquids with large spinon Fermi surfaces
NASA Astrophysics Data System (ADS)
Ma, Yuan-Fei; Ng, Tai-Kai
2016-06-01
In this paper we study the nonlinear current response of U (1 ) spin liquids with large spinon Fermi surfaces under the perturbation of a time-dependent ac electric field E (t ) within the framework of an effective U (1 ) gauge theory. In particular, the third-order nonlinear current response to ac electric fields is derived. We show that as in the case of linear current response, an in-gap power-law (˜ωη ) response is found for the nonlinear current at low frequency. The nonlinear susceptibility may also induce through process of third harmonic generation propagating EM wave with frequency 3 ω inside the spin liquids.
NASA Astrophysics Data System (ADS)
Lan, Yue; Wang, Shiqiang; Yin, Xianpeng; Liang, Yun; Dong, Hao; Gao, Ning; Li, Jian; Wang, Hui; Li, Guangtao
2016-07-01
Recently, it has been demonstrated that the combination of periodic dielectric structures with metallic structures provides an efficient means to yield a synergetic optical response or functionality in the resultant hybrid plasmonic-photonic systems. In this work, a new hybrid plasmonic-photonic structure of 2D-ordered dielectric sub-micron bowls on a flat gold surface was proposed, prepared, and theoretically and experimentally characterized. This hybrid structure supports two types of modes: surface plasmon polaritons bound at the metallic surface and waveguided mode of light confined in the cavity of bowls. Optical responses of this hybrid structure as well as the spatial electric field distribution of each mode are found to be strongly dependent on the structural parameters of this system, and thus could be widely modified on demand. Importantly, compared to the widely studied hybrid systems, namely the flat metallic surface coated with a monolayer array of latex spheres, the waveguided mode with strong field enhancement appearing in the cavities of bowls is more facilely accessible and thus suitable for practical use. For demonstration, a 2D-ordered silica sub-micron bowl array deposited on a flat gold surface was fabricated and used as a regenerable platform for fluorescence enhancement by simply accommodating emitters in bowls. All the simulation and experiment results indicate that the 2D-ordered dielectric sub-micron bowls on a metal surface should be a useful hybrid plasmonic-photonic system with great potential for applications such as sensors or tunable emitting devices if appropriate periods and materials are employed.Recently, it has been demonstrated that the combination of periodic dielectric structures with metallic structures provides an efficient means to yield a synergetic optical response or functionality in the resultant hybrid plasmonic-photonic systems. In this work, a new hybrid plasmonic-photonic structure of 2D-ordered dielectric sub
Surface origin of quasi-2D Shubnikov-de Haas oscillations in Bi2Te2Se
NASA Astrophysics Data System (ADS)
Kapustin, A. A.; Stolyarov, V. S.; Bozhko, S. I.; Borisenko, D. N.; Kolesnikov, N. N.
2015-08-01
Transport measurements at liquid helium temperatures were done on a number of Bi2Te2Se samples with thicknesses ranging from 30 to 200 μm in order to detect surface states. In each sample we observed Shubnikov-de Haas (SdH) oscillations and sublinear dependence of off-diagonal component of magnetoresistance tensor on magnetic field. The periods of SdH oscillations in inverse magnetic field were found to be the same within 15%. The positions of SdH oscillations are determined by the normal to surface component of magnetic field. We found that the measured conductivity can be well described by a model with two groups of electrons, 2D and 3D. The conductivity of 2D electrons was found to be relatively weakly varying from sample to sample and not depending on thickness in a systematic manner. This behavior can be explained only by their localization on the surface. Comparison of the results of magnetotransport measurements with our scanning tunneling spectroscopy results on atomically smooth Bi2Te2Se surface in ultrahigh vacuum led us to conclude that the surface electrons are separated from the bulk electrons by a depletion layer approximately 100 nm thick. This effect could provide the dominant contribution of surface electrons to conductivity in samples with thicknesses less than 200 nm.
Maximal Cherenkov γ-radiation on Fermi-surface of compact stars
Akbari-Moghanjoughi, M.
2014-05-15
The quantum magnetohydrodynamic model is employed in this paper to study the extraordinary (XO) elliptically polarized electromagnetic wave dispersion in quantum plasmas with spin-1/2 magnetization and relativistic degeneracy effects, considering also the electron-exchange and quantum diffraction of electrons. From the lower and upper calculated XO-modes, it is observed that, for electrons on the surface of the Fermi-sphere, the lower XO-mode can excite the Cherenkov radiation by crossing the Fermi-line, with some proper conditions depending on the values of independent plasma parameters, such as the relativistic-degeneracy, the atomic-number of constituent ions, and the magnetic field strength. Particularly, a lower electron number-density and Cherenkov radiation frequency limits are found to exist, for instance, for given values of the plasma ions atomic-number and the magnetic field strength below which the radiation can not be excited by the electrons on the Fermi-surface. This lower density limit increases by decrease in the atomic-number but decreases with decrease in the strength of the ambient magnetic field. It is remarkable that in this research it is discovered that the maximal Cherenkov-radiation per unit-length (the energy radiated by superluminal electrons traveling through the dielectric medium) coincides with the plasma number-densities, which is present in compact stars with the maximal radiation frequency lying in the gamma-ray spectrum. Current study can provide an important plasma diagnostic tool for a wide plasma density range, be it the solid density, the warm dense matter, the inertial confined or the astrophysical compact plasmas and may reveal an important cooling mechanism for white dwarfs. Current findings may also answer the fundamental astrophysical question on the mysterious origin of intense cosmic gamma-ray emissions.
Lan, Yue; Wang, Shiqiang; Yin, Xianpeng; Liang, Yun; Dong, Hao; Gao, Ning; Li, Jian; Wang, Hui; Li, Guangtao
2016-07-21
Recently, it has been demonstrated that the combination of periodic dielectric structures with metallic structures provides an efficient means to yield a synergetic optical response or functionality in the resultant hybrid plasmonic-photonic systems. In this work, a new hybrid plasmonic-photonic structure of 2D-ordered dielectric sub-micron bowls on a flat gold surface was proposed, prepared, and theoretically and experimentally characterized. This hybrid structure supports two types of modes: surface plasmon polaritons bound at the metallic surface and waveguided mode of light confined in the cavity of bowls. Optical responses of this hybrid structure as well as the spatial electric field distribution of each mode are found to be strongly dependent on the structural parameters of this system, and thus could be widely modified on demand. Importantly, compared to the widely studied hybrid systems, namely the flat metallic surface coated with a monolayer array of latex spheres, the waveguided mode with strong field enhancement appearing in the cavities of bowls is more facilely accessible and thus suitable for practical use. For demonstration, a 2D-ordered silica sub-micron bowl array deposited on a flat gold surface was fabricated and used as a regenerable platform for fluorescence enhancement by simply accommodating emitters in bowls. All the simulation and experiment results indicate that the 2D-ordered dielectric sub-micron bowls on a metal surface should be a useful hybrid plasmonic-photonic system with great potential for applications such as sensors or tunable emitting devices if appropriate periods and materials are employed. PMID:27349558
NASA Astrophysics Data System (ADS)
Pillai, S.; Hemmersam, A. G.; Mukhopadhyay, R.; Meyer, R. L.; Moghimi, S. M.; Besenbacher, F.; Kingshott, P.
2009-01-01
Polymer-based nanopatterning on metal surfaces is of increasing importance to a number of applications, including biosensors, bioelectronic devices and medical implants. Here we show that polycrystalline gold surfaces can be functionalized with monocomponent nanoparticle (NP) assemblies by a simple drop deposition method. Ordered 3D hexagonal close-packed structures consisting of 350 nm polystyrene (PS) NPs on hydrophobically modified gold surfaces from solutions of very low volume fraction (phiv = 0.0006) were obtained as a result of capillary force induced self-assembly, whilst 2D self-assembly of PS NPs was generated over large area on hydrophilic gold and TiO2 surfaces by spin coating. Furthermore, we show that when Triton X-100 is added to the PS NP suspending medium longer range ordering is obtained. Our observations may initiate interesting applications in the areas of nanoengineering of metal-based sensors and as a means to design new nanostructures for biocompatible implant surfaces.
Single reconstructed Fermi surface pocket in an underdoped single-layer cuprate superconductor
Chan, M. K.; Harrison, N.; McDonald, R. D.; Ramshaw, B. J.; Modic, K. A.; Barišić, N.; Greven, M.
2016-01-01
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 of 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. PMID:27448102
Single reconstructed Fermi surface pocket in an underdoped single-layer cuprate superconductor.
Chan, M K; Harrison, N; McDonald, R D; Ramshaw, B J; Modic, K A; Barišić, N; Greven, M
2016-01-01
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 of 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. PMID:27448102
Single reconstructed Fermi surface pocket in an underdoped single-layer cuprate superconductor
NASA Astrophysics Data System (ADS)
Chan, M. K.; Harrison, N.; McDonald, R. D.; Ramshaw, B. J.; Modic, K. A.; Barišić, N.; Greven, M.
2016-07-01
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 of 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.
Origin of Fermi-level pinning and its control on the n -type Ge(100) surface
NASA Astrophysics Data System (ADS)
Kuzmin, Mikhail; Laukkanen, Pekka; Mäkelä, Jaakko; Tuominen, Marjukka; Yasir, Muhammad; Dahl, Johnny; Punkkinen, Marko P. J.; Kokko, Kalevi
2016-07-01
Strong Fermi-level pinning (FLP) near the valence-band maximum on n -type Ge surfaces has been a long-standing challenge in semiconductor physics, and the nature of this phenomenon has been heavily debated for years. Here, we report a systematic synchrotron-based photoemission study of atomically well-defined Ge(100) surfaces and interfaces to elucidate the origin of FLP in such systems. It is experimentally shown that the FLP on n -Ge is not due to the dangling-bond, back-bond, and defect states, but is strongly contributed by the evanescent state of the Ge bulk. The conditions required for alleviating the FLP and even the implementation of a flatband structure on Ge(100) are formulated. Such a structure is realized in the BaO/Ge(100) system where one can obtain control over the Fermi-level position in the Ge gap. These findings are not only important from a fundamental viewpoint, but also open a route to producing Ohmic metal-insulator-semiconductor contacts for n -type Ge-based technology.
Magnetic and Fermi Surface Properties of Ferromagnets EuPd2 and EuPt2
NASA Astrophysics Data System (ADS)
Nakamura, Ai; Akamine, Hiromu; Ashitomi, Yousuke; Honda, Fuminori; Aoki, Dai; Takeuchi, Tetsuya; Matsubayashi, Kazuyuki; Uwatoko, Yoshiya; Tatetsu, Yasutomi; Maehira, Takahiro; Hedo, Masato; Nakama, Takao; Ōnuki, Yoshichika
2016-08-01
We succeeded in growing single crystals of the ferromagnets EuPd2 and EuPt2 with the Laves-type cubic structure by the Bridgman method, namely, heating constituting materials in a Mo crucible up to a high temperature of about 1500 °C. The ferromagnetic properties of EuPd2 and EuPt2 with Curie temperatures of 74 and 100 K, respectively, were confirmed from the results of electrical resistivity, specific heat, and magnetization measurements. The ordered moment is 7 μB/Eu, revealing the Eu-divalent ferromagnetism. The present Eu-divalent electronic state is found to be robust against high pressures of up to 8 GPa and is not changed into the Eu-trivalent state. We also carried out de Haas-van Alphen (dHvA) experiments for EuPd2. The detected dHvA branches in EuPd2 are well explained by the relativistic linearized augmented plane wave (RLAPW) energy band calculations for SrPd2, revealing a closed hole Fermi surface and compensated four closed electron Fermi surfaces.
Fermi surface reconstruction and multiple quantum phase transitions in the antiferromagnet CeRhIn5
Jiao, Lin; Chen, Ye; Kohama, Yoshimitsu; Graf, David; Bauer, E. D.; Singleton, John; Zhu, Jian-Xin; Weng, Zongfa; Pang, Guiming; Shang, Tian; Zhang, Jinglei; Lee, Han-Oh; Park, Tuson; Jaime, Marcelo; Thompson, J. D.; Steglich, Frank; Si, Qimiao; Yuan, H. Q.
2015-01-01
Conventional, thermally driven continuous phase transitions are described by universal critical behavior that is independent of the specific microscopic details of a material. However, many current studies focus on materials that exhibit quantum-driven continuous phase transitions (quantum critical points, or QCPs) at absolute zero temperature. The classification of such QCPs and the question of whether they show universal behavior remain open issues. Here we report measurements of heat capacity and de Haas–van Alphen (dHvA) oscillations at low temperatures across a field-induced antiferromagnetic QCP (Bc0 ≈ 50 T) in the heavy-fermion metal CeRhIn5. A sharp, magnetic-field-induced change in Fermi surface is detected both in the dHvA effect and Hall resistivity at B0* ≈ 30 T, well inside the antiferromagnetic phase. Comparisons with band-structure calculations and properties of isostructural CeCoIn5 suggest that the Fermi-surface change at B0* is associated with a localized-to-itinerant transition of the Ce-4f electrons in CeRhIn5. Taken in conjunction with pressure experiments, our results demonstrate that at least two distinct classes of QCP are observable in CeRhIn5, a significant step toward the derivation of a universal phase diagram for QCPs. PMID:25561536
Silicon quantum wires on Ag(1 1 0): Fermi surface and quantum well states
NASA Astrophysics Data System (ADS)
Valbuena, M. A.; Avila, J.; Dávila, M. E.; Leandri, C.; Aufray, B.; Le Lay, G.; Asensio, M. C.
2007-10-01
One-dimensional Si quantum wires have been grown on silver single crystals upon deposition of ˜0.25 monolayer of Si on Ag(1 1 0) surfaces. Scanning tunneling microscopy (STM) clearly shows parallel 1D Si chains along the [-1 1 0] Ag crystallographic direction. Low Energy Electron Diffraction (LEED) confirms the massively parallel assembly of these selforganized Nanowires (NWs). We have characterized these nano-objects by measuring the dispersion of the NWs valence band at room temperature using Angle-Resolved PhotoEmission Spectroscopy (ARPES). Also, the Fermi Surface (FS) of the Ag(1 1 0) substrate has been mapped before and after the silicon deposition, trying to put in evidence the metallic or semiconductor character of the NWs silicon's states close to the Fermi level. Our results show the existence of well-defined quantum states associated to the silicon super-structure. Both LEED and ARUPS results confirm that the NWs have typical 1D features, however their metallic or semiconductor character could not be confirmed.
NASA Astrophysics Data System (ADS)
Ye, Z. R.; Zhang, Y.; Chen, F.; Xu, M.; Jiang, J.; Niu, X. H.; Wen, C. H. P.; Xie, B. P.; Feng, D. L.; Xing, L. Y.; Wang, X. C.; Jin, C. Q.
2014-03-01
The superconductivity in Fe-based superconductors could be achieved by doping the parent compounds. Previous researches were focusing on the charge carrier density or Fermi surface alteration by doping only. However, the dominating factors based on Fermiology have many inconsistencies, which indicates that some other effects induced by doping are neglected. Using ARPES, we have established the microscopic and more comprehensive picture of doping on the electronic structure beyond altering Fermi surface. We have figured out other two critical effects of doping, scattering and changing correlation. With doping, the dxy-related band around the zone center is found to be much more sensitive than the dxz/dyz-related bands and the strength of the impurity scattering strongly depends on the position of dopants, which resembles the case in cuprates. On the other hand, we observed that the electron correlation decreases with doping, which is universal in various systems of Fe-based superconductors. Moderate electron correlation is critical for the high Tc. The two effects we observed here both are very important for the superconductivity, and explain a lot of previous mysteries and unresolved issues.
NASA Astrophysics Data System (ADS)
Olmez, O.; Ozbulut, M.; Yildiz, M.; Goren, O.
2016-06-01
The present study investigates the vortical and nonlinear effects in the roll motion of a 2-D body with square cross-sections by using Smoothed Particle Hydrodynamics (SPH). A 2-D rigid body with square cross-section is taken into account for the benchmark study and subjected to the oscillatory roll motion with a given angular frequency. The governing equations are continuity equation and Euler's equation with artificial viscosity term. Weakly Compressible SPH (WCSPH) scheme is employed for the discretization of the governing equations. Velocities of the fluid particles are updated by means of XSPH+Artificial Particle Displacement (VXSPH+APD) algorithm. In this method only the free surface fluid particles are subjected to VXSPH algorithm while the APD algorithm is employed for the fully populated flow regions. The hybrid usage of numerical treatment keeps free surface particles together by creating an artificial surface tension on the free surface. VXSPH+APD is a proven numerical treatment to provide the most accurate results for this type of free surface flows (Ozbulut et al. 2014). The results of the present study are compared with those of the experimental studies as well as with those of the numerical methods obtained from the current literature.
Investigation of ellipsometric parameters of 2D microrough surfaces by FDTD.
Qiu, J; Ran, D F; Liu, Y B; Liu, L H
2016-07-10
Ellipsometry is a powerful method for measuring the optical constants of materials and is very sensitive to surface roughness. In previous ellipsometric measurement of optical constants of solid materials with rough surfaces, researchers frequently used effective medium approximation (EMA) with roughness already known to fit the complex refractive index of the material. However, the ignored correlation length, the other important parameter of rough surfaces, will definitely result in fitting errors. Hence it is necessary to consider the influence of surface roughness and correlation length on the ellipsometric parameters Δ (phase difference) and Ψ (azimuth) characterizing practical systems. In this paper, the influence of roughness of two-dimensional randomly microrough surfaces (relative roughness σ/λ ranges from 0.001 to 0.025) of silicon on ellipsometric parameters was simulated by the finite-difference time-domain method which was validated with experimental results. The effects of incident angle, relative roughness, and correlation length were numerically investigated for two-dimensional Gaussian distributed randomly microrough surfaces, respectively. The simulated results showed that compared with the smooth surface, only tiny changes of the ellipsometric parameter Δ could be observed for microrough silicon surface in the vicinity of the Brewster angle, but obviously changes of Ψ occur especially in the vicinity of the Brewster angle. More differences between the ellipsometric parameters of the rough surface and smooth surface can been seen especially in the vicinity of the Brewster angle as the relative roughness σ/λ increases or correlation length τ decreases. The results reveal that when we measure the optical constants of solid materials by ellipsometry, the smaller roughness, larger correlation length and larger incident wavelength will lead to the higher precision of measurements. PMID:27409321
Electronic structure and Fermi surfaces of transition metal carbides with rocksalt structure
NASA Astrophysics Data System (ADS)
Paduani, C.
2008-06-01
First-principles calculations were carried out to investigate the structural and electronic properties of the metal carbides FeC, CoC, NiC, and PtC in the rocksalt structure. The full-potential linearized augmented-plane wave (FP-LAPW) method was used in the framework of the density-functional theory with the generalized gradient approximation (GGA) for the exchange-correlation potential. Ground state properties are determined and compared with available experimental data. The energy band structures, densities of states, and Fermi surface structures are obtained, which show that these compounds are metallic like the conventional transition metal carbides. There is an extensive hybridization between the metal-d and C-2p states for all the studied carbides, which can form bonding and antibonding states. From FeC to PtC a band narrowing for the hybridized metal-d and C-2p states near to the Fermi level takes place, which is expected to lead to smaller interactions between adjacent atoms. The largest bulk modulus of FeC is expected to be associated with the behavior of valence electrons near the Fermi level, i.e. a higher degree of hybridization between p-d states that are responsible for the chemical bonding results in strengthened interactions. The decrease in the number of bonding orbitals or decrease in metallic valence with the increase in number of 3d electrons from FeC to PtC provides a mechanism for weaker interactions due to the filling of antibonding bands.
A Coupled Finite-Volume Model for 2-D Surface and 3-D Subsurface Flows
Technology Transfer Automated Retrieval System (TEKTRAN)
Surface-subsurface interactions are an intrinsic component of the hydrologic response within a watershed; therefore, hydrologic modeling tools should consider these interactions to provide reliable predictions, especially during rainfall-runoff processes. This paper presents a fully implicit coupled...
Evolution of the Fermi surface of a doped topological insulator with carrier concentration
NASA Astrophysics Data System (ADS)
Lahoud, E.; Maniv, E.; Petrushevsky, M. Shaviv; Naamneh, M.; Ribak, A.; Wiedmann, S.; Petaccia, L.; Salman, Z.; Chashka, K. B.; Dagan, Y.; Kanigel, A.
2013-11-01
In an ideal bulk topological insulator (TI) conducting surface states protected by time-reversal symmetry enfold an insulating crystal. However, the archetypical TI, Bi2Se3, is actually never insulating; it is in fact a relatively good metal. Nevertheless, it is the most studied system among all the TIs, mainly due to its simple band structure and large spin-orbit gap. Recently, it was shown that copper intercalated Bi2Se3 becomes superconducting and it was suggested as a realization of a topological superconductor. Here we use a combination of techniques that are sensitive to the shape of the Fermi surface (FS): the Shubnikov-de Haas effect and angle-resolved photoemission spectroscopy to study the evolution of the FS shape with carrier concentration, n. We find that as n increases, the FS becomes two-dimensional-like. These results are of crucial importance for understanding the superconducting properties of CuxBi2Se3.
Measurement of residual radioactive surface contamination by 2-D laser heated TLD
Jones, S.C.
1997-06-01
The feasibility of applying and adapting a two-dimensional laser heated thermoluminescence dosimetry system to the problem of surveying for radioactive surface contamination was studied. The system consists of a CO{sub 2} laser-based reader and monolithic arrays of thin dosimeter elements. The arrays consist of 10,201 thermoluminescent phosphor elements of 40 micron thickness, covering a 900 cm{sup 2} area. Array substrates are 125 micron thick polyimide sheets, enabling them to easily conform to regular surface shapes, especially for survey of surfaces that are inaccessible for standard survey instruments. The passive, integrating radiation detectors are sensitive to alpha and beta radiation at contamination levels below release guideline limits. Required contact times with potentially contaminated surfaces are under one hour to achieve detection of transuranic alpha emission at 100 dpm/100 cm{sup 2}. Positional information obtained from array evaluation is useful for locating contamination zones. Unique capabilities of this system for survey of sites, facilities and material include measurement inside pipes and other geometrical configurations that prevent standard surveys, and below-surface measurement of alpha and beta emitters in contaminated soils. These applications imply a reduction of material that must be classified as radioactive waste by virtue of its possibility of contamination, and cost savings in soil sampling at contaminated sites.
Singleton, John; Mc Donald, Ross D; Cox, Susan
2008-01-01
The authors give a brief review of high-magnetic-field quantum-oscillation measurements on cuprate superconductors. In the case of the underdoped cuprates, a number of small Fermi-surface pockets are observed, probably due to the incommensurate nesting of the predicted (large) hole Fermi surface. The Fermi-surface instabilities that drive this nesting are also likely to result in the incommensurate spin fluctuations observed in inelastic neutron-scattering measurements. They suggest that the unusually high superconducting transitions in the cuprates are driven by an exact mapping of these incommensurate spin fluctuations onto the d{sub x{sup 2}-y{sup 2}} Cooper-pair wavefunction. The maximum energy of the fluctuations {approx} 100s of Kelvin gives an appropriate energy scale for the superconducting transition temperature.
A Weyl Fermion semimetal with surface Fermi arcs in the transition metal monopnictide TaAs class.
Huang, Shin-Ming; Xu, Su-Yang; Belopolski, Ilya; Lee, Chi-Cheng; Chang, Guoqing; Wang, BaoKai; Alidoust, Nasser; Bian, Guang; Neupane, Madhab; Zhang, Chenglong; Jia, Shuang; Bansil, Arun; Lin, Hsin; Hasan, M Zahid
2015-01-01
Weyl fermions are massless chiral fermions that play an important role in quantum field theory but have never been observed as fundamental particles. A Weyl semimetal is an unusual crystal that hosts Weyl fermions as quasiparticle excitations and features Fermi arcs on its surface. Such a semimetal not only provides a condensed matter realization of the anomalies in quantum field theories but also demonstrates the topological classification beyond the gapped topological insulators. Here, we identify a topological Weyl semimetal state in the transition metal monopnictide materials class. Our first-principles calculations on TaAs reveal its bulk Weyl fermion cones and surface Fermi arcs. Our results show that in the TaAs-type materials the Weyl semimetal state does not depend on fine-tuning of chemical composition or magnetic order, which opens the door for the experimental realization of Weyl semimetals and Fermi arc surface states in real materials. PMID:26067579
Itinerant 5 f Electrons and the Fermi Surface Properties in an Enhanced Pauli Paramagnet NpGe3
NASA Astrophysics Data System (ADS)
Aoki, Dai; Yamagami, Hiroshi; Homma, Yoshiya; Shiokawa, Yoshinobu; Yamamoto, Etsuji; Nakamura, Akio; Haga, Yoshinori; Settai, Rikio; Ōnuki, Yoshichika
2005-08-01
We succeeded in growing a high-quality single crystal of an enhanced Pauli paramagnet, NpGe3, by the Bi-flux method, and observed the de Haas-van Alphen (dHvA) effect. The topology of a Fermi surface is well explained by the relativistic linear augmented-plane-wave (LAPW) band calculations based on the 5 f itinerant band model. The Fermi surface consists of a nearly spherical electron-Fermi surface with necks along the < 100 > direction, forming a hollow ball, centered at the R point, which is derived from the single band. The cyclotron effective mass is in the range from 2.6 to 16 m0, which is enhanced approximately 3.5 times from the corresponding band mass.
A Weyl Fermion semimetal with surface Fermi arcs in the transition metal monopnictide TaAs class
Huang, Shin-Ming; Xu, Su-Yang; Belopolski, Ilya; Lee, Chi-Cheng; Chang, Guoqing; Wang, BaoKai; Alidoust, Nasser; Bian, Guang; Neupane, Madhab; Zhang, Chenglong; Jia, Shuang; Bansil, Arun; Lin, Hsin; Hasan, M. Zahid
2015-01-01
Weyl fermions are massless chiral fermions that play an important role in quantum field theory but have never been observed as fundamental particles. A Weyl semimetal is an unusual crystal that hosts Weyl fermions as quasiparticle excitations and features Fermi arcs on its surface. Such a semimetal not only provides a condensed matter realization of the anomalies in quantum field theories but also demonstrates the topological classification beyond the gapped topological insulators. Here, we identify a topological Weyl semimetal state in the transition metal monopnictide materials class. Our first-principles calculations on TaAs reveal its bulk Weyl fermion cones and surface Fermi arcs. Our results show that in the TaAs-type materials the Weyl semimetal state does not depend on fine-tuning of chemical composition or magnetic order, which opens the door for the experimental realization of Weyl semimetals and Fermi arc surface states in real materials. PMID:26067579
Billington, David; Ernsting, David; Millichamp, Thomas E.; Lester, Christopher; Dugdale, Stephen B.; Kersh, David; Duffy, Jonathan A.; Giblin, Sean R.; Taylor, Jonathan W.; Manuel, Pascal; Khalyavin, Dmitry D.; Takatsu, Hiroshi
2015-01-01
Frustrated interactions exist throughout nature, with examples ranging from protein folding through to frustrated magnetic interactions. Whilst magnetic frustration is observed in numerous electrically insulating systems, in metals it is a rare phenomenon. The interplay of itinerant conduction electrons mediating interactions between localised magnetic moments with strong spin-orbit coupling is likely fundamental to these systems. Therefore, knowledge of the precise shape and topology of the Fermi surface is important in any explanation of the magnetic behaviour. PdCrO2, a frustrated metallic magnet, offers the opportunity to examine the relationship between magnetic frustration, short-range magnetic order and Fermi surface topology. By mapping the short-range order in reciprocal space and experimentally determining the electronic structure, we have identified the dual role played by the Cr electrons in which the itinerant ones on the nested paramagnetic Fermi surface mediate the frustrated magnetic interactions between local moments. PMID:26206589
Billington, David; Ernsting, David; Millichamp, Thomas E; Lester, Christopher; Dugdale, Stephen B; Kersh, David; Duffy, Jonathan A; Giblin, Sean R; Taylor, Jonathan W; Manuel, Pascal; Khalyavin, Dmitry D; Takatsu, Hiroshi
2015-01-01
Frustrated interactions exist throughout nature, with examples ranging from protein folding through to frustrated magnetic interactions. Whilst magnetic frustration is observed in numerous electrically insulating systems, in metals it is a rare phenomenon. The interplay of itinerant conduction electrons mediating interactions between localised magnetic moments with strong spin-orbit coupling is likely fundamental to these systems. Therefore, knowledge of the precise shape and topology of the Fermi surface is important in any explanation of the magnetic behaviour. PdCrO2, a frustrated metallic magnet, offers the opportunity to examine the relationship between magnetic frustration, short-range magnetic order and Fermi surface topology. By mapping the short-range order in reciprocal space and experimentally determining the electronic structure, we have identified the dual role played by the Cr electrons in which the itinerant ones on the nested paramagnetic Fermi surface mediate the frustrated magnetic interactions between local moments. PMID:26206589
NASA Astrophysics Data System (ADS)
Mechler, S.; Pershan, P. S.; Yahel, E.; Stoltz, S. E.; Shpyrko, O. G.; Lin, B.; Meron, M.; Sellner, S.
2010-10-01
The structural and mechanical properties of 2D crystalline surface phases that form at the surface of liquid eutectic Au82Si18 are studied using synchrotron x-ray scattering over a large temperature range. In the vicinity of the eutectic temperature the surface consists of a 2D atomic bilayer crystalline phase that transforms into a 2D monolayer crystalline phase during heating. The latter phase eventually melts into a liquidlike surface on further heating. We demonstrate that the short wavelength capillary wave fluctuations are suppressed due to the bending rigidity of 2D crystalline phases. The corresponding reduction in the Debye-Waller factor allows for measured reflectivity to be explained in terms of an electron density profile that is consistent with the 2D surface crystals.
Collapse of Ferromagnetism and Fermi Surface Instability near Reentrant Superconductivity of URhGe.
Gourgout, A; Pourret, A; Knebel, G; Aoki, D; Seyfarth, G; Flouquet, J
2016-07-22
We present thermoelectric power and resistivity measurements in the ferromagnetic superconductor URhGe for a magnetic field applied along the hard magnetization b axis of the orthorhombic crystal. Reentrant superconductivity is observed near the spin reorientation transition at H_{R}=12.75 T, where a first order transition from the ferromagnetic to the polarized paramagnetic state occurs. Special focus is given to the longitudinal configuration, where both the electric and heat current are parallel to the applied field. The validity of the Fermi-liquid T^{2} dependence of the resistivity through H_{R} demonstrates clearly that no quantum critical point occurs at H_{R}. Thus, the ferromagnetic transition line at H_{R} becomes first order implying the existence of a tricritical point at finite temperature. The enhancement of magnetic fluctuations in the vicinity of the tricritical point stimulates the reentrance of superconductivity. The abrupt sign change observed in the thermoelectric power with the thermal gradient applied along the b axis together with the strong anomalies in the other directions is definitive macroscopic evidence that in addition a significant change of the Fermi surface appears through H_{R}. PMID:27494485
Collapse of Ferromagnetism and Fermi Surface Instability near Reentrant Superconductivity of URhGe
NASA Astrophysics Data System (ADS)
Gourgout, A.; Pourret, A.; Knebel, G.; Aoki, D.; Seyfarth, G.; Flouquet, J.
2016-07-01
We present thermoelectric power and resistivity measurements in the ferromagnetic superconductor URhGe for a magnetic field applied along the hard magnetization b axis of the orthorhombic crystal. Reentrant superconductivity is observed near the spin reorientation transition at HR=12.75 T , where a first order transition from the ferromagnetic to the polarized paramagnetic state occurs. Special focus is given to the longitudinal configuration, where both the electric and heat current are parallel to the applied field. The validity of the Fermi-liquid T2 dependence of the resistivity through HR demonstrates clearly that no quantum critical point occurs at HR. Thus, the ferromagnetic transition line at HR becomes first order implying the existence of a tricritical point at finite temperature. The enhancement of magnetic fluctuations in the vicinity of the tricritical point stimulates the reentrance of superconductivity. The abrupt sign change observed in the thermoelectric power with the thermal gradient applied along the b axis together with the strong anomalies in the other directions is definitive macroscopic evidence that in addition a significant change of the Fermi surface appears through HR.
Evolution of the Fermi surface of Weyl semimetals in the transition metal pnictide family
NASA Astrophysics Data System (ADS)
Liu, Z. K.; Yang, L. X.; Sun, Y.; Zhang, T.; Peng, H.; Yang, H. F.; Chen, C.; Zhang, Y.; Guo, Y. F.; Prabhakaran, D.; Schmidt, M.; Hussain, Z.; Mo, S.-K.; Felser, C.; Yan, B.; Chen, Y. L.
2016-01-01
Topological Weyl semimetals (TWSs) represent a novel state of topological quantum matter which not only possesses Weyl fermions (massless chiral particles that can be viewed as magnetic monopoles in momentum space) in the bulk and unique Fermi arcs generated by topological surface states, but also exhibits appealing physical properties such as extremely large magnetoresistance and ultra-high carrier mobility. Here, by performing angle-resolved photoemission spectroscopy (ARPES) on NbP and TaP, we directly observed their band structures with characteristic Fermi arcs of TWSs. Furthermore, by systematically investigating NbP, TaP and TaAs from the same transition metal monopnictide family, we discovered their Fermiology evolution with spin-orbit coupling (SOC) strength. Our experimental findings not only reveal the mechanism to realize and fine-tune the electronic structures of TWSs, but also provide a rich material base for exploring many exotic physical phenomena (for example, chiral magnetic effects, negative magnetoresistance, and the quantum anomalous Hall effect) and novel future applications.
NASA Astrophysics Data System (ADS)
Hwang, Sungyoul; Kim, Youngkeol; Kwon, Dokyeong; Char, Kookheon
There have been many studies to investigate the phase behavior of block copolymers (BCPs) in cylindrical confinement. In the nanometer scale 2D confinement, the phase behavior of BCPs is mainly dependent upon commensurability and interfacial interaction. However, most studies have focused only on the effects of commensurability on the microdomains of BCP. In this study, we employed organosilicate (OS) which has tunable surface energy upon adjusting curing temperature as interlayer to examine the phase behavior of BCPs as a function of interfacial energy. The OS interlayer was coated in the inner surface of anodized aluminum oxide (AAO) pores by template-wetting method and cured in a range of temperature to control the surface energy of the interlayer. Lamellae-forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) (SMA) in the melt was injected into the OS-coated AAO pores by capillary forces. With the detailed analysis, we note that the self-assembly of SMA within 2D confinement is competitively affected by both entropic and enthalpic effects as the contact interfacial energy is varied. Simply by controlling the curing temperature of the OS interlayer, various morphologies arising from both preferential and neutral wetting were identified.
Tracey, John; Miyazawa, Keisuke; Spijker, Peter; Miyata, Kazuki; Reischl, Bernhard; Canova, Filippo Federici; Rohl, Andrew L; Fukuma, Takeshi; Foster, Adam S
2016-10-14
Frequency modulation atomic force microscopy (FM-AFM) experiments were performed on the calcite (10[Formula: see text]4) surface in pure water, and a detailed analysis was made of the 2D images at a variety of frequency setpoints. We observed eight different contrast patterns that reproducibly appeared in different experiments and with different measurement parameters. We then performed systematic free energy calculations of the same system using atomistic molecular dynamics to obtain an effective force field for the tip-surface interaction. By using this force field in a virtual AFM simulation we found that each experimental contrast could be reproduced in our simulations by changing the setpoint, regardless of the experimental parameters. This approach offers a generic method for understanding the wide variety of contrast patterns seen on the calcite surface in water, and is generally applicable to AFM imaging in liquids. PMID:27609045
NASA Astrophysics Data System (ADS)
Mani, Prashant; Tyagi, Chandra Shekhar; Srivastav, Nishant
2016-03-01
In this paper the analytical solution of the 2D Poisson's equation for single gate Fully Depleted SOI (FDSOI) MOSFET's is derived by using a Green's function solution technique. The surface potential is calculated and the threshold voltage of the device is minimized for the low power consumption. Due to minimization of threshold voltage the short channel effect of device is suppressed and after observation we obtain the device is kink free. The structure and characteristics of SingleGate FDSOI MOSFET were matched by using MathCAD and silvaco respectively.
Rayleigh surface wave interaction with the 2D exciton Bose-Einstein condensate
Boev, M. V.; Kovalev, V. M.
2015-06-15
We describe the interaction of a Rayleigh surface acoustic wave (SAW) traveling on the semiconductor substrate with the excitonic gas in a double quantum well located on the substrate surface. We study the SAW attenuation and its velocity renormalization due to the coupling to excitons. Both the deformation potential and piezoelectric mechanisms of the SAW-exciton interaction are considered. We focus on the frequency and excitonic density dependences of the SAW absorption coefficient and velocity renormalization at temperatures both above and well below the critical temperature of Bose-Einstein condensation of the excitonic gas. We demonstrate that the SAW attenuation and velocity renormalization are strongly different below and above the critical temperature.
Surface-confined 2D polymerization of a brominated copper-tetraphenylporphyrin on Au(111).
Smykalla, Lars; Shukrynau, Pavel; Korb, Marcus; Lang, Heinrich; Hietschold, Michael
2015-03-01
A coupling-limited approach for the Ullmann reaction-like on-surface synthesis of a two-dimensional covalent organic network starting from a halogenated metallo-porphyrin is demonstrated. Copper-octabromo-tetraphenylporphyrin molecules can diffuse and self-assemble when adsorbed on the inert Au(111) surface. Splitting-off of bromine atoms bonded at the macrocyclic core of the porphyrin starts at room temperature after the deposition and is monitored by X-ray photoelectron spectroscopy for different annealing steps. Direct coupling between the reactive carbon sites of the molecules is, however, hindered by the molecular shape. This leads initially to an ordered non-covalently interconnected supramolecular structure. Further heating to 300 °C and an additional hydrogen dissociation step is required to link the molecular macrocycles via a phenyl group and form large ordered polymeric networks. This approach leads to a close-packed covalently bonded network of overall good quality. The structures are characterized using scanning tunneling microscopy. Different kinds of lattice defects and, furthermore, the impact of polymerization on the HOMO-LUMO gap are discussed. Density functional theory calculations corroborate the interpretations and give further insight into the adsorption of the debrominated molecule on the surface and the geometry and coupling reaction of the polymeric structure. PMID:25672486
Surface-confined 2D polymerization of a brominated copper-tetraphenylporphyrin on Au(111)
NASA Astrophysics Data System (ADS)
Smykalla, Lars; Shukrynau, Pavel; Korb, Marcus; Lang, Heinrich; Hietschold, Michael
2015-02-01
A coupling-limited approach for the Ullmann reaction-like on-surface synthesis of a two-dimensional covalent organic network starting from a halogenated metallo-porphyrin is demonstrated. Copper-octabromo-tetraphenylporphyrin molecules can diffuse and self-assemble when adsorbed on the inert Au(111) surface. Splitting-off of bromine atoms bonded at the macrocyclic core of the porphyrin starts at room temperature after the deposition and is monitored by X-ray photoelectron spectroscopy for different annealing steps. Direct coupling between the reactive carbon sites of the molecules is, however, hindered by the molecular shape. This leads initially to an ordered non-covalently interconnected supramolecular structure. Further heating to 300 °C and an additional hydrogen dissociation step is required to link the molecular macrocycles via a phenyl group and form large ordered polymeric networks. This approach leads to a close-packed covalently bonded network of overall good quality. The structures are characterized using scanning tunneling microscopy. Different kinds of lattice defects and, furthermore, the impact of polymerization on the HOMO-LUMO gap are discussed. Density functional theory calculations corroborate the interpretations and give further insight into the adsorption of the debrominated molecule on the surface and the geometry and coupling reaction of the polymeric structure.
Raj, S; Hashimoto, D; Matsui, H; Souma, S; Sato, T; Takahashi, T; Sarma, D D; Mahadevan, Priya; Oishi, S
2006-04-14
The electronic structure of the insulating sodium tungsten bronze, Na(0.025)WO(3), is investigated by high-resolution angle-resolved photoemission spectroscopy. We find that near-E(F) states are localized due to the strong disorder arising from random distribution of Na+ ions in the WO(3) lattice, which makes the system insulating. The temperature dependence of photoemission spectra provides direct evidence for polaron formation. The remnant Fermi surface of the insulator is found to be the replica of the real Fermi surface in the metallic system. PMID:16712121
Effect of Fermi surface nesting on resonant spin excitations in Ba{<_1-x}K{<_x}Fe{<_2}As{<_2}.
Castellan, J.-P.; Rosenkranz, S.; Goremychkin, E.A.; Chung, D.Y.; Todorov, I.S.; Kanatzidis, M.G.; Eremin, I.; Knolle, J.; Chubukov, A.V.; Maiti, s.; Norman, M.R.; Weber, F.; Claus, H.; Guidi, T.; Bewley, R.I.; Osborn, R.
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.
CNNEDGEPOT: CNN based edge detection of 2D near surface potential field data
NASA Astrophysics Data System (ADS)
Aydogan, D.
2012-09-01
All anomalies are important in the interpretation of gravity and magnetic data because they indicate some important structural features. One of the advantages of using gravity or magnetic data for searching contacts is to be detected buried structures whose signs could not be seen on the surface. In this paper, a general view of the cellular neural network (CNN) method with a large scale nonlinear circuit is presented focusing on its image processing applications. The proposed CNN model is used consecutively in order to extract body and body edges. The algorithm is a stochastic image processing method based on close neighborhood relationship of the cells and optimization of A, B and I matrices entitled as cloning template operators. Setting up a CNN (continues time cellular neural network (CTCNN) or discrete time cellular neural network (DTCNN)) for a particular task needs a proper selection of cloning templates which determine the dynamics of the method. The proposed algorithm is used for image enhancement and edge detection. The proposed method is applied on synthetic and field data generated for edge detection of near-surface geological bodies that mask each other in various depths and dimensions. The program named as CNNEDGEPOT is a set of functions written in MATLAB software. The GUI helps the user to easily change all the required CNN model parameters. A visual evaluation of the outputs due to DTCNN and CTCNN are carried out and the results are compared with each other. These examples demonstrate that in detecting the geological features the CNN model can be used for visual interpretation of near surface gravity or magnetic anomaly maps.
Application of 2D surface ERT to on-site wastewater treatment survey
NASA Astrophysics Data System (ADS)
Forquet, N.; French, H. K.
2012-05-01
Interest in the functioning of on-site wastewater treatment in rural areas has grown both among authorities and private companies in France and elsewhere in Europe. This is partly due to the enforcement of a new law that obliges communities to control on-site wastewater treatment systems. For extensive systems—mostly Vertical Flow Sand Filters (VFSF)—the introduction of this law revealed the absence of reliable methods to assess if a system was built according to recommendations and is operating well. The aim of this paper is to examine whether surface Electrical Resistivity Tomography (ERT) is a reliable method for mapping outline of filter dimensions and reveal clogging effects. Using forward modeling of synthetic models, we created sensitivity maps of ideal resistivity maps revealing that the method is well-suited to outline the horizontal extent of the filter but not necessarily its constitutive layers because the coarse gravel layer near the surface reduces the sensitivity to features below this layer. Hence whatever the geophysical signal is produced by clogging, this will be difficult to detect. The most appropriate inversion procedure, i.e. the L1-norm inversion, reveals the filter extent with an error less than the electrode spacing independent of noise levels. Finally, the procedure is illustrated for a real case ERT survey on a full scale VFSF. This study reveals that simple surface ERT measurements provide a good estimate of the filter area, but additional methods are required for more detailed vertical analysis including potential detection of clogging effects.
Volterra Series Approach for Nonlinear Aeroelastic Response of 2-D Lifting Surfaces
NASA Technical Reports Server (NTRS)
Silva, Walter A.; Marzocca, Piergiovanni; Librescu, Liviu
2001-01-01
The problem of the determination of the subcritical aeroelastic response and flutter instability of nonlinear two-dimensional lifting surfaces in an incompressible flow-field via Volterra series approach is addressed. The related aeroelastic governing equations are based upon the inclusion of structural nonlinearities, of the linear unsteady aerodynamics and consideration of an arbitrary time-dependent external pressure pulse. Unsteady aeroelastic nonlinear kernels are determined, and based on these, frequency and time histories of the subcritical aeroelastic response are obtained, and in this context the influence of geometric nonlinearities is emphasized. Conclusions and results displaying the implications of the considered effects are supplied.
NASA Astrophysics Data System (ADS)
Souma, S.; Wang, Zhiwei; Kotaka, H.; Sato, T.; Nakayama, K.; Tanaka, Y.; Kimizuka, H.; Takahashi, T.; Yamauchi, K.; Oguchi, T.; Segawa, Kouji; Ando, Yoichi
2016-04-01
We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on noncentrosymmetric Weyl semimetal candidate NbP, and determined the electronic states of both Nb- and P-terminated surfaces. We revealed a drastic difference in the Fermi-surface topology between two types of surfaces, whereas the Fermi arcs on both surfaces are likely terminated at the surface projection of the same bulk Weyl nodes. A comparison of the ARPES data with our first-principles band calculations suggests a notable difference in the electronic structure at the Nb-terminated surface between theory and experiment. The present result opens a platform for realizing exotic quantum phenomena arising from the unusual surface properties of Weyl semimetals.
NASA Astrophysics Data System (ADS)
Liu, Shuang; Hu, Xiangyun; Xi, Yufei; Liu, Tianyou
2015-03-01
The regular grid discretization is prevalent in the inverse modeling for gravity and magnetic data. However, this subdivision strategy performs lower precision to represent the rugged observation surface. To deal with this problem, we evaluate a non-structured discretization method in which the subsurface with rolling terrain is divided into numbers of Delaunay triangular cells and each mesh has the uniform physical property distributions. The gravity and magnetic anomalies of a complex-shaped anomalous body are represented as the summaries of the single anomaly produced by each triangle field source. When inverting for the potential field data, we specify a minimization objective function composed of data constraints and then use the preconditioned conjugate gradient algorithm to iteratively solve the matrix minimization equations, where the preconditioner is determined by the distances between triangular cells and surface observers. We test our method using synthetic data; all tests return favorable results. In the case studies involving the gravity and magnetic anomalies of the Mengku and Pobei deposits in Xinjiang, northwest China, the inferred magnetite orebodies and ultrabasic rocks distributions are verified by the additional drilling and geological information. The discretization of constrained Delaunay triangulation provides an useful approach of computing and inverting the potential field data on the situations of undulate topography and complicated objects.
Gudelli, Vijay Kumar Kanchana, V.
2014-04-24
Structural, elastic, electronic and Fermi surface studies of AuTe{sub 2} have been carried out by means of first principles calculations based on density functional theory. The calculated ground state properties agree well with the experiment. Fermi surface and elastic constants are predicted for the first time and from the calculated elastic constants we find the compound to be mechanically stable satisfying the stability criteria of monoclinic structure. In addition, we also find the c-axis to be more compressible than the other two which is also speculated from the present work. The metallic behaviour of this compound is confirmed from the electronic band structure calculation as we find the bands to cross the Fermi level (E{sub F}). In addition, we also observe a FS topology change under pressure which is also explained in the present work.
Fermi surface study of ScAu{sub 2}(Al, In) and ScPd{sub 2}(Sn, Pb) compounds
Reddy, P. V. Sreenivasa; Kanchana, V.; Vaitheeswaran, G.
2015-06-24
A detailed study on the electronic structure and Fermi surface (FS) of superconducting Heusler compounds ScAu{sub 2}(Al, In) and ScPd{sub 2}(Sn, Pb) has been carried out using first principles electronic structure calculations. The spin orbit coupling is found to play a major role in understanding the band structure and FS. Analysis of the data shows the importance of spin orbit coupling effect in the above compounds. The bands which cross Fermi level (EF) are found to be dominated by the Sc d{sub t2g}-states. The calculated total density of states are in good agreement with the experimentally reported value for ScPd{sub 2}Sn. Under compression we find a change in the Fermi surface topology of ScPd{sub 2}Sn at V/V{sub 0} = 0.95 (pressure of≈15 GPa), which is explained using the band structure calculations.
NASA Astrophysics Data System (ADS)
Baireuther, P.; Hutasoit, J. A.; Tworzydło, J.; Beenakker, C. W. J.
2016-04-01
We formulate a linear response theory of the chiral magnetic effect in a finite Weyl semimetal, expressing the electrical current density j induced by a slowly oscillating magnetic field B or chiral chemical potential μ in terms of the scattering matrix of Weyl fermions at the Fermi level. Surface conduction can be neglected in the infinite-system limit for δ j/δ μ , but not for δ j/δ B: the chirally circulating surface Fermi arcs give a comparable contribution to the bulk Weyl cones no matter how large the system is, because their smaller number is compensated by an increased flux sensitivity. The Fermi arc contribution to {μ }-1δ j/δ B has the universal value {(e/h)}2, protected by chirality against impurity scattering—unlike the bulk contribution of opposite sign.
Anomalous Fermi-Surface Dependent Pairing in a Self-Doped High-Tc Superconductor
Chen, Y.
2010-05-03
We report the discovery of a self-doped multi-layer high T{sub c} superconductor Ba{sub 2}Ca{sub 3}Cu{sub 4}O{sub 8}F{sub 2} (F0234) which contains distinctly different superconducting gap magnitudes along its two Fermi surface(FS) sheets. While formal valence counting would imply this material to be an undoped insulator, it is a self-doped superconductor with a T{sub c} of 60K, possessing simultaneously both electron- and hole-doped FS sheets. Intriguingly, the FS sheet characterized by the much larger gap is the electron-doped one, which has a shape disfavoring two electronic features considered to be important for the pairing mechanism: the van Hove singularity and the antiferromagnetic ({pi}/a, {pi}/a) scattering.
Fermi surface distortion induced by interaction between Rashba and Zeeman effects
Choi, Won Young; Koo, Hyun Cheol; Chang, Joonyeon; Kim, Hyung-jun; Lee, Kyung-Jin
2015-05-07
To evaluate Fermi surface distortion induced by interaction between Rashba and Zeeman effects, the channel resistance in an InAs quantum well layer is investigated with an in-plane magnetic field transverse to the current direction. In the magnetoresistance curve, the critical point occurs at ∼3.5 T, which is approximately half of the independently measured Rashba field. To get an insight into the correlation between the critical point in magnetoresistance curve and the Rashba strength, the channel conductivity is calculated using a two-dimensional free-electron model with relaxation time approximation. The critical point obtained from the model calculation is in agreement with the experiment, suggesting that the observation of critical point can be an alternative method to experimentally determine the Rashba parameter.
Fermi surface in local-density-functional theory and in gradient expansions
NASA Astrophysics Data System (ADS)
Mearns, Daniel; Kohn, Walter
1989-05-01
It has recently been shown that the Kohn-Sham (KS) equations, even with the exact exchange-correlation potential, vxc(r), in general do not yield the exact physical Fermi surface (FS). The latter may be obtained either from the discontinuities of the momentum distribution in the exact ground state or, equally well, from the locus of singularities in q space of the exact density-density response function, χ(q,q) (Kohn effect). The present paper considers approximations in which the exact exchange-correlation energy functional is replaced by a gradient expansion of arbitrary finite order m [e.g., Exc(2)[n] =Fd3 [exc(n(r))n(r)+gxc (n(r))||∇n(r)||2
NASA Astrophysics Data System (ADS)
Prentice, Joseph C. A.; Coldea, Amalia I.
2016-06-01
By solving the Boltzmann transport equation we investigate theoretically the general form of oscillations in the resistivity caused by varying the direction of an applied magnetic field for the case of quasi-two-dimensional systems on hexagonal lattices. The presence of the angular magnetoresistance oscillations can be used to map out the topology of the Fermi surface and we study how this effect varies as a function of the degree of interplane warping as well as a function of the degree of isotropic scattering. We find that the angular-dependent effect due to in-plane rotation follows the symmetry imposed by the lattice whereas for interplane rotation the degree of warping dictates the dominant features observed in simulations. Our calculations make predictions for specific angle-dependent magnetotransport signatures in magnetic fields expected for quasi-two-dimensional hexagonal compounds similar to PdCoO2 and PtCoO2.
Fermi Surface Evolution Across Multiple Charge Density Wave Transitions in ErTe3
Moore, R.G.; Brouet, V.; He, R.; Lu, D.H.; Ru, N.; Chu, J.-H.; Fisher, I.R.; Shen, Z.-X.; /SLAC, SSRL /Stanford U., Geballe Lab.
2010-02-15
The Fermi surface (FS) of ErTe{sub 3} is investigated using angle-resolved photoemission spectroscopy (ARPES). Low temperature measurements reveal two incommensurate charge density wave (CDW) gaps created by perpendicular FS nesting vectors. A large {Delta}{sub 1} = 175 meV gap arising from a CDW with c* - q{sub CDW1} {approx} 0.70(0)c* is in good agreement with the expected value. A second, smaller {Delta}{sub 2} = 50 meV gap is due to a second CDW with a* - q{sub CDW2} {approx} 0.68(5)a*. The temperature dependence of the FS, the two gaps and possible interaction between the CDWs are examined.
Band structure and fermi surface of Electron-Doped C{sub 60} Monolayers
Yang, W.L.; Brouet, V.; Zhou, X.J.; Choi, Hyoung J.; Louie, Steven G.; Cohen, Marvin L.; Kellar, S.A.; Bogdanov, P.V.; Lanzara, A.; Goldoni, A.; Parmigiani, F.; Hussain, Z.; Shen, Z-X.
2003-11-06
C60 fullerides are challenging systems because both the electron-phonon and electron-electron interactions are large on the energy scale of the expected narrow band width. We report angle-resolved photoemission data on the band dispersion for an alkali doped C60 monolayer and a detailed comparison with theory. Compared to the maximum bare theoretical band width of 170 meV, the observed 100-meV dispersion is within the range of renormalization by electron-phonon coupling. This dispersion is only a fraction of the integrated peak width, revealing the importance of many-body effects. Additionally, measurements on the Fermi surface indicate the robustness of the Luttinger theorem even for materials with strong interactions.
Bulk Fermi surface and electronic properties of Cu0.07Bi2Se3
NASA Astrophysics Data System (ADS)
Martin, C.; Craciun, V.; Miller, K. H.; Uzakbaiuly, B.; Buvaev, S.; Berger, H.; Hebard, A. F.; Tanner, D. B.
2013-05-01
The electronic properties of Cu0.07Bi2Se3 have been investigated using Shubnikov-de Haas and optical reflectance measurements. Quantum oscillations reveal a bulk, three-dimensional Fermi surface with anisotropy kFc/kFab≈ 2 and a modest increase in free-carrier concentration and in scattering rate with respect to the undoped Bi2Se3, also confirmed by reflectivity data. The effective mass is almost identical to that of Bi2Se3. Optical conductivity reveals a strong enhancement of the bound impurity bands with Cu addition, suggesting that a significant number of Cu atoms enter the interstitial sites between Bi and Se layers or may even substitute for Bi. This conclusion is also supported by x-ray diffraction measurements, where a significant increase of microstrain was found in Cu0.07Bi2Se3, compared to Bi2Se3.
Band structure and Fermi surface of electron-doped C60 monolayers.
Yang, W L; Brouet, V; Zhou, X J; Choi, Hyoung J; Louie, Steven G; Cohen, Marvin L; Kellar, S A; Bogdanov, P V; Lanzara, A; Goldoni, A; Parmigiani, F; Hussain, Z; Shen, Z-X
2003-04-11
C60 fullerides are challenging systems because both the electron-phonon and electron-electron interactions are large on the energy scale of the expected narrow band width. We report angle-resolved photoemission data on the band dispersion for an alkali-doped C60 monolayer and a detailed comparison with theory. Compared to the maximum bare theoretical band width of 170 meV, the observed 100-meV dispersion is within the range of renormalization by electron-phonon coupling. This dispersion is only a fraction of the integrated peak width, revealing the importance of many-body effects. Additionally, measurements on the Fermi surface indicate the robustness of the Luttinger theorem even for materials with strong interactions. PMID:12690192
Fermi surface topology and negative longitudinal magnetoresistance observed in the semimetal NbAs2
NASA Astrophysics Data System (ADS)
Shen, Bing; Deng, Xiaoyu; Kotliar, Gabriel; Ni, Ni
2016-05-01
We report transverse and longitudinal magnetotransport properties of NbAs2 single crystals. Attributing to the electron-hole compensation, nonsaturating large transverse magnetoresistance reaches up to 8000 at 9 T at 1.8 K with mobility around 1 to 2 m2V-1S-1 . We present a thorough study of angular-dependent Shubnikov-de Haas (SdH) quantum oscillations of NbAs2. Three distinct oscillation frequencies are identified. First-principles calculations reveal four types of Fermi-surface pockets: electron α pocket, hole β pocket, hole γ pocket, and small electron δ pocket. Although the angular dependence of α ,β , and δ agree well with the SdH data, γ pocket is missing in SdH. Negative longitudinal magnetoresistance is observed which may be linked to novel topological states in this material, although systematic study is necessary to ascertain its origin.
Magnetic-field- and temperature-dependent Fermi surface of CeBiPt
NASA Astrophysics Data System (ADS)
Wosnitza, J.; Goll, G.; Bianchi, A. D.; Bergk, B.; Kozlova, N.; Opahle, I.; Elgazzar, S.; Richter, Manuel; Stockert, O.; Löhneysen, H. v.; Yoshino, T.; Takabatake, T.
2006-09-01
The half-Heusler compounds CeBiPt and LaBiPt are semimetals with very low charge-carrier concentrations as evidenced by Shubnikov de Haas (SdH) and Hall-effect measurements. Neutron-scattering results reveal a simple antiferromagnetic structure in CeBiPt below TN = 1.15 K. The band structure of CeBiPt sensitively depends on temperature, magnetic field and stoichiometry. Above a certain, sample-dependent, threshold field (B>25 T), the SdH signal disappears and the Hall coefficient reduces significantly. These effects are absent in the non-4f compound LaBiPt. Electronic-band-structure calculations can well explain the observed behaviour by a 4f-polarization-induced Fermi-surface modification.
Predicting 2D geotechnical parameter fields in near-surface sedimentary environments
NASA Astrophysics Data System (ADS)
Rumpf, M.; Tronicke, J.
2014-02-01
For a detailed characterization of near-surface environments, geophysical techniques are increasingly used to support more conventional point-based techniques such as borehole and direct-push logging. Because the underlying parameter relations are often complex, site-specific, or even poorly understood, a remaining challenging task is to link the geophysical parameter models to the actual geotechnical target parameters measured only at selected points. We propose a workflow based on nonparametric regression to establish functional relationships between jointly inverted geophysical parameters and selected geotechnical parameters as measured, for example, by different borehole and direct-push tools. To illustrate our workflow, we present field data collected to characterize a near-surface sedimentary environment. Our field data base includes crosshole ground penetrating radar (GPR), seismic P-, and S-wave data sets collected between 25 m deep boreholes penetrating sand- and gravel dominated sediments. Furthermore, different typical borehole and direct-push logs are available. We perform a global joint inversion of traveltimes extracted from the crosshole geophysical data using a recently proposed approach based on particle swarm optimization. Our inversion strategy allows for generating consistent models of GPR, P-wave, and S-wave velocities including an appraisal of uncertainties. We analyze the observed complex relationships between geophysical velocities and target parameter logs using the alternating conditional expectation (ACE) algorithm. This nonparametric statistical tool allows us to perform multivariate regression analysis without assuming a specific functional relation between the variables. We are able to explain selected target parameters such as characteristic grain size values or natural gamma activity by our inverted geophysical data and to extrapolate these parameters to the inter-borehole plane covered by our crosshole experiments. We conclude that
Renosh, P. R.; Schmitt, Francois G.; Loisel, Hubert
2015-01-01
Satellite remote sensing observations allow the ocean surface to be sampled synoptically over large spatio-temporal scales. The images provided from visible and thermal infrared satellite observations are widely used in physical, biological, and ecological oceanography. The present work proposes a method to understand the multi-scaling properties of satellite products such as the Chlorophyll-a (Chl-a), and the Sea Surface Temperature (SST), rarely studied. The specific objectives of this study are to show how the small scale heterogeneities of satellite images can be characterised using tools borrowed from the fields of turbulence. For that purpose, we show how the structure function, which is classically used in the frame of scaling time series analysis, can be used also in 2D. The main advantage of this method is that it can be applied to process images which have missing data. Based on both simulated and real images, we demonstrate that coarse-graining (CG) of a gradient modulus transform of the original image does not provide correct scaling exponents. We show, using a fractional Brownian simulation in 2D, that the structure function (SF) can be used with randomly sampled couple of points, and verify that 1 million of couple of points provides enough statistics. PMID:26017551
NASA Astrophysics Data System (ADS)
Kalisperakis, I.; Stentoumis, Ch.; Grammatikopoulos, L.; Karantzalos, K.
2015-08-01
The indirect estimation of leaf area index (LAI) in large spatial scales is crucial for several environmental and agricultural applications. To this end, in this paper, we compare and evaluate LAI estimation in vineyards from different UAV imaging datasets. In particular, canopy levels were estimated from i.e., (i) hyperspectral data, (ii) 2D RGB orthophotomosaics and (iii) 3D crop surface models. The computed canopy levels have been used to establish relationships with the measured LAI (ground truth) from several vines in Nemea, Greece. The overall evaluation indicated that the estimated canopy levels were correlated (r2 > 73%) with the in-situ, ground truth LAI measurements. As expected the lowest correlations were derived from the calculated greenness levels from the 2D RGB orthomosaics. The highest correlation rates were established with the hyperspectral canopy greenness and the 3D canopy surface models. For the later the accurate detection of canopy, soil and other materials in between the vine rows is required. All approaches tend to overestimate LAI in cases with sparse, weak, unhealthy plants and canopy.
Renosh, P R; Schmitt, Francois G; Loisel, Hubert
2015-01-01
Satellite remote sensing observations allow the ocean surface to be sampled synoptically over large spatio-temporal scales. The images provided from visible and thermal infrared satellite observations are widely used in physical, biological, and ecological oceanography. The present work proposes a method to understand the multi-scaling properties of satellite products such as the Chlorophyll-a (Chl-a), and the Sea Surface Temperature (SST), rarely studied. The specific objectives of this study are to show how the small scale heterogeneities of satellite images can be characterised using tools borrowed from the fields of turbulence. For that purpose, we show how the structure function, which is classically used in the frame of scaling time series analysis, can be used also in 2D. The main advantage of this method is that it can be applied to process images which have missing data. Based on both simulated and real images, we demonstrate that coarse-graining (CG) of a gradient modulus transform of the original image does not provide correct scaling exponents. We show, using a fractional Brownian simulation in 2D, that the structure function (SF) can be used with randomly sampled couple of points, and verify that 1 million of couple of points provides enough statistics. PMID:26017551
NASA Astrophysics Data System (ADS)
Chaabouni, H.; Minissale, M.; Manicò, G.; Congiu, E.; Noble, J. A.; Baouche, S.; Accolla, M.; Lemaire, J. L.; Pirronello, V.; Dulieu, F.
2012-12-01
The formation of the first monolayer of water molecules on bare dust grains is of primary importance to understand the growth of the icy mantles that cover dust in the interstellar medium. In this work, we explore experimentally the formation of water molecules from O2 + D reaction on bare silicate surfaces that simulates the grains present in the diffuse interstellar clouds at visual extinctions (AV < 3 mag). For comparison, we also study the formation of water molecules on surfaces covered with amorphous water ice representing the dense clouds (AV ⩾ 3 mag). Our studies focus on the formation of water molecules in the sub-monolayer and monolayer regimes using reflection absorption infrared spectroscopy and temperature-programmed desorption techniques. We provide the fractions of the products, such as D2O and D2O2 molecules formed on three astrophysically relevant surfaces held at 10 K (amorphous olivine-type silicate, porous amorphous water ice, and nonporous amorphous water ice). Our results showed that the formation of D2O molecules occurs with an efficiency of about 55%-60% on nonporous amorphous water ice and about 18% on bare silicate grains surfaces. We explain the low efficiency of D2O water formation on the silicate surfaces by the desorption upon formation of certain products once the reaction occurs between O2 and D atoms on the surface. A kinetic model taking into account the chemical desorption of newly formed water supports our conclusions.
NASA Astrophysics Data System (ADS)
Calvo-Velasco, D. M.; Porras-Montenegro, N.
2016-04-01
Using the revised plane wave method, we calculated the photonic band structure (PBS) considering TE polarization of a square 2D photonic crystal made of rectangular metallic rods embedded in air. In case of square rods and comparing different plasma frequencies, we found a characteristic band distribution related with the existence of localized plasmons on the rod surfaces, and also we found that this type of rod shape contributes to a high concentration of the electromagnetic field close to the rod corners. Considering rectangular rods and varying one of the sides of the rods, we found a PBS that presents a reorganization of the bands in comparing with the low dispersion present in the square rod case, related with a high localization of the radiation on the rod surfaces.
Study of Unsteady Flow Actuation Produced by Surface Plasma Actuator on 2-D Airfoil
NASA Astrophysics Data System (ADS)
Phan, Minh Khang; Shin, Jichul
2014-10-01
Effect of flow actuation driven by low current continuous or pulsed DC surface glow discharge plasma actuator is studied. Schlieren image of induced flow on flat plate taken at a high repetition rate reveals that the actuation is mostly initiated near the cathode. Assuming that the actuation is mostly achieved by ions in the cathode sheath region, numerical model for the source of flow actuation is obtained by analytical estimation of ion pressure force created in DC plasma sheath near the cathode and added in momentum equation as a body force term. Modeled plasma flow actuator is simulated with NACA0012 airfoil oscillating over a certain range of angle of attack (AoA) at specific reduced frequencies of airfoil. By changing actuation authority according to the change in AoA, stabilization of unsteady flow field is improved and hence steady aerodynamic performance can be maintained. Computational result shows that plasma actuation is only effective in modifying aerodynamic characteristics of separated flow. It turns out that plasma pulse frequency should be tuned for optimal performance depending on phase angle and rotating speed. The actuation authority can be parameterized by a ratio between plasma pulse frequency and reduced frequency.
Two-dimensional Fermi surfaces in Kondo insulating SmB6
NASA Astrophysics Data System (ADS)
Li, Gang
There has been renewed interest in Samarium Hexaboride, which is a strongly correlated heavy Fermion material. Hybridization between itinerant electrons and localized orbitals lead to an opening of charge gap at low temperature. However, the resistivity of SmB6 does not diverge at low temperature. Former studies suggested that this residual conductance is contributed by various origins. Recent theoretical developments suggest that the particular symmetry of energy bands of SmB6 may host a topologically non-trivial surface state, i.e., a topological Kondo insulator. To probe the Fermiology of the possible metallic surface state, we use sensitive torque magnetometry to detect the de Haas van Alphen (dHvA) effect due to Landau level quantization on flux-grown crystals, down to He-3 temperature and up to 45 Tesla. Our angular and temperature dependent data suggest two-dimensional Fermi Surfaces lie in both crystalline (001) and (101) surface planes of SmB6.
Simonds, Brian J.; Kheraj, Vipul; Palekis, Vasilios; Ferekides, Christos; Scarpulla, Michael A.
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. 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.
Large Exciton Energy Shifts by Reversible Surface Exchange in 2D II-VI Nanocrystals.
Zhou, Yang; Wang, Fudong; Buhro, William E
2015-12-01
Reaction of n-octylamine-passivated {CdSe[n-octylamine](0.53±0.06)} quantum belts with anhydrous metal carboxylates M(oleate)2 (M = Cd, Zn) results in a rapid exchange of the L-type amine passivation for Z-type M(oleate)2 passivation. The cadmium-carboxylate derivative is determined to have the composition {CdSe[Cd(oleate)2](0.19±0.02)}. The morphologies and crystal structures of the quantum belts are largely unaffected by the exchange processes. Addition of n-octylamine or oleylamine to the M(oleate)2-passivated quantum belts removes M(oleate)2 and restores the L-type amine passivation. Analogous, reversible surface exchanges are also demonstrated for CdS quantum platelets. The absorption and emission spectra of the quantum belts and platelets are reversibly shifted to lower energy by M(oleate)2 passivation vs amine passivation. The largest shift of 140 meV is observed for the Cd(oleate)2-passivated CdSe quantum belts. These shifts are attributed entirely to changes in the strain states in the Zn(oleate)2-passivated nanocrystals, whereas changes in strain states and confinement dimensions contribute roughly equally to the shifts in the Cd(oleate)2-passivated nanocrystals. Addition of Cd(oleate)2, which electronically couples to the nanocrystal lattices, increases the effective thickness of the belts and platelets by approximately a half of a monolayer, thus increasing the confinement dimension. PMID:26568026
Dynamic Linkages Between the Transition Zone & Surface Plate Motions in 2D Models of Subduction
NASA Astrophysics Data System (ADS)
Arredondo, K.; Billen, M. I.
2013-12-01
feedback to other added processes remain important, which could encourage mineralogical research into multiphase systems. Feedback from the compositionally complex slab to the dynamic trench may improve understanding on the mechanics of slab behavior in the upper and lower mantle and surface behavior of the subducting and overriding plates. Běhounková, M., and H. Cízková, Long-wavelength character of subducted slabs in the lower mantle, Earth and Planetary Science Letters, 275, 43-53, 2008. Fukao, Y., M. Obayashi, T. Nakakuki, and the Deep Slab Project Group, Stagnant slab: A review, Annual Reviews of Earth and Planetary Science, 37, 19-46, 2009. Ricard, Y., E. Mattern, and J. Matas, Synthetic tomographic images of slabs from mineral physics, in Earth's Deep Mantle: Structure, Composition, and Evolution, Geophysical Monograph Series, vol. 160, American Geophysical Union, 2005.
Lee, Eunsook; Kim, D. H.; Kim, Hyun Woo; Denlinger, J. D.; Kim, Heejung; Kim, Junwon; Kim, Kyoo; Min, B. I.; Min, B. H.; Kwon, Y. S.; Kang, J.-S.
2016-01-01
The electronic structure of a charge density wave (CDW) system PrTe3 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 PrTe3 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 kz, demonstrating the nearly 2D character for the near-EF states. The observed linear dichroism reveals the in-plane orbital character of the near-EF Te 5p states. PMID:27453329
NASA Astrophysics Data System (ADS)
Lee, Eunsook; Kim, D. H.; Kim, Hyun Woo; Denlinger, J. D.; Kim, Heejung; Kim, Junwon; Kim, Kyoo; Min, B. I.; Min, B. H.; Kwon, Y. S.; Kang, J.-S.
2016-07-01
The electronic structure of a charge density wave (CDW) system PrTe3 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 PrTe3 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 kz, demonstrating the nearly 2D character for the near-EF states. The observed linear dichroism reveals the in-plane orbital character of the near-EF Te 5p states.