Time-evolution of electronic states in a Rashba anisotropic two-dimensional quantum dot
NASA Astrophysics Data System (ADS)
Amiri, F.; Shirkani, H.; Golshan, M. M.
2011-10-01
In this article we present the time evolution of the electronic spin and subbands states, of an electron in an anisotropic two dimensional Rashba quantum dot, to which a magnetic field of arbitrary strength is applied. We also explicitly include the confining (gate) effects as a two dimensional anisotropic harmonic oscillator. From the governing Hamiltonian we compute the time evolution of the initial state, leading to spin and subbands averages as functions of time. Our results indicate that the spin, on the average, precesses about the magnetic field, on an ellipse with intrinsic wobbling. The subbands populations, similar to the case of atom-photon interaction, follow the pattern of collapse-revivals.
Large Tunable Rashba Spin Splitting of a Two-Dimensional Electron Gas in Bi2Se3
NASA Astrophysics Data System (ADS)
King, P. D. C.; Hatch, R. C.; Bianchi, M.; Ovsyannikov, R.; Lupulescu, C.; Landolt, G.; Slomski, B.; Dil, J. H.; Guan, D.; Mi, J. L.; Rienks, E. D. L.; Fink, J.; Lindblad, A.; Svensson, S.; Bao, S.; Balakrishnan, G.; Iversen, B. B.; Osterwalder, J.; Eberhardt, W.; Baumberger, F.; Hofmann, Ph.
2011-08-01
We report a Rashba spin splitting of a two-dimensional electron gas in the topological insulator Bi2Se3 from angle-resolved photoemission spectroscopy. We further demonstrate its electrostatic control, and show that spin splittings can be achieved which are at least an order-of-magnitude larger than in other semiconductors. Together these results show promise for the miniaturization of spintronic devices to the nanoscale and their operation at room temperature.
Chiral-like tunneling of electrons in two-dimensional semiconductors with Rashba spin-orbit coupling
Ang, Yee Sin; Ma, Zhongshui; Zhang, C.
2014-01-01
The unusual tunneling effects of massless chiral fermions (mCF) and massive chiral fermions (MCF) in a single layer graphene and bilayer graphene represent some of the most bizarre quantum transport phenomena in condensed matter system. Here we show that in a two-dimensional semiconductor with Rashba spin-orbit coupling (R2DEG), the real-spin chiral-like tunneling of electrons at normal incidence simultaneously exhibits features of mCF and MCF. The parabolic branch of opposite spin in R2DEG crosses at a Dirac-like point and has a band turning point. These features generate transport properties not found in usual two-dimensional electron gas. Albeit its π Berry phase, electron backscattering is present in R2DEG. An electron mimics mCF if its energy is in the vicinity of the subband crossing point or it mimics MCF if its energy is near the subband minima. PMID:24445394
Perpendicular magnetic anisotropy of two-dimensional Rashba ferromagnets
NASA Astrophysics Data System (ADS)
Kim, Kyoung-Whan; Lee, Kyung-Jin; Lee, Hyun-Woo; Stiles, M. D.
2016-11-01
We compute the magnetocrystalline anisotropy energy within two-dimensional Rashba models. For a ferromagnetic free-electron Rashba model, the magnetic anisotropy is exactly zero regardless of the strength of the Rashba coupling, unless only the lowest band is occupied. For this latter case, the model predicts in-plane anisotropy. For a more realistic Rashba model with finite band width, the magnetic anisotropy evolves from in-plane to perpendicular and back to in-plane as bands are progressively filled. This evolution agrees with first-principles calculations on the interfacial anisotropy, suggesting that the Rashba model captures energetics leading to anisotropy originating from the interface provided that the model takes account of the finite Brillouin zone. The results show that the electron density modulation by doping or an external voltage is more important for voltage-controlled magnetic anisotropy than the modulation of the Rashba parameter.
Biswas, Tutul; Ghosh, Tarun Kanti
2013-07-03
We theoretically study the phonon-drag contribution to the thermoelectric power and the hot-electron energy-loss rate in a Rashba spin-orbit coupled two-dimensional electron system in the Bloch-Gruneisen (BG) regime. We assume that electrons interact with longitudinal acoustic phonons through a deformation potential and with both longitudinal and transverse acoustic phonons through a piezoelectric potential. The effect of the Rashba spin-orbit interaction on the magnitude and temperature dependence of the phonon-drag thermoelectric power and hot-electron energy-loss rate is discussed. We numerically extract the exponent of temperature dependence of the phonon-drag thermopower and the energy-loss rate. We find that the exponents are suppressed due to the presence of the Rashba spin-orbit coupling.
NASA Astrophysics Data System (ADS)
Yudin, Dmitry; Shelykh, Ivan A.
2016-10-01
A nonperturbative interaction of an electronic system with a laser field can substantially modify its physical properties. In particular, in two-dimensional (2D) materials with a lack of inversion symmetry, the achievement of a regime of strong light-matter coupling allows direct optical tuning of the strength of the Rashba spin-orbit interaction (SOI). Capitalizing on these results, we build a theory of the dynamical conductivity of a 2D electron gas with both Rashba and Dresselhaus SOIs coupled to an off-resonant high-frequency electromagnetic wave. We argue that strong light-matter coupling modifies qualitatively the dispersion of the electrons and can be used as a powerful tool to probe and manipulate the coupling strengths and adjust the frequency range where optical conductivity is essentially nonzero.
2014-09-26
linear electronic specific heat disappears in strong magnetic fields if Landau levels are not broadened. Thus, the amplitude of the magnetothermal...Molec. Crys. Liq. Crys. 121, 169 (1984). In consideration of mixing of low-lying Landau levels, the magneto- conductance of two-dimensional electrons...and narrowing can be explained when the Landau level filling factor v is larger than 1. Actually, we have shown that the resonance phenomena are
Eremeev, Sergey V; Tsirkin, Stepan S; Nechaev, Ilya A; Echenique, Pedro M; Chulkov, Evgueni V
2015-08-04
Intriguing phenomena and novel physics predicted for two-dimensional (2D) systems formed by electrons in Dirac or Rashba states motivate an active search for new materials or combinations of the already revealed ones. Being very promising ingredients in themselves, interplaying Dirac and Rashba systems can provide a base for next generation of spintronics devices, to a considerable extent, by mixing their striking properties or by improving technically significant characteristics of each other. Here, we demonstrate that in BiTeI@PbSb2Te4 composed of a BiTeI trilayer on top of the topological insulator (TI) PbSb2Te4 weakly- and strongly-coupled Dirac-Rashba hybrid systems are realized. The coupling strength depends on both interface hexagonal stacking and trilayer-stacking order. The weakly-coupled system can serve as a prototype to examine, e.g., plasmonic excitations, frictional drag, spin-polarized transport, and charge-spin separation effect in multilayer helical metals. In the strongly-coupled regime, within ~100 meV energy interval of the bulk TI projected bandgap a helical state substituting for the TI surface state appears. This new state is characterized by a larger momentum, similar velocity, and strong localization within BiTeI. We anticipate that our findings pave the way for designing a new type of spintronics devices based on Rashba-Dirac coupled systems.
Eremeev, Sergey V.; Tsirkin, Stepan S.; Nechaev, Ilya A.; Echenique, Pedro M.; Chulkov, Evgueni V.
2015-01-01
Intriguing phenomena and novel physics predicted for two-dimensional (2D) systems formed by electrons in Dirac or Rashba states motivate an active search for new materials or combinations of the already revealed ones. Being very promising ingredients in themselves, interplaying Dirac and Rashba systems can provide a base for next generation of spintronics devices, to a considerable extent, by mixing their striking properties or by improving technically significant characteristics of each other. Here, we demonstrate that in BiTeI@PbSb2Te4 composed of a BiTeI trilayer on top of the topological insulator (TI) PbSb2Te4 weakly- and strongly-coupled Dirac-Rashba hybrid systems are realized. The coupling strength depends on both interface hexagonal stacking and trilayer-stacking order. The weakly-coupled system can serve as a prototype to examine, e.g., plasmonic excitations, frictional drag, spin-polarized transport, and charge-spin separation effect in multilayer helical metals. In the strongly-coupled regime, within ~100 meV energy interval of the bulk TI projected bandgap a helical state substituting for the TI surface state appears. This new state is characterized by a larger momentum, similar velocity, and strong localization within BiTeI. We anticipate that our findings pave the way for designing a new type of spintronics devices based on Rashba-Dirac coupled systems. PMID:26239268
NASA Astrophysics Data System (ADS)
Dolcini, Fabrizio
2017-02-01
The effects of Rashba interaction and electromagnetic field on the edge states of a two-dimensional topological insulator are investigated in a nonperturbative way. We show that the electron dynamics is equivalent to a problem of massless Dirac fermions propagating with an inhomogeneous velocity, enhanced by the Rashba profile with respect to the bare Fermi value vF. Despite the inelastic and time-reversal breaking processes induced by the electromagnetic field, no backscattering occurs without interaction. The photoexcited electron densities are explicitly obtained in terms of the electric field and the Rashba interaction, and are shown to fulfill generalized chiral anomaly equations. The case of a Gaussian electromagnetic pulse is analyzed in detail. When the photoexcitation occurs far from the Rashba region, the latter effectively acts as a "superluminal gate" boosting the photoexcited wave packet outside the light-cone determined by vF. In contrast, for an electric pulse overlapping the Rashba region, the emerging wave packets are squeezed in a manner that depends on the overlap area. The electron-electron interaction effects are also discussed, for both intraspin and interspin density-density coupling. The results suggest that Rashba interaction, often considered as an unwanted disorder effect, may be exploited to tailor the shape and the propagation time of photoexcited spin-polarized wave packets.
Quantum ratchet in two-dimensional semiconductors with Rashba spin-orbit interaction
Ang, Yee Sin; Ma, Zhongshui; Zhang, Chao
2015-01-01
Ratchet is a device that produces direct current of particles when driven by an unbiased force. We demonstrate a simple scattering quantum ratchet based on an asymmetrical quantum tunneling effect in two-dimensional electron gas with Rashba spin-orbit interaction (R2DEG). We consider the tunneling of electrons across a square potential barrier sandwiched by interface scattering potentials of unequal strengths on its either sides. It is found that while the intra-spin tunneling probabilities remain unchanged, the inter-spin-subband tunneling probabilities of electrons crossing the barrier in one direction is unequal to that of the opposite direction. Hence, when the system is driven by an unbiased periodic force, a directional flow of electron current is generated. The scattering quantum ratchet in R2DEG is conceptually simple and is capable of converting a.c. driving force into a rectified current without the need of additional symmetry breaking mechanism or external magnetic field. PMID:25598490
Zhang, Shou-Juan; Ji, Wei-Xiao; Zhang, Chang-Wen; Li, Ping; Wang, Pei-Ji
2017-04-03
The coexistence of nontrivial topology and giant Rashba splitting, however, has rare been observed in two-dimensional (2D) films, limiting severely its potential applications at room temperature. Here, we through first-principles calculations to propose a series of inversion-asymmetric group-IV films, ABZ2 (A ≠ B = Si, Ge, Sn, Pb; Z = F, Cl, Br), whose stability are confirmed by phonon spectrum calculations. The analyses of electronic structures reveal that they are intrinsic 2D TIs with a bulk gap as large as 0.74 eV, except for GeSiF2, SnSiCl2, GeSiCl2 and GeSiBr2 monolayers which can transform from normal to topological phases under appropriate tensile strain of 4, 4, 5, and 4%, respectively. The nontrivial topology is identified by Z2 topological invariant together with helical edge states, as well as the berry curvature of these systems. Another prominent intriguing feature is the giant Rashba spin splitting with a magnitude reaching 0.15 eV, the largest value reported in 2D films so far. The tunability of Rashba SOC and band topology can be realized through achievable compressive/tensile strains (-4 ~ 6%). Also, the BaTe semiconductor is an ideal substrate for growing ABZ2 films without destroying their nontrivial topology.
Zhang, Shou-juan; Ji, Wei-xiao; Zhang, Chang-wen; Li, Ping; Wang, Pei-ji
2017-01-01
The coexistence of nontrivial topology and giant Rashba splitting, however, has rare been observed in two-dimensional (2D) films, limiting severely its potential applications at room temperature. Here, we through first-principles calculations to propose a series of inversion-asymmetric group-IV films, ABZ2 (A ≠ B = Si, Ge, Sn, Pb; Z = F, Cl, Br), whose stability are confirmed by phonon spectrum calculations. The analyses of electronic structures reveal that they are intrinsic 2D TIs with a bulk gap as large as 0.74 eV, except for GeSiF2, SnSiCl2, GeSiCl2 and GeSiBr2 monolayers which can transform from normal to topological phases under appropriate tensile strain of 4, 4, 5, and 4%, respectively. The nontrivial topology is identified by Z2 topological invariant together with helical edge states, as well as the berry curvature of these systems. Another prominent intriguing feature is the giant Rashba spin splitting with a magnitude reaching 0.15 eV, the largest value reported in 2D films so far. The tunability of Rashba SOC and band topology can be realized through achievable compressive/tensile strains (−4 ~ 6%). Also, the BaTe semiconductor is an ideal substrate for growing ABZ2 films without destroying their nontrivial topology. PMID:28368035
Photocontrol of Dirac electrons in a bulk Rashba semiconductor
NASA Astrophysics Data System (ADS)
Ogawa, N.; Bahramy, M. S.; Kaneko, Y.; Tokura, Y.
2014-09-01
We demonstrate the generation of circularly polarized light induced current of bulk Dirac electrons at room temperature by exploiting a giant Rashba effect in a bulk semiconductor. The photocurrent is spin polarized due to the spin-momentum locking of the electronic states, which is manifested by a sign reversal upon flipping either the photon helicity or the sign of the Rashba parameter, without any stray current. The action spectra revealed the photon energy range, where the photocurrent is carried by the Dirac electrons at the inner Fermi surface. This photogalvanic control is enabled by the sizable spin splittings both at the valence and conduction bands with the same helicity, and also by a number of optical transition pathways compared to those in the two-dimensional Rashba systems. An efficient coupling between the photon field and large spin-orbit interaction is accordingly proposed to allow the universal control of Dirac electrons.
Unconventional dc Transport in Rashba Electron Gases.
Brosco, Valentina; Benfatto, Lara; Cappelluti, Emmanuele; Grimaldi, Claudio
2016-04-22
We discuss the transport properties of a disordered two-dimensional electron gas with strong Rashba spin-orbit coupling. We show that in the high-density regime where the Fermi energy overcomes the energy associated with spin-orbit coupling, dc transport is accurately described by a standard Drude's law, due to a nontrivial compensation between the suppression of backscattering and the relativistic correction to the quasiparticle velocity. On the contrary, when the system enters the opposite dominant spin-orbit regime, Drude's paradigm breaks down and the dc conductivity becomes strongly sensitive to the spin-orbit coupling strength, providing a suitable tool to test the entanglement between spin and charge degrees of freedom in these systems.
Theory of Hall effect in two-dimensional giant Rashba systems
NASA Astrophysics Data System (ADS)
Suzuura, Hidekatsu; Ando, Tsuneya
2016-07-01
The weak-field Hall conductivity of disordered two-dimensional systems with strong Rashba spin-orbit interaction is studied in a self-consistent Born approximation. Explicit numerical results are obtained for scatterers with a Gaussian potential and for charged impurities. The singular behavior associated with a conelike crossing band appears only in the case of scatterers with a long-range Gaussian potential, which do not cause mixing with the outer band. In the case of more realistic scatterers such as charged impurities, the singularity is completely removed except the presence of a weak steplike feature. The Hall conductivity associated with the spin-Zeeman energy is also strongly reduced by interband mixing and generally remains much smaller than the orbital contribution.
Spin Hall conductivity in the impure two-dimensional Rashba s-wave superconductor
NASA Astrophysics Data System (ADS)
Biderang, M.; Yavari, H.
2016-06-01
Based on the Kubo formula approach, the spin Hall conductivity (SHC) of a two-dimensional (2D) Rashba s-wave superconductor in the presence of nonmagnetic impurities is calculated. We will show that by increasing the superconducting gap, the SHC decreases monotonically to zero, while by decreasing the concentration of impurities at zero gap, the SHC closes to the clean limit universal value - e/8 π. As a function of the impurity relaxation rate τ at Tc = 0.1 and γ = 0.01 (γ is the spin-orbit coupling in unit of eV · m), we will show that in the dirty limit (τ → 0) the SHC vanishes, and by increasing the relaxation time (τ → ∞) the SHC depends on the value of superconducting gap (Δ = 1.76Tc√{ 1 -T/Tc }), is changed from zero for full gap to -e/8 π in zero gap. At low temperatures, the SHC goes to zero exponentially and near the critical temperature depending on the concentration of the scattering centers, the SHC will tend to the value of normal state. We will also show that the SHC is independent of spin-orbit coupling (γ) in the clean limit.
Electronics based on two-dimensional materials.
Fiori, Gianluca; Bonaccorso, Francesco; Iannaccone, Giuseppe; Palacios, Tomás; Neumaier, Daniel; Seabaugh, Alan; Banerjee, Sanjay K; Colombo, Luigi
2014-10-01
The compelling demand for higher performance and lower power consumption in electronic systems is the main driving force of the electronics industry's quest for devices and/or architectures based on new materials. Here, we provide a review of electronic devices based on two-dimensional materials, outlining their potential as a technological option beyond scaled complementary metal-oxide-semiconductor switches. We focus on the performance limits and advantages of these materials and associated technologies, when exploited for both digital and analog applications, focusing on the main figures of merit needed to meet industry requirements. We also discuss the use of two-dimensional materials as an enabling factor for flexible electronics and provide our perspectives on future developments.
Two-Dimensional Superconductor with a Giant Rashba Effect: One-Atom-Layer Tl-Pb Compound on Si(111)
NASA Astrophysics Data System (ADS)
Matetskiy, A. V.; Ichinokura, S.; Bondarenko, L. V.; Tupchaya, A. Y.; Gruznev, D. V.; Zotov, A. V.; Saranin, A. A.; Hobara, R.; Takayama, A.; Hasegawa, S.
2015-10-01
A one-atom-layer compound made of one monolayer of Tl and one-third monolayer of Pb on a Si(111) surface having √{3 }×√{3 } periodicity was found to exhibit a giant Rashba-type spin splitting of metallic surface-state bands together with two-dimensional superconducting transport properties. Temperature-dependent angle-resolved photoelectron spectroscopy revealed an enhanced electron-phonon coupling for one of the spin-split bands. In situ micro-four-point-probe conductivity measurements with and without magnetic field demonstrated that the (Tl, Pb)/Si(111) system transformed into the superconducting state at 2.25 K, followed by the Berezinskii-Kosterlitz-Thouless mechanism. The 2D Tl-Pb compound on Si(111) is believed to be the prototypical object for prospective studies of intriguing properties of the superconducting 2D system with lifted spin degeneracy, bearing in mind that its composition, atomic and electron band structures, and spin texture are already well established.
Kirigami for Two-Dimensional Electronic Membranes
NASA Astrophysics Data System (ADS)
Qi, Zenan; Bahamon, Dario; Campbell, David; Park, Harold
2015-03-01
Two-dimensional materials have recently drawn tremendous attention because of their unique properties. In this work, we introduce the notion of two-dimensional kirigami, where concepts that have been used almost exclusively for macroscale structures are applied to dramatically enhance their stretchability. Specifically, we show using classical molecular dynamics simulations that the yield and fracture strains of graphene and MoS2 can be enhanced by about a factor of three using kirigami as compared to standard monolayers. Finally, using graphene as an example, we demonstrate that the kirigami structure may open up interesting opportunities in coupling to the electronic behavior of 2D materials. Authors acknowledge Mechanical Engineering and Physics departments at Boston University, and Mackgrafe at Mackenzie Presbyterian University.
Interplay between Rashba spin-orbit coupling and adiabatic rotation in a two-dimensional Fermi gas
NASA Astrophysics Data System (ADS)
Doko, E.; Subaşı, A. L.; Iskin, M.
2017-01-01
We explore the trap profiles of a two-dimensional atomic Fermi gas in the presence of a Rashba spin-orbit coupling and under an adiabatic rotation. We first consider a noninteracting gas and show that the competition between the effects of Rashba coupling on the local density of single-particle states and the Coriolis effects caused by rotation gives rise to a characteristic ring-shaped density profile that survives at experimentally accessible temperatures. Furthermore, Rashba splitting of the Landau levels gives the density profiles a ziggurat shape in the rapid-rotation limit. We then consider an interacting gas under the BCS mean-field approximation for local pairing, and study the pair-breaking mechanism that is induced by the Coriolis effects on superfluidity, where we calculate the critical rotation frequencies both for the onset of pair breaking and for the complete destruction of superfluidity in the system. In particular, by comparing the results of a fully-quantum-mechanical Bogoliubov-de Gennes approach with those of a semiclassical local-density approximation, we construct extensive phase diagrams for a wide range of parameter regimes in the trap where the aforementioned competition may, e.g., favor an outer normal edge that is completely phase separated from the central superfluid core by vacuum.
Two-dimensional vibrational-electronic spectroscopy
Courtney, Trevor L.; Fox, Zachary W.; Slenkamp, Karla M.; Khalil, Munira
2015-10-21
Two-dimensional vibrational-electronic (2D VE) spectroscopy is a femtosecond Fourier transform (FT) third-order nonlinear technique that creates a link between existing 2D FT spectroscopies in the vibrational and electronic regions of the spectrum. 2D VE spectroscopy enables a direct measurement of infrared (IR) and electronic dipole moment cross terms by utilizing mid-IR pump and optical probe fields that are resonant with vibrational and electronic transitions, respectively, in a sample of interest. We detail this newly developed 2D VE spectroscopy experiment and outline the information contained in a 2D VE spectrum. We then use this technique and its single-pump counterpart (1D VE) to probe the vibrational-electronic couplings between high frequency cyanide stretching vibrations (ν{sub CN}) and either a ligand-to-metal charge transfer transition ([Fe{sup III}(CN){sub 6}]{sup 3−} dissolved in formamide) or a metal-to-metal charge transfer (MMCT) transition ([(CN){sub 5}Fe{sup II}CNRu{sup III}(NH{sub 3}){sub 5}]{sup −} dissolved in formamide). The 2D VE spectra of both molecules reveal peaks resulting from coupled high- and low-frequency vibrational modes to the charge transfer transition. The time-evolving amplitudes and positions of the peaks in the 2D VE spectra report on coherent and incoherent vibrational energy transfer dynamics among the coupled vibrational modes and the charge transfer transition. The selectivity of 2D VE spectroscopy to vibronic processes is evidenced from the selective coupling of specific ν{sub CN} modes to the MMCT transition in the mixed valence complex. The lineshapes in 2D VE spectra report on the correlation of the frequency fluctuations between the coupled vibrational and electronic frequencies in the mixed valence complex which has a time scale of 1 ps. The details and results of this study confirm the versatility of 2D VE spectroscopy and its applicability to probe how vibrations modulate charge and energy transfer in a
Microscopic theory of the residual surface resistivity of Rashba electrons
NASA Astrophysics Data System (ADS)
Bouaziz, Juba; Lounis, Samir; Blügel, Stefan; Ishida, Hiroshi
2016-07-01
A microscopic expression of the residual electrical resistivity tensor is derived in linear response theory for Rashba electrons scattering at a magnetic impurity with cylindrical or noncylindrical potential. The behavior of the longitudinal and transversal residual resistivity is obtained analytically and computed for an Fe impurity at the Au(111) surface. We studied the evolution of the resistivity tensor elements as a function of the Rashba spin-orbit strength and the magnetization direction of the impurity. We found that the absolute values of longitudinal resistivity reduce with increasing spin-orbit strength of the substrate and that the scattering of the conduction electrons at magnetic impurities with magnetic moments pointing in directions not perpendicular to the surface plane produce a planar Hall effect and an anisotropic magnetoresistance even if the impurity carries no spin-orbit interaction. Functional forms are provided describing the anisotropy of the planar Hall effect and the anisotropic magnetoresistance with respect to the direction of the impurity moment. In the limit of no spin-orbit interaction and a nonmagnetic impurity of cylindrical symmetry, the expression of the residual resistivity of a two-dimensional electron gas has the same simplicity and form as for the three-dimensional electron gas [J. Friedel, J. Nuovo. Cim. 7, 287 (1958), 10.1007/BF02751483] and can also be expressed in terms of scattering phase shifts.
NASA Astrophysics Data System (ADS)
Lee, Juhee; Kim, Dong-Hee
2017-03-01
We investigate the Gorkov-Melik-Barkhudarov (GM) correction to superfluid transition temperature in two-dimensional Fermi gases with Rashba spin-orbit coupling (SOC) across the SOC-driven BCS-BEC crossover. In the calculation of the induced interaction, we find that the spin-component mixing due to SOC can induce both of the conventional screening and additional antiscreening contributions that interplay significantly in the strong SOC regime. While the GM correction generally lowers the estimate of transition temperature, it turns out that at a fixed weak interaction, the correction effect exhibits a crossover behavior where the ratio between the estimates without and with the correction first decreases with SOC and then becomes insensitive to SOC when it goes into the strong SOC regime. We demonstrate the applicability of the GM correction by comparing the zero-temperature condensate fraction with the recent quantum Monte Carlo results.
Two dimensional electron gas at oxide interfaces
NASA Astrophysics Data System (ADS)
Janicka, Karolina
2011-12-01
Extraordinary phenomena can occur at the interface between two oxide materials. A spectacular example is a formation of a two-dimensional electron gas (2DEG) at the SrTiO3/LaAlO3 interface. In this dissertation the properties of the 2DEG are investigated from first principles. The spatial extent of the 2DEG formed at the SrTiO3/LaAlO 3 n-type interface is studied. It is shown that the confinement of the 2DEG is controlled by metal induced gap states formed in the band gap of SrTiO 3. The confinement width is then determined by the attenuation length of the metal induced gap states into SrTiO3 which is governed by the lowest decay rate evanescent states of bulk SrTiO3 which in turn can be found from the complex band structure of bulk SrTiO3. Magnetic properties of the 2DEG formed at the n-type interface of the SrTiO3/LaAlO3 superlattices are investigated. It is found that for a thin SrTiO3 film the interface is ferromagnetic but for a thicker SrTiO3 film the magnetic moment decreases and eventually disappears. This is a result of delocalization of the 2DEG that spreads over thicker SrTiO3 film which leads to violation of the Stoner criterion. Further, it is shown that inclusion of the Hubbard U interaction enhances the Stoner parameter and stabilizes the magnetism. The effect of the 2DEG and the polar interfaces for the thin film ferroelectricity is investigated using both first principles and model calculations. Using a TiO2-terminated BaTiO3 film with LaO monolayers at the two interfaces it is shown that the intrinsic electric field produced by the polar interface forces ionic displacements in BaTiO3 to produce the electric polarization directed into the interior of the BaTiO 3 layer. This creates a ferroelectric dead layer near the interfaces that is non-switchable and thus detrimental to ferroelectricity. It is found that the effect is stronger for a larger effective ionic charge at the interface and longer screening length due to a stronger intrinsic electric
Two-dimensional materials and their prospects in transistor electronics.
Schwierz, F; Pezoldt, J; Granzner, R
2015-05-14
During the past decade, two-dimensional materials have attracted incredible interest from the electronic device community. The first two-dimensional material studied in detail was graphene and, since 2007, it has intensively been explored as a material for electronic devices, in particular, transistors. While graphene transistors are still on the agenda, researchers have extended their work to two-dimensional materials beyond graphene and the number of two-dimensional materials under examination has literally exploded recently. Meanwhile several hundreds of different two-dimensional materials are known, a substantial part of them is considered useful for transistors, and experimental transistors with channels of different two-dimensional materials have been demonstrated. In spite of the rapid progress in the field, the prospects of two-dimensional transistors still remain vague and optimistic opinions face rather reserved assessments. The intention of the present paper is to shed more light on the merits and drawbacks of two-dimensional materials for transistor electronics and to add a few more facets to the ongoing discussion on the prospects of two-dimensional transistors. To this end, we compose a wish list of properties for a good transistor channel material and examine to what extent the two-dimensional materials fulfill the criteria of the list. The state-of-the-art two-dimensional transistors are reviewed and a balanced view of both the pros and cons of these devices is provided.
Triola, Christopher; Badiane, Driss M; Balatsky, Alexander V; Rossi, E
2016-06-24
We obtain the general conditions for the emergence of odd-frequency superconducting pairing in a two-dimensional (2D) electronic system proximity coupled to a superconductor, making minimal assumptions about both the 2D system and the superconductor. Using our general results we show that a simple heterostructure formed by a monolayer of a group VI transition metal dichalcogenide, such as molybdenum disulfide, and an s-wave superconductor with Rashba spin-orbit coupling exhibits odd-frequency superconducting pairing. Our results allow the identification of a new class of systems among van der Waals heterostructures in which odd-frequency superconductivity should be present.
NASA Astrophysics Data System (ADS)
Rossi, Enrico; Triola, Christopher; Badiane, Driss; Balatsky, Alexander V.
We obtain the general conditions for the emergence of odd-frequency superconducting pairing in a two-dimensional (2D) electronic system proximity-coupled to a superconductor, making minimal assumptions about both the 2D system and the superconductor. Using our general results we show that a simple heterostructure formed by a monolayer of a group VI transition metal dichalcogenide, such as molybdenum disulfide, and an s-wave superconductor with Rashba spin-orbit coupling will exhibit odd-frequency superconducting pairing. Work supported by US DOE BES E304, KAW, ACS-PRF-53581-DNI5, and NSF-DMR-1455233.
Electronic properties of two-dimensional carbon
Peres, N.M.R.; Guinea, F.; Castro Neto, A.H. . E-mail: neto@bu.edu
2006-07-15
We present a theoretical description of the electronic properties of graphene in the presence of disorder, electron-electron interactions, and particle-hole symmetry breaking. We show that while particle-hole asymmetry, long-range Coulomb interactions, and extended defects lead to the phenomenon of self-doping, local defects determine the transport and spectroscopic properties. Our results explain recent experiments in graphitic devices and predict new electronic behavior.
Herranz, Gervasi; Singh, Gyanendra; Bergeal, Nicolas; ...
2015-01-13
We find the discovery of two-dimensional electron gases (2DEGs) at oxide interfaces—involving electrons in narrow d-bands—has broken new ground, enabling the access to correlated states that are unreachable in conventional semiconductors based on s- and p- electrons. There is a growing consensus that emerging properties at these novel quantum wells—such as 2D superconductivity and magnetism—are intimately connected to specific orbital symmetries in the 2DEG sub-band structure. Here we show that crystal orientation allows selective orbital occupancy, disclosing unprecedented ways to tailor the 2DEG properties. By carrying out electrostatic gating experiments in LaAlO3/SrTiO3 wells of different crystal orientations, we show thatmore » the spatial extension and anisotropy of the 2D superconductivity and the Rashba spin–orbit field can be largely modulated by controlling the 2DEG sub-band filling. Such an orientational tuning expands the possibilities for electronic engineering of 2DEGs at LaAlO3/SrTiO3 interfaces.« less
Herranz, Gervasi; Singh, Gyanendra; Bergeal, Nicolas; Jouan, Alexis; Lesueur, Jérôme; Gázquez, Jaume; Varela, María; Scigaj, Mateusz; Dix, Nico; Sánchez, Florencio; Fontcuberta, Josep
2015-01-01
The discovery of two-dimensional electron gases (2DEGs) at oxide interfaces—involving electrons in narrow d-bands—has broken new ground, enabling the access to correlated states that are unreachable in conventional semiconductors based on s- and p- electrons. There is a growing consensus that emerging properties at these novel quantum wells—such as 2D superconductivity and magnetism—are intimately connected to specific orbital symmetries in the 2DEG sub-band structure. Here we show that crystal orientation allows selective orbital occupancy, disclosing unprecedented ways to tailor the 2DEG properties. By carrying out electrostatic gating experiments in LaAlO3/SrTiO3 wells of different crystal orientations, we show that the spatial extension and anisotropy of the 2D superconductivity and the Rashba spin–orbit field can be largely modulated by controlling the 2DEG sub-band filling. Such an orientational tuning expands the possibilities for electronic engineering of 2DEGs at LaAlO3/SrTiO3 interfaces. PMID:25583368
Two Dimensional Synthetic Electron Cyclotron Emission Imaging
NASA Astrophysics Data System (ADS)
Shi, Lei; Valeo, Ernest J.; Tobias, Benjamin J.; Kramer, Gerrit J.; Liu, Chang; Tang, William M.
2016-10-01
Electron Cyclotron Emission (ECE) has been widely used as a measurement of the electron temperature profile in magnetically confined plasmas. The ECE Imaging (ECEI) system provides additional vertical resolutions, and is used to measure the electron temperature fluctuations. The vertical resolution is typically a few centi-meters which is sometimes comparable to the vertical wave length of the underlying fluctuations. The ray-tracing technique used in most synthetic ECE codes to determine the origin and spatial extent of the ECE radiations is not accurate when the refraction and diffraction due to the fluctuations are important. In this presentation, we introduce a new synthetic ECEI code which solves the wave propagation up to the 2nd order of the WKB approximation, and provides full 2D information of the ECE source. We'll show that when the ECE frequency is near the cutoff, the refraction due to the fluctuations is important. A ``trapping'' of the ECE source by the density fluctuations is identified, and is potentially useful for determining the cross phase between electron temperature and density fluctuations. The new formalism is also used to study the Runaway Electrons contribution to the ECE signal, and provides insights to the measured ECE spectrum on DIII-D. This work has been funded by the US Department of Energy under Contract Number DE-AC02-09CH11466.
Two-Dimensional Electron-Spin Resonance
NASA Astrophysics Data System (ADS)
Freed, Jack H.
2000-03-01
The extension of the concepts of 2D-NMR to ESR posed significant technological challenges, especially for liquids. ESR relaxation times are very short, as low as 10-15 ns. for T_2's. Spectral bandwidths are 100-250 MHz for nitroxide spin labels. Adequate coverage is obtained with 3-5 ns. π/2 (9-17 GHz) microwave pulses into a small low Q resonator. Dead-times are currently 25-30 ns. Additional requirements are rapid phase shifting for phase cycling, nsec. data acquisition, and fast repetition rates (10-100 kHz). 2D-ELDOR (electron-electron double resonance), which is a 3-pulse 2D-exchange experiment, takes about 30 minutes with just 0.5 nanomole spin-probe in solution (SNR 200). 2D-ELDOR is very useful in studies of molecular dynamics and local structure in complex fluids. For such media, the slow rotational dynamics requires a theory based upon the stochastic Liouville equation which enables quantitative interpretation of 2D-ELDOR experiments. In studies of spin-probes in a liquid crystal new insights could be obtained on the dynamic structure in different phases. One obtains, in addition to ordering and reorientation rates of the probes, details of the local dynamic cage: its orienting potential and (slow) relaxation rate. 2D-ELDOR overcomes the loss of resolution resulting from microscopically ordered but macroscopically disordered complex fluids. This is illustrated by studies of the dynamic structure of lipid membrane vesicles, and the effects of adding a peptide. The short dead times enable the observation of both the bulk lipids and the more immobilized lipids that coat (or are trapped) by the (aggregates of) peptides. Also, new developments of multi-quantum (2D) FT-ESR from nitroxide spin labels interacting by dipolar interactions show considerable promise in measuring distances of ca. 15-70A in macromolecules.
Two-dimensional electronic spectroscopy using incoherent light: theoretical analysis.
Turner, Daniel B; Howey, Dylan J; Sutor, Erika J; Hendrickson, Rebecca A; Gealy, M W; Ulness, Darin J
2013-07-25
Electronic energy transfer in photosynthesis occurs over a range of time scales and under a variety of intermolecular coupling conditions. Recent work has shown that electronic coupling between chromophores can lead to coherent oscillations in two-dimensional electronic spectroscopy measurements of pigment-protein complexes measured with femtosecond laser pulses. A persistent issue in the field is to reconcile the results of measurements performed using femtosecond laser pulses with physiological illumination conditions. Noisy-light spectroscopy can begin to address this question. In this work we present the theoretical analysis of incoherent two-dimensional electronic spectroscopy, I((4)) 2D ES. Simulations reveal diagonal peaks, cross peaks, and coherent oscillations similar to those observed in femtosecond two-dimensional electronic spectroscopy experiments. The results also expose fundamental differences between the femtosecond-pulse and noisy-light techniques; the differences lead to new challenges and new opportunities.
General solution of the Dirac equation for quasi-two-dimensional electrons
Eremko, Alexander; Brizhik, Larissa; Loktev, Vadim
2016-06-15
The general solution of the Dirac equation for quasi-two-dimensional electrons confined in an asymmetric quantum well, is found. The energy spectrum of such a system is exactly calculated using special unitary operator and is shown to depend on the electron spin polarization. This solution contains free parameters, whose variation continuously transforms one known particular solution into another. As an example, two different cases are considered in detail: electron in a deep and in a strongly asymmetric shallow quantum well. The effective mass renormalized by relativistic corrections and Bychkov–Rashba coefficients are analytically obtained for both cases. It is demonstrated that the general solution transforms to the particular solutions, found previously (Eremko et al., 2015) with the use of spin invariants. The general solution allows to establish conditions at which a specific (accompanied or non-accompanied by Rashba splitting) spin state can be realized. These results can prompt the ways to control the spin degree of freedom via the synthesis of spintronic heterostructures with the required properties.
Two-dimensional optimization of free electron laser designs
Prosnitz, Donald; Haas, Roger A.
1985-01-01
Off-axis, two-dimensional designs for free electron lasers that maintain correspondence of a light beam with a "synchronous electron" at an optimal transverse radius r>0 to achieve increased beam trapping efficiency and enhanced laser beam wavefront control so as to decrease optical beam diffraction and other deleterious effects.
Two-dimensional optimization of free-electron-laser designs
Prosnitz, D.; Haas, R.A.
1982-05-04
Off-axis, two-dimensional designs for free electron lasers are described that maintain correspondence of a light beam with a synchronous electron at an optimal transverse radius r > 0 to achieve increased beam trapping efficiency and enhanced laser beam wavefront control so as to decrease optical beam diffraction and other deleterious effects.
Melting of a two-dimensional crystal of electrons
NASA Astrophysics Data System (ADS)
Grimes, C. C.
1981-03-01
Experiments show that a sheet of electrons in image-potential-induced states outside a helium surface forms at low temperatures a two-dimensional crystal (the classical, two-dimensional analog of a Wigner crystal). At higher temperatures the electron crystal melts to form a two-dimensional, classical, one-component plasma. The melting transition occurs at Γm = 131 ± 7 where Γ is a measure of the ratio of Coulomb potential energy to kinetic energy per electron. This measured value of Γm is consistent with a value obtained by Morf from a calculation based on the Kosterlitz and Thouless theory of dislocation mediated melting in two-dimensions.
Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.
Wang, Qing Hua; Kalantar-Zadeh, Kourosh; Kis, Andras; Coleman, Jonathan N; Strano, Michael S
2012-11-01
The remarkable properties of graphene have renewed interest in inorganic, two-dimensional materials with unique electronic and optical attributes. Transition metal dichalcogenides (TMDCs) are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness. Although TMDCs have been studied for decades, recent advances in nanoscale materials characterization and device fabrication have opened up new opportunities for two-dimensional layers of thin TMDCs in nanoelectronics and optoelectronics. TMDCs such as MoS(2), MoSe(2), WS(2) and WSe(2) have sizable bandgaps that change from indirect to direct in single layers, allowing applications such as transistors, photodetectors and electroluminescent devices. We review the historical development of TMDCs, methods for preparing atomically thin layers, their electronic and optical properties, and prospects for future advances in electronics and optoelectronics.
Control of spin dynamics in a two-dimensional electron gas by electromagnetic dressing
NASA Astrophysics Data System (ADS)
Pervishko, A. A.; Kibis, O. V.; Morina, S.; Shelykh, I. A.
2015-11-01
We solved the Schrödinger problem for a two-dimensional electron gas (2DEG) with the Rashba spin-orbit interaction in the presence of a strong high-frequency electromagnetic field (dressing field). The found eigenfunctions and eigenenergies of the problem are used to describe the spin dynamics of the dressed 2DEG within the formalism of the density matrix response function. Solving the equations of spin dynamics, we show that the dressing field can switch the spin relaxation in the 2DEG between the cases corresponding to the known Elliott-Yafet and D'yakonov-Perel' regimes. As a result, the spin properties of the 2DEG can be tuned by a high-frequency electromagnetic field. The present effect opens an unexplored way for controlling the spin with light and, therefore, forms the physical prerequisites for creating light-tuned spintronics devices.
Landau level crossing in a spin-orbit coupled two-dimensional electron gas
Wu, Xing-Jun; Li, Ting-Xin; Zhang, Chi; Du, Rui-Rui
2015-01-05
We have studied experimentally the Landau level (LL) spectrum of a two-dimensional electron gas (2DEG) in an In{sub 0.53}Ga{sub 0.47}As/InP quantum well structure by means of low-temperature magneto-transport coincidence measurement in vector magnetic fields. It is well known that LL crossing occurs in tilted magnetic fields due to a competition between cyclotron energy and Zeeman effect. Remarkably, here we observe an additional type of level-crossing resulting from a competition between Rashba and Zeeman splitting in a small magnetic field, consistent with the theoretical prediction for strongly spin-orbit coupled 2DEG.
Biswas, Tutul; Ghosh, Tarun Kanti
2013-01-23
We study the interaction between electron and acoustic phonons in a Rashba spin-orbit coupled two-dimensional electron gas using Boltzmann transport theory. Both the deformation potential and piezoelectric scattering mechanisms are considered in the Bloch-Grüneisen (BG) regime as well as in the equipartition (EP) regime. The effect of the Rashba spin-orbit interaction on the temperature dependence of the resistivity in the BG and EP regimes is discussed. We find that the effective exponent of the temperature dependence of the resistivity in the BG regime decreases due to spin-orbit coupling.
Toward the Accurate Simulation of Two-Dimensional Electronic Spectra
NASA Astrophysics Data System (ADS)
Giussani, Angelo; Nenov, Artur; Segarra-Martí, Javier; Jaiswal, Vishal K.; Rivalta, Ivan; Dumont, Elise; Mukamel, Shaul; Garavelli, Marco
2015-06-01
Two-dimensional pump-probe electronic spectroscopy is a powerful technique able to provide both high spectral and temporal resolution, allowing the analysis of ultrafast complex reactions occurring via complementary pathways by the identification of decay-specific fingerprints. [1-2] The understanding of the origin of the experimentally recorded signals in a two-dimensional electronic spectrum requires the characterization of the electronic states involved in the electronic transitions photoinduced by the pump/probe pulses in the experiment. Such a goal constitutes a considerable computational challenge, since up to 100 states need to be described, for which state-of-the-art methods as RASSCF and RASPT2 have to be wisely employed. [3] With the present contribution, the main features and potentialities of two-dimensional electronic spectroscopy are presented, together with the machinery in continuous development in our groups in order to compute two-dimensional electronic spectra. The results obtained using different level of theory and simulations are shown, bringing as examples the computed two-dimensional electronic spectra for some specific cases studied. [2-4] [1] Rivalta I, Nenov A, Cerullo G, Mukamel S, Garavelli M, Int. J. Quantum Chem., 2014, 114, 85 [2] Nenov A, Segarra-Martí J, Giussani A, Conti I, Rivalta I, Dumont E, Jaiswal V K, Altavilla S, Mukamel S, Garavelli M, Faraday Discuss. 2015, DOI: 10.1039/C4FD00175C [3] Nenov A, Giussani A, Segarra-Martí J, Jaiswal V K, Rivalta I, Cerullo G, Mukamel S, Garavelli M, J. Chem. Phys. submitted [4] Nenov A, Giussani A, Fingerhut B P, Rivalta I, Dumont E, Mukamel S, Garavelli M, Phys. Chem. Chem. Phys. Submitted [5] Krebs N, Pugliesi I, Hauer J, Riedle E, New J. Phys., 2013,15, 08501
Spatially resolved two-dimensional Fourier transform electron spin resonance
NASA Astrophysics Data System (ADS)
Ewert, Uwe; Crepeau, Richard H.; Lee, Sanghyuk; Dunnam, Curt R.; Xu, Dajiang; Freed, Jack H.
1991-09-01
Fourier transform ESR methods have been extended to permit spatially resolved two-dimensional (2D)-ESR experiments. This is illustrated for the case of 2D-electron-electron double resonance (2D-ELDOR) spectra of nitroxides in a liquid that exhibits appreciable cross-peaks due to Heisenberg spin exchange. The use of spin-echo decays in spatially resolved FT-ESR is also demonstrated.
Manifestations of two-dimensional electron gas in molecular crystals
NASA Astrophysics Data System (ADS)
Kuklja, Maija M.; Sharia, Onise; Tsyshevsky, Roman
2017-03-01
The existence of two-dimensional electron gas in molecular materials has not been reported or discussed. Intriguing properties of two-dimensional electron gas observed on interfaces of polar and nonpolar oxides spurred oxide electronics and advanced nanotechnology. Here we discover how an electrostatic instability occurs on polar surfaces of molecular crystals and explore its manifestations, chemical degradation of surfaces, charge separation, electrical conductivity, optical band-gap closure and surface metallization. A thin layer of polar surface of a dielectric molecular crystal becomes metallic due to interactions of polar molecules. Our findings are illustrated with two polymorphs of cyclotetramethylene-tetranitramine crystals, the polar δ-phase and nonpolar β-phase. Our theory offers an explanation to a relative stability of the β-phase versus the explosive reactivity of δ-phase and to the experimentally observed difference in conductivity of these crystals. We predict that the electrostatic instability takes place on all polar molecular materials.
Suspended two-dimensional electron and hole gases
Kazazis, D.; Bourhis, E.; Gierak, J.; Gennser, U.; Bourgeois, O.; Antoni, T.
2013-12-04
We report on the fabrication of fully suspended two-dimensional electron and hole gases in III-V heterostructures. Low temperature transport measurements verify that the properties of the suspended gases are only slightly degraded with respect to the non-suspended gases. Focused ion beam technology is used to pattern suspended nanostructures with minimum damage from the ion beam, due to the small width of the suspended membrane.
Two-dimensional electron gas magnetic field sensors
NASA Astrophysics Data System (ADS)
Heremans, J.; Partin, D. L.; Morelli, D. T.; Fuller, B. K.; Thrush, C. M.
1990-07-01
We describe the use of accumulation layers of electron charge in applications as magnetoresistive devices. We consider two such systems: an InGaAs/InP heterostructure in which we identify a two-dimensional electron gas from the observation of the quantum Hall effect, and InAs films, in which a strong surface accumulation of charge is inferred from depth profiling studies of the galvanomagnetic coefficients. Magnetoresistive devices fabricated from these materials exhibit outstanding field sensitivity and temperature stability due to the existence of electrons of relatively high density and mobility in the accumulation regions. We also model the magnetosensitivity of our devices.
Two-dimensional electronic spectroscopy of molecular excitons.
Milota, Franz; Sperling, Jaroslaw; Nemeth, Alexandra; Mancal, Tomás; Kauffmann, Harald F
2009-09-15
Understanding of the nuclear and electronic structure and dynamics of molecular systems has advanced considerably through probing the nonlinear response of molecules to sequences of pulsed electromagnetic fields. The ability to control various degrees of freedom of the excitation pulses-such as duration, sequence, frequency, polarization, and shape-has led to a variety of time-resolved spectroscopic methods. The various techniques that researchers use are commonly classified by their dimensionality, which refers to the number of independently variable time delays between the pulsed fields that induce the signal. Though pico- and femtosecond time-resolved spectroscopies of electronic transitions have come of age, only recently have researchers been able to perform two-dimensional electronic spectroscopy (2D-ES) in the visible frequency regime and correlate transition frequencies that evolve in different time intervals. The two-dimensional correlation plots and their temporal evolution allow one to access spectral information that is not exposed directly in other one-dimensional nonlinear methods. In this Account, we summarize our studies of a series of increasingly complex molecular chromophores. We examine noninteracting dye molecules, a monomer-dimer equilibrium of a prototypical dye molecule, and finally a supramolecular assembly of electronically coupled absorbers. By tracing vibronic signal modulations, differentiating line-broadening mechanisms, analyzing distinctly different relaxation dynamics, determining electronic coupling strengths, and directly following excitation energy transfer pathways, we illustrate how two-dimensional electronic spectroscopy can image physical phenomena that underlie the optical response of a particular system. Although 2D-ES is far from being a "turn-key" method, we expect that experimental progress and potential commercialization of instrumentation will make 2D-ES accessible to a much broader scientific audience, analogous to
Intersubband relaxation of two-dimensional electrons in heterostructures
NASA Astrophysics Data System (ADS)
Fal'ko, Vladimir I.
1993-05-01
We calculate the lifetime of a nonequilibrium electron in the first excited subband in the low-density heterostructure where this photocreated carrier occurs at the last stage of its cooling. The electron interaction with acoustic phonons gives the dominant intersubband relaxation mechanism, if the intersubband energy splitting and the Fermi energy splitting are relatively small, 1>ɛF/Δ10>0.7-0.8. In GaAs-AlxGa1-xAs heterostructures the intersubband relaxation determines the excited-electron lifetime to be of the order of τphon~nanoseconds, which depends slightly on the value of the two-dimensional electron density. When the ratio ɛF/Δ10 is smaller, the intersubband relaxation is determined by the Auger-like electron-electron scattering whose rate can increase up to the value τ-1Aug~10-10 sec-1.
Anisotropic electronic conduction in stacked two-dimensional titanium carbide
Hu, Tao; Zhang, Hui; Wang, Jiemin; Li, Zhaojin; Hu, Minmin; Tan, Jun; Hou, Pengxiang; Li, Feng; Wang, Xiaohui
2015-01-01
Stacked two-dimensional titanium carbide is an emerging conductive material for electrochemical energy storage which requires an understanding of the intrinsic electronic conduction. Here we report the electronic conduction properties of stacked Ti3C2T2 (T = OH, O, F) with two distinct stacking sequences (Bernal and simple hexagonal). On the basis of first-principles calculations and energy band theory analysis, both stacking sequences give rise to metallic conduction with Ti 3d electrons contributing most to the conduction. The conduction is also significantly anisotropic due to the fact that the effective masses of carriers including electrons and holes are remarkably direction-dependent. Such an anisotropic electronic conduction is evidenced by the I−V curves of an individual Ti3C2T2 particulate, which demonstrates that the in-plane electrical conduction is at least one order of magnitude higher than that vertical to the basal plane. PMID:26548439
Anisotropic electronic conduction in stacked two-dimensional titanium carbide
NASA Astrophysics Data System (ADS)
Hu, Tao; Zhang, Hui; Wang, Jiemin; Li, Zhaojin; Hu, Minmin; Tan, Jun; Hou, Pengxiang; Li, Feng; Wang, Xiaohui
2015-11-01
Stacked two-dimensional titanium carbide is an emerging conductive material for electrochemical energy storage which requires an understanding of the intrinsic electronic conduction. Here we report the electronic conduction properties of stacked Ti3C2T2 (T = OH, O, F) with two distinct stacking sequences (Bernal and simple hexagonal). On the basis of first-principles calculations and energy band theory analysis, both stacking sequences give rise to metallic conduction with Ti 3d electrons contributing most to the conduction. The conduction is also significantly anisotropic due to the fact that the effective masses of carriers including electrons and holes are remarkably direction-dependent. Such an anisotropic electronic conduction is evidenced by the I-V curves of an individual Ti3C2T2 particulate, which demonstrates that the in-plane electrical conduction is at least one order of magnitude higher than that vertical to the basal plane.
Anisotropic electronic conduction in stacked two-dimensional titanium carbide.
Hu, Tao; Zhang, Hui; Wang, Jiemin; Li, Zhaojin; Hu, Minmin; Tan, Jun; Hou, Pengxiang; Li, Feng; Wang, Xiaohui
2015-11-09
Stacked two-dimensional titanium carbide is an emerging conductive material for electrochemical energy storage which requires an understanding of the intrinsic electronic conduction. Here we report the electronic conduction properties of stacked Ti3C2T2 (T = OH, O, F) with two distinct stacking sequences (Bernal and simple hexagonal). On the basis of first-principles calculations and energy band theory analysis, both stacking sequences give rise to metallic conduction with Ti 3d electrons contributing most to the conduction. The conduction is also significantly anisotropic due to the fact that the effective masses of carriers including electrons and holes are remarkably direction-dependent. Such an anisotropic electronic conduction is evidenced by the I-V curves of an individual Ti3C2T2 particulate, which demonstrates that the in-plane electrical conduction is at least one order of magnitude higher than that vertical to the basal plane.
NASA Astrophysics Data System (ADS)
Zhu, Song; Liu, Hui-Ping; Yi, Lin
2010-09-01
A generalized finite element formulation is proposed for the study of the spin-dependent ballistic transport of electron through the two-dimensional quantum structures with Rashba spin-orbit interactions (SOI). The transmission coefficient, conductance, the total and local polarization are numerically calculated and discussed as the Rashba coefficient, the geometric sizes, and incident energy are changed in the T-shaped devices. Some interesting features are found in the proper parameter regime. The polarization has an enhancement as the Rashba coefficient becomes stronger. The polarization valley is rigid in the regime of the conductance plateaus since the local interference among the polarized multi-wave modes. The Rashba interactions coupling to geometry in sizes could form the structure-induced Fano-Rashba resonance. In the wider stub, the localized spin lattice of electron could be produced. The conductance plateaus correspond to weak polarizations. Strong polarizations appear when the stub sizes, incident energy, and the Rashba coupling coefficient are matched. The resonances are formed in a wide Fermi energy segment easily.
Disordered two-dimensional electron systems with chiral symmetry
NASA Astrophysics Data System (ADS)
Markoš, P.; Schweitzer, L.
2012-10-01
We review the results of our recent numerical investigations on the electronic properties of disordered two dimensional systems with chiral unitary, chiral orthogonal, and chiral symplectic symmetry. Of particular interest is the behavior of the density of states and the logarithmic scaling of the smallest Lyapunov exponents in the vicinity of the chiral quantum critical point in the band center at E=0. The observed peaks or depressions in the density of states, the distribution of the critical conductances, and the possible non-universality of the critical exponents for certain chiral unitary models are discussed.
Sakaguchi, Hidetsugu; Sherman, E Ya; Malomed, Boris A
2016-09-01
We present an analysis of two-dimensional (2D) matter-wave solitons, governed by the pseudospinor system of Gross-Pitaevskii equations with self- and cross attraction, which includes the spin-orbit coupling (SOC) in the general Rashba-Dresselhaus form, and, separately, the Rashba coupling and the Zeeman splitting. Families of semivortex (SV) and mixed-mode (MM) solitons are constructed, which exist and are stable in free space, as the SOC terms prevent the onset of the critical collapse and create the otherwise missing ground states in the form of the solitons. The Dresselhaus SOC produces a destructive effect on the vortex solitons, while the Zeeman term tends to convert the MM states into the SV ones, which eventually suffer delocalization. Existence domains and stability boundaries are identified for the soliton families. For physically relevant parameters of the SOC system, the number of atoms in the 2D solitons is limited by ∼1.5×10^{4}. The results are obtained by means of combined analytical and numerical methods.
NASA Astrophysics Data System (ADS)
Sakaguchi, Hidetsugu; Sherman, E. Ya.; Malomed, Boris A.
2016-09-01
We present an analysis of two-dimensional (2D) matter-wave solitons, governed by the pseudospinor system of Gross-Pitaevskii equations with self- and cross attraction, which includes the spin-orbit coupling (SOC) in the general Rashba-Dresselhaus form, and, separately, the Rashba coupling and the Zeeman splitting. Families of semivortex (SV) and mixed-mode (MM) solitons are constructed, which exist and are stable in free space, as the SOC terms prevent the onset of the critical collapse and create the otherwise missing ground states in the form of the solitons. The Dresselhaus SOC produces a destructive effect on the vortex solitons, while the Zeeman term tends to convert the MM states into the SV ones, which eventually suffer delocalization. Existence domains and stability boundaries are identified for the soliton families. For physically relevant parameters of the SOC system, the number of atoms in the 2D solitons is limited by ˜1.5 ×104 . The results are obtained by means of combined analytical and numerical methods.
Dispersion-free continuum two-dimensional electronic spectrometer
Zheng, Haibin; Caram, Justin R.; Dahlberg, Peter D.; Rolczynski, Brian S.; Viswanathan, Subha; Dolzhnikov, Dmitriy S.; Khadivi, Amir; Talapin, Dmitri V.; Engel, Gregory S.
2015-01-01
Electronic dynamics span broad energy scales with ultrafast time constants in the condensed phase. Two-dimensional (2D) electronic spectroscopy permits the study of these dynamics with simultaneous resolution in both frequency and time. In practice, this technique is sensitive to changes in nonlinear dispersion in the laser pulses as time delays are varied during the experiment. We have developed a 2D spectrometer that uses broadband continuum generated in argon as the light source. Using this visible light in phase-sensitive optical experiments presents new challenges in implementation. We demonstrate all-reflective interferometric delays using angled stages. Upon selecting an ~180 nm window of the available bandwidth at ~10 fs compression, we probe the nonlinear response of broadly absorbing CdSe quantum dots and electronic transitions of Chlorophyll a. PMID:24663470
Two-dimensional electron beam charging model for polymer films
NASA Technical Reports Server (NTRS)
Reeves, R. D.; Balmain, K. G.
1981-01-01
A two-dimensional model is developed to describe the charging of strips of thin polymer films above a grounded substrate exposed to a uniform mono-energetic electron beam. The study is motivated by the observed anomalous behavior of geosynchronous satellites, which has been attributed to differential charging of the satellite surfaces exposed to magnetospheric electrons. Surface and bulk electric fields are calcuated at steady state in order to identify regions of high electrical stress, with emphasis on behavior near the material's edge. The model is used to study the effects of some of the experimental parameters, notably beam energy, beam angle of incidence, beam current density, material thickness and material width. Also examined are the consequences of a central gap in the material and a discontinuity in the material thickness.
Dirac Fermions in a Nanopatterned Two-Dimensional Electron Gas
NASA Astrophysics Data System (ADS)
Park, Cheol-Hwan
2013-03-01
If a lateral periodic potential with triangular (or honeycomb) lattice symmetry is applied to a conventional two-dimensional electron gas (2DEG), the charge carriers behave like massless Dirac ferions. A very interesting and useful point of these newly-generated massless Dirac fermions is that, unlike the case of graphene, their properties can be tuned through the external periodic potential. In this presentation, I will review the electronic properties of those newly-generated massless Dirac fermions in an artificial 2DEG superlattice system and will discuss how the elecctronic structure of those massless Dirac fermions changes depending on the external periodic potential. This work was partly supported by Research Settlement Fund for the new faculty of SNU.
Microwave Reflection Spectroscopy of a Two-Dimensional Electron Gas
NASA Astrophysics Data System (ADS)
Zhang, Jie; Liu, Ruiyuan; Du, Lingjie; Du, Rui-Rui; Pfeiffer, Loren; West, Ken
Cyclotron resonance (CR) is a standard method to determine the carrier effective mass in two-dimensional electron systems, typically by measuring/analyzing the absorption or transmission signal. Here we report a microwave spectrometer utilizing the reflection signal. In our experiment setup based on a top-loading helium3 cryostat and a superconducting solenoid, the microwave (up to 40GHz) is sent down via a coax cable to the sample surface, and the reflection signal is then collected by the same cable and fed upward to a directional coupler, and being detected. We demonstrate the applicability of the spectrometer by measuring the CR of high-mobility electrons or holes in GaAs/AlGaAs quantum wells. The construction of spectrometer, preliminary data, and brief discussions will be presented. The work at Rice was supported by Welch Foundation Grant C-1682.
Electronic transport in two-dimensional high dielectric constant nanosystems
Ortuño, M.; Somoza, A. M.; Vinokur, V. M.; Baturina, T. I.
2015-01-01
There has been remarkable recent progress in engineering high-dielectric constant two dimensional (2D) materials, which are being actively pursued for applications in nanoelectronics in capacitor and memory devices, energy storage, and high-frequency modulation in communication devices. Yet many of the unique properties of these systems are poorly understood and remain unexplored. Here we report a numerical study of hopping conductivity of the lateral network of capacitors, which models two-dimensional insulators, and demonstrate that 2D long-range Coulomb interactions lead to peculiar size effects. We find that the characteristic energy governing electronic transport scales logarithmically with either system size or electrostatic screening length depending on which one is shorter. Our results are relevant well beyond their immediate context, explaining, for example, recent experimental observations of logarithmic size dependence of electric conductivity of thin superconducting films in the critical vicinity of superconductor-insulator transition where a giant dielectric constant develops. Our findings mark a radical departure from the orthodox view of conductivity in 2D systems as a local characteristic of materials and establish its macroscopic global character as a generic property of high-dielectric constant 2D nanomaterials. PMID:25860804
Electronic transport in two-dimensional high dielectric constant nanosystems
Ortuño, M.; Somoza, A. M.; Vinokur, V. M.; ...
2015-04-10
There has been remarkable recent progress in engineering high-dielectric constant two dimensional (2D) materials, which are being actively pursued for applications in nanoelectronics in capacitor and memory devices, energy storage, and high-frequency modulation in communication devices. Yet many of the unique properties of these systems are poorly understood and remain unexplored. Here we report a numerical study of hopping conductivity of the lateral network of capacitors, which models two-dimensional insulators, and demonstrate that 2D long-range Coulomb interactions lead to peculiar size effects. We find that the characteristic energy governing electronic transport scales logarithmically with either system size or electrostatic screeningmore » length depending on which one is shorter. Our results are relevant well beyond their immediate context, explaining, for example, recent experimental observations of logarithmic size dependence of electric conductivity of thin superconducting films in the critical vicinity of superconductor-insulator transition where a giant dielectric constant develops. Our findings mark a radical departure from the orthodox view of conductivity in 2D systems as a local characteristic of materials and establish its macroscopic global character as a generic property of high-dielectric constant 2D nanomaterials.« less
Electronic transport in two-dimensional high dielectric constant nanosystems
Ortuño, M.; Somoza, A. M.; Vinokur, V. M.; Baturina, T. I.
2015-04-10
There has been remarkable recent progress in engineering high-dielectric constant two dimensional (2D) materials, which are being actively pursued for applications in nanoelectronics in capacitor and memory devices, energy storage, and high-frequency modulation in communication devices. Yet many of the unique properties of these systems are poorly understood and remain unexplored. Here we report a numerical study of hopping conductivity of the lateral network of capacitors, which models two-dimensional insulators, and demonstrate that 2D long-range Coulomb interactions lead to peculiar size effects. We find that the characteristic energy governing electronic transport scales logarithmically with either system size or electrostatic screening length depending on which one is shorter. Our results are relevant well beyond their immediate context, explaining, for example, recent experimental observations of logarithmic size dependence of electric conductivity of thin superconducting films in the critical vicinity of superconductor-insulator transition where a giant dielectric constant develops. Our findings mark a radical departure from the orthodox view of conductivity in 2D systems as a local characteristic of materials and establish its macroscopic global character as a generic property of high-dielectric constant 2D nanomaterials.
Electronic transport in two-dimensional high dielectric constant nanosystems
NASA Astrophysics Data System (ADS)
Ortuño, M.; Somoza, A. M.; Vinokur, V. M.; Baturina, T. I.
2015-04-01
There has been remarkable recent progress in engineering high-dielectric constant two dimensional (2D) materials, which are being actively pursued for applications in nanoelectronics in capacitor and memory devices, energy storage, and high-frequency modulation in communication devices. Yet many of the unique properties of these systems are poorly understood and remain unexplored. Here we report a numerical study of hopping conductivity of the lateral network of capacitors, which models two-dimensional insulators, and demonstrate that 2D long-range Coulomb interactions lead to peculiar size effects. We find that the characteristic energy governing electronic transport scales logarithmically with either system size or electrostatic screening length depending on which one is shorter. Our results are relevant well beyond their immediate context, explaining, for example, recent experimental observations of logarithmic size dependence of electric conductivity of thin superconducting films in the critical vicinity of superconductor-insulator transition where a giant dielectric constant develops. Our findings mark a radical departure from the orthodox view of conductivity in 2D systems as a local characteristic of materials and establish its macroscopic global character as a generic property of high-dielectric constant 2D nanomaterials.
Exploring two-dimensional electron gases with two-dimensional Fourier transform spectroscopy
Paul, J.; Dey, P.; Tokumoto, T.; ...
2014-10-07
The dephasing of excitons in a modulation doped single quantum well was carefully measured using time integrated four-wave mixing (FWM) and two-dimensional Fourier transform (2DFT) spectroscopy. These are the first 2DFT measurements performed on a modulation doped single quantum well. The inhomogeneous and homogeneous excitonic line widths were obtained from the diagonal and cross-diagonal profiles of the 2DFT spectra. The laser excitation density and temperature were varied and 2DFT spectra were collected. A very rapid increase of the dephasing decay, and as a result, an increase in the cross-diagonal 2DFT linewidths with temperature was observed. Furthermore, the lineshapes of themore » 2DFT spectra suggest the presence of excitation induced dephasing and excitation induced shift.« less
Exploring two-dimensional electron gases with two-dimensional Fourier transform spectroscopy
Paul, J.; Dey, P.; Tokumoto, T.; Reno, J. L.; Hilton, D. J.; Karaiskaj, D.
2014-10-07
The dephasing of excitons in a modulation doped single quantum well was carefully measured using time integrated four-wave mixing (FWM) and two-dimensional Fourier transform (2DFT) spectroscopy. These are the first 2DFT measurements performed on a modulation doped single quantum well. The inhomogeneous and homogeneous excitonic line widths were obtained from the diagonal and cross-diagonal profiles of the 2DFT spectra. The laser excitation density and temperature were varied and 2DFT spectra were collected. A very rapid increase of the dephasing decay, and as a result, an increase in the cross-diagonal 2DFT linewidths with temperature was observed. Furthermore, the lineshapes of the 2DFT spectra suggest the presence of excitation induced dephasing and excitation induced shift.
Electron-Spin Filters Based on the Rashba Effect
NASA Technical Reports Server (NTRS)
Ting, David Z.-Y.; Cartoixa, Xavier; McGill, Thomas C.; Moon, Jeong S.; Chow, David H.; Schulman, Joel N.; Smith, Darryl L.
2004-01-01
Semiconductor electron-spin filters of a proposed type would be based on the Rashba effect, which is described briefly below. Electron-spin filters more precisely, sources of spin-polarized electron currents have been sought for research on, and development of, the emerging technological discipline of spintronics (spin-based electronics). There have been a number of successful demonstrations of injection of spin-polarized electrons from diluted magnetic semiconductors and from ferromagnetic metals into nonmagnetic semiconductors. In contrast, a device according to the proposal would be made from nonmagnetic semiconductor materials and would function without an applied magnetic field. The Rashba effect, named after one of its discoverers, is an energy splitting, of what would otherwise be degenerate quantum states, caused by a spin-orbit interaction in conjunction with a structural-inversion asymmetry in the presence of interfacial electric fields in a semiconductor heterostructure. The magnitude of the energy split is proportional to the electron wave number. The present proposal evolved from recent theoretical studies that suggested the possibility of devices in which electron energy states would be split by the Rashba effect and spin-polarized currents would be extracted by resonant quantum-mechanical tunneling. Accordingly, a device according to the proposal would be denoted an asymmetric resonant interband tunneling diode [a-RITD]. An a-RITD could be implemented in a variety of forms, the form favored in the proposal being a double-barrier heterostructure containing an asymmetric quantum well. It is envisioned that a-RITDs would be designed and fabricated in the InAs/GaSb/AlSb material system for several reasons: Heterostructures in this material system are strong candidates for pronounced Rashba spin splitting because InAs and GaSb exhibit large spin-orbit interactions and because both InAs and GaSb would be available for the construction of highly asymmetric
Two-Dimensional Electronic Spectroscopy in the Ultraviolet Wavelength Range.
West, Brantley A; Moran, Andrew M
2012-09-20
Coherent two-dimensional (2D) spectroscopies conducted at visible and infrared wavelengths are having a transformative impact on the understanding of numerous processes in condensed phases. The extension of 2D spectroscopy to the ultraviolet spectral range (2DUV) must contend with several challenges, including the attainment of adequate laser bandwidth, interferometric phase stability, and the suppression of undesired nonlinearities in the sample medium. Solutions to these problems are motivated by the study of a wide range of biological systems whose lowest-frequency electronic resonances are found in the UV. The development of 2DUV spectroscopy also makes possible the attainment of new insights into elementary chemical reaction dynamics (e.g., electrocyclic ring opening in cycloalkenes). Substantial progress has been made in both the implementation and application of 2DUV spectroscopy in the past several years. In this Perspective, we discuss 2DUV methodology, review recent applications, and speculate on what the future will hold.
Two-dimensional electronic spectroscopy with birefringent wedges
Réhault, Julien; Maiuri, Margherita; Oriana, Aurelio; Cerullo, Giulio
2014-12-15
We present a simple experimental setup for performing two-dimensional (2D) electronic spectroscopy in the partially collinear pump-probe geometry. The setup uses a sequence of birefringent wedges to create and delay a pair of phase-locked, collinear pump pulses, with extremely high phase stability and reproducibility. Continuous delay scanning is possible without any active stabilization or position tracking, and allows to record rapidly and easily 2D spectra. The setup works over a broad spectral range from the ultraviolet to the near-IR, it is compatible with few-optical-cycle pulses and can be easily reconfigured to two-colour operation. A simple method for scattering suppression is also introduced. As a proof of principle, we present degenerate and two-color 2D spectra of the light-harvesting complex 1 of purple bacteria.
A ballistic two-dimensional-electron-gas Andreev interferometer
Amado, M. Fornieri, A.; Sorba, L.; Giazotto, F.; Biasiol, G.
2014-06-16
We report the realization and investigation of a ballistic Andreev interferometer based on an InAs two dimensional electron gas coupled to a superconducting Nb loop. We observe strong magnetic modulations in the voltage drop across the device due to quasiparticle interference within the weak-link. The interferometer exhibits flux noise down to ∼80 μΦ{sub 0}/√(Hz) and a robust behavior in temperature with voltage oscillations surviving up to ∼7 K. Besides this remarkable performance, the device represents a crucial first step for the realization of a fully-tunable ballistic superconducting magnetometer and embodies a potential advanced platform for the investigation of Majorana bound states, non-local entanglement of Cooper pairs, as well as the manipulation and control of spin triplet correlations.
Exploring two-dimensional electron gases with two-dimensional Fourier transform spectroscopy
Paul, J.; Dey, P.; Karaiskaj, D.; Tokumoto, T.; Hilton, D. J.; Reno, J. L.
2014-10-07
The dephasing of the Fermi edge singularity excitations in two modulation doped single quantum wells of 12 nm and 18 nm thickness and in-well carrier concentration of ∼4 × 10{sup 11} cm{sup −2} was carefully measured using spectrally resolved four-wave mixing (FWM) and two-dimensional Fourier transform (2DFT) spectroscopy. Although the absorption at the Fermi edge is broad at this doping level, the spectrally resolved FWM shows narrow resonances. Two peaks are observed separated by the heavy hole/light hole energy splitting. Temperature dependent “rephasing” (S{sub 1}) 2DFT spectra show a rapid linear increase of the homogeneous linewidth with temperature. The dephasing rate increases faster with temperature in the narrower 12 nm quantum well, likely due to an increased carrier-phonon scattering rate. The S{sub 1} 2DFT spectra were measured using co-linear, cross-linear, and co-circular polarizations. Distinct 2DFT lineshapes were observed for co-linear and cross-linear polarizations, suggesting the existence of polarization dependent contributions. The “two-quantum coherence” (S{sub 3}) 2DFT spectra for the 12 nm quantum well show a single peak for both co-linear and co-circular polarizations.
Giant spin splitting of the two-dimensional electron gas at the surface of SrTiO3
NASA Astrophysics Data System (ADS)
Santander-Syro, A. F.; Fortuna, F.; Bareille, C.; Rödel, T. C.; Landolt, G.; Plumb, N. C.; Dil, J. H.; Radović, M.
2014-12-01
Two-dimensional electron gases (2DEGs) forming at the interfaces of transition metal oxides exhibit a range of properties, including tunable insulator-superconductor-metal transitions, large magnetoresistance, coexisting ferromagnetism and superconductivity, and a spin splitting of a few meV (refs , ). Strontium titanate (SrTiO3), the cornerstone of such oxide-based electronics, is a transparent, non-magnetic, wide-bandgap insulator in the bulk, and has recently been found to host a surface 2DEG (refs , , , ). The most strongly confined carriers within this 2DEG comprise two subbands, separated by an energy gap of 90 meV and forming concentric circular Fermi surfaces. Using spin- and angle-resolved photoemission spectroscopy (SARPES), we show that the electron spins in these subbands have opposite chiralities. Although the Rashba effect might be expected to give rise to such spin textures, the giant splitting of almost 100 meV at the Fermi level is far larger than anticipated. Moreover, in contrast to a simple Rashba system, the spin-polarized subbands are non-degenerate at the Brillouin zone centre. This degeneracy can be lifted by time-reversal symmetry breaking, implying the possible existence of magnetic order. These results show that confined electronic states at oxide surfaces can be endowed with novel, non-trivial properties that are both theoretically challenging to anticipate and promising for technological applications.
Giant spin splitting of the two-dimensional electron gas at the surface of SrTiO3.
Santander-Syro, A F; Fortuna, F; Bareille, C; Rödel, T C; Landolt, G; Plumb, N C; Dil, J H; Radović, M
2014-12-01
Two-dimensional electron gases (2DEGs) forming at the interfaces of transition metal oxides exhibit a range of properties, including tunable insulator-superconductor-metal transitions, large magnetoresistance, coexisting ferromagnetism and superconductivity, and a spin splitting of a few meV (refs 10, 11). Strontium titanate (SrTiO3), the cornerstone of such oxide-based electronics, is a transparent, non-magnetic, wide-bandgap insulator in the bulk, and has recently been found to host a surface 2DEG (refs 12-15). The most strongly confined carriers within this 2DEG comprise two subbands, separated by an energy gap of 90 meV and forming concentric circular Fermi surfaces. Using spin- and angle-resolved photoemission spectroscopy (SARPES), we show that the electron spins in these subbands have opposite chiralities. Although the Rashba effect might be expected to give rise to such spin textures, the giant splitting of almost 100 meV at the Fermi level is far larger than anticipated. Moreover, in contrast to a simple Rashba system, the spin-polarized subbands are non-degenerate at the Brillouin zone centre. This degeneracy can be lifted by time-reversal symmetry breaking, implying the possible existence of magnetic order. These results show that confined electronic states at oxide surfaces can be endowed with novel, non-trivial properties that are both theoretically challenging to anticipate and promising for technological applications.
Stability and electronic properties of two-dimensional indium iodide
NASA Astrophysics Data System (ADS)
Wang, Jizhang; Dong, Baojuan; Guo, Huaihong; Yang, Teng; Zhu, Zhen; Hu, Gan; Saito, Riichiro; Zhang, Zhidong
2017-01-01
Based on ab initio density functional calculations, we studied the stability and electronic properties of two-dimensional indium iodide (InI). The calculated results show that monolayer and few-layer InI can be as stable as its bulk counterpart. The stability of the monolayer structure is further supported by examining the electronic and dynamic stability. The interlayer interaction is found to be fairly weak (˜160 meV/atom) and mechanical exfoliation to obtain monolayer and few-layer structures will be applicable. A direct band gap of 1.88 eV of the bulk structure is obtained from the hybrid functional method, and is comparable to the experimental one (˜2.00 eV). The electronic structure can be tuned by layer stacking and external strain. The size of the gap is a linear function of an inverse number of layers, suggesting that we can design few-layer structures for optoelectronic applications in the visible optical range. In-plane tensile or hydrostatic compressive stress is found to be useful not only in varying the gap size to cover the whole visible optical range, but also in inducing a semiconductor-metal transition with an experimentally accessible stress. The present result strongly supports the strategy of broadening the scope of group-V semiconductors by looking for isoelectronic III-VII atomic-layered materials.
Electronic nanobiosensors based on two-dimensional materials
NASA Astrophysics Data System (ADS)
Ping, Jinglei
Atomically-thick two-dimensional (2D) nanomaterials have tremendous potential to be applied as transduction elements in biosensors and bioelectronics. We developed scalable methods for synthesis and large-area transfer of two-dimensional nanomaterials, particularly graphene and metal dichalcogenides (so called ``MX2'' materials). We also developed versatile fabrication methods for large arrays of field-effect transistors (FETs) and micro-electrodes with these nanomaterials based on either conventional photolithography or innovative approaches that minimize contamination of the 2D layer. By functionalizing the FETs with a computationally redesigned water-soluble mu-opioid receptor, we created selective and sensitive biosensors suitable for detection of the drug target naltrexone and the neuropeptide enkephalin at pg/mL concentrations. We also constructed DNA-functionalized biosensors and nano-particle decorated biosensors by applying related bio-nano integration techniques. Our methodology paves the way for multiplexed nanosensor arrays with all-electronic readout suitable for inexpensive point-of-care diagnostics, drug-development and biomedical research. With graphene field-effect transistors, we investigated the graphene/solution interface and developed a quantitative model for the effect of ionic screening on the graphene carrier density based on theories of the electric double layer. Finally, we have developed a technique for measuring low-level Faradaic charge-transfer current (fA) across the graphene/solution interface via real-time charge monitoring of graphene microelectrodes in ionic solution. This technique enables the development of flexible and transparent pH sensors that are promising for in vivo applications. The author acknowledges the support from the Defense Advanced Research Projects Agency (DARPA) and the U. S. Army Research Office under Grant Number W911NF1010093.
Quantum holographic encoding in a two-dimensional electron gas
Moon, Christopher
2010-05-26
The advent of bottom-up atomic manipulation heralded a new horizon for attainable information density, as it allowed a bit of information to be represented by a single atom. The discrete spacing between atoms in condensed matter has thus set a rigid limit on the maximum possible information density. While modern technologies are still far from this scale, all theoretical downscaling of devices terminates at this spatial limit. Here, however, we break this barrier with electronic quantum encoding scaled to subatomic densities. We use atomic manipulation to first construct open nanostructures - 'molecular holograms' - which in turn concentrate information into a medium free of lattice constraints: the quantum states of a two-dimensional degenerate Fermi gas of electrons. The information embedded in the holograms is transcoded at even smaller length scales into an atomically uniform area of a copper surface, where it is densely projected into both two spatial degrees of freedom and a third holographic dimension mapped to energy. In analogy to optical volume holography, this requires precise amplitude and phase engineering of electron wavefunctions to assemble pages of information volumetrically. This data is read out by mapping the energy-resolved electron density of states with a scanning tunnelling microscope. As the projection and readout are both extremely near-field, and because we use native quantum states rather than an external beam, we are not limited by lensing or collimation and can create electronically projected objects with features as small as {approx}0.3 nm. These techniques reach unprecedented densities exceeding 20 bits/nm{sup 2} and place tens of bits into a single fermionic state.
Two-Dimensional Halide Perovskites: Tuning Electronic Activities of Defects.
Liu, Yuanyue; Xiao, Hai; Goddard, William A
2016-05-11
Two-dimensional (2D) halide perovskites are emerging as promising candidates for nanoelectronics and optoelectronics. To realize their full potential, it is important to understand the role of those defects that can strongly impact material properties. In contrast to other popular 2D semiconductors (e.g., transition metal dichalcogenides MX2) for which defects typically induce harmful traps, we show that the electronic activities of defects in 2D perovskites are significantly tunable. For example, even with a fixed lattice orientation one can change the synthesis conditions to convert a line defect (edge or grain boundary) from electron acceptor to inactive site without deep gap states. We show that this difference originates from the enhanced ionic bonding in these perovskites compared with MX2. The donors tend to have high formation energies and the harmful defects are difficult to form at a low halide chemical potential. Thus, we unveil unique properties of defects in 2D perovskites and suggest practical routes to improve them.
Two-dimensional material electronics and photonics (Presentation Recording)
NASA Astrophysics Data System (ADS)
Zhu, Wenjuan
2015-09-01
Two-dimensional (2D) materials has attracted intense interest in research in recent years. As compared to their bulk counterparts, these 2D materials have many unique properties due to their reduced dimensionality and symmetry. A key difference is the band structures, which lead to distinct electronic and photonic properties. The 2D nature of the materials also plays an important role in defining their exceptional properties of mechanical strength, surface sensitivity, thermal conductivity, tunable band-gap and interaction with light. These unique properties of 2D materials open up broad territories of applications in computing, communication, energy, and medicine. In this talk, I will present our work on understanding the electrical properties of graphene and MoS2, in particular current transport and band-gap engineering in graphene, interface between gate dielectrics and graphene, and gap states in MoS2. I will also present our work on the nano-scale electronic devices (RF and logic devices) and photonic devices (plasmonic devices and photo-detectors) based on graphene and transition metal dichalcogenides.
Two-Dimensional Plasmonics in Massive and Massless Electron Gases
NASA Astrophysics Data System (ADS)
Yoon, Hosang
Plasmonic waves in solid-state are caused by collective oscillation of mobile charges inside or at the surface of conductors. In particular, surface plasmonic waves propagating at the skin of metals have recently attracted interest, as they reduce the wavelength of electromagnetic waves coupled to them by up to ˜10 times, allowing one to create miniaturized wave devices at optical frequencies. In contrast, plasmonic waves on two-dimensional (2D) conductors appear at much lower infrared and THz-GHz frequencies, near or in the electronics regime, and can achieve far stronger wavelength reduction factor reaching well above 100. In this thesis, we study the unique machinery of 2D plasmonic waves behind this ultra-subwavelength confinement and explore how it can be used to create various interesting devices. To this end, we first develop a physically intuitive theoretical formulation of 2D plasmonic waves, whose two main components---the Coulomb restoration force and inertia of the collectively oscillating charges---are combined into a transmission-line-like model. We then use this formulation to create various ultra-subwavelength 2D plasmonic devices. For the 2D conductor, we first choose GaAs/AlGaAs heterostructure---a 2D electron gas consisting of massive (m* > 0) electrons---demonstrating plasmonic bandgap crystals, interferometers, and negatively refracting metamaterials. We then examine a 2D plasmonic device based on graphene, a 2D electron gas consisting of effectively massless (m* = 0) electrons. We theoretically show and experimentally demonstrate that the massless electrons in graphene can surprisingly exhibit a collective mass when subjected to a collective excitation, providing the inertia that is essential for the propagation of 2D plasmonic waves. Lastly, we theoretically investigate the thermal current fluctuation behaviors in massive and massless electron gases. While seemingly unrelated on first sight, we show that the thermal current fluctuation is
Two-dimensional electronic spectroscopy signatures of the glass transition
Lewis, K. L. .. M.; Myers, J. A.; Fuller, F.; ...
2010-01-01
Two-dimensional electronic spectroscopy is a sensitive probe of solvation dynamics. Using a pump–probe geometry with a pulse shaper [ Optics Express 15 (2007), 16681-16689; Optics Express 16 (2008), 17420-17428], we present temperature dependent 2D spectra of laser dyes dissolved in glass-forming solvents. At low waiting times, the system has not yet relaxed, resulting in a spectrum that is elongated along the diagonal. At longer times, the system loses its memory of the initial excitation frequency, and the 2D spectrum rounds out. As the temperature is lowered, the time scale of this relaxation grows, and the elongation persists for longermore » waiting times. This can be measured in the ratio of the diagonal width to the anti-diagonal width; the behavior of this ratio is representative of the frequency–frequency correlation function [ Optics Letters 31 (2006), 3354–3356]. Near the glass transition temperature, the relaxation behavior changes. Understanding this change is important for interpreting temperature-dependent dynamics of biological systems.« less
Two dimensional electron spin resonance: Structure and dynamics of biomolecules
NASA Astrophysics Data System (ADS)
Saxena, Sunil; Freed, Jack H.
1998-03-01
The potential of two dimensional (2D) electron spin resonance (ESR) for measuring the structural properties and slow dynamics of labeled biomolecules will be presented. Specifically, it will be shown how the recently developed method of double quantum (DQ) 2D ESR (S. Saxena and J. H. Freed, J. Chem. Phys. 107), 1317, (1997) can be used to measure large interelectron distances in bilabeled peptides. The need for DQ ESR spectroscopy, as well as the challenges and advantages of this method will be discussed. The elucidation of the slow reorientational dynamics of this peptide (S. Saxena and J. H. Freed, J. Phys. Chem. A, 101) 7998 (1997) in a glassy medium using COSY and 2D ELDOR ESR spectroscopy will be demonstrated. The contributions to the homogeneous relaxation time, T_2, from the overall and/or internal rotations of the nitroxide can be distinguished from the COSY spectrum. The growth of spectral diffusion cross-peaks^2 with mixing time in the 2D ELDOR spectra can be used to directly determine a correlation time from the experiment which can be related to the rotational correlation time.
NASA Astrophysics Data System (ADS)
Sawada, A.; Koga, T.
2017-02-01
We have developed a method to calculate the weak localization and antilocalization corrections based on the real-space simulation, where we provide 147 885 predetermined return orbitals of quasi-two-dimensional electrons with up to 5000 scattering events that are repeatedly used. Our model subsumes that of Golub [L. E. Golub, Phys. Rev. B 71, 235310 (2005), 10.1103/PhysRevB.71.235310] when the Rashba spin-orbit interaction (SOI) is assumed. Our computation is very simple, fast, and versatile, where the numerical results, obtained all at once, cover wide ranges of the magnetic field under various one-electron interactions H' exactly. Thus, it has straightforward extensibility to incorporate interactions other than the Rashba SOI, such as the linear and cubic Dresselhaus SOIs, Zeeman effect, and even interactions relevant to the valley and pseudo spin degrees of freedom, which should provide a unique tool to study new classes of materials like emerging 2D materials. Using our computation, we also demonstrate the robustness of a persistent spin helix state against the cubic Dresselhaus SOI.
Electronic, Vibrational and Thermoelectric Properties of Two-Dimensional Materials
NASA Astrophysics Data System (ADS)
Wickramaratne, Darshana
The discovery of graphene's unique electronic and thermal properties has motivated the search for new two-dimensional materials. Examples of these materials include the layered two-dimensional transition metal dichalcogenides (TMDC) and metal mono-chalcogenides. The properties of the TMDCs (eg. MoS 2, WS2, TaS2, TaSe2) and the metal mono-chalcogenides (eg. GaSe, InSe, SnS) are diverse - ranging from semiconducting, semi-metallic and metallic. Many of these materials exhibit strongly correlated phenomena and exotic collective states such as exciton condensates, charge density waves, Lifshitz transitions and superconductivity. These properties change as the film thickness is reduced down to a few monolayers. We use first-principles simulations to discuss changes in the electronic and the vibrational properties of these materials as the film thickness evolves from a single atomic monolayer to the bulk limit. In the semiconducting TMDCs (MoS2, MoSe2, WS2 and WSe2) and monochalcogenides (GaS, GaSe, InS and InSe) we show confining these materials to their monolayer limit introduces large band degeneracies or non-parabolic features in the electronic structure. These changes in the electronic structure results in increases in the density of states and the number of conducting modes. Our first-principles simulations combined with a Landauer approach show these changes can lead to large enhancements up to an order of magnitude in the thermoelectric performance of these materials when compared to their bulk structure. Few monolayers of the TMDCs can be misoriented with respect to each other due to the weak van-der-Waals (vdW) force at the interface of two monolayers. Misorientation of the bilayer semiconducting TMDCs increases the interlayer van-der-Waals gap distance, reduces the interlayer coupling and leads to an increase in the magnitude of the indirect bandgap by up to 100 meV compared to the registered bilayer. In the semi-metallic and metallic TMDC compounds (TiSe2, Ta
NASA Astrophysics Data System (ADS)
Shen, Ka; Raimondi, R.; Vignale, G.
2014-12-01
Spin-orbit interactions in two-dimensional electron liquids are responsible for many interesting transport phenomena in which particle currents are converted to spin polarizations and spin currents and vice versa. Prime examples are the spin Hall effect, the Edelstein effect, and their inverses. By similar mechanisms, it is also possible to partially convert an optically induced electron-hole density wave to a spin density wave and vice versa. In this paper, we present a unified theoretical treatment of these effects based on quantum kinetic equations that include not only the intrinsic spin-orbit coupling from the band structure of the host material, but also the spin-orbit coupling due to an external electric field and a random impurity potential. The drift-diffusion equations we derive in the diffusive regime are applicable to a broad variety of experimental situations, both homogeneous and nonhomogeneous, and include on equal footing "skew scattering" and "side jump" from electron-impurity collisions. As a demonstration of the strength and usefulness of the theory we apply it to the study of several effects of current experimental interest: the inverse Edelstein effect, the spin-current swapping effect, and the partial conversion of an electron-hole density wave to a spin density wave in a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit couplings, subject to an electric field.
Herranz, Gervasi; Singh, Gyanendra; Bergeal, Nicolas; Jouan, Alexis; Lesueur, Jérôme; Gázquez, Jaume; Varela, María; Scigaj, Mateusz; Dix, Nico; Sánchez, Florencio; Fontcuberta, Josep
2015-01-13
We find the discovery of two-dimensional electron gases (2DEGs) at oxide interfaces—involving electrons in narrow d-bands—has broken new ground, enabling the access to correlated states that are unreachable in conventional semiconductors based on s- and p- electrons. There is a growing consensus that emerging properties at these novel quantum wells—such as 2D superconductivity and magnetism—are intimately connected to specific orbital symmetries in the 2DEG sub-band structure. Here we show that crystal orientation allows selective orbital occupancy, disclosing unprecedented ways to tailor the 2DEG properties. By carrying out electrostatic gating experiments in LaAlO_{3}/SrTiO_{3} wells of different crystal orientations, we show that the spatial extension and anisotropy of the 2D superconductivity and the Rashba spin–orbit field can be largely modulated by controlling the 2DEG sub-band filling. Such an orientational tuning expands the possibilities for electronic engineering of 2DEGs at LaAlO_{3}/SrTiO_{3} interfaces.
NASA Astrophysics Data System (ADS)
McKeown Walker, S.; Riccò, S.; Bruno, F. Y.; de la Torre, A.; Tamai, A.; Golias, E.; Varykhalov, A.; Marchenko, D.; Hoesch, M.; Bahramy, M. S.; King, P. D. C.; Sánchez-Barriga, J.; Baumberger, F.
2016-06-01
We reinvestigate the putative giant spin splitting at the surface of SrTiO3 reported by Santander-Syro et al. [Nat. Mater. 13, 1085 (2014), 10.1038/nmat4107]. Our spin- and angle-resolved photoemission experiments on fractured (001) oriented surfaces supporting a two-dimensional electron liquid with high carrier density show no detectable spin polarization in the photocurrent. We demonstrate that this result excludes a giant spin splitting while it is consistent with the unconventional Rashba-like splitting seen in band structure calculations that reproduce the experimentally observed ladder of quantum confined subbands.
Kondo spin screening cloud in two-dimensional electron gas with spin-orbit couplings.
Feng, Xiao-Yong; Zhang, Fu-Chun
2011-03-16
A spin-1/2 Anderson impurity in a semiconductor quantum well with Rashba and Dresselhaus spin-orbit couplings is studied by using a variational wavefunction method. The local magnetic moment is found to be quenched at low temperatures. The spin-spin correlations of the impurity and the conduction electron density show anisotropy in both spatial and spin spaces, which interpolates the Kondo spin screenings of a conventional metal and of a surface of three-dimensional topological insulators.
Ferroelectric control of two dimensional electron gas in oxide heterointerface
NASA Astrophysics Data System (ADS)
Thanh, Tra Vu; Chen, Jhih-Wei; Yeh, Chao-Hui; Chen, Yi-Chun; Wu, Chung-Lin; Lin, Jiunn Yuan; Chu, Ying-Hao
2012-02-01
Oxide heterointerfaces are emerging as one of the most exciting materials systems in condensed-matter science. One remarkable example is the LaAlO3 /SrTiO3 (LAO/STO) interface, a model system in which a highly mobile electron gas forms between two band insulators. Our study to manipulate the conductivity at this interface by using ferroeletricity of Pb(Zr,Ti)O3. Our transport data strongly suggests that down polarization direction depletes the conducting interface of LAO/STO. After switching the polarization direction (up), it becomes accumulation. In addition, our experiments show there is obvious the band structure changed by cross-sectional scanning tunneling microscopy and combining with X-ray photoelectron spectroscopy (XPS) measurements. The transport properties are measured to build up the connection between macroscopic properties and local electronic structures that have been applied to study this structure. Controlling the conductivity of this oxide interface suggests that this technique may not only extend more generally to other oxide systems but also open much potential to ferroelectric field effect transistors.
Tunneling Under Microwave Illumination in Bilayer Two Dimensional Electron Systems
NASA Astrophysics Data System (ADS)
Bonetti, J. A.; Pfeiffer, L. N.
2005-03-01
The striking Josephson-like effect recently observed [1] in bilayer 2D electron systems at νt=1 raises important questions about the nature of photon-assisted tunneling in this system. For instance, it is unknown whether Shapiro steps will arise, or whether the presence of radiation will lead to photon- assisted sidebands [2]. In order to address these questions, we have examined the effect of microwave radiation on tunneling in bilayer electron systems. Several aspects of coupling radiation into the sample will be presented, including issues of heating and gating. Preliminary results demonstrate a conduction enhancement near gate voltages corresponding to top and bottom layer depletion. The frequency and power dependence of this enhancement will be presented. This work is supported by the NSF and DOE. [1] I.B. Spielman, J.P. Eisenstein, L.N. Pfeiffer, and K.W. West, Phys. Rev. Lett. 84, 5808 (2000). [2]Ady Stern, S. M. Girvin, A. H. MacDonald, and Ning Ma, Phys. Rev. Lett. 86, 1829 (2001)
Electronic and magnetic properties of Fe and Mn doped two dimensional hexagonal germanium sheets
Soni, Himadri R. Jha, Prafulla K.
2014-04-24
Using first principles density functional theory calculations, the present paper reports systematic total energy calculations of the electronic properties such as density of states and magnetic moment of pristine and iron and manganese doped two dimensional hexagonal germanium sheets.
Heterodyne Hall effect in a two-dimensional electron gas
NASA Astrophysics Data System (ADS)
Oka, Takashi; Bucciantini, Leda
2016-10-01
We study the hitherto unaddressed phenomenon of the quantum Hall effect with a magnetic and electric field oscillating in time with resonant frequencies. This phenomenon highlights an example of a heterodyne device with the magnetic field acting as a driving force, and it is analyzed in detail in its classical and quantum versions using Floquet theory. A bulk current flowing perpendicularly to the applied electric field is found, with a frequency shifted by integer multiples of the driving frequency. When the ratio of the cyclotron and driving frequency takes special values, the electron's classical trajectory forms a loop and the effective mass diverges, while in the quantum case we find an analog of the Landau quantization. A possible realization using metamaterial plasmonics is discussed.
Limit of the electrostatic doping in two-dimensional electron gases of LaXO3(X = Al, Ti)/SrTiO3
Biscaras, J.; Hurand, S.; Feuillet-Palma, C.; Rastogi, A.; Budhani, R. C.; Reyren, N.; Lesne, E.; Lesueur, J.; Bergeal, N.
2014-01-01
In LaTiO3/SrTiO3 and LaAlO3/SrTiO3 heterostructures, the bending of the SrTiO3 conduction band at the interface forms a quantum well that contains a superconducting two-dimensional electron gas (2-DEG). Its carrier density and electronic properties, such as superconductivity and Rashba spin-orbit coupling can be controlled by electrostatic gating. In this article we show that the Fermi energy lies intrinsically near the top of the quantum well. Beyond a filling threshold, electrons added by electrostatic gating escape from the well, hence limiting the possibility to reach a highly-doped regime. This leads to an irreversible doping regime where all the electronic properties of the 2-DEG, such as its resistivity and its superconducting transition temperature, saturate. The escape mechanism can be described by the simple analytical model we propose. PMID:25346028
Chiral magnetism of magnetic adatoms generated by Rashba electrons
NASA Astrophysics Data System (ADS)
Bouaziz, Juba; dos Santos Dias, Manuel; Ziane, Abdelhamid; Benakki, Mouloud; Blügel, Stefan; Lounis, Samir
2017-02-01
We investigate long-range chiral magnetic interactions among adatoms mediated by surface states spin-splitted by spin–orbit coupling. Using the Rashba model, the tensor of exchange interactions is extracted wherein a thepseudo-dipolar interaction is found, in addition to the usual isotropic exchange interaction and the Dzyaloshinskii–Moriya interaction. We find that, despite the latter interaction, collinear magnetic states can still be stabilized by the pseudo-dipolar interaction. The interadatom distance controls the strength of these terms, which we exploit to design chiral magnetism in Fe nanostructures deposited on a Au(111) surface. We demonstrate that these magnetic interactions are related to superpositions of the out-of-plane and in-plane components of the skyrmionic magnetic waves induced by the adatoms in the surrounding electron gas. We show that, even if the interatomic distance is large, the size and shape of the nanostructures dramatically impacts on the strength of the magnetic interactions, thereby affecting the magnetic ground state. We also derive an appealing connection between the isotropic exchange interaction and the Dzyaloshinskii–Moriya interaction, which relates the latter to the first-order change of the former with respect to spin–orbit coupling. This implies that the chirality defined by the direction of the Dzyaloshinskii–Moriya vector is driven by the variation of the isotropic exchange interaction due to the spin–orbit interaction.
Nonlinear effects in two-dimensional & layered electronic systems
NASA Astrophysics Data System (ADS)
Lee, Changjin
In this dissertation, nonlinear effects of strongly correlated 2D and layered electronic system are focused on within the framework of quasi-localized charge approximation (QLCA) and dynamic mean field theory (DMFT). In Part I, it is shown that QLCA scheme can be generalized beyond the harmonic approximation into the nonlinear regime, as a powerful tool to handle with not only the liquid phase but also the solid phase of the strongly correlated classical bilayer system. (a) The quadratic order equation of a single quasi-localized charge (QLC) for the strongly coupled classical bilayer system interacting via any general isotropic scalar potential has been derived in real space from first principle, and it is applied to the strongly coupled Coulomb bilayer system (b) The quadratic order collective mode QLCA equation has been derived in real space. (c) The Fourier space representation of quadratic QLCA equation is obtained. (d) Some difficulties for solving quadratic order QLCA equation are emphasized for the future study. In Part II, (a) the formal derivation of the longitudinal quadratic Density Response Function (qDRF) will be given in terms of the modified three-point Density Correlation Function (DCF: symbolized as F-function) not only to extract the naive symmetry of 2D qDRF in imaginary frequency space, but also to point out that the modified DCF does not stand alone because it can violate Pauli principle. (b) The modified three-point longitudinal DCF (F-function) has been calculated with the mathematical rigor. (c) It is shown that the static qDRF develops strong peaks as well as fore-reported properties of vanishing and discontinuity. (d) The mathematical mechanism of vanishing and discontinuity of static qDRF will be given. (e) The vanishing of qDRF is shown not limited to the static qDRF.
Kohda, M.; Altmann, P.; Salis, G.; Schuh, D.; Ganichev, S. D.; Wegscheider, W.
2015-10-26
A method is presented that enables the measurement of spin-orbit coefficients in a diffusive two-dimensional electron gas without the need for processing the sample structure, applying electrical currents or resolving the spatial pattern of the spin mode. It is based on the dependence of the average electron velocity on the spatial distance between local excitation and detection of spin polarization, resulting in a variation of spin precession frequency that in an external magnetic field is linear in the spatial separation. By scanning the relative positions of the exciting and probing spots in a time-resolved Kerr rotation microscope, frequency gradients along the [100] and [010] crystal axes of GaAs/AlGaAs QWs are measured to obtain the Rashba and Dresselhaus spin-orbit coefficients, α and β. This simple method can be applied in a variety of materials with electron diffusion for evaluating spin-orbit coefficients.
Dong, Hui; Lewis, Nicholas H. C.; Oliver, Thomas A. A.; ...
2015-05-07
Changes in the electronic structure of pigments in protein environments and of polar molecules in solution inevitably induce a re-adaption of molecular nuclear structure. Both changes of electronic and vibrational energies can be probed with visible or infrared lasers, such as two-dimensional electronic spectroscopy or vibrational spectroscopy. The extent to which the two changes are correlated remains elusive. The recent demonstration of two-dimensional electronic-vibrational (2DEV) spectroscopy potentially enables a direct measurement of this correlation experimentally. However, it has hitherto been unclear how to characterize the correlation from the spectra. In this report, we present a theoretical formalism to demonstrate themore » slope of the nodal line between the excited state absorption and ground state bleach peaks in the spectra as a characterization of the correlation between electronic and vibrational transition energies. In conclusion, we also show the dynamics of the nodal line slope is correlated to the vibrational spectral dynamics. Additionally, we demonstrate the fundamental 2DEV spectral line-shape of a monomer with newly developed response functions« less
Dong, Hui; Lewis, Nicholas H. C.; Oliver, Thomas A. A.; Fleming, Graham R.
2015-05-07
Changes in the electronic structure of pigments in protein environments and of polar molecules in solution inevitably induce a re-adaption of molecular nuclear structure. Both changes of electronic and vibrational energies can be probed with visible or infrared lasers, such as two-dimensional electronic spectroscopy or vibrational spectroscopy. The extent to which the two changes are correlated remains elusive. The recent demonstration of two-dimensional electronic-vibrational (2DEV) spectroscopy potentially enables a direct measurement of this correlation experimentally. However, it has hitherto been unclear how to characterize the correlation from the spectra. In this paper, we present a theoretical formalism to demonstrate the slope of the nodal line between the excited state absorption and ground state bleach peaks in the spectra as a characterization of the correlation between electronic and vibrational transition energies. We also show the dynamics of the nodal line slope is correlated to the vibrational spectral dynamics. Additionally, we demonstrate the fundamental 2DEV spectral line-shape of a monomer with newly developed response functions.
Dong, Hui; Lewis, Nicholas H C; Oliver, Thomas A A; Fleming, Graham R
2015-05-07
Changes in the electronic structure of pigments in protein environments and of polar molecules in solution inevitably induce a re-adaption of molecular nuclear structure. Both changes of electronic and vibrational energies can be probed with visible or infrared lasers, such as two-dimensional electronic spectroscopy or vibrational spectroscopy. The extent to which the two changes are correlated remains elusive. The recent demonstration of two-dimensional electronic-vibrational (2DEV) spectroscopy potentially enables a direct measurement of this correlation experimentally. However, it has hitherto been unclear how to characterize the correlation from the spectra. In this paper, we present a theoretical formalism to demonstrate the slope of the nodal line between the excited state absorption and ground state bleach peaks in the spectra as a characterization of the correlation between electronic and vibrational transition energies. We also show the dynamics of the nodal line slope is correlated to the vibrational spectral dynamics. Additionally, we demonstrate the fundamental 2DEV spectral line-shape of a monomer with newly developed response functions.
Dong, Hui; Lewis, Nicholas H. C.; Oliver, Thomas A. A.; Fleming, Graham R.
2015-05-07
Changes in the electronic structure of pigments in protein environments and of polar molecules in solution inevitably induce a re-adaption of molecular nuclear structure. Both changes of electronic and vibrational energies can be probed with visible or infrared lasers, such as two-dimensional electronic spectroscopy or vibrational spectroscopy. The extent to which the two changes are correlated remains elusive. The recent demonstration of two-dimensional electronic-vibrational (2DEV) spectroscopy potentially enables a direct measurement of this correlation experimentally. However, it has hitherto been unclear how to characterize the correlation from the spectra. In this report, we present a theoretical formalism to demonstrate the slope of the nodal line between the excited state absorption and ground state bleach peaks in the spectra as a characterization of the correlation between electronic and vibrational transition energies. In conclusion, we also show the dynamics of the nodal line slope is correlated to the vibrational spectral dynamics. Additionally, we demonstrate the fundamental 2DEV spectral line-shape of a monomer with newly developed response functions
Two-Dimensional Crystallization of the Ca(2+)-ATPase for Electron Crystallography.
Glaves, John Paul; Primeau, Joseph O; Young, Howard S
2016-01-01
Electron crystallography of two-dimensional crystalline arrays is a powerful alternative for the structure determination of membrane proteins. The advantages offered by this technique include a native membrane environment and the ability to closely correlate function and dynamics with crystalline preparations and structural data. Herein, we provide a detailed protocol for the reconstitution and two-dimensional crystallization of the sarcoplasmic reticulum calcium pump (also known as Ca(2+)-ATPase or SERCA) and its regulatory subunits phospholamban and sarcolipin.
Taylor, R J E; Childs, D T D; Ivanov, P; Stevens, B J; Babazadeh, N; Crombie, A J; Ternent, G; Thoms, S; Zhou, H; Hogg, R A
2015-08-20
We demonstrate a semiconductor PCSEL array that uniquely combines an in-plane waveguide structure with nano-scale patterned PCSEL elements. This novel geometry allows two-dimensional electronically controllable coherent coupling of remote vertically emitting lasers. Mutual coherence of the PCSEL elements is verified through the demonstration of a two-dimensional Young's Slits experiment. In addition to allowing the all-electronic control of the interference pattern, this type of device offers new routes to power and brightness scaling in semiconductor lasers, and opportunities for all-electronic beam steering.
NASA Astrophysics Data System (ADS)
Taylor, R. J. E.; Childs, D. T. D.; Ivanov, P.; Stevens, B. J.; Babazadeh, N.; Crombie, A. J.; Ternent, G.; Thoms, S.; Zhou, H.; Hogg, R. A.
2015-08-01
We demonstrate a semiconductor PCSEL array that uniquely combines an in-plane waveguide structure with nano-scale patterned PCSEL elements. This novel geometry allows two-dimensional electronically controllable coherent coupling of remote vertically emitting lasers. Mutual coherence of the PCSEL elements is verified through the demonstration of a two-dimensional Young’s Slits experiment. In addition to allowing the all-electronic control of the interference pattern, this type of device offers new routes to power and brightness scaling in semiconductor lasers, and opportunities for all-electronic beam steering.
Two-dimensional electron gas in monolayer InN quantum wells
Pan, Wei; Dimakis, Emmanouil; Wang, George T.; Moustakas, Theodore D.; Tsui, Daniel C.
2014-11-24
We report in this letter experimental results that confirm the two-dimensional nature of the electron systems in monolayer InN quantum wells embedded in GaN barriers. The electron density and mobility of the two-dimensional electron system (2DES) in these InN quantum wells are 5×10^{15} cm^{-2} and 420 cm^{2 }/Vs, respectively. Moreover, the diagonal resistance of the 2DES shows virtually no temperature dependence in a wide temperature range, indicating the topological nature of the 2DES.
Taylor, R. J. E.; Childs, D. T. D.; Ivanov, P.; Stevens, B. J.; Babazadeh, N.; Crombie, A. J.; Ternent, G.; Thoms, S.; Zhou, H.; Hogg, R. A.
2015-01-01
We demonstrate a semiconductor PCSEL array that uniquely combines an in-plane waveguide structure with nano-scale patterned PCSEL elements. This novel geometry allows two-dimensional electronically controllable coherent coupling of remote vertically emitting lasers. Mutual coherence of the PCSEL elements is verified through the demonstration of a two-dimensional Young’s Slits experiment. In addition to allowing the all-electronic control of the interference pattern, this type of device offers new routes to power and brightness scaling in semiconductor lasers, and opportunities for all-electronic beam steering. PMID:26289621
Two-dimensional electron gas in monolayer InN quantum wells
Pan, Wei; Dimakis, Emmanouil; Wang, George T.; ...
2014-11-24
We report in this letter experimental results that confirm the two-dimensional nature of the electron systems in monolayer InN quantum wells embedded in GaN barriers. The electron density and mobility of the two-dimensional electron system (2DES) in these InN quantum wells are 5×1015 cm-2 and 420 cm2 /Vs, respectively. Moreover, the diagonal resistance of the 2DES shows virtually no temperature dependence in a wide temperature range, indicating the topological nature of the 2DES.
NASA Astrophysics Data System (ADS)
Muraguchi, M.; Endoh, T.; Takada, Y.; Sakurai, Y.; Nomura, S.; Shiraishi, K.; Ikeda, M.; Makihara, K.; Miyazaki, S.; Shigeta, Y.
2010-09-01
We report the unexpected temperature dependence of electron tunneling from the two-dimensional electron gas (2DEG) to the Si-dot in a Si-dots floating gate metal-oxide-semiconductor (MOS) capacitor. We indicate that this temperature dependence of the electron tunneling cannot be explained by the conventional one-dimensional tunneling model, and show that it is necessary for a new model which includes the geometrical factor of the system. To extract a mechanism of the electron injection process from the 2DEG to the nano-structure, we have employed the numerical simulation, which includes both the geometrical condition of the system and the experimental setup. We suggest in our new tunneling model that the main contribution to the electron tunneling is induced by the wave-packet-like state of the electron below the Si-dots. We successfully show that the temperature dependence of the electron injection voltage in the Si-dots floating gate MOS capacitor fits our model. This indicates that the spatial distribution of electron density in the two-dimensional electron gas would play a crucial role in the electron tunneling.
Wan, Zhong; Kazakov, Aleksandr; Manfra, Michael J; Pfeiffer, Loren N; West, Ken W; Rokhinson, Leonid P
2015-06-11
Search for Majorana fermions renewed interest in semiconductor-superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime. Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor-superconductor ohmic contacts. Supercurrent with characteristic temperature dependence of a ballistic junction has been observed across 0.6 μm, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields.
Wan, Zhong; Kazakov, Aleksandr; Manfra, Michael J.; Pfeiffer, Loren N.; West, Ken W.; Rokhinson, Leonid P.
2015-01-01
Search for Majorana fermions renewed interest in semiconductor–superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime. Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor–superconductor ohmic contacts. Supercurrent with characteristic temperature dependence of a ballistic junction has been observed across 0.6 μm, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields. PMID:26067452
Two-dimensional electronic spectra of the photosynthetic apparatus of green sulfur bacteria
Kramer, Tobias; Rodriguez, Mirta
2017-01-01
Advances in time resolved spectroscopy have provided new insight into the energy transmission in natural photosynthetic complexes. Novel theoretical tools and models are being developed in order to explain the experimental results. We provide a model calculation for the two-dimensional electronic spectra of Cholorobaculum tepidum which correctly describes the main features and transfer time scales found in recent experiments. From our calculation one can infer the coupling of the antenna chlorosome with the environment and the coupling between the chlorosome and the Fenna-Matthews-Olson complex. We show that environment assisted transport between the subunits is the required mechanism to reproduce the experimental two-dimensional electronic spectra. PMID:28345621
Effects of finite laser pulse width on two-dimensional electronic spectroscopy
NASA Astrophysics Data System (ADS)
Leng, Xuan; Yue, Shuai; Weng, Yu-Xiang; Song, Kai; Shi, Qiang
2017-01-01
We combine the hierarchical equations of motion method and the equation-of-motion phase-matching approach to calculate two-dimensional electronic spectra of model systems. When the laser pulse is short enough, the current method reproduces the results based on third-order response function calculations in the impulsive limit. Finite laser pulse width is found to affect both the peak positions and shapes, as well as the time evolution of diagonal and cross peaks. Simulations of the two-color two-dimensional electronic spectra also show that, to observe quantum beats in the diagonal and cross peaks, it is necessary to excite the related excitonic states simultaneously.
2013-01-01
In J-aggregates of cyanine dyes, closely packed molecules form mesoscopic tubes with nanometer-diameter and micrometer-length. Their efficient energy transfer pathways make them suitable candidates for artificial light harvesting systems. This great potential calls for an in-depth spectroscopic analysis of the underlying energy deactivation network and coherence dynamics. We use two-dimensional electronic spectroscopy with sub-10 fs laser pulses in combination with two-dimensional decay-associated spectra analysis to describe the population flow within the aggregate. Based on the analysis of Fourier-transform amplitude maps, we distinguish between vibrational or vibronic coherence dynamics as the origin of pronounced oscillations in our two-dimensional electronic spectra. PMID:23461650
King, P.D.C.
2012-03-01
We demonstrate the formation of a two-dimensional electron gas (2DEG) at the (100) surface of the 5d transition-metal oxide KTaO{sub 3}. From angle-resolved photoemission, we find that quantum confinement lifts the orbital degeneracy of the bulk band structure and leads to a 2DEG composed of ladders of subband states of both light and heavy carriers. Despite the strong spin-orbit coupling, we find no experimental signatures of a Rashba spin splitting, which has important implications for the interpretation of transport measurements in both KTaO{sub 3}- and SrTiO{sub 3}-based 2DEGs. The polar nature of the KTaO{sub 3}(100) surface appears to help mediate formation of the 2DEG as compared to non-polar SrTiO{sub 3}(100).
NASA Astrophysics Data System (ADS)
Wang, Zhigang; Zhang, Wei; Zhang, Ping
2009-06-01
We study the de Haas-van Alphen (dHvA) oscillations in the magnetization of a two-dimensional electron gas under the influence of the edge states and/or the Rashba spin-orbit interaction (SOI). The boundaries of the systems lift partially the degeneracies of Landau levels (LLs) and the resulting edge states lead to the changes in both the center and the amplitude of the sawtoothlike magnetization oscillation. The SOI mixes the spin-up and spin-down states of neighboring LLs into two unequally spaced energy branches. The inclusion of SOI changes the well-defined sawtooth pattern of the dHvA oscillations in the magnetization. The weaker the magnetic field is, the larger the change in the dHvA oscillations is due to the edge effect and/or the spin-orbit coupling. Some theoretical results are compared with the experimental data.
NASA Astrophysics Data System (ADS)
Yang, Chunlei
This thesis focuses on the two important parts in the spintronic devices based on the spin field effect transistor: (1) the ferromagnetic GaMnAs thin films which is the source-drain material and (2) the Rashba spin-orbit coupling in a two-dimensional electron gas in which the spin can be transported and its polarization can be tuned by external gates. We have grown GaMnAs samples using molecular beam epitaxy (MBE) with Mn composition up to 8%. The intrinsic and extrinsic contribution to the lattice parameter of the low temperature grown GaMnAs is discussed. The influence of the defects on the electrical and magnetic properties of GaMnAs thin films is presented. We have also studied optical properties of GaMnAs thin films and quantum wells using absorption spectrum. The magnetic circular dichroism is employed to study the p-d exchange interaction induced spin splitting. The Rashba spin-orbit coupling in a InGaAs/InAlAs two dimensional electron gas (2DEG) is demonstrated by the beating patterns in the Shubnikov de Hass oscillation. Based on the coupling between the spin and orbit momentum, we demonstrate the ways to use spin to drive current by the circular photo galvanic effect (CPGE) and the spin galvanic effect (SGE) with interband excitation. And conversely we show, for the first time, that an electric current can induce spin polarization in a 2DEG, which provides us the opportunity to manipulate spin using electric field instead of magnetic field for the future spintronic devices.
Numerical Studies of Collective Phenomena in Two-Dimensional Electron and Cold Atom Systems
Rezayi, Edward
2013-07-25
Numerical calculations were carried out to investigate a number of outstanding questions in both two-dimensional electron and cold atom systems. These projects aimed to increase our understanding of the properties of and prospects for non-Abelian states in quantum Hall matter.
THE TWO-DIMENSIONAL VALENCE ELECTRONIC STRUCTURE OF A MONOLYAER OF Ag ON Cu(00l)
Tobin, J.G.; Robey, S.W.; Shirley, D.A.
1985-05-01
The metal overlayer system c(10x2)Ag/Cu(001) was studied at coverages near one monolayer with angle-resolved photoemission. The observed spectroscopic features indicate a two-dimensional d-band electronic structure that can be interpreted using a model with planar, hexagonal symmetry in which crystal field effects dominate over spin-orbit effects.
Raman scattering in a two-dimensional electron gas: Boltzmann equation approach
NASA Astrophysics Data System (ADS)
Mishchenko, E. G.
1999-06-01
The inelastic light scattering in a two-dimensional electron gas is studied theoretically using the Boltzmann equation techniques. Electron-hole excitations produce the Raman spectrum essentially different from the one predicted for the 3D case. In the clean limit it has the form of a strong nonsymmetric resonance due to the square-root singularity at the electron-hole frequency ω=vk, while in the opposite dirty limit the usual Lorentzian shape of the cross section is reestablished. The effects of electromagnetic field are considered self-consistently, and the contribution from collective plasmon modes is found. It is shown that unlike 3D metals where plasmon excitations are unobservable (because of very large required transferred frequencies), the two-dimensional electron system gives rise to a low-frequency (ω~k1/2) plasmon peak. A measurement of the width of this peak can provide data on the magnitude of the electron-scattering rate.
Two-dimensional electromagnetic Child-Langmuir law of a short-pulse electron flow
Chen, S. H.; Tai, L. C.; Liu, Y. L.; Ang, L. K.; Koh, W. S.
2011-02-15
Two-dimensional electromagnetic particle-in-cell simulations were performed to study the effect of the displacement current and the self-magnetic field on the space charge limited current density or the Child-Langmuir law of a short-pulse electron flow with a propagation distance of {zeta} and an emitting width of W from the classical regime to the relativistic regime. Numerical scaling of the two-dimensional electromagnetic Child-Langmuir law was constructed and it scales with ({zeta}/W) and ({zeta}/W){sup 2} at the classical and relativistic regimes, respectively. Our findings reveal that the displacement current can considerably enhance the space charge limited current density as compared to the well-known two-dimensional electrostatic Child-Langmuir law even at the classical regime.
NASA Astrophysics Data System (ADS)
Marocchino, A.; Lapenta, G.; Evstatiev, E. G.; Nebel, R. A.; Park, J.
2006-10-01
Theoretical works by Barnes and Nebel [D. C. Barnes and R. A. Nebel, Phys. Plasmas 5, 2498 (1998); R. A. Nebel and D. C. Barnes, Fusion Technol. 38, 28 (1998)] have suggested that a tiny oscillating ion cloud (referred to as the periodically oscillating plasma sphere or POPS) may undergo a self-similar collapse in a harmonic oscillator potential formed by a uniform electron background. A major uncertainty in this oscillating plasma scheme is the stability of the virtual cathode that forms the harmonic oscillator potential. The electron-electron two-stream stability of the virtual cathode has previously been studied with a fluid model, a slab kinetic model, a spherically symmetric kinetic model, and experimentally [R. A. Nebel and J. M. Finn, Phys. Plasmas 8, 1505 (2001); R. A. Nebel et al., Phys. Plasmas 12, 040501 (2005)]. Here the mode is studied with a two-dimensional particle-in-cell code. Results indicate stability limits near those of the previously spherically symmetric case.
The effect of depolarization fields on the electronic properties of two-dimensional materials
NASA Astrophysics Data System (ADS)
Shin, Young-Han; Kim, Hye Jung; Noor-A-Alam, Mohammad
2015-03-01
Graphene is a two-dimensional semimetal with a zero band gap. By weakening the sp2 covalent bonding of graphene with additional elements such as hydrogen or fluorine, however, it is possible to make it insulating. We can expect that the band gap converges to that of a three-dimensional analogue by repeating such two-dimensional layers along the normal to the layer. If we control the position of additional elements to make a dipole monolayer, the system will have an intrinsic internal field decreases as the number of layers increases. But, for two-dimensional bilayers, depolarization field is so strong that its electronic properties can be much different from its monolayer analogue. In this presentation, we show that the internal fields induced by dipole moments can change electronic properties of two-dimensional materials such as graphene-like structures and complex metal oxides. This work was supported by the National Research Foundation of Korea Grant by the Ministry of Education, Science, and Technology (2009-0093818, 2012-014007, 2014M3A7B4049367)
Two-dimensional electron gas in monolayer InN quantum wells
Pan, W. E-mail: e.dimakis@hzdr.de; Wang, G. T.; Dimakis, E. E-mail: e.dimakis@hzdr.de; Moustakas, T. D.; Tsui, D. C.
2014-11-24
We report in this letter experimental results that confirm the two-dimensional nature of the electron systems in a superlattice structure of 40 InN quantum wells consisting of one monolayer of InN embedded between 10 nm GaN barriers. The electron density and mobility of the two-dimensional electron system (2DES) in these InN quantum wells are 5 × 10{sup 15 }cm{sup −2} (or 1.25 × 10{sup 14 }cm{sup −2} per InN quantum well, assuming all the quantum wells are connected by diffused indium contacts) and 420 cm{sup 2}/Vs, respectively. Moreover, the diagonal resistance of the 2DES shows virtually no temperature dependence in a wide temperature range, indicating the topological nature of the 2DES.
Finite two-dimensional electron gas in a patterned semiconductor system
NASA Astrophysics Data System (ADS)
Ciftja, Orion; Livingston, Victoria; Thomas, Elsa; Saganti, Seth
On various occasions, fabrication of a two-dimensional semiconductor quantum dot leads to a small system of electrons confined in a domain that is not circular and may have a pronounced square (or rectangular) shape. In this work we consider a square-shaped semiconductor quantum dot configuration and treat the system of electrons as a finite two-dimensional electron gas. Within this framework, we adopt a Hartree-Fock approach and study the properties of a small two-dimensional system of electrons confined in a finite square region. We calculate the energy for various finite systems of fully spin-polarized (spinless) electrons interacting with a Coulomb potential. The results give a fairly accurate picture of how the energy of the finite system evolves towards the bulk value as the size of the system increases. The calculations for a square domain are challenging since expressions depend in each component of particle's position and not the radial distance from the center of the square-shaped semiconductor quantum dot. Therefore, we also consider a possible circularly symmetric approximation to the problem. We assess the quality of this approximation and discuss instances where its use is not only desirable, but also accurate. This research was supported in part by U.S. Army Research Office (ARO) Grant No. W911NF-13-1-0139 and National Science Foundation (NSF) Grant No. DMR-1410350.
Electron compound nature in a surface atomic layer of a two-dimensional hexagonal lattice
NASA Astrophysics Data System (ADS)
Matsuda, Iwao; Nakamura, Fumitaka; Kubo, Keisuke; Hirahara, Toru; Yamazaki, Shiro; Choi, Won Hoon; Yeom, Han Woong; Narita, Hisashi; Fukaya, Yuki; Hashimoto, Mie; Kawasuso, Atsuo; Ono, Masanori; Hasegawa, Yukio; Hasegawa, Shuji; Kobayashi, Katsuyoshi
2010-10-01
The two-dimensional (2D) ordered phase of monovalent metal alloy, 21×21 , is formed on the Si(111) surface with the constant electron/atom ratio, indicating electron compound nature. Two conventional theories of the Hume-Rothery compounds, Jones model (nearly-free-electron model), and pseudopotential model (interionic interaction model), were applied to examine stability of the 2D phase. We found breakdown of the former and confirmation of the latter approaches with importance of medium-range interatomic interaction, mediated by the 2D surface-state electrons, in the latter approach.
Transport signatures of correlated disorder in a two-dimensional electron gas
NASA Astrophysics Data System (ADS)
Heinzel, T.; Jäggi, R.; Ribeiro, E.; Waldkirch, M. v.; Ensslin, K.; Ulloa, S. E.; Medeiros-Ribeiro, G.; Petroff, P. M.
2003-03-01
We report electronic transport measurements on two-dimensional electron gases in a Ga[Al]As heterostructure with an embedded layer of InAs self-assembled quantum dots. At high InAs dot densities, pronounced Altshuler-Aronov-Spivak magnetoresistance oscillations are observed, which indicate short-range ordering of the potential landscape formed by the charged dots and the strain fields. The presence of these oscillations coincides with the observation of a metal-insulator transition, and a maximum in the electron mobility as a function of the electron density. Within a model based on correlated disorder, we establish a relation between these effects.
Generalov, Alexander; Otrokov, Mikhail M; Chikina, Alla; Kliemt, Kristin; Kummer, Kurt; Höppner, Marc; Güttler, Monika; Seiro, Silvia; Fedorov, Alexander; Schulz, Susanne; Danzenbächer, Steffen; Chulkov, Evgueni V; Geibel, Christoph; Laubschat, Clemens; Dudin, Pavel; Hoesch, Moritz; Kim, Timur; Radovic, Milan; Shi, Ming; Plumb, Nicholas C; Krellner, Cornelius; Vyalikh, Denis V
2017-02-08
Finding ways to create and control the spin-dependent properties of two-dimensional electron states (2DESs) is a major challenge for the elaboration of novel spin-based devices. Spin-orbit and exchange-magnetic interactions (SOI and EMI) are two fundamental mechanisms that enable access to the tunability of spin-dependent properties of carriers. The silicon surface of HoRh2Si2 appears to be a unique model system, where concurrent SOI and EMI can be visualized and controlled by varying the temperature. The beauty and simplicity of this system lie in the 4f moments, which act as a multiple tuning instrument on the 2DESs, as the 4f projections parallel and perpendicular to the surface order at essentially different temperatures. Here we show that the SOI locks the spins of the 2DESs exclusively in the surface plane when the 4f moments are disordered: the Rashba-Bychkov effect. When the temperature is gradually lowered and the system experiences magnetic order, the rising EMI progressively competes with the SOI leading to a fundamental change in the spin-dependent properties of the 2DESs. The spins rotate and reorient toward the out-of-plane Ho 4f moments. Our findings show that the direction of the spins and the spin-splitting of the two-dimensional electrons at the surface can be manipulated in a controlled way by using only one parameter: the temperature.
Two-Dimensional Crystallization of Integral Membrane Proteins for Electron Crystallography
Stokes, David L.; Rice, William J.; Hu, Minghui; Kim, Changki; Ubarretxena, Iban
2011-01-01
Although membrane proteins make up 30% of the proteome and are a common target for therapeutic drugs, determination of their atomic structure remains a technical challenge. Electron crystallography represents an alternative to the conventional methods of X-ray diffraction and NMR and relies on the formation of two-dimensional crystals. These crystals are produced by reconstituting purified, detergent-solubilized membrane proteins back into the native environment of a lipid bilayer. This chapter reviews methods for producing two-dimensional crystals and for screening them by negative stain electron microscopy. In addition, we show examples of the different morphologies that are commonly obtained and describe basic image analysis procedures that can be used to evaluate their promise for structure determination by cryoelectron microsopy. PMID:20665267
NASA Astrophysics Data System (ADS)
Kim, Young Cheol; Jang, Sung Ho; Kim, Gun Ho; Chung, Chin Wook
2009-10-01
Real time two-dimensional spatial distribution measurement method of electron temperature and plasma density was developed. It is based on a floating probe method [1] because the floating probe has high time resolution. Two-dimensional array of sensors on a 300 mm diameter wafer-shaped printed circuit board (PCB) and a high speed multiplexer circuit were used for real time distribution measurement. The method was tested at various powers and pressures, spatial distributions of the electron temperature and the plasma density could be obtained. And in the measurement results, asymmetric plasma density distributions caused by pumping port effect could be observed. This method can measure spatial distribution of plasma parameters on the wafer in real time without plasma perturbation, therefore it will be expected to improve the uniformity of processing plasmas such as etching and deposition. [4pt] [1] M. H. Lee, S. H. Jang, C. W. Chung, J. Appl. Phys. 101, 033305 (2007).
NASA Astrophysics Data System (ADS)
Ma, Hui; Jiang, Chongyun; Liu, Yu; Zhu, Laipan; Qin, Xudong; Chen, Yonghai
2013-06-01
We investigate the circular photogalvanic effect (CPGE) excited by sub-bandgap radiation in a GaAs/Al0.3Ga0.7As two dimensional electron gas and tune its amplitude by synchronously imposing an above-bandgap unpolarized light at normal incidence. With this photo-modulation technique, we identify two microscopic mechanisms of CPGE according to the dramatic change of apparent Rashba and Dresselhaus effects. We suggest the optical transitions to be Franz-Keldysh and intraband regime, respectively. Both regimes coexist in conventional CPGE and the intraband regime dominates at sufficient modulation power.
Consistent calculation of the stopping power for slow ions in two-dimensional electron gases
Wang, You-Nian |; Ma, Teng-Gai
1997-03-01
Within the framework of quantum scattering theory, we present a consistent calculation of the stopping power for slow protons and antiprotons moving in two-dimensional electron gases. The Friedel sum rule is used to determine the screening constant in the scattering potential. For the stopping power our results are compared with that of the random-phase approximation dielectric theory and that predicted by the linear Thomas-Fermi potential. {copyright} {ital 1997} {ital The American Physical Society}
Ohm-Kirchhoff's law and screening in two-dimensional electron liquid
NASA Astrophysics Data System (ADS)
Mareš, J. J.; Krištofik, J.; Hubík, P.
2002-01-01
Analysing some experimental facts, a modification of Ohm-Kirchhoff's constitutive transport relation containing diffusion-related term was established. Simultaneous application of this formula and basic ideas of stochastic electrodynamics to a two-dimensional electron liquid (2DEL) enabled us to obtain relations coupling screening and transport properties of the 2DEL, which may be useful, e.g. for the interpretation of low-temperature magneto-capacitance experiments.
Layer-by-Layer Evolution of a Two-Dimensional Electron Gas Near an Oxide Interface
NASA Astrophysics Data System (ADS)
Chang, Young Jun; Moreschini, Luca; Bostwick, Aaron; Gaines, Geoffrey A.; Kim, Yong Su; Walter, Andrew L.; Freelon, Byron; Tebano, Antonello; Horn, Karsten; Rotenberg, Eli
2013-09-01
We report the momentum-resolved measurement of a two-dimensional electron gas at the LaTiO3/SrTiO3 interface by angle-resolved photoemission spectroscopy (ARPES). Thanks to an advanced sample preparation technique, the orbital character of the conduction electrons and the electronic correlations can be accessed quantitatively as each unit cell layer is added. We find that all of these quantities change dramatically with distance from the interface. These findings open the way to analogous studies on other heterostructures, which are traditionally a forbidden field for ARPES.
Collective Tunneling Model between Two-Dimensional Electron Gas to Si-Nano Dot
NASA Astrophysics Data System (ADS)
Muraguchi, M.; Sakurai, Y.; Takada, Y.; Nomura, S.; Shiraishi, K.; Makihara, K.; Ikeda, M.; Miyazaki, S.; Shigeta, Y.; Endoh, T.
2011-12-01
We study the temperature dependence of electron injection voltage in Si-Nano-Dot (Si-NDs) Floating Gate MOS capacitor by using the collective tunneling model, which models the tunneling between two-dimensional electron gas (2DEG) and the Si-NDs. We clarify the temperature dependence by numerical calculation, which emulate the experiment in this system, and we obtained a new insight into the origin of the temperature dependence. We have revealed that the collective tunneling model can reproduce the temperature dependence of electron tunneling.
NASA Astrophysics Data System (ADS)
Armigliato, A.; Balboni, R.; Carnevale, G. P.; Pavia, G.; Piccolo, D.; Frabboni, S.; Benedetti, A.; Cullis, A. G.
2003-03-01
A method of obtaining quantitative two-dimensional (2D) maps of strain by the convergent beam electron diffraction technique in a transmission electron microscope is described. It is based on the automatic acquisition of a series of diffraction patterns generated from digital rastering the electron spot in a matrix of points within a selected area of the sample. These patterns are stored in a database and the corresponding strain tensor at each point is calculated, thus yielding a 2D strain map. An example of application of this method to cross-sectioned cells fabricated for the 0.15 μm technology of flash memories is reported.
Interaction induced staggered spin-orbit order in two-dimensional electron gas
Das, Tanmoy
2012-06-05
Decoupling spin and charge transports in solids is among the many prerequisites for realizing spin electronics, spin caloritronics, and spin-Hall effect. Beyond the conventional method of generating and manipulating spin current via magnetic knob, recent advances have expanded the possibility to optical and electrical method which are controllable both internally and externally. Yet, due to the inevitable presence of charge excitations and electrical polarizibility in these methods, the separation between spin and charge degrees of freedom of electrons remains a challenge. Here we propose and formulate an interaction induced staggered spin-orbit order as a new emergent phase of matter. We show that when some form of inherent spin-splitting via Rashba-type spin-orbit coupling renders two helical Fermi surfaces to become significantly nested, a Fermi surface instability arises. To lift this degeneracy, a spontaneous symmetry breaking spin-orbit density wave develops, causing a surprisingly large quasiparticle gapping with chiral electronic states, with no active charge excitations. Since the staggered spin-orbit order is associated with a condensation energy, quantified by the gap value, destroying such spin-orbit interaction costs sufficiently large perturbation field or temperature or de-phasing time. BiAg2 surface state is shown to be a representative system for realizing such novel spin-orbit interaction with tunable and large strength, and the spin-splitting is decoupled from charge excitations.
Fidler, Andrew F.; Singh, Ved P.; Long, Phillip D.; Dahlberg, Peter D.; Engel, Gregory S.
2014-01-01
Time-resolved ultrafast optical probes of chiral dynamics provide a new window allowing us to explore how interactions with such structured environments drive electronic dynamics. Incorporating optical activity into time-resolved spectroscopies has proven challenging due to the small signal and large achiral background. Here, we demonstrate that two-dimensional electronic spectroscopy can be adapted to detect chiral signals and that these signals reveal how excitations delocalize and contract following excitation. We dynamically probe the evolution of chiral electronic structure in the light harvesting complex 2 of purple bacteria following photoexcitation by creating a chiral two-dimensional mapping. The dynamics of the chiral two-dimensional signal directly reports on changes in the degree of delocalization of the excitonic state following photoexcitation. The mechanism of energy transfer in this system may enhance transfer probability due to the coherent coupling among chromophores while suppressing fluorescence that arises from populating delocalized states. This generally applicable spectroscopy will provide an incisive tool to probe ultrafast transient molecular fluctuations that are obscured in non-chiral experiments. PMID:24504144
A CMOS VLSI IC for real-time opto-electronic two-dimensional histogram generation
NASA Astrophysics Data System (ADS)
Richstein, James K.
1993-12-01
Histogram generation, a standard image processing operation, is a record of the intensity distribution in the image. Histogram generation has straightforward implementations on digital computers using high level languages. A prototype of an optical-electronic histogram generator was designed and tested for 1-D objects using wirewrapped MSI TTL components. The system has shown to be fairly modular in design. The aspects of the extension to two dimensions and the VLSI implementation of this design are discussed. In this paper, we report a VLSI design to be used in a two-dimensional real-time histogram generation scheme. The overall system design is such that the electronic signal obtained from the optically scanned two-dimensional semi-opaque image is processed and displayed within a period of one cycle of the scanning process. Specifically, in the VLSI implementation of the two-dimensional histogram generator, modifications were made to the original design. For the two-dimensional application, the output controller was analyzed as a finite state machine. The process used to describe the required timing signals and translate them to a VLSI finite state machine using Computer Aided Design Tools is discussed. In addition, the circuitry for sampling, binning, and display were combined with the timing circuitry on one IC. In the original design, the pulse width of the electronically sampled photodetector is limited with an analog one-shot. The high sampling rates associated with the extension to two dimensions requires significant reduction in the original 1-D prototype's sample pulse width of approximately 75 ns. The alternate design using VLSI logic gates will provide one-shot pulse widths of approximately 3 ns.
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.
Edge spin accumulation in a two-dimensional electron gas with two subbands
NASA Astrophysics Data System (ADS)
Khaetskii, Alexander; Egues, J. Carlos
We have studied the edge spin accumulation in 2D electron gas due to the intrinsic mechanism of spin-orbit interaction for the case of a two-subband structure. This study is strongly motivated by recent experiments which observed the spin accumulation near the edges of a high mobility 2D electron system in a bilayer symmetric GaAs structure in contrast to zero effect in a single-layer configuration. Our theoretical explanation is based on the Rashba-like spin-orbit interaction which arises as a result of the coupling between two subband states of opposite parities in a symmetric quantum well. Following the method developed in, we have calculated the edge spin density in a quasi-ballistic regime, and explained the experimental results, in particular, a large magnitude of the edge spin density. We showed that one can easily proceed from the regime of strong spin accumulation to the regime of weak one. It opens up a possibility to construct an interesting new spintronic device Supported by FAPESP (Brazil).
Electromagnetic Aharonov-Bohm effect in a two-dimensional electron gas ring
NASA Astrophysics Data System (ADS)
van der Wiel, W. G.; Nazarov, Yu. V.; de Franceschi, S.; Fujisawa, T.; Elzerman, J. M.; Huizeling, E. W.; Tarucha, S.; Kouwenhoven, L. P.
2003-01-01
We define a mesoscopic ring in a two-dimensional electron gas interrupted by two tunnel barriers, enabling us to apply a well-defined potential difference between the two halves of the ring. The electron interference in the ring is modified using a perpendicular magnetic field and a bias voltage. We observe clear Aharonov-Bohm oscillations up to the quantum Hall regime as a function of both parameters. The electron travel time between the barriers is found to increase with the applied magnetic field. Introducing a scattering model, we develop a method to measure the nonequilibrium electron dephasing time, which becomes very short at high voltages and magnetic fields. The relevance of electron-electron interactions is discussed.
Quantum Hall effect in black phosphorus two-dimensional electron system.
Li, Likai; Yang, Fangyuan; Ye, Guo Jun; Zhang, Zuocheng; Zhu, Zengwei; Lou, Wenkai; Zhou, Xiaoying; Li, Liang; Watanabe, Kenji; Taniguchi, Takashi; Chang, Kai; Wang, Yayu; Chen, Xian Hui; Zhang, Yuanbo
2016-07-01
The development of new, high-quality functional materials has been at the forefront of condensed-matter research. The recent advent of two-dimensional black phosphorus has greatly enriched the materials base of two-dimensional electron systems (2DESs). Here, we report the observation of the integer quantum Hall effect in a high-quality black phosphorus 2DES. The high quality is achieved by embedding the black phosphorus 2DES in a van der Waals heterostructure close to a graphite back gate; the graphite gate screens the impurity potential in the 2DES and brings the carrier Hall mobility up to 6,000 cm(2) V(-1) s(-1). The exceptional mobility enabled us to observe the quantum Hall effect and to gain important information on the energetics of the spin-split Landau levels in black phosphorus. Our results set the stage for further study on quantum transport and device application in the ultrahigh mobility regime.
Amplification and directional emission of surface acoustic waves by a two-dimensional electron gas
Shao, Lei; Pipe, Kevin P.
2015-01-12
Amplification of surface acoustic waves (SAWs) by electron drift in a two-dimensional electron gas (2DEG) is analyzed analytically and confirmed experimentally. Calculations suggest that peak power gain per SAW radian occurs at a more practical carrier density for a 2DEG than for a bulk material. It is also shown that SAW emission with tunable directionality can be achieved by modulating a 2DEG's carrier density (to effect SAW generation) in the presence of an applied DC field that amplifies SAWs propagating in a particular direction while attenuating those propagating in the opposite direction.
Giant Andreev reflection in a two-dimensional electron gas coupled to superconductors
NASA Astrophysics Data System (ADS)
van Wees, Bart J.
1998-03-01
I will review recent experiments on ballistic transport in a two- dimensional electron gas coupled to superconductors (Work done in collaboration with A.F. Morpurgo, S.G. den Hartog, and T.M. Klapwijk). This system has made it possible to study various aspects of coherent Andreev reflection, such as the modification of the conductance of a quantum point contact due to retroreflected holes, and the formation of Andreev bound states between two superconductors. In particular I will discuss the phenomenon of giant Andreev reflection, where, despite of the presence of disorder, electrons injected through a quantum point contact can be Andreev reflected with probability one.
Zener tunneling between landau orbits in a high-mobility two-dimensional electron gas.
Yang, C L; Zhang, J; Du, R R; Simmons, J A; Reno, J L
2002-08-12
Magnetotransport in a laterally confined two-dimensional electron gas (2DEG) can exhibit modified scattering channels owing to a tilted Hall potential. Transitions of electrons between Landau levels with shifted guiding centers can be accomplished through a Zener tunneling mechanism, and make a significant contribution to the magnetoresistance. A remarkable oscillation effect in weak field magnetoresistance has been observed in high-mobility 2DEGs in GaAs -Al Ga 0.3As (0.7) heterostructures, and can be well explained by the Zener mechanism.
Electron spin rotations induced by oscillating Rashba interaction in a quantum wire
NASA Astrophysics Data System (ADS)
Pawłowski, J.; Szumniak, P.; Bednarek, S.
2016-01-01
A method and nanodevice are introduced that allows us to rotate the single electron spin confined in a gated electrostatic InSb nanowire quantum dot. The proposed method does not require the application of any (oscillating or static) external magnetic fields. Our proposal instead employs spatial and time modulation of confining potential induced by electric gates, which, in turn leads to oscillating Rashba-type spin-orbit coupling. Moving electron back and forth in such a variable Rashba field allows for the realization of spin rotations around two different axes separately without using an external magnetic field. The results are supported by realistic three-dimensional time-dependent Poisson-Schrödinger calculations for systems and material parameters corresponding to experimentally accessible structures.
NASA Astrophysics Data System (ADS)
Ramaswamy, Rahul
Two-dimensional electron gas (2DEG) in semiconductor heterostructures was identified as a promising medium for hot-electron bolometers (HEB) in the early 90s. Up until now all research based on 2DEG HEBs is done using high mobility AlGaAs/GaAs heterostructures. These systems have demonstrated very good performance, but only in the sub terahertz (THz) range. However, above ˜0.5 THz the performance of AlGaAs/GaAs detectors drastically deteriorates. It is currently understood, that detectors fabricated from standard AlGaAs/GaAs heterostructures do not allow for reasonable coupling to THz radiation while maintaining high conversion efficiency. In this work we have developed 2DEG HEBs based on disordered Gallium Nitride (GaN) semiconductor, that operate at frequencies beyond 1THz at room temperature. We observe strong free carrier absorption at THz frequencies in our disordered 2DEG film due to Drude absorption. We show the design and fabrication procedures of novel micro-bolometers having ultra-low heat capacities. In this work the mechanism of 2DEG response to THz radiation is clearly identified as bolometric effect through our direct detection measurements. With optimal doping and detector geometry, impedances of 10--100 O have been achieved, which allow integration of these devices with standard THz antennas. We also demonstrate performance of the antennas used in this work in effectively coupling THz radiation to the micro-bolometers through polarization dependence and far field measurements. Finally heterodyne mixing due to hot electrons in the 2DEG micro-bolometer has been performed at sub terahertz frequencies and a mixing bandwidth greater than 3GHz has been achieved. This indicates that the characteristic cooling time in our detectors is fast, less than 50ps. Due to the ultra-low heat capacity; these detectors can be used in a heterodyne system with a quantum cascade laser (QCL) as a local oscillator (LO) which typically provides output powers in the micro
The Instability of Terahertz Plasma Waves in Two Dimensional Gated and Ungated Quantum Electron Gas
NASA Astrophysics Data System (ADS)
Zhang, Liping
2016-04-01
The instability of terahertz (THz) plasma waves in two-dimensional (2D) quantum electron gas in a nanometer field effect transistor (FET) with asymmetrical boundary conditions has been investigated. We analyze THz plasma waves of two parts of the 2D quantum electron gas: gated and ungated regions. The results show that the radiation frequency and the increment (radiation power) in 2D ungated quantum electron gas are much higher than that in 2D gated quantum electron gas. The quantum effects always enhance the radiation power and enlarge the region of instability in both cases. This allows us to conclude that 2D quantum electron gas in the transistor channel is important for the emission and detection process and both gated and ungated parts take part in that process. supported by National Natural Science Foundation of China (No. 10975114)
Resistance oscillations of two-dimensional electrons in crossed electric and tilted magnetic fields
NASA Astrophysics Data System (ADS)
Mayer, William; Vitkalov, Sergey; Bykov, A. A.
2016-06-01
The effect of dc electric field on transport of highly mobile two-dimensional electrons is studied in wide GaAs single quantum wells placed in titled magnetic fields. The study shows that in perpendicular magnetic field resistance oscillates due to electric-field induced Landau-Zener transitions between quantum levels that correspond to geometric resonances between cyclotron orbits and periodic modulation of electron density of states. Magnetic field tilt inverts these oscillations. Surprisingly the strongest inverted oscillations are observed at a tilt corresponding to nearly absent modulation of the electron density of states in regime of magnetic breakdown of semiclassical electron orbits. This phenomenon establishes an example of quantum resistance oscillations due to Landau quantization, which occur in electron systems with a constant density of states.
Quantum transport in Rashba spin-orbit materials: a review.
Bercioux, Dario; Lucignano, Procolo
2015-10-01
In this review article we describe spin-dependent transport in materials with spin-orbit interaction of Rashba type. We mainly focus on semiconductor heterostructures, however we consider topological insulators, graphene and hybrid structures involving superconductors as well. We start from the Rashba Hamiltonian in a two dimensional electron gas and then describe transport properties of two- and quasi-one-dimensional systems. The problem of spin current generation and interference effects in mesoscopic devices is described in detail. We address also the role of Rashba interaction on localisation effects in lattices with nontrivial topology, as well as on the Ahronov-Casher effect in ring structures. A brief section, in the end, describes also some related topics including the spin-Hall effect, the transition from weak localisation to weak anti localisation and the physics of Majorana fermions in hybrid heterostructures involving Rashba materials in the presence of superconductivity.
Hydrostatic pressure response of an oxide-based two-dimensional electron system
NASA Astrophysics Data System (ADS)
Zabaleta, J.; Borisov, V. S.; Wanke, R.; Jeschke, H. O.; Parks, S. C.; Baum, B.; Teker, A.; Harada, T.; Syassen, K.; Kopp, T.; Pavlenko, N.; Valentí, R.; Mannhart, J.
2016-06-01
Two-dimensional electron systems with fascinating properties exist in multilayers of standard semiconductors, on helium surfaces, and in oxides. Compared to the two-dimensional (2D) electron gases of semiconductors, the 2D electron systems in oxides are typically more strongly correlated and more sensitive to the microscopic structure of the hosting lattice. This sensitivity suggests that the oxide 2D systems are highly tunable by hydrostatic pressure. Here we explore the effects of hydrostatic pressure on the well-characterized 2D electron system formed at LaAlO3-SrTiO3 interfaces [A. Ohtomo and H. Y. Hwang, Nature (London) 427, 423 (2004), 10.1038/nature02308] and measure a pronounced, unexpected response. Pressure of ˜2 GPa reversibly doubles the 2D carrier density ns at 4 K. Along with the increase of ns, the conductivity and mobility are reduced under pressure. First-principles pressure simulations reveal the same behavior of the carrier density and suggest a possible mechanism of the mobility reduction, based on the dielectric properties of both materials and their variation under external pressure.
High-mobility capacitively-induced two-dimensional electrons in a lateral superlattice potential
Lu, T. M.; Laroche, D.; Huang, S.-H.; Chuang, Y.; Li, J.-Y.; Liu, C. W.
2016-01-01
In the presence of a lateral periodic potential modulation, two-dimensional electrons may exhibit interesting phenomena, such as a graphene-like energy-momentum dispersion, Bloch oscillations, or the Hofstadter butterfly band structure. To create a sufficiently strong potential modulation using conventional semiconductor heterostructures, aggressive device processing is often required, unfortunately resulting in strong disorder that masks the sought-after effects. Here, we report a novel fabrication process flow for imposing a strong lateral potential modulation onto a capacitively induced two-dimensional electron system, while preserving the host material quality. Using this process flow, the electron density in a patterned Si/SiGe heterostructure can be tuned over a wide range, from 4.4 × 1010 cm−2 to 1.8 × 1011 cm−2, with a peak mobility of 6.4 × 105 cm2/V·s. The wide density tunability and high electron mobility allow us to observe sequential emergence of commensurability oscillations as the density, the mobility, and in turn the mean free path, increase. Magnetic-field-periodic quantum oscillations associated with various closed orbits also emerge sequentially with increasing density. We show that, from the density dependence of the quantum oscillations, one can directly extract the steepness of the imposed superlattice potential. This result is then compared to a conventional lateral superlattice model potential. PMID:26865160
High-mobility capacitively-induced two-dimensional electrons in a lateral superlattice potential
Lu, Tzu -Ming; Laroche, Dominique; Huang, S. -H.; Chuang, Y.; Li, J. -Y.; Liu, C. W.
2016-01-01
In the presence of a lateral periodic potential modulation, two-dimensional electrons may exhibit interesting phenomena, such as a graphene-like energy-momentum dispersion, Bloch oscillations, or the Hofstadter butterfly band structure. To create a sufficiently strong potential modulation using conventional semiconductor heterostructures, aggressive device processing is often required, unfortunately resulting in strong disorder that masks the sought-after effects. Here, we report a novel fabrication process flow for imposing a strong lateral potential modulation onto a capacitively induced two-dimensional electron system, while preserving the host material quality. Using this process flow, the electron density in a patterned Si/SiGe heterostructure can be tuned over a wide range, from 4.4 × 10^{10} cm^{–2} to 1.8 × 10^{11} cm^{–2}, with a peak mobility of 6.4 × 10^{5} cm^{2}/V·s. The wide density tunability and high electron mobility allow us to observe sequential emergence of commensurability oscillations as the density, the mobility, and in turn the mean free path, increase. Magnetic-field-periodic quantum oscillations associated with various closed orbits also emerge sequentially with increasing density. We show that, from the density dependence of the quantum oscillations, one can directly extract the steepness of the imposed superlattice potential. Lastly, this result is then compared to a conventional lateral superlattice model potential.
High-mobility capacitively-induced two-dimensional electrons in a lateral superlattice potential
Lu, Tzu -Ming; Laroche, Dominique; Huang, S. -H.; ...
2016-01-01
In the presence of a lateral periodic potential modulation, two-dimensional electrons may exhibit interesting phenomena, such as a graphene-like energy-momentum dispersion, Bloch oscillations, or the Hofstadter butterfly band structure. To create a sufficiently strong potential modulation using conventional semiconductor heterostructures, aggressive device processing is often required, unfortunately resulting in strong disorder that masks the sought-after effects. Here, we report a novel fabrication process flow for imposing a strong lateral potential modulation onto a capacitively induced two-dimensional electron system, while preserving the host material quality. Using this process flow, the electron density in a patterned Si/SiGe heterostructure can be tuned overmore » a wide range, from 4.4 × 1010 cm–2 to 1.8 × 1011 cm–2, with a peak mobility of 6.4 × 105 cm2/V·s. The wide density tunability and high electron mobility allow us to observe sequential emergence of commensurability oscillations as the density, the mobility, and in turn the mean free path, increase. Magnetic-field-periodic quantum oscillations associated with various closed orbits also emerge sequentially with increasing density. We show that, from the density dependence of the quantum oscillations, one can directly extract the steepness of the imposed superlattice potential. Lastly, this result is then compared to a conventional lateral superlattice model potential.« less
High-mobility capacitively-induced two-dimensional electrons in a lateral superlattice potential.
Lu, T M; Laroche, D; Huang, S-H; Chuang, Y; Li, J-Y; Liu, C W
2016-02-11
In the presence of a lateral periodic potential modulation, two-dimensional electrons may exhibit interesting phenomena, such as a graphene-like energy-momentum dispersion, Bloch oscillations, or the Hofstadter butterfly band structure. To create a sufficiently strong potential modulation using conventional semiconductor heterostructures, aggressive device processing is often required, unfortunately resulting in strong disorder that masks the sought-after effects. Here, we report a novel fabrication process flow for imposing a strong lateral potential modulation onto a capacitively induced two-dimensional electron system, while preserving the host material quality. Using this process flow, the electron density in a patterned Si/SiGe heterostructure can be tuned over a wide range, from 4.4 × 10(10) cm(-2) to 1.8 × 10(11) cm(-2), with a peak mobility of 6.4 × 10(5) cm(2)/V·s. The wide density tunability and high electron mobility allow us to observe sequential emergence of commensurability oscillations as the density, the mobility, and in turn the mean free path, increase. Magnetic-field-periodic quantum oscillations associated with various closed orbits also emerge sequentially with increasing density. We show that, from the density dependence of the quantum oscillations, one can directly extract the steepness of the imposed superlattice potential. This result is then compared to a conventional lateral superlattice model potential.
NASA Astrophysics Data System (ADS)
Wu, Xiaoguang
2016-11-01
The exchange effect and the magneto-plasmon mode dispersion are studied theoretically for an anisotropic two-dimensional electronic system in the presence of a uniform perpendicular magnetic field. Employing an effective low-energy model with anisotropic effective masses, which is relevant for a monolayer of phosphorus, the exchange effect due to the electron-electron interaction is treated within the self-consistent Hartree-Fock approximation. The magneto-plasmon mode dispersion is obtained by solving a Bethe-Salpeter equation for the electron density-density correlation function within the ladder diagram approximation. It is found that the exchange effect is reduced in the anisotropic system in comparison with the isotropic one. The magneto-plasmon mode dispersion shows a clear dependence on the direction of the wave vector. Project supported by the National Natural Science Foundation of China (Grant Nos. 61076092 and 61290303).
Temperature Distribution in Two-Dimensional Electron Gases under a Strong Magnetic Field
NASA Astrophysics Data System (ADS)
Hirayama, Naomi; Endo, Akira; Fujita, Kazuhiro; Hasegawa, Yasuhiro; Hatano, Naomichi; Nakamura, Hiroaki; Shirasaki, Ryōen; Yonemitsu, Kenji
2011-05-01
Two-dimensional electron gases having an electrochemical potential gradient under a magnetic field are numerically examined using the finite-difference method. The temperature, voltage, electric current, and heat flux are calculated from transport equations describing thermoelectric and thermomagnetic effects, namely the Hall, Nernst, Ettingshausen, and Righi-Leduc effects. The results show that a magnetic field distorts equipotential lines and generates an uneven temperature distribution. In particular, a part of the system is found to become colder than the temperature of the heat baths. The cooling effect under a strong magnetic field is due primarily to the Ettingshausen and Hall effects.
Zhang, Miao-Lei; Deng, Guang-Wei; Li, Shu-Xiao; Li, Hai-Ou; Cao, Gang; Tu, Tao; Xiao, Ming; Guo, Guang-Can; Guo, Guo-Ping; Jiang, Hong-Wen; Siddiqi, Irfan
2014-02-24
We have designed and fabricated a half-wavelength reflection line resonator that consists of a pair of coupled microstrip lines on a GaAs/AlGaAs heterostructure. By changing the top gate voltage on a small square with a two-dimensional electron gas under the resonator, the quality factor was tuned over a large range from 2700 to below 600. Apart from being of fundamental interest, this gate modulation technique has the potential for use in on-chip resonator applications.
New edge magnetoplasmon for a two-dimensional electron gas in a ring geometry
NASA Astrophysics Data System (ADS)
Proetto, C. R.
1992-09-01
The dynamical response of a classical two-dimensional electron gas confined in a ring geometry under a perpendicular magnetic field is analyzed. Within the hydrodynamical approach and in the strong magnetic field limit, a new set of antidot edge magnetoplasmons is obtained, corresponding to density oscillations circulating along the inner boundary of the ring and whose frequency increases with magnetic field. The associated self-induced distribution of densities and currents are presented, together with an analysis of the size dependence of these perimeter waves.
Lo, C. C.; Lang, V.; George, R. E.; Morton, J. J. L.; Tyryshkin, A. M.; Lyon, A.; Bokor, J.; Schenkel, T.
2011-04-20
We have measured the electrically detected magnetic resonance of donor-doped silicon field-effect transistors in resonant X- (9.7 GHz) and W-band (94 GHz) microwave cavities. The two-dimensional electron gas (2DEG) resonance signal increases by two orders of magnitude from X- to W-band, while the donor resonance signals are enhanced by over one order of magnitude. Bolometric effects and spin-dependent scattering are inconsistent with the observations. We propose that polarization transfer from the donor to the 2DEG is the main mechanism giving rise to the spin resonance signals.
NASA Astrophysics Data System (ADS)
Wu, Mingyu; Lu, Quanming; Huang, Can; Wang, Shui
2010-10-01
A multidimensional electron phase-space hole (electron hole) is considered to be unstable to the transverse instability. In this paper, we perform two-dimensional (2D) particle-in-cell (PIC) simulations to study the evolution of electron holes at different plasma conditions; we find that the evolution is determined by combined actions between the transverse instability and the stabilization by the background magnetic field. In very weakly magnetized plasma (Ωe $\\ll$ ωpe, where Ωe and ωpe are the electron gyrofrequency and plasma frequency, respectively), the transverse instability dominates the evolution of the electron holes. The parallel cut of the perpendicular electric field (E$\\perp$) has bipolar structures, accompanied by the kinking of the electron holes. Such structures last for only tens of electron plasma periods. With the increase of the background magnetic field, the evolution of the electron holes becomes slower. The bipolar structures of the parallel cut of E$\\perp$ in the electron holes can evolve into unipolar structures. In very strongly magnetized plasma (Ωe $\\gg$ ωpe), the unipolar structures of the parallel cut of E$\\perp$ can last for thousands of electron plasma periods. At the same time, the perpendicular electric field (E$\\perp$) in the electron holes can also influence electron trajectories passing through the electron holes, which results in variations of charge density along the direction perpendicular to the background magnetic field outside of the electron holes. When the amplitude of the electron hole is sufficiently strong, streaked structures of E$\\perp$ can be formed outside of the electron holes, which then emit electrostatic whistler waves because of the interactions between the streaked structures of E$\\perp$ and vibrations of the kinked electron holes.
NASA Astrophysics Data System (ADS)
Wu, M.; Lu, Q.; Huang, C.; Wang, S.
2010-12-01
A multi-dimensional electron phase-space hole (electron hole) is considered to be unstable to the transverse instability. In this paper, we perform two-dimensional (2D) particle-in-cell (PIC) simulations to study the evolution of electron holes at different plasma conditions, and find that the evolution is determined by combined actions between the transverse instability and the stabilization of the background magnetic field. In very weakly magnetized plasma (Ωe<<ωpe, where Ωe andωpe are the electron gyrofrequency and plasma frequency, respectively), the transverse instability dominates the evolution of the electron holes. Accompanied by the kinking of the electron holes, the parallel cut of the perpendicular electric field (E⊥) has bipolar structures. Such structures last for only tens of electron plasma periods. With the increase of the background magnetic field, the evolution of the electron holes becomes slower. The bipolar structures of the parallel cut of E⊥ in the electron holes can evolve into unipolar structures. In very strongly magnetized plasma (Ωe>>ωpe), the unipolar structures of the parallel cut of E⊥ can last for thousands of electron plasma periods. At the same time, the perpendicular electric field (E⊥) in electron holes can also influence electron trajectories passing through the electron holes, which results in the variations of charge density along the direction perpendicular to the background magnetic field outside of the electron holes. When the amplitude of the electron hole is sufficiently strong, streaked structures of E⊥ can be formed outside of the electron holes, which then emit electrostatic whistler waves due to the interactions between the streaked structures of E⊥ and vibrations of the kinked electron hole.
NASA Astrophysics Data System (ADS)
Richter, Nils; Hernandez, Yenny R.; Schweitzer, Sebastian; Kim, June-Seo; Patra, Ajit Kumar; Englert, Jan; Lieberwirth, Ingo; Liscio, Andrea; Palermo, Vincenzo; Feng, Xinliang; Hirsch, Andreas; Müllen, Klaus; Kläui, Mathias
2017-02-01
We report on the electronic properties of turbostratic graphitic microdisks, rotationally stacked systems of graphene layers, where interlayer twisting leads to electronic decoupling resulting in charge-transport properties that retain the two dimensionality of graphene, despite the presence of a large number of layers. A key fingerprint of this reduced dimensionality is the effect of weak charge-carrier localization that we observe at low temperatures. The disks' resistivity measured as a function of magnetic field changes its shape from parabolic at room temperature to linear at a temperature of 2.7 K indicating further this type of two-dimensional transport. Compared to Bernal stacked graphite, turbostratic graphene is mechanically much more robust, and it exhibits almost negligible variations of the electrical properties between samples. We demonstrate a reproducible resistivity of (3.52 ±0.11 ) ×10-6 Ω m , which is a particularly low value for graphitic systems. Combined with large charge-carrier mobilities demonstrated at low temperatures of up to 7 ×104 cm2/V s , typical for carbon-based crystalline conductors, such disks are highly interesting from a scientific point of view and, in particular, for applications where robust electronic properties are required.
Observation of anomalous Hall effect in a non-magnetic two-dimensional electron system
Maryenko, D.; Mishchenko, A. S.; Bahramy, M. S.; Ernst, A.; Falson, J.; Kozuka, Y.; Tsukazaki, A.; Nagaosa, N.; Kawasaki, M.
2017-01-01
Anomalous Hall effect, a manifestation of Hall effect occurring in systems without time-reversal symmetry, has been mostly observed in ferromagnetically ordered materials. However, its realization in high-mobility two-dimensional electron system remains elusive, as the incorporation of magnetic moments deteriorates the device performance compared to non-doped structure. Here we observe systematic emergence of anomalous Hall effect in various MgZnO/ZnO heterostructures that exhibit quantum Hall effect. At low temperatures, our nominally non-magnetic heterostructures display an anomalous Hall effect response similar to that of a clean ferromagnetic metal, while keeping a large anomalous Hall effect angle θAHE≈20°. Such a behaviour is consistent with Giovannini–Kondo model in which the anomalous Hall effect arises from the skew scattering of electrons by localized paramagnetic centres. Our study unveils a new aspect of many-body interactions in two-dimensional electron systems and shows how the anomalous Hall effect can emerge in a non-magnetic system. PMID:28300133
Camargo, Franco V A; Anderson, Harry L; Meech, Stephen R; Heisler, Ismael A
2015-01-08
In this work we present experimental and calculated two-dimensional electronic spectra for a 5,15-bisalkynyl porphyrin chromophore. The lowest energy electronic Qy transition couples mainly to a single 380 cm(-1) vibrational mode. The two-dimensional electronic spectra reveal diagonal and cross peaks which oscillate as a function of population time. We analyze both the amplitude and phase distribution of this main vibronic transition as a function of excitation and detection frequencies. Even though Feynman diagrams provide a good indication of where the amplitude of the oscillating components are located in the excitation-detection plane, other factors also affect this distribution. Specifically, the oscillation corresponding to each Feynman diagram is expected to have a phase that is a function of excitation and detection frequencies. Therefore, the overall phase of the experimentally observed oscillation will reflect this phase dependence. Another consequence is that the overall oscillation amplitude can show interference patterns resulting from overlapping contributions from neighboring Feynman diagrams. These observations are consistently reproduced through simulations based on third order perturbation theory coupled to a spectral density described by a Brownian oscillator model.
Quantum point contacts on two-dimensional electron gases with a strong spin-orbit coupling
NASA Astrophysics Data System (ADS)
Lee, Joon Sue; Pendaharkar, Mihir; Shojaei, Borzoyeh; McFadden, Anthony P.; Palmstrøm, Chris
Studies of electrical transport in one-dimensional semiconductors in a presence of a strong spin-orbit interaction are crucial not only for exploring the emergent phenomena, such as topological superconductivity, but also for potential spintronic applications by controlling of the electron spins. We investigate the electrical transport properties of one-dimensional confinement defined by electrostatic potentials on large area two-dimensional electron gases of InAs and InSb, which have a strong spin-orbit coupling. The high-quality InAs and InSb quantum wells are grown on antimonide buffers by molecular beam epitaxy, and the gate-tunable regions are created using Al2O3 or HfO2 gate dielectrics by atomic layer deposition. We will discuss the modulation of spin-orbit coupling in the two-dimensional electron gases and the spin-orbit-induced spin splitting by the split-gate quantum point contacts. This work was supported by Microsoft Research.
Observation of anomalous Hall effect in a non-magnetic two-dimensional electron system
NASA Astrophysics Data System (ADS)
Maryenko, D.; Mishchenko, A. S.; Bahramy, M. S.; Ernst, A.; Falson, J.; Kozuka, Y.; Tsukazaki, A.; Nagaosa, N.; Kawasaki, M.
2017-03-01
Anomalous Hall effect, a manifestation of Hall effect occurring in systems without time-reversal symmetry, has been mostly observed in ferromagnetically ordered materials. However, its realization in high-mobility two-dimensional electron system remains elusive, as the incorporation of magnetic moments deteriorates the device performance compared to non-doped structure. Here we observe systematic emergence of anomalous Hall effect in various MgZnO/ZnO heterostructures that exhibit quantum Hall effect. At low temperatures, our nominally non-magnetic heterostructures display an anomalous Hall effect response similar to that of a clean ferromagnetic metal, while keeping a large anomalous Hall effect angle θAHE~20°. Such a behaviour is consistent with Giovannini-Kondo model in which the anomalous Hall effect arises from the skew scattering of electrons by localized paramagnetic centres. Our study unveils a new aspect of many-body interactions in two-dimensional electron systems and shows how the anomalous Hall effect can emerge in a non-magnetic system.
NASA Astrophysics Data System (ADS)
Lu, Q.; Wu, M.; Huang, C.; Wang, S.
2011-12-01
A multi-dimensional electron phase-space hole (electron hole) is considered to be unstable to the transverse instability. We perform two-dimensional (2D) particle-in-cell (PIC) simulations to study the evolution of electron holes at different plasma conditions; we find that the evolution is determined by combined actions between the transverse instability and the stabilization by the ackground magnetic field. In very weakly magnetized plasma, the transverse instability dominates the evolution of the electron holes. The parallel cut of the perpendicular electric field has bipolar structures, accompanied by the kinking of the electron holes. Such structures last for only tens of electron plasma periods. With the increase of the background magnetic field, the evolution of the electron holes becomes slower. The bipolar structures of the parallel cut of the perpendicular electric field in the electron holes can evolve into unipolar structures. In very strongly magnetized plasma, the unipolar structures of the parallel cut of the perpendicular electric field can last for thousands of electron plasma periods. At the same time, the perpendicular electric field in the electron holes can also influence electron trajectories passing through the electron holes, which results in variations of charge density along the direction perpendicular to the background magnetic field outside of the electron holes. When the amplitude of the electron hole is sufficiently strong, streaked structures of the perpendicular electric field can be formed outside of the electron holes, which then emit electrostatic whistler waves because of the interactions between the streaked structures of the perpendicular electric field and vibrations of the kinked electron holes.
NASA Astrophysics Data System (ADS)
Andreev, Pavel A.
2017-02-01
The hydrodynamics analysis of waves in a two-dimensional degenerate electron gas with a separate spin evolution is presented. The transverse electric field is included along with the longitudinal electric field. The Coulomb exchange interaction is included in the analysis. In contrast with the three-dimensional plasma-like media, the contribution of the transverse electric field is rather small, but it decreases the frequency of the extraordinary wave at small wave vectors. We show the decrease in the frequency of both the extraordinary (Langmuir) wave and the spin-electron acoustic wave due to the exchange interaction. Moreover, spin-electron acoustic waves have negative dispersion at the relatively large spin-polarization. The corresponding dispersion dependencies are presented and analyzed.
Albert, Julian; Falge, Mirjam; Hildenbrand, Heiko; Engel, Volker; Gomez, Sandra; Sola, Ignacio R.
2015-07-28
We theoretically investigate the photon-echo spectroscopy of coupled electron-nuclear quantum dynamics. Two situations are treated. In the first case, the Born-Oppenheimer (adiabatic) approximation holds. It is then possible to interpret the two-dimensional (2D) spectra in terms of vibrational motion taking place in different electronic states. In particular, pure vibrational coherences which are related to oscillations in the time-dependent third-order polarization can be identified. This concept fails in the second case, where strong non-adiabatic coupling leads to the breakdown of the Born-Oppenheimer-approximation. Then, the 2D-spectra reveal a complicated vibronic structure and vibrational coherences cannot be disentangled from the electronic motion.
Dynamical correlation effects on structure factor of spin-polarized two-dimensional electron gas
Singh, Gurvinder; Moudgil, R. K.; Kumar, Krishan; Garg, Vinayak
2015-06-24
We report a theoretical study on static density structure factor S(q) of a spin-polarized two-dimensional electron gas over a wide range of electron number density r{sub s}. The electron correlations are treated within the dynamical version of the self-consistent mean-field theory of Singwi, Tosi, Land, and Sjolander, the so-called qSTLS approach. The calculated S(q) exhibits almost perfect agreement with the quantum Monte Carlo simulation data at r{sub s}=1. However, the extent of agreement somewhat diminishes with increasing r{sub s}, particularly for q around 2k{sub F}. Seen in conjunction with the success of qSTLS theory in dealing with correlations in the unpolarized phase, our study suggests that the otherwise celebrated qSTLS theory is not that good in treating the like-spin correlations.
Two-dimensional electron gas in GaAs/SrHfO3 heterostructure
NASA Astrophysics Data System (ADS)
Wang, Jianli; Yuan, Mengqi; Tang, Gang; Li, Huichao; Zhang, Junting; Guo, Sandong
2016-06-01
The III-V/perovskite-oxide system can potentially create new material properties and new device applications by combining the rich properties of perovskite-oxides together with the superior optical and electronic properties of III-Vs. The structural and electronic properties of the surface and interface are studied using first-principles calculations for the GaAs/SrHfO3 heterostructure. We investigate the specific adsorption sites and the atomic structure at the initial growth stage of GaAs on the SrHfO3 (001) substrate. Ga and As adsorption atoms preferentially adsorb at the top sites of oxygen atoms under different coverage. The energetically favorable interfaces are presented among the atomic arrangements of the GaAs/SrHfO3 interfaces. Our calculations predict the existing of the two-dimensional electron gas in the GaAs/SrHfO3 heterostructure.
Rajan, Arunkumar Chitteth; Rezapour, Mohammad Reza; Yun, Jeonghun; Cho, Yeonchoo; Cho, Woo Jong; Min, Seung Kyu; Lee, Geunsik; Kim, Kwang S
2014-02-25
Laser-driven molecular spectroscopy of low spatial resolution is widely used, while electronic current-driven molecular spectroscopy of atomic scale resolution has been limited because currents provide only minimal information. However, electron transmission of a graphene nanoribbon on which a molecule is adsorbed shows molecular fingerprints of Fano resonances, i.e., characteristic features of frontier orbitals and conformations of physisorbed molecules. Utilizing these resonance profiles, here we demonstrate two-dimensional molecular electronics spectroscopy (2D MES). The differential conductance with respect to bias and gate voltages not only distinguishes different types of nucleobases for DNA sequencing but also recognizes methylated nucleobases which could be related to cancerous cell growth. This 2D MES could open an exciting field to recognize single molecule signatures at atomic resolution. The advantages of the 2D MES over the one-dimensional (1D) current analysis can be comparable to those of 2D NMR over 1D NMR analysis.
NASA Astrophysics Data System (ADS)
Albert, Julian; Falge, Mirjam; Gomez, Sandra; Sola, Ignacio R.; Hildenbrand, Heiko; Engel, Volker
2015-07-01
We theoretically investigate the photon-echo spectroscopy of coupled electron-nuclear quantum dynamics. Two situations are treated. In the first case, the Born-Oppenheimer (adiabatic) approximation holds. It is then possible to interpret the two-dimensional (2D) spectra in terms of vibrational motion taking place in different electronic states. In particular, pure vibrational coherences which are related to oscillations in the time-dependent third-order polarization can be identified. This concept fails in the second case, where strong non-adiabatic coupling leads to the breakdown of the Born-Oppenheimer-approximation. Then, the 2D-spectra reveal a complicated vibronic structure and vibrational coherences cannot be disentangled from the electronic motion.
NASA Astrophysics Data System (ADS)
Ogilvie, Jennifer
2010-03-01
Two-dimensional (2D) Fourier transform electronic spectroscopy has recently emerged as a powerful tool for the study of energy transfer in complex condensed-phase systems. Its experimental implementation is challenging but can be greatly simplified by implementing a pump-probe geometry, where the two phase-stable collinear pump pulses are created with an acousto-optic pulse-shaper. This approach also allows the use of a continuum probe pulse, expanding the available frequency range of the detection axis and allowing studies of energy transfer and electronic coupling over a broad range of frequencies. We discuss several benefits of 2D electronic spectroscopy and present 2D data on the D1-D2 reaction center complex of Photosystem II from spinach. We discuss the ability of 2D spectroscopy to distinguish between current models of energy and charge transfer in this system.
Anisotropic two-dimensional electron gas at SrTiO3(110)
Wang, Zhiming; Zhong, Zhicheng; Hao, Xianfeng; Gerhold, Stefan; Stöger, Bernhard; Schmid, Michael; Sánchez-Barriga, Jaime; Varykhalov, Andrei; Franchini, Cesare; Held, Karsten; Diebold, Ulrike
2014-01-01
Two-dimensional electron gases (2DEGs) at oxide heterostructures are attracting considerable attention, as these might one day substitute conventional semiconductors at least for some functionalities. Here we present a minimal setup for such a 2DEG––the SrTiO3(110)-(4 × 1) surface, natively terminated with one monolayer of tetrahedrally coordinated titania. Oxygen vacancies induced by synchrotron radiation migrate underneath this overlayer; this leads to a confining potential and electron doping such that a 2DEG develops. Our angle-resolved photoemission spectroscopy and theoretical results show that confinement along (110) is strikingly different from the (001) crystal orientation. In particular, the quantized subbands show a surprising “semiheavy” band, in contrast with the analog in the bulk, and a high electronic anisotropy. This anisotropy and even the effective mass of the (110) 2DEG is tunable by doping, offering a high flexibility to engineer the properties of this system. PMID:24591596
Single-electron tunneling by using a two-dimensional Corbino nano-scale disk
Taira, H.; Suzuki, A.
2015-09-15
We investigate a single-electron tunneling effect of two-dimensional electron systems formed in the Corbino nano-scale disk. By controlling bias and gate voltages, the transistor using this effect is able to control electrons one by one. The present study focuses on the electronic transmission probability affected by the charging energy in the Corbino-type single-electron transistor. We reformulated the Schrödinger equation for an electron in the Corbino disk in order to consider the effect of the curvature of the disk, taking into account the charging effect on the performance of the Corbino-type single-electron transistor. We formulated the transmission probability of the electron by applying the Wentzel-Kramers-Brillouin (WKB) method. The electron’s energy in the formula of the transmission probability is then associated to the energy eigenvalue of the Schrödinger equation for an electron in an effective confining potential. We numerically solved the Schrödinger equation to evaluate the transmission probability. Our results show that the transmission probability strongly depends on the charging energy stored in the Corbino disk depending on its size.
NASA Astrophysics Data System (ADS)
Khan, Mahtab; Erementchouk, Mikhail; Leuenberger, Michael
Defects play an important role in tailoring electronic and optical properties of two-dimensional monolayer transition metal dichalcogenides (TMDCs). Recently it has been shown that the presence of vacancy defects (VDs) in two-dimensional monolayer MoS_2 induces localized states which give rise to extra resonance peaks in both in-plane χ∥ and out-of-plane χ⊥ susceptibilities.1 In-plane χ∥ and out-of-plane χ⊥ susceptibilities are related to the presence of even and odd states with respect to the Mo plane, respectively1. Moreover, monolayer TMDCs have a large spin orbit coupling (SOC), originating from d-orbitals of heavy transition metals and being of the order of a few 100 meV. We present a more general picture of the electronic and optical properties of defected monolayer TMDCs. In particular, we consider MoS2, MoSe2, WS2 and WSe2 with three types of VDs (i) Mo, W vacancy, (ii) S2, Se2 vacancy, and (iii) S, Se vacancy. In addition, we investigate the effects of SOC on the band structures and the optical susceptibilities of VDs in TMDCs. 1. Mikhail Erementchouk, M. A. Khan, and Michael N. Leuenberger, Phys. Rev. B 92, 121401(R) (2015).
Electron counting and a large family of two-dimensional semiconductors
NASA Astrophysics Data System (ADS)
Miao, Maosheng; Botana, Jorge; Zurek, Eva; Liu, Jingyao; Yang, Wen
Two-dimensional semiconductors (2DSC) are currently the focus of many studies, thanks to their novel and superior transport properties that may greatly influence future electronic devices. The potential applications of 2DSCs range from low-dimensional electronics, topological insulators and vallytronics all the way to novel photolysis. However, compared with the conventional semiconductors that are comprised of main group elements and cover a large range of band gaps and lattice constants, the choice of 2D materials is very limited. In this work, we propose and demonstrate a large family of 2DSCs, all adopting the same structure and consisting of only main group elements. Using advanced density functional calculations, we demonstrate the attainability of these materials, and show that they cover a large range of lattice constants, band gaps and band edge states, making them good candidate materials for heterojunctions. This family of two dimensional materials may be instrumental in the fabrication of 2DSC devices that may rival the currently employed 3D semiconductors.
Torres, Manuel; Kunold, Alejandro
2006-04-26
In this work we study the microwave photoconductivity of a two-dimensional electron system (2DES) in the presence of a magnetic field and a two-dimensional modulation (2D). The model includes the microwave and Landau contributions in a non-perturbative exact way; the periodic potential is treated perturbatively. The Landau-Floquet states provide a convenient base with respect to which the lattice potential becomes time dependent, inducing transitions between the Landau-Floquet levels. Based on this formalism, we provide a Kubo-like formula that takes into account the oscillatory Floquet structure of the problem. The total longitudinal conductivity and resistivity exhibit strong oscillations, determined by ϵ = ω/ω(c), with ω the radiation frequency and ω(c) the cyclotron frequency. The oscillations follow a pattern with minima centred at [Formula: see text], and maxima centred at [Formula: see text], where j = 1,2,3..., δ∼1/5 is a constant shift and l is the dominant multipole contribution. Negative resistance states (NRSs) develop as the electron mobility and the intensity of the microwave power are increased. These NRSs appear in a narrow window region of values of the lattice parameter (a), around a∼l(B), where l(B) is the magnetic length. It is proposed that these phenomena may be observed in artificially fabricated arrays of periodic scatterers at the interface of ultraclean GaAs /Al(x)Ga(1-x)As heterostructures.
Two-dimensional group-IV monochalcogenides: structural, electronic and optical properties
NASA Astrophysics Data System (ADS)
Gomes, Lidia; Carvalho, Alexandra; Castro Neto, A. H.
Two-dimensional materials have attracted a massive attention of the scientific and industrial communities due to their unusual and interesting properties. The layered group-IV monochalcogenides-SnS, SnSe, GeS and GeSe- has gained attention as a promising group with potentially useful applications in diverse fields. The bulk SnS, a naturally occurring mineral, has been considered as an alternative to be used in film PV cells, due to its electronic and optical properties. We use first principles calculations to explore structural, electronic and optical properties of this group, with focus in their two-dimensional forms. We show that all those binary compounds are semiconducting, with bandgap energies covering most of the visible range. They have multiple valleys in the valence and conduction bands, with spin-orbit splitting of the order of 19-86 meV. An enhanced static dielectric permittivity is found for the monolayers. Structural analysis shows that the 2D form of these materials presents very high piezoelectric constants, exceeding values recently observed for other 2D-systems. The existence of a negative Poisson ratio is predicted for the GeS compound. We acknowledge the NRF-CRP award ``Novel 2D materials with tailored properties: beyond graphene'' (R-144-000-295-281).
NASA Astrophysics Data System (ADS)
Moskalenko, S. A.; Liberman, M. A.; Moskalenko, E. S.; Dumanov, E. V.; Podlesny, I. V.
2013-08-01
The spontaneous breaking of the continuous symmetries of a two-dimensional electron-hole system in a strong magnetic field perpendicular to the plane leads to the formation of new ground states and determines the energy spectrum of collective elementary excitations that appear above these new ground states. In this review, the main attention is paid to the electron-hole system formed from coplanar magnetoexcitons under conditions of Bose-Einstein condensation in the ground state with the wave vector k = 0 taking into account the influence of excited Landau levels, when exciton-type elementary excitations coexist with plasmon-type oscillations. At the same time, the properties of a two-component system consisting of a two-dimensional electron gas and a two-dimensional hole gas spatially separated in a double quantum well under conditions of the fractional quantum Hall effect are of great interest, because these properties can affect the quantum states of magnetic excitons that are formed when the distance between the layers tends to zero. Bilayer electron systems are also considered under conditions of the fractional quantum Hall effect with the one-half filling factor for each layer and the total filling factor equal to unity for both layers. The coherence between the electron states in the two layers is equivalent to the formation of excitons in a macroscopic coherent state. This makes it possible to compare the energy spectrum of collective elementary excitations of Bose-Einstein condensed excitons under conditions of the quantum Hall effect and coplanar magnetoexcitons. The breaking of the global gauge symmetry or of the continuous rotational symmetry leads to the formation of a gapless spectrum of the Nambu-Goldstone type, whereas the breaking of the local gauge symmetry is accompanied by the appearance of a gap in the energy spectrum (Higgs phenomenon). These phenomena are equivalent to the formation of massless and massive particles in the relativistic
Rashba diamond in an Aharonov-Casher ring
NASA Astrophysics Data System (ADS)
Wang, Xuhui; Manchon, Aurelien
2011-10-01
Spin interference due to Rashba spin-orbit interaction (SOI) in a ballistic two-dimensional electron gas ring conductor submitted to a bias voltage is investigated theoretically. We calculate the scattering matrices and differential conductance with lead-ring junction coupling as an adjustable parameter. Due to the interference of electronic waves traversing the ring, the differential conductance modulated by both bias voltage and SOI exhibits a diamond-shaped pattern, thus termed as Rashba diamond. This feature offers a supplementary degree of freedom to manipulate phase interference.
Two-body problem for two-dimensional electrons in the Bernervig-Hughes-Zhang model
NASA Astrophysics Data System (ADS)
Sablikov, Vladimir A.
2017-02-01
We study the two-body problem for two-dimensional electron systems in a symmetrized Bernevig-Hughes-Zhang model, which is widely used to describe topological and conventional insulators. The main result is that two interacting electrons can form bound states with the energy in the gap of the band spectrum. The pairing mechanism can be interpreted as the formation of a negative reduced effective mass of two electrons. The problem is complicated because the relative motion of the electrons is coupled to the center-of-mass motion. We consider the case of zero total momentum. Detail calculations are carried out for the repulsive interaction potential of steplike form. The states are classified according to their spin structure and two-particle basis functions that form a given bound state. We analyze the spectra and electronic structure of the bound states in the case of both topological and trivial phases and especially focus on effects originating from the band inversion and the coupling of the electron and hole bands. In the trivial phase and the topological phase with the large coupling parameter a , the bound state spectra are qualitatively similar. However, when a is less a certain value, the situation changes dramatically. In the topological phase, new states arise with a higher binding energy at lower interaction potential, which evidences that the band inversion can favor pairing the electrons.
NASA Astrophysics Data System (ADS)
Lee, H.; Campbell, N.; Ryu, S.; Chang, W.; Irwin, J.; Lindemann, S.; Mahanthappa, M. K.; Rzchowski, M. S.; Eom, C. B.
2016-11-01
Reversible control over the electrical properties of the two-dimensional electron gas (2DEG) in oxide heterostructures is a key capability enabling practical applications. Herein, we report an efficient method to reversibly tune the charge carrier density of the 2DEG by surface modification. We demonstrate both increasing and decreasing the carrier density of the LaAlO3/SrTiO3 2DEG interface via application of functional phosphonic acids with molecular dipoles pointing either toward or away from the interface, respectively. In addition, in the case of the enhanced 2DEG, we recovered the initial conduction properties by exposing the samples to a basic solution. The tuning processes were highly reversible over repetitive cycles. These results reveal that the surface modification is an efficient way to tune the carrier density of 2DEG in oxide heterostructures. This simple chemical approach offers a vast range of fabrication possibilities in versatile electronic device applications.
Two-Dimensional Electronic-Vibrational Spectroscopy of Chlorophyll a and b.
Lewis, Nicholas H C; Fleming, Graham R
2016-03-03
We present two-dimensional electronic-vibrational (2DEV) spectra of isolated chlorophyll a and b in deuterated ethanol. We excite the Q-band electronic transitions and measure the effects on the carbonyl and C ═ C double-bond stretch region of the infrared spectrum. With the aid of density functional theory calculations, we provide assignments for the major features of the spectrum. We show how the 2DEV spectra can be used to readily distinguish different solvation states of the chlorophyll, with features corresponding to the minority pentacoordinate magnesium (Mg) species being resolved along each dimension of the 2DEV spectra from the dominant hexacoordinate Mg species. These assignments represent a crucial first step toward the application of 2DEV spectroscopy to chlorophyll-containing pigment-protein complexes.
Terahertz time-domain spectroscopy of two-dimensional electron gasses at high magnetic fields
NASA Astrophysics Data System (ADS)
Curtis, Jeremy A.
This dissertation covers two projects that were in the logical path to studying decoherence in a high mobility GaAs two--dimensional electron gas at high magnetic fields. The first project is the ultrafast non--degenerate pump--probe spectroscopic study of bulk GaAs in the Split Florida Helix at the National High Magnetic Field Laboratory at Florida State University. This project was undertaken as a proof of concept that ultrafast optics could be done in the Split Florida Helix so that we might study a high mobility two dimensional electron gas using THz time--domain spectroscopy at high magnetic fields, which is a much more complicated measurement than the pump--probe discussed here. This demonstration was a success. We completed the first ultrafast optical study of any kind in the Florida Split Helix. We collected differential reflection data from this bulk sample that exhibited electronic and oscillatory components. These components were treated independently in the analysis by treating the electronic dynamics with a four level approximation. The electronic transition rates were extracted and agreed well with published values. This agreement is a demonstration that the spectrometer functioned as desired. The oscillatory response was found to be a result of the emission of coherent phonons upon electronic transition between the four levels. The frequency of the oscillatory response was extracted and agreed well with the theoretical value. The second project is the study of the temperature dependence of the cyclotron decay lifetimes in a Landau quantized GaAs high mobility two dimensional electron gas using THz time--domain spectroscopy at relatively low magnetic field (1.25 T). We find that the cyclotron decay lifetimes decrease monotonically with increasing temperature from 0.4 K to 100 K and that the primary pulse amplitudes increase from 0.4 K to 1.2 K, saturates above 1.2 K up to 50 K, and decreases rapidly above 50 K. We attribute this rapid drop in
Current switching of electronic structures in two-dimensional 1 T -Ta S2 crystals
NASA Astrophysics Data System (ADS)
Yoshida, Masaro; Gokuden, Takashi; Suzuki, Ryuji; Nakano, Masaki; Iwasa, Yoshihiro
2017-03-01
We report that a high electric field and current triggers the switching of multiple states in two-dimensional (2D) crystals of 1 T -Ta S2 , accompanying a metamorphosis of the electronic structure. We fabricated four-terminal devices of nanometer-thick crystals of 1 T -Ta S2 with charge-density-wave (CDW) phases. By applying in-plane electric fields and concomitantly injecting currents, we realized nonvolatile switching among normal metals, Mott insulators, and thermally inaccessible semimetals. The field and current not only interact with the CDW but also generate Joule heat, and both effects contribute to the switching. The results indicate the potential existence of multiple electronic states accessible only in 2D crystals.
Evidence for Two Different Solid Phases of Two-Dimensional Electrons in High Magnetic Fields
NASA Astrophysics Data System (ADS)
Chen, Yong P.; Lewis, R. M.; Engel, L. W.; Tsui, D. C.; Ye, P. D.; Wang, Z. H.; Pfeiffer, L. N.; West, K. W.
2004-11-01
We have observed two different rf resonances in the frequency dependent real diagonal conductivity of very high quality two-dimensional electron systems in the high magnetic field insulating phase and interpret them as coming from two different pinned electron solid phases (labeled as “A” and “B”). The “A” resonance is observable for Landau level filling ν<2/9 [reentrant around the ν=1/5 fractional quantum Hall effect (FQHE)] and then crosses over to the different “B” resonance which dominates at sufficiently low ν. Moreover, the “A” resonance is found to show dispersion with respect to the size of the transmission line, indicating that the “A” phase has a large correlation length. We suggest that quantum correlations such as those responsible for FQHE may play an important role in giving rise to such different solids.
Cavity quantum electrodynamics with many-body states of a two-dimensional electron gas.
Smolka, Stephan; Wuester, Wolf; Haupt, Florian; Faelt, Stefan; Wegscheider, Werner; Imamoglu, Ataç
2014-10-17
Light-matter interaction has played a central role in understanding as well as engineering new states of matter. Reversible coupling of excitons and photons enabled groundbreaking results in condensation and superfluidity of nonequilibrium quasiparticles with a photonic component. We investigated such cavity-polaritons in the presence of a high-mobility two-dimensional electron gas, exhibiting strongly correlated phases. When the cavity was on resonance with the Fermi level, we observed previously unknown many-body physics associated with a dynamical hole-scattering potential. In finite magnetic fields, polaritons show distinct signatures of integer and fractional quantum Hall ground states. Our results lay the groundwork for probing nonequilibrium dynamics of quantum Hall states and exploiting the electron density dependence of polariton splitting so as to obtain ultrastrong optical nonlinearities.
Terahertz Radiation Heterodyne Detector Using Two-Dimensional Electron Gas in a GaN Heterostructure
NASA Technical Reports Server (NTRS)
Karasik, Boris S.; Gill, John J.; Mehdi, Imran; Crawford, Timothy J.; Sergeev, Andrei V.; Mitin, Vladimir V.
2012-01-01
High-resolution submillimeter/terahertz spectroscopy is important for studying atmospheric and interstellar molecular gaseous species. It typically uses heterodyne receivers where an unknown (weak) signal is mixed with a strong signal from the local oscillator (LO) operating at a slightly different frequency. The non-linear mixer devices for this frequency range are unique and are not off-the-shelf commercial products. Three types of THz mixers are commonly used: Schottky diode, superconducting hot-electron bolometer (HEB), and superconductor-insulation-superconductor (SIS) junction. A HEB mixer based on the two-dimensional electron gas (2DEG) formed at the interface of two slightly dissimilar semiconductors was developed. This mixer can operate at temperatures between 100 and 300 K, and thus can be used with just passive radiative cooling available even on small spacecraft.
Bizimana, Laurie A; Brazard, Johanna; Carbery, William P; Gellen, Tobias; Turner, Daniel B
2015-10-28
Coherent multidimensional optical spectroscopy is an emerging technique for resolving structure and ultrafast dynamics of molecules, proteins, semiconductors, and other materials. A current challenge is the quality of kinetics that are examined as a function of waiting time. Inspired by noise-suppression methods of transient absorption, here we incorporate shot-by-shot acquisitions and balanced detection into coherent multidimensional optical spectroscopy. We demonstrate that implementing noise-suppression methods in two-dimensional electronic spectroscopy not only improves the quality of features in individual spectra but also increases the sensitivity to ultrafast time-dependent changes in the spectral features. Measurements on cresyl violet perchlorate are consistent with the vibronic pattern predicted by theoretical models of a highly displaced harmonic oscillator. The noise-suppression methods should benefit research into coherent electronic dynamics, and they can be adapted to multidimensional spectroscopies across the infrared and ultraviolet frequency ranges.
Bizimana, Laurie A.; Brazard, Johanna; Carbery, William P.; Gellen, Tobias; Turner, Daniel B.
2015-10-28
Coherent multidimensional optical spectroscopy is an emerging technique for resolving structure and ultrafast dynamics of molecules, proteins, semiconductors, and other materials. A current challenge is the quality of kinetics that are examined as a function of waiting time. Inspired by noise-suppression methods of transient absorption, here we incorporate shot-by-shot acquisitions and balanced detection into coherent multidimensional optical spectroscopy. We demonstrate that implementing noise-suppression methods in two-dimensional electronic spectroscopy not only improves the quality of features in individual spectra but also increases the sensitivity to ultrafast time-dependent changes in the spectral features. Measurements on cresyl violet perchlorate are consistent with the vibronic pattern predicted by theoretical models of a highly displaced harmonic oscillator. The noise-suppression methods should benefit research into coherent electronic dynamics, and they can be adapted to multidimensional spectroscopies across the infrared and ultraviolet frequency ranges.
Energy Transfer Observed in Live Cells Using Two-Dimensional Electronic Spectroscopy
Dahlberg, Peter D.; Fidler, Andrew F.; Caram, Justin R.; Long, Phillip D.; Engel, Gregory S.
2013-01-01
Two-dimensional electronic spectroscopy (2DES) elucidates electronic structure and dynamics on a femtosecond time scale and has proven to be an incisive tool for probing congested linear spectra of biological systems. However, samples that scatter light intensely frustrate 2DES analysis, necessitating the use of isolated protein chromophore complexes when studying photosynthetic energy transfer processes. We present a method for conducting 2DES experiments that takes only seconds to acquire thousands of 2DES spectra and permits analysis of highly scattering samples, specifically whole cells of the purple bacterium Rhodobacter sphaeroides. These in vivo 2DES experiments reveal similar timescales for energy transfer within the antennae complex (light harvesting complex 2, LH2) both in the native photosynthetic membrane environment and in isolated detergent micelles. PMID:24478821
Denteneer, P J H; Scalettar, R T
2003-06-20
The effect of a Zeeman magnetic field coupled to the spin of the electrons on the conducting properties of the disordered Hubbard model is studied. Using the determinant quantum Monte Carlo method, the temperature- and magnetic-field-dependent conductivity is calculated, as well as the degree of spin polarization. We find that the Zeeman magnetic field suppresses the metallic behavior present for certain values of interaction and disorder strength and is able to induce a metal-insulator transition at a critical field strength. It is argued that the qualitative features of magnetoconductance in this microscopic model containing both repulsive interactions and disorder are in agreement with experimental findings in two-dimensional electron and hole gases in semiconductor structures.
Structural and electronic properties of two-dimensional stanene and graphene heterostructure.
Wu, Liyuan; Lu, Pengfei; Bi, Jingyun; Yang, Chuanghua; Song, Yuxin; Guan, Pengfei; Wang, Shumin
2016-12-01
Structural and electronic properties of two-dimensional stanene and graphene heterostructure (Sn/G) are studied by using first-principles calculations. Various supercell models are constructed in order to reduce the strain induced by the lattice mismatch. The results show that stanene interacts overall weakly with graphene via van der Waals (vdW) interactions. Multiple phases of different crystalline orientation of stanene and graphene could coexist at room temperature. Moreover, interlayer interactions in stanene and graphene heterostructure can induce tunable band gaps at stanene's Dirac point, and weak p-type and n-type doping of stanene and graphene, respectively, generating a small amount of electron transfer from stanene to graphene. Interestingly, for model [Formula: see text] , there emerges a band gap about 34 meV overall the band structure, indicating it shows semiconductor feature.
A new equation in two dimensional fast magnetoacoustic shock waves in electron-positron-ion plasmas
Masood, W.; Jehan, Nusrat; Mirza, Arshad M.
2010-03-15
Nonlinear properties of the two dimensional fast magnetoacoustic waves are studied in a three-component plasma comprising of electrons, positrons, and ions. In this regard, Kadomtsev-Petviashvili-Burger (KPB) equation is derived using the small amplitude perturbation expansion method. Under the condition that the electron and positron inertia are ignored, Burger-Kadomtsev-Petviashvili (Burger-KP) for a fast magnetoacoustic wave is derived for the first time, to the best of author's knowledge. The solutions of both KPB and Burger-KP equations are obtained using the tangent hyperbolic method. The effects of positron concentration, kinematic viscosity, and plasma beta are explored both for the KPB and the Burger-KP shock waves and the differences between the two are highlighted. The present investigation may have relevance in the study of nonlinear electromagnetic shock waves both in laboratory and astrophysical plasmas.
Interface states in two-dimensional electron systems with spin-orbital interaction.
Sukhanov, Aleksei A; Sablikov, Vladimir A
2011-10-05
Interface states at a boundary between regions with different spin-orbit interactions (SOIs) in two-dimensional (2D) electron systems are investigated within the one-band effective mass method with generalized boundary conditions for envelope functions. We have found that the interface states unexpectedly exist even if the effective interface potential equals zero. Depending on the system parameters, the energy of these states can lie in either or both forbidden and conduction bands of bulk states. The interface states have chiral spin texture similar to that of the edge states in 2D topological insulators. However, their energy spectrum is more sensitive to the interfacial potential, the largest effect being produced by the spin-dependent component of the interfacial potential. We have also studied the size quantization of the interface states in a strip of 2D electron gas with SOI and found an unusual (non-monotonic) dependence of the quantization energy on the strip width.
NASA Astrophysics Data System (ADS)
Cui, Juan; Yang, Yong-Hong; Wang, Jun
2009-11-01
We report a theoretic study on modulating the spin polarization of charge current in a mesoscopic four-terminal device of cross structure by using the inverse spin hall effect. The scattering region of device is a two-dimensional electron gas (2DEG) with Rashba spin orbital interaction (RSOI), one of lead is ferromagnetic metal and other three leads are spin-degenerate normal metals. By using Landauer-Büttiker formalism, we found that when a longitudinal charge current flows through 2DEG scattering region from FM lead by external bias, the transverse current can be either a pure spin current or full-polarized charge current due to the combined effect of spin hall effect and its inverse process, and the polarization of this transverse current can be easily controlled by several device parameters such as the Fermi energy, ferromagnetic magnetization, and the RSOI constant. Our method may pave a new way to control the spin polarization of a charge current.
Two-dimensional Fourier transform electronic spectroscopy at a conical intersection
Kitney-Hayes, Katherine A.; Ferro, Allison A.; Tiwari, Vivek; Jonas, David M.
2014-03-28
We report measurement and modeling of two-dimensional (2D) electronic spectra of a silicon naphthalocyanine (SiNc) in benzonitrile, a system for which the polarization anisotropy reveals passage through a square-symmetric Jahn-Teller conical intersection in ∼100 fs [D. A. Farrow, W. Qian, E. R. Smith, A. A. Ferro, and D. M. Jonas, J. Chem. Phys. 128, 144510 (2008)]. The measured 2D Fourier transform (FT) spectra indicate loss of electronic coherence on a similar timescale. The 2D spectra arising from femtosecond vibronic dynamics through the conical funnel are modeled by full non-adiabatic treatment of the coupled electronic and vibrational dynamics for a pair of un-damped Jahn-Teller active vibrations responsible for both electronic decoherence and population transfer. Additional damped Jahn-Teller active modes that can cause only decoherence or population transfer are treated with analytical response functions that can be incorporated into the numerical non-adiabatic calculation by exploiting symmetry assignment of degenerate vibronic eigenstates to one of two electronic states. Franck-Condon active totally symmetric modes are incorporated analytically. The calculations reveal that these conical intersection dynamics alone are incapable of destroying the coherence of the initially prepared wavepacket on the experimentally observed timescale and predict an unobserved recurrence in the photon echo slice at ∼200 fs. Agreement with the experimental two-dimensional electronic spectra necessitates a role for totally symmetric vibrational dynamics in causing the echo slice to decay on a ∼100 fs timescale. This extended model also reproduces the ∼100 fs ultrafast electronic anisotropy decay in SiNc when an “asymmetric solvation mode” with a small stabilization energy of ∼2 cm{sup −1} is included. Although calculations show that inhomogeneities in the energy gap between excited states can broaden the anti-diagonal 2D lineshape, the anti-diagonal width is
Jiang Chongyun; Chen Yonghai; Ma Hui; Yu Jinling; Liu Yu
2011-06-06
In this letter we investigated the InAs/InAlAs quantum wires (QWRs) superlattice by optically exciting the structure with near-infrared radiation. By varying the helicity of the radiation at room temperature we observed the circular photogalvanic effect related to the C{sub 2v} symmetry of the structure, which could be attributed to the formation of a quasi-two-dimensional system underlying in the vicinity of the QWRs pattern. The ratio of Rashba and Dresselhaus terms shows an evolution of the spin-orbit interaction in quasi-two-dimensional structure with the QWR layer deposition thickness.
NASA Astrophysics Data System (ADS)
Ren, Li; Mi, Yi-Ming
2010-09-01
Based on the Kubo formalism, the anomalous Hall effect in a magnetic two-dimensional hole gas with cubic-Rashba spin-orbit coupling is studied in the presence of δ-function scattering potential. When the weak, short-ranged disorder scattering is considered in the Born approximation, we find that the self-energy becomes diagonal in the helicity basis and its value is independent of the wave number, and the vertex correction to the anomalous Hall conductivity due to impurity scattering vanishes when both subbands are occupied. That is to say, the anomalous Hall effect is not vanishing or influenced by the vertex correction for two-dimensional heavy-hole system, which is in sharp contrast to the case of linear-Rashba spin-orbit coupling in the electron band when the short-range disorder scattering is considered and the extrinsic mechanism as well as the effect of external electric field on the SO interaction are ignored.
High-Current Gain Two-Dimensional MoS₂-Base Hot-Electron Transistors.
Torres, Carlos M; Lan, Yann-Wen; Zeng, Caifu; Chen, Jyun-Hong; Kou, Xufeng; Navabi, Aryan; Tang, Jianshi; Montazeri, Mohammad; Adleman, James R; Lerner, Mitchell B; Zhong, Yuan-Liang; Li, Lain-Jong; Chen, Chii-Dong; Wang, Kang L
2015-12-09
The vertical transport of nonequilibrium charge carriers through semiconductor heterostructures has led to milestones in electronics with the development of the hot-electron transistor. Recently, significant advances have been made with atomically sharp heterostructures implementing various two-dimensional materials. Although graphene-base hot-electron transistors show great promise for electronic switching at high frequencies, they are limited by their low current gain. Here we show that, by choosing MoS2 and HfO2 for the filter barrier interface and using a noncrystalline semiconductor such as ITO for the collector, we can achieve an unprecedentedly high-current gain (α ∼ 0.95) in our hot-electron transistors operating at room temperature. Furthermore, the current gain can be tuned over 2 orders of magnitude with the collector-base voltage albeit this feature currently presents a drawback in the transistor performance metrics such as poor output resistance and poor intrinsic voltage gain. We anticipate our transistors will pave the way toward the realization of novel flexible 2D material-based high-density, low-energy, and high-frequency hot-carrier electronic applications.
NASA Astrophysics Data System (ADS)
Inada, Yuki; Ono, Ryo; Kumada, Akiko; Hidaka, Kunihiko; Maeyama, Mitsuaki
2016-09-01
The electron density of streamer discharges propagating in atmospheric-pressure air is crucially important for systematic understanding of the production mechanisms of reactive species utilized in wide ranging applications such as medical treatment, plasma-assisted ignition and combustion, ozone production and environmental pollutant processing. However, electron density measurement during the propagation of the atmospheric-pressure streamers is extremely difficult by using the conventional localized type measurement systems due to the streamer initiation jitters and the irreproducibility in the discharge paths. In order to overcome the difficulties, single-shot two-dimensional electron density measurement was conducted by using a Shack-Hartmann type laser wavefront sensor. The Shack-Hartmann sensor with a temporal resolution of 2 ns was applied to pulsed positive streamer discharges generated in an air gap between pin-to-plate electrodes. The electron density a few ns after the streamer initiation was 7*1021m-3 and uniformly distributed along the streamer channel. The electron density and its distribution profile were compared with a previous study simulating similar streamers, demonstrating good agreement. This work was supported in part by JKA and its promotion funds from KEIRIN RACE. The authors like to thank Mr. Kazuaki Ogura and Mr. Kaiho Aono of The University of Tokyo for their support during this work.
NASA Astrophysics Data System (ADS)
Guo, Hongxuan; Gao, Jianhua; Ishida, Nobuyuki; Xu, Mingsheng; Fujita, Daisuke
2014-01-01
Characterization of the structural and physical properties of two-dimensional (2D) materials, such as layer number and inelastic mean free path measurements, is very important to optimize their synthesis and application. In this study, we characterize the layer number and morphology of hexagonal boron nitride (h-BN) nanosheets on a metallic substrate using field emission scanning electron microscopy (FE-SEM) and scanning helium ion microscopy (HIM). Using scanning beams of various energies, we could analyze the dependence of the intensities of secondary electrons on the thickness of the h-BN nanosheets. Based on the interaction between the scanning particles (electrons and helium ions) and h-BN nanosheets, we deduced an exponential relationship between the intensities of secondary electrons and number of layers of h-BN. With the attenuation factor of the exponential formula, we calculate the inelastic mean free path of electrons and helium ions in the h-BN nanosheets. Our results show that HIM is more sensitive and consistent than FE-SEM for characterizing the number of layers and morphology of 2D materials.
Guo, Hongxuan E-mail: msxu@zju.edu.cn; Gao, Jianhua; Ishida, Nobuyuki; Xu, Mingsheng E-mail: msxu@zju.edu.cn; Fujita, Daisuke
2014-01-20
Characterization of the structural and physical properties of two-dimensional (2D) materials, such as layer number and inelastic mean free path measurements, is very important to optimize their synthesis and application. In this study, we characterize the layer number and morphology of hexagonal boron nitride (h-BN) nanosheets on a metallic substrate using field emission scanning electron microscopy (FE-SEM) and scanning helium ion microscopy (HIM). Using scanning beams of various energies, we could analyze the dependence of the intensities of secondary electrons on the thickness of the h-BN nanosheets. Based on the interaction between the scanning particles (electrons and helium ions) and h-BN nanosheets, we deduced an exponential relationship between the intensities of secondary electrons and number of layers of h-BN. With the attenuation factor of the exponential formula, we calculate the inelastic mean free path of electrons and helium ions in the h-BN nanosheets. Our results show that HIM is more sensitive and consistent than FE-SEM for characterizing the number of layers and morphology of 2D materials.
Automated Electron Microscopy for Evaluating Two-dimensional Crystallization of Membrane Proteins
Hu, Minghui; Vink, Martin; Kim, Changki; Derr, KD; Koss, John; D'Amico, Kevin; Cheng, Anchi; Pulokas, James; Ubarretxena-Belandia, Iban; Stokes, David
2010-01-01
Membrane proteins fulfill many important roles in the cell and represent the target for a large number of therapeutic drugs. Although structure determination of membrane proteins has become a major priority, it has proven to be technically challenging. Electron microscopy of two-dimensional (2D) crystals has the advantage of visualizing membrane proteins in their natural lipidic environment, but has been underutilized in recent structural genomics efforts. To improve the general applicability of electron crystallography, high-throughput methods are needed for screening large numbers of conditions for 2D crystallization, thereby increasing the chances of obtaining well ordered crystals and thus achieving atomic resolution. Previous reports describe devices for growing 2D crystals on a 96-well format. The current report describes a system for automated imaging of these screens with an electron microscope. Samples are inserted with a two-part robot: a SCARA robot for loading samples into the microscope holder, and a Cartesian robot for placing the holder into the electron microscope. A standard JEOL 1230 electron microscope was used, though a new tip was designed for the holder and a toggle switch controlling the airlock was rewired to allow robot control. A computer program for controlling the robots was integrated with the Leginon program, which provides a module for automated imaging of individual samples. The resulting images are uploaded into the Sesame laboratory information management system database where they are associated with other data relevant to the crystallization screen. PMID:20197095
NASA Astrophysics Data System (ADS)
Buchner, M.; Kuczmik, T.; Oltscher, M.; Ciorga, M.; Korn, T.; Loher, J.; Schuh, D.; Schüller, C.; Bougeard, D.; Weiss, D.; Back, C. H.
2017-01-01
We report on electrical spin injection from (Ga,Mn)As into a high-mobility two-dimensional electron gas confined at an (Al,Ga)As/GaAs interface. Besides standard nonlocal electrical detection, we use a magneto-optical approach which provides cross-sectional images of the spin accumulation at the cleaved edge of the sample, yielding spin decay lengths on the order of 2 μ m . In some cases we find a nonmonotonic bias voltage dependence of the spin signal, which may be linked to ballistic tunneling effects during spin injection. We observe a clear Hanle depolarization using a technique which is free of dynamic nuclear polarization effects. Fitting the data with the standard drift-diffusion model of spin injection suggests averaged in-plane spin lifetimes on the order of 1 ns.
Tunable electronic and optical behaviors of two-dimensional germanium carbide
NASA Astrophysics Data System (ADS)
Xu, Zhuo; Li, Yangping; Li, Chenxi; Liu, Zhengtang
2016-03-01
The electronic and optical properties of two-dimensional graphene-like germanium carbide (2D-GeC) are calculated using first-principle calculation based on density functional theory. Monolayer GeC has a direct band gap of 2.19 eV. The imaginary part of the dielectric function shows a wide energy range of absorption spectrum for monolayer GeC. Tunable band structures are found for monolayer GeC through in-plane strain. In addition, the band structures and optical properties of bilayer GeC under strain along the c axis are analyzed. Multilayer GeC exhibits a direct band gap like monolayer GeC, and new options of interband transitions are found between layers. The results suggest that 2D-GeC could be a good candidate for optoelectronic such as light-emitting diodes, photodiodes, and solar cells.
A conceptual design study for a two-dimensional, electronically scanned thinned array radiometer
NASA Technical Reports Server (NTRS)
Mutton, Philip; Chromik, Christopher C.; Dixon, Iain; Statham, Richard B.; Stillwagen, Frederic H.; Vontheumer, Alfred E.; Sasamoto, Washito A.; Garn, Paul A.; Cosgrove, Patrick A.; Ganoe, George G.
1993-01-01
A conceptual design for the Two-Dimensional, Electronically Steered Thinned Array Radiometer (ESTAR) is described. This instrument is a synthetic aperture microwave radiometer that operates in the L-band frequency range for the measurement of soil moisture and ocean salinity. Two auxiliary instruments, an 8-12 micron, scanning infrared radiometer and a 0.4-1.0 micron, charge coupled device (CCD) video camera, are included to provided data for sea surface temperature measurements and spatial registration of targets respectively. The science requirements were defined by Goddard Space Flight Center. Instrument and the spacecraft configurations are described for missions using the Pegasus and Taurus launch vehicles. The analyses and design trades described include: estimations of size, mass and power, instrument viewing coverage, mechanical design trades, structural and thermal analyses, data and communications performance assessments, and cost estimation.
Dielectric response of metal/SrTiO{sub 3}/two-dimensional electron liquid heterostructures
Mikheev, Evgeny; Raghavan, Santosh; Stemmer, Susanne
2015-08-17
Maximizing the effective dielectric constant of the gate dielectric stack is important for electrostatically controlling high carrier densities inherent to strongly correlated materials. SrTiO{sub 3} is uniquely suited for this purpose, given its extremely high dielectric constant, which can reach 10{sup 4}. Here, we present a systematic study of the thickness dependence of the dielectric response and leakage of SrTiO{sub 3} that is incorporated into a vertical structure on a high-carrier-density two-dimensional electron liquid (2DEL). A simple model can be used to interpret the data. The results show a need for improved interface control in the design of metal/SrTiO{sub 3}/2DEL devices.
Local interlayer tunneling between two-dimensional electron systems in the ballistic regime
NASA Astrophysics Data System (ADS)
Luna, Katherine; Kim, Eun-Ah; Oreto, Paul; Kivelson, Steven A.; Goldhaber-Gordon, David
2010-12-01
We study a theoretical model of virtual scanning tunneling microscopy (VSTM) [A. Sciambi, M. Pelliccione, M. Lilly, S. Bank, A. Gossard, L. Pfeiffer, K. West, and D. Goldhaber-Gordon, arXiv:1008.0668 (unpublished); A. Sciambi, M. Pelliccione, S. R. Bank, A. C. Gossard, and D. Goldhaber-Gordon, Appl. Phys. Lett. 97, 132103 (2010)10.1063/1.3492440]: a proposed application of interlayer tunneling in a bilayer system to locally probe a two-dimensional electron system (2DES) in a semiconductor heterostructure. We consider tunneling for the case where transport in the 2DESs is ballistic and show that the zero-bias anomaly is suppressed by extremely efficient screening. Since such an anomaly would complicate the interpretation of data from VSTM, this result is encouraging for efforts to implement such a microscopy technique.
Thermoelectric and electrical transport in mesoscopic two-dimensional electron gases
NASA Astrophysics Data System (ADS)
Narayan, Vijay; Pepper, Michael; Ritchie, David A.
2016-12-01
We review some of our recent experimental studies on low-carrier concentration, mesoscopic two-dimensional electron gases (m2DEGs). The m2DEGs show a range of striking characteristics, including a complete avoidance of the strongly localised regime even when the electrical resistivity ρ > > h /e2, giant thermoelectric response, and an apparent decoupling of charge and thermoelectric transport. We analyse the results and demonstrate that these observations can be explained based on the assumption that the charge carriers retain phase coherence over the m2DEG dimensions. Intriguingly, this would imply phase coherence on lengthscales of up to 10 μm and temperature T up to 10 K, which is significantly greater than conventionally expected in GaAs-based 2DEGs. We critically assess this assumption and explore other possible explanations to the data. Such unprecedentedly large phase coherence lengths open up several possibilities in quantum information and computation schemes. xml:lang="fr"
Imaginary time density-density correlations for two-dimensional electron gases at high density
Motta, M.; Galli, D. E.; Moroni, S.; Vitali, E.
2015-10-28
We evaluate imaginary time density-density correlation functions for two-dimensional homogeneous electron gases of up to 42 particles in the continuum using the phaseless auxiliary field quantum Monte Carlo method. We use periodic boundary conditions and up to 300 plane waves as basis set elements. We show that such methodology, once equipped with suitable numerical stabilization techniques necessary to deal with exponentials, products, and inversions of large matrices, gives access to the calculation of imaginary time correlation functions for medium-sized systems. We discuss the numerical stabilization techniques and the computational complexity of the methodology and we present the limitations related to the size of the systems on a quantitative basis. We perform the inverse Laplace transform of the obtained density-density correlation functions, assessing the ability of the phaseless auxiliary field quantum Monte Carlo method to evaluate dynamical properties of medium-sized homogeneous fermion systems.
NASA Astrophysics Data System (ADS)
Heisler, Ismael A.; Moca, Roberta; Camargo, Franco V. A.; Meech, Stephen R.
2014-06-01
We report an improved experimental scheme for two-dimensional electronic spectroscopy (2D-ES) based solely on conventional optical components and fast data acquisition. This is accomplished by working with two choppers synchronized to a 10 kHz repetition rate amplified laser system. We demonstrate how scattering and pump-probe contributions can be removed during 2D measurements and how the pump probe and local oscillator spectra can be generated and saved simultaneously with each population time measurement. As an example the 2D-ES spectra for cresyl violet were obtained. The resulting 2D spectra show a significant oscillating signal during population evolution time which can be assigned to an intramolecular vibrational mode.
NASA Technical Reports Server (NTRS)
Schacham, S. E.; Mena, R. A.; Haugland, E. J.; Alterovitz, S. A.
1993-01-01
A technique for determination of room-temperature two-dimensional electron gas (2DEG) concentration and mobility in heterostructures is presented. Using simultaneous fits of the longitudinal and transverse voltages as a function of applied magnetic field, we were able to separate the parameters associated with the 2DEG from those of the parallel layer. Comparison with the Shubnikov-de Haas data derived from measurements at liquid helium temperatures proves that the analysis of the room-temperature data provides an excellent estimate of the 2DEG concentration. In addition we were able to obtain for the first time the room-temperature mobility of the 2DEG, an important parameter to device application. Both results are significantly different from those derived from conventional Hall analysis.
Scattering of two-dimensional massless Dirac electrons by a circular potential barrier
NASA Astrophysics Data System (ADS)
Wu, Jhih-Sheng; Fogler, Michael M.
2014-12-01
We calculate the differential, total, and transport cross-sections for scattering of two-dimensional massless Dirac electrons by a circular barrier. For scatterer of a small radius, the cross-sections are dominated by quantum effects such as resonant scattering that can be computed using the partial-wave series. Scattering by larger size barriers is better described within the classical picture of reflection and refraction of rays, which leads to phenomena of caustics, rainbow, and critical scattering. Refraction can be negative if the potential of the scatterer is repulsive, so that a p -n junction forms at its boundary. Qualitative differences of this case from the n -N doping case are examined. Quantum interference effects beyond the classical ray picture are also considered, such as normal and anomalous diffraction, and also whispering-gallery resonances. Implications of these results for transport and scanned-probe experiments in graphene and topological insulators are discussed.
Instability of the QHE induced by a nearby two-dimensional-electron system
NASA Astrophysics Data System (ADS)
Jörger, C.; Wegscheider, W.; Dietsche, W.; von Klitzing, K.
1998-12-01
We report on the observation of structures in the longitudinal resistance of a single two-dimensional electron gas (2DEG) in a perpendicular magnetic field at filling factor ν≅1±0.1. The observed structures can be switched off by depleting a second 2DEG, which is separated from the first by a 30 nm or a 60 nm AlGaAs-barrier, using a gate-electrode. Measurements as a function of temperature and drive current indicate that these structures are precursors of the breakdown of the quantum-Hall-effect (QHE) at ν=1. The QHE becomes unstable when another 2DEG is in the neighbourhood. This effect is most likely due to macroscopic effects like screening, because other coupling processes between these two 2DEGs, such as frictional drag, are weak.
Enabling two-dimensional fourier transform electronic spectroscopy on quantum dots
NASA Astrophysics Data System (ADS)
Hill, Robert John, Jr.
Colloidal semiconductor nanocrystals exhibit unique properties not seen in their bulk counterparts. Quantum confinement of carriers causes a size-tunable bandgap, making them attractive candidates for solar cells. Fundamental understanding of their spectra and carrier dynamics is obscured by inhomogeneous broadening arising from the size distribution. Because quantum dots have long excited state lifetimes and are sensitive to both air and moisture, there are many potential artifacts in femtosecond experiments. Two-dimensional electronic spectroscopy promises insight into the photo-physics, but required key instrumental advances. Optics that can process a broad bandwidth without distortion are required for a two-dimensional optical spectrometer. To control pathlength differences for femtosecond time delays, hollow retro-reflectors are used on actively stabilized delay lines in interferometers. The fabrication of rigid, lightweight, precision hollow rooftop retroreflectors that allow beams to be stacked while preserving polarization is described. The rigidity and low mass enable active stabilization of an interferometer to within 0.6 nm rms displacement, while the return beam deviation is sufficient for Fourier transform spectroscopy with a frequency precision of better than 1 cm -1. Keeping samples oxygen and moisture free while providing fresh sample between laser shots is challenging in an interferometer. A low-vibration spinning sample cell was designed and built to keep samples oxygen free for days while allowing active stabilization of interferometer displacement to ˜1 nm. Combining these technologies has enabled 2D short-wave infrared spectroscopy on colloidal PbSe nanocrystals. 2D spectra demonstrate the advantages of this key instrumentation while providing valuable insight into the low-lying electronic states of colloidal quantum dots.
NASA Astrophysics Data System (ADS)
Ullrich, C. A.; Vignale, G.
1998-09-01
It is well known that high-frequency collective excitations in electronic systems are not Landau damped, i.e., they cannot decay effectively into single particle-hole pairs. The leading damping mechanism in this regime is instead provided by dynamical exchange and correlation effects, such as multipair production. These effects are not captured by the widely used adiabatic local-density approximation (ALDA), which accounts for Landau damping only. In the recently developed time-dependent current density-functional formalism [G. Vignale, C. A. Ullrich, and S. Conti, Phys. Rev. Lett. 79, 4878 (1997)], exchange and correlation enter as viscoelastic stresses in the electron fluid, causing an additional damping that is not contained in the ALDA. We use this theory to derive an explicit formula for the linewidth of collective electronic excitations that are not Landau damped. The formula is then applied to calculate the linewidth of collective modes in two-dimensional (2D) quantum strips. In comparison with the corresponding modes in the homogeneous 2D electron gas, we find an order-of-magnitude enhancement of the linewidth due to the nonuniformity of the system.
NASA Astrophysics Data System (ADS)
Dabiran, Amir Massoud
The electrochemical potential (ECP) oscillations of the two-dimensional electron gas (2DEG) in GaAs/AlGaAs heterostructures (GaAs-HET) in high magnetic fields and at low temperatures has been measured by a 'floating-gate' technique. We have carried out these measurements using a very high impedence circuit (~10 ^{13}Omega) at temperatures between 1.2 and 4.2 K and in magnetic fields up to 15 Tesla on conventional GaAs-HETs by monitoring the voltage difference between the contacts to the 2DEG and a metal 'gate' evaporated on top of the samples. We have also used novel GaAs-HET samples with a substrate contact to the 2DEG through a thin (~50 nm) tunneling barrier thereby eliminating the problems associated with circulating eddy currents induced in the 2DEG layer by the changing magnetic field. These quantitative measurements of the ECP oscillations have allowed us to extract the 2DEG thermodynamic density of states (DOS) in quantizing magnetic fields. Our experimental findings bring out a dynamic picture of the DOS where nonlinear screening of the long-range scatterers (e.g. ionized impurities) causes a filling factor dependent 'breathing' of the Landau levels and the electron-electron interactions result in an oscillatory enhancement of the g-factor of electrons in GaAs.
Graphene/MoS2 hybrid technology for large-scale two-dimensional electronics.
Yu, Lili; Lee, Yi-Hsien; Ling, Xi; Santos, Elton J G; Shin, Yong Cheol; Lin, Yuxuan; Dubey, Madan; Kaxiras, Efthimios; Kong, Jing; Wang, Han; Palacios, Tomás
2014-06-11
Two-dimensional (2D) materials have generated great interest in the past few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2), and insulating boron nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency, and favorable transport properties for realizing electronic, sensing, and optical systems on arbitrary surfaces. In this paper, we demonstrate a novel technology for constructing large-scale electronic systems based on graphene/molybdenum disulfide (MoS2) heterostructures grown by chemical vapor deposition. We have fabricated high-performance devices and circuits based on this heterostructure, where MoS2 is used as the transistor channel and graphene as contact electrodes and circuit interconnects. We provide a systematic comparison of the graphene/MoS2 heterojunction contact to more traditional MoS2-metal junctions, as well as a theoretical investigation, using density functional theory, of the origin of the Schottky barrier height. The tunability of the graphene work function with electrostatic doping significantly improves the ohmic contact to MoS2. These high-performance large-scale devices and circuits based on this 2D heterostructure pave the way for practical flexible transparent electronics.
Dahlberg, Peter D.; Norris, Graham J.; Wang, Cheng; Viswanathan, Subha; Singh, Ved P.; Engel, Gregory S.
2015-01-01
Energy transfer through large disordered antenna networks in photosynthetic organisms can occur with a quantum efficiency of nearly 100%. This energy transfer is facilitated by the electronic structure of the photosynthetic antennae as well as interactions between electronic states and the surrounding environment. Coherences in time-domain spectroscopy provide a fine probe of how a system interacts with its surroundings. In two-dimensional electronic spectroscopy, coherences can appear on both the ground and excited state surfaces revealing detailed information regarding electronic structure, system-bath coupling, energy transfer, and energetic coupling in complex chemical systems. Numerous studies have revealed coherences in isolated photosynthetic pigment-protein complexes, but these coherences have not been observed in vivo due to the small amplitude of these signals and the intense scatter from whole cells. Here, we present data acquired using ultrafast video-acquisition gradient-assisted photon echo spectroscopy to observe quantum beating signals from coherences in vivo. Experiments were conducted on isolated light harvesting complex II (LH2) from Rhodobacter sphaeroides, whole cells of R. sphaeroides, and whole cells of R. sphaeroides grown in 30% deuterated media. A vibronic coherence was observed following laser excitation at ambient temperature between the B850 and the B850∗ states of LH2 in each of the 3 samples with a lifetime of ∼40-60 fs. PMID:26373989
A deterministic computational model for the two dimensional electron and photon transport
NASA Astrophysics Data System (ADS)
Badavi, Francis F.; Nealy, John E.
2014-12-01
A deterministic (non-statistical) two dimensional (2D) computational model describing the transport of electron and photon typical of space radiation environment in various shield media is described. The 2D formalism is casted into a code which is an extension of a previously developed one dimensional (1D) deterministic electron and photon transport code. The goal of both 1D and 2D codes is to satisfy engineering design applications (i.e. rapid analysis) while maintaining an accurate physics based representation of electron and photon transport in space environment. Both 1D and 2D transport codes have utilized established theoretical representations to describe the relevant collisional and radiative interactions and transport processes. In the 2D version, the shield material specifications are made more general as having the pertinent cross sections. In the 2D model, the specification of the computational field is in terms of a distance of traverse z along an axial direction as well as a variable distribution of deflection (i.e. polar) angles θ where -π/2<θ<π/2, and corresponding symmetry is assumed for the range of azimuth angles (0<φ<2π). In the transport formalism, a combined mean-free-path and average trajectory approach is used. For candidate shielding materials, using the trapped electron radiation environments at low Earth orbit (LEO), geosynchronous orbit (GEO) and Jupiter moon Europa, verification of the 2D formalism vs. 1D and an existing Monte Carlo code are presented.
NASA Astrophysics Data System (ADS)
Dahlberg, Peter D.; Norris, Graham J.; Wang, Cheng; Viswanathan, Subha; Singh, Ved P.; Engel, Gregory S.
2015-09-01
Energy transfer through large disordered antenna networks in photosynthetic organisms can occur with a quantum efficiency of nearly 100%. This energy transfer is facilitated by the electronic structure of the photosynthetic antennae as well as interactions between electronic states and the surrounding environment. Coherences in time-domain spectroscopy provide a fine probe of how a system interacts with its surroundings. In two-dimensional electronic spectroscopy, coherences can appear on both the ground and excited state surfaces revealing detailed information regarding electronic structure, system-bath coupling, energy transfer, and energetic coupling in complex chemical systems. Numerous studies have revealed coherences in isolated photosynthetic pigment-protein complexes, but these coherences have not been observed in vivo due to the small amplitude of these signals and the intense scatter from whole cells. Here, we present data acquired using ultrafast video-acquisition gradient-assisted photon echo spectroscopy to observe quantum beating signals from coherences in vivo. Experiments were conducted on isolated light harvesting complex II (LH2) from Rhodobacter sphaeroides, whole cells of R. sphaeroides, and whole cells of R. sphaeroides grown in 30% deuterated media. A vibronic coherence was observed following laser excitation at ambient temperature between the B850 and the B850∗ states of LH2 in each of the 3 samples with a lifetime of ˜40-60 fs.
Two-Dimensional Electronic Spectroscopy of Chlorophyll a: Solvent Dependent Spectral Evolution.
Moca, Roberta; Meech, Stephen R; Heisler, Ismael A
2015-07-09
The interaction of the monomeric chlorophyll Q-band electronic transition with solvents of differing physical-chemical properties is investigated through two-dimensional electronic spectroscopy (2DES). Chlorophyll constitutes the key chromophore molecule in light harvesting complexes. It is well-known that the surrounding protein in the light harvesting complex fine-tunes chlorophyll electronic transitions to optimize energy transfer. Therefore, an understanding of the influence of the environment on the monomeric chlorophyll electronic transitions is important. The Q-band 2DES is inhomogeneous at early times, particularly in hydrogen bonding polar solvents, but also in nonpolar solvents like cyclohexane. Interestingly this inhomogeneity persists for long times, even up to the nanosecond time scale in some solvents. The reshaping of the 2DES occurs over multiple time scales and was assigned mainly to spectral diffusion. At early times the reshaping is Gaussian-like, hinting at a strong solvent reorganization effect. The temporal evolution of the 2DES response was analyzed in terms of a Brownian oscillator model. The spectral densities underpinning the Brownian oscillator fitting were recovered for the different solvents. The absorption spectra and Stokes shift were also properly described by this model. The extent and nature of inhomogeneous broadening was a strong function of solvent, being larger in H-bonding and viscous media and smaller in nonpolar solvents. The fastest spectral reshaping components were assigned to solvent dynamics, modified by interactions with the solute.
Dahlberg, Peter D.; Norris, Graham J.; Wang, Cheng; Viswanathan, Subha; Singh, Ved P.; Engel, Gregory S.
2015-09-14
Energy transfer through large disordered antenna networks in photosynthetic organisms can occur with a quantum efficiency of nearly 100%. This energy transfer is facilitated by the electronic structure of the photosynthetic antennae as well as interactions between electronic states and the surrounding environment. Coherences in time-domain spectroscopy provide a fine probe of how a system interacts with its surroundings. In two-dimensional electronic spectroscopy, coherences can appear on both the ground and excited state surfaces revealing detailed information regarding electronic structure, system-bath coupling, energy transfer, and energetic coupling in complex chemical systems. Numerous studies have revealed coherences in isolated photosynthetic pigment-protein complexes, but these coherences have not been observed in vivo due to the small amplitude of these signals and the intense scatter from whole cells. Here, we present data acquired using ultrafast video-acquisition gradient-assisted photon echo spectroscopy to observe quantum beating signals from coherences in vivo. Experiments were conducted on isolated light harvesting complex II (LH2) from Rhodobacter sphaeroides, whole cells of R. sphaeroides, and whole cells of R. sphaeroides grown in 30% deuterated media. A vibronic coherence was observed following laser excitation at ambient temperature between the B850 and the B850{sup ∗} states of LH2 in each of the 3 samples with a lifetime of ∼40-60 fs.
Bordács, Sándor; Checkelsky, Joseph G; Murakawa, Hiroshi; Hwang, Harold Y; Tokura, Yoshinori
2013-10-18
Optical excitations of BiTeI with large Rashba spin splitting have been studied in an external magnetic field (B) applied parallel to the polar axis. A sequence of transitions between the Landau levels (LLs), whose energies are in proportion to √B were observed, being characteristic of massless Dirac electrons. The large separation energy between the LLs makes it possible to detect the strongest cyclotron resonance even at room temperature in moderate fields. Unlike in 2D Dirac systems, the magnetic field induced rearrangement of the conductivity spectrum is directly observed.
Electronic and magnetism properties of two-dimensional stacked nickel hydroxides and nitrides
Wei, Xiao-Lin; Tang, Zhen-Kun; Guo, Gen-Cai; Ma, Shangyi; Liu, Li-Min
2015-01-01
Two-dimensional (2D) layered materials receive a lot of attention because of their outstanding intrinsic properties and wide applications. In this work, the structural, electronic and magnetic properties of nickel hydroxides (Ni(OH)2) and nitrides XN (X = B, Al, and Ga) heterostructures are studied by first-principles calculations. The results show that the pristine monolayer Ni(OH)2 owns no macro magnetism with antiferromagnetic (AFM) coupling between two nearest Ni atoms, the electronic structure can be modulated through the heterostructures. The Ni(OH)2-GaN and Ni(OH)2-AlN heterostructures retain the AFM coupling, while Ni(OH)2-BN heterostructure have a larger magnetic moment with ferromagnetic (FM) coupling. The complete electron–hole separation is found in the Ni(OH)2-GaN heterostructure. The tunable electronic and magnetic properties of the Ni(OH)2-XN heterostructures open a new door to design the spintronic devices in the 2D stacked nanostructures. PMID:26111476
Origin of Hund's multiplicity rule in quasi-two-dimensional two-electron quantum dots
Sako, Tokuei; Paldus, Josef; Diercksen, Geerd H. F.
2010-02-15
The origin of Hund's multiplicity rules has been studied for a system of two electrons confined by a quasi-two-dimensional harmonic-oscillator potential by relying on a full configuration interaction wave function and Cartesian anisotropic Gaussian basis sets. In terms of appropriate normal-mode coordinates the wave function factors into a product of the center-of-mass and the internal components. The {sup 1{Pi}}{sub u} singlet state and the {sup 3{Pi}}{sub u} triplet state represent the energetically lowest pair of states to which Hund's multiplicity rule applies. They are shown to involve excitations into different degrees of freedom, namely, into the center-of-mass angular mode and the internal angular mode for the singlet and triplet states, respectively. The presence of an angular nodal line in the internal space allows then the triplet state to avoid the singularity in the electron-electron interaction potential, leading to the energy lowering of the triplet state relative to its counterpart singlet state.
Stability and Electronic Properties of Two-Dimensional Silicene and Germanene on Graphene
NASA Astrophysics Data System (ADS)
Chuu, Chih-Piao; Cai, Yongmao; Wei, C.-M.; Chou, M.-Y.
2014-03-01
Recently, there have been experimental attempts to synthesize silicene, a two-dimensional (2D) graphene-like form of silicon on metal surfaces such as Ag(111) and Ir(0001). The possibility of preparing silicene on ZrB2 thin films grown on silicon wafers has also been reported. This suggests new perspectives for the applications of massless fermions in materials that are compatible with Si-based electronics. It is expected that many of the unique electronic properties of graphene can also be realized in this new 2D system. However, the interaction between the 2D silicon structure and the metal substrate is found to be quite strong, leading to distortion in the adlayer and consequently the disappearance of the Dirac cone. Therefore, finding a suitable substrate that interacts with silicene weakly and preserves the sublattice symmetry is of ultimate importance. We have performed first-principles calculations of silicene and germanene on graphene in order to understand the effect of substrate interaction on the physical properties of these systems. Of particular interest is the induced change in the electronic structure, the modification of the Fermi velocity, the gap opening, the charge doping from the substrate, and the stability of the combined system. The energetics of forming the 2D silicone structure on a substrate is carefully evaluated in comparison with possible three-dimensional cluster structures.
Non-linear Resistivity of a Two-Dimensional Electron Gas in a Magnetic Field
NASA Astrophysics Data System (ADS)
Vavilov, Maxim G.; Aleiner, Igor L.; Glazman, Leonid I.
2007-03-01
We develop a theory of nonlinear response to an electric field of a two-dimensional electron gas (2DEG) placed in a classically strong magnetic field. The latter leads to a non-linear current-voltage characteristic at a relatively weak electric field. The origin of the non-linearity is two-fold: the formation of a non-equilibrium electron distribution function, and the geometrical resonance in the inter-Landau-levels transitions rates. We find the dependence of the current-voltage characteristics on the electron relaxation rates in the 2DEG. Our results can be applied for analysis of measurements at low [1] and high [2,3] current densities. [1] J. Zhang, S. Vitkalov, A. A. Bykov, A. K. Kalagin and A. K. Bakarov, cond-mat/0607741. [2] C. L. Yang, J. Zhang, R. R. Du, J. A. Simmons and J. L. Reno, Phys. Rev. Lett. 89, 076801 (2002). [3] W. Zhang, H. -S. Chiang, M. A. Zudov, L. N. Pfeiffer and K. W. West, cond-mat/0608727.
Postek, Michael T; Vladár, András E; Lowney, Jeremiah R; Keery, William J
2002-01-01
Traditional Monte Carlo modeling of the electron beam-specimen interactions in a scanning electron microscope (SEM) produces information about electron beam penetration and output signal generation at either a single beam-landing location, or multiple landing positions. If the multiple landings lie on a line, the results can be graphed in a line scan-like format. Monte Carlo results formatted as line scans have proven useful in providing one-dimensional information about the sample (e.g., linewidth). When used this way, this process is called forward line scan modeling. In the present work, the concept of image simulation (or the first step in the inverse modeling of images) is introduced where the forward-modeled line scan data are carried one step further to construct theoretical two-dimensional (2-D) micrographs (i.e., theoretical SEM images) for comparison with similar experimentally obtained micrographs. This provides an ability to mimic and closely match theory and experiment using SEM images. Calculated and/or measured libraries of simulated images can be developed with this technique. The library concept will prove to be very useful in the determination of dimensional and other properties of simple structures, such as integrated circuit parts, where the shape of the features is preferably measured from a single top-down image or a line scan. This paper presents one approach to the generation of 2-D simulated images and presents some suggestions as to their application to critical dimension metrology.
NASA Astrophysics Data System (ADS)
Inada, Y.; Matsuoka, S.; Kumada, A.; Ikeda, H.; Hidaka, K.
2017-03-01
Electrode material dependence of intense-mode vacuum arc behaviour was systematically investigated by using the Shack-Hartmann method capable of simultaneously visualising two-dimensional electron and metal vapour density distributions from single-shot recordings. The electrode materials studied included Cu, CuCr (Cu75Cr25 wt. %), WC, and AgWC (Ag40WC60 wt. %). A comparison between the Cu and CuCr electrodes showed that the metal vapour densities for the CuCr decreased in an even shorter time scale than for the Cu. In the case of the WC electrodes, the widths of the electron density distributions became narrower as the arc current decreased although the electron densities hardly decreased in the decaying process of the arc current. The density measurements conducted at the late stage of the vacuum arcs demonstrated that the metal vapour densities around the anode were maintained at the highest value for the AgWC among the electrode materials in this study.
High Pressure Studies of the Second Landau Level Region of a Two-Dimensional Electron System
NASA Astrophysics Data System (ADS)
Schreiber, Katherine; Samkharadze, Nodar; Gardner, Geoffrey; Fradkin, Eduardo; Manfra, Michael; Csathy, Gabor
Hydrostatic pressure has become a prevalent tool in condensed matter systems because the application of pressure to crystalline structures results in the shrinking of the lattice constant. This allows one to tune the Bloch wavefunction of the electrons and therefore all band parameters such as effective carrier mass, carrier density, and effective g-factor. In this manner, pressure acts as a probe into various strongly interacting electronic states. Motivated in particular by the capability to discern the spin polarization of quantum Hall states, we apply hydrostatic pressure up to 10 kbar to a two dimensional electron system (2DES) in a high-mobility GaAs/AlGaAs quantum well. This 2DES is subjected to milliKelvin temperatures and strong magnetic fields to observe the effect of pressure on fractional quantum Hall states, especially those in higher Landau levels, a regime not previously studied under pressure. We report our findings, focusing on the observation of a pressure-driven transition from a fractional quantum Hall state to the quantum Hall nematic phase in the second Landau level. Grants: Researchers at Purdue and N. Samkharadze: US DOE, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, DE-SC0006671. E. Fradkin: US NSF, DMR 1408713.
Schmidt-Krey, Ingeborg; Rubinstein, John L.
2010-01-01
Membrane protein structure and function can be studied by two powerful and highly complementary electron cryomicroscopy (cryo-EM) methods: electron crystallography of two-dimensional (2D) crystals and single particle analysis of detergent-solubilized protein complexes. To obtain the highest-possible resolution data from membrane proteins, whether prepared as 2D crystals or single particles, cryo-EM samples must be vitrified with great care. Grid preparation for cryo-EM of 2D crystals is possible by back-injection, the carbon sandwich technique, drying in sugars before cooling in the electron microscope, or plunge-freezing. Specimen grids for single particle cryo-EM studies of membrane proteins are usually produced by plunge-freezing protein solutions, supported either by perforated or a continuous carbon film substrate. This review outlines the different techniques available and the suitability of each method for particular samples and studies. Experimental considerations in sample preparation and preservation include the protein itself and the presence of lipid or detergent. The appearance of cryo-EM samples in different conditions is also discussed. PMID:20678942
Controlling the two-dimensional electron gas at complex oxide interfaces
NASA Astrophysics Data System (ADS)
Janotti, Anderson
2014-03-01
Heterostructures of complex oxides have attracted great interest since the demonstration of a high-density two-dimensional electron gas (2DEG) at the SrTiO3/LaAlO3 (STO/LAO) interface. Still, the density of the 2DEG is only one tenth of what was expected from simple electron counting, i.e., 1/2 electron per unit-cell area. Since then, the origin and amount of the charge, the electrical properties of the 2DEG, the role of native defects, and the abrupt variation of the electron density with the thickness of the LAO top layer have been the subject of numerous theoretical and experimental studies. More recently, a 2DEG with the full density of 1/2 electron per unit cell area has been observed at the interface between the band insulator STO and the Mott insulator GdTiO3 (GTO), shedding additional light on the origin of the 2DEG, and raising important questions on the differences between the STO/LAO and STO/GTO heterostructures. Here we will discuss the similarities of the 2DEG at the STO/LAO and STO/GTO heterostructures from the perspective of first-principles simulations. We will address the differences in band alignments in the STO/LAO and STO/GTO heterostructures, and how the 2DEG is affected by the surface of the LAO top layer in the STO/LAO, but apparently not in the STO/GTO case. Finally, we will also discuss how heterostructures can be used to drastically alter the electronic structure of STO, transforming it from a band insulator into a Mott insulator. This work was performed in collaboration with Lars Bjaalie, Luke Gordon, Burak Himmetoglu, and Chris G. Van de Walle, and supported by ARO and NSF.
High mobility one- and two-dimensional electron systems in nanowire-based quantum heterostructures.
Funk, Stefan; Royo, Miguel; Zardo, Ilaria; Rudolph, Daniel; Morkötter, Stefanie; Mayer, Benedikt; Becker, Jonathan; Bechtold, Alexander; Matich, Sonja; Döblinger, Markus; Bichler, Max; Koblmüller, Gregor; Finley, Jonathan J; Bertoni, Andrea; Goldoni, Guido; Abstreiter, Gerhard
2013-01-01
Free-standing semiconductor nanowires in combination with advanced gate-architectures hold an exceptional promise as miniaturized building blocks in future integrated circuits. However, semiconductor nanowires are often corrupted by an increased number of close-by surface states, which are detrimental with respect to their optical and electronic properties. This conceptual challenge hampers their potentials in high-speed electronics and therefore new concepts are needed in order to enhance carrier mobilities. We have introduced a novel type of core-shell nanowire heterostructures that incorporate modulation or remote doping and hence may lead to high-mobility electrons. We demonstrate the validity of such concepts using inelastic light scattering to study single modulation-doped GaAs/Al0.16Ga0.84As core-multishell nanowires grown on silicon. We conclude from a detailed experimental study and theoretical analysis of the observed spin and charge density fluctuations that one- and two-dimensional electron channels are formed in a GaAs coaxial quantum well spatially separated from the donor ions. A total carrier density of about 3 × 10(7) cm(-1) and an electron mobility in the order of 50,000 cm(2)/(V s) are estimated. Spatial mappings of individual GaAs/Al0.16Ga0.84As core-multishell nanowires show inhomogeneous properties along the wires probably related to structural defects. The first demonstration of such unambiguous 1D- and 2D-electron channels and the respective charge carrier properties in these advanced nanowire-based quantum heterostructures is the basis for various novel nanoelectronic and photonic devices.
NASA Astrophysics Data System (ADS)
Zhang, Jing-Jing; Liang, Feng; Yang, Yong-Hong; Wang, Jun
2009-12-01
We report a theoretic study on the inverse spin-Hall effect (ISHE) in a two-terminal nano-device that consists of a two-dimensional electron gas (2DEG) with Rashba spin-orbit coupling (RSOC) and two ideal leads. Based on a two-site toy model and Keldysh Green's function method, we derive an analytic result of ISHE, which shows clearly that a nonzero transverse charge current stems from the combined effect of the RSOC, the spin bias, and its spin polarization direction in spin space. Our further numerical calculations in a larger system other than two-site lattice model demonstrate that the transverse charge current, dependent on the strength of the RSOC, the Fermi energy of the system, as well as the system size, can exhibit oscillating behavior and even reverse its sign due to Rashba spin precession. These properties may be helpful for efficient detection of the spin current (spin bias) by measuring the transverse charge current in a spin-orbital coupling system.
Kanou, Manabu; Sasagawa, Takao
2013-04-03
3D Rashba materials can be a leading player in spin-related novel phenomena, ranging from the metallic extreme (unconventional superconductivity) to the transport intermediate (spin Hall effects) to the novel insulating variant (3D topological insulating states). As the essential backbone for both fundamental and applied research of such a 3D Rashba material, this study established the growth of sizeable single crystals of a candidate compound BiTeI with adjusted carrier concentrations. Three techniques (standard vertical Bridgman, modified horizontal Bridgman, and vapour transport) were employed, and BiTeI crystals (>1 × 1 × 0.2 mm(3)) with fundamentally different electronic states from metallic to insulating were successfully grown by the chosen technique. The 3D Rashba electronic states, including the Fermi surface topology, for the corresponding carrier concentrations of the obtained BiTeI crystals were revealed by relativistic first-principles calculations.
Hexagonal boron nitride: Ubiquitous layered dielectric for two-dimensional electronics
NASA Astrophysics Data System (ADS)
Jain, Nikhil
Hexagonal boron nitride (h-BN), a layer-structured dielectric with very similar crystalline lattice to that of graphene, has been studied as a ubiquitous dielectric for two-dimensional electronics. While 2D materials may lead to future platform for electronics, traditional thin-film dielectrics (e.g., various oxides) make highly invasive interface with graphene. Multiple key roles of h-BN in graphene electronics are explored in this thesis. 2D graphene/h-BN heterostructures are designed and implemented in diverse configurations in which h-BN is evaluated as a supporting substrate, a gate dielectric, a passivation layer, or an interposing barrier in "3D graphene" superlattice. First, CVD-grown graphene on h-BN substrate shows improved conductivity and resilience to thermally induced breakdown, as compared with graphene on SiO2, potentially useful for high-speed graphene devices and on-chip interconnects. h-BN is also explored as a gate dielectric for graphene field-effect transistor with 2D heterostructure design. The dielectric strength and tunneling behavior of h-BN are investigated, confirming its robust nature. Next, h-BN is studied as a passivation layer for graphene electronics. In addition to significant improvement in current density and breakdown threshold, fully encapsulated graphene exhibits minimal environmental sensitivity, a key benefit to 2D materials which have only surfaces. Lastly, reduction in interlayer carrier scattering is observed in a double-layered graphene setup with ultrathin h-BN multilayer as an interposing layer. The DFT simulation and Raman spectral analysis indicate reduction in interlayer scattering. The decoupling of the two graphene monolayers is further confirmed by electrical characterization, as compared with other referencing mono- and multilayer configurations. The heterostructure serves as the building element in "3D graphene", a versatile platform for future electronics.
Electron Localization and Superconductivity in Two-Dimensional Metal Film Systems
NASA Astrophysics Data System (ADS)
Burns, Michael Joseph
The low temperature electrical transport properties of very thin polycrystalline palladium and palladium-gold films grown on glass or fused quartz, and also the properties of epitaxially grown silver films on germanium (001), have been investigated. These, and other two dimensional electronic systems, display a nonmetallic conductivity which has been attributed to electron localization and/or electron-electron interaction effects. According to the various theories, some of the transport properties should display different behaviors, thus allowing one to distinguish between the two effects. Measurements of Pd and Pd-Au film resistivities as a function of temperature, electric and magnetic field, plus the thermopower, were performed on films ranging from 18 to 30 Angstroms in thickness, having resistivities from 600 to 500000 ohms/square at 10(DEGREES)K. The magnetotransport properties imply the presence of strong spin-orbit coupling, although the temperature dependence of the resistivity of these films does not. All 'metallic' samples (resistivities less than 30000 ohms/square) have a material specific thermopower which tends to zero as the temperature goes to zero. Samples whose resistivities increase above 30000 ohms/square have thermopowers which diverge as the temperature approaches zero. Thus the density of states for the electronic transport is zero at the Fermi energy (i.e. an energy gap opens in the density of states at 30000 ohms/square) for high -resistivity films. The very thin (2.5 monolayer) silver films epitaxially grown on Ge (001) consist of a monolayer coverage plus isolated three dimensional islands. Below 70(DEGREES)K the conductivity is dominated by the metal film and displays the temperature and electric and magnetic field dependencies characteristic of metallic weak localization in two dimensions. Below about 2(DEGREES)K, the resistance drops rapidly in a manner resembling an incomplete superconducting transition. The resistance is restored by
The Two-dimensional Electron Gas at the Lanthanum Aluminate - Strontium Titanate Heterointerface
NASA Astrophysics Data System (ADS)
Hernandez, Tomas
Since the discovery of the conducting two dimensional electron gas (2DEG) at the interface of insulating oxide materials LaAlO3 and SrTiO 3 in 2004, this system has shown a vast diversity of physical properties including superconductivity, ferromagnetism and field induced metal-insulator phase transitions. In this dissertation we discuss three advances by our collaboration that bolster applicability of this interfacial system. In an all thin-film LaAlO3/SrTiO3 system we identified two different conduction regimes, where the effect of oxygen partial pressure during growth directly affects the carrier density of the system. In the lower carrier density regime (˜ 1013 cm-2) we found a metallic to insulating temperature dependence and strong localization by disorder, whereas in the higher regime (> 1014 cm-2) we find metallic dependence with signs of weak localization. This is understood in the occupation of Ti 3d bands farther from the interface as carrier density is increased. Our work in the electronic properties of all-thin-film LaAlO 3/SrTiO3 pioneers the understanding and implementations of this 2DEG to harness the richness of this oxide interface and its integration to industry standard substrates. Furthermore, we showed the first conducting LaAlO3/SrTiO3 two-dimensional system grown by the physical vapor deposition technique: 90° off-axis sputtering. Films grown with this scalable technique have excellent crystalline quality and transport characteristics similar to those grown by (mostly research employed) pulsed laser deposition. Writing of circuits on the nanoscale using conducting-tip atomic force microscopy is demonstrated in sputtered LaAlO3 on SrTiO3, opening the doors to nanoelectronic application of this novel interfacial system. Moreover, we are among the first groups in the world to demonstrate conducting [111]-oriented LaAlO3/SrTiO3 heterostructures, a crystallographic orientation that proves to be challenging due to the large polar nature of
NASA Astrophysics Data System (ADS)
Farina, L. A.; Lewis, K. M.; Kurdak, C.; Ghosh, S.; Krishna, S.; Bhattacharya, P.
2003-03-01
We studied the coupling between vortices in a superconducting film and electrons in a two-dimensional electron gas (2DEG). We measured structures consisting of a thin Al film evaporated on a GaAs/AlGaAs heterostructure containing a 2DEG patterned in a 1mmx4μm Hall bar structure. This structure allowed us to vary the carrier density of the 2DEG in situ, by applying a voltage with respect to the Al. We looked for evidence of interaction by passing a current through the Al film and measuring the drag voltage induced in the 2DEG, near the superconducting transition temperature. Al films with sheet resistances of 20 Ω and 1 k Ω, located 650 Åand 1000 Åabove the 2DEG, respectively, were measured. The initial results indicate no drag resistance measured down to the 10nV range with 10nA-250 μA drive currents, for various 2DEG carrier densities. The opposite configuration was also measured, and showed no drag voltage. The theoretical implications of these results will be discussed. *current address: Center for High Technology Materials, Electrical and Computer Engineering Dept., Albuquerque, NM, 87106
High-k Dielectric Nanosheets for Two-Dimensional material Electronics
NASA Astrophysics Data System (ADS)
Hao, Yufeng; Cui, Xu; Yin, Jun; Lee, Gwan-Hyoung; Arefe, Ghidewon; Osada, Minoru; Sasaki, Takayoshi; Hone, James
2015-03-01
Two-dimensional (2D) materials, such as graphene, hexagonal boron nitride (hBN), transition metal dichalcogenides, have shown great potential in nano-electronics because of their unique and superior physical properties. Among them, hBN has been known as an alternative dielectric that is atomically flat and free of trapped charges, which drastically enhance the mobility of graphene or MoS2. However, low dielectric constant (k ~ 3.5) of hBN limits its use in transistors as gate lengths are scaled down to tens of nanometers. Here we demonstrate high performance graphene and MoS2 field effect transistors by using ultrathin Ca2NaNb4O13 nanosheet as a dielectric and mechanically stacking 2D materials. We developed a facile transfer strategy to build 2D materials devices based on the Ca2NaNb4O13 nanosheets. We measured and found that the oxide nanosheet has high dielectric strength, along with high dielectric constant at thickness of a few tens of nanometer. Therefore, multiple-stacked heterostructure of 2D materials shows high mobility at small operating voltage. This study shows possibility of high-k dielectric nanosheets for 2D electronics.
NASA Astrophysics Data System (ADS)
Massote, Daniel V. P.; Liang, Liangbo; Kharche, Neerav; Meunier, Vincent
2016-11-01
Compared to graphene, the synthesis of large area atomically thin boron materials is particularly challenging, owing to the electronic shell structure of B, which does not lend itself to the straightforward assembly of pure B materials. This difficulty is evidenced by the fact that the first synthesis of a pure two-dimensional boron was only very recently reported, using silver as a growing substrate. In addition to experimentally observed 2D boron allotropes, a number of other stable and metastable 2D boron materials are predicted to exist, depending on growth conditions and the use of a substrate during growth. This first-principles study based on density functional theory aims at providing guidelines for the identification of these materials. To this end, this report presents a comparative description of a number of possible 2D B allotropes. Electronic band structures, phonon dispersion curves, Raman scattering spectra, and scanning tunneling microscopy images are simulated to highlight the differences between five distinct realizations of these B systems. The study demonstrates the existence of clear experimental signatures that constitute a solid basis for the unambiguous experimental identification of layered B materials.
Massote, Daniel V. P.; Liang, Liangbo; Kharche, Neerav; ...
2016-11-11
Compared to graphene, the synthesis of large area atomically thin boron materials is particularly challenging, owing to the electronic shell structure of B, which does not lend itself to the straightforward assembly of pure B materials. This difficulty is evidenced by the fact that the first synthesis of a pure two-dimensional boron was only very recently reported, using silver as a growing substrate. In addition to experimentally observed 2D boron allotropes, a number of other stable and metastable 2D boron materials are predicted to exist, depending on growth conditions and the use of a substrate during growth. This first-principles studymore » based on density functional theory aims at providing guidelines for the identification of these materials. To this end, this report presents a comparative description of a number of possible 2D B allotropes. Electronic band structures, phonon dispersion curves, Raman scattering spectra, and scanning tunneling microscopy images are simulated to highlight the differences between five distinct realizations of these B systems. In conclusion, this study demonstrates the existence of clear experimental signatures that constitute a solid basis for the unambiguous experimental identification of layered B materials.« less
NASA Astrophysics Data System (ADS)
Carr, Stephen; Massatt, Daniel; Fang, Shiang; Cazeaux, Paul; Luskin, Mitchell; Kaxiras, Efthimios
2017-02-01
The ability in experiments to control the relative twist angle between successive layers in two-dimensional (2D) materials offers an approach to manipulating their electronic properties; we refer to this approach as "twistronics." A major challenge to theory is that, for arbitrary twist angles, the resulting structure involves incommensurate (aperiodic) 2D lattices. Here, we present a general method for the calculation of the electronic density of states of aperiodic 2D layered materials, using parameter-free Hamiltonians derived from ab initio density-functional theory. We use graphene, a semimetal, and MoS2, a representative of the transition-metal dichalcogenide family of 2D semiconductors, to illustrate the application of our method, which enables fast and efficient simulation of multilayered stacks in the presence of local disorder and external fields. We comment on the interesting features of their density of states as a function of twist angle and local configuration and on how these features can be experimentally observed.
Plasmonic terahertz modulator based on a grating-coupled two-dimensional electron system
NASA Astrophysics Data System (ADS)
Huang, Y. D.; Yu, Y.; Qin, H.; Sun, J. D.; Zhang, Z. P.; Li, X. X.; Huang, J. J.; Cai, Y.
2016-11-01
Electrically driven broadband modulator with large modulation depth and high speed is in high demand to meet the technical advancing and applications in terahertz fields recently. So far, the single-particle non-resonant absorption mechanism described by the Drude conductivity has been utilized in most of the related researches but is still not efficient enough. Here we proposed and demonstrated a terahertz modulator based on the collective electron plasma excitations (plasmons) in a grating-coupled two-dimensional electron gas in GaN/AlGaN heterostructure. By switching between the resonant and non-resonant conditions of the 2D plasmon excitation enabled by applying proper gate biases, the transmission of terahertz electromagnetic waves can be efficiently manipulated. Taking advantage of its resonant characteristic combined with the strong electric field enhancement in the active region, we experimentally achieved a maximum intensity modulation depth of 93%, a 3 dB operation bandwidth of ˜400 kHz, and a small required driving voltage amplitude of 2 V at a cryogenic temperature of 8.7 K. Owing to its excellent performances, this active plasmon-based terahertz modulator may offer some promising solutions in several fields of terahertz technology in the future.
Density-Functional Study of the Two-Dimensional Electron Gas at the Perovskite Titanate Interface
NASA Astrophysics Data System (ADS)
Nanda, Ranjit; Popovic, Zoran; Thulasi, Sunita; Satpathy, Sashi
2006-03-01
Oxide superlattices and microstructures hold the promise for creating a new class of devices with unprecedented functionalities. Density-functional studies^1 of the recently fabricated, lattice-matched perovskite titanates^2 (SrTiO3)n/(LaTiO3)m reveal a classic wedge-shaped potential well for the monolayer structure, originating from the Coulomb potential of a charged La sheet. The potential in turn confines the electrons in the Airy-function-localized states. This resulting two-dimensional electron gas may be described in terms of the simplified jellium model^3 and it describes reasonably well the observed charge modulation of the Ti atoms near the interface. Concerning magnetism, it is suppressed for the monolayer LaTiO3 structure, while in structures with a thicker LaTiO3 part, bulk antiferromagnetism is recovered, with a narrow transition region separating the magnetic LaTiO3 and the non-magnetic SrTiO3. 1. Z. S. Popovic and S. Satpathy, Phys. Rev. Lett. 94, 176805 (2005) 2. A. Ohtomo et al., Nature 419, 378 (2002) 3. S. Thulasi and S. Satpathy, Phys. Rev. B (2006)
Pelliccione, M.; Bartel, J.; Goldhaber-Gordon, D.; Sciambi, A.; Pfeiffer, L. N.; West, K. W.
2014-11-03
Correlated electron states in high mobility two-dimensional electron systems (2DESs), including charge density waves and microemulsion phases intermediate between a Fermi liquid and Wigner crystal, are predicted to exhibit complex local charge order. Existing experimental studies, however, have mainly probed these systems at micron to millimeter scales rather than directly mapping spatial organization. Scanning probes should be well-suited to study the spatial structure of these states, but high mobility 2DESs are found at buried semiconductor interfaces, beyond the reach of conventional scanning tunneling microscopy. Scanning techniques based on electrostatic coupling to the 2DES deliver important insights, but generally with resolution limited by the depth of the 2DES. In this letter, we present our progress in developing a technique called “virtual scanning tunneling microscopy” that allows local tunneling into a high mobility 2DES. Using a specially designed bilayer GaAs/AlGaAs heterostructure where the tunnel coupling between two separate 2DESs is tunable via electrostatic gating, combined with a scanning gate, we show that the local tunneling can be controlled with sub-250 nm resolution.
Massote, Daniel V. P.; Liang, Liangbo; Kharche, Neerav; Meunier, Vincent
2016-11-11
Compared to graphene, the synthesis of large area atomically thin boron materials is particularly challenging, owing to the electronic shell structure of B, which does not lend itself to the straightforward assembly of pure B materials. This difficulty is evidenced by the fact that the first synthesis of a pure two-dimensional boron was only very recently reported, using silver as a growing substrate. In addition to experimentally observed 2D boron allotropes, a number of other stable and metastable 2D boron materials are predicted to exist, depending on growth conditions and the use of a substrate during growth. This first-principles study based on density functional theory aims at providing guidelines for the identification of these materials. To this end, this report presents a comparative description of a number of possible 2D B allotropes. Electronic band structures, phonon dispersion curves, Raman scattering spectra, and scanning tunneling microscopy images are simulated to highlight the differences between five distinct realizations of these B systems. In conclusion, this study demonstrates the existence of clear experimental signatures that constitute a solid basis for the unambiguous experimental identification of layered B materials.
Wu, Jingbo; Mayorov, Alexander S.; Wood, Christopher D.; Mistry, Divyang; Li, Lianhe; Muchenje, Wilson; Rosamond, Mark C.; Chen, Li; Linfield, Edmund H.; Davies, A. Giles; Cunningham, John E.
2015-01-01
Terahertz frequency time-domain spectroscopy employing free-space radiation has frequently been used to probe the elementary excitations of low-dimensional systems. The diffraction limit, however, prevents its use for the in-plane study of individual laterally-defined nanostructures. Here, we demonstrate a planar terahertz frequency plasmonic circuit in which photoconductive material is monolithically integrated with a two-dimensional electron system. Plasmons with a broad spectral range (up to ~ 400 GHz) are excited by injecting picosecond-duration pulses, generated and detected by a photoconductive semiconductor, into a high mobility two-dimensional electron system. Using voltage modulation of a Schottky gate overlying the two-dimensional electron system, we form a tuneable plasmonic cavity, and observe electrostatic manipulation of the plasmon resonances. Our technique offers a direct route to access the picosecond dynamics of confined electron transport in a broad range of lateral nanostructures. PMID:26487263
Doppler Velocimetry of Current Driven Spin Helices in a Two-Dimensional Electron Gas
Yang, Luyi
2013-05-17
Spins in semiconductors provide a pathway towards the development of spin-based electronics. The appeal of spin logic devices lies in the fact that the spin current is even under time reversal symmetry, yielding non-dissipative coupling to the electric field. To exploit the energy-saving potential of spin current it is essential to be able to control it. While recent demonstrations of electrical-gate control in spin-transistor configurations show great promise, operation at room temperature remains elusive. Further progress requires a deeper understanding of the propagation of spin polarization, particularly in the high mobility semiconductors used for devices. This dissertation presents the demonstration and application of a powerful new optical technique, Doppler spin velocimetry, for probing the motion of spin polarization at the level of 1 nm on a picosecond time scale. We discuss experiments in which this technique is used to measure the motion of spin helices in high mobility n-GaAs quantum wells as a function of temperature, in-plane electric field, and photoinduced spin polarization amplitude. We find that the spin helix velocity changes sign as a function of wave vector and is zero at the wave vector that yields the largest spin lifetime. This observation is quite striking, but can be explained by the random walk model that we have developed. We discover that coherent spin precession within a propagating spin density wave is lost at temperatures near 150 K. This finding is critical to understanding why room temperature operation of devices based on electrical gate control of spin current has so far remained elusive. We report that, at all temperatures, electron spin polarization co-propagates with the high-mobility electron sea, even when this requires an unusual form of separation of spin density from photoinjected electron density. Furthermore, although the spin packet co-propagates with the two-dimensional electron gas, spin diffusion is strongly
NASA Astrophysics Data System (ADS)
Heisz, Jeffrey Maurice
This thesis is a theoretical examination of zero temperature electronic transport in the two dimensional electron gas (2DEG) contained at the interfaces of GaAs/ Al/it xGa1-[/it x]As heterostructures. The transport calculations have been performed in the semi-classical limit (i.e., neglecting Landau level quantization) using a multi-subband Boltzmann equation. The standard transport equations have been reformulated in order to deal with the anisotropic electronic structures found in the 2DEGs in the presence of a parallel magnetic field. In addition, a realistic model of impurity scattering has been utilized, with a simple extension of the RPA-based screening potential that accounts for the effects of the magnetic field. As a first application, the transport behaviour of a conventional heterojunction is examined. Distinct anisotropies in the energy structures are found, along with very anisotropic scattering rates and lifetimes. These translate into complex parallel field transport phenomena, which are in most cases similar to those found experimentally. In addition, the occurrence of a transverse magnetoresistance dip is fully quantified and excellent agreement with experiment is obtained. We next perform similar transport calculations for single and double quantum well models. In this situation, virtual crossings of the Fermi contours are possible, and significant anisotropies are found in the scattering rates and lifetimes. Magnetically induced resistance resonances (attributable to these crossings) are observed, in qualitative agreement with experiment. The resistance resonance effect has also been probed using external gating voltages, and the relationship between the symmetrized condition and the observed resonance has been explained. Calculations of the transverse magnetoresistance have suggested a method to experimentally determine the exact gated symmetrization point. Finally, the effects of a parallel magnetic field on the gated resonance condition have
Huang, Bing; Zhuang, Houlong L.; Yoon, Mina; ...
2015-01-01
We report that the discovery of stable two-dimensional, earth-abundant, semiconducting materials is of great interest and may impact future electronic technologies. By combining global structural prediction and first-principles calculations, we have theoretically discovered several previously unknown semiconducting silicon phosphides (SixPy) monolayers, which could be formed stably at the stoichiometries of y/x ≥1. Unexpectedly, some of these compounds, i.e., P-6m2 Si1P1 and Pm Si1P2, have comparable or even lower formation enthalpies than their previously known bulk allotropes. The band gaps (Eg) of SixPy compounds can be dramatically tuned in an extremely wide range (0< Eg < 3 eV) by simply changingmore » the number of layers or applying an in-plane strain. Furthermore, we find that carrier doping can drive the ground state of C2/m Si1P3 from a nonmagnetic state into a robust half-metallic spin-polarized state, originating from its unique valence band structure, which can extend the use of Si-related compounds for spintronics.« less
Huang, Bing; Zhuang, Houlong L.; Yoon, Mina; Sumpter, Bobby G.; Wei, Su-Huai
2015-01-01
We report that the discovery of stable two-dimensional, earth-abundant, semiconducting materials is of great interest and may impact future electronic technologies. By combining global structural prediction and first-principles calculations, we have theoretically discovered several previously unknown semiconducting silicon phosphides (Si_{x}P_{y}) monolayers, which could be formed stably at the stoichiometries of y/x ≥1. Unexpectedly, some of these compounds, i.e., P-6m2 Si_{1}P_{1} and Pm Si_{1}P_{2}, have comparable or even lower formation enthalpies than their previously known bulk allotropes. The band gaps (Eg) of Si_{x}P_{y} compounds can be dramatically tuned in an extremely wide range (0< E_{g} < 3 eV) by simply changing the number of layers or applying an in-plane strain. Furthermore, we find that carrier doping can drive the ground state of C2/m Si_{1}P_{3} from a nonmagnetic state into a robust half-metallic spin-polarized state, originating from its unique valence band structure, which can extend the use of Si-related compounds for spintronics.
Fateev, D. V. Mashinsky, K. V.; Bagaeva, T. Yu.; Popov, V. V.
2015-01-15
The problem of the rectification of terahertz radiation due to plasmonic nonlinearities in a periodic two-dimensional electron system upon the excitation of plasma oscillations by the attenuated total reflection method is solved. This model allows the independent study of different plasmonic rectification mechanisms, i.e., plasmonic electron drag and plasmonic ratchet effects.
Bordajandi, Luisa R; Ramos, Lourdes; González, María José
2006-09-01
Comprehensive two-dimensional gas chromatography with micro electron-capture detection (GC x GC-microECD) has been evaluated for the enantioseparation of five chiral toxaphenes typically found in real-life samples (Parlar 26, 32, 40, 44 and 50). From the two enantioselective beta-cyclodextrin-based columns evaluated as first dimension column, BGB-176SE and BGB-172, the latter provided the best results and was further combined with three non-enantioselective columns in the second dimension: HT-8, BPX-50 and Supelcowax-10. The combination BGB-172 x BPX-50 was finally selected because it provided a complete separation among all enantiomers. A satisfactory repeatability and reproducibility of the retention times in both the first and the second dimension were observed for all target compounds (RSDs below 0.8%, n = 4). Linear responses in the tested range of 10-200 pg/microl and limits of detection in the range of 2-6 pg/microl were obtained. The repeatability and reproducibility at a concentration of 100 pg/microl, evaluated as the RSDs calculated for the enantiomeric fraction (EF), was better than 11% (n = 4) in all instances. The feasibility of the method developed for real-life analyses was illustrated by the determination of the enantiomeric ratios and concentration levels of the test compounds in four commercial fish oil samples. These results were compared to those obtained by heart-cut multidimensional gas chromatography using the same enantioselective column.
Current-Induced Cooling Phenomenon in a Two-Dimensional Electron Gas Under a Magnetic Field
NASA Astrophysics Data System (ADS)
Hirayama, Naomi; Endo, Akira; Fujita, Kazuhiro; Hasegawa, Yasuhiro; Hatano, Naomichi; Nakamura, Hiroaki; Shirasaki, Ryōen; Yonemitsu, Kenji
2013-07-01
We investigate the spatial distribution of temperature induced by a dc current in a two-dimensional electron gas (2DEG) subjected to a perpendicular magnetic field. We numerically calculate the distributions of the electrostatic potential ϕ and the temperature T in a 2DEG enclosed in a square area surrounded by insulated-adiabatic (top and bottom) and isopotential-isothermal (left and right) boundaries (with ϕ left< ϕ right and T left= T right), using a pair of nonlinear Poisson equations (for ϕ and T) that fully take into account thermoelectric and thermomagnetic phenomena, including the Hall, Nernst, Ettingshausen, and Righi-Leduc effects. We find that, in the vicinity of the left-bottom corner, the temperature becomes lower than the fixed boundary temperature, contrary to the naive expectation that the temperature is raised by the prevalent Joule heating effect. The cooling is attributed to the Ettingshausen effect at the bottom adiabatic boundary, which pumps up the heat away from the bottom boundary. In order to keep the adiabatic condition, downward temperature gradient, hence the cooled area, is developed near the boundary, with the resulting thermal diffusion compensating the upward heat current due to the Ettingshausen effect.
Yu, P.N.; Ginzburg, N.S.; Sergeev, A.S.
1995-12-31
In the report we present a time domain approach to the theory of FELs with one and two dimensional Bragg resonators. It is demonstrated that traditional 1-D Bragg resonators provide possibilities for effective longitudinal mode control. In particular, simulation of the FEL realized in the joint experiment of JINR (Dybna) and IAP (N. Novgord) confirms achievement of the single mode operating regime with high efficiency of about 20%. However, 1-D Bragg resonators lose their selectivity as the transverse size of the system is increased. We simulate mode competition in FELs with coaxial 1-D Bragg resonators and observe a progressively more complicated azimuthal mode competition pattern as the perimeter of the resonator is increased. At the same time, using 2-D Bragg resonators for the same electron beam and microwave system perimeter gives very fast establishment of the single frequency regime with an azimuthally symmetric operating mode. Therefore, FELs utilising 2-D Bragg resonators with coaxial and planar geometry may be considered as attractive sources of high power spatially coherent radiation in the mm and sub-mm wave bands.
NASA Astrophysics Data System (ADS)
Lim, James; Ing, David J.; Rosskopf, Joachim; Jeske, Jan; Cole, Jared H.; Huelga, Susana F.; Plenio, Martin B.
2017-01-01
We investigate how correlated fluctuations affect oscillatory features in rephasing and non-rephasing two-dimensional (2D) electronic spectra of a model dimer system. Based on a beating map analysis, we show that non-secular environmental couplings induced by uncorrelated fluctuations lead to oscillations centered at both cross- and diagonal-peaks in rephasing spectra as well as in non-rephasing spectra. Using an analytical approach, we provide a quantitative description of the non-secular effects in terms of the Feynman diagrams and show that the environment-induced mixing of different inter-excitonic coherences leads to oscillations in the rephasing diagonal-peaks and non-rephasing cross-peaks. We demonstrate that as correlations in the noise increase, the lifetime of oscillatory 2D signals is enhanced at rephasing cross-peaks and non-rephasing diagonal-peaks, while the other non-secular oscillatory signals are suppressed. We discuss that the asymmetry of 2D lineshapes in the beating map provides information on the degree of correlations in environmental fluctuations. Finally we investigate how the oscillatory features in 2D spectra are affected by inhomogeneous broadening.
NASA Astrophysics Data System (ADS)
Wiebe, Jens
2011-03-01
Magnetic atoms doped into a semiconductor are the building blocks for bottom up spintronic and quantum logic devices. They also provide model systems for the investigation of fundamental effects. In order to correlate the dopant's atomic structure with its magnetism magnetically sensitive techniques with atomic resolution are a prerequisite. Here, I show electrical excitation and read-out [ 1 ] of single magnetic dopant associated spins in a two-dimensional electron gas (2DEG) confined to a semiconductor surface [ 2 ] using spin-resolved scanning tunneling spectroscopy [ 3 ] . I will review our real-space study of the quantum Hall transition in the 2DEG [ 2 ] and of the magnetic properties of the dopants [ 1 ] . Finally, I will demonstrate that the dopant serves as an atomic scale probe for local magnetometry of the 2DEG. This work was done in collaboration with A. A. Khajetoorians, B. Chillian, S. Schuwalow, F. Lechermann, K. Hashimoto, C. Sohrmann, T. Inaoka, F. Meier, Y. Hirayama, R. A. Römer, M. Morgenstern, and R. Wiesendanger. [ 1 ] A. A. Khajetoorians et al., Nature 467, 1084 (2010). [ 2 ] K. Hashimoto et al., Phys. Rev. Lett. 101, 256802 (2008). [ 3 ] J. Wiebe et al., Rev. Sci. Instrum. 75, 4871 (2004). We acknowledge financial support from ERC Advanced Grant ``FURORE'', by the DFG via SFB668 and GrK1286, and by the city of Hamburg via the cluster of excellence ``Nanospintronics''.
Dark States in the Light-Harvesting complex 2 Revealed by Two-dimensional Electronic Spectroscopy
Ferretti, Marco; Hendrikx, Ruud; Romero, Elisabet; ...
2016-02-09
Energy transfer and trapping in the light harvesting antennae of purple photosynthetic bacteria is an ultrafast process, which occurs with a quantum efficiency close to unity. However the mechanisms behind this process have not yet been fully understood. Recently it was proposed that low-lying energy dark states, such as charge transfer states and polaron pairs, play an important role in the dynamics and directionality of energy transfer. However, it is difficult to directly detect those states because of their small transition dipole moment and overlap with the B850/B870 exciton bands. Here we present a new experimental approach, which combines themore » selectivity of two-dimensional electronic spectroscopy with the availability of genetically modified light harvesting complexes, to reveal the presence of those dark states in both the genetically modified and the wild-type light harvesting 2 complexes of Rhodopseudomonas palustris. In conclusion, we suggest that Nature has used the unavoidable charge transfer processes that occur when LH pigments are concentrated to enhance and direct the flow of energy.« less
NASA Astrophysics Data System (ADS)
Levin, A. D.; Gusev, G. M.; Raichev, O. E.; Momtaz, Z. S.; Bakarov, A. K.
2016-07-01
To study the influence of microwave irradiation on two-dimensional electrons, we apply a method based on capacitance measurements in GaAs quantum well samples where the gate covers a central part of the layer. We find that the capacitance oscillations at high magnetic fields, caused by the oscillations of thermodynamic density of states, are not essentially modified by microwaves. However, in the region of fields below 1 T, we observe another set of oscillations, with the period and the phase identical to those of microwave-induced resistance oscillations. The phenomenon of microwave-induced capacitance oscillations is explained in terms of violation of the Einstein relation between conductivity and the diffusion coefficient in the presence of microwaves, which leads to a dependence of the capacitor charging on the anomalous conductivity. We also observe microwave-induced oscillations in the capacitive response to periodic variations of external heating. These oscillations appear due to the thermoelectric effect and are in antiphase with microwave-induced resistance oscillations because of the Corbino-like geometry of our experimental setup.
Stability and electronic structure of two-dimensional allotropes of group-IV materials
NASA Astrophysics Data System (ADS)
Matusalem, Filipe; Marques, Marcelo; Teles, Lara K.; Bechstedt, Friedhelm
2015-07-01
We study six different two-dimensional (2D) allotropes of carbon, silicon, germanium, and tin by means of the ab initio density functional theory for the ground state and approximate methods to calculate their electronic structures, including quasiparticle effects. Four of the investigated allotropes are based on dumbbell geometries, one on a kagome lattice, and one on the graphenelike hexagonal structure for comparison. Concerning carbon, our calculations of the cohesive energies clearly show that the hexagonal structure (graphene) is most stable. However, in the case of Si and Ge, the dumbbell structures, particularly the large honeycomb dumbbell (LHD) geometries, are energetically favored compared to the s p2/s p3 -bonded hexagonal lattice (i.e., silicene and germanene). The main reason for this is the opening of a band gap in the honeycomb dumbbell arrangements. The LHD sheet crystals represent indirect semiconductors with a K →Γ gap of about 0.5 eV. In the Sn case we predict the MoS2-like symmetry to be more stable, in contrast to the stanene and LHD geometries predicted in literature. Our results for freestanding group-IV layers shine new light on recent experimental studies of group-IV overlayers on various substrates.
Dark States in the Light-Harvesting complex 2 Revealed by Two-dimensional Electronic Spectroscopy
Ferretti, Marco; Hendrikx, Ruud; Romero, Elisabet; Southall, June; Cogdell, Richard J.; Novoderezhkin, Vladimir I.; Scholes, Gregory D.; van Grondelle, Rienk
2016-02-09
Energy transfer and trapping in the light harvesting antennae of purple photosynthetic bacteria is an ultrafast process, which occurs with a quantum efficiency close to unity. However the mechanisms behind this process have not yet been fully understood. Recently it was proposed that low-lying energy dark states, such as charge transfer states and polaron pairs, play an important role in the dynamics and directionality of energy transfer. However, it is difficult to directly detect those states because of their small transition dipole moment and overlap with the B850/B870 exciton bands. Here we present a new experimental approach, which combines the selectivity of two-dimensional electronic spectroscopy with the availability of genetically modified light harvesting complexes, to reveal the presence of those dark states in both the genetically modified and the wild-type light harvesting 2 complexes of Rhodopseudomonas palustris. In conclusion, we suggest that Nature has used the unavoidable charge transfer processes that occur when LH pigments are concentrated to enhance and direct the flow of energy.
Dark States in the Light-Harvesting complex 2 Revealed by Two-dimensional Electronic Spectroscopy
NASA Astrophysics Data System (ADS)
Ferretti, Marco; Hendrikx, Ruud; Romero, Elisabet; Southall, June; Cogdell, Richard J.; Novoderezhkin, Vladimir I.; Scholes, Gregory D.; van Grondelle, Rienk
2016-02-01
Energy transfer and trapping in the light harvesting antennae of purple photosynthetic bacteria is an ultrafast process, which occurs with a quantum efficiency close to unity. However the mechanisms behind this process have not yet been fully understood. Recently it was proposed that low-lying energy dark states, such as charge transfer states and polaron pairs, play an important role in the dynamics and directionality of energy transfer. However, it is difficult to directly detect those states because of their small transition dipole moment and overlap with the B850/B870 exciton bands. Here we present a new experimental approach, which combines the selectivity of two-dimensional electronic spectroscopy with the availability of genetically modified light harvesting complexes, to reveal the presence of those dark states in both the genetically modified and the wild-type light harvesting 2 complexes of Rhodopseudomonas palustris. We suggest that Nature has used the unavoidable charge transfer processes that occur when LH pigments are concentrated to enhance and direct the flow of energy.
Two-Dimensional Fourier Transform Electronic Spectroscopy of Peridinin and Peridinin Analogs
NASA Astrophysics Data System (ADS)
Khosravi, Soroush; Bishop, Michael; Obaid, Razib; Whitelock, Hope; Carroll, Ann Marie; Lafountain, Amy; Frank, Harry; Beck, Warren; Gibson, George; Berrah, Nora
2016-05-01
The peridinin chlorophyll- a protein (PCP) is a light harvesting complex in dinoflagellates that exhibits a carotenoid-to-chlorophyll (Chl) a excitation energy transfer (EET) efficiency of 85-95%. Unlike most light harvesting complexes, where the number of carotenoids is less than Chl, each subunit of PCP contains eight tightly-packed peridinins surrounding two Chl a molecules. The unusual solvent polarity dependence of the lowest excited S1 state of peridinin suggests the presence of an intramolecular charge-transfer (ICT) state. The nature of the ICT state, its coupling to the S1 of peridinin, and whether it enables the high EET efficiency is still unclear. Two-dimensional electronic Fourier transform spectroscopy (2DES) is a powerful method capable of examining these issues. The present work examines the ICT state of peridinin and peridinin analogs that have diminished ICT character. 2DES data adding new insight into the spectral signatures and nature of the ICT state in peridinin will be presented. Funded by the DoE-BES, Grant No. DE-SC0012376.
Electrical detection of spin transport in Si two-dimensional electron gas systems
NASA Astrophysics Data System (ADS)
Chang, Li-Te; Fischer, Inga Anita; Tang, Jianshi; Wang, Chiu-Yen; Yu, Guoqiang; Fan, Yabin; Murata, Koichi; Nie, Tianxiao; Oehme, Michael; Schulze, Jörg; Wang, Kang L.
2016-09-01
Spin transport in a semiconductor-based two-dimensional electron gas (2DEG) system has been attractive in spintronics for more than ten years. The inherent advantages of high-mobility channel and enhanced spin-orbital interaction promise a long spin diffusion length and efficient spin manipulation, which are essential for the application of spintronics devices. However, the difficulty of making high-quality ferromagnetic (FM) contacts to the buried 2DEG channel in the heterostructure systems limits the potential developments in functional devices. In this paper, we experimentally demonstrate electrical detection of spin transport in a high-mobility 2DEG system using FM Mn-germanosilicide (Mn(Si0.7Ge0.3)x) end contacts, which is the first report of spin injection and detection in a 2DEG confined in a Si/SiGe modulation doped quantum well structure (MODQW). The extracted spin diffusion length and lifetime are l sf = 4.5 μm and {τ }{{s}}=16 {{ns}} at 1.9 K respectively. Our results provide a promising approach for spin injection into 2DEG system in the Si-based MODQW, which may lead to innovative spintronic applications such as spin-based transistor, logic, and memory devices.
Radiation modulation of circular photogalvanic effect in two-dimensional electron gas system
NASA Astrophysics Data System (ADS)
Jiang, Chongyun; Ma, Hui; Yu, Jinling; Liu, Yu; Chen, Yonghai
2012-12-01
We report on the observation of a modulation on the circular photogalvanic effect (CPGE) imposed by an extra optical radiation in a GaAs-based two dimensional electron gas system. The wavelength of the radiation for exciting the CPGE is 1064 nm and the wavelength of the modulation is 532 nm. The experiment is carried out from 77 K up to room temperature. The 1064 nm induced CPGE modulated by the 532 nm radiation increases as the increasing temperature. We also vary the power of the modulation beam to investigate the intensity dependence of the modulation effect. The modulation exhibits a linear dependence at low intensity. As the intensity increasing, we observe a saturation at certain level of the intensity and a suppression of the modulation when the intensity is further increased. The investigation of photoconductivity reveals that the change of the photoexcited charge carrier density has little contribution to the radiation modulation effect. Therefore, the microscopic mechanism of the radiation modulation effect can be attributed to the modulation of spin-orbit interaction in the structure.
Simulated two-dimensional electronic spectroscopy of the eight-bacteriochlorophyll FMO complex
Yeh, Shu-Hao; Kais, Sabre
2014-12-21
The Fenna-Matthews-Olson (FMO) protein-pigment complex acts as a molecular wire conducting energy between the outer antenna system and the reaction center; it is an important photosynthetic system to study the transfer of excitonic energy. Recent crystallographic studies report the existence of an additional (eighth) bacteriochlorophyll a (BChl a) in some of the FMO monomers. To understand the functionality of this eighth BChl, we simulated the two-dimensional electronic spectra of both the 7-site (apo form) and the 8-site (holo form) variant of the FMO complex from green sulfur bacteria, Prosthecochloris aestuarii. By comparing the spectrum, it was found that the eighth BChl can affect two different excitonic energy transfer pathways: (1) it is directly involved in the first apo form pathway (6 → 3 → 1) by passing the excitonic energy to exciton 6; and (2) it facilitates an increase in the excitonic wave function overlap between excitons 4 and 5 in the second pathway (7 → 4,5 → 2 → 1) and thus increases the possible downward sampling routes across the BChls.
Lim, James; Ing, David J; Rosskopf, Joachim; Jeske, Jan; Cole, Jared H; Huelga, Susana F; Plenio, Martin B
2017-01-14
We investigate how correlated fluctuations affect oscillatory features in rephasing and non-rephasing two-dimensional (2D) electronic spectra of a model dimer system. Based on a beating map analysis, we show that non-secular environmental couplings induced by uncorrelated fluctuations lead to oscillations centered at both cross- and diagonal-peaks in rephasing spectra as well as in non-rephasing spectra. Using an analytical approach, we provide a quantitative description of the non-secular effects in terms of the Feynman diagrams and show that the environment-induced mixing of different inter-excitonic coherences leads to oscillations in the rephasing diagonal-peaks and non-rephasing cross-peaks. We demonstrate that as correlations in the noise increase, the lifetime of oscillatory 2D signals is enhanced at rephasing cross-peaks and non-rephasing diagonal-peaks, while the other non-secular oscillatory signals are suppressed. We discuss that the asymmetry of 2D lineshapes in the beating map provides information on the degree of correlations in environmental fluctuations. Finally we investigate how the oscillatory features in 2D spectra are affected by inhomogeneous broadening.
Separation of transport lifetimes in SrTi O3 -based two-dimensional electron liquids
NASA Astrophysics Data System (ADS)
Mikheev, Evgeny; Freeze, Christopher R.; Isaac, Brandon J.; Cain, Tyler A.; Stemmer, Susanne
2015-04-01
Deviations from Landau Fermi-liquid behavior are ubiquitous features of the normal state of unconventional superconductors. Despite several decades of investigation, the underlying mechanisms of these properties are still not completely understood. In this work, we show that two-dimensional electron liquids at SrTi O3/R Ti O3 (R =Gd or Sm) interfaces reveal strikingly similar physics. Analysis of Hall and resistivity data shows a clear separation of transport and Hall scattering rates, also known as "two-lifetime" behavior. This framework gives a remarkably simple and general description of the temperature dependence of the Hall coefficient. Distinct transport lifetimes accurately describe the transport phenomena irrespective of the nature of incipient magnetic ordering, the degree of disorder, confinement, or the emergence of non-Fermi-liquid behavior. The Hall scattering rate diverges at a critical quantum well thickness, coinciding with a quantum phase transition. Collectively, these results introduce constraints on the existing microscopic theories of lifetime separation and point to the need for unified understanding.
NASA Astrophysics Data System (ADS)
Gorcester, Jeff; Rananavare, Shankar B.; Freed, Jack H.
1989-05-01
Electron spin-echo (ESE) and two-dimensional electron-electron double resonance (2D ELDOR) experiments have been performed as a function of director orientation and temperature in the smectic A phase of the liquid crystal S2 for the spin-probe PD-tempone(2×10-3 M). Over the entire temperature range studied (288-323 K) we observe significant 2D ELDOR cross peaks only for ΔMI =±1 indicative of 14N spin-relaxation and negligible Heisenberg exchange. From the angular dependent 14N spin-relaxation rates we obtain the dipolar spectral densities at the hyperfine (hf) frequency, whereas from a combination of ESE and 2D ELDOR we obtain the dipolar and Zeeman-dipolar spectral densities at zero frequency. The angular dependent spectral densities were successfully decomposed into their basic components in accordance with theory. The angular dependent spectral densities at the hf frequency are not predicted by a model of anisotropic rotational diffusion in a nematic orienting potential, but are consistent with predictions of a model due to Moro and Nordio of solute rototranslational diffusion in a McMillan-type potential. The angular dependence also indicates that order director fluctuations in the smectic phase are suppressed at frequencies on the order of 10 MHz. An additional contribution to solute reorientation due to cooperative hydrocarbon chain fluctuations is suggested to account for the behavior of the observed spectral densities at zero frequency. An evaluation of the relevance of several other dynamical models to this experimental work is also presented.
Time domain capacitance spectroscopy of tunneling electrons in the two-dimensional electron gas
NASA Astrophysics Data System (ADS)
Ashoori, R. C.; Chan, H. B.
2003-07-01
We have developed a technique capable of measuring the tunneling current into both localized and conducting states in a 2D electron system (2DES). The method yields I-V characteristics for tunneling with no distortions arising from low 2D in-plane conductivity. We have used the technique to determine the pseudogap energy spectrum for electron tunneling into and out of a 2D system and, further, we have demonstrated that such tunneling measurements reveal spin relaxation times within the 2DEG. Pseudogap: In a 2DEG in perpendicular magnetic field, a pseudogap develops in the tunneling density of states at the Fermi energy. We resolve a linear energy dependence of this pseudogap at low excitations. The slopes of this linear gap are strongly field dependent. No existing theory predicts the observed behavior. Spin relaxation: We explore the characteristics of equilibrium tunneling of electrons from a 3D electrode into a high mobility 2DES. For most 2D Landau level filling factors, we find that electrons tunnel with a single, well-defined tunneling rate. However, for spin-polarized quantum Hall states ( ν=1, 3 and 1/3) tunneling occurs at two distinct rates that differ by up to two orders of magnitude. The dependence of the two rates on temperature and tunnel barrier thickness suggests that slow in-plane spin relaxation creates a bottleneck for tunneling of electrons.
Ogilvie, Jennifer P.
2016-11-22
Photosystem II (PSII) is the only known natural enzyme that uses solar energy to split water, making the elucidation of its design principles critical for our fundamental understanding of photosynthesis and for our ability to mimic PSII’s remarkable properties. This report discusses progress towards addressing key open questions about the PSII RC. It describes new spectroscopic methods that were developed to answer these questions, and summarizes the outcomes of applying these methods to study the PSII RC. Using 2D electronic spectroscopy and 2D electronic Stark spectroscopy, models for the PSII RC were tested and refined. Work is ongoing to use the collected data to elucidate the charge separation mechanism in the PSII RC. Coherent dynamics were also observed in the PSII RC for the first time. Through extensive characterization and modeling we have assigned these coherences as vibronic in nature, and believe that they reflect resonances between key vibrational pigment modes and electronic energy gaps that may facilitate charge separation. Work is ongoing to definitively test the functional relevance of electronic-vibrational resonances.
Strain-modulated electronic and thermal transport properties of two-dimensional O-silica
NASA Astrophysics Data System (ADS)
Han, Yang; Qin, Guangzhao; Jungemann, Christoph; Hu, Ming
2016-07-01
Silica is one of the most abundant materials in the Earth’s crust and is a remarkably versatile and important engineering material in various modern science and technology. Recently, freestanding and well-ordered two-dimensional (2D) silica monolayers with octahedral (O-silica) building blocks were found to be theoretically stable by (Wang G et al 2015 J. Phys. Chem. C 119 15654-60). In this paper, by performing first-principles calculations, we systematically investigated the electronic and thermal transport properties of 2D O-silica and also studied how these properties can be tuned by simple mechanical stretching. Unstrained 2D O-silica is an insulator with an indirect band gap of 6.536 eV. The band gap decreases considerably with bilateral strain up to 29%, at which point a semiconductor-metal transition occurs. More importantly, the in-plane thermal conductivity of freestanding 2D O-silica is found to be unusually high, which is around 40 to 50 times higher than that of bulk α-quartz and more than two orders of magnitude higher than that of amorphous silica. The thermal conductivity of O-silica decreases by almost two orders of magnitude when the bilateral stretching strain reaches 10%. By analyzing the mode-dependent phonon properties and phonon-scattering channel, the phonon lifetime is found to be the dominant factor that leads to the dramatic decrease of the lattice thermal conductivity under strain. The very sensitive response of both band gap and phonon transport properties to the external mechanical strain will enable 2D O-silica to easily adapt to the different environment of realistic applications. Our study is expected to stimulate experimental exploration of further physical and chemical properties of 2D silica systems, and offers perspectives on modulating the electronic and thermal properties of related low-dimensional structures for applications such as thermoelectric, photovoltaic, and optoelectronic devices.
Stability of Quasi-Two-Dimensional Electron-Hole Liquid in Semiconductor Structures of the Type-II
NASA Astrophysics Data System (ADS)
Vasilchenko, A. A.; Kopytov, G. F.; Krivobok, V. S.; Ermokhin, D. A.
2017-02-01
Analytical expressions are obtained for the energy of a quasi-two-dimensional electron-hole liquid (EHL) and the threshold value of the barrier height for electrons, above which formation of the direct EHL is impossible. It is shown that the state with a quasi-two-dimensional EHL can be energetically favorable in semiconductors with the anisotropy of masses and (or) a large number of equivalent valleys. A comparison of the calculation results with the experimental data for the Si/SiGe/Si structure is made.
NASA Astrophysics Data System (ADS)
de A. Camargo, Franco V.; Grimmelsmann, Lena; Anderson, Harry L.; Meech, Stephen R.; Heisler, Ismael A.
2017-01-01
The observation of coherent quantum effects in photosynthetic light-harvesting complexes prompted the question whether quantum coherence could be exploited to improve the efficiency in new energy materials. The detailed characterization of coherent effects relies on sensitive methods such as two-dimensional electronic spectroscopy (2D-ES). However, the interpretation of the results produced by 2D-ES is challenging due to the many possible couplings present in complex molecular structures. In this work, we demonstrate how the laser spectral profile can induce electronic coherencelike signals in monomeric chromophores, potentially leading to data misinterpretation. We argue that the laser spectrum acts as a filter for certain coherence pathways and thus propose a general method to differentiate vibrational from electronic coherences.
Spin Filtering in a Rashba Electron Waveguide Induced by Edge Disorder
NASA Astrophysics Data System (ADS)
Xiao, Xian-Bo; Li, Fei; Liu, Nian-Hua
2012-08-01
We theoretically study the spin-dependent electron transport in a Rashba electron waveguide with rough edges, attached to ideal leads without spin-orbit interaction. The influence of the edge disorder on the charge and spin conductances is clarified by using the spin-resolved lattice Green function method. It is found that a spin-polarized current can be generated in the output lead and its spin polarization can be manipulated by varying the waveguide length. The underlying physics is attributed to the broken longitudinal symmetry and the spin-dependent quantum interference induced by the rough boundaries. Our results may provide a new method to design a spin filter without using magnetic materials or applying a magnetic field.
Functional two-dimensional electronic gases at interfaces of oxide heterostructures
NASA Astrophysics Data System (ADS)
Wang, Yong
2011-12-01
A quasi-two dimensional electron gas (2DEG) in oxide heterostructures such as LaAlO3/SrTiO3 has unique properties that are promising for applications in all-oxide electronic devices. In this dissertation, we focus on understanding and predicting novel properties of the 2DEG by performing first-principles electronic calculations within the frame work of density-functional theory (DFT). The investigation is made upon adding new functionalities in oxide heterostructures, such as ferroelectric polarization, epitaxial strain, and spin polarization that can be employed to control 2DEG properties. Based on first-principles calculations the effects of different polarization magnitudes and alignments in all-oxide heterostructures incorporating different ferroelectric constituents, such as KNbO3/ATiO3 (A = Sr, Ba, Pb), are investigated. It is found that screening charge at the interface that counteracts the depolarizing electric field in the ferroelectric material significantly changes the free electron density of 2DEG at the interface. Using this mechanism, nonvolatile metal-insulating transition can be achieved at the interface by switching the ferroelectric spontaneous polarization. Growing on different substrates, LaAlO3/SrTiO3 heterostructures experience different epitaxial strains. Our first-principles calculations reveal that compressive epitaxial strain introduces a polarization in SrTiO3 pointing away from the interface, which is consistent with the experimental observations. This polarization strongly affects the 2DEG carrier density through a polarization charge formed at the interface. Our theoretical investigation finds that the critical thickness to form a 2DEG at the interface of the heterostructure increases with the compressive strain, while the saturated carrier density decreases which is consistent with the experimental results. Adding a spin degree of freedom to 2DEG may be interesting for the application of 2DEGs in a spintronic device. We explore a La
Two-dimensional electronic-vibrational spectra: modeling correlated electronic and nuclear motion.
Terenziani, F; Painelli, A
2015-05-21
We calculate 2D electronic-vibrational (2D-EV) spectra of solvated organic dyes modeled in terms of a reduced set of electronic diabatic states (the essential states) non-adiabatically coupled to molecular vibrations. An effective overdamped coordinate, whose dynamics is described by the Smoluchowski diffusion equation, accounts for polar solvation. Results are discussed for two dyes with distinctively different spectroscopic behavior: 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) and 8-(N,N-dibutylamino)-2-azachrysene (AAC). Linear absorption and fluorescence spectra of DCM are well reproduced based on a minimal two-state model. The same model leads to 2D-EV spectra in good agreement with the recent experimental data reported by Oliver and coworkers for DCM in DMSO. In contrast, linear spectra of AAC show a subtle interplay between a locally-excited (LE) and a charge-transfer (CT) excitation, calling for a three-state model. Calculated 2D-EV spectra for AAC show a qualitatively different behavior, demonstrating that the experimental data for DCM do not support a LE/CT interplay. This resolves the long-lasting discussion about the nature of low-lying excitations of DCM in favor of the simplest picture.
NASA Astrophysics Data System (ADS)
Xiao, Xian-Bo; Li, Xiao-Mao; Chen, Yu-Guang
2009-12-01
We investigate theoretically the spin-dependent electron transport in a straight waveguide with Rashba spin-orbit coupling (SOC) under the irradiation of a transversely polarized electromagnetic (EM) field. Spin-dependent electron conductance and spin polarization are calculated as functions of the emitting energy of electrons or the strength of the EM field by adopting the mode matching approach. It is shown that the spin polarization can be manipulated by external parameters when the strength of Rashba SOC is strong. Furthermore, a sharp step structure is found to exist in the total electron conductance. These results can be understood by the nontrivial Rashba subbands intermixing and the electron intersubband transition when a finite-range transversely polarized EM field irradiates a straight waveguide.
NASA Astrophysics Data System (ADS)
Neogi, Suneeta Shamanna
The purpose of this research has been to develop a methodologoy to map two-dimensional dopant distributions in silicon and investigate the factors that influence the interpretation of the results. The analysis exploits the image contrast obtained by transmission electron microscopy (TEM) using cross-section specimens which have undergone selective chemical etching. The appearance of iso-thickness contours in a selectively etched TEM sample must represent iso-concentration contours when imaged under constant diffraction conditions. The application of this technique is two-fold: (1) to establish a physical metrology of semiconductor devices for the purpose of research and development efforts that impact on future nodes outlined in the semiconductor roadmap and (2) to provide physical data for validation of simulation tools in technology computer aided design (TCAD). The research involves an investigation into the selective removal of doped regions for both test and device structures, followed by an analysis to obtain two-dimensional (2-D) dopant profiles. The critical issues which arise in the development of a methodology to profile dopant distributions and which are addressed in this investigation are, wedge technique versus conventional dimple and ion-mill procedures for thin-film preparation, thin-film versus bulk chemical etching, data acquisition using TEM and choice of diffraction conditions, sensitivity in terms of the etch detection limit, resolution influenced by the effective extinction length of the operating reflection, digital image processing to extract profiles from thickness contours, calibration of the 2-D profiles using a one-dimensional (1-D) calibrator and role of structure/dopant interactions such as stress, interfaces and point defects in test structures and real device structures containing additional processing sequences. Selective chemical etching in combination with TEM has the sensitivity, resolution and reproducibility required to be used
Understanding and engineering two-dimensional electron gases in complex oxides
NASA Astrophysics Data System (ADS)
Bjaalie, Lars Gunnar Tangen
The next generation of electronic devices faces the challenge of adequately containing and controlling extremely high charge densities within structures of nanometer dimensions. Atomic-scale transistors must be thin and be able to control extremely high charge densities (>10e13/cm. 2). Silicon devicestypically have two-dimensional electron gas (2DEG) densities around 10e12/cm. 2.Nitride-based devices can sustain densities an order of magnitude higher. The "complex oxides" have recently emerged as an attractive materials system to support these developments. The demonstration of a 2DEG at the SrTiO 3/LaAlO3 interface has triggered an avalanche of research, including the unprecedentedly high density of 3x10e14/cm. 2 at SrTiO3/GdTiO3and SrTiO3/SmTiO3 interfaces. Metal-insulator (Mott) transitions that are inherent to some of these complex oxides could offer even greater prospects for enhanced functionality or novel device concepts. The materials and heterostructures that have been explored to date are clearly only a small subset of the vast number of materials combinations that could lead to interesting phenomena. In this work we use first-principles methods to build greater understanding of the interface phenomena, so that searches can be better informed and more focused. We also develop a set of criteria that the materials and their heterostructures should satisfy to develop a high-performance 2DEG-based device. We focus in particular on the band alignment, calculating it for a variety of different potential materials. Next, we study GdTiO3/SrTiO3/GdTiO3 heterostructures in depth, where each interface contributes excess electrons into the SrTiO3. We calculate the 2DEG formation for a superlattice containing six layers of SrTiO3, and compare with angle-resolved photoemission spectroscopy results. Together, the experimental and theoretical results conclusively show that the 2DEG results from the interface itself, and does not originate from a secondary source such as
Temperature Dependent Transport of Two-Dimensional Electrons in the Integral Quantum Hall Regime.
NASA Astrophysics Data System (ADS)
Wei, Hsuang-Ping
This thesis is concerned with the temperature (T) dependent electronic transport properties of a two dimensional electron gas subject to background potential fluctuations and a perpendicular magnetic field. We have carried out an extensive temperature dependent study of the transport coefficients, in the region of an integral quantum plateau, in an In(,x)Ga(,1-x)As/InP heterostructure for 4.2K < T < 50K. By assuming a simple thermal activation picture, we demonstrate a quantitative deduction of the electron density of states. Our results indicate that there exists a significant number of states (1 x 10('10)cm(' -2)meV('-1)) even at the middle between two Landau levels, which is unexpected from model calculations based on short ranged randomness. In addition, the different T dependent behavior of (rho)(,xx) between the states in the tails and those near the center of a Landau level, indicates the existence of different electron states in a Landau level. Moreover, we have performed T dependent trans- port measurements in the transition region between two quantum plateaus, in several different materials. In the In(,x)Ga(,1-x)As/InP sample, when T(, )> 4K, the transport behavior can be attributed to the T dependent distribution function. When T(, )< 4K, our experi- mental T-driven (sigma)(,xx) vs. (sigma)(,xy) flow diagram is consistent with the pre- dicted theoretical renormalization group flow diagram, and suggests the existence of a critical point related to the localization to delocali- zation transition. However, in the GaAs/Al(,x)Ga(,1-x)As samples there is a difference in the T dependent behavior of (sigma)(,xx), between N = 1(UPARR) and 1(DARR) electrons. First, (sigma)(,xx)(1(UPARR)) decreases with decreasing T; whereas (sigma)(,xx)(1(DARR)) increases with decreasing T for 0.3K(, )< T < 4.2K. Second, (sigma)(,xx)('max) (1(DARR)) ('(TURN)) 3(sigma)(,xx)('max) (1(UPARR)) at T('(TURN))0.5K in all of our samples. These results indicate the existence of spin
Lewis, Nicholas H. C.; Dong, Hui; Oliver, Thomas A. A.; Fleming, Graham R.
2015-05-07
Two-dimensional electronic-vibrational (2DEV) spectroscopy is an experimental technique that shows great promise in its ability to provide detailed information concerning the interactions between the electronic and vibrational degrees of freedom in molecular systems. The physical quantities 2DEV is particularly suited for measuring have not yet been fully determined, nor how these effects manifest in the spectra. In this work, we investigate the use of the center line slope of a peak in a 2DEV spectrum as a measure of both the dynamic and static correlations between the electronic and vibrational states of a dye molecule in solution. We show how this center line slope is directly related to the solvation correlation function for the vibrational degrees of freedom. We also demonstrate how the strength with which the vibration on the electronic excited state couples to its bath can be extracted from a set of 2DEV spectra. These analytical techniques are then applied to experimental data from the laser dye 3,3′-diethylthiatricarbocyanine iodide in deuterated chloroform, where we determine the lifetime of the correlation between the electronic transition frequency and the transition frequency for the backbone C = C stretch mode to be ∼1.7 ps. Furthermore, we find that on the electronic excited state, this mode couples to the bath ∼1.5 times more strongly than on the electronic ground state.
Study of two-dimensional device-error-redundant single-electron oscillator system
NASA Astrophysics Data System (ADS)
Murakami, Yoshisato; Oya, Takahide
2012-10-01
This paper reports the study of a two-dimensional device-error-redundant single-electron (SE) circuit. The circuit is an SE reaction-diffusion (RD) circuit that imitates the unique behavior of the chemical RD system and is expected to be a new information processing system. The original RD system is a complex chemical system that is said to express selforganizing dynamics in nature. It can also be assumed to operate as parallel information processing systems. Therefore, by imitating the original RD system for SE circuits, this SE-RD circuit can perform parallel information processing that is based on a natural phenomenon. However, the circuit is very sensitive to noise because it is controlled by a very small amount of energy. It is also sensitive to device errors (e.g., circuit parameter fluctuations in the fabrication process). Generally, fluctuations caused by errors introduced in manufacturing the circuit components trigger incorrect circuit operations, including noises. To overcome such noises, the circuit requires redundant properties for noise. To address this issue, we consider mimicking the information processing method of the natural world for the circuit to obtain noise redundancy. Actually, we previously proposed a unique method based on a model of neural networks with a stochastic resonance (SR) for the circuit. The SR phenomenon, which was discovered in studies of living things (e.g., insects), can be considered a type of noise-energy-harnessing system. Many researchers have proposed SR-based applications for novel electronic devices or systems. In networks where SR exists, signals can generally be distinguished from noise by harnessing noise energy. We previously designed SE-SR systems and succeeded in making an architecture for an SE circuit that has thermal noise redundancy. At the time, we applied an SR model proposed by Collins to our circuit. Prior to our current study, however, it had not yet been confirmed whether SE circuits have device
Li, Zhenyu; Abramavicius, Darius; Zhuang, Wei; Mukamel, Shaul
2007-11-15
The two dimensional (2D) photon echo spectrum of the amide ultraviolet (UV) bands of proteins are simulated. Two effective exciton Hamiltonian parameter sets developed by Woody and Hirst, which predict similar CD spectra, may be distinguished by their very different 2DUV spectra. These differences are enhanced in specific configurations of pulse polarizations which provide chirality-induced signals.
Magnetoabsorption and radiation-induced resistance oscillations in two-dimensional electron systems
Inarrea, J.; Platero, G.
2011-12-23
Magnetoabsorption and resistance oscillations in two-dimensional systems are calculated in the framework of the same theory: the microwave driven Larmor orbit model. On the one hand, this theory allows to obtain resistance oscillations with multiple peaks, depending on the microwave frequency. On the other hand, it permits also to calculate the microwave magnetoabsorption..
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.
Masutomi, Ryuichi; Triyama, Naotaka; Okamoto, Tohru
2013-12-04
We have performed not only magnetotransport measurements on two-dimensional electron systems (2DESs) formed at the cleaved surfaces of p-InAs but also observations of the surface morphology of the adsorbate atoms, which induced the 2DES at the surfaces of narrow band-gap semiconductors, with use of a scanning tunneling microscopy. The electron density of the 2DESs is compared to the atomic density of the isolated Ag adatoms on InAs surfaces.
Theory of two-dimensional Fourier transform electron spin resonance for ordered and viscous fluids
NASA Astrophysics Data System (ADS)
Lee, Sanghyuk; Budil, David E.; Freed, Jack H.
1994-10-01
A comprehensive theory for interpreting two-dimensional Fourier transform (2D-FT) electron spin resonance (ESR) experiments that is based on the stochastic Liouville equation is presented. It encompasses the full range of motional rates from fast through very slow motions, and it also provides for microscopic as well as macroscopic molecular ordering. In these respects it is as sophisticated in its treatment of molecular dynamics as the theory currently employed for analyzing cw ESR spectra. The general properties of the pulse propagator superoperator, which describes the microwave pulses in Liouville space, are analyzed in terms of the coherence transfer pathways appropriate for COSY (correlation spectroscopy), SECSY (spin-echo correlation spectroscopy), and 2D-ELDOR (electron-electron double resonance) sequences wherein either the free-induction decay (FID) or echo decay is sampled. Important distinctions are made among the sources of inhomogeneous broadening, which include (a) incomplete spectral averaging in the slow-motional regime, (b) unresolved superhyperfine structure and related sources, and (c) microscopic molecular ordering but macroscopic disorder (MOMD). The differing effects these sources of inhomogeneous broadening have on the two mirror image coherence pathways observed in the dual quadrature 2D experiments, as well as on the auto vs crosspeaks of 2D-ELDOR, is described. The theory is applied to simulate experiments of nitroxide spin labels in complex fluids such as membrane vesicles, where the MOMD model applies and these distinctions are particularly relevant, in order to extract dynamic and ordering parameters. The recovery of homogeneous linewidths from FID-based COSY experiments on complex fluids with significant inhomogeneous broadening is also described. The theory is applied to the ultraslow motional regime, and a simple method is developed to determine rotational rates from the broadening of the autopeaks of the 2D-ELDOR spectra as a
Exploring size and state dynamics in CdSe quantum dots using two-dimensional electronic spectroscopy
Caram, Justin R.; Zheng, Haibin; Rolczynski, Brian S.; Griffin, Graham B.; Engel, Gregory S.; Dahlberg, Peter D.; Dolzhnikov, Dmitriy S.; Talapin, Dmitri V.
2014-02-28
Development of optoelectronic technologies based on quantum dots depends on measuring, optimizing, and ultimately predicting charge carrier dynamics in the nanocrystal. In such systems, size inhomogeneity and the photoexcited population distribution among various excitonic states have distinct effects on electron and hole relaxation, which are difficult to distinguish spectroscopically. Two-dimensional electronic spectroscopy can help to untangle these effects by resolving excitation energy and subsequent nonlinear response in a single experiment. Using a filament-generated continuum as a pump and probe source, we collect two-dimensional spectra with sufficient spectral bandwidth to follow dynamics upon excitation of the lowest three optical transitions in a polydisperse ensemble of colloidal CdSe quantum dots. We first compare to prior transient absorption studies to confirm excitation-state-dependent dynamics such as increased surface-trapping upon excitation of hot electrons. Second, we demonstrate fast band-edge electron-hole pair solvation by ligand and phonon modes, as the ensemble relaxes to the photoluminescent state on a sub-picosecond time-scale. Third, we find that static disorder due to size polydispersity dominates the nonlinear response upon excitation into the hot electron manifold; this broadening mechanism stands in contrast to that of the band-edge exciton. Finally, we demonstrate excitation-energy dependent hot-carrier relaxation rates, and we describe how two-dimensional electronic spectroscopy can complement other transient nonlinear techniques.
Exploring size and state dynamics in CdSe quantum dots using two-dimensional electronic spectroscopy
Caram, Justin R.; Zheng, Haibin; Dahlberg, Peter D.; Rolczynski, Brian S.; Griffin, Graham B.; Dolzhnikov, Dmitriy S.; Talapin, Dmitri V.; Engel, Gregory S.
2014-01-01
Development of optoelectronic technologies based on quantum dots depends on measuring, optimizing, and ultimately predicting charge carrier dynamics in the nanocrystal. In such systems, size inhomogeneity and the photoexcited population distribution among various excitonic states have distinct effects on electron and hole relaxation, which are difficult to distinguish spectroscopically. Two-dimensional electronic spectroscopy can help to untangle these effects by resolving excitation energy and subsequent nonlinear response in a single experiment. Using a filament-generated continuum as a pump and probe source, we collect two-dimensional spectra with sufficient spectral bandwidth to follow dynamics upon excitation of the lowest three optical transitions in a polydisperse ensemble of colloidal CdSe quantum dots. We first compare to prior transient absorption studies to confirm excitation-state-dependent dynamics such as increased surface-trapping upon excitation of hot electrons. Second, we demonstrate fast band-edge electron-hole pair solvation by ligand and phonon modes, as the ensemble relaxes to the photoluminescent state on a sub-picosecond time-scale. Third, we find that static disorder due to size polydispersity dominates the nonlinear response upon excitation into the hot electron manifold; this broadening mechanism stands in contrast to that of the band-edge exciton. Finally, we demonstrate excitation-energy dependent hot-carrier relaxation rates, and we describe how two-dimensional electronic spectroscopy can complement other transient nonlinear techniques. PMID:24588185
Caram, Justin R; Zheng, Haibin; Dahlberg, Peter D; Rolczynski, Brian S; Griffin, Graham B; Dolzhnikov, Dmitriy S; Talapin, Dmitri V; Engel, Gregory S
2014-02-28
Development of optoelectronic technologies based on quantum dots depends on measuring, optimizing, and ultimately predicting charge carrier dynamics in the nanocrystal. In such systems, size inhomogeneity and the photoexcited population distribution among various excitonic states have distinct effects on electron and hole relaxation, which are difficult to distinguish spectroscopically. Two-dimensional electronic spectroscopy can help to untangle these effects by resolving excitation energy and subsequent nonlinear response in a single experiment. Using a filament-generated continuum as a pump and probe source, we collect two-dimensional spectra with sufficient spectral bandwidth to follow dynamics upon excitation of the lowest three optical transitions in a polydisperse ensemble of colloidal CdSe quantum dots. We first compare to prior transient absorption studies to confirm excitation-state-dependent dynamics such as increased surface-trapping upon excitation of hot electrons. Second, we demonstrate fast band-edge electron-hole pair solvation by ligand and phonon modes, as the ensemble relaxes to the photoluminescent state on a sub-picosecond time-scale. Third, we find that static disorder due to size polydispersity dominates the nonlinear response upon excitation into the hot electron manifold; this broadening mechanism stands in contrast to that of the band-edge exciton. Finally, we demonstrate excitation-energy dependent hot-carrier relaxation rates, and we describe how two-dimensional electronic spectroscopy can complement other transient nonlinear techniques.
Vortex Core Structure in Multilayered Rashba Superconductors
NASA Astrophysics Data System (ADS)
Higashi, Y.; Nagai, Y.; Yoshida, T.; Yanase, Y.
2014-12-01
We numerically study the electronic structure of a single vortex in two dimensional superconducting bilayer systems within the range of the mean-field theory. The lack of local inversion symmetry in the system is taken into account through the layer dependent Rashba spin-orbit coupling. The spatial profiles of the pair potential and the local quasiparticle density of states are calculated in the clean spin-singlet superconductor on the basis of the quasiclassical theory. In particular, we discuss the characteristic core structure in the pair-density wave state, which is spatially modulated exotic superconducting phase in a high magnetic field.
Bian, Guang; Wang, Zhengfei; Wang, Xiao-Xiong; Xu, Caizhi; Xu, SuYang; Miller, Thomas; Hasan, M Zahid; Liu, Feng; Chiang, Tai-Chang
2016-03-22
We report on the fabrication of a two-dimensional topological insulator Bi(111) bilayer on Sb nanofilms via a sequential molecular beam epitaxy growth technique. Our angle-resolved photoemission measurements demonstrate the evolution of the electronic band structure of the heterostructure as a function of the film thickness and reveal the existence of a two-dimensional spinful massless electron gas within the top Bi bilayer. Interestingly, our first-principles calculation extrapolating the observed band structure shows that, by tuning down the thickness of the supporting Sb films into the quantum dimension regime, a pair of isolated topological edge states emerges in a partial energy gap at 0.32 eV above the Fermi level as a consequence of quantum confinement effect. Our results and methodology of fabricating nanoscale heterostructures establish the Bi bilayer/Sb heterostructure as a platform of great potential for both ultra-low-energy-cost electronics and surface-based spintronics.
NASA Astrophysics Data System (ADS)
Wu, Haiping; Qian, Yan; Lu, Ruifeng; Tan, Weishi
2016-02-01
Motivated by the recent synthesis of bulk MoN2 which exhibits the layered structure just like the bulk MoS2, the monolayered MoN2 exfoliated from the bulk counterpart is investigated systematically by using density-functional calculations in this work. The result shows that the ground-state two-dimensional monolayered MoN2 behaves as an indirect band gap semiconductor with the energy gap of ∼0.12 eV. Subsequently, the external strain from -6% to 6% is employed to engineer the band structure, and the energy gap can be efficiently tuned from 0 to 0.70 eV. Notably, when the strain is beyond 5% or -3%, the two-dimensional monolayered MoN2 would transfer from an indirect band gap to a direct band gap semiconductor. This work introduces a new member of two-dimensional transition-metal family, which is important for industry applications, especially for the utilization in the long-wavelength infrared field.
NASA Astrophysics Data System (ADS)
Deng, Nianpei
The two dimensional electron gas subjected to a magnetic field has been a model system in contemporary condensed matter physics which generated many beautiful experiments as well as novel fundamental concepts. These novel concepts are of broad interests and have benefited other fields of research. For example, the observations of conventional odd-denominator fractional quantum Hall states have enriched many-body physics with important concepts such as fractional statistics and composite fermions. The subsequent discovery of the enigmatic even-denominator nu=5/2 fractional quantum Hall state has led to more interesting concepts such as non-Abelian statistics and pairing of composite fermions which can be intimately connected to the electron pairing in superconductivity. Moreover, the observations of stripe phases and reentrant integer quantum Hall states have stimulated research on exotic electron solids which have more intricate structures than the Wigner Crystal. In contrast to fractional quantum Hall states and stripes phases, the reentrant integer quantum Hall states are very little studied and their ground states are the least understood. There is a lack of basic information such as exact filling factors, temperature dependence and energy scales for the reentrant integer quantum Hall states. A critical experimental condition in acquiring this information is a stable ultra-low temperature environment. In the first part of this dissertation, I will discuss our unique setup of 3He immersion cell in a state-of-art dilution refrigerator which achieves the required stability of ultra-low temperature. With this experimental setup, we are able to observe for the first time very sharp magnetotransport features of reentrant integer quantum Hall states across many Landau levels for the first time. I will firstly present our results in the second Landau level. The temperature dependence measurements reveal a surprisingly sharp peak signature that is unique to the reentrant
NASA Astrophysics Data System (ADS)
Avetisyan, Siranush; Chakraborty, Tapash; Pietiläinen, Pekka
2016-07-01
Magnetization of anisotropic quantum dots in the presence of the Rashba spin-orbit interaction has been studied for three and four interacting electrons in the dot for non-zero values of the applied magnetic field. We observe unique behaviors of magnetization that are direct reflections of the anisotropy and the spin-orbit interaction parameters independently or concurrently. In particular, there are saw-tooth structures in the magnetic field dependence of the magnetization, as caused by the electron-electron interaction, that are strongly modified in the presence of large anisotropy and high strength of the spin-orbit interactions. We also report the temperature dependence of magnetization that indicates the temperature beyond which these structures due to the interactions disappear. Additionally, we found the emergence of a weak sawtooth structure in magnetization for three electrons in the high anisotropy and large spin-orbit interaction limit that was explained as a result of merging of two low-energy curves when the level spacings evolve with increasing values of the anisotropy and the spin-orbit interaction strength.
NASA Astrophysics Data System (ADS)
Singh, Vijeta; Pulikkotil, J. J.
2017-02-01
The origin of quasi-two dimensional electron gas at the interface of polar-nonpolar oxide hetero-structure, such as LaAlO3/SrTiO3, is debated over electronic reconstruction and defects/disorder models. Common to these models is the partial valence transformation of substrate Ti ions from its equilibrium 4 + state to an itinerant 3 + state. Given that the Hf ions have a lower ionization potential than Ti due to the 4 f orbital screening, one would expect a hetero-interface conductivity in the polar-nonpolar LaAlO3/SrHfO3 system as well. However, our first principles calculations show the converse. Unlike the Ti3+ -Ti4+ valence transition which occur at a nominal energy cost, the barrier energy associated with its isoelectronic Hf3+ -Hf4+ counterpart is very high, hence suppressing the formation of quasi-two dimensional electron gas at LaAlO3/SrHfO3 hetero-interface. These calculations, therefore, emphasize on the propensity of mixed valence at the interface as a necessary condition for an oxide hetero-structure to exihibit quasi two-dimensional electron gas.
Yeager, Mark; Dryden, Kelly A; Ganser-Pornillos, Barbie K
2013-01-01
Electron microscopy provides an efficient method for rapidly assessing whether a solution of macromolecules is homogeneous and monodisperse. If the macromolecules can be induced to form two-dimensional crystals that are a single layer in thickness, then electron crystallography of frozen-hydrated crystals has the potential of achieving three-dimensional density maps at sub-nanometer or even atomic resolution. Here we describe the lipid monolayer and sparse matrix screening methods for growing two-dimensional crystals and present successful applications to soluble macromolecular complexes: carboxysome shell proteins and HIV CA, respectively. Since it is common to express recombinant proteins with poly-His tags for purification by metal affinity chromatography, the monolayer technique using bulk lipids doped with Ni(2+) lipids has the potential for broad application. Likewise, the sparse matrix method uses screening conditions for three-dimensional crystallization and is therefore of broad applicability.
Shamim, S; Mahapatra, S; Scappucci, G; Klesse, W M; Simmons, M Y; Ghosh, A
2014-06-13
We report experimental evidence of a remarkable spontaneous time-reversal symmetry breaking in two-dimensional electron systems formed by atomically confined doping of phosphorus (P) atoms inside bulk crystalline silicon (Si) and germanium (Ge). Weak localization corrections to the conductivity and the universal conductance fluctuations were both found to decrease rapidly with decreasing doping in the Si:P and Ge:P delta layers, suggesting an effect driven by Coulomb interactions. In-plane magnetotransport measurements indicate the presence of intrinsic local spin fluctuations at low doping, providing a microscopic mechanism for spontaneous lifting of the time-reversal symmetry. Our experiments suggest the emergence of a new many-body quantum state when two-dimensional electrons are confined to narrow half-filled impurity bands.
NASA Astrophysics Data System (ADS)
Huang, Zhishuo; Zhang, Wenxu; Zhang, Wanli
2016-08-01
We calculated the electron mobility of 14 two dimensional semiconductors with composition of MX$_2$, where M (= Mo, W, Sn, Hf, Zr and Pt) is the transition metal, and X is S, Se and Te. We treated the scattering matrix by deformation potential approximation. Long wave longitudinal acoustical and optical phonon scatterings are included. Piezoelectric scattering in the compounds without inversion symmetry is also taken into account. We found that out of the 14 compounds, WSe$_2$, PtS$_2$ and PtSe$_2$, are promising regarding to the possible high electron mobility and finite band gap. The phonon limited mobility in PtSe$_2$ reaches about 3000 cm$^2$V$^{-1}$s$^{-1}$ at room temperature which is the highest among the compounds. The bandgap under the local density approximation is 1.25 eV. Our results can be a guide for experiments to search for better two-dimensional materials for future semiconductor devices.
Gaynor, James D; Courtney, Trevor L; Balasubramanian, Madhumitha; Khalil, Munira
2016-06-15
The development of coherent Fourier transform two-dimensional electronic-vibrational (2D EV) spectroscopy with acousto-optic pulse-shaper-generated near-UV pump pulses and an octave-spanning broadband mid-IR probe pulse is detailed. A 2D EV spectrum of a silicon wafer demonstrates the full experimental capability of this experiment, and a 2D EV spectrum of dissolved hexacyanoferrate establishes the viability of our 2D EV experiment for studying condensed phase molecular ensembles.
Anomalous-circular photogalvanic effect in a GaAs/AlGaAs two-dimensional electron gas
NASA Astrophysics Data System (ADS)
Tang, C. G.; Chen, Y. H.; Liu, Y.; Wang, Z. G.
2009-09-01
We have studied the circular photogalvanic effect (CPGE) in a GaAs/AlGaAs two-dimensional electron gas excited by near infrared light at room temperature. The anomalous CPGE observed under normal incidence indicates a swirling current which is realized by a radial spin current via the reciprocal spin-Hall effect. The anomalous CPGE exhibits a cubic cosine dependence on the incidence angle, which is discussed in line with the above interpretation.
Anomalous-circular photogalvanic effect in a GaAs/AlGaAs two-dimensional electron gas.
Tang, C G; Chen, Y H; Liu, Y; Wang, Z G
2009-09-16
We have studied the circular photogalvanic effect (CPGE) in a GaAs/AlGaAs two-dimensional electron gas excited by near infrared light at room temperature. The anomalous CPGE observed under normal incidence indicates a swirling current which is realized by a radial spin current via the reciprocal spin-Hall effect. The anomalous CPGE exhibits a cubic cosine dependence on the incidence angle, which is discussed in line with the above interpretation.
Yu, Chao; Wei, Hui; Wang, Xu; ...
2015-10-27
Imaging the transient process of molecules has been a basic way to investigate photochemical reactions and dynamics. Based on laser-induced electron diffraction and partial one-dimensional molecular alignment, here we provide two effective methods for reconstructing two-dimensional structure of polyatomic molecules. We demonstrate that electron diffraction images in both scattering angles and broadband energy can be utilized to retrieve complementary structure information, including positions of light atoms. Lastly, with picometre spatial resolution and the inherent femtosecond temporal resolution of lasers, laser-induced electron diffraction method offers significant opportunities for probing atomic motion in a large molecule in a typical pump-probe measurement.
Vostokov, N. V. Shashkin, V. I.
2015-11-28
We consider the problem of non-resonant detection of terahertz signals in a short gate length field-effect transistor having a two-dimensional electron channel with zero external bias between the source and the drain. The channel resistance, gate-channel capacitance, and quadratic nonlinearity parameter of the transistor during detection as a function of the gate bias voltage are studied. Characteristics of detection of the transistor connected in an antenna with real impedance are analyzed. The consideration is based on both a simple one-dimensional model of the transistor and allowance for the two-dimensional distribution of the electric field in the transistor structure. The results given by the different models are discussed.
Rashba-type spin splitting and the electronic structure of ultrathin Pb/MoTe2 heterostructure
NASA Astrophysics Data System (ADS)
Du, X.; Wang, Z. Y.; Huang, G. Q.
2016-11-01
The spin-polarized band structures of the Pb(111)/MoTe2 heterostructure are studied by the first-principles calculations. Due to strong spin-orbit coupling and space inversion asymmetry, large Rashba spin splitting of electronic bands appears in this hybrid system. The spin splitting is completely out-of-plane and opposite at \\bar{K} and {\\bar{K}}\\prime points. Rashba spin splitting also appears along the in-plane momentum direction around the \\bar{{{Γ }}} point due to the existence of surface potential gradient induced by charge transfer at interface. Furthermore, our calculations show that the spin-polarized bands closely approach the Fermi level in Pb/MoTe2 heterostructure, showing that this heterostructure may be a good candidate in valleytronics or spintronics.
Strongly enhanced Rashba splittings in an oxide heterostructure: A tantalate monolayer on BaHfO3
NASA Astrophysics Data System (ADS)
Kim, Minsung; Ihm, Jisoon; Chung, Suk Bum
2016-09-01
In the two-dimensional electron gas emerging at the transition metal oxide surface and interface, various exotic electronic ordering and topological phases can become experimentally more accessible with the stronger Rashba spin-orbit interaction. Here, we present a promising route to realize significant Rashba-type band splitting using a thin film heterostructure. Based on first-principles methods and analytic model analyses, a tantalate monolayer on BaHfO3 is shown to host two-dimensional bands originating from Ta t2 g states with strong Rashba spin splittings, nearly 10% of the bandwidth, at both the band minima and saddle points. An important factor in this enhanced splitting is the significant t2 g-eg interband coupling, which can generically arise when the inversion symmetry is maximally broken due to the strong confinement of the 2DEG on a transition metal oxide surface. Our results could be useful in realizing topological superconductivity at oxide surfaces.
Nersisyan, Hrachya B.; Das, Amal K.
2009-07-15
The results of a theoretical investigation on the stopping power of ions moving in a two-dimensional degenerate electron gas are presented. The stopping power for an ion is calculated employing linear-response theory using the dielectric function approach. The collisions, which lead to a damping of plasmons and quasiparticles in the electron gas, is taken into account through a relaxation-time approximation in the linear-response function. The stopping power for an ion is calculated in both the low- and high-velocity limits. In order to highlight the effects of damping, we present a comparison of our analytical and numerical results, in the case of pointlike ions, obtained for a nonzero damping with those for a vanishing damping. It is shown that the equipartition sum rule first formulated by Lindhard and Winther for three-dimensional degenerate electron gas does not necessarily hold in two dimensions. We have generalized this rule introducing an effective dielectric function. In addition, some results for two-dimensional interacting electron gas have been obtained. In this case, the exchange-correlation interactions of electrons are considered via local-field corrected dielectric function.
NASA Astrophysics Data System (ADS)
Kang, Sungmu
In this thesis, devices using the ballistic transport of two dimensional electron gas (2DEG) in GaAs High Electron Mobility Transistor(HEMT) structure is fabricated and their dc and ac properties are characterized. This study gives insight on operation and applications of modern submicron devices with ever reduced gate length comparable to electron mean free path. The ballistic transport is achieved using both temporal and spatial limits in this thesis. In temporal limit, when frequency is higher than the scattering frequency (1/(2pitau)), ballistic transport can be achieved. At room temperature, generally the scattering frequency is around 500 GHz but at cryogenic temperature (≤4K) with high mobility GaAs HEMT structure, the frequency is much lower than 2 GHz. On this temporal ballistic transport regime, effect of contact impedance and different dc mobility on device operation is characterized with the ungated 2DEG of HEMT structure. In this ballistic regime, impedance and responsivity of plasma wave detector are investigated using the gated 2DEG of HEMT at different ac boundary conditions. Plasma wave is generated at asymmetric ac boundary conditions of HEMTs, where source is short to ground and drain is open while rf power is applied to gate. The wave velocity can be tuned by gate bias voltage and induced drain to source voltage(Vds ) shows the resonant peak at odd number of fundamental frequency. Quantitative power coupling to plasma wave detector leads to experimental characterization of resonant response of plasma wave detector as a function of frequency. Because plasma wave resonance is not limited by transit time, the physics learned in this study can be directly converted to room temperature terahertz detection by simply reducing gate length(Lgate) to submicron for the terahertz application such as non destructive test, bio medical analysis, homeland security, defense and space. In same HEMT structure, the dc and rf characterization on device is also
Effects of finite pulse width on two-dimensional Fourier transform electron spin resonance
NASA Astrophysics Data System (ADS)
Liang, Zhichun; Crepeau, Richard H.; Freed, Jack H.
2005-12-01
Two-dimensional (2D) Fourier transform ESR techniques, such as 2D-ELDOR, have considerably improved the resolution of ESR in studies of molecular dynamics in complex fluids such as liquid crystals and membrane vesicles and in spin labeled polymers and peptides. A well-developed theory based on the stochastic Liouville equation (SLE) has been successfully employed to analyze these experiments. However, one fundamental assumption has been utilized to simplify the complex analysis, viz. the pulses have been treated as ideal non-selective ones, which therefore provide uniform irradiation of the whole spectrum. In actual experiments, the pulses are of finite width causing deviations from the theoretical predictions, a problem that is exacerbated by experiments performed at higher frequencies. In the present paper we provide a method to deal with the full SLE including the explicit role of the molecular dynamics, the spin Hamiltonian and the radiation field during the pulse. The computations are rendered more manageable by utilizing the Trotter formula, which is adapted to handle this SLE in what we call a "Split Super-Operator" method. Examples are given for different motional regimes, which show how 2D-ELDOR spectra are affected by the finite pulse widths. The theory shows good agreement with 2D-ELDOR experiments performed as a function of pulse width.
Two-dimensional plasma expansion in a magnetic nozzle: Separation due to electron inertia
Ahedo, Eduardo; Merino, Mario
2012-08-15
A previous axisymmetric model of the supersonic expansion of a collisionless, hot plasma in a divergent magnetic nozzle is extended here in order to include electron-inertia effects. Up to dominant order on all components of the electron velocity, electron momentum equations still reduce to three conservation laws. Electron inertia leads to outward electron separation from the magnetic streamtubes. The progressive plasma filling of the adjacent vacuum region is consistent with electron-inertia being part of finite electron Larmor radius effects, which increase downstream and eventually demagnetize the plasma. Current ambipolarity is not fulfilled and ion separation can be either outwards or inwards of magnetic streamtubes, depending on their magnetization. Electron separation penalizes slightly the plume efficiency and is larger for plasma beams injected with large pressure gradients. An alternative nonzero electron-inertia model [E. Hooper, J. Propul. Power 9, 757 (1993)] based on cold plasmas and current ambipolarity, which predicts inwards electron separation, is discussed critically. A possible competition of the gyroviscous force with electron-inertia effects is commented briefly.
Spectroscopy and Dynamics of a Two-Dimensional Electron Gas on Ultrathin Helium Films on Cu(111)
NASA Astrophysics Data System (ADS)
Armbrust, N.; Güdde, J.; Höfer, U.; Kossler, S.; Feulner, P.
2016-06-01
Electrons in image-potential states on the surface of bulk helium represent a unique model system of a two-dimensional electron gas. Here, we investigate their properties in the extreme case of reduced film thickness: a monolayer of helium physisorbed on a single-crystalline (111)-oriented Cu surface. For this purpose we have utilized a customized setup for time-resolved two-photon photoemission at very low temperatures under ultrahigh vacuum conditions. We demonstrate that the highly polarizable metal substrate increases the binding energy of the first (n =1 ) image-potential state by more than 2 orders of magnitude as compared to the surface of liquid helium. An electron in this state is still strongly decoupled from the metal surface due to the large negative electron affinity of helium and we find that even 1 monolayer of helium increases its lifetime by 1 order of magnitude compared to the bare Cu(111) surface.
Pugachev, L. P. Andreev, N. E. Levashov, P. R.; Malkov, Yu. A. Stepanov, A. N. Yashunin, D. A.
2015-07-15
The electron acceleration mechanism associated with the generation of a plasma wave due to self-modulation instability of laser radiation in a subcritical plasma produced by a laser prepulse coming 10 ns before the arrival of the main intense femtosecond pulse is considered. Three-dimensional particle-in-cell simulations of the interaction of laser radiation with two-dimensionally inhomogeneous subcritical plasma have shown that, for a sufficiently strong plasma inhomogeneity and a sharp front of the laser pulse, efficient plasma wave excitation, electron trapping, and generation of collimated electron beams with energies on the order of 0.2–0.5 MeV can occur. The simulation results agree with experiments on the generation of collimated beams of accelerated electrons from metal targets irradiated by intense femtosecond laser pulses.
Cai, Libing; Wang, Jianguo; Zhu, Xiangqin; Wang, Yue; Zhang, Dianhui
2015-01-15
Based on the secondary electron emission avalanche (SEEA) model, the SEEA discharge on the vacuum insulator surface is simulated by using a 2D PIC-MCC code developed by ourselves. The evolutions of the number of discharge electrons, insulator surface charge, current, and 2D particle distribution are obtained. The effects of the strength of the applied electric field, secondary electron yield coefficient, rise time of the pulse, length of the insulator on the discharge are investigated. The results show that the number of the SEEA electrons presents a quadratic dependence upon the applied field strength. The SEEA current, which is on the order of Ampere, is directly proportional to the field strength and secondary electron yield coefficient. Finally, the electron-stimulated outgassing is included in the simulation code, and a three-phase discharge curve is presented by the simulation, which agrees with the experimental data.
NASA Astrophysics Data System (ADS)
Cai, Libing; Wang, Jianguo; Zhu, Xiangqin; Wang, Yue; Zhang, Dianhui
2015-01-01
Based on the secondary electron emission avalanche (SEEA) model, the SEEA discharge on the vacuum insulator surface is simulated by using a 2D PIC-MCC code developed by ourselves. The evolutions of the number of discharge electrons, insulator surface charge, current, and 2D particle distribution are obtained. The effects of the strength of the applied electric field, secondary electron yield coefficient, rise time of the pulse, length of the insulator on the discharge are investigated. The results show that the number of the SEEA electrons presents a quadratic dependence upon the applied field strength. The SEEA current, which is on the order of Ampere, is directly proportional to the field strength and secondary electron yield coefficient. Finally, the electron-stimulated outgassing is included in the simulation code, and a three-phase discharge curve is presented by the simulation, which agrees with the experimental data.
Roslyak, O.; Gumbs, Godfrey; Mukamel, S.
2012-01-01
We study the localization of dressed Dirac electrons in a cylindrical quantum dot (QD) formed on monolayer and bilayer graphene by spatially different potential profiles. Short lived excitonic states which are too broad to be resolved in linear spectroscopy are revealed by cross peaks in the photon-echo nonlinear technique. Signatures of the dynamic gap in the two-dimensional spectra are discussed. The effect of the Coulomb induced exciton-exciton scattering and the formation of biexciton molecules are demonstrated. PMID:22612079
Long wavelength plasmon damping in the two-dimensional electron gas
NASA Astrophysics Data System (ADS)
Bachlechner, Martina E.; Böhm, Helga M.; Schinner, Andreas
1993-07-01
The damping of the long wavelength plasmon in a homogeneos electron layer is determined by two-electron-hole excitations. Analytical and numerical results for the corresponding imaginary part of the dielectric function and the plasmon half width are presented for various densities and different manners of screening.
Very large capacitance enhancement in a two-dimensional electron system.
Li, Lu; Richter, C; Paetel, S; Kopp, T; Mannhart, J; Ashoori, R C
2011-05-13
Increases in the gate capacitance of field-effect transistor structures allow the production of lower-power devices that are compatible with higher clock rates, driving the race for developing high-κ dielectrics. However, many-body effects in an electronic system can also enhance capacitance. Onto the electron system that forms at the LaAlO(3)/SrTiO(3) interface, we fabricated top-gate electrodes that can fully deplete the interface of all mobile electrons. Near depletion, we found a greater than 40% enhancement of the gate capacitance. Using an electric-field penetration measurement method, we show that this capacitance originates from a negative compressibility of the interface electron system. Capacitance enhancement exists at room temperature and arises at low electron densities, in which disorder is strong and the in-plane conductance is much smaller than the quantum conductance.
Atomic structure and electronic properties of the two-dimensional (Au ,Al )/Si (111 )2 ×2 compound
NASA Astrophysics Data System (ADS)
Gruznev, D. V.; Bondarenko, L. V.; Matetskiy, A. V.; Tupchaya, A. Y.; Chukurov, E. N.; Hsing, C. R.; Wei, C. M.; Eremeev, S. V.; Zotov, A. V.; Saranin, A. A.
2015-12-01
A combination of scanning tunneling microscopy, angle-resolved photoelectron spectroscopy, ab initio random structure searching, and density functional theory electronic structure calculations was applied to elucidate the atomic arrangement and electron band structure of the (Au ,Al )/Si (111 )2 ×2 two-dimensional compound formed upon Al deposition onto the mixed 5 ×2 /√{3 }×√{3 } Au/Si(111) surface. It was found that the most stable 2 ×2 -(Au, Al) compound incorporates four Au atoms, three Al atoms, and two Si atoms per 2 ×2 unit cell. Its atomic arrangement can be visualized as an array of meandering Au atomic chains with two-thirds of the Al atoms incorporated into the chains and one-third of the Al atoms interconnecting the chains. The compound is metallic and its electronic properties can be controlled by appropriate Al dosing since energetic location of the bands varies by ˜0.5 eV during increasing of Al contents. The 2 ×2 -(Au, Al) structure appears to be lacking the C3 v symmetry typical for the hexagonal lattices. The consequence of the peculiar atomic structure of the two-dimensional alloy is spin splitting of the metallic states, which should lead to anisotropy of the current-induced in-plane spin polarization.
The electronic properties of bare and alkali metal adsorbed two-dimensional GeSi alloy sheet
NASA Astrophysics Data System (ADS)
Qiu, Wenhao; Ye, Han; Yu, Zhongyuan; Liu, Yumin
2016-09-01
In this paper, the structural and electronic properties of both bare and alkali metal (AM) atoms adsorbed two-dimensional GeSi alloy sheet (GeSiAS) are investigated by means of first-principles calculations. The band gaps of bare GeSiAS are shown slightly opened at Dirac point with the energy dispersion remain linear due to the spin-orbit coupling effect at all concentrations of Ge atoms. For metal adsorption, AM atoms (including Li, Na and K) prefer to occupy the hexagonal hollow site of GeSiAS and the primary chemical bond between AM adatom and GeSiAS is ionic. The adsorption energy has an increase tendency with the increase of the Ge concentration in supercell. Besides, single-side adsorption of AM atoms introduces band gap at Dirac point, which can be tuned by the Ge concentration and the species of AM atoms. The strong relation between the band gaps and the distribution of Si and Ge atoms inside GeSiAS are also demonstrated. The opened band gaps of AM covered GeSiAS range from 14.8 to 269.1 meV along with the effective masses of electrons ranging from 0.013 to 0.109 me, indicating the high tunability of band gap as well as high mobility of carriers. These results provide a development in two-dimensional alloys and show potential applications in novel micro/nano-electronic devices.
NASA Astrophysics Data System (ADS)
Hai, Guo-Qiang; Peeters, François M.
2015-01-01
Based on the metastable electron-pair energy band in a two-dimensional (2D) periodic potential obtained previously by Hai and Castelano [J. Phys.: Condens. Matter 26, 115502 (2014)], we present in this work a Hamiltonian of many electrons consisting of single electrons and electron pairs in the 2D system. The electron-pair states are metastable of energies higher than those of the single-electron states at low electron density. We assume two different scenarios for the single-electron band. When it is considered as the lowest conduction band of a crystal, we compare the obtained Hamiltonian with the phenomenological model Hamiltonian of a boson-fermion mixture proposed by Friedberg and Lee [Phys. Rev. B 40, 6745 (1989)]. Single-electron-electron-pair and electron-pair-electron-pair interaction terms appear in our Hamiltonian and the interaction potentials can be determined from the electron-electron Coulomb interactions. When we consider the single-electron band as the highest valence band of a crystal, we show that holes in this valence band are important for stabilization of the electron-pair states in the system.
Heating and cooling of a two-dimensional electron gas by terahertz radiation
Budkin, G. V.; Tarasenko, S. A.
2011-04-15
The absorption of terahertz radiation by free charge carriers in n-type semiconductor quantum wells accompanied by the interaction of electrons with acoustic and optical phonons is studied. It is shown that intrasubband optical transitions can cause both heating and cooling of the electron gas. The cooling of charge carriers occurs in a certain temperature and radiation frequency region where light is most efficiently absorbed due to intrasubband transitions with emission of optical phonons. In GaAs quantum wells, the optical cooling of electrons occurs most efficiently at liquid nitrogen temperatures, while cooling is possible even at room temperature in GaN heterostructures.
Two-dimensional numerical model of electron cyclotron resonance discharge with pointwise mappings
Eruhimov, V.; Semenov, V.
2006-03-15
We suggest a new approach to numerical modeling of electron distribution function in an electron cyclotron resonance (ECR) discharge. The method is based on a pointwise mapping of electron velocity over a single bounce oscillation. We limit our consideration to ECR heating, collisions, ionization, and ambipolar losses from the trap although other processes can be accounted for as well. The method gives a solution close to the brute-force particle-in-cell integration but is incomparably faster. Initial results of experiments are presented.
Electronic, Mechanical, and Dielectric Properties of Two-Dimensional Atomic Layers of Noble Metals
NASA Astrophysics Data System (ADS)
Kapoor, Pooja; Kumar, Jagdish; Kumar, Arun; Kumar, Ashok; Ahluwalia, P. K.
2017-01-01
We present density functional theory-based electronic, mechanical, and dielectric properties of monolayers and bilayers of noble metals (Au, Ag, Cu, and Pt) taken with graphene-like hexagonal structure. The Au, Ag, and Pt bilayers stabilize in AA-stacked configuration, while the Cu bilayer favors the AB stacking pattern. The quantum ballistic conductance of the noble-metal mono- and bilayers is remarkably increased compared with their bulk counterparts. Among the studied systems, the tensile strength is found to be highest for the Pt monolayer and bilayer. The noble metals in mono- and bilayer form show distinctly different electron energy loss spectra and reflectance spectra due to the quantum confinement effect on going from bulk to the monolayer limit. Such tunability of the electronic and dielectric properties of noble metals by reducing the degrees of freedom of electrons offers promise for their use in nanoelectronics and optoelectronics applications.
Two-dimensional single-stream electron motion in a coaxial diode with magnetic insulation
Fuks, Mikhail I.; Schamiloglu, Edl
2014-05-15
One of the most widespread models of electrons drifting around the cathode in magnetrons is the single-stream state, which is the Brillouin stream with purely azimuthal motion. We describe a single-stream state in which electrons not only move in the azimuthal direction, but also along the axial direction, which is useful for consideration, for example, of relativistic magnetrons, MILOs, and coaxial transmission lines. Relations are given for the conditions of magnetic insulation for 2D electron motion, for 1D azimuthal and axial motion, and for synchronism of these streams with the operating waves of M-type microwave sources. Relations are also provided for the threshold of generation in magnetrons with 2D electron motion.
Goswami, Srijit; Aamir, Mohammed Ali; Shamim, Saquib; Ghosh, Arindam; Siegert, Christoph; Farrer, Ian; Ritchie, David A.; Pepper, Michael
2013-12-04
We use a dual gated device structure to introduce a gate-tuneable periodic potential in a GaAs/AlGaAs two dimensional electron gas (2DEG). Using only a suitable choice of gate voltages we can controllably alter the potential landscape of the bare 2DEG, inducing either a periodic array of antidots or quantum dots. Antidots are artificial scattering centers, and therefore allow for a study of electron dynamics. In particular, we show that the thermovoltage of an antidot lattice is particularly sensitive to the relative positions of the Fermi level and the antidot potential. A quantum dot lattice, on the other hand, provides the opportunity to study correlated electron physics. We find that its current-voltage characteristics display a voltage threshold, as well as a power law scaling, indicative of collective Coulomb blockade in a disordered background.
Studies of scattering mechanisms in gate tunable InAs/(Al,Ga)Sb two dimensional electron gases
Shojaei, B.; McFadden, A.; Schultz, B. D.; Shabani, J.; Palmstrøm, C. J.
2015-06-01
A study of scattering mechanisms in gate tunable two dimensional electron gases confined to InAs/(Al,Ga)Sb heterostructures with varying interface roughness and dislocation density is presented. By integrating an insulated gate structure the evolution of the low temperature electron mobility and single-particle lifetime was determined for a previously unexplored density regime, 10{sup 11}–10{sup 12 }cm{sup −2}, in this system. Existing theoretical models were used to analyze the density dependence of the electron mobility and single particle lifetime in InAs quantum wells. Scattering was found to be dominated by charged dislocations and interface roughness. It was demonstrated that the growth of InAs quantum wells on nearly lattice matched GaSb substrate results in fewer dislocations, lower interface roughness, and improved low temperature transport properties compared to growth on lattice mismatched GaAs substrates.
NASA Astrophysics Data System (ADS)
Gredeskul, S. A.; Zusman, M.; Avishai, Y.; Azbel', M. Ya.
1997-09-01
Electron spectral properties and localization in a two-dimensional system with point potentials subject to a perpendicular magnetic field are studied. A brief review of the known results concerning electron dynamics in such systems is presented. For a set of periodic point potentials, exact dispersion laws and energy-flux diagram (Hofstadter-type butterfly) are obtained. It is shown that, in the case of one-dimensional disorder, the electron localization in a strong magnetic field is described by the random Harper equation. Energy-flux diagram for the localization length is presented and the fractal structure of the localization length is demonstrated. Near the Landau levels an exact formula for the localization length as a function of energy and disorder is obtained. The corresponding critical exponent is equal to unity which is reminiscent of one-dimensional characteristics.
Idrobo Tapia, Juan Carlos; Zhou, Wu
2017-03-01
Here we present a short historical account of when single adatom impurities where first identified in two-dimensional materials by scanning transmission electron microscopy (STEM). We also present a study of the graphene low-loss (below 50 eV) response as a function of number of layers using electron energy-loss spectroscopy (EELS). The study shows that as few as three layers of graphene behave as bulk graphite for losses above 10 eV We also show examples of how point and extended defects can easily be resolved and structural dynamics can be readily capture by using aberration-corrected STEM imaging. Lastly, we show that themore » new generation of monochromators has opened up possibilities to explore new physics with an electron microscope. All these capabilities were enabled by the development of spherical aberration correctors and monochromators, where Ondrej Krivanek has played a key role.« less
Shi, Likun; Lou, Wenkai; Cheng, F.; Zou, Y. L.; Yang, Wen; Chang, Kai
2015-01-01
Based on the Born-Oppemheimer approximation, we divide the total electron Hamiltonian in a spin-orbit coupled system into the slow orbital motion and the fast interband transition processes. We find that the fast motion induces a gauge field on the slow orbital motion, perpendicular to the electron momentum, inducing a topological phase. From this general designing principle, we present a theory for generating artificial gauge field and topological phase in a conventional two-dimensional electron gas embedded in parabolically graded GaAs/InxGa1−xAs/GaAs quantum wells with antidot lattices. By tuning the etching depth and period of the antidot lattices, the band folding caused by the antidot potential leads to the formation of minibands and band inversions between neighboring subbands. The intersubband spin-orbit interaction opens considerably large nontrivial minigaps and leads to many pairs of helical edge states in these gaps. PMID:26471126
Zeng, Shengwei; Lü, Weiming; Huang, Zhen; Liu, Zhiqi; Han, Kun; Gopinadhan, Kalon; Li, Changjian; Guo, Rui; Zhou, Wenxiong; Ma, Haijiao Harsan; Jian, Linke; Venkatesan, Thirumalai; Ariando
2016-04-26
Electric field effect in electronic double layer transistor (EDLT) configuration with ionic liquids as the dielectric materials is a powerful means of exploring various properties in different materials. Here, we demonstrate the modulation of electrical transport properties and extremely high mobility of two-dimensional electron gas at LaAlO3/SrTiO3 (LAO/STO) interface through ionic liquid-assisted electric field effect. With a change of the gate voltages, the depletion of charge carrier and the resultant enhancement of electron mobility up to 19 380 cm(2)/(V s) are realized, leading to quantum oscillations of the conductivity at the LAO/STO interface. The present results suggest that high-mobility oxide interfaces, which exhibit quantum phenomena, could be obtained by ionic liquid-assisted field effect.
Idrobo, Juan C; Zhou, Wu
2017-03-01
Here we present a short historical account of when single adatom impurities where first identified in two-dimensional materials by scanning transmission electron microscopy (STEM). We also present a study of the graphene low-loss (below 50eV) response as a function of number of layers using electron energy-loss spectroscopy (EELS). The study shows that as few as three layers of graphene behave as bulk graphite for losses above 10eV We also show examples of how point and extended defects can easily be resolved and structural dynamics can be readily capture by using aberration-corrected STEM imaging. Finally, we show that the new generation of monochromators has opened up possibilities to explore new physics with an electron microscope. All these capabilities were enabled by the development of spherical aberration correctors and monochromators, where Ondrej Krivanek has played a key role.
Two-Dimensional Simulations of Electron Shock Ignition at the Megajoule Scale
NASA Astrophysics Data System (ADS)
Shang, W.; Betti, R.
2016-10-01
Shock ignition uses a late strong shock to ignite the hot spot of an inertial confinement fusion capsule. In the standard shock-ignition scheme, an ignitor shock is launched by the ablation pressure from a spike in laser intensity. Recent experiments on OMEGA have shown that focused beams with intensity up to 6 ×1015 W /cm2 can produce copious amounts of hot electrons. The hot electrons are produced by laser-plasma instabilities (LPI's) and can carry up to 15 % of the instantaneous laser power. Megajoule-scale targets will likely produce even more hot electrons because of the large plasma scale length. We show that it is possible to design ignition targets with low implosion velocities that can be shock ignited using LPI-generated hot electrons to obtain high energy gains. These designs are robust to low-mode asymmetries and they ignite even for highly distorted implosions. Electron shock ignition requires tens of kilojoules of hot electrons, which can only be produced on a large laser facility like the National Ignition Facility. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.
Oliver, Thomas A. A.; Lewis, Nicholas H. C.; Fleming, Graham R.
2014-01-01
Multidimensional nonlinear spectroscopy, in the electronic and vibrational regimes, has reached maturity. To date, no experimental technique has combined the advantages of 2D electronic spectroscopy and 2D infrared spectroscopy, monitoring the evolution of the electronic and nuclear degrees of freedom simultaneously. The interplay and coupling between the electronic state and vibrational manifold is fundamental to understanding ensuing nonradiative pathways, especially those that involve conical intersections. We have developed a new experimental technique that is capable of correlating the electronic and vibrational degrees of freedom: 2D electronic–vibrational spectroscopy (2D-EV). We apply this new technique to the study of the 4-(di-cyanomethylene)-2-methyl-6-p-(dimethylamino)styryl-4H-pyran (DCM) laser dye in deuterated dimethyl sulfoxide and its excited state relaxation pathways. From 2D-EV spectra, we elucidate a ballistic mechanism on the excited state potential energy surface whereby molecules are almost instantaneously projected uphill in energy toward a transition state between locally excited and charge-transfer states, as evidenced by a rapid blue shift on the electronic axis of our 2D-EV spectra. The change in minimum energy structure in this excited state nonradiative crossing is evident as the central frequency of a specific vibrational mode changes on a many-picoseconds timescale. The underlying electronic dynamics, which occur on the hundreds of femtoseconds timescale, drive the far slower ensuing nuclear motions on the excited state potential surface, and serve as a excellent illustration for the unprecedented detail that 2D-EV will afford to photochemical reaction dynamics. PMID:24927586
Electronic transport in two-dimensional systems in the quantum hall regime
NASA Astrophysics Data System (ADS)
Tarquini, Vinicio
The integer and the fractional quantum Hall effects are essential to the exploration of quantum matters characterized by topological phases. A quantum Hall system hosts one-dimensional (1D) chiral edge channels that manifest zero magnetoresistance, dissipationless due to the broken time reversal symmetry, and quantized Hall resistance vhe2 with v being the topological invariant (or Chern number). The 1-1 correspondence between the conducting gapless edge channels to the gapped incompressible bulk states is a defining character of a topological insulator (TI). Understanding this correspondence in real systems, especially the origin of its robustness (in terms of the limit of breakdown), is important both fundamentally and practically (i.e. in relation to spintronics). However, the breakdown mechanism, especially in light of the edge-bulk correlation, is still an open question. We adopt GaAs two-dimensional (2D) high-mobility hole systems confined in a 20 nm wide (100)-GaAs quantum wells and have perform transport measurement for a range of charge densities between 4 and 5 x 1010 cm -2 with a carrier mobility of 2 - 4 x 106 cm 2/V·s down to millikelvin temperatures. Systematic characterization of the 2D systems through Shubnikov-de Haas (SdH) oscillations yields an effective mass between 0.30 and 0.50me, in good agreement with the cyclotron resonance results. We then modify a regular Hall bar system into a unique anti-Hall bar geometry that provides an extra set of independent chiral edge channels without altering the topological invariant. We perform systematic measurement of quantum oscillations via chiral edges while simultaneously probing the bulk dynamics, through measuring across independent edges, in respond to the edge excitations. The edge-bulk correspondence reveals a non-equilibrium dynamical development of the incompressible bulk states that leads to a novel asymmetrical 1-0 Hall potential distribution. Moreover, probing the breakdown via inner and outer
Franck, John M.; Dzikovski, Boris; Freed, Jack H.
2015-01-01
The development, applications, and current challenges of the pulsed ESR technique of two-dimensional Electron-Electron Double Resonance (2D ELDOR) are described. This is a three-pulse technique akin to 2D Exchange Nuclear Magnetic Resonance, but involving electron spins, usually in the form of spin-probes or spin-labels. As a result, it required the extension to much higher frequencies, i.e., microwaves, and much faster time scales, with π/2 pulses in the 2-3 ns range. It has proven very useful for studying molecular dynamics in complex fluids, and spectral results can be explained by fitting theoretical models (also described) that provide a detailed analysis of the molecular dynamics and structure. We discuss concepts that also appear in other forms of 2D spectroscopy but emphasize the unique advantages and difficulties that are intrinsic to ESR. Advantages include the ability to tune the resonance frequency, in order to probe different motional ranges, while challenges include the high ratio of the detection dead time vs. the relaxation times. We review several important 2D ELDOR studies of molecular dynamics. (1) The results from a spin probe dissolved in a liquid crystal are followed throughout the isotropic → nematic → liquid-like smectic → solid-like smectic → crystalline phases as the temperature is reduced and are interpreted in terms of the slowly relaxing local structure model. Here, the labeled molecule is undergoing overall motion in the macroscopically aligned sample, as well as responding to local site fluctuations. (2) Several examples involving model phospholipid membranes are provided, including the dynamic structural characterization of the boundary lipid that coats a transmembrane peptide dimer. Additionally, subtle differences can be elicited for the phospholipid membrane phases: liquid disordered, liquid ordered, and gel, and the subtle effects upon the membrane, of antigen cross-linking of receptors on the surface of plasma membrane
NASA Astrophysics Data System (ADS)
Franck, John M.; Chandrasekaran, Siddarth; Dzikovski, Boris; Dunnam, Curt R.; Freed, Jack H.
2015-06-01
The development, applications, and current challenges of the pulsed ESR technique of two-dimensional Electron-Electron Double Resonance (2D ELDOR) are described. This is a three-pulse technique akin to 2D Exchange Nuclear Magnetic Resonance, but involving electron spins, usually in the form of spin-probes or spin-labels. As a result, it required the extension to much higher frequencies, i.e., microwaves, and much faster time scales, with π/2 pulses in the 2-3 ns range. It has proven very useful for studying molecular dynamics in complex fluids, and spectral results can be explained by fitting theoretical models (also described) that provide a detailed analysis of the molecular dynamics and structure. We discuss concepts that also appear in other forms of 2D spectroscopy but emphasize the unique advantages and difficulties that are intrinsic to ESR. Advantages include the ability to tune the resonance frequency, in order to probe different motional ranges, while challenges include the high ratio of the detection dead time vs. the relaxation times. We review several important 2D ELDOR studies of molecular dynamics. (1) The results from a spin probe dissolved in a liquid crystal are followed throughout the isotropic → nematic → liquid-like smectic → solid-like smectic → crystalline phases as the temperature is reduced and are interpreted in terms of the slowly relaxing local structure model. Here, the labeled molecule is undergoing overall motion in the macroscopically aligned sample, as well as responding to local site fluctuations. (2) Several examples involving model phospholipid membranes are provided, including the dynamic structural characterization of the boundary lipid that coats a transmembrane peptide dimer. Additionally, subtle differences can be elicited for the phospholipid membrane phases: liquid disordered, liquid ordered, and gel, and the subtle effects upon the membrane, of antigen cross-linking of receptors on the surface of plasma membrane
Franck, John M; Chandrasekaran, Siddarth; Dzikovski, Boris; Dunnam, Curt R; Freed, Jack H
2015-06-07
The development, applications, and current challenges of the pulsed ESR technique of two-dimensional Electron-Electron Double Resonance (2D ELDOR) are described. This is a three-pulse technique akin to 2D Exchange Nuclear Magnetic Resonance, but involving electron spins, usually in the form of spin-probes or spin-labels. As a result, it required the extension to much higher frequencies, i.e., microwaves, and much faster time scales, with π/2 pulses in the 2-3 ns range. It has proven very useful for studying molecular dynamics in complex fluids, and spectral results can be explained by fitting theoretical models (also described) that provide a detailed analysis of the molecular dynamics and structure. We discuss concepts that also appear in other forms of 2D spectroscopy but emphasize the unique advantages and difficulties that are intrinsic to ESR. Advantages include the ability to tune the resonance frequency, in order to probe different motional ranges, while challenges include the high ratio of the detection dead time vs. the relaxation times. We review several important 2D ELDOR studies of molecular dynamics. (1) The results from a spin probe dissolved in a liquid crystal are followed throughout the isotropic → nematic → liquid-like smectic → solid-like smectic → crystalline phases as the temperature is reduced and are interpreted in terms of the slowly relaxing local structure model. Here, the labeled molecule is undergoing overall motion in the macroscopically aligned sample, as well as responding to local site fluctuations. (2) Several examples involving model phospholipid membranes are provided, including the dynamic structural characterization of the boundary lipid that coats a transmembrane peptide dimer. Additionally, subtle differences can be elicited for the phospholipid membrane phases: liquid disordered, liquid ordered, and gel, and the subtle effects upon the membrane, of antigen cross-linking of receptors on the surface of plasma membrane
Franck, John M.; Chandrasekaran, Siddarth; Dzikovski, Boris; Dunnam, Curt R.; Freed, Jack H.
2015-06-07
The development, applications, and current challenges of the pulsed ESR technique of two-dimensional Electron-Electron Double Resonance (2D ELDOR) are described. This is a three-pulse technique akin to 2D Exchange Nuclear Magnetic Resonance, but involving electron spins, usually in the form of spin-probes or spin-labels. As a result, it required the extension to much higher frequencies, i.e., microwaves, and much faster time scales, with π/2 pulses in the 2-3 ns range. It has proven very useful for studying molecular dynamics in complex fluids, and spectral results can be explained by fitting theoretical models (also described) that provide a detailed analysis of the molecular dynamics and structure. We discuss concepts that also appear in other forms of 2D spectroscopy but emphasize the unique advantages and difficulties that are intrinsic to ESR. Advantages include the ability to tune the resonance frequency, in order to probe different motional ranges, while challenges include the high ratio of the detection dead time vs. the relaxation times. We review several important 2D ELDOR studies of molecular dynamics. (1) The results from a spin probe dissolved in a liquid crystal are followed throughout the isotropic → nematic → liquid-like smectic → solid-like smectic → crystalline phases as the temperature is reduced and are interpreted in terms of the slowly relaxing local structure model. Here, the labeled molecule is undergoing overall motion in the macroscopically aligned sample, as well as responding to local site fluctuations. (2) Several examples involving model phospholipid membranes are provided, including the dynamic structural characterization of the boundary lipid that coats a transmembrane peptide dimer. Additionally, subtle differences can be elicited for the phospholipid membrane phases: liquid disordered, liquid ordered, and gel, and the subtle effects upon the membrane, of antigen cross-linking of receptors on the surface of plasma membrane
LaTiO₃/KTaO₃ interfaces: A new two-dimensional electron gas system
Zou, K.; Ismail-Beigi, Sohrab; Kisslinger, Kim; ...
2015-03-01
We report a new 2D electron gas (2DEG) system at the interface between a Mott insulator, LaTiO₃, and a band insulator, KTaO₃. For LaTiO₃/KTaO₃ interfaces, we observe metallic conduction from 2 K to 300 K. One serious technological limitation of SrTiO₃-based conducting oxide interfaces for electronics applications is the relatively low carrier mobility (0.5-10 cm²/V s) of SrTiO₃ at room temperature. By using KTaO₃, we achieve mobilities in LaTiO₃/KTaO₃ interfaces as high as 21 cm²/V s at room temperature, over a factor of 3 higher than observed in doped bulk SrTiO₃. By density functional theory, we attribute the higher mobilitymore » in KTaO₃ 2DEGs to the smaller effective mass for electrons in KTaO₃.« less
LaTiO₃/KTaO₃ interfaces: A new two-dimensional electron gas system
Zou, K.; Ismail-Beigi, Sohrab; Kisslinger, Kim; Shen, Xuan; Su, Dong; Walker, F. J.; Ahn, C. H.
2015-03-01
We report a new 2D electron gas (2DEG) system at the interface between a Mott insulator, LaTiO₃, and a band insulator, KTaO₃. For LaTiO₃/KTaO₃ interfaces, we observe metallic conduction from 2 K to 300 K. One serious technological limitation of SrTiO₃-based conducting oxide interfaces for electronics applications is the relatively low carrier mobility (0.5-10 cm²/V s) of SrTiO₃ at room temperature. By using KTaO₃, we achieve mobilities in LaTiO₃/KTaO₃ interfaces as high as 21 cm²/V s at room temperature, over a factor of 3 higher than observed in doped bulk SrTiO₃. By density functional theory, we attribute the higher mobility in KTaO₃ 2DEGs to the smaller effective mass for electrons in KTaO₃.
NASA Astrophysics Data System (ADS)
Matsumoto, Yosuke; Amano, Takanobu; Hoshino, Masahiro
2012-08-01
Electron accelerations at high Mach number collisionless shocks are investigated by means of two-dimensional electromagnetic particle-in-cell simulations with various Alfvén Mach numbers, ion-to-electron mass ratios, and the upstream electron β e (the ratio of the thermal pressure to the magnetic pressure). We find electrons are effectively accelerated at a super-high Mach number shock (MA ~ 30) with a mass ratio of M/m = 100 and β e = 0.5. The electron shock surfing acceleration is an effective mechanism for accelerating the particles toward the relativistic regime even in two dimensions with a large mass ratio. Buneman instability excited at the leading edge of the foot in the super-high Mach number shock results in a coherent electrostatic potential structure. While multi-dimensionality allows the electrons to escape from the trapping region, they can interact with the strong electrostatic field several times. Simulation runs in various parameter regimes indicate that the electron shock surfing acceleration is an effective mechanism for producing relativistic particles in extremely high Mach number shocks in supernova remnants, provided that the upstream electron temperature is reasonably low.
NASA Astrophysics Data System (ADS)
Matsumoto, Y.; Amano, T.; Hoshino, M.
2012-12-01
Electron accelerations at high Mach number collision-less shocks are investigated by means of two-dimensional electromagnetic Particle-in-Cell simulations with various Alfven Mach numbers, ion-to-electron mass ratios, and the upstream electron βe (the ratio of the thermal pressure to the magnetic pressure). We found electrons are effectively accelerated at a super-high Mach number shock (MA ~ 30) with a mass ratio of M/m=100 and βe=0.5. The electron shock surfing acceleration is an effective mechanism for accelerating the particles toward the relativistic regime even in two dimensions with the large mass ratio. Buneman instability excited at the leading edge of the foot in the super-high Mach number shock results in a coherent electrostatic potential structure. While multi-dimensionality allows the electrons to escape from the trapping region, they can interact with the strong electrostatic field several times. Simulation runs in various parameter regimes indicate that the electron shock surfing acceleration is an effective mechanism for producing relativistic particles in extremely-high Mach number shocks in supernova remnants, provided that the upstream electron temperature is reasonably low. Matsumoto et al., Astrophys. J., 755, 109, 2012.
Matsumoto, Yosuke; Amano, Takanobu; Hoshino, Masahiro
2012-08-20
Electron accelerations at high Mach number collisionless shocks are investigated by means of two-dimensional electromagnetic particle-in-cell simulations with various Alfven Mach numbers, ion-to-electron mass ratios, and the upstream electron {beta}{sub e} (the ratio of the thermal pressure to the magnetic pressure). We find electrons are effectively accelerated at a super-high Mach number shock (M{sub A} {approx} 30) with a mass ratio of M/m = 100 and {beta}{sub e} = 0.5. The electron shock surfing acceleration is an effective mechanism for accelerating the particles toward the relativistic regime even in two dimensions with a large mass ratio. Buneman instability excited at the leading edge of the foot in the super-high Mach number shock results in a coherent electrostatic potential structure. While multi-dimensionality allows the electrons to escape from the trapping region, they can interact with the strong electrostatic field several times. Simulation runs in various parameter regimes indicate that the electron shock surfing acceleration is an effective mechanism for producing relativistic particles in extremely high Mach number shocks in supernova remnants, provided that the upstream electron temperature is reasonably low.
High resolution electron energy loss spectroscopy with two-dimensional energy and momentum mapping.
Zhu, Xuetao; Cao, Yanwei; Zhang, Shuyuan; Jia, Xun; Guo, Qinlin; Yang, Fang; Zhu, Linfan; Zhang, Jiandi; Plummer, E W; Guo, Jiandong
2015-08-01
High resolution electron energy loss spectroscopy (HREELS) is a powerful technique to probe vibrational and electronic excitations at surfaces. The dispersion relation of surface excitations, i.e., energy as a function of momentum, has in the past, been obtained by measuring the energy loss at a fixed angle (momentum) and then rotating sample, monochromator, or analyzer. Here, we introduce a new strategy for HREELS, utilizing a specially designed lens system with a double-cylindrical Ibach-type monochromator combined with a commercial VG Scienta hemispherical electron energy analyzer, which can simultaneously measure the energy and momentum of the scattered electrons. The new system possesses high angular resolution (<0.1°), detecting efficiency and sampling density. The capabilities of this system are demonstrated using Bi2Sr2CaCu2O(8+δ). The time required to obtain a complete dispersion spectrum is at least one order of magnitude shorter than conventional spectrometers, with improved momentum resolution and no loss in energy resolution.
High resolution electron energy loss spectroscopy with two-dimensional energy and momentum mapping
Zhu, Xuetao; Cao, Yanwei; Zhang, Shuyuan; Jia, Xun; Guo, Qinlin; Yang, Fang; Zhu, Linfan; Zhang, Jiandi; Plummer, E. W.; Guo, Jiandong
2015-08-15
High resolution electron energy loss spectroscopy (HREELS) is a powerful technique to probe vibrational and electronic excitations at surfaces. The dispersion relation of surface excitations, i.e., energy as a function of momentum, has in the past, been obtained by measuring the energy loss at a fixed angle (momentum) and then rotating sample, monochromator, or analyzer. Here, we introduce a new strategy for HREELS, utilizing a specially designed lens system with a double-cylindrical Ibach-type monochromator combined with a commercial VG Scienta hemispherical electron energy analyzer, which can simultaneously measure the energy and momentum of the scattered electrons. The new system possesses high angular resolution (<0.1°), detecting efficiency and sampling density. The capabilities of this system are demonstrated using Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+δ}. The time required to obtain a complete dispersion spectrum is at least one order of magnitude shorter than conventional spectrometers, with improved momentum resolution and no loss in energy resolution.
NASA Astrophysics Data System (ADS)
Regeta, K.; Allan, M.
2015-05-01
Detailed experimental information on the motion of a nuclear packet on a complex (resonant) anion potential surface is obtained by measuring 2-dimensional (2D) electron energy loss spectra. The cross section is plotted as a function of incident electron energy, which determines which resonant anion state is populated, i.e., along which normal coordinate the wave packet is launched, and of the electron energy loss, which reveals into which final states each specific resonant state decays. The 2D spectra are presented for acrylonitrile and methacrylonitrile, at the incident energy range 0.095-1.0 eV, where the incoming electron is temporarily captured in the lowest π∗ orbital. The 2D spectra reveal selectivity patterns with respect to which vibrations are excited in the attachment and de-excited in the detachment. Further insight is gained by recording 1D spectra measured along horizontal, vertical, and diagonal cuts of the 2D spectrum. The methyl group in methacrylonitrile increases the resonance width 7 times. This converts the sharp resonances of acrylonitrile into boomerang structures but preserves the essence of the selectivity patterns. Selectivity of vibrational excitation by higher-lying shape resonances up to 8 eV is also reported.
Regeta, K. Allan, M.
2015-05-14
Detailed experimental information on the motion of a nuclear packet on a complex (resonant) anion potential surface is obtained by measuring 2-dimensional (2D) electron energy loss spectra. The cross section is plotted as a function of incident electron energy, which determines which resonant anion state is populated, i.e., along which normal coordinate the wave packet is launched, and of the electron energy loss, which reveals into which final states each specific resonant state decays. The 2D spectra are presented for acrylonitrile and methacrylonitrile, at the incident energy range 0.095-1.0 eV, where the incoming electron is temporarily captured in the lowest π{sup ∗} orbital. The 2D spectra reveal selectivity patterns with respect to which vibrations are excited in the attachment and de-excited in the detachment. Further insight is gained by recording 1D spectra measured along horizontal, vertical, and diagonal cuts of the 2D spectrum. The methyl group in methacrylonitrile increases the resonance width 7 times. This converts the sharp resonances of acrylonitrile into boomerang structures but preserves the essence of the selectivity patterns. Selectivity of vibrational excitation by higher-lying shape resonances up to 8 eV is also reported.
Tur, A.; Fruit, G.; Louarn, P.
2014-03-15
In the general context of understanding the possible destabilization of a current sheet with applications to magnetospheric substorms or solar flares, a kinetic model is proposed for studying the resonant interaction between electromagnetic fluctuations and trapped bouncing electrons in a 2D current sheet. Tur et al. [A. Tur et al., Phys. Plasmas 17, 102905 (2010)] and Fruit et al. [G. Fruit et al., Phys. Plasmas 20, 022113 (2013)] already used this model to investigate the possibilities of electrostatic instabilities. Here, the model is completed for full electromagnetic perturbations. Starting with a modified Harris sheet as equilibrium state, the linearized gyrokinetic Vlasov equation is solved for electromagnetic fluctuations with period of the order of the electron bounce period. The particle motion is restricted to its first Fourier component along the magnetic field and this allows the complete time integration of the non local perturbed distribution functions. The dispersion relation for electromagnetic modes is finally obtained through the quasineutrality condition and the Ampere's law for the current density. It is found that for mildly strechted current, undamped modes oscillate at typical electron bounce frequency with wavelength of the order of the plasma sheet half thickness. As the stretching of the plasma sheet becomes more intense, the frequency of these normal modes decreases and beyond a certain threshold in ε = B{sub z}/B{sub lobes}, the mode becomes explosive with typical growth rate of a few tens of seconds. The free energy contained in the bouncing motion of the electrons may trigger an electromagnetic instability able to disrupt the cross-tail current in a few seconds. This new instability–electromagnetic electron-bounce instability–may explain fast and global scale destabilization of current sheets as required to describe substorm phenomena.
Calandra, Matteo; Zoccante, Paolo; Mauri, Francesco
2015-02-20
In two-dimensional multivalley semiconductors, at low doping, even a moderate electron-electron interaction enhances the response to any perturbation inducing a valley polarization. If the valley polarization is due to the electron-phonon coupling, the electron-electron interaction results in an enhancement of the superconducting critical temperature. By performing first-principles calculations beyond density functional theory, we prove that this effect accounts for the unconventional doping dependence of the superconducting transition temperature (T(c)) and of the magnetic susceptibility measured in Li(x)ZrNCI. Finally, we discuss what are the conditions for a maximal T(c) enhancement in weakly doped two-dimensional semiconductors.
Lateral quantization of two-dimensional electron states by embedded Ag nanocrystals
NASA Astrophysics Data System (ADS)
Van Haesendonck, Chris; Schouteden, Koen
2013-03-01
We show that quantization of image-potential state (IS)electrons above the surface of nanostructures can be experimentally achieved by Ag nanocrystals that appear as stacking fault tetrahedrons (SFTs) at Ag(111) surfaces. By means of cryogenic scanning tunneling spectroscopy the n = 1 IS of the Ag(111) surface is revealed to split up in discrete energy levels, which is accompanied by the formation of pronounced standing wave patterns that directly reflect the eigenstates of the SFT surface. The IS confinement behavior is compared to that of the surface state electrons in the SFT surface and can be directly linked to the particle-in-a-box model. ISs provide a novel playground for investigating quantum size effects and defect induced scattering above nanostructured surfaces. This work has been supported by the Research Foundation - Flanders (FWO, Belgium). K.S. is a postdoctoral researcher of the FWO.
New two-dimensional boron nitride allotropes with attractive electronic and optical properties
NASA Astrophysics Data System (ADS)
Shahrokhi, Masoud; Mortazavi, Bohayra; Berdiyorov, Golibjon R.
2017-03-01
Using first principles calculations, structural, electronic and optical properties of five new 2D boron nitride (BN) allotropes have been studied. The results exhibit that the cohesive energy for all these five new allotrope is positive such as all these systems are stable; therefore, it is possible to synthesize these structures in experiments. It is found that the band gap of all new 2D BN allotropes is smaller than the h-BN sheet. In our calculations the dielectric tensor is derived within the random phase approximation (RPA). Specifically, the dielectric function, refraction index and the loss function, of the 2D BN allotropes are calculated for both parallel and perpendicular electric field polarizations. The results show that the optical spectra are anisotropic along these two polarizations. The results obtained from our calculations are beneficial to practical applications of these 2D BN allotropes in optoelectronics and electronics.
Spin-susceptibility of the Two-Dimensional Electron Gas: effect of weak disorder
NASA Astrophysics Data System (ADS)
Gaetano, Senatore; Palo Stefania, De; Michela, Botti; Saverio, Moroni
2004-03-01
Recent measurements[1] on a 2DEG in a GaAs HIGHFET yield a spin susceptibility that is as much as 30% lower than the most accurate theoretical prediction[2] for a one-valley, strictly 2D, clean electron gas. We therefore consider the effect of a model random disorder, in the weak regime, employing the response function formalism. We find that the weak disorder yields an enhancement (depression) of the magnetic response of interacting (noninteracting) electrons in 2D. The implications of such a result are briefly discussed. [1] J.Zhu, H. L. Stormer, L. N. Pfeiffer,K. W. Baldwin, and K. W. West, Phys. Rev. Lett. 90, 056805 (2003) [2]C. Attaccalite, S. Moroni, P. Gori-Giorgi, and G. B. Bachelet, Phys. Rev. Lett. 88, 256601 (2002
Lateral quantization of two-dimensional electron states by embedded Ag nanocrystals.
Schouteden, K; Van Haesendonck, C
2012-02-17
We show that quantization of image-potential state (IS) electrons above the surface of nanostructures can be experimentally achieved by Ag nanocrystals that appear as stacking-fault tetrahedrons (SFTs) at Ag(111) surfaces. By means of cryogenic scanning tunneling spectroscopy, the n=1 IS of the Ag(111) surface is revealed to split up in discrete energy levels, which is accompanied by the formation of pronounced standing wave patterns that directly reflect the eigenstates of the SFT surface. The IS confinement behavior is compared to that of the surface state electrons in the SFT surface and can be directly linked to the particle-in-a-box model. ISs provide a novel playground for investigating quantum size effects and defect-induced scattering above nanostructured surfaces.
Design of transparent conductors and periodic two-dimensional electron gases without doping
NASA Astrophysics Data System (ADS)
Zhang, Xiuwen; Zhang, Lijun; Zunger, Alex; Perkins, John; Materials by Design Team; John D. Perkins Collaboration
The functionality of transparency plus conductivity plays an important role in renewable energy and information technologies, including applications such as solar cells, touch-screen sensors, and flat panel display. However, materials with such seemingly contraindicated properties are difficult to come by. The traditional strategy for designing bulk transparent conductors (TCs) starts from a wide-gap insulator and finds ways to make it conductive by extensive doping. We propose a different strategy for TC design--starting with a metallic conductor and designing transparency by control of intrinsic interband transitions and intraband plasmonic frequency. We identified specific design principles for prototypical intrinsic TC classes and searched computationally for materials that satisfy them. The electron gases in the 3D intrinsic TCs demonstrate intriguing properties, such as periodic 2D electron gas regions with very high carrier density. We will discuss a more extended search of these functionalities, in parallel with stability and growability calculations
Surface Acoustic-Wave-Induced Magnetoresistance Oscillations in a Two-Dimensional Electron Gas
NASA Astrophysics Data System (ADS)
Robinson, John P.; Kennett, Malcolm P.; Cooper, Nigel R.; Fal'Ko, Vladimir I.
2004-07-01
We study the geometrical commensurability oscillations imposed on the resistivity of 2D electrons in a perpendicular magnetic field by a propagating surface acoustic wave (SAW). We show that, for ω<ωc, this effect contains an anisotropic dynamical classical contribution increasing the resistivity and a nonequilibrium quantum contribution isotropically decreasing the resistivity, and we predict zero-resistance states associated with geometrical commensurability at large SAW amplitude.
Deep Learning the Quantum Phase Transitions in Random Two-Dimensional Electron Systems
NASA Astrophysics Data System (ADS)
Ohtsuki, Tomoki; Ohtsuki, Tomi
2016-12-01
Random electron systems show rich phases such as Anderson insulator, diffusive metal, quantum Hall and quantum anomalous Hall insulators, Weyl semimetal, as well as strong/weak topological insulators. Eigenfunctions of each matter phase have specific features, but owing to the random nature of systems, determining the matter phase from eigenfunctions is difficult. Here, we propose the deep learning algorithm to capture the features of eigenfunctions. Localization-delocalization transition, as well as disordered Chern insulator-Anderson insulator transition, is discussed.
Shukla, Chandrasekhar; Das, Amita; Patel, Kartik
2015-11-15
Relativistic electron beam propagation in plasma is fraught with several micro instabilities like two stream, filamentation, etc., in plasma. This results in severe limitation of the electron transport through a plasma medium. Recently, however, there has been an experimental demonstration of improved transport of Mega Ampere of electron currents (generated by the interaction of intense laser with solid target) in a carbon nanotube structured solid target [G. Chatterjee et al., Phys. Rev. Lett. 108, 235005 (2012)]. This then suggests that the inhomogeneous plasma (created by the ionization of carbon nanotube structured target) helps in containing the growth of the beam plasma instabilities. This manuscript addresses this issue with the help of a detailed analytical study and 2-D Particle-In-Cell simulations. The study conclusively demonstrates that the growth rate of the dominant instability in the 2-D geometry decreases when the plasma density is chosen to be inhomogeneous, provided the scale length 1/k{sub s} of the inhomogeneous plasma is less than the typical plasma skin depth (c/ω{sub 0}) scale. At such small scale lengths channelization of currents is also observed in simulation.
Quenching Plasma Waves in Two Dimensional Electron Gas by a Femtosecond Laser Pulse
NASA Astrophysics Data System (ADS)
Shur, Michael; Rudin, Sergey; Greg Rupper Collaboration; Andrey Muraviev Collaboration
Plasmonic detectors of terahertz (THz) radiation using the plasma wave excitation in 2D electron gas are capable of detecting ultra short THz pulses. To study the plasma wave propagation and decay, we used femtosecond laser pulses to quench the plasma waves excited by a short THz pulse. The femtosecond laser pulse generates a large concentration of the electron-hole pairs effectively shorting the 2D electron gas channel and dramatically increasing the channel conductance. Immediately after the application of the femtosecond laser pulse, the equivalent circuit of the device reduces to the source and drain contact resistances connected by a short. The total response charge is equal to the integral of the current induced by the THz pulse from the moment of the THz pulse application to the moment of the femtosecond laser pulse application. This current is determined by the plasma wave rectification. Registering the charge as a function of the time delay between the THz and laser pulses allowed us to follow the plasmonic wave decay. We observed the decaying oscillations in a sample with a partially gated channel. The decay depends on the gate bias and reflects the interplay between the gated and ungated plasmons in the device channel. Army Research Office.
Enhanced thermopower in ZnO two-dimensional electron gas
Shimizu, Sunao; Bahramy, Mohammad Saeed; Iizuka, Takahiko; Ono, Shimpei; Miwa, Kazumoto; Tokura, Yoshinori; Iwasa, Yoshihiro
2016-01-01
Control of dimensionality has proven to be an effective way to manipulate the electronic properties of materials, thereby enabling exotic quantum phenomena, such as superconductivity, quantum Hall effects, and valleytronic effects. Another example is thermoelectricity, which has been theoretically proposed to be favorably controllable by reducing the dimensionality. Here, we verify this proposal by performing a systematic study on a gate-tuned 2D electron gas (2DEG) system formed at the surface of ZnO. Combining state-of-the-art electric-double-layer transistor experiments and realistic tight-binding calculations, we show that, for a wide range of carrier densities, the 2DEG channel comprises a single subband, and its effective thickness can be reduced to ∼ 1 nm at sufficiently high gate biases. We also demonstrate that the thermoelectric performance of the 2DEG region is significantly higher than that of bulk ZnO. Our approach opens up a route to exploit the peculiar behavior of 2DEG electronic states and realize thermoelectric devices with advanced functionalities. PMID:27222585
Enhanced thermopower in ZnO two-dimensional electron gas.
Shimizu, Sunao; Bahramy, Mohammad Saeed; Iizuka, Takahiko; Ono, Shimpei; Miwa, Kazumoto; Tokura, Yoshinori; Iwasa, Yoshihiro
2016-06-07
Control of dimensionality has proven to be an effective way to manipulate the electronic properties of materials, thereby enabling exotic quantum phenomena, such as superconductivity, quantum Hall effects, and valleytronic effects. Another example is thermoelectricity, which has been theoretically proposed to be favorably controllable by reducing the dimensionality. Here, we verify this proposal by performing a systematic study on a gate-tuned 2D electron gas (2DEG) system formed at the surface of ZnO. Combining state-of-the-art electric-double-layer transistor experiments and realistic tight-binding calculations, we show that, for a wide range of carrier densities, the 2DEG channel comprises a single subband, and its effective thickness can be reduced to [Formula: see text] 1 nm at sufficiently high gate biases. We also demonstrate that the thermoelectric performance of the 2DEG region is significantly higher than that of bulk ZnO. Our approach opens up a route to exploit the peculiar behavior of 2DEG electronic states and realize thermoelectric devices with advanced functionalities.
Transport-Coefficient Dependence of Current-Induced Cooling Effect in a Two-Dimensional Electron Gas
NASA Astrophysics Data System (ADS)
Hirayama, Naomi; Endo, Akira; Fujita, Kazuhiro; Hasegawa, Yasuhiro; Hatano, Naomichi; Nakamura, Hiroaki; Shirasaki, Ryōen; Yonemitsu, Kenji
2012-06-01
The dependence of the current-induced cooling effect on the electron mobility μ e is explored for a two-dimensional electron gas (2DEG) subjected to a perpendicular magnetic field. We calculate the distributions of the electrochemical potentials and the temperatures under a magnetic field, fully taking account of thermoelectric and thermomagnetic phenomena. Whereas the electrochemical potential and the electric current remain qualitatively unchanged, the temperature distribution exhibits drastic mobility dependence. The lower-mobility system has cold and hot areas at opposite corners, which results from the heat current brought about by the Ettingshausen effect in the vicinity of the adiabatic boundaries. The cooling effect is intensified by an increase in μ e. Intriguingly, the cold and hot areas change places with each other as the mobility μ e is further increased. This is because the heating current on the adiabatic edges due to the Righi-Leduc effect exceeds that due to the Ettingshausen effect in the opposite direction.
Zhukov, Alexander V. Bouffanais, Roland; Fedorov, E. G.; Belonenko, Mikhail B.
2014-05-28
Propagation of ultrashort laser pulses through various nano-objects has recently became an attractive topic for both theoretical and experimental studies due to its promising perspectives in a variety of problems of modern nanoelectronics. Here, we study the propagation of extremely short two-dimensional bipolar electromagnetic pulses in a heterogeneous array of semiconductor carbon nanotubes. Heterogeneity is defined as a region of enhanced electron density. The electromagnetic field in an array of nanotubes is described by Maxwell's equations, reduced to a multidimensional wave equation. Our numerical analysis shows the possibility of stable propagation of an electromagnetic pulse in a heterogeneous array of nanotubes. Furthermore, we establish that, depending on its speed of propagation, the pulse can pass through the area of increased electron concentration or be reflected therefrom.
NASA Astrophysics Data System (ADS)
Mauchamp, Vincent; Bugnet, Matthieu; Bellido, Edson P.; Botton, Gianluigi A.; Moreau, Philippe; Magne, Damien; Naguib, Michael; Cabioc'h, Thierry; Barsoum, Michel W.
2014-06-01
The dielectric response of two-dimensional (2D) Ti3C2 stacked sheets was investigated by high-resolution transmission electron energy-loss spectroscopy and ab initio calculations in the 0.2-30-eV energy range. Intense surface plasmons (SPs), evidenced at the nanometer scale at energies as low as 0.3 eV, are shown to be the dominant screening process up to at least 45-nm-thick stacks. This domination results from a combination of efficient free-electron dynamics, begrenzungs effect, and reduced interband damping. It is shown that, in principle, the SPs energies can be tuned in the mid-infrared, from 0.2 to 0.7 eV, by controlling the sheets' functionalization and/or thickness. This point evidences a new attribute of this new class of 2D materials.
NASA Astrophysics Data System (ADS)
Drichko, I. L.; Smirnov, I. Yu.; Suslov, A. V.; Galperin, Y. M.; Pfeiffer, L. N.; West, K. W.
2017-01-01
We review our work on high-frequency conductance in two-dimensional high-mobility electronic systems in wide n-AlGaAs/GaAs/AlGaAs quantum wells. Using simultaneous measurements of the attenuation and velocity of a surface acoustic wave we obtained both real and imaginary components of the complex high-frequency conductance. Based on the experimental results and their analysis we conclude that close to the filling factor ν = 1/5, as well as in the interval 0.18 > ν > 0.125, a Wigner crystal pinned by disorder is formed. Both the melting temperature and the correlation length of the pinning-induced domains in the Wigner crystal were found. In close vicinities of ν = 1 and 2, transitions from single-electron localization to a Wigner crystal were observed.
NASA Astrophysics Data System (ADS)
Chang, Yuan-Ming; Lin, Che-Yi; Lin, Yen-Fu; Tsukagoshi, Kazuhito
2016-11-01
We present a review of recent developments in the synthesis, thickness identification, electronic properties, and possible applications of layered MoTe2 flakes. Special emphasis is made on two-dimensional (2D) MoTe2 semiconductors and the extensive research in recent years on their applications in electronics. Layered MoTe2 flakes have been the focus of substantial interest in the research community because of their fascinating characteristics, including an appropriate band gap and a simple fabrication method (exfoliation) to form layered nanomaterials. Our aim is to provide the readers an overview of layered MoTe2 flakes and to understand their properties, which may lead to their applications in micro- and nanoelectronics.
ZUDOV,M.A.; PONOMAREV,I.V.; EFROS,A.L.; DU,R.R.; SIMMONS,JERRY A.; RENO,JOHN L.
2000-05-11
The authors report a new type of oscillations in magnetoresistance observed in high-mobility two-dimensional electron gas (2DEG), in GaAs-AIGaAs heterostructures. Being periodic in 1/B these oscillations appear in weak magnetic field (B < 0.3 T) and only in a narrow temperature range (3 K < T < 7 K). Remarkably, these oscillations can be understood in terms of magneto-phonon resonance originating from the interaction of 2DEG and leaky interface-acoustic phonon modes. The existence of such modes on the GaAs:AIGaAs interface is demonstrated theoretically and their velocities are calculated. It is shown that the electron-phonon scattering matrix element exhibits a peak for the phonons carrying momentum q = 2k{sub F} (k{sub F} is the Fermi wave-vector of 2DEG).
Rancova, Olga; Abramavicius, Darius; Jankowiak, Ryszard
2015-06-07
Two-dimensional (2D) electronic spectroscopy at cryogenic and room temperatures reveals excitation energy relaxation and transport, as well as vibrational dynamics, in molecular systems. These phenomena are related to the spectral densities of nuclear degrees of freedom, which are directly accessible by means of hole burning and fluorescence line narrowing approaches at low temperatures (few K). The 2D spectroscopy, in principle, should reveal more details about the fluctuating environment than the 1D approaches due to peak extension into extra dimension. By studying the spectral line shapes of a dimeric aggregate at low temperature, we demonstrate that 2D spectra have the potential to reveal the fluctuation spectral densities for different electronic states, the interstate correlation of static disorder and, finally, the time scales of spectral diffusion with high resolution.
Monisha, P. J.; Sankar, I. V.; Sil, Shreekantha; Chatterjee, Ashok
2016-01-01
Persistent current in a correlated quantum ring threaded by an Aharonov-Bohm flux is studied in the presence of electron-phonon interactions and Rashba spin-orbit coupling. The quantum ring is modeled by the Holstein-Hubbard-Rashba Hamiltonian and the energy is calculated by performing the conventional Lang-Firsov transformation followed by the diagonalization of the effective Hamiltonian within a mean-field approximation. The effects of Aharonov-Bohm flux, temperature, spin-orbit and electron-phonon interactions on the persistent current are investigated. It is shown that the electron-phonon interactions reduce the persistent current, while the Rashba coupling enhances it. It is also shown that temperature smoothens the persistent current curve. The effect of chemical potential on the persistent current is also studied. PMID:26831831
Monisha, P J; Sankar, I V; Sil, Shreekantha; Chatterjee, Ashok
2016-02-01
Persistent current in a correlated quantum ring threaded by an Aharonov-Bohm flux is studied in the presence of electron-phonon interactions and Rashba spin-orbit coupling. The quantum ring is modeled by the Holstein-Hubbard-Rashba Hamiltonian and the energy is calculated by performing the conventional Lang-Firsov transformation followed by the diagonalization of the effective Hamiltonian within a mean-field approximation. The effects of Aharonov-Bohm flux, temperature, spin-orbit and electron-phonon interactions on the persistent current are investigated. It is shown that the electron-phonon interactions reduce the persistent current, while the Rashba coupling enhances it. It is also shown that temperature smoothens the persistent current curve. The effect of chemical potential on the persistent current is also studied.
Electron in a Two Dimensional System with Point Scatterers Amd Magnetic Field
NASA Astrophysics Data System (ADS)
Gredeskul, S. A.; Avishai, Y.; Azbel, M. Ya.; Zusman, M.
We consider non-interacting electrons in two dimensions subject to a perpendicular magnetic field and interacting with point impurities with random strength located on the sites of a square lattice. For very strong magnetic field (such that there are more than n+1 flux quanta per plaque), we obtain (in closed analytic form) extended solutions, which are independent on the strength of disorder, for each Landau level with a number smaller or equal to n. If the rational magnetic flux per plaque is less than one unit, we construct dispersion laws and Hofstadter-like butterfly for all denominators less than ten.
Two-dimensional boron based nanomaterials: electronic, vibrational, Raman, and STM signatures
NASA Astrophysics Data System (ADS)
Massote, Daniel V. P.; Liang, Liangbo; Kharche, Neerav; Meunier, Vincent
Because boron has only three electrons on its outer shell, planar mono-elemental boron nanostructures are expected to be much more challenging to assemble than their carbon counterparts. Several studies proposed schemes in which boron is stabilized to form flat semiconducting sheets consisting of a hexagonal lattice of boron atoms with partial hexagon filling (PRL 99 115501, ACSNano 6 7443-7453) . Other structures were proposed based on results from an evolutionary algorithm (PRL 112 085502). These structures are metallic and one even features a distorted Dirac cone near the Fermi level. Experimental evidence for 2D boron is still lacking but the recently proposed molecular synthesis of a flat all-boron molecule is a promising route to achieve this goal (Nat.Comms. 5 3113). Our research aims at providing a first-principles based description of these materials' properties to help in their identification. DFT is used to calculate phonon dispersion and associated Raman scattering spectra. We report some marked discrepancy between our findings and results from the recent literature and address the deviation using two methods for phonon dispersion. We also simulated STM images at various bias potentials to reveal the electronic symmetry of each material.
Two-dimensional crystal CuS—electronic and structural properties
NASA Astrophysics Data System (ADS)
Soares, Antonio L., Jr.; Dos Santos, Egon C.; Morales-García, A.; Heine, Thomas; De Abreu, Heitor A.; Duarte, Hélio A.
2017-03-01
Covellite is a metallic layered mineral with rather strong interlayer interaction. Recently, synthesis of covellite nanosheets of 3.2 nm thickness was reported (Du et al 2012 Nat. Commun. 3 1177), which raises the question: ‘What is the thinnest possible covellite nanosheet?’ Based on density functional/plane waves calculations, we have shown that graphene-like structure CuS (1L-CuS) is unstable but can be stabilized on a support. Here, however, we demonstrate that the three layered CuS (3L-CuS) with thickness of 0.773 nm (including the atomic radius of the outer plans atoms) is predicted to be intrinsically stable, as confirmed by phonon analysis and Born-Oppenheimer molecular dynamics simulations, with 3L-CuS about 0.15 eV per CuS less stable than the bulk. Interestingly, the electronic band structure shows metallic character with four bands crossing the Fermi level. The nature of chemical bonding is confirmed by a detailed topological analysis of the electron density.
Ma, H. J. Harsan E-mail: ariando@nus.edu.sg; Zeng, S. W.; Annadi, A.; Ariando E-mail: ariando@nus.edu.sg; Huang, Z.; Venkatesan, T.
2015-08-15
The two-dimensional electron gas (2DEG) formed at the perovskite oxides heterostructures is of great interest because of its potential applications in oxides electronics and nanoscale multifunctional devices. A canonical example is the 2DEG at the interface between a polar oxide LaAlO{sub 3} (LAO) and non-polar SrTiO{sub 3} (STO). Here, the LAO polar oxide can be regarded as the modulating or doping layer and is expected to define the electronic properties of 2DEG at the LAO/STO interface. However, to practically implement the 2DEG in electronics and device design, desired properties such as tunable 2D carrier density are necessary. Here, we report the tuning of conductivity threshold, carrier density and electronic properties of 2DEG in LAO/STO heterostructures by insertion of a La{sub 0.5}Sr{sub 0.5}TiO{sub 3} (LSTO) layer of varying thicknesses, and thus modulating the amount of polarization of the oxide over layers. Our experimental result shows an enhancement of carrier density up to a value of about five times higher than that observed at the LAO/STO interface. A complete thickness dependent metal-insulator phase diagram is obtained by varying the thickness of LAO and LSTO providing an estimate for the critical thickness needed for the metallic phase. The observations are discussed in terms of electronic reconstruction induced by polar oxides.
NASA Astrophysics Data System (ADS)
Dubinskii, Alexander A.; Maresch, Günter G.; Spiess, Hans-Wolfgang
1994-02-01
The combination of concepts of two-dimensional (2D) spectroscopy with the well-known field step electron-electron double resonance (ELDOR) method offers a practical route to recording 2D ELDOR spectra covering the full spectral range needed for electron paramagnetic resonance (EPR) of nitroxide spin labels in the solid state. The 2D ELDOR pattern provides information about molecular reorientation measured in real time, the anisotropies of electron phase, and electron spin-lattice relaxation as well as nuclear spin-lattice relaxation all of which are connected with the detailed geometry of the molecular reorientation. Thus, in 2D ELDOR the same electron spin probes the motional behavior over a wide range of correlation times from 10-4 to 10-12 s. An efficient algorithm for simulating 2D ELDOR spectra is derived, based on analytical solutions of the spin relaxation behavior for small-angle fluctuations and offers a means of quantitatively analyzing experimental data. As an example, the motion of nitroxide spin labels in a liquid-crystalline side-group polymer well below its glass transition is determined as a β-relaxation process with a mean angular amplitude of 5° and a distribution of correlation times with a mean correlation time of 0.9×10-10 s and a width of 2.5 decades.
NASA Astrophysics Data System (ADS)
Côté, R.; Bazier-Matte, Xavier
2016-11-01
The combined effect of a lateral square superlattice potential and the Coulomb interaction on the ground state of a two-dimensional electron gas in a perpendicular magnetic field is studied for different rational values of Γ , the inverse of the number of flux quanta per unit cell of the external potential, at filling factor ν =1 in Landau-level N =0 . When Landau-level mixing and disorder effects are neglected, increasing the strength W0 of the potential induces a transition at a critical strength of W0(c ) from a uniform and fully spin-polarized state to a two-dimensional charge density wave (CDW) with a meronlike spin texture at each maximum and minimum of the CDW. The collective excitations of this "vortex CDW" are similar to those of the Skyrme crystal that is expected to be the ground-state near filling factor ν =1 . In particular, a broken U (1 ) symmetry in the vortex CDW results in an extra gapless phase mode that could provide a fast channel for the relaxation of nuclear spins. The average spin-polarization Sz changes in a continuous or discontinuous manner as W0 is increased depending on whether Γ ∈[1 /2 ,1 ] or Γ ∈[0 ,1 /2 ] . The phase mode and the meronlike spin texture disappear at a large value of W0 leaving as the ground state a partially spin-polarized CDW if Γ ≠1 /2 or a spin-unpolarized CDW if Γ =1 /2 .
Seibt, Joachim; Pullerits, Tõnu
2014-09-21
While the theoretical description of population transfer subsequent to electronic excitation in combination with a line shape function description of vibrational dynamics in the context of 2D-spectroscopy is well-developed under the assumption of different timescales of population transfer and fluctuation dynamics, the treatment of the interplay between both kinds of processes lacks a comprehensive description. To bridge this gap, we use the cumulant expansion approach to derive response functions, which account for fluctuation dynamics and population transfer simultaneously. We compare 2D-spectra of a model system under different assumptions about correlations between fluctuations and point out under which conditions a simplified treatment is justified. Our study shows that population transfer and dissipative fluctuation dynamics cannot be described independent of each other in general. Advantages and limitations of the proposed calculation method and its compatibility with the modified Redfield description are discussed.
A high density two-dimensional electron gas in an oxide heterostructure on Si (001)
Jin, E. N.; Kornblum, L.; Kumah, D. P.; Zou, K.; Walker, F. J.; Broadbridge, C. C.; Ngai, J. H.; Ahn, C. H.
2014-11-01
We present the growth and characterization of layered heterostructures comprised of LaTiO{sub 3} and SrTiO{sub 3} epitaxially grown on Si (001). Magnetotransport measurements show that the sheet carrier densities of the heterostructures scale with the number of LaTiO{sub 3}/SrTiO{sub 3} interfaces, consistent with the presence of an interfacial 2-dimensional electron gas (2DEG) at each interface. Sheet carrier densities of 8.9 × 10{sup 14} cm{sup −2} per interface are observed. Integration of such high density oxide 2DEGs on silicon provides a bridge between the exceptional properties and functionalities of oxide 2DEGs and microelectronic technologies.
Spin-susceptibility of the Two-Dimensional Electron Gas: effect of the finite layer thickness
NASA Astrophysics Data System (ADS)
de Palo, Stefania; Gaetano, Senatore; Michela, Botti; Moroni, Saverio
2004-03-01
Recent measurements[1] on a 2DEG in a GaAs HIGHFET yield a spin susceptibility that is as much as 30most accurate theoretical prediction[2] for a one-valley, strictly 2D, clean electron gas (1V2DEG). We consider here the effect of the finite thickness of the 2DEG on the magnetic response, starting from the known energy and pair correlations of the 1V2DEG[2]. We find a substantial reduction of the discrepancies, with the theoretical prediction however still remaining above the experimental estimate. [1] J. Zhu, H. L. Stormer, L. N. Pfeiffer,K. W. Baldwin, and K.W. West, Phys. Rev. Lett. 90, 056805 (2003) [2]C. Attaccalite, S. Moroni, P. Gori-Giorgi, and G. B. Bachelet, Phys. Rev. Lett. 88, 256601 (2002)
Interaction-induced magnetoresistance in a two-dimensional electron gas
NASA Astrophysics Data System (ADS)
Gornyi, I. V.; Mirlin, A. D.
2004-04-01
We study the interaction-induced quantum correction δσαβ to the conductivity tensor of electrons in two dimensions for arbitrary Tτ (where T is the temperature and τ the transport scattering time), magnetic field, and type of disorder. A general theory is developed, allowing us to express δσαβ in terms of classical propagators (“ballistic diffusons”). The formalism is used to calculate the interaction contribution to the longitudinal and the Hall resistivities in a transverse magnetic field in the whole range of temperature from the diffusive ( Tτ≪1) to the ballistic ( Tτ≳1) regime, both in smooth disorder and in the presence of short-range scatterers.
NASA Astrophysics Data System (ADS)
Du, Xiang
As the sizes of individual components in electronic and optoelectronic devices approach nano scale, the performance of the devices is often determined by surface properties due to their large surface-to-volume ratio. Surface phenomena have become one of the cornerstones in nanoelectronic industry. For this reason, research on the surface functionalization has been tremendous amount of growth over the past decades, and promises to be an increasingly important field in the future. Surface functionalization, as an effective technique to modify the surface properties of a material through a physical or chemical approach, exhibits great potential to solve the problems and challenges, and modulate the performance of nanomaterials based functional devices. Surface functionalization drives the developments and applications of modern electronic and optoelectronic devices fabricated by nanomaterials. In this thesis, I demonstrate two surface functionalization approaches, namely, surface transfer doping and H2 annealing, to effectively solve the problems and significantly enhance the performance of 2D (single structure black phosphorus (BP) and heterostructure graphene/Si Schottky junction), and quasi-1D (molybdenum trioxide (MoO 3) nanobelt) nanomaterials based functional devices, respectively. In situ photoelectron spectroscopy (PES) measurements were also carried out to explore the interfacial charge transfer occurring at the interface between the nanostructures and doping layers, and the gap states in MoO 3 thin films, which provides the underlying mechanism to understand and support our device measurement results. In the first part of this thesis, I will discuss the first surface functionalization approach, namely, surface transfer doping, to effectively modulate the ambipolar characteristics of 2D few-layer BP flakes based FETs. The ambipolar characteristics of BP transistors were effectively modulated through in situ surface functionalization with cesium carbonate (Cs2
Li, Shuai; Qiu, Wen-Xuan; Gao, Jin-Hua
2016-07-07
Recently, a new kind of artificial two dimensional (2D) electron lattice on the nanoscale, i.e. molecular graphene, has drawn a lot of interest, where the metal surface electrons are transformed into a honeycomb lattice via absorbing a molecular lattice on the metal surface [Gomes et al., Nature, 2012, 438, 306; Wang et al., Phys. Rev. Lett., 2014, 113, 196803]. In this work, we theoretically demonstrate that this technique can be readily used to build other complex 2D electron lattices on a metal surface, which are of high interest in the field of condensed matter physics. The main challenge to build a complex 2D electron lattice is that this is a quantum antidot system, where the absorbed molecule normally exerts a repulsive potential on the surface electrons. Thus, there is no straightforward corresponding relation between the molecular lattice pattern and the desired 2D lattice of surface electrons. Here, we give an interesting example about the Kagome lattice, which has exotic correlated electronic states. We design a special molecular pattern and show that this molecular lattice can transform the surface electrons into a Kagome-like lattice. The numerical simulation is conducted using a Cu(111) surface and CO molecules. We first estimate the effective parameters of the Cu/CO system by fitting experimental data of the molecular graphene. Then, we calculate the corresponding energy bands and LDOS of the surface electrons in the presence of the proposed molecular lattice. Finally, we interpret the numerical results by the tight binding model of the Kagome lattice. We hope that our work can stimulate further theoretical and experimental interest in this novel artificial 2D electron lattice system.
NASA Astrophysics Data System (ADS)
Hoffman, David Paul
Chemical reactions are defined by the change in the relative positions and bonding of nuclei in molecules. I have used femtosecond stimulated Raman spectroscopy (FSRS) to probe these transformations with structural specificity and high time precision revealing the mechanisms of two important classes of reactions; isomerization about an N=N bond and interfacial/intermolecular electron transfer. Isomerization about a double bond is one of the simplest, yet most important, photochemical reactions. In contrast to carbon double bonds, nitrogen double bonds can react via two possible mechanisms; rotation or inversion. To determine which pathway is predominant, I studied an azobenzene derivative using both FSRS and impulsive stimulated Raman spectroscopy (ISRS). The FSRS experiments demonstrated that the photochemical reaction occurs concomitantly with the 700 fs non-radiative decay of the excited state; because no major change in N=N stretching frequency was measured, I surmised that the reaction proceeds through an inversion pathway. My subsequent ISRS experiments confirmed this hypothesis; I observed a highly displaced, low frequency, inversion-like mode, indicating that initial movement out of the Franck-Condon region proceeds along an inversion coordinate. To probe which nuclear motions facilitate electron transfer and charge recombination, I used FSRS and the newly developed 2D-FSRS techniques to study two model systems, triphenylamine dyes bound to TiO2 nanoparticles and a molecular charge transfer (CT) dimer. In the dye-nanoparticle system I discovered that charge separation persists much longer (> 100 ps) than previously thought by using the juxtaposition of the FSRS and transient absorption data to separate the dynamics of the dye from that of the injected electron. Additionally, I discovered that dye constructs with an added vinyl group were susceptible to quenching via isomerization. The CT dimer offered an opportunity to study a system in which charge
Quinn, John
2009-11-30
Work related to this project introduced the idea of an effective monopole strength Q* that acted as the effective angular momentum of the lowest shell of composite Fermions (CF). This allowed us to predict the angular momentum of the lowest band of energy states for any value of the applied magnetic field simply by determining N{sub QP} the number of quasielectrons (QE) or quasiholes (QH) in a partially filled CF shell and adding angular momenta of the N{sub QP} Fermions excitations. The approach reported treated the filled CF level as a vacuum state which could support QE and QH excitations. Numerical diagonalization of small systems allowed us to determine the angular momenta, the energy, and the pair interaction energies of these elementary excitations. The spectra of low energy states could then be evaluated in a Fermi liquid-like picture, treating the much smaller number of quasiparticles and their interactions instead of the larger system of N electrons with Coulomb interactions.
Two-dimensional Vlasov Simulation of Driven, Nonlinear Electron Plasma Waves
NASA Astrophysics Data System (ADS)
Hittinger, J. A.; Banks, J. W.; Berger, R. L.; Cohen, B. I.; Brunner, S.
2010-11-01
In the VALHALLA project at LLNL, we are developing advanced, scalable algorithms for the continuum solution of Vlasov-Maxwell that differ from traditional approaches to continuum Vlasov methods.ootnotetextJ. Banks and J.Hittinger, sub. to IEEE Trans. Plas. Sci. (Dec 2009), LLNL-JRNL-420843. Here, continuum solution of the Vlasov-Maxwell system using these techniques is extended to two spatial dimensions and two velocity dimensions. We report Vlasov simulation studies of ponderomotively driven electron plasma waves (EPW) with fixed ions. Motivated plasma waves driven by SRS in light speckles, we consider a driving potential with a finite transverse width. This localization introduces losses as the waves propagate transversely out of the driven region and the particles are only transiently trapped. Linearly, the transverse localization leads to constant phase surfaces that defocus the EPW while nonlinearly, the constant phase surfaces from trapping-induced frequency shifts focus the EPW. We show how these processes are affected by the system length and the boundary conditions.
Cao, Duc; Moses, Gregory; Delettrez, Jacques
2015-08-15
An implicit, non-local thermal conduction algorithm based on the algorithm developed by Schurtz, Nicolai, and Busquet (SNB) [Schurtz et al., Phys. Plasmas 7, 4238 (2000)] for non-local electron transport is presented and has been implemented in the radiation-hydrodynamics code DRACO. To study the model's effect on DRACO's predictive capability, simulations of shot 60 303 from OMEGA are completed using the iSNB model, and the computed shock speed vs. time is compared to experiment. Temperature outputs from the iSNB model are compared with the non-local transport model of Goncharov et al. [Phys. Plasmas 13, 012702 (2006)]. Effects on adiabat are also examined in a polar drive surrogate simulation. Results show that the iSNB model is not only capable of flux-limitation but also preheat prediction while remaining numerically robust and sacrificing little computational speed. Additionally, the results provide strong incentive to further modify key parameters within the SNB theory, namely, the newly introduced non-local mean free path. This research was supported by the Laboratory for Laser Energetics of the University of Rochester.
Luo, Xiaoguang Long, Kailin; Wang, Jun; Qiu, Teng; He, Jizhou; Liu, Nian
2014-06-28
Theoretical thermoelectric nanophysics models of low-dimensional electronic heat engine and refrigerator devices, comprising two-dimensional hot and cold reservoirs and an interconnecting filtered electron transport mechanism have been established. The models were used to numerically simulate and evaluate the thermoelectric performance and energy conversion efficiencies of these low-dimensional devices, based on three different types of electron transport momentum-dependent filters, referred to herein as k{sub x}, k{sub y}, and k{sub r} filters. Assuming the Fermi-Dirac distribution of electrons, expressions for key thermoelectric performance parameters were derived for the resonant transport processes, in which the transmission of electrons has been approximated as a Lorentzian resonance function. Optimizations were carried out and the corresponding optimized design parameters have been determined, including but not limited to the universal theoretical upper bound of the efficiency at maximum power for heat engines, and the maximum coefficient of performance for refrigerators. From the results, it was determined that k{sub r} filter delivers the best thermoelectric performance, followed by the k{sub x} filter, and then the k{sub y} filter. For refrigerators with any one of three filters, an optimum range for the full width at half maximum of the transport resonance was found to be <2k{sub B}T.
Nenov, Artur; Segarra-Martí, Javier; Giussani, Angelo; Conti, Irene; Rivalta, Ivan; Dumont, Elise; Jaiswal, Vishal K; Altavilla, Salvatore Flavio; Mukamel, Shaul; Garavelli, Marco
2015-01-01
The SOS//QM/MM [Rivalta et al., Int. J. Quant. Chem., 2014, 114, 85] method consists of an arsenal of computational tools allowing accurate simulation of one-dimensional (1D) and bi-dimensional (2D) electronic spectra of monomeric and dimeric systems with unprecedented details and accuracy. Prominent features like doubly excited local and excimer states, accessible in multi-photon processes, as well as charge-transfer states arise naturally through the fully quantum-mechanical description of the aggregates. In this contribution the SOS//QM/MM approach is extended to simulate time-resolved 2D spectra that can be used to characterize ultrafast excited state relaxation dynamics with atomistic details. We demonstrate how critical structures on the excited state potential energy surface, obtained through state-of-the-art quantum chemical computations, can be used as snapshots of the excited state relaxation dynamics to generate spectral fingerprints for different de-excitation channels. The approach is based on high-level multi-configurational wavefunction methods combined with non-linear response theory and incorporates the effects of the solvent/environment through hybrid quantum mechanics/molecular mechanics (QM/MM) techniques. Specifically, the protocol makes use of the second-order Perturbation Theory (CASPT2) on top of Complete Active Space Self Consistent Field (CASSCF) strategy to compute the high-lying excited states that can be accessed in different 2D experimental setups. As an example, the photophysics of the stacked adenine-adenine dimer in a double-stranded DNA is modeled through 2D near-ultraviolet (NUV) spectroscopy.
NASA Astrophysics Data System (ADS)
John, Sajeev; Golubentsev, Andrey
1995-01-01
It is suggested that an interacting many-electron system in a two-dimensional lattice may condense into a topological magnetic state distinct from any discussed previously. This condensate exhibits local spin-1/2 magnetic moments on the lattice sites but is composed of a Slater determinant of single-electron wave functions which exist in an orthogonal sector of the electronic Hilbert space from the sector describing traditional spin-density-wave or spiral magnetic states. These one-electron spinor wave functions have the distinguishing property that they are antiperiodic along a closed path encircling any elementary plaquette of the lattice. This corresponds to a 2π rotation of the internal coordinate frame of the electron as it encircles the plaquette. The possibility of spinor wave functions with spatial antiperiodicity is a direct consequence of the two-valuedness of the internal electronic wave function defined on the space of Euler angles describing its spin. This internal space is the topologically, doubly-connected, group manifold of SO(3). Formally, these antiperiodic wave functions may be described by passing a flux which couples to spin (rather than charge) through each of the elementary plaquettes of the lattice. When applied to the two-dimensional Hubbard model with one electron per site, this new topological magnetic state exhibits a relativistic spectrum for charged, quasiparticle excitations with a suppressed one-electron density of states at the Fermi level. For a topological antiferromagnet on a square lattice, with the standard Hartree-Fock, spin-density-wave decoupling of the on-site Hubbard interaction, there is an exact mapping of the low-energy one-electron excitation spectrum to a relativistic Dirac continuum field theory. In this field theory, the Dirac mass gap is precisely the Mott-Hubbard charge gap and the continuum field variable is an eight-component Dirac spinor describing the components of physical electron-spin amplitude on each of
Two dimensional, electronic particle tracking in liquids with a graphene-based magnetic sensor array
NASA Astrophysics Data System (ADS)
Neumann, Rodrigo F.; Engel, Michael; Steiner, Mathias
2016-07-01
nanoparticles in the liquid with high accuracy and (b) the reconstruction of a particle's flow-driven trajectory across the integrated sensor array with sub-pixel precision as a function of time, in what we call the ``Magnetic nanoparticle velocimetry'' technique. Since the method does not rely on optical detection, potential lab-on-chip applications include particle tracking and flow analysis in opaque media at the sub-micron scale. Electronic supplementary information (ESI) available: Movie that illustrates the reconstruction of particle trajectories and additional results to be used as a guideline for the choice of operational parameters such as nanoparticle diameter, nanoparticle concentration and magnetic field components. See DOI: 10.1039/C6NR03434A
NASA Astrophysics Data System (ADS)
Li, Jia; Li, Hongdong; Yin, Hong
2014-08-01
By first-principles calculations, we study the layer number (n) dependent structural evolution, electronic and magnetic properties of two-dimensional bare and semi-hydrogenated (1 1 1)-oriented cubic boron nitride (c-BN) nanosheets. After energy optimization, there is a threshold of n = 7 for maintaining the cubic phase of the bare BN nanosheets. The hydrogenation on either B-end or N-end surface sites would be helpful for stabilizing the c-BN nanosheets with small n, and the structural relaxations on both two outermost surface sides are asymmetric due to different electronegativity between B and N atoms. The bare nanosheets are ferrimagnetic metallic and further turn into ferrimagnetic semiconductor after semi-hydrogenation.
A New Method to Modify Two-Dimensional Electron Gas Density by GaN Cap Etching
NASA Astrophysics Data System (ADS)
Li, Zhongda; Chow, T. Paul
2013-08-01
We have experimentally demonstrated a new method for modifying the two-dimensional electron density (2DEG) at the AlGaN/GaN interface by etching of the GaN cap layer on top of the AlGaN. GaN MOS capacitors have been fabricated on samples with partially or fully etched GaN cap, and the 2DEG density has been extracted. The results show a linear relation between the 2DEG density and the thickness of the GaN cap being etched. We have also fabricated van der Pauw structures and obtained the 2DEG density using Hall measurements, and the results are consistent with that from the GaN MOS capacitors.
Du, Juan; Xia, Congxin; Xiong, Wenqi; Zhao, Xu; Wang, Tianxing; Jia, Yu
2016-08-10
Based on first-principles calculations, the electronic structures and magnetism are investigated in 3d transition metal (TM)-embedded porous two-dimensional (2D) C2N monolayers. Numerical results indicate that except Mn and Co atoms, other TM atoms can be embedded stably in the 2D C2N monolayer. Moreover, the magnetic moments of the TM-embedded C2N monolayer depend highly on the atomic number of the TM atoms. The Sc, Ti, V, Cr, Mn, Fe, Co and Ni atom-embedded C2N monolayers possess a ferromagnetic ground state, while embedding Cu can induce paramagnetic characteristics in the 2D C2N monolayer. Meanwhile, the Zn-embedded C2N monolayer exhibits a nonmagnetic ground state. These results indicate that the magnetism of 2D C2N monolayers can be tuned via embedding TM atoms.
NASA Astrophysics Data System (ADS)
Kolasiński, K.; Szafran, B.; Hackens, B.
2015-05-01
We consider conductance mapping of systems based on the two-dimensional electron gas with scanning gate microscopy using two or more tips of an atomic force microscope. The paper contains the results of numerical simulations for a model tip potential. In addition, a few procedures are proposed for the extraction and manipulation of ballistic transport properties. In particular, we demonstrate that the multitip techniques can be used to obtain a readout of the Fermi wavelength, to detect potential defects, to filter specific transverse modes, and to tune the system into resonant conditions under which a stable map of the local density of states can be extracted from conductance maps using a third tip.
Arzhannikov, A V; Ginzburg, N S; Kalinin, P V; Kuznetsov, S A; Malkin, A M; Peskov, N Yu; Sergeev, A S; Sinitsky, S L; Stepanov, V D; Thumm, M; Zaslavsky, V Yu
2016-09-09
A spatially extended planar 75 GHz free-electron maser with a hybrid two-mirror resonator consisting of two-dimensional upstream and traditional one-dimensional downstream Bragg reflectors and driven by two parallel-sheet electron beams 0.8 MeV/1 kA has been elaborated. For the highly oversized interaction space (cross section 45×2.5 vacuum wavelengths), the two-dimensional distributed feedback allowed realization of stable narrow-band generation that includes synchronization of emission from both electron beams. As a result, spatially coherent radiation with the output power of 30-50 MW and a pulse duration of ∼100 ns was obtained in each channel.
NASA Astrophysics Data System (ADS)
Arzhannikov, A. V.; Ginzburg, N. S.; Kalinin, P. V.; Kuznetsov, S. A.; Malkin, A. M.; Peskov, N. Yu.; Sergeev, A. S.; Sinitsky, S. L.; Stepanov, V. D.; Thumm, M.; Zaslavsky, V. Yu.
2016-09-01
A spatially extended planar 75 GHz free-electron maser with a hybrid two-mirror resonator consisting of two-dimensional upstream and traditional one-dimensional downstream Bragg reflectors and driven by two parallel-sheet electron beams 0.8 MeV /1 kA has been elaborated. For the highly oversized interaction space (cross section 45 ×2.5 vacuum wavelengths), the two-dimensional distributed feedback allowed realization of stable narrow-band generation that includes synchronization of emission from both electron beams. As a result, spatially coherent radiation with the output power of 30-50 MW and a pulse duration of ˜100 ns was obtained in each channel.
NASA Astrophysics Data System (ADS)
Shlimak, I.; Butenko, A.; Golosov, D. I.; Friedland, K.-J.; Kravchenko, S. V.
2012-02-01
Positive magnetoresistance (PMR) of a silicon MOSFET in parallel magnetic fields B has been measured at high electron densities n nc where nc is the critical density of the metal-insulator transition (MIT). It turns out that the normalized PMR curves, R(B)/R(0), merge together when the field is scaled according to B/Bc(n) where Bc is the field in which electrons become fully spin polarized. The values of Bc have been calculated from the simple equality between the Zeeman splitting energy and the Fermi energy taking into account the experimentally measured dependence of the spin susceptibility on the electron density. This extends the range of validity of the scaling all the way to a deeply metallic regime far away from MIT. The subseqent analysis of PMR for low n>˜ nc demonstrated that the merging of the initial parts of curves can bee achieved only with taking into account the temperature dependence of Bc. It is shown that the shape of the PMR curves at strong magnetic fields is affected by a crossover from a purely two-dimensional (2D) electron transport to a regime where out-of-plane carrier motion becomes important (quasi-three-dimensional regime).
NASA Astrophysics Data System (ADS)
Shlimak, I.; Butenko, A.; Golosov, D. I.; Friedland, K.-J.; Kravchenko, S. V.
2012-02-01
Positive magnetoresistance (PMR) of a silicon MOSFET in parallel magnetic fields B has been measured at high electron densities nGtnc, where nc is the critical density of the metal-insulator transition (MIT). It turns out that the normalized PMR curves, R(B)/R(0), merge together when the field is scaled according to B/Bc(n), where Bc is the field in which electrons become fully spin polarized. The values of Bc have been calculated from the simple equality between the Zeeman splitting energy and the Fermi energy taking into account the experimentally measured dependence of the spin susceptibility on the electron density. This extends the range of validity of the scaling all the way to a deeply metallic regime far away from MIT. The subsequent analysis of PMR for low n\\gtrsim n_{c} demonstrated that the merging of the initial parts of curves can be achieved only with taking into account the temperature dependence of Bc. It is also shown that the shape of the PMR curves at strong magnetic fields is affected by a crossover from a purely two-dimensional (2D) electron transport to a regime where out-of-plane carrier motion becomes important (quasi-three-dimensional regime).
Anisotropic two-dimensional electron gas at the LaAlO3/SrTiO3 (110) interface
Annadi, A.; Zhang, Q.; Renshaw Wang, X.; Tuzla, N.; Gopinadhan, K.; Lü, W. M.; Roy Barman, A.; Liu, Z. Q.; Srivastava, A.; Saha, S.; Zhao, Y. L.; Zeng, S. W.; Dhar, S.; Olsson, E.; Gu, B.; Yunoki, S.; Maekawa, S.; Hilgenkamp, H.; Venkatesan, T.; Ariando
2013-01-01
The observation of a high-mobility two-dimensional electron gas between two insulating complex oxides, especially LaAlO3/SrTiO3, has enhanced the potential of oxides for electronics. The occurrence of this conductivity is believed to be driven by polarization discontinuity, leading to an electronic reconstruction. In this scenario, the crystal orientation has an important role and no conductivity would be expected, for example, for the interface between LaAlO3 and (110)-oriented SrTiO3, which should not have a polarization discontinuity. Here we report the observation of unexpected conductivity at the LaAlO3/SrTiO3 interface prepared on (110)-oriented SrTiO3, with a LaAlO3-layer thickness-dependent metal-insulator transition. Density functional theory calculation reveals that electronic reconstruction, and thus conductivity, is still possible at this (110) interface by considering the energetically favourable (110) interface structure, that is, buckled TiO2/LaO, in which the polarization discontinuity is still present. The conductivity was further found to be strongly anisotropic along the different crystallographic directions with potential for anisotropic superconductivity and magnetism, leading to possible new physics and applications. PMID:23673623
Bykov, A. A.; Rodyakina, E. E.; Latyshev, A. V.; Strygin, I. S.; Goran, A. V.; Kalagin, A. K.
2016-01-04
In this study we fabricated lateral superlattices (LSLs) based on the selectively doped GaAs/AlAs heterostructures with a high-mobility two-dimensional (2D) electron gas. The LSLs were formed using the electron-beam lithography and lift-off techniques, which produced a set of metallic strips on top of a heterojunction. The amplitude of the 2D electron gas modulation in the LSL was controlled by the gate voltage applied to the metallic strips. The LSLs with two different periods (a = 200 nm and 500 nm) were used to investigate the influence of microwave radiation with the frequency of 110–150 GHz on the 2D electron transport at the temperature T = 1.6 K in the magnetic field B < 1 T. We have found that zero-resistance states (ZRSs) appear under the microwave radiation in the 2D systems with a unidirectional periodic modulation. These ZRSs are located at the minima of commensurability oscillations.
NASA Astrophysics Data System (ADS)
Rödel, T. C.; Fortuna, F.; Bertran, F.; Gabay, M.; Rozenberg, M. J.; Santander-Syro, A. F.; Le Fèvre, P.
2015-07-01
We report the existence of metallic two-dimensional electron gases (2DEGs) at the (001) and (101) surfaces of bulk-insulating TiO2 anatase due to local chemical doping by oxygen vacancies in the near-surface region. Using angle-resolved photoemission spectroscopy, we find that the electronic structure at both surfaces is composed of two occupied subbands of dx y orbital character. While the Fermi surface observed at the (001) termination is isotropic, the 2DEG at the (101) termination is anisotropic and shows a charge carrier density three times larger than at the (001) surface. Moreover, we demonstrate that intense UV synchrotron radiation can alter the electronic structure and stoichiometry of the surface up to the complete disappearance of the 2DEG. These results open a route for the nanoengineering of confined electronic states, the control of their metallic or insulating nature, and the tailoring of their microscopic symmetry, using UV illumination at different surfaces of anatase.
NASA Astrophysics Data System (ADS)
Zhongxin, Zheng; Jiandong, Sun; Yu, Zhou; Zhipeng, Zhang; Hua, Qin
2015-10-01
The broadband terahertz (THz) emission from drifting two-dimensional electron gas (2DEG) in an AlGaN/GaN heterostructure at 6 K is reported. The devices are designed as THz plasmon emitters according to the Smith-Purcell effect and the ‘shallow water’ plasma instability mechanism in 2DEG. Plasmon excitation is excluded since no signature of electron-density dependent plasmon mode is observed. Instead, the observed THz emission is found to come from the heated lattice and/or the hot electrons. Simulated emission spectra of hot electrons taking into account the THz absorption in air and Fabry-Pérot interference agree well with the experiment. It is confirmed that a blackbody-like THz emission will inevitably be encountered in similar devices driven by a strong in-plane electric field. A conclusion is drawn that a more elaborate device design is required to achieve efficient plasmon excitation and THz emission. Project supported by the National Basic Research Program of China (No. G2009CB929303), the National Natural Science Foundation of China (No. 61271157), the China Postdoctoral Science Foundation (No. 2014M551678), and the Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1301054B).
NASA Astrophysics Data System (ADS)
Romé, M.; Lepreti, F.; Maero, G.; Pozzoli, R.; Vecchio, A.; Carbone, V.
2013-03-01
Highly magnetized, pure electron plasmas confined in a Penning-Malmberg trap allow one to perform experiments on the two-dimensional (2D) fluid dynamics under conditions where non-ideal effects are almost negligible. Recent results on the freely decaying 2D turbulence obtained from experiments with electron plasmas performed in the Penning-Malmberg trap ELTRAP are presented. The analysis has been applied to experimental sequences with different types of initial density distributions. The dynamical properties of the system have been investigated by means of wavelet transforms and Proper Orthogonal Decomposition (POD). The wavelet analysis shows that most of the enstrophy is contained at spatial scales corresponding to the typical size of the persistent vortices in the 2D electron plasma flow. The POD analysis allows one to identify the coherent structures which give the dominant contribution to the plasma evolution. The statistical properties of the turbulence have been investigated by means of Probability Density Functions (PDFs) and structure functions of spatial vorticity increments. The analysis evidences how the shape and evolution of the dominant coherent structures and the intermittency properties of the turbulence strongly depend on the initial conditions for the electron density.
Sang, Ling; Yang, Xuelin Cheng, Jianpeng; Guo, Lei; Hu, Anqi; Xiang, Yong; Yu, Tongjun; Xu, Fujun; Tang, Ning; Jia, Lifang; He, Zhi; Wang, Maojun; Wang, Xinqiang; Shen, Bo; Ge, Weikun
2015-08-03
High-temperature transport properties in high-mobility lattice-matched InAlN/GaN heterostructures have been investigated. An interesting hysteresis phenomenon of the two dimensional electron gas (2DEG) density is observed in the temperature-dependent Hall measurements. After high-temperature thermal cycles treatment, the reduction of the 2DEG density is observed, which is more serious in thinner InAlN barrier samples. This reduction can then be recovered by light illumination. We attribute these behaviors to the shallow trap states with energy level above the Fermi level in the GaN buffer layer. The electrons in the 2DEG are thermal-excited when temperature is increased and then trapped by these shallow trap states in the buffer layer, resulting in the reduction and hysteresis phenomenon of their density. Three trap states are observed in the GaN buffer layer and C{sub Ga} may be one of the candidates responsible for the observed behaviors. Our results provide an alternative approach to assess the quality of InAlN/GaN heterostructures for applications in high-temperature electronic devices.
Two-dimensional electron gas with six-fold symmetry at the (111) surface of KTaO3.
Bareille, C; Fortuna, F; Rödel, T C; Bertran, F; Gabay, M; Cubelos, O Hijano; Taleb-Ibrahimi, A; Le Fèvre, P; Bibes, M; Barthélémy, A; Maroutian, T; Lecoeur, P; Rozenberg, M J; Santander-Syro, A F
2014-01-07
Two-dimensional electron gases (2DEGs) at transition-metal oxide (TMO) interfaces, and boundary states in topological insulators, are being intensively investigated. The former system harbors superconductivity, large magneto-resistance, and ferromagnetism. In the latter, honeycomb-lattice geometry plus bulk spin-orbit interactions lead to topologically protected spin-polarized bands. 2DEGs in TMOs with a honeycomb-like structure could yield new states of matter, but they had not been experimentally realized, yet. We successfully created a 2DEG at the (111) surface of KTaO3, a strong insulator with large spin-orbit coupling. Its confined states form a network of weakly-dispersing electronic gutters with 6-fold symmetry, a topology novel to all known oxide-based 2DEGs. If those pertain to just one Ta-(111) bilayer, model calculations predict that it can be a topological metal. Our findings demonstrate that completely new electronic states, with symmetries not realized in the bulk, can be tailored in oxide surfaces, promising for TMO-based devices.
NASA Astrophysics Data System (ADS)
Steiner, Christian; Gebhardt, Julian; Ammon, Maximilian; Yang, Zechao; Heidenreich, Alexander; Hammer, Natalie; Görling, Andreas; Kivala, Milan; Maier, Sabine
2017-03-01
The fabrication of nanostructures in a bottom-up approach from specific molecular precursors offers the opportunity to create tailored materials for applications in nanoelectronics. However, the formation of defect-free two-dimensional (2D) covalent networks remains a challenge, which makes it difficult to unveil their electronic structure. Here we report on the hierarchical on-surface synthesis of nearly defect-free 2D covalent architectures with carbonyl-functionalized pores on Au(111), which is investigated by low-temperature scanning tunnelling microscopy in combination with density functional theory calculations. The carbonyl-bridged triphenylamine precursors form six-membered macrocycles and one-dimensional (1D) chains as intermediates in an Ullmann-type coupling reaction that are subsequently interlinked to 2D networks. The electronic band gap is narrowed when going from the monomer to 1D and 2D surface-confined π-conjugated organic polymers comprising the same building block. The significant drop of the electronic gap from the monomer to the polymer confirms an efficient conjugation along the triphenylamine units within the nanostructures.
Steiner, Christian; Gebhardt, Julian; Ammon, Maximilian; Yang, Zechao; Heidenreich, Alexander; Hammer, Natalie; Görling, Andreas; Kivala, Milan; Maier, Sabine
2017-01-01
The fabrication of nanostructures in a bottom-up approach from specific molecular precursors offers the opportunity to create tailored materials for applications in nanoelectronics. However, the formation of defect-free two-dimensional (2D) covalent networks remains a challenge, which makes it difficult to unveil their electronic structure. Here we report on the hierarchical on-surface synthesis of nearly defect-free 2D covalent architectures with carbonyl-functionalized pores on Au(111), which is investigated by low-temperature scanning tunnelling microscopy in combination with density functional theory calculations. The carbonyl-bridged triphenylamine precursors form six-membered macrocycles and one-dimensional (1D) chains as intermediates in an Ullmann-type coupling reaction that are subsequently interlinked to 2D networks. The electronic band gap is narrowed when going from the monomer to 1D and 2D surface-confined π-conjugated organic polymers comprising the same building block. The significant drop of the electronic gap from the monomer to the polymer confirms an efficient conjugation along the triphenylamine units within the nanostructures. PMID:28322232
NASA Astrophysics Data System (ADS)
Wei, Su-Huai; Yang, Ji-Hui; Zhang, Yueyu; Yin, Wan-Jian; Gong, X. G.; Yakobson, Boris I.
Two-dimensional (2D) semiconductors have many unique electronic and optoelectronic properties that is suitable for novel device applications. Most of the current study are focused on group IV or transition metal chalcogenides. In this study, using atomic transmutation and global optimization methods, we identified two group IV-VI 2D materials, Pma2-SiS and silicene sulfide that can overcome shortcomings encountered in conventional 2D semiconducttord. Pma2-SiS is found to be both chemically, energetically, and thermally stable. Most importantly, Pma2-SiS has unique electronic and optoelectronic properties, including direct bandgaps suitable for solar cells, good mobility for nanoelectronics, good flexibility of property tuning by layer thickness and strain appliance, and good air stability as well. Therefore, Pma2-SiS is expected to be a very promising 2D material in the field of 2D electronics and optoelectronics. Silicene sulfide also shows similar properties. We believe that the designing principles and approaches used to identify these materials have great potential to accelerate future finding of new functional materials within the 2D families.
Borisov, A. G.; Juaristi, J. I.
2006-01-15
Time-dependent density-functional theory is used to calculate quantum-size effects in the energy loss of antiprotons interacting with a confined two-dimensional electron gas. The antiprotons follow a trajectory normal to jellium circular clusters of variable size, crossing every cluster at its geometrical center. Analysis of the characteristic time scales that define the process is made. For high-enough velocities, the interaction time between the projectile and the target electrons is shorter than the time needed for the density excitation to travel along the cluster. The finite-size object then behaves as an infinite system, and no quantum-size effects appear in the energy loss. For small velocities, the discretization of levels in the cluster plays a role and the energy loss does depend on the system size. A comparison to results obtained using linear theory of screening is made, and the relative contributions of electron-hole pair and plasmon excitations to the total energy loss are analyzed. This comparison also allows us to show the importance of a nonlinear treatment of the screening in the interaction process.
Chakrabarti, S; Chatterjee, B; Debbarma, S; Ghatak, K P
2015-09-01
In this paper we study the influence of strong electric field on the two dimensional (2D)effective electron mass (EEM) at the Fermi level in quantum wells of III-V, ternary and quaternary semiconductors within the framework of k x p formalism by formulating a new 2D electron energy spectrum. It appears taking quantum wells of InSb, InAs, Hg(1-x)Cd(x)Te and In(1-x)Ga(x)As(1-y)P(y) lattice matched to InP as examples that the EEM increases with decreasing film thickness, increasing electric field and increases with increasing surface electron concentration exhibiting spikey oscillations because of the crossing over of the Fermi level by the quantized level in quantum wells and the quantized oscillation occurs when the Fermi energy touches the sub-band energy. The electric field makes the mass quantum number dependent and the oscillatory mass introduces quantum number dependent mass anisotropy in addition to energy. The EEM increases with decreasing alloy composition where the variations are totally band structure dependent. Under certain limiting conditions all the results for all the cases get simplified into the well-known parabolic energy bands and thus confirming the compatibility test. The content of this paper finds three applications in the fields of nano-science and technology.
Conductivity of the two-dimensional electron gas at LaAlO3/SrTiO3 interface
NASA Astrophysics Data System (ADS)
Kirichenko, E. V.; Stephanovich, V. A.; Dugaev, V. K.
2017-02-01
We propose an analytical theory of metallic conductivity in the two-dimensional (2D) LaAlO3/SrTiO3 (LAO/STO) interface. For that we consider the electron-phonon interaction at the interface. The electronic part is taken from our previous work [Phys. Chem. Chem. Phys. 18, 2104 (2016), 10.1039/C5CP06627A], considering the conditions for the interfacial charge carrier (electron or hole) to become itinerant. The second ingredient deals with the atomic oscillations localized near the interface and decaying rapidly at its both sides, which can be regarded as 2D acoustic phonons. The dispersion of such phonons depends on the characteristics of phonon spectra of LAO and STO. Calculating the corresponding scattering rate by Fermi's golden rule, we show that the resulting resistivity (i.e., inverse conductivity) has typical metallic character, growing linearly with temperature and tending to zero (without defects forming so-called residual resistivity) at T →0 . The results of our calculations are in agreement with available experimental data.
Teleportation with two-dimensional electron gas formed at the interface of a GaAs heterostructure
NASA Astrophysics Data System (ADS)
Adepoju, Adenike Grace; Falaye, Babatunde James; Sun, Guo-Hua; Camacho-Nieto, Oscar; Dong, Shi-Hai
2017-03-01
Inspired by the scenario proposed by Bennett et al, a teleportation protocol of qubits formed in a two-dimensional electron gas formed at the interface of a GaAs heterostructure is presented. The teleportation is carried out using three GaAs quantum dots (say P{{P}\\prime} , Q{{Q}\\prime} , R{{R}\\prime} ) and three electrons. The electron spin on GaAs quantum dots P{{P}\\prime} is used to encode the unknown qubit. The GaAs quantum dot Q{{Q}\\prime} and R{{R}\\prime} combine to form an entangled state. Alice (the sender) performs a Bell measurement on pairs (P,Q ) and ({{P}\\prime},{{Q}\\prime} ). Depending on the outcome of the measurement, a suitable Hamiltonian for the quantum gate can be used by Bob (the receiver) to transform the information based on spin to charge-based information. This work offers relevant corrections to the misconception in Weng and Kais (2006 Chem. Phys. Lett. 421 338).
Yuan, Hongtao; Wang, Haotian; Cui, Yi
2015-01-20
Electron occupation of orbitals in two-dimensional (2D) layered materials controls the magnitude and anisotropy of the interatomic electron transfer and exerts a key influence on the chemical bonding modes of 2D layered lattices. Therefore, their orbital occupations are believed to be responsible for massive variations of the physical and chemical properties from electrocatalysis and energy storage, to charge density waves, superconductivity, spin-orbit coupling, and valleytronics. Especially in nanoscale structures such as nanoribbons, nanoplates, and nanoflakes, 2D layered materials provide opportunities to exploit new quantum phenomena. In this Account, we report our recent progress in the rational design and chemical, electrochemical, and electrical modulations of the physical and chemical properties of layered nanomaterials via modification of the electron occupation in their electronic structures. Here, we start with the growth and fabrication of a group of layered chalcogenides with varied orbital occupation (from 4d/5d electron configuration to 5p/6p electron configuration). The growth techniques include bottom-up methods, such as vapor-liquid-solid growth and vapor-solid growth, and top-down methods, such as mechanical exfoliation with tape and AFM tip scanning. Next, we demonstrate the experimental strategies for the tuning of the chemical potential (orbital occupation tuned with electron filling) and the resulting modulation of the electronic states of layered materials, such as electric-double-layer gating, electrochemical intercalation, and chemical intercalation with molecule and zerovalence metal species. Since the properties of layered chalcogenides are normally dominated by the specific band structure around which the chemical potential is sitting, their desired electronic states and properties can be modulated in a large range, showing unique phenomena including quantum electronic transport and extraordinary optical transmittance. As the most
Physics of a two-dimensional electron gas with cold atoms in non-Abelian gauge potentials
NASA Astrophysics Data System (ADS)
Satija, Indubala I.; Dakin, Daniel C.; Vaishnav, J. Y.; Clark, Charles W.
2008-04-01
Motivated by the possibility of creating non-Abelian fields using cold atoms in optical lattices, we explore the richness and complexity of noninteracting two-dimensional electron gases (2DEGs) in a lattice, subjected to such fields. In the continuum limit, a non-Abelian system characterized by a two-component “magnetic flux” describes a harmonic oscillator existing in two different charge states (mimicking a particle-hole pair) where the coupling between the states is determined by the non-Abelian parameter, namely, the difference between the two components of the “magnetic flux.” A key feature of the non-Abelian system is a splitting of the Landau energy levels, which broaden into bands, as the spectrum depends explicitly on the transverse momentum. These Landau bands result in a coarse-grained “moth,” a continuum version of the generalized Hofstadter butterfly. Furthermore, the bands overlap, leading to effective relativistic effects. Importantly, similar features also characterize the corresponding two-dimensional lattice problem when at least one of the components of the magnetic flux is an irrational number. The lattice system with two competing “magnetic fluxes” penetrating the unit cell provides a rich environment in which to study localization phenomena. Some unique aspects of the transport properties of the non-Abelian system are the possibility of inducing localization by varying the quasimomentum, and the absence of localization of certain zero-energy states exhibiting a linear energy-momentum relation. Furthermore, non-Abelian systems provide an interesting localization scenario where the localization transition is accompanied by a transition from relativistic to nonrelativistic theory.
NASA Astrophysics Data System (ADS)
Schöche, S.; Shi, Junxia; Boosalis, A.; Kühne, P.; Herzinger, C. M.; Woollam, J. A.; Schaff, W. J.; Eastman, L. F.; Schubert, M.; Hofmann, T.
2011-02-01
The free-charge carrier mobility, sheet density, and effective mass of a two-dimensional electron gas are exemplarily determined in the spectral range from 640 GHz to 1 THz in a AlGaN/GaN heterostructure using the optical-Hall effect at room temperature. Complementary midinfrared spectroscopic ellipsometry measurements are performed for analysis of heterostructure constituents layer thickness, phonon mode, and free-charge carrier parameters. The electron effective mass is determined to be (0.22±0.04)m0. The high-frequency sheet density and carrier mobility parameters are in good agreement with results from dc electrical Hall effect measurements, indicative for frequency-independent carrier scattering mechanisms of the two-dimensional carrier distribution.
Electric field controlled spin interference in a system with Rashba spin-orbit coupling
NASA Astrophysics Data System (ADS)
Ciftja, Orion
2016-05-01
There have been intense research efforts over the last years focused on understanding the Rashba spin-orbit coupling effect from the perspective of possible spintronics applications. An important component of this line of research is aimed at control and manipulation of electron's spin degrees of freedom in semiconductor quantum dot devices. A promising way to achieve this goal is to make use of the tunable Rashba effect that relies on the spin-orbit interaction in a two-dimensional electron system embedded in a host semiconducting material that lacks inversion-symmetry. This way, the Rashba spin-orbit coupling effect may potentially lead to fabrication of a new generation of spintronic devices where control of spin, thus magnetic properties, is achieved via an electric field and not a magnetic field. In this work we investigate theoretically the electron's spin interference and accumulation process in a Rashba spin-orbit coupled system consisting of a pair of two-dimensional semiconductor quantum dots connected to each other via two conducting semi-circular channels. The strength of the confinement energy on the quantum dots is tuned by gate potentials that allow "leakage" of electrons from one dot to another. While going through the conducting channels, the electrons are spin-orbit coupled to a microscopically generated electric field applied perpendicular to the two-dimensional system. We show that interference of spin wave functions of electrons travelling through the two channels gives rise to interference/conductance patterns that lead to the observation of the geometric Berry's phase. Achieving a predictable and measurable observation of Berry's phase allows one to control the spin dynamics of the electrons. It is demonstrated that this system allows use of a microscopically generated electric field to control Berry's phase, thus, enables one to tune the spin-dependent interference pattern and spintronic properties with no need for injection of spin
Liboiron, Barry D.; Thompson, Katherine H.; Vera, Erika; Yuen, Violet G.; McNeill, John H.
2003-01-01
The biological fate of a chelated vanadium source is investigated by/n vivo spectroscopic methods to elucidate the chemical form in which the metal ion is accumulated. A pulsed electron paramagnetic resonance study of vanadyl ions in kidney tissue, taken from rats previously treated with bis(ethylmaltolato)oxovanadium(IV) (BEOV) in drinking water, is presented. A combined approach using stimulated echo (3-pulse) electron spin echo envelope modulation (ESEEM) and the two dimensional 4-pulse hyperfine sublevel correlation (HYSCORE) spectroscopies has shown that at least some of the VO2+ ions are involved in the coordination with nitrogen-containing ligands. From the experimental spectra, a 4N hyperfine coupling constant of 4.9 MHz and a quadrupole coupling constant of 0.6 + 0.04 MHz were determined, consistent with amine coordination of the vanadyl ions. Study of VO-histidine model complexes allowed for a determination of the percentage of nitrogen-coordinated VO2+ ions in the tissue sample that is found nitrogen-coordinated. By taking into account the bidentate nature of histidine coordination to VO2+ ions, a more accurate determination of this value is reported. The biological fate of chelated versus free (i.e. salts) vanadyl ion sources has been deduced by comparison to earlier reports. In contrast to its superior pharmacological efficacy over VOSO4, BEOV shares a remarkably similar biological fate after uptake into kidney tissue. PMID:18365044
Sharma, Munish E-mail: pk-ahluwalia7@yahoo.com; Kumar, Ashok; Ahluwalia, P. K. E-mail: pk-ahluwalia7@yahoo.com; Pandey, Ravindra
2014-08-14
Tunability of the electronic properties of two-dimensional bilayer hetero structures of transition-metal dichalcogenides (i.e., MX{sub 2}-M′X′{sub 2} with (M, M′ = Mo, W; X, X′ = S, Se) is investigated. Application of both strain and electric field is found to modify the band gap and carrier effective mass in the hybrid bilayers considered. The calculated results based on density functional theory suggest that the tensile strain considerably changes the band gap of semiconducting bilayers; it makes the band gap to be indirect, and later initiates the semiconductor-to-metal transition. Application of the external electric fields, on the other hand, shows asymmetric variation in the band gap leading to the closure of the gap at about 0.5–1.0 V/Å. Tuning of the band gap and carrier effective mass in such a controlled manner makes the hybrid bilayers of transition metal dichalcogenides to be promising candidates for application in electronic devices at nanoscale.
Egorova, Dassia
2015-06-07
Several recent experiments report on possibility of dark-state detection by means of so called beating maps of two-dimensional photon-echo spectroscopy [Ostroumov et al., Science 340, 52 (2013); Bakulin et al., Ultrafast Phenomena XIX (Springer International Publishing, 2015)]. The main idea of this detection scheme is to use coherence induced upon the laser excitation as a very sensitive probe. In this study, we investigate the performance of ground-state coherence in the detection of dark electronic states. For this purpose, we simulate beating maps of several models where the excited-state coherence can be hardly detected and is assumed not to contribute to the beating maps. The models represent strongly coupled electron-nuclear dynamics involving avoided crossings and conical intersections. In all the models, the initially populated optically accessible excited state decays to a lower-lying dark state within few hundreds femtoseconds. We address the role of Raman modes and of interstate-coupling nature. Our findings suggest that the presence of low-frequency Raman active modes significantly increases the chances for detection of dark states populated via avoided crossings, whereas conical intersections represent a more challenging task.
Cresti, Alessandro . E-mail: cresti@df.unipi.it; Grosso, Giuseppe . E-mail: grosso@df.unipi.it; Parravicini, Giuseppe Pastori . E-mail: pastori@fisicavolta.unipv.it
2006-05-15
We have derived closed analytic expressions for the Green's function of an electron in a two-dimensional electron gas threaded by a uniform perpendicular magnetic field, also in the presence of a uniform electric field and of a parabolic spatial confinement. A workable and powerful numerical procedure for the calculation of the Green's functions for a large infinitely extended quantum wire is considered exploiting a lattice model for the wire, the tight-binding representation for the corresponding matrix Green's function, and the Peierls phase factor in the Hamiltonian hopping matrix element to account for the magnetic field. The numerical evaluation of the Green's function has been performed by means of the decimation-renormalization method, and quite satisfactorily compared with the analytic results worked out in this paper. As an example of the versatility of the numerical and analytic tools here presented, the peculiar semilocal character of the magnetic Green's function is studied in detail because of its basic importance in determining magneto-transport properties in mesoscopic systems.
Dual-Gate Modulation of Carrier Density and Disorder in an Oxide Two-Dimensional Electron System
Chen, Zhuoyu; Yuan, Hongtao; Xie, Yanwu; Lu, Di; Inoue, Hisashi; Hikita, Yasuyuki; Bell, Christopher; Hwang, Harold Y.
2016-09-08
Carrier density and disorder are two crucial parameters that control the properties of correlated two-dimensional electron systems. Furthermore, in order to disentangle their individual contributions to quantum phenomena, independent tuning of these two parameters is required. By utilizing a hybrid liquid/solid electric dual-gate geometry acting on the conducting LaAlO_{3}/SrTiO_{3} heterointerface, we obtain an additional degree of freedom to strongly modify the electron confinement profile and thus the strength of interfacial scattering, independent from the carrier density. A dual-gate controlled nonlinear Hall effect is a direct manifestation of this profile, which can be quantitatively understood by a Poisson–Schrödinger sub-band model. In particular, the large nonlinear dielectric response of SrTiO_{3} enables a very wide range of tunable density and disorder, far beyond that for conventional semiconductors. This study provides a broad framework for understanding various reported phenomena at the LaAlO_{3}/SrTiO_{3} interface.
Dual-Gate Modulation of Carrier Density and Disorder in an Oxide Two-Dimensional Electron System
Chen, Zhuoyu; Yuan, Hongtao; Xie, Yanwu; ...
2016-09-08
Carrier density and disorder are two crucial parameters that control the properties of correlated two-dimensional electron systems. Furthermore, in order to disentangle their individual contributions to quantum phenomena, independent tuning of these two parameters is required. By utilizing a hybrid liquid/solid electric dual-gate geometry acting on the conducting LaAlO3/SrTiO3 heterointerface, we obtain an additional degree of freedom to strongly modify the electron confinement profile and thus the strength of interfacial scattering, independent from the carrier density. A dual-gate controlled nonlinear Hall effect is a direct manifestation of this profile, which can be quantitatively understood by a Poisson–Schrödinger sub-band model. Inmore » particular, the large nonlinear dielectric response of SrTiO3 enables a very wide range of tunable density and disorder, far beyond that for conventional semiconductors. This study provides a broad framework for understanding various reported phenomena at the LaAlO3/SrTiO3 interface.« less
NASA Astrophysics Data System (ADS)
Zhang, Qi; Gao, Weilu; Watson, John; Manfra, Michael; Kono, Junichiro
2015-03-01
Density-dependent Coulomb interactions can drive electron-hole (e - h) pairs in semiconductors through an excitonic Mott transition from an excitonic gas into an e - h plasma. Theoretical studies suggest that these interactions can be strongly modified by an external magnetic field, including the absence of inter-exciton interactions in the high magnetic field limit in two dimensions, due to an e - h charge symmetry, which results in ultrastable magneto-excitons. Here, we present a systematic experimental study of e - h pairs in photo-excited undoped GaAs quantum wells in magnetic fields with ultrafast terahertz spectroscopy. We simultaneously monitored the dynamics of the intraexcitonic 1 s-2 p transition (which splits into 1 s-2p+ and 1 s-2p- transitions in a magnetic field) and the cyclotron resonance of unbound electrons and holes up to 10 Tesla. We found that the 1 s-2p- absorption feature is robust at high magnetic fields even under high excitation fluences, indicating magnetically enhanced stability of excitons. We will discuss the Mott physics of magneto-excitons as a function of temperature, e - h pair density, optical pump delay time, as well as magnetic field, and also compare two-dimensional excitons in GaAs quantum wells with three-dimensional excitons in bulk GaAs.
Jun, Sunhong; Yang, Cheolhee; Kim, Tae Wu; Isaji, Megumi; Tamiaki, Hitoshi; Ihee, Hyotcherl; Kim, Jeongho
2015-07-21
Chlorosomes are the largest light harvesting complexes in nature and consist of many bacteriochlorophyll pigments forming self-assembled J-aggregates. In this work, we use two-dimensional electronic spectroscopy (2D-ES) to investigate ultrafast dynamics of excitation energy transfer (EET) in chlorosomes and their temperature dependence. From time evolution of the measured 2D electronic spectra of chlorosomes, we directly map out the distribution of the EET rate among the manifold of exciton states in a 2D energy space. In particular, it is found that the EET rate varies gradually depending on the energies of energy-donor and energy-acceptor states. In addition, from comparative 2D-ES measurements at 77 K and room temperature, we show that the EET rate exhibits subtle dependence on both the exciton energy and temperature, demonstrating the effect of thermal excitation on the EET rate. This observation suggests that active thermal excitation at room temperature prevents the excitation trapping at low-energy states and thus promotes efficient exciton diffusion in chlorosomes at ambient temperature.
NASA Astrophysics Data System (ADS)
Yang, Kesong; Nazir, Safdar; Behtash, Maziar; Cheng, Jianli
2016-10-01
The two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides such as LaAlO3 and SrTiO3 (STO) is of fundamental and practical interest because of its novel interfacial conductivity and its promising applications in next-generation nanoelectronic devices. Here we show that a group of combinatorial descriptors that characterize the polar character, lattice mismatch, band gap, and the band alignment between the perovskite-oxide-based band insulators and the STO substrate, can be introduced to realize a high-throughput (HT) design of SrTiO3-based 2DEG systems from perovskite oxide quantum database. Equipped with these combinatorial descriptors, we have carried out a HT screening of all the polar perovskite compounds, uncovering 42 compounds of potential interests. Of these, Al-, Ga-, Sc-, and Ta-based compounds can form a 2DEG with STO, while In-based compounds exhibit a strain-induced strong polarization when deposited on STO substrate. In particular, the Ta-based compounds can form 2DEG with potentially high electron mobility at (TaO2)+/(SrO)0 interface. Our approach, by defining materials descriptors solely based on the bulk materials properties, and by relying on the perovskite-oriented quantum materials repository, opens new avenues for the discovery of perovskite-oxide-based functional interface materials in a HT fashion.
NASA Astrophysics Data System (ADS)
Irie, Hiroshi; Todt, Clemens; Kumada, Norio; Harada, Yuichi; Sugiyama, Hiroki; Akazaki, Tatsushi; Muraki, Koji
2016-10-01
We study coherent transport and bound state formation of Bogoliubov quasiparticles in a high-mobility I n0.75G a0.25As two-dimensional electron gas (2DEG) coupled to a superconducting Nb electrode by means of a quantum point contact (QPC) as a tunable single-mode probe. Below the superconducting critical temperature of Nb, the QPC shows a single-channel conductance greater than the conductance quantum 2 e2/h at zero bias, which indicates the presence of Andreev-reflected quasiparticles, time-reversed states of the injected electron, returning back through the QPC. The marked sensitivity of the conductance enhancement to voltage bias and perpendicular magnetic field suggests a mechanism analogous to reflectionless tunneling—a hallmark of phase-coherent transport, with the boundary of the 2DEG cavity playing the role of scatterers. When the QPC transmission is reduced to the tunneling regime, the differential conductance vs bias voltage probes the single-particle density of states in the proximity area. Measured conductance spectra show a double peak within the superconducting gap of Nb, demonstrating the formation of Andreev bound states in the 2DEG. Both of these results, obtained in the open and closed geometries, underpin the coherent nature of quasiparticles, i.e., phase-coherent Andreev reflection at the InGaAs/Nb interface and coherent propagation in the ballistic 2DEG.
Trevisanutto, Paolo E; Vignale, Giovanni
2016-05-28
Ab initio electronic structure calculations of two-dimensional layered structures are typically performed using codes that were developed for three-dimensional structures, which are periodic in all three directions. The introduction of a periodicity in the third direction (perpendicular to the layer) is completely artificial and may lead in some cases to spurious results and to difficulties in treating the action of external fields. In this paper we develop a new approach, which is "native" to quasi-2D materials, making use of basis function that are periodic in the plane, but atomic-like in the perpendicular direction. We show how some of the basic tools of ab initio electronic structure theory - density functional theory, GW approximation and Bethe-Salpeter equation - are implemented in the new basis. We argue that the new approach will be preferable to the conventional one in treating the peculiarities of layered materials, including the long range of the unscreened Coulomb interaction in insulators, and the effects of strain, corrugations, and external fields.
Yang, Kesong; Nazir, Safdar; Behtash, Maziar; Cheng, Jianli
2016-01-01
The two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides such as LaAlO3 and SrTiO3 (STO) is of fundamental and practical interest because of its novel interfacial conductivity and its promising applications in next-generation nanoelectronic devices. Here we show that a group of combinatorial descriptors that characterize the polar character, lattice mismatch, band gap, and the band alignment between the perovskite-oxide-based band insulators and the STO substrate, can be introduced to realize a high-throughput (HT) design of SrTiO3-based 2DEG systems from perovskite oxide quantum database. Equipped with these combinatorial descriptors, we have carried out a HT screening of all the polar perovskite compounds, uncovering 42 compounds of potential interests. Of these, Al-, Ga-, Sc-, and Ta-based compounds can form a 2DEG with STO, while In-based compounds exhibit a strain-induced strong polarization when deposited on STO substrate. In particular, the Ta-based compounds can form 2DEG with potentially high electron mobility at (TaO2)+/(SrO)0 interface. Our approach, by defining materials descriptors solely based on the bulk materials properties, and by relying on the perovskite-oriented quantum materials repository, opens new avenues for the discovery of perovskite-oxide-based functional interface materials in a HT fashion. PMID:27708415
NASA Astrophysics Data System (ADS)
Tumino, F.; Casari, C. S.; Passoni, M.; Bottani, C. E.; Li Bassi, A.
2016-11-01
Two-dimensional (2D) ZnO structures have been deposited on the Au(111) surface by means of the pulsed laser deposition technique. In situ scanning tunneling microscopy and scanning tunneling spectroscopy measurements have been performed to characterize morphological, structural and electronic properties of 2D ZnO at the nanoscale. Starting from a sub-monolayer coverage, we investigated the growth of ZnO, identifying different atomic layers (up to the fifth). At low coverage, we observed single- and bi-layer nanocrystals, characterized by a surface moiré pattern that is associated to a graphene-like ZnO structure. By increasing the coverage, we revealed a morphological change starting from the fourth layer, which was attributed to a transition toward a bulk-like structure. Investigation of the electronic properties revealed the semiconducting character of 2D ZnO. We observed a dependence of the density of states (DOS) and, in particular, of the conduction band (CB) on the ZnO thickness, with a decreasing of the CB onset energy for increasing thickness. The CB DOS of 2D ZnO shows a step-like behaviour which may be interpreted as due to a 2D quantum confinement effect in ZnO atomic layers.
NASA Astrophysics Data System (ADS)
Sharma, Munish; Kumar, Ashok; Ahluwalia, P. K.; Pandey, Ravindra
2014-08-01
Tunability of the electronic properties of two-dimensional bilayer hetero structures of transition-metal dichalcogenides (i.e., MX2-M'X'2 with (M, M' = Mo, W; X, X' = S, Se) is investigated. Application of both strain and electric field is found to modify the band gap and carrier effective mass in the hybrid bilayers considered. The calculated results based on density functional theory suggest that the tensile strain considerably changes the band gap of semiconducting bilayers; it makes the band gap to be indirect, and later initiates the semiconductor-to-metal transition. Application of the external electric fields, on the other hand, shows asymmetric variation in the band gap leading to the closure of the gap at about 0.5-1.0 V/Å. Tuning of the band gap and carrier effective mass in such a controlled manner makes the hybrid bilayers of transition metal dichalcogenides to be promising candidates for application in electronic devices at nanoscale.
NASA Astrophysics Data System (ADS)
Yang, Zihao; Kent, Thomas F.; Yang, Jing; Jin, Hyungyu; Heremans, Joseph P.; Myers, Roberto C.
2015-12-01
Here we report on the effect of rare-earth Gd doping on the magnetic properties and magnetotransport of GaN two-dimensional electron gasses (2DEGs). Samples are grown by plasma-assisted molecular-beam epitaxy and consist of AlN/GaN heterostructures where Gd is δ doped within a polarization-induced 2DEG. Ferromagnetism is observed in these Gd-doped 2DEGs with a Curie temperature above room temperature and an anisotropic spontaneous magnetization preferring an out-of-plane (c -axis) orientation. At magnetic fields up to 50 kOe, the magnetization remains smaller for the in-plane configuration than for the out-of-plane configuration, which is indicative of exchange-coupled spins locked along the polar c axis. The sample with the lowest Gd concentration (2.3 ×1014c m-2 ) exhibits a saturation magnetization of 41.1 μB/ G d3 + at 5 K revealing that the Gd ion spins (7 μB ) alone do not account for the magnetization. Surprisingly, control samples grown without any Gd display inconsistent magnetic properties; in some control samples weak ferromagnetism is observed, and in others paramagnetism is observed. The ferromagnetic 2DEGs do not exhibit the anomalous Hall effect; the Hall resistance varies nonlinearly with the magnetic field but does not track the magnetization, indicating the lack of coupling between the ferromagnetic phase and the conduction-band electrons within the 2DEG.
Nenov, Artur; Mukamel, Shaul; Garavelli, Marco; Rivalta, Ivan
2015-08-11
First-principles simulations of two-dimensional electronic spectroscopy in the ultraviolet region (2DUV) require computationally demanding multiconfigurational approaches that can resolve doubly excited and charge transfer states, the spectroscopic fingerprints of coupled UV-active chromophores. Here, we propose an efficient approach to reduce the computational cost of accurate simulations of 2DUV spectra of benzene, phenol, and their dimer (i.e., the minimal models for studying electronic coupling of UV-chromophores in proteins). We first establish the multiconfigurational recipe with the highest accuracy by comparison with experimental data, providing reference gas-phase transition energies and dipole moments that can be used to construct exciton Hamiltonians involving high-lying excited states. We show that by reducing the active spaces and the number of configuration state functions within restricted active space schemes, the computational cost can be significantly decreased without loss of accuracy in predicting 2DUV spectra. The proposed recipe has been successfully tested on a realistic model proteic system in water. Accounting for line broadening due to thermal and solvent-induced fluctuations allows for direct comparison with experiments.
NASA Astrophysics Data System (ADS)
Taut, M.
2001-10-01
The ground state energy and the lowest excitations of a two-dimensional Wigner crystal in a perpendicular magnetic field with one and two electrons per cell is investigated. In the case of two electrons per lattice site, the interaction of the electrons within each cell is taken into account exactly (including exchange and correlation effects), and the interaction between the cells is in second order (dipole) van der Waals approximation. No further approximations are made, in particular Landau level mixing and incomplete spin polarization are accounted for. Therefore, our calculation comprises a, roughly speaking, complementary description of the bubble phase (in the special case of one and two electrons per bubble), which was proposed by Koulakov, Fogler, and Shklovskii on the basis of a Hartree Fock calculation. The phase diagram shows that in GaAs the paired phase is energetically more favorable than the single electron phase for, roughly speaking, filling factor f larger than 0.3 and density parameter rs smaller than 19 effective Bohr radii (for a more precise statement see Figs. 3 and 4). If we start within the paired phase and increase magnetic field or decrease density, the pairs first undergo some singlet-triplet transitions before they break.
NASA Astrophysics Data System (ADS)
Taut, M. G.
2001-03-01
The ground state energy and the lowest excitations of a two dimensional Wigner crystal with one -- and two electrons per cell are investigated. In case of two electrons per lattice site, the interaction of the electrons within each cell is taken into account exactly (including exchange and the full Coulomb correlation effects). The interaction between the cells is in second order (dipole) van der Waals approximation. No further approximations are made, in particular Landau level mixing is accounted for. Therefore, our calculation comprises a supplementary description of the bubble phase in the special case of one and two electrons per bubble, as proposed by Koulakov, Fogler and Shklovskii in Phys.Rev.Lett. 76, 499 (1996) on the basis of a Hartree Fock calculation. The phase diagram shows for which filling factors and densities which phase: single electrons versus singlet or triplet pairs on the Wigner crystal lattice sites, is energetically favored. Comparison with other approximations for the Wigner crystal and with the Laughlin liquid is made as well.
Yan, Zhequan; Chen, Liang; Yoon, Mina; Kumar, Satish
2016-12-07
We investigate the role of interfacial electronic properties on the phonon transport in two-dimensional MoS2 adsorbed on metal substrates (Au and Sc) using first-principles density functional theory and the atomistic Green's function method. Our study reveals that the different degree of orbital hybridization and electronic charge distribution between MoS2 and metal substrates play a significant role in determining the overall phonon-phonon coupling and phonon transmission. The charge transfer caused by the adsorption of MoS2 on Sc substrate can significantly weaken the Mo-S bond strength and change the phonon properties of MoS2, which result in a significant change in thermal boundary conductance (TBC) from one lattice-stacking configuration to another for same metallic substrate. In a lattice-stacking configuration of MoS2/Sc, weakening of the Mo-S bond strength due to charge redistribution results in decrease in the force constant between Mo and S atoms and substantial redistribution of phonon density of states to low-frequency region which affects overall phonon transmission leading to 60% decrease in TBC compared to another configuration of MoS2/Sc. Strong chemical coupling between MoS2 and the Sc substrate leads to a significantly (∼19 times) higher TBC than that of the weakly bound MoS2/Au system. Our findings demonstrate the inherent connection among the interfacial electronic structure, the phonon distribution, and TBC, which helps us understand the mechanism of phonon transport at the MoS2/metal interfaces. The results provide insights for the future design of MoS2-based electronics and a way of enhancing heat dissipation at the interfaces of MoS2-based nanoelectronic devices.
Ferretti, Marco; Novoderezhkin, Vladimir I; Romero, Elisabet; Augulis, Ramunas; Pandit, Anjali; Zigmantas, Donatas; van Grondelle, Rienk
2014-06-07
Light-harvesting in photosynthesis is determined by the excitonic interactions in disordered antennae and the coupling of collective electronic excitations to fast nuclear motions, producing efficient energy transfer with a complicated interplay between exciton and vibrational coherences. Two-dimensional electronic spectroscopy (2DES) is a powerful tool to study the presence of these coherences in photosynthetic complexes. However, the unambiguous assignment of the nature of the observed coherences is still under debate. In this paper we apply 2DES to an excitonically coupled bacteriochlorophyll dimer, the B820 subunit of the light harvesting complex 1 (LH1-RC) of R. rubrum G9. Fourier analysis of the measured kinetics and modeling of the spectral responses in a complete basis of electronic and vibrational states allow us to distinguish between pure vibrational, mixed exciton-vibrational (vibronic), and predominantly exciton coherences. The mixed coherences have been found in a wide range of oscillation frequencies, whereas exciton coherences give the biggest contributions for the frequencies in the 400-550 cm(-1) range, corresponding to the exciton splitting energy of the B820 dimer. Significant exciton coherences are also present at higher frequencies, i.e., up to 800 cm(-1), which are determined by realizations of the disorder with a large energy gap between the two pigments (which increases the apparent value of the exciton splitting). Although the B820 dimer is a model system, the approach presented here represents a basis for further analyses of more complicated systems, providing a tool for studying the interplay between electronic and vibrational coherences in disordered photosynthetic antennae and reaction centres.
Yan, Zhequan; Chen, Liang; Yoon, Mina; ...
2016-11-08
In this paper, we investigate the role of interfacial electronic properties on the phonon transport in two-dimensional MoS2 adsorbed on metal substrates (Au and Sc) using first-principles density functional theory and the atomistic Green’s function method. Our study reveals that the different degree of orbital hybridization and electronic charge distribution between MoS2 and metal substrates play a significant role in determining the overall phonon–phonon coupling and phonon transmission. The charge transfer caused by the adsorption of MoS2 on Sc substrate can significantly weaken the Mo–S bond strength and change the phonon properties of MoS2, which result in a significant changemore » in thermal boundary conductance (TBC) from one lattice-stacking configuration to another for same metallic substrate. In a lattice-stacking configuration of MoS2/Sc, weakening of the Mo–S bond strength due to charge redistribution results in decrease in the force constant between Mo and S atoms and substantial redistribution of phonon density of states to low-frequency region which affects overall phonon transmission leading to 60% decrease in TBC compared to another configuration of MoS2/Sc. Strong chemical coupling between MoS2 and the Sc substrate leads to a significantly (~19 times) higher TBC than that of the weakly bound MoS2/Au system. Our findings demonstrate the inherent connection among the interfacial electronic structure, the phonon distribution, and TBC, which helps us understand the mechanism of phonon transport at the MoS2/metal interfaces. Finally, the results provide insights for the future design of MoS2-based electronics and a way of enhancing heat dissipation at the interfaces of MoS2-based nanoelectronic devices.« less
Disorder-induced topological phase transitions in two-dimensional spin-orbit coupled superconductors
NASA Astrophysics Data System (ADS)
Qin, Wei; Xiao, Di; Chang, Kai; Shen, Shun-Qing; Zhang, Zhenyu
2016-12-01
Normal superconductors with Rashba spin-orbit coupling have been explored as candidate systems of topological superconductors. Here we present a comparative theoretical study of the effects of different types of disorder on the topological phases of two-dimensional Rashba spin-orbit coupled superconductors. First, we show that a topologically trivial superconductor can be driven into a chiral topological superconductor upon diluted doping of isolated magnetic disorder, which close and reopen the quasiparticle gap of the paired electrons in a nontrivial manner. Secondly, the superconducting nature of a topological superconductor is found to be robust against Anderson disorder, but the topological nature is not, converting the system into a topologically trivial state even in the weak scattering limit. These topological phase transitions are distinctly characterized by variations in the topological invariant. We discuss the central findings in connection with existing experiments, and provide new schemes towards eventual realization of topological superconductors.
Disorder-induced topological phase transitions in two-dimensional spin-orbit coupled superconductors
Qin, Wei; Xiao, Di; Chang, Kai; Shen, Shun-Qing; Zhang, Zhenyu
2016-01-01
Normal superconductors with Rashba spin-orbit coupling have been explored as candidate systems of topological superconductors. Here we present a comparative theoretical study of the effects of different types of disorder on the topological phases of two-dimensional Rashba spin-orbit coupled superconductors. First, we show that a topologically trivial superconductor can be driven into a chiral topological superconductor upon diluted doping of isolated magnetic disorder, which close and reopen the quasiparticle gap of the paired electrons in a nontrivial manner. Secondly, the superconducting nature of a topological superconductor is found to be robust against Anderson disorder, but the topological nature is not, converting the system into a topologically trivial state even in the weak scattering limit. These topological phase transitions are distinctly characterized by variations in the topological invariant. We discuss the central findings in connection with existing experiments, and provide new schemes towards eventual realization of topological superconductors. PMID:27991541
NASA Technical Reports Server (NTRS)
Yngvesson, K. Sigfrid; Lau, Kei-May
1992-01-01
This final report describes a three-year research effort, aimed at developing new types of THz low noise receivers, based on bulk effect ('hot electron') nonlinearities in the Two-Dimensional Electron Gas (2DEG) Medium, and the inclusion of such receivers in focal plane arrays. 2DEG hot electron mixers have been demonstrated at 35 and 94 GHz with three orders of magnitude wider bandwidth than previous hot electron mixers, which use bulk InSb. The 2DEG mixers employ a new mode of operation, which was invented during this program. Only moderate cooling is required for this mode, to temperatures in the range 20-77 K. Based on the results of this research, it is now possible to design a hot electron mixer focal plane array for the THz range, which is anticipated to have a DSB receiver noise temperature of 500-1000K. In our work on this grant, we have found similar results the the Cronin group (resident at the University of Bath, UK). Neither group has so far demonstrated heterodyne detection in this mode, however. We discovered and explored some new effects in the magnetic field mode, and these are described in the report. In particular, detection of 94 GHz and 238 GHz, respectively, by a new effect, 'Shubnikov de Haas detection', was found to be considerably stronger in our materials than the cyclotron resonance detection. All experiments utilized devices with an active 2DEG region of size of the order of 10-40 micrometers long, and 20-200 micrometers wide, formed at the heterojunction between AlGaAs and GaAs. All device fabrication was performed in-house. The materials for the devices were also grown in-house, utilizing OMCVD (Organo Metallic Chemical Vapor Deposition). In the course of this grant, we developed new techniques for growing AlGaAs/GaAs with mobilities equalling the highest values published by any laboratory. We believe that the field of hot electron mixers and detectors will grow substantially in importance in the next few years, partly as a result of
Formation of Ideal Rashba States on Layered Semiconductor Surfaces Steered by Strain Engineering
Ming, Wenmei; Wang, Z. F.; Zhou, Miao; Yoon, Mina; Liu, Feng
2015-12-10
Spin splitting of Rashba states in two-dimensional electron system provides a mechanism of spin manipulation for spintronics applications. However, Rashba states realized experimentally to date are often outnumbered by spin-degenerated substrate states at the same energy range, hindering their practical applications. Here, by density functional theory calculation, we show that Au one monolayer film deposition on a layered semiconductor surface β-InSe(0001) can possess “ideal” Rashba states with large spin splitting, which are completely situated inside the large band gap of the substrate. The position of the Rashba bands can be tuned over a wide range with respect to the substrate band edges by experimentally accessible strain. Furthermore, our nonequilibrium Green’s function transport calculation shows that this system may give rise to the long-sought strong current modulation when made into a device of Datta-Das transistor. Similar systems may be identified with other metal ultrathin films and layered semiconductor substrates to realize ideal Rashba states.
Formation of Ideal Rashba States on Layered Semiconductor Surfaces Steered by Strain Engineering
Ming, Wenmei; Wang, Z. F.; Zhou, Miao; ...
2015-12-10
Spin splitting of Rashba states in two-dimensional electron system provides a mechanism of spin manipulation for spintronics applications. However, Rashba states realized experimentally to date are often outnumbered by spin-degenerated substrate states at the same energy range, hindering their practical applications. Here, by density functional theory calculation, we show that Au one monolayer film deposition on a layered semiconductor surface β-InSe(0001) can possess “ideal” Rashba states with large spin splitting, which are completely situated inside the large band gap of the substrate. The position of the Rashba bands can be tuned over a wide range with respect to the substratemore » band edges by experimentally accessible strain. Furthermore, our nonequilibrium Green’s function transport calculation shows that this system may give rise to the long-sought strong current modulation when made into a device of Datta-Das transistor. Similar systems may be identified with other metal ultrathin films and layered semiconductor substrates to realize ideal Rashba states.« less
Lei, X. L.; Liu, S. Y.
2014-06-21
We analyze a phase-sensitive contribution to the oscillating magnetoresistance induced by the combined driving of two microwave fields having commensurate frequencies ω{sub 1} and ω{sub 2} (m{sub 1}ω{sub 1} + m{sub 2}ω{sub 2} = 0 for at least a set of nonzero integers m{sub 1} and m{sub 2}), based on the balance-equation approach to magnetotransport for high-density two-dimensional electron systems. This commensurate oscillating photoresistance not only depends on the frequencies and polarizations of both microwaves, but varies drastically when changing the relative phases of two incident radiation fields. It shows up most significantly in the case of ω{sub 2}/ω{sub 1} = 3 and may lead to a phase-controllable change of more than a factor of two in the total magnetoresistivity in the vicinity of ω{sub 1}/ω{sub c} = 1.5 and 2.5 (ω{sub c} is the cyclotron frequency), when both radiation fields are linearly x-direction polarized.
NASA Astrophysics Data System (ADS)
Kah, Cherno Baba; Yu, M.; Jayanthi, C. S.; Wu, S. Y.
2014-03-01
Our previous study on one-dimensional icosahedral B12 cluster (α-B12) based chain [Bulletin of APS Annual Meeting, p265 (2013)] and ring structures has prompted us to study the two-dimensional (2D) α-B12 based structures. Recently, we have carried out a systematic molecular dynamics study on the structural stabilities and electronic properties of the 2D α-B12 based structures using the SCED-LCAO method [PRB 74, 15540 (2006)]. We have considered several types of symmetry for these 2D structures such as δ3, δ4, δ6 (flat triangular), and α' types. We have found that the optimized structures are energetically in the order of δ6 < α' < δ3 < δ4 which is different from the energy order of α'< δ6 < δ4 < δ3 found in the 2D boron monolayer sheets [ACS Nano 6, 7443 (2012)]. A detailed discussion of this study will be presented. The first author acknowledges the McSweeny Fellowship for supporting his research in this work.
Dynamical spin injection into a two-dimensional electron gas in an AlGaAs/GaAs structure
NASA Astrophysics Data System (ADS)
Ohtomo, Kenro; Ando, Yuichiro; Shinjo, Teruya; Uemura, Tetsuya; Shiraishi, Masashi
A two-dimensional electron system in a GaAs-based heterostructure is the attractive platform for spintronics since it has high mobility and spin-orbit interaction can be modulated by the gate voltage1. Thus, it is a possible platform to realize electric gate-controlled spin transistor2. However, room-temperature spin transport through GaAs-based heterostructure has yet to be shown. We report first spin transport through the quantum well at GaAs/AlGaAs interface at room temperature. We used spin pumping under ferromagnetic resonance to inject spins from the Ni80Fe20 to the GaAs/AlGaAs quantum well. Generated spin current propagated through the 1 μm channel and was detected using spin-charge conversion inverse spin Hall effect in the Pt electrode. In agreement with spin pumping theory, polarity of the spin transport signal was reversed together with magnetization of the Ni80Fe20. This first demonstration of spin transport through a quantum well at a semiconductor heterostructure interface at room temperature opens a way to realize Datta-Das spin-based transistor.1 J. Nitta, et al., PRL 78, 1335 (1997). 2 S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990).
NASA Astrophysics Data System (ADS)
Saxena, Sunil; Freed, Jack H.
1997-02-01
A methodology for obtaining pure absorption two-dimensional electron spin resonance spectra is presented for the case of large inhomogeneous broadening and/or slow motions. For slow motions, the spectra consist of "complex Lorentzians" superimposed with complex weighting factors, presenting a challenge to obtaining absorption spectra. It is shown how absorption-type spectra can be recovered for the two-pulse COSY and SECSY experiments in such cases. For three-pulse 2D ELDOR experiments, absorption lineshapes can be obtained for the autopeaks, whereas the cross peaks would be of mixed-mode character, in general. However, for practical cases the dispersive components in the cross peaks will be relatively small. Theoretical and experimental absorption spectra are provided to illustrate the method and to show the improved resolution obtained from absorption lineshapes. In particular, the variation in linewidths across a SECSY spectrum, which is a key component in elucidating motional dynamics, is clearly rendered in the pure absorption mode. A convenient method for introducing the necessary phase corrections for the slow-motional spectra is also provided.
NASA Astrophysics Data System (ADS)
Chen, Guo-Xiang; Li, Xiang-Guo; Wang, Yun-Peng; Fry, James N.; Cheng, Hai-Ping
2017-01-01
We study the electronic and magnetic structures of quasi-one-dimensional interfaces along the zigzag direction in two-dimensional GaN/SiC heterostructures using first-principles calculations. Four representative heterostructures with six inequivalent interfaces are discussed in detail. Our results indicate that a bulk electric field will develop only when both interfaces feature no gap states and the total net charge at interfaces are of opposite sign. All the geometries studied exhibit an intriguing quasi-one-dimensional conductor character, of which three show finite nonzero magnetic moment. Furthermore, the magnetic moment in one of the systems can be tuned by applying an electric field along the normal direction of monolayer indicating a strong magnetoelectric coupling. Our analysis shows that the magnetization at the interfaces is closely related to the density of states at the Fermi level due to the Stoner instability, and the ribbon-width-dependent magnetization for different geometries implies the existence of an in-bulk electric field.
Ab initio electronic transport study of two-dimensional silicon carbide-based p–n junctions
NASA Astrophysics Data System (ADS)
Zhou, Hanming; Lin, Xiao; Guo, Hongwei; Lin, Shisheng; Sun, Yiwei; Xu, Yang
2017-03-01
Two-dimensional silicon carbide (2d-SiC) is a viable material for next generation electronics due to its moderate, direct bandgap with huge potential. In particular, its potential for p–n junctions is yet to be explored. In this paper, three types of 2d-SiC-based p–n junctions with different doping configuration are modeled. The doping configurations refer to partially replacing carbon with boron or nitrogen atoms along the zigzag or armchair direction, respectively. By employing density functional theory, we calculate the transport properties of the SiC based p–n junctions and obtain negative differential resistance and high rectification ratio. We also find that the junction along the zigzag direction with lower doping density exhibits optimized rectification performance. Our study suggests that 2d-SiC is a promising candidate as a material platform for future nano-devices. Project supported by the National Science Foundation of China (Nos. 61474099, 61674127) and the ZJ-NSF (No. Z17F04003).
Das, Amit K. Misra, P.; Ajimsha, R. S.; Joshi, M. P.; Kukreja, L. M.
2015-09-07
We report the effects of electron interference on temperature dependent transport properties of two dimensional electron gas (2DEG) confined at the interface in polycrystalline MgZnO/ZnO heterostructures grown by pulsed laser deposition on c-alumina substrates. On increasing Mg concentration in the MgZnO layer, the sheet electron concentration was found to increase and the sheet resistance was found to decrease. In addition, the electron concentration and mobility were almost temperature independent in the range from 4.2 to 300 K, indicating the formation of 2DEG at the interface. The temperature dependent resistivity measurements showed a negative temperature coefficient of resistivity at low temperatures together with negative magnetoresistance. These were found to be caused by electron interference effects, and the experimental data could be explained using the models of quantum corrections to conductivity.
Hong, Jinhua; Jin, Chuanhong; Yuan, Jun; Zhang, Ze
2017-04-01
Two-dimensional layered graphene-like crystals including transition-metal dichalcogenides (TMDs) have received extensive research interest due to their diverse electronic, valleytronic, and chemical properties, with the corresponding optoelectronics and catalysis application being actively explored. However, the recent surge in two-dimensional materials science is accompanied by equally great challenges, such as defect engineering in large-scale sample synthesis. It is necessary to elucidate the effect of structural defects on the electronic properties in order to develop an application-specific strategy for defect engineering. Here, two aspects of the existing knowledge of native defects in two-dimensional crystals are reviewed. One is the point defects emerging in graphene and hexagonal boron nitride, as probed by atomically resolved electron microscopy, and their local electronic properties, as measured by single-atom electron energy-loss spectroscopy. The other will focus on the point defects in TMDs and their influence on the electronic structure, photoluminescence, and electric transport properties. This review of atomic defects in two-dimensional materials will offer a clear picture of the defect physics involved to demonstrate the local modulation of the electronic properties and possible benefits in potential applications in magnetism and catalysis.
Two-Dimensional Electronic Spectroscopy of the Photosystem II D1D2-cyt.b559 Reaction Center Complex
NASA Astrophysics Data System (ADS)
Myers, Jeffrey Allen
Two-dimensional electronic spectroscopy (2DES) is a powerful new technique for examining the electronic and vibronic couplings and dynamics of chemical, semiconductor, and biological samples. We present several technical innovations in the implementation of 2DES. We have performed two-color 2DES experiments, extending the technique's ability to study energy transfer to states at frequencies far from the initial absorption. We have demonstrated 2DES in the pump-probe geometry using a pulse-shaper. This method eliminates many technical challenges inherent to previous implementations of 2DES, making it a more widely accessible technique. To broaden the available frequency information, we have demonstrated 2DES with a continuum probe pulse. We have utilized this method to observe vibrational wavepacket dynamics in a laser dye, demonstrating that these dynamics modulate 2D lineshapes and must be accounted for in modelling 2DES data. We perform 2DES studies on the Qy band of the D1D2-cyt.b559 reaction center of plant photosystem II. This reaction center is the core oxygen-evolving complex in plant photosynthesis, taking in light energy and forming a charge separated state capable of splitting water. Understanding the relationship between the structure and function has both fundamental importance and applications to improving artificial light-harvesting. Traditional spectroscopy methods have been unable to completely resolve the time-ordering of energy and charge transfer events or the degree of electronic coupling between chromophores due to severe spectral congestion in the Q y band. 2DES extends previous methods by frequency-resolving an additional dimension to reveal the degree of static disorder and electronic coupling, as well as a detailed picture of energy and charge transfer dynamics that will allow tests of excitonic models of the reaction center. Our data show direct evidence of electronic coupling and rapid sub-ps energy transfer between "blue" and "red
Thermopower enhancement in quantum wells with the Rashba effect
Wu, Lihua; Yang, Jiong; Wang, Shanyu; Wei, Ping; Yang, Jihui E-mail: wqzhang@mail.sic.ac.cn; Zhang, Wenqing E-mail: wqzhang@mail.sic.ac.cn; Chen, Lidong
2014-11-17
We theoretically demonstrate that the thermopower in two-dimensional quantum wells (QWs) can be significantly enhanced by its Rashba spin-splitting effect, governed by the one-dimensional density of states in the low Fermi energy region. The thermopower enhancement is due to the lower Fermi level for a given carrier concentration in Rashba QWs, as compared with that in normal two-dimensional systems without the spin-splitting effect. The degenerate approximation directly shows that larger strength of Rashba effect leads to higher thermopower and consequently better thermoelectric performance in QWs.
Krishtopenko, S. S.; Malyzhenkov, A. V.; Kalinin, K. P.; Ikonnikov, A. V.; Maremyanin, K. V.; Gavrilenko, V. I.; Goiran, M.
2013-12-04
We report a study of electron spin resonance (ESR) in a perpendicular magnetic field in n-type narrow-gap quantum well (QW) heterostructures. Using the Hartree-Fock approximation, based on the 8×8 k⋅p Hamiltonian, the many-body corrections to the ESR energy are found to be nonzero in symmetric and asymmetric narrow-gap QWs. We demonstrate a significant enhancement of the ESR energy in asymmetric QWs, induced by the Rashba spin splitting and exchange interaction, as well as the exchange-induced enhancement of the ESR energy in symmetric QWs. The ESR energies estimated for 2DEG in InAs/AlSb QWs are compared with experimental results in weak magnetic fields.
NASA Astrophysics Data System (ADS)
Choi, M. J.; Park, H. K.; Yun, G. S.; Nam, Y. B.; Choe, G. H.; Lee, W.; Jardin, S.
2016-01-01
The electron cyclotron emission imaging (ECEI) instrument is widely used to study the local electron temperature (Te) fluctuations by measuring the ECE intensity IECE ∝ Te in tokamak plasmas. The ECEI measurement is often processed in a normalized fluctuation quantity against the time averaged value due to complication in absolute calibration. In this paper, the ECEI channels are relatively calibrated using the flat Te assumption of the sawtooth crash or the tearing mode island and a proper extrapolation. The 2-D relatively calibrated electron temperature (Te,rel) images are reconstructed and the displacement amplitude of the magnetohydrodynamic modes can be measured for the accurate quantitative growth analysis.
Fermionic boundary modes in two-dimensional noncentrosymmetric superconductors
NASA Astrophysics Data System (ADS)
Samokhin, K. V.; Mukherjee, S. P.
2016-09-01
We calculate the spectrum of the Andreev boundary modes in a two-dimensional superconductor formed at an interface between two different nonsuperconducting materials, e.g., insulating oxides. Inversion symmetry is absent in this system, and both the electron band structure and the superconducting pairing are strongly affected by the spin-orbit coupling of the Rashba type. We consider isotropic s -wave pairing states, both with and without time-reversal symmetry breaking, as well as various d -wave states. In all cases, there exist subgap Andreev boundary states, whose properties, in particular, the number and location of the zero-energy modes, qualitatively depend on the gap symmetry and the spin-orbit coupling strength.
Jarrett, J W; Yi, C; Stoll, T; Rehault, J; Oriana, A; Branchi, F; Cerullo, G; Knappenberger, K L
2017-03-30
Exciton relaxation dynamics of CdSe and quasi-type-II CdSe/CdS core/shell nanocrystals were examined using femtosecond two-dimensional electronic spectroscopy (2DES). The use of 2DES allowed for determination of structure-specific and state-resolved carrier dynamics for CdSe nanocrystals formed with five, or fewer, CdS passivation monolayers (ML). For CdSe and CdSe/CdS nanocrystals formed with one through three MLs of CdS, excitation using broad bandwidth femtosecond visible laser pulses generated electron-hole pairs among the |X1〉 = 2.14 eV and |X2〉 = 2.27 eV exciton states. For both excitations, the electron is promoted to the lowest energy excited (1Se) conduction-band state and the hole is in the 1S3/2 (X1) or 2S3/2 (X2) valence-band state. Therefore, the relaxation dynamics of the hot hole were isolated by monitoring the-time-dependent amplitude of 2DES cross peaks. The time constant for hot hole relaxation within the CdSe valence band was 150 ± 45 fs. Upon passivation by CdS, this hole relaxation time constant increased to 170 ± 30 fs (CdSe/CdS-3ML). This small increase was attributed to the formation of a graded, or alloyed, interfacial region that precedes the growth of a uniform CdS capping layer. The small increase in hole relaxation time reflects the larger nanocrystal volume of the CdSe/CdS system with respect to the CdSe nanocrystal core. In contrast, the dynamics of larger core/shell nanocrystals (≥4ML CdS) exhibited a picosecond buildup in 2DES cross-peak amplitude. This time-dependent response was attributed to interfacial hole transfer from CdS to CdSe valence-band states. Importantly, the 2DES data distinguish CdSe exciton relaxation from interfacial carrier transfer dynamics. In combination, isolation of structurally well-defined nanocrystals and state-resolved 2DES can be used to examine directly the influence of nanoscale structural modifications on electronic carrier dynamics, which are critical for developing nanocluster
Choi, M. J.; Park, H. K.; Yun, G. S.; Nam, Y. B.; Choe, G. H.; Lee, W.; Jardin, S.
2016-01-15
The electron cyclotron emission imaging (ECEI) instrument is widely used to study the local electron temperature (T{sub e}) fluctuations by measuring the ECE intensity I{sub ECE} ∝ T{sub e} in tokamak plasmas. The ECEI measurement is often processed in a normalized fluctuation quantity against the time averaged value due to complication in absolute calibration. In this paper, the ECEI channels are relatively calibrated using the flat T{sub e} assumption of the sawtooth crash or the tearing mode island and a proper extrapolation. The 2-D relatively calibrated electron temperature (T{sub e,rel}) images are reconstructed and the displacement amplitude of the magnetohydrodynamic modes can be measured for the accurate quantitative growth analysis.
Seebeck effects in two-dimensional spin transistors
NASA Astrophysics Data System (ADS)
Alomar, M. I.; Serra, Llorenç; Sánchez, David
2015-02-01
We consider a spin-orbit-coupled two-dimensional electron system under the influence of a thermal gradient externally applied to two attached reservoirs. We discuss the generated voltage bias (charge Seebeck effect), spin bias (spin Seebeck effect), and magnetization-dependent thermopower (magneto-Seebeck effect) in the ballistic regime of transport at linear response. We find that the charge thermopower is an oscillating function of both the spin-orbit strength and the quantum well width. We also observe that it is always negative for normal leads. We carefully compare the exact results for the linear response coefficients and a Sommerfeld approximation. When the contacts are ferromagnetic, we calculate the spin-resolved Seebeck coefficient for parallel and antiparallel magnetization configuration. Remarkably, the thermopower can change its sign by tuning the Fermi energy. This effect disappears when the Rashba coupling is absent. Additionally, we determine the magneto-Seebeck ratio, which shows dramatic changes in the presence of a the Rashba potential.
NASA Astrophysics Data System (ADS)
Khalaf, E.; Skvortsov, M. A.; Ostrovsky, P. M.
2016-03-01
We study electron transport at the edge of a generic disordered two-dimensional topological insulator, where some channels are topologically protected from backscattering. Assuming the total number of channels is large, we consider the edge as a quasi-one-dimensional quantum wire and describe it in terms of a nonlinear sigma model with a topological term. Neglecting localization effects, we calculate the average distribution function of transmission probabilities as a function of the sample length. We mainly focus on the two experimentally relevant cases: a junction between two quantum Hall (QH) states with different filling factors (unitary class) and a relatively thick quantum well exhibiting quantum spin Hall (QSH) effect (symplectic class). In a QH sample, the presence of topologically protected modes leads to a strong suppression of diffusion in the other channels already at scales much shorter than the localization length. On the semiclassical level, this is accompanied by the formation of a gap in the spectrum of transmission probabilities close to unit transmission, thereby suppressing shot noise and conductance fluctuations. In the case of a QSH system, there is at most one topologically protected edge channel leading to weaker transport effects. In order to describe `topological' suppression of nearly perfect transparencies, we develop an exact mapping of the semiclassical limit of the one-dimensional sigma model onto a zero-dimensional sigma model of a different symmetry class, allowing us to identify the distribution of transmission probabilities with the average spectral density of a certain random-matrix ensemble. We extend our results to other symmetry classes with topologically protected edges in two dimensions.
Tailoring the Two Dimensional Electron Gas at Polar ABO3/SrTiO3 Interfaces for Oxide Electronics
Li, Changjian; Liu, Zhiqi; Lü, Weiming; Wang, Xiao Renshaw; Annadi, Anil; Huang, Zhen; Zeng, Shengwei; Ariando; Venkatesan, T.
2015-01-01
The 2D electron gas at the polar/non-polar oxide interface has become an important platform for several novel oxide electronic devices. In this paper, the transport properties of a wide range of polar perovskite oxide ABO3/SrTiO3 (STO) interfaces, where ABO3 includes LaAlO3, PrAlO3, NdAlO3, NdGaO3 and LaGaO3 in both crystalline and amorphous forms, were investigated. A robust 4 unit cell (uc) critical thickness for metal insulator transition was observed for crystalline polar layer/STO interface while the critical thickness for amorphous ones was strongly dependent on the B site atom and its oxygen affinity. For the crystalline interfaces, a sharp transition to the metallic state (i.e. polarization catastrophe induced 2D electron gas only) occurs at a growth temperature of 515 °C which corresponds to a critical relative crystallinity of ~70 ± 10% of the LaAlO3 overlayer. This temperature is generally lower than the metal silicide formation temperature and thus offers a route to integrate oxide heterojunction based devices on silicon. PMID:26307382
Tailoring the Two Dimensional Electron Gas at Polar ABO3/SrTiO3 Interfaces for Oxide Electronics.
Li, Changjian; Liu, Zhiqi; Lü, Weiming; Wang, Xiao Renshaw; Annadi, Anil; Huang, Zhen; Zeng, Shengwei; Ariando; Venkatesan, T
2015-08-26
The 2D electron gas at the polar/non-polar oxide interface has become an important platform for several novel oxide electronic devices. In this paper, the transport properties of a wide range of polar perovskite oxide ABO3/SrTiO3 (STO) interfaces, where ABO3 includes LaAlO3, PrAlO3, NdAlO3, NdGaO3 and LaGaO3 in both crystalline and amorphous forms, were investigated. A robust 4 unit cell (uc) critical thickness for metal insulator transition was observed for crystalline polar layer/STO interface while the critical thickness for amorphous ones was strongly dependent on the B site atom and its oxygen affinity. For the crystalline interfaces, a sharp transition to the metallic state (i.e. polarization catastrophe induced 2D electron gas only) occurs at a growth temperature of 515 °C which corresponds to a critical relative crystallinity of ~70 ± 10% of the LaAlO3 overlayer. This temperature is generally lower than the metal silicide formation temperature and thus offers a route to integrate oxide heterojunction based devices on silicon.
Tailoring the Two Dimensional Electron Gas at Polar ABO3/SrTiO3 Interfaces for Oxide Electronics
NASA Astrophysics Data System (ADS)
Li, Changjian; Liu, Zhiqi; Lü, Weiming; Wang, Xiao Renshaw; Annadi, Anil; Huang, Zhen; Zeng, Shengwei; Ariando; Venkatesan, T.
2015-08-01
The 2D electron gas at the polar/non-polar oxide interface has become an important platform for several novel oxide electronic devices. In this paper, the transport properties of a wide range of polar perovskite oxide ABO3/SrTiO3 (STO) interfaces, where ABO3 includes LaAlO3, PrAlO3, NdAlO3, NdGaO3 and LaGaO3 in both crystalline and amorphous forms, were investigated. A robust 4 unit cell (uc) critical thickness for metal insulator transition was observed for crystalline polar layer/STO interface while the critical thickness for amorphous ones was strongly dependent on the B site atom and its oxygen affinity. For the crystalline interfaces, a sharp transition to the metallic state (i.e. polarization catastrophe induced 2D electron gas only) occurs at a growth temperature of 515 °C which corresponds to a critical relative crystallinity of ~70 ± 10% of the LaAlO3 overlayer. This temperature is generally lower than the metal silicide formation temperature and thus offers a route to integrate oxide heterojunction based devices on silicon.
Random walk approach to spin dynamics in a two-dimensional electron gas with spin-orbit coupling
Yang, Luyi; Orenstein, J.; Lee, Dung-Hai
2010-09-27
We introduce and solve a semiclassical random walk (RW) model that describes the dynamics of spin polarization waves in zinc-blende semiconductor quantum wells. We derive the dispersion relations for these waves, including the Rashba, linear and cubic Dresselhaus spin-orbit interactions, as well as the effects of an electric field applied parallel to the spin polarization wave vector. In agreement with calculations based on quantum kinetic theory [P. Kleinert and V. V. Bryksin, Phys. Rev. B 76, 205326 (2007)], the RW approach predicts that spin waves acquire a phase velocity in the presence of the field that crosses zero at a nonzero wave vector, q{sub 0}. In addition, we show that the spin-wave decay rate is independent of field at q{sub 0} but increases as (q-q{sub 0}){sup 2} for q {ne} q{sub 0}. These predictions can be tested experimentally by suitable transient spin grating experiments.
Ye, Tianyu; Mani, Ramesh G.; Wegscheider, Werner
2013-12-04
We present the results of a concurrent experimental study of microwave reflection and transport in the GaAs/AlGaAs two dimensional electron gas system and correlate observed features in the reflection with the observed transport features. The experimental results are compared with expectations based on theory.
Guo, Rui; Fournier, Frederic; Donaldson, Paul M; Gardner, Elizabeth M; Gould, Ian R; Klug, David R
2009-10-14
Electrical interactions between molecular vibrations can be non-linear and thereby produce intermolecular coupling even in the absence of a chemical bond. We use this fact to detect the formation of an intermolecular complex using electron-vibration-vibration two-dimensional infrared spectroscopy (EVV 2DIR) and also to determine the distance and angle between the two molecular species.
Shibata, Y. Manabe, T.; Ohno, N.; Takagi, M.; Kajita, S.; Tsuchiya, H.; Morisaki, T.
2014-09-15
A compact and high-particle-flux thermal-lithium-beam source for two-dimensional measurement of electron density profiles has been developed. The thermal-lithium-beam oven is heated by a carbon heater. In this system, the maximum particle flux of the thermal lithium beam was ∼4 × 10{sup 19} m{sup −2} s{sup −1} when the temperature of the thermal-lithium-beam oven was 900 K. The electron density profile was evaluated in the small tokamak device HYBTOK-II. The electron density profile was reconstructed using the thermal-lithium-beam probe data and this profile was consistent with the electron density profile measured with a Langmuir electrostatic probe. We confirm that the developed thermal-lithium-beam probe can be used to measure the two-dimensional electron density profile with high time and spatial resolutions.
Two-dimensional electron gas at the Ti-diffused BiFeO{sub 3}/SrTiO{sub 3} interface
Chen, Chunlin; Li, Junjie; Wang, Zhongchang Liang, Xiaobin; Nakajima, Ken; Lv, Shuhui; Li, Yanxi; Viehland, Dwight; Ikuhara, Yuichi
2015-07-20
Oxide heterostructures with the broken translational symmetry often trigger a two-dimensional quantum confinement and associated unique electronic properties that cannot be observed in bulk constituents. Particular interest is devoted to the formation of two-dimensional electron gas (2DEG) at heterointerfaces between two insulators, which offers a fertile ground for fabricating advanced electronic devices. Here, we combine atomic force microscopy, transmission electron microscopy, and atomistic first-principles calculations to demonstrate that the (100) BiFeO{sub 3}/SrTiO{sub 3} interface takes on a metallic nature and a 2DEG is generated at this interface. Our findings also reveal that the electronic reconstruction due to the polar discontinuity and the variation in valence state of Ti arising from diffusion of Ti cations in SrTiO{sub 3} to Fe sites of BiFeO{sub 3} are critical to the formation of 2DEG at the heterointerface.
NASA Astrophysics Data System (ADS)
Pakmehr, Mehdi; McCombe, B. D.; Bruene, C.; Buhmann, H.; Molenkamp, L. W.
2015-10-01
HgTe quantum wells (QWs) have shown a number of interesting phenomena, recently the first two-dimensional topological insulating state. We have studied thermoelectric photovoltages of two-dimensional electrons in a 6.1 nm wide HgTe QW induced by cyclotron resonance absorption ( B = 2 to 5 T) of a THz laser beam. We have estimated thermopower coefficients by detailed analysis of the photovoltage signals developed across various contacts of a large Hall bar structure at a bath temperature of 1.6 K. The photovoltage signals are washed out at bath temperature of 18 K.
NASA Astrophysics Data System (ADS)
Lewis, Nicholas H. C.; Dong, Hui; Oliver, Thomas A. A.; Fleming, Graham R.
2015-09-01
Two dimensional electronic spectroscopy has proved to be a valuable experimental technique to reveal electronic excitation dynamics in photosynthetic pigment-protein complexes, nanoscale semiconductors, organic photovoltaic materials, and many other types of systems. It does not, however, provide direct information concerning the spatial structure and dynamics of excitons. 2D infrared spectroscopy has become a widely used tool for studying structural dynamics but is incapable of directly providing information concerning electronic excited states. 2D electronic-vibrational (2DEV) spectroscopy provides a link between these domains, directly connecting the electronic excitation with the vibrational structure of the system under study. In this work, we derive response functions for the 2DEV spectrum of a molecular dimer and propose a method by which 2DEV spectra could be used to directly measure the electronic site populations as a function of time following the initial electronic excitation. We present results from the response function simulations which show that our proposed approach is substantially valid. This method provides, to our knowledge, the first direct experimental method for measuring the electronic excited state dynamics in the spatial domain, on the molecular scale.
Lewis, Nicholas H. C.; Dong, Hui; Oliver, Thomas A. A.; Fleming, Graham R.
2015-09-28
Two dimensional electronic spectroscopy has proved to be a valuable experimental technique to reveal electronic excitation dynamics in photosynthetic pigment-protein complexes, nanoscale semiconductors, organic photovoltaic materials, and many other types of systems. It does not, however, provide direct information concerning the spatial structure and dynamics of excitons. 2D infrared spectroscopy has become a widely used tool for studying structural dynamics but is incapable of directly providing information concerning electronic excited states. 2D electronic-vibrational (2DEV) spectroscopy provides a link between these domains, directly connecting the electronic excitation with the vibrational structure of the system under study. In this work, we derive response functions for the 2DEV spectrum of a molecular dimer and propose a method by which 2DEV spectra could be used to directly measure the electronic site populations as a function of time following the initial electronic excitation. We present results from the response function simulations which show that our proposed approach is substantially valid. This method provides, to our knowledge, the first direct experimental method for measuring the electronic excited state dynamics in the spatial domain, on the molecular scale.
Interaction of two-dimensional magnetoexcitons
NASA Astrophysics Data System (ADS)
Dumanov, E. V.; Podlesny, I. V.; Moskalenko, S. A.; Liberman, M. A.
2017-04-01
We study interaction of the two-dimensional magnetoexcitons with in-plane wave vector k→∥ = 0 , taking into account the influence of the excited Landau levels (ELLs) and of the external electric field perpendicular to the surface of the quantum well and parallel to the external magnetic field. It is shown that the account of the ELLs gives rise to the repulsion between the spinless magnetoexcitons with k→∥ = 0 in the Fock approximation, with the interaction constant g decreasing inverse proportional to the magnetic field strength B (g (0) ∼ 1 / B) . In the presence of the perpendicular electric field the Rashba spin-orbit coupling (RSOC), Zeeman splitting (ZS) and nonparabolicity of the heavy-hole dispersion law affect the Landau quantization of the electrons and holes. They move along the new cyclotron orbits, change their Coulomb interactions and cause the interaction between 2D magnetoexcitons with k→∥ = 0 . The changes of the Coulomb interactions caused by the electrons and by the holes moving with new cyclotron orbits are characterized by some coefficients, which in the absence of the electric field turn to be unity. The differences between these coefficients of the electron-hole pairs forming the magnetoexcitons determine their affinities to the interactions. The interactions between the homogeneous, semihomogeneous and heterogeneous magnetoexcitons forming the symmetric states with the same signs of their affinities are attractive whereas in the case of different sign affinities are repulsive. In the heterogeneous asymmetric states the interactions have opposite signs in comparison with the symmetric states. In all these cases the interaction constant g have the dependence g (0) 1 /√{ B} .
An electrical characterization of a two-dimensional electron gas in GaN/AlGaN on silicon substrates
NASA Astrophysics Data System (ADS)
Elhamri, S.; Berney, R.; Mitchel, W. C.; Mitchell, W. D.; Roberts, J. C.; Rajagopal, P.; Gehrke, T.; Piner, E. L.; Linthicum, K. J.
2004-06-01
We present results of transport measurements performed on AlGaN/GaN heterostructures grown on silicon substrates. Variable temperature Hall effect measurements revealed that the temperature dependence of the carrier density and mobility were characteristic of a two-dimensional electron gas (2DEG). Carrier densities greater than 1×1013cm-2 and Hall mobilities in excess of 1500 cm2/V s were measured at room temperature. Variable field Hall measurements at low temperatures, and in magnetic fields up to 6 T, indicated that conduction is dominated by a single carrier type in these samples. Shubnikov-de Haas (SdH) measurements were also performed, but no oscillations were observed in fields up to 8 T and at temperatures as low as 1.2 K. Illuminating some of the samples with a blue (λ=470 nm) light emitting diode (LED) induced a persistent increase in the carrier density. SdH measurements were repeated and again no oscillations were present following illumination. However, exposing the samples to radiation from an UV (λ=395 nm) LED induced well-defined SdH oscillations in fields as low as 4 T. The observation of SdH oscillations confirmed the presence of a 2DEG in these structures. It is hypothesized that small angle scattering suppressed the oscillations before exposure to UV light. This conclusion is supported by the observed increase in the quantum scattering time, τq, with the carrier density and the calculated quantum to transport scattering times ratio, τq/τc. For instance, in one of the samples the τq increased by 32% while the τc changed by only 3% as the carrier density increased; an indication of an increase in the screening of small angle scattering. The absence of SdH oscillations in fields up to 8 T and at temperatures as low as 1.2 K is not unique to AlGaN/GaN on silicon. This behavior was observed in AlGaN/GaN on sapphire and on silicon carbide. SdH oscillations were observed in one AlGaN/GaN on silicon carbide sample following exposure to radiation
NASA Astrophysics Data System (ADS)
Gascooke, Jason R.; Alexander, Ula N.; Lawrance, Warren D.
2011-05-01
We demonstrate the power of high resolution, two dimensional laser induced fluorescence (2D-LIF) spectroscopy for observing rovibronic transitions of polyatomic molecules. The technique involves scanning a tunable laser over absorption features in the electronic spectrum while monitoring a segment, in our case 100 cm-1 wide, of the dispersed fluorescence spectrum. 2D-LIF images separate features that overlap in the usual laser induced fluorescence spectrum. The technique is illustrated by application to the S1-S0 transition in fluorobenzene. Images of room temperature samples show that overlap of rotational contours by sequence band structure is minimized with 2D-LIF allowing a much larger range of rotational transitions to be observed and high precision rotational constants to be extracted. A significant advantage of 2D-LIF imaging is that the rotational contours separate into their constituent branches and these can be targeted to determine the three rotational constants individually. The rotational constants determined are an order of magnitude more precise than those extracted from the analysis of the rotational contour and we find the previously determined values to be in error by as much as 5% [G. H. Kirby, Mol. Phys. 19, 289 (1970), 10.1080/00268977000101291]. Comparison with earlier ab initio calculations of the S0 and S1 geometries [I. Pugliesi, N. M. Tonge, and M. C. R. Cockett, J. Chem. Phys. 129, 104303 (2008), 10.1063/1.2970092] reveals that the CCSD/6-311G** and RI-CC2/def2-TZVPP levels of theory predict the rotational constants, and hence geometries, with comparable accuracy. Two ground state Fermi resonances were identified by the distinctive patterns that such resonances produce in the images. 2D-LIF imaging is demonstrated to be a sensitive method capable of detecting weak spectral features, particularly those that are otherwise hidden beneath stronger bands. The sensitivity is demonstrated by observation of the three isotopomers of fluorobenzene
Gascooke, Jason R; Alexander, Ula N; Lawrance, Warren D
2011-05-14
We demonstrate the power of high resolution, two dimensional laser induced fluorescence (2D-LIF) spectroscopy for observing rovibronic transitions of polyatomic molecules. The technique involves scanning a tunable laser over absorption features in the electronic spectrum while monitoring a segment, in our case 100 cm(-1) wide, of the dispersed fluorescence spectrum. 2D-LIF images separate features that overlap in the usual laser induced fluorescence spectrum. The technique is illustrated by application to the S(1)-S(0) transition in fluorobenzene. Images of room temperature samples show that overlap of rotational contours by sequence band structure is minimized with 2D-LIF allowing a much larger range of rotational transitions to be observed and high precision rotational constants to be extracted. A significant advantage of 2D-LIF imaging is that the rotational contours separate into their constituent branches and these can be targeted to determine the three rotational constants individually. The rotational constants determined are an order of magnitude more precise than those extracted from the analysis of the rotational contour and we find the previously determined values to be in error by as much as 5% [G. H. Kirby, Mol. Phys. 19, 289 (1970)]. Comparison with earlier ab initio calculations of the S(0) and S(1) geometries [I. Pugliesi, N. M. Tonge, and M. C. R. Cockett, J. Chem. Phys. 129, 104303 (2008)] reveals that the CCSD∕6-311G∗∗ and RI-CC2∕def2-TZVPP levels of theory predict the rotational constants, and hence geometries, with comparable accuracy. Two ground state Fermi resonances were identified by the distinctive patterns that such resonances produce in the images. 2D-LIF imaging is demonstrated to be a sensitive method capable of detecting weak spectral features, particularly those that are otherwise hidden beneath stronger bands. The sensitivity is demonstrated by observation of the three isotopomers of fluorobenzene-d(1) in natural abundance in
NASA Astrophysics Data System (ADS)
Toyoda, Tadashi; Hiraiwa, Nobuyoshi; Fukuda, Taturo; Koizumi, Hideki
2008-01-01
The filling-factor-dependent plateau-type dispersion of the long-wavelength magnetoplasmon in high-mobility two-dimensional electron system observed by Holland et al. [Phys. Rev. Lett. 93, 186804 (2004)PRLTAO0031-900710.1103/PhysRevLett.93.186804] can be explained by the well-established semiclassical dispersion, by adopting the electron reservoir hypothesis previously proposed in order to explain the integer quantum Hall effects.
Extrinsic Rashba spin-orbit coupling effect on silicene spin polarized field effect transistors.
Pournaghavi, Nezhat; Esmaeilzadeh, Mahdi; Abrishamifar, Adib; Ahmadi, Somaieh
2017-04-12
Regarding the spin field effect transistor (spin FET) challenges such as mismatch effect in spin injection and insufficient spin life time, we propose a silicene based device which can be a promising candidate to overcome some of those problems. Using non-equilibrium Green's function method, we investigate the spin-dependent conductance in a zigzag silicene nanoribbon connected to two magnetized leads which are supposed to be either in parallel or anti-parallel configurations. For both configurations, a controllable spin current can be obtained when the Rashba effect is present; thus, we can have a spin filter device. In addition, for anti-parallel configuration, in the absence of Rashba effect, there is an intrinsic energy gap in the system (OFF-state); while, in the presence of Rashba effect, electrons with flipped spin can pass through the channel and make the ON-state. The current voltage (I-V) characteristics which can be tuned by changing the gate voltage or Rashba strength, are studied. More importantly, reducing the mismatch conductivity as well as energy consumption make the silicene based spin FET more efficient relative to the spin FET based on two-dimensional electron gas proposed by Datta and Das. Also, we show that, at the same conditions, the current and [Formula: see text] ratio of silicene based spin FET are significantly greater than that of the graphene based one.
Extrinsic Rashba spin–orbit coupling effect on silicene spin polarized field effect transistors
NASA Astrophysics Data System (ADS)
Pournaghavi, Nezhat; Esmaeilzadeh, Mahdi; Abrishamifar, Adib; Ahmadi, Somaieh
2017-04-01
Regarding the spin field effect transistor (spin FET) challenges such as mismatch effect in spin injection and insufficient spin life time, we propose a silicene based device which can be a promising candidate to overcome some of those problems. Using non-equilibrium Green’s function method, we investigate the spin-dependent conductance in a zigzag silicene nanoribbon connected to two magnetized leads which are supposed to be either in parallel or anti-parallel configurations. For both configurations, a controllable spin current can be obtained when the Rashba effect is present; thus, we can have a spin filter device. In addition, for anti-parallel configuration, in the absence of Rashba effect, there is an intrinsic energy gap in the system (OFF-state); while, in the presence of Rashba effect, electrons with flipped spin can pass through the channel and make the ON-state. The current voltage (I–V) characteristics which can be tuned by changing the gate voltage or Rashba strength, are studied. More importantly, reducing the mismatch conductivity as well as energy consumption make the silicene based spin FET more efficient relative to the spin FET based on two-dimensional electron gas proposed by Datta and Das. Also, we show that, at the same conditions, the current and {{I}\\text{on}}/{{I}\\text{off}} ratio of silicene based spin FET are significantly greater than that of the graphene based one.
A Study of Electron and Phonon Dynamics by Broadband Two-Dimensional THz Time-Domain Spectroscopy
NASA Astrophysics Data System (ADS)
Fu, Zhengping
Terahertz (THz) wave interacts with semiconductors in many ways, such as resonant excitation of lattice vibration, intraband transition and polaron formation. Different from the optical waves, THz wave has lower photon energy (1 THz = 4.14 meV) and is suitable for studying dynamics of low-energy excitations. Recently the studies of the interaction of THz wave and semiconductors have been extending from the linear regime to the nonlinear regime, owing to the advance of the high-intensity THz generation and detection methods. Two-dimensional (2D) spectroscopy, as a useful tool to unravel the nonlinearity of materials, has been well developed in nuclear magnetic resonance and infrared region. However, the counterpart in THz region has not been well developed and was only demonstrated at frequency around 20 THz due to the lack of intense broadband THz sources. Using laser-induced plasma as the THz source, we developed collinear broadband 2D THz time-domain spectroscopy covering from 0.5 THz to 20 THz. Broadband intense THz pulses emitted from laser-induced plasma provide access to a variety of nonlinear properties of materials. Ultrafast optical and THz pulses make it possible to resolve the transient change of the material properties with temporal resolution of tens of femtoseconds. This thesis focuses on the linear and nonlinear interaction of the THz wave with semiconductors. Since a great many physical processes, including vibrational motion of lattice and plasma oscillation, has resonant frequency in the THz range, rich physics can be studies in our experiment. The thesis starts from the linear interaction of the THz wave with semiconductors. In the narrow band gap semiconductor InSb, the plasma absorption edge, Restrahlen band and dispersion of polaritons are observed. The nonlinear response of InSb in high THz field is verified in the frequency-resolved THz Z-scan experiment. The third harmonic generations due to the anharmonicity of plasma oscillation and the
NASA Astrophysics Data System (ADS)
Ahadi, Kaveh; Shoron, Omor F.; Marshall, Patrick B.; Mikheev, Evgeny; Stemmer, Susanne
2017-02-01
SmTiO3/SrTiO3 interfaces exhibit a two-dimensional electron system with carrier densities in the order of 3 × 1014 cm-2 due to the polar discontinuity at the interface. Here, electric field effect is used to investigate an electron system at this interface whose carrier density has been depleted substantially by the gate metal and by reducing the thickness of the SmTiO3. At zero applied gate voltage, the sheet resistance exceeds the quantum resistance, h/e2, by more than an order of magnitude, and the SrTiO3 channel is in the hopping transport regime. The electric field modulates the carrier density in the channel, which approaches the transition to a metal at positive gate bias. The channel resistances are found to scale by a single parameter that depends on the gate voltage, similar to two-dimensional electron systems in high-quality semiconductors.
NASA Astrophysics Data System (ADS)
Chang, C.-P.; Chu, M.-W.; Jeng, H. T.; Cheng, S.-L.; Lin, J. G.; Yang, J.-R.; Chen, C. H.
2014-03-01
The success of semiconductor technology is largely ascribed to controlled impacts of strains and defects on the two-dimensional interfacial charges. Interfacial charges also appear in oxide heterojunctions such as LaAlO3/SrTiO3 and (Nd0.35Sr0.65)MnO3/SrTiO3. How the localized strain field of one-dimensional misfit dislocations, defects resulting from the intrinsic misfit strains, would affect the extended oxide-interfacial charges is intriguing and remains unresolved. Here we show the atomic-scale observation of one-dimensional electron chains formed in (Nd0.35Sr0.65)MnO3/SrTiO3 by the condensation of characteristic two-dimensional interfacial charges into the strain field of periodically arrayed misfit dislocations, using chemical mapping and quantification by scanning transmission electron microscopy. The strain-relaxed inter-dislocation regions are readily charge depleted, otherwise decorated by the pristine charges, and the corresponding total-energy calculations unravel the undocumented charge-reservoir role played by the dislocation-strain field. This two-dimensional-to-one-dimensional electronic condensation represents a novel electronic-inhomogeneity mechanism at oxide interfaces and could stimulate further studies of one-dimensional electron density in oxide heterostructures.
Spin-Orbit Interaction in High-κ Dielectric Gated Rashba-2D Electron Gas and Mesoscopic Rings
NASA Astrophysics Data System (ADS)
Dai, Yanhua; Yuan, Zhuoquan; Stone, Kristjan; Du, Rui-Rui; Xu, Min; Ye, Peide
2008-03-01
There is increasing current interest in the quantum interference effect in mesoscopic devices fabricated on a Rashba-2D electron gas (2DEG), where the spin-orbit interaction parameters can be tuned by a potential gate. We explore ring structures that use a gate consisting of thin (5nm-50nm) high-κ dielectric Al2O3 or HfO2 layer and nano-patterned metals. The 2DEG is provided by lattice-matched In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum wells that have a typical electron density n of 1.5x10^12/cm^2 and mobility μ>=2x10^4cm^2/Vs. The dielectric material was grown by atomic layer deposition. We will present the gate characteristics of Hall bars as well as magnetic transport data from gated mesoscopic rings. The work at Rice is funded by NSF DMR-0706634. Reference: M. Konig et al, Phys. Rev. Lett. 96, 076804 (2006); T. Bergsten et al, Phys. Rev. Lett. 97, 196803 (2006); B. Grbic et al, Phys. Rev. Lett. 99, 176803 (2007).
Li, Zhaoguo; Chen, Taishi; Pan, Haiyang; Song, Fengqi; Wang, Baigeng; Han, Junhao; Qin, Yuyuan; Wang, Xuefeng; Zhang, Rong; Wan, Jianguo; Xing, Dingyu; Wang, Guanghou
2012-01-01
The universal conductance fluctuations (UCFs), one of the most important manifestations of mesoscopic electronic interference, have not yet been demonstrated for the two-dimensional surface state of topological insulators (TIs). Even if one delicately suppresses the bulk conductance by improving the quality of TI crystals, the fluctuation of the bulk conductance still keeps competitive and difficult to be separated from the desired UCFs of surface carriers. Here we report on the experimental evidence of the UCFs of the two-dimensional surface state in the bulk insulating Bi2Te2Se microflakes. The solely-B⊥-dependent UCF is achieved and its temperature dependence is investigated. The surface transport is further revealed by weak antilocalizations. Such survived UCFs of the surface states result from the limited dephasing length of the bulk carriers in ternary crystals. The electron-phonon interaction is addressed as a secondary source of the surface state dephasing based on the temperature-dependent scaling behavior. PMID:22916331
Li, Zhaoguo; Chen, Taishi; Pan, Haiyang; Song, Fengqi; Wang, Baigeng; Han, Junhao; Qin, Yuyuan; Wang, Xuefeng; Zhang, Rong; Wan, Jianguo; Xing, Dingyu; Wang, Guanghou
2012-01-01
The universal conductance fluctuations (UCFs), one of the most important manifestations of mesoscopic electronic interference, have not yet been demonstrated for the two-dimensional surface state of topological insulators (TIs). Even if one delicately suppresses the bulk conductance by improving the quality of TI crystals, the fluctuation of the bulk conductance still keeps competitive and difficult to be separated from the desired UCFs of surface carriers. Here we report on the experimental evidence of the UCFs of the two-dimensional surface state in the bulk insulating Bi2Te2Se microflakes. The solely-B⊥-dependent UCF is achieved and its temperature dependence is investigated. The surface transport is further revealed by weak antilocalizations. Such survived UCFs of the surface states result from the limited dephasing length of the bulk carriers in ternary crystals. The electron-phonon interaction is addressed as a secondary source of the surface state dephasing based on the temperature-dependent scaling behavior.
Two-dimensional electron gas in an n(+)-GaAs/undoped AlGaAs/undoped GaAs SIS structure
NASA Astrophysics Data System (ADS)
Wada, T.; Matsumoto, K.; Ogura, M.; Shida, K.; Yao, T.; Igarashi, T.; Hashizume, N.; Hayashi, Y.
1985-03-01
The two-dimensionality of the electronic system in a new self-aligned GaAs MIS-like FET having an n(+)-GaAs/undoped AlGaAs/undoped GaAs SIS structure is demonstrated by the angular-dependent characteristics of SdH effects. The mobility of a two-dimensional electron gas in a GaAs-SISFET is shown to be 120,000 sq cm/V x s with a sheet-carrier concentration of 6.6 x 10 to the 11th/sq cm at 4.2 K and VG = -/0.6 V. The quantized Hall effect is realized by changing gate voltages at as low a magnetic field as 3.5 T.
Bhadauria, P. P. S.; Gupta, Anurag; Kumar, Pramod; Dogra, Anjana; Budhani, R. C.
2015-05-15
A fiber optic based probe is designed and developed for electrical transport measurements in presence of quasi-monochromatic (360–800 nm) light, varying temperature (T = 1.8–300 K), and magnetic field (B = 0–7 T). The probe is tested for the resistivity and Hall measurements performed on a LaAlO{sub 3}–SrTiO{sub 3} heterointerface system with a conducting two dimensional electron gas.
NASA Astrophysics Data System (ADS)
Fang, Cheng; Wang, Zhi-Gang; Li, Shu-Shen; Zhang, Ping
2009-10-01
The magnetisation of heavy holes in III-V semiconductor quantum wells with Rashba spin-orbit coupling (SOC) in an external perpendicular magnetic field is studied theoretically. We concentrate on the effects on the magnetisation induced by the system boundary, the Rashba SOC and the temperature. It is found that the sawtooth-like de Haasvan Alphen (dHvA) oscillations of the magnetisation will change dramatically in the presence of such three factors. Especially, the effects of the edge states and Rashba SOC on the magnetisation are more evident when the magnetic field is smaller. The oscillation center will shift when the boundary effect is considered and the Rashba SOC will bring beating patterns to the dHvA oscillations. These effects on the dHvA oscillations are preferably observed at low temperatures. With increasing temperature, the dHvA oscillations turn to be blurred and eventually disappear.
NASA Astrophysics Data System (ADS)
Wadsworth, D. C.; Mudawar, I.
1990-11-01
Experiments were performed to investigate single-phase heat transfer from a smooth 12.7 x 12.7-sq-mm simulated chip to a two-dimensional jet of dielectric FC-72 liquid issuing from a thin rectangular slot into a channel confined between the chip surface and nozzle plate. The effects of jet width, confinement channel height, and impingement velocity have been examined. Channel height had a negligible effect on the heat-transfer performance of the jet. A correlation for the convective heat-transfer coefficient is presented as a function of jet width, heater length, flow velocity, and fluid properties. A self-contained multichip cooling module consisting of a 3 x 3 array of heat sources confirmed the uniformity and predictability of cooling for each of the nine chips, and proved the cooling module is well suited for packaging large arrays of high-power-density chips.
Wadsworth, D.C.; Mudawar, I. )
1990-11-01
Experiments were performed to investigate single-phase heat transfer froma smooth 12.7 {times} 12.7 mm{sup 2} simulated chip to a two-dimensional jet of dielectric Fluorinert FC-72 liquid issuing from a thin rectangular slot into a channel confined between the chip surface and nozzle plate. The effects of jet width, confined channel height, and impingement velocity have been examined. Channel height had a negligible effect ont eh theat transfer performance of the jet for the conditions of the present study. A correlation for the convective heat transfer coefficient is presented as a function of jet, width, heat length, flow velocity, and fluid properties. A self-contained multichip cooling module consisting of a 3 {times} 3 array of heat sources confirmed the uniformity and predictability of cooling for each of the nine chips, and proved the cooling module is well suited for packaging large arrays of high-power density chips.
Charge-density oscillations on Be(10{bar 1}0): Screening in a non-free two-dimensional electron gas
Briner, B.G.; Hofmann, P. ||; Doering, M.; Rust, H.; Plummer, E.W. |; Bradshaw, A.M.
1998-11-01
The surface state on Be(10{bar 1}0) has been investigated using a low-temperature scanning tunneling microscope (STM). The Fermi contour of this surface state is located at one boundary of the surface Brillouin zone, and surface-state electrons provide the main part of the charge density near the Fermi energy. Be(10{bar 1}0), therefore, corresponds closely to a non-free two-dimensional electron gas. We have observed standing waves of the surface charge density on Be(10{bar 1}0) near step edges and point defects. Such wave patterns derive from the interference of incoming and scattered electrons; they demonstrate the screening characteristics of the surface state. On Be(10{bar 1}0) these waves were found to be highly anisotropic. It is shown that calculating the Fourier transforms of topographic STM images is a powerful method for determining the Fermi contour of the surface state. This method could even be applied to images that display a complex wave pattern arising from a random distribution of point scatterers. Fourier analysis also revealed that the charge density oscillations on Be(10{bar 1}0) contain multiple periods that differ by reciprocal lattice vectors. These multiperiodic oscillations relate to the non-free character of the surface-state electrons and constitute an interference pattern of Bloch states. Fourier filtering was used to separate the charge-density oscillations from the topographic corrugation and to visualize their shape and spatial range. The experimental data are qualitatively discussed using a model calculation based on the scattering of Bloch electrons from planar obstacles in a two-dimensional conductor. Experimental results and model calculations highlight how the screening characteristics on Be(10{bar 1}0) significantly deviate from the behavior expected for a free two-dimensional electron gas. {copyright} {ital 1998} {ital The American Physical Society}
NASA Astrophysics Data System (ADS)
Nishio, Yui; Tange, Takahiro; Hirayama, Naomi; Iida, Tsutomu; Takanashi, Yoshifumi
2014-01-01
The energy states of a two-dimensional electron gas (2DEG) in high-electron-mobility transistors with a pseudomorphically strained InAs channel (PHEMTs) were analyzed rigorously using a recently established theory that takes into account the nonparabolicity of the conduction band of the channel layer. The sheet density of the 2DEG in InxGa1-xAs-PHEMTs and the drain I-V characteristics of those devices were calculated theoretically and compared with the density and characteristics obtained experimentally. Not only the calculated threshold voltage (VTH) but also the calculated transconductance agreed fairly well with the corresponding values obtained experimentally. When the effects of the compositions of the InxGa1-xAs subchannel layer in the composite channel and the channel layer on energy states of 2DEG were investigated in order to establish a guiding principle for a design of the channel structure in PHEMTs, it was found that VTH is determined by the effective conduction-band offset energy ΔEC between the InAlAs barrier and the channel layers.
NASA Astrophysics Data System (ADS)
Enyashin, A. N.; Ivanovskii, A. L.
2013-11-01
The structural, electronic properties and stability of the new MXene compounds—two-dimensional pristine carbonitrides Ti3C2-xNx and their hydroxylated derivatives Ti3C2-xNx(OH)2 are studied by means of DFTB calculations. The genesis of the properties is discussed in the sequence: binary MXenes Ti3C2 (Ti3N2)→hydroxylated forms Ti3C2(OH)2 (Ti3N2(OH)2)→pristine MXene Ti3C2-xNx→hydroxylated Ti3C2-xNx(OH)2. All examined materials are metallic-like. The most favorable type of OH-covering is presented by the occupation of the hollow sites between three neighboring carbon (nitrogen) atoms. Two-dimensional MXene carbonitrides with random distribution of C and N atoms are found to be thermodynamically more favorable.
Lou, Zaizhu; Fujitsuka, Mamoru; Majima, Tetsuro
2017-02-16
Two-dimensional Au-nanoprism/reduced graphene oxide (rGO)/Pt-nanoframe was synthesized as plasmonic photocatalyt, exhibiting activity of photocatalytic hydrogen generation greater than those of Au-nanorod/rGO/Pt-nanoframe and metallic plasmonic photocatalyst Pt-Au. The single-particle plasmonic photoluminescence study demonstrated that Au-nanorod has only a longitudinal plasmon resonance mode for hot electron transfer to rGO, while Au-nanoprism has in-plane dipole and multipole surface plasmon resonance modes for hot electron transfer, leading to highly efficient charge separation for hydrogen generation.
Dorozhkin, S. I. Sychev, D. V.; Kapustin, A. A.
2014-11-28
We have implemented a new bolometric method to detect resonances in magneto-absorption of microwave radiation by two-dimensional electron systems (2DES) in selectively doped GaAs/AlGaAs heterostructures. Radiation is absorbed by the 2DES and the thermally activated conductivity of the doping layer supplying electrons to the 2DES serves as a thermometer. The resonant absorption brought about by excitation of the confined magnetoplasma modes appears as peaks in the magnetic field dependence of the low-frequency impedance measured between the Schottky gate and 2DES.
Song, Qi; Zhang, Hongrui; Su, Tang; Yuan, Wei; Chen, Yangyang; Xing, Wenyu; Shi, Jing; Sun, Jirong; Han, Wei
2017-03-01
The Rashba physics has been intensively studied in the field of spin orbitronics for the purpose of searching novel physical properties and the ferromagnetic (FM) magnetization switching for technological applications. We report our observation of the inverse Edelstein effect up to room temperature in the Rashba-split two-dimensional electron gas (2DEG) between two insulating oxides, SrTiO3 and LaAlO3, with the LaAlO3 layer thickness from 3 to 40 unit cells (UC). We further demonstrate that the spin voltage could be markedly manipulated by electric field effect for the 2DEG between SrTiO3 and 3-UC LaAlO3. These results demonstrate that the Rashba-split 2DEG at the complex oxide interface can be used for efficient charge-and-spin conversion at room temperature for the generation and detection of spin current.
Song, Qi; Zhang, Hongrui; Su, Tang; Yuan, Wei; Chen, Yangyang; Xing, Wenyu; Shi, Jing; Sun, Jirong; Han, Wei
2017-01-01
The Rashba physics has been intensively studied in the field of spin orbitronics for the purpose of searching novel physical properties and the ferromagnetic (FM) magnetization switching for technological applications. We report our observation of the inverse Edelstein effect up to room temperature in the Rashba-split two-dimensional electron gas (2DEG) between two insulating oxides, SrTiO3 and LaAlO3, with the LaAlO3 layer thickness from 3 to 40 unit cells (UC). We further demonstrate that the spin voltage could be markedly manipulated by electric field effect for the 2DEG between SrTiO3 and 3-UC LaAlO3. These results demonstrate that the Rashba-split 2DEG at the complex oxide interface can be used for efficient charge-and-spin conversion at room temperature for the generation and detection of spin current. PMID:28345050
Valley spin polarization by using the extraordinary Rashba effect on silicon
NASA Astrophysics Data System (ADS)
Sakamoto, Kazuyuki; Kim, Tae-Hwan; Kuzumaki, Takuya; Müller, Beate; Yamamoto, Yuta; Ohtaka, Minoru; Osiecki, Jacek R.; Miyamoto, Koji; Takeichi, Yasuo; Harasawa, Ayumi; Stolwijk, Sebastian D.; Schmidt, Anke B.; Fujii, Jun; Uhrberg, R. I. G.; Donath, Markus; Yeom, Han Woong; Oda, Tatsuki
2013-06-01
The addition of the valley degree of freedom to a two-dimensional spin-polarized electronic system provides the opportunity to multiply the functionality of next-generation devices. So far, however, such devices have not been realized due to the difficulty to polarize the valleys, which is an indispensable step to activate this degree of freedom. Here we show the formation of 100% spin-polarized valleys by a simple and easy way using the Rashba effect on a system with C3 symmetry. This polarization, which is much higher than those in ordinary Rashba systems, results in the valleys acting as filters that can suppress the backscattering of spin-charge. The present system is formed on a silicon substrate, and therefore opens a new avenue towards the realization of silicon spintronic devices with high efficiency.