GaAs Two-dimensional Electron Gas Research Articles

Calculation method for the conductance of mesoscopic systems: assembly of Tight Binding Hamiltonians with spin effects—revisited

We discretize the Schrödinger equation in the approximation of the effective mass for the two-dimensional electron gas of GaAs, without magnetic field and on the other hand, with magnetic field. This discretization leads naturally to Tight Binding (TB) Hamiltonians in the approximation of the effective mass. An analysis of this discretization allows us to gain insight into the role of site and hopping energies, which allows us to model the TB Hamiltonian assembly with spin: Zeeman and spin–orbit coupling effects, especially the case Rashba. With this tool we can assemble Hamiltonians of quantum boxes, Aharanov–Bohm interferometers, anti-dots lattices and effects of imperfections, as well as disorder in the system. The extension to mount quantum billiards is natural. We also explain here how to adapt the recursive equations of Green’s functions for the case of spin modes, apart from transverse modes, for the calculation of conductance in these mesoscopic systems. The assembled Hamiltonians allow to identify the matrix elements (depending on the different parameters of the system) associated with splitting or spin flipping, which gives a starting point to model specific systems of interest, manipulating certain parameters. In general, the approach of this work allows us to clearly see the relationship between the wave and matrix description of quantum mechanics. We discuss here also, the extension of the method for 1D and 3D systems, for the extension apart from the first neighbors and for the inclusion of other types of interaction. The way we approach the method, has the objective of showing how specifically the site and hopping energies change in the presence of new interactions. This is very important in the case of spin interactions, because by looking at the matrix elements (site or hopping) we can directly identify the conditions that can lead to splitting, flipping or a mixture of these effects. Which is essential for the design of devices based on spintronics. Finally, we discuss spin-conductance modulation (Rashba spin precession) for the states of an open quantum dot (resonant states). Unlike the case of a quantum wire, the spin-flipping observed in the conductance is not perfectly sinusoidal, there is an envelope that modulates the sinusoidal component, which depends on the discrete-continuous coupling of the resonant states.

Unified approach to cyclotron and plasmon resonances in a periodic two-dimensional GaAs electron gas hosting the Hofstadter butterfly

We present theoretical calculations for the cyclotron resonance and various magnetoplasmon modes of a Coulomb interacting two-dimensional GaAs electron gas (2DEG) modulated as a lateral superlattice of quantum dots subjected to an external perpendicular constant magnetic field. We use a real-time excitation approach based on the Liouville-von Neumann equation for the density operator, that can go beyond linear response delivering information of all longitudinal and transverse collective modes of interest to the same order. We perform an extensive analysis of the coexisting collective modes due to the lateral confinement and the magnetic field for a different number of electrons in each dot. In the limit of vanishing dot modulation of the 2DEG we find signs of the structure of the Hofstadter butterfly in the excitation spectra.

Control of spin relaxation anisotropy by spin-orbit-coupled diffusive spin motion

Spatiotemporal spin dynamics under spin-orbit interaction is investigated in a (001) GaAs two-dimensional electron gas using magneto-optical Kerr rotation microscopy. Spin polarized electrons are diffused away from the excited position, resulting in spin precession because of the diffusion-induced spin-orbit field. Near the cancellation between spin-orbit field and external magnetic field, the induced spin precession frequency depends nonlinearly on the diffusion velocity, which is unexpected from the conventional linear relation between the spin-orbit field and the electron velocity.This behavior originates from an enhancement of the spin relaxation anisotropy by the electron velocity perpendicular to the diffused direction. We demonstrate that the spin relaxation anisotropy, which has been regarded as a material constant, can be controlled via diffusive electron motion.

Spatial and energy resolution of electronic states by shot noise

Shot noise measurements are widely used for the characterization of nonequilibrium configurations in electronic conductors. The recently introduced quantum tomography approach was implemented for the studies of electronic wavefunctions of few-electron excitations created by periodic voltage pulses in phase-coherent ballistic conductors based on the high-quality GaAs two-dimensional electron gas. Still relying on the manifestation of Fermi correlations in noise, we focus on the simpler and more general approach beneficial for the local measurements of energy distribution (ED) in electronic systems with arbitrary excitations with well-defined energies and random phases. Using biased diffusive metallic wire as a testbed, we demonstrate the power of this approach and extract the well-known double-step ED from the shot noise of a weakly coupled tunnel junction. Our experiment paves the way for the local measurements of generic nonequilibrium configurations applicable to virtually any conductor.

Edge probes of topological order

According to the bulk-edge correspondence principle, the physics of the gapless edge in the quantum Hall effect determines the topological order in the gapped bulk. As the bulk is less accessible, the last two decades saw the emergence of several experimental techniques that invoke the study of the compressible edge. We review the properties of the edge, and describe several experimental techniques that include shot noise and thermal noise measurements, interferometry, and energy (thermal) transport at the edge. We pay special attention to the filling factor 5/2 in the first excited Landau level (in two-dimensional electron gas in GaAs), where experimental evidence of a non-Abelian topological order was found. A brief discussion is devoted to recent interferometry experiments that uncovered unexpected physics in the integer quantum Hall effect. The article also addresses the theory of edge states, for systems with Abelian and non-Abelian topological orders.

Anomalous Coulomb drag between bilayer graphene and a GaAs electron gas

We report on Coulomb drag experiments between a bilayer graphene flake and a GaAs two-dimensional electron gas, where the charge-carrier densities of both systems can be tuned independently. For both p- and n-type graphene charge carriers, we observe that the Coulomb drag unexpectedly changes direction when the temperature is lowered. We find this phenomenon to be dominant when the Fermi wave vector in graphene is larger than in GaAs. At temperatures above , the drag signal is consistent with momentum exchange. In all discussed regimes, the Onsager relation is respected.

Effect of cubic Dresselhaus spin-orbit interaction in a persistent spin helix state including phonon scattering in semiconductor quantum wells

We performed a numerical simulation of the spatial behavior of spin precession in a persistent spin helix (PSH) state at high temperatures (>150 K) in a two-dimensional electron gas of GaAs and InGaAs (001)-semiconductor quantum wells (QWs). To describe the spin dynamics of the PSH state at high temperatures, the effect of a cubic Dresselhaus spin-orbit interaction (SOI) that destroys the PSH state was added to the balanced Rashba and linear Dresselhaus SOI. Furthermore, longitudinal optical and acoustic phonon scattering were taken into account in the momentum scattering calculations. The simulation results indicate that the PSH state in the InGaAs QW persists for over 500 ps because of the small effective mass of the electron, even at room temperature. We also reveal that it is closer to the ideal PSH state when the Rashba strength (α) is controlled to the renormalized linear Dresselhaus SOI strength (−β̃) rather than the linear Dresselhaus SOI strength (−β).

Rapid scanning terahertz time-domain magnetospectroscopy with a table-top repetitive pulsed magnet.

We have performed terahertz time-domain magnetospectroscopy by combining a rapid scanning terahertz time-domain spectrometer based on the electronically controlled optical sampling method with a table-top minicoil pulsed magnet capable of producing magnetic fields up to 30T. We demonstrate the capability of this system by measuring coherent cyclotron resonance oscillations in a high-mobility two-dimensional electron gas in GaAs and interference-induced terahertz transmittance modifications in a magnetoplasma in lightly doped n-InSb.

Direct measurement of the spin gaps in a gated GaAs two-dimensional electron gas

We have performed magnetotransport measurements on gated GaAs two-dimensional electron gases in which electrons are confined in a layer of the nanoscale. From the slopes of a pair of spin-split Landau levels (LLs) in the energy-magnetic field plane, we can perform direct measurements of the spin gap for different LLs. The measured g-factor g is greatly enhanced over its bulk value in GaAs (0.44) due to electron–electron (e-e) interactions. Our results suggest that both the spin gap and g determined from conventional activation energy studies can be very different from those obtained by direct measurements.

Spin polarization in a two-dimensional electron gas in GaAs

In this paper, positive magnetoresistance of a dilute two-dimensional electron gas in GaAs is studied in a parallel magnetic field B. It is found that the normalized resistivity curves, ρ(B)/ρ(0), merge together when we scale the field according to B/Bχ, where Bχ is assumed to be the field in which full spin polarization of electrons is reached. It is also shown that the crossing field, Bcross, determined by the crossover of the B2 dependence of the resistivity, becomes lower than Bχ with increasing electron density, which cannot be explained in terms of filling of the upper electron subbands in the fully spin-polarized regime. The spin susceptibility, χ, is assessed by extracting the product g*m*, where g* and m* are the effective Landé factor and electron mass, respectively. The behavior of χ with increasing electron density, however, deserves further theoretical and experimental study.

Probing temperature-driven flow lines in a gated two-dimensional electron gas with tunable spin-splitting

We study the temperature flow of conductivities in a gated GaAs two-dimensional electron gas (2DEG) containing self-assembled InAs dots and compare the results with recent theoretical predictions. By changing the gate voltage, we are able to tune the 2DEG density and thus vary disorder and spin-splitting. Data for both the spin-resolved and spin-degenerate phase transitions are presented, the former collapsing to the latter with decreasing gate voltage and/or decreasing spin-splitting. The experimental results support a recent theory, based on modular symmetry, which predicts how the critical Hall conductivity varies with spin-splitting.

Terahertz magneto-optical polarization modulation spectroscopy

We report the development of new terahertz techniques for rapidly measuring the complex Faraday angle in systems with broken time-reversal symmetry using the cyclotron resonance of a GaAs two-dimensional electron gas in a magnetic field as a system for demonstration of performance. We have made polarization modulation, high sensitivity (< 1 mrad) narrow band rotation measurements with a CW optically pumped molecular gas laser, and by combining the distinct advantages of terahertz (THz) time domain spectroscopy and polarization modulation techniques, we have demonstrated rapid broadband rotation measurements to < 5 mrad precision.

Numerical study of resonant spin relaxation in quasi-one-dimensional channels

Recent experiments demonstrate that a ballistic version of spin resonance, mediated by spin-orbit interaction, can be induced in narrow channels of a high-mobility GaAs two-dimensional electron gas by matching the spin precession frequency with the frequency of bouncing trajectories in the channel. Contrary to the typical suppression of Dyakonov-Perel' spin relaxation in confined geometries, the spin relaxation rate increases by orders of magnitude on resonance. Here, we present Monte Carlo simulations of this effect to explore the roles of varying degrees of disorder and strength of spin-orbit interaction. These simulations help to extract quantitative spin-orbit parameters from experimental measurements of ballistic spin resonance, and may guide the development of future spintronic devices.

Measurement of spin-dependent conductivities in a two-dimensional electron gas

Spin accumulation is generated by injecting an unpolarized charge current into a channel of GaAs two-dimensional electron gas subject to an in-plane magnetic field, then measured in a nonlocal geometry. Unlike previous measurements that have used spin-polarized nanostructures, here the spin accumulation arises simply from the difference in bulk conductivities for spin-up and spin-down carriers. Comparison to a diffusive model that includes spin subband splitting in magnetic field suggests a significantly enhanced electron spin susceptibility in the two-dimensional electron gas.

Interplay among spin, orbital effects, and localization in a GaAs two-dimensional electron gas in a strong in-plane magnetic field

The magnetoresistance of a low carrier density disordered GaAs based two-dimensional (2D) electron gas has been measured in parallel magnetic fields up to 32 T. The feature in the resistance associated with the complete spin polarization of the carriers shifts down by more than 20 T as the electron density is reduced, consistent with recent theories taking into account the enhancement of the electron-electron interactions at low densities. Nevertheless, the magnetic field for complete polarization, ${B}_{p}$, remains 2--3 times smaller than predicted for a disorder-free system. We show, in particular, by studying the temperature dependence of ${B}_{p}$ to probe the effective size of the Fermi sea that localization plays an important role in determining the spin polarization of a 2D electron gas.

Electrical Generation of Pure Spin Currents in a Two-Dimensional Electron Gas

Pure spin currents are generated and detected in micron-wide channels of a GaAs two-dimensional electron gas, using quantum point contacts in an in-plane magnetic field as injectors and detectors. The enhanced sensitivity to spin transport offered by a nonlocal measurement geometry enables accurate spin current measurements in this widely studied physical system. The polarization of the contacts is used to extract the quantum point contact g factor and provides a test for spontaneous polarization at 0.7 structure. The spin relaxation length in the channel is 30-50 microm over the magnetic field range 3-10 T, much longer than has been reported in GaAs two-dimensional electron gases but shorter than that expected from Dyakonov-Perel relaxation.

Time-resolved photoluminescence of type-II Ga(As)Sb/GaAs quantum dots embedded inan InGaAs quantum well

Optical properties and carrier dynamics in type-II Ga(As)Sb/GaAs quantum dots (QDs)embedded in an InGaAs quantum well (QW) are reported. A large blueshift of thephotoluminescence (PL) peak is observed with increased excitation densities. This blueshiftis due to the Coulomb interaction between physically separated electrons and holescharacteristic of the type-II band alignment, along with a band-filling effect of electrons inthe QW. Low-temperature (4 K) time-resolved PL measurements show a decay time of ns from the transition between Ga(As)Sb QDs and InGaAs QW which is longer than thatof the transition between Ga(As)Sb QDs and GaAs two-dimensional electron gas ( ns).

System-Size Dependence of Quantum Hall Transitions

The quantum Hall plateau-to-insulator (plateau) transition is studied in small GaAs two-dimensional electron gas systems, where the system size L is comparable to or smaller than the correlation length L cor of potential fluctuation. At low temperatures, the transition depends on the system size but the dependence is not explained in terms of Anderson localization. At elevated temperatures, the transition width exhibits a power-law dependence but the exponent κ takes nonuniversal values. It is suggested that the transition crosses over from the well-known, Anderson localization dominated regime to the semiclassical percolation-dominated regime when the system size is reduced below L cor .

‘Mobility gap’ of a spin-split GaAs two-dimensional electron gas

We have performed magnetotransport measurements of the electron g-factor in a two-dimensional GaAs electron gas. In order to obtain the spin gap Δs, we measure the spin-split longitudinal resistivity minimum which shows an activated behavior. From the spin gaps at different odd filling factors, we can obtain the effective g-factor which is greatly enhanced over its bare value (0.44) in GaAs. This enhancement is due to many-body electron–electron interactions. Our experimental results provide compelling evidence that conventional activation energy studies yield a ‘mobility gap’ which can be very different from the real spin gap in the energy spectrum.

Magnetoresistively Detected Electron Spin Resonance in Low-Density Two-Dimensional Electron Gas in GaAs–AlGaAs Single Quantum Wells

Electron spin resonance (ESR) is a natural candidate for quantum bit manipulation, provided that the confinement of a small number of electrons in a sufficiently small volume can be achieved. An important step is the development of low carrier density materials and structures in which the electron spins are isolated and can be controlled by ESR. We report on the realization of three low-density (n/sub 1/=1.77/spl times/10/sup 10/, n/sub 2/=4.5/spl times/10/sup 10/, and n/sub 3/=9/spl times/10/sup 10/ cm/sup -2/ without the help of a gate to deplete the channel) two-dimensional electron systems in GaAs-AlGaAs single quantum wells (QWs) and on the magnetoresistively detected electron spin resonance (MDESR) measurements in these samples. The MDESR has been characterized at /spl nu/=1 and /spl nu/=3 and the current intensity, microwave power, and temperature dependence have been studied. The structures that have been investigated represent the lowest density single QW samples in which MDESR has been detected. The implications of detection of the MDESR at such low electron density to coupled quantum-dot spin device technology will be presented.