Kinetic magnetoelectric effect in laterally boundary-confined ballistic two-dimensional hole gases

A theoretical investigation is presented on the characteristics of the kinetic magnetoelectric effect in laterally boundary-confined ballistic two-dimensional hole gases.It was shown that,though the momentum-dependent effective magnetic fields felt by charge carriers due to the spin-orbit interaction are in-plane orientated in such systems,both in-plane polarized and normal polarized nonequilibrium spin polarization densities could be electrically induced by the kinetic magnetoelectric effect,and the induced nonequilibrium spin polarizations exhibit some interesting characteristics.The characteristics we found indicate that there may be some possible relation between this effect and some recent experimental findings.     

Intrinsic spin Hall effect in the two-dimensional hole gas

We show that two types of spin-orbit coupling in the 2 dimensional hole gas, with and without inversion symmetry breaking, contribute to the intrinsic spin-Hall effect. Furthermore, the vertex correction due to impurity scattering vanishes in both cases, in sharp contrast to the case of usual Rashba coupling in the electron band. Recently, the spin-Hall effect in a hole doped GaAs semiconductor has been observed experimentally by Wunderlich et al. [ Phys. Rev. Lett. 94, 047204 (2005).]. From the fact that the lifetime broadening is smaller than the spin splitting, and the fact impurity vertex corrections vanish in this system, we argue that the observed spin-Hall effect should be in the intrinsic regime.     

Nonequilibrium state of the two-dimensional electron gas in the integer quantum Hall effect regime

We have compared the properties of the nonequilibrium state of the two-dimensional electron gas observed in the samples of different types by means of magnetotransport and magnetization measurements in the quantum Hall effect regime at integer filling factors n. It has been found that the range of filling factors corresponding to the nonequilibrium state is universal for the samples of different types and different measurement techniques and varies from 0.1 to 0.3 for n changing from 1 to 4. The comparison indicates that the observed nonequilibrium state is not directly caused by the appearance of eddy currents and the dielectric phase in the two-dimensional electron gas but is probably associated with the magnetic field-induced phase transition.     

PLE studies of two-dimensional hole systems in the quantum Hall regime

We report a spectroscopic investigation of the densities of both occupied and unoccupied states of a high-quality two-dimensional hole system, in the regime of the quantum Hall effects (QHEs). Photoluminescence and photoluminescence excitation spectroscopies are used to elucidate the complicated valence band structure of the holes, and to establish their optical response to the QHEs.     

Magnetic barrier in a two-dimensional hole gas

Transport properties of a magnetic barrier in a Ga_xAl_1−xAs based two-dimensional hole gas are reported. A ferromagnetic cobalt film, separated by an AlO_x layer from the semiconductor in order to prevent leakage currents, is magnetized in-plane, such that the fringe field generates a localized perpendicular magnetic field acting as a magnetic barrier. The resistance as a function of the in-plane magnetic field shows a characteristic minimum at the coercive field of the ferromagnetic film. Semiclassical simulations based on the Landauer–Büttiker formalism show good agreement with the experiment.     

Observation of the presuperfluid regime in a two-dimensional Bose gas

In complementary images of coordinate-space and momentum-space density in a trapped 2D Bose gas, we observe the emergence of presuperfluid behavior. As phase-space density ρ increases toward degenerate values, we observe a gradual divergence of the compressibility κ from the value predicted by a bare-atom model, κ(ba). κ/κ(ba) grows to 1.7 before ρ reaches the value for which we observe the sudden emergence of a spike at p = 0 in momentum space. Momentum-space images are acquired by means of a 2D focusing technique. Our data represent the first observation of non-mean-field physics in the presuperfluid but degenerate 2D Bose gas.     

Temperature and magnetic-field-enhanced hall slope of a dilute 2D hole system in the ballistic regime

We report the temperature (T) and perpendicular magnetic-field (B) dependence of the Hall resistivity rho(xy)(B) of dilute metallic 2D holes in GaAs over a broad range of temperature (0.02-1.25 K). The low B Hall coefficient, R(H), is found to be enhanced when T decreases. Strong magnetic fields further enhance the slope of rho(xy)(B) at all temperatures studied. Coulomb interaction corrections of a Fermi liquid (FL) in the ballistic regime can not explain the enhancement of rho(xy) which occurs in the same regime as the anomalous metallic longitudinal conductivity. In particular, although the metallic conductivity in 2D systems has been attributed to electron interactions in a FL, these same interactions should reduce, not enhance, the slope of rho(xy)(B) as T decreases and/or B increases.     

Microwave photoresistance of a two-dimensional electron gas in a ballistic microbar

The effect of millimeter microwave radiation on the electron transport of two-dimensional (2D) ballistic microbars formed on the basis of individual GaAs quantum wells at a temperature of T = 4.2 K in magnetic fields B < 0.6 T has been investigated. Differences have been revealed in the magnetic field dependences of the microwave photoresistance of a 2D electron gas in Hall bars with a length L and a width W for the cases L, W > l _p and L, W < l _p , where l _p is the electron mean free path for momentum. The microwave photoresistance in macroscopic bars (L, W > l _p ) is a periodic alternating function of the inverse magnetic field; in microbars (L, W < l _p ), it is a periodic positive function of 1/B. The experimental results indicate that the mechanisms of the microwave photoresistance of a 2D electron gas are different for macroscopic and microscopic bars.     

Experimental observation of the spin-Hall effect in a spin–orbit coupled two-dimensional hole gas

Electrically induced ordering and manipulation of electron spins in semiconductors has a number of practical advantages over the established techniques using circularly polarized light sources, external magnetic fields and spin injection from a ferromagnet. The spin-Hall effect utilizes spin–orbit coupling to induce edge spin accumulation in response to a longitudinal electric field which can be applied locally and lead to low energy consumption devices. We study spin accumulation near the edge of a weakly disordered two-dimensional hole gas (2DHG) in a GaAs/AlGaAs heterostructure where the magnitude of the transverse spin current approaches the intrinsic, disorder independent value, in contrast to the impurity dominated regime observed in 3D electron doped systems. In our experiment, the induced spin polarization is detected by the electroluminescence resulting from two p–n junctions bordering the 2DHG channel. When an electric field is applied across the 2DHG channel, a non-zero out-of-plane component of the spin is optically detected. The sign of the spin depends on the direction of the field and is opposite for the two edges, consistent with theory predictions. We also report and analyze an in-plane spin-polarization effect induced in the device by asymmetric electron–hole recombination.     

The two-antidot system in the ballistic regime

A tunable two-antidot device is studied in the cyclotron-trapping regime. Periodic quantum oscillations are found to be superimposed on the peaks reminiscent of those observed in antidot lattices. The results are compared to quantum and classical simulations and Feynman path integral analysis. Published by Elsevier Science B.V.     

Energy relaxation of two-dimensional electrons in the quantum Hall effect regime

The mm-wave spectroscopy with high temporal resolution is used to measure the energy relaxation times τ_e of 2D electrons in GaAs/AlGaAs heterostructures in magnetic fields B=0–4 T under quasi-equilibrium conditions at T=4.2 K. With increasing B, a considerable increase in τe from 0.9 to 25 ns is observed. For high B and low values of the filling factor ν, the energy relaxation rate τ _e ^?1 oscillates. The depth of these oscillations and the positions of maxima depend on the filling factor ν. For ν>5, the relaxation rate τ _e ^?1 is maximum when the Fermi level lies in the region of the localized states between the Landau levels. For lower values of ν, the relaxation rate is maximum at half-integer values of τ _e ^?1 when the Fermi level is coincident with the Landau level. The characteristic features of the dependence τ _e ^?1 (B) are explained by different contributions of the intralevel and interlevel electron-phonon transitions to the process of the energy relaxation of 2D electrons.