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Magneto-intersubband resistance oscillations (MISO) of highly mobile 2D electrons in symmetric GaAs quantum wells with two populated subbands are studied in magnetic fields tilted from the normal to the 2D electron layer at different temperatures $T$. Decrease of MISO amplitude with temperature increase is observed. At moderate tilts the temperature decrease of MISO amplitude is consistent with decrease of Dingle factor due to reduction of quantum electron lifetime at high temperatures. At large tilts new regime of strong MISO suppression with the temperature is observed. Proposed model relates this suppression to magnetic entanglement between subbands, leading to beating in oscillating density of states. The model yields corresponding temperature damping factor: $A_{MISO}(T)=X/\sinh(X)$, where $X=2\pi^2kT\delta f$ and $\delta f$ is difference frequency of oscillations of density of states in two subbands. This factor is in agreement with experiment. Fermi liquid enhancement of MISO amplitude is observed., Comment: 15 pages, 10 figures

The interaction of electrons with a flux of ballistic phonons leads to excitation of many angular harmonics of an electron distribution function. We show that this property dramatically modifies the magnetothermoelectric phenomena in two-dimensional electron systems with boundaries. By considering classical magnetotransport of electrons in a narrow channel with partly diffusive boundary scattering, we show that the phonon flux excites a pattern of current stripes with alternating directions of propagation along the channel. The Nernst voltage due to phonon drag appears already in the classical transport regime and can be comparable with the Seebeck voltage, while the latter acquires a dependence on the magnetic field. The temperature dependence of these voltages shows an unusual behavior determined by relaxation of higher-order harmonics of the distribution function via electron-electron scattering. Our experimental studies of mesoscopic samples based on high-quality GaAs quantum wells confirm the main properties of the thermoelectric response suggested by the theory.

The dynamics of exciton recombination and spin relaxation in thin (Ga,Al)(Sb,As)/AlAs quantum wells (QWs) with indirect band gap are studied. The band alignment in these QWs is identified as type I. The exciton recombination time exceeds hundreds of microseconds, while the spin relaxation times of the electron and the heavy hole in an exciton do not exceed hundreds of nanoseconds. The heavy-hole longitudinal $g$ factor is determined to be $+2.5$. Despite the long exciton lifetimes, the photoluminescence circular polarization degree induced by a magnetic field is unexpectedly small and does not exceed $25%$.

Hall viscosity is a nondissipative response function describing momentum transport in two-dimensional (2D) systems with broken time-reversal symmetry. In the classical regime, Hall viscosity contributes to the viscous flow of 2D electrons in the presence of a magnetic field. We observe a pronounced, negative Hall resistivity at low magnetic field in a mesoscopic size, two-dimensional electron system, which is attributed to Hall viscosity in the inhomogeneous charge flow. Experimental results supported by a theoretical analysis confirm that the conditions for observation of Hall viscosity are correlated with predictions., Comment: 8 pages, 7 figures

By measuring the thermoelectric effect in high-mobility quantum wells with two occupied subbands in perpendicular magnetic field, we detect magnetophonon oscillations due to interaction of electrons with acoustic phonons. These oscillations contain specific features identified as combined resonances caused by intersubband phonon-assisted transitions of electrons in the presence of Landau quantization. The quantum theory of phonon-drag magnetothermoelectric effect, generalized to the case of multi-subband occupation, describes our experimental findings., 7 pages, 6 figures

The magnetotransport of highly mobile 2D electrons in wide GaAs single quantum wells with three populated subbands placed in titled magnetic fields is studied. The bottoms of the lower two subbands have nearly the same energy while the bottom of the third subband has a much higher energy ($E_1\approx E_2<, Comment: 14 pages, 10 figures, to be published in Phys. Rev. B

We show that the electron spin polarization generated by an electrical current may have its direction controlled and magnitude amplified by periodic optical excitation. The electrical and optical spin control methods were combined and implemented in a two-dimensional electron gas. By Kerr rotation in an external transverse magnetic field, we demonstrate unexpected long-lived coherent spin oscillations of the current-induced signal in a system with large spin-orbit interaction. Using a single linearly polarized pulse for spin manipulation and detection, we found a strong dependence on the pulse optical power and sample temperature indicating the relevance of the hole spin in the electron spin initialization. The signal was mapped in a Hall bar as function of the position relative to the injection contact. Finally, the presence of an in-plane spin polarization was directly verified by rotating the experimental geometry., Comment: 5 pages, 5 figures

We observe microwave induced nonlocal resistance in magnetotransport in single and bilayer electronic systems. The obtained results provide evidence for an edge state current stabilized by microwave irradiation due to nonlinear resonances. Our observation are closely related to microwave induced oscillations and zero resistance states in a two-dimensional (2D) electron system., Comment: 5 pages, 4 figures

Vibrational spectra of self-assembled GaAs and AlAs islands grown by molecular-beam epitaxy on InAs (001) substrates have been investigated by Raman spectroscopy. We observed large strain-induced shifts of optical-phonon frequencies in GaAs and AlAs islands with respect to the bulk materials. The values of shifts are 36 and $24 {\mathrm{cm}}^{\ensuremath{-}1}$ for GaAs longitudinal-optical (LO) and transverse-optical (TO) phonons and 55 and $28 {\mathrm{cm}}^{\ensuremath{-}1}$ for AlAs LO and TO phonons, respectively. Comparison of experimental data with calculated optical-phonon frequencies in strained GaAs and AlAs islands of different shape demonstrates that the structures studied are coherently strained, i.e., they do not contain dislocations that could lead to strain relaxation. The features of interface phonons were observed in the polarized Raman spectra between the peaks of InAs TO and LO phonons. The frequency positions of interface phonon lines are well described by the dielectric continuum model and also give evidence of three-dimensional island formation. Doublets of folded acoustic phonons appear in the low-frequency region of Raman spectra of multilayer structures with GaAs and AlAs islands. These doublets are very similar to those typically observed in planar superlattices and well described by the elastic continuum model.

We study the Hall effect of a nonplanar stripe-shaped two-dimensional electron gas (2DEG) with stripes oriented perpendicular to the current in the presence of an in-plane external magnetic field ${B}_{\mathrm{ext}}.$ Since the 2DEG is sensitive only to the normal component of ${B}_{\mathrm{ext}},$ the electrons move in a sign alternating magnetic field with zero mean $〈B〉=0$ and nonzero variance, which is proportional to ${B}_{\mathrm{ext}}.$ A zero Hall resistance is observed at low ${B}_{\mathrm{ext}},$ as expected for $〈B〉=0.$ However, for ${B}_{\mathrm{ext}}g5$ T, a nonzero Hall voltage is found. In this regime the electron transport is characterized by the propagation of chiral snakelike trajectories. These states move in positive and negative directions along stripes and are confined to one-dimensional channels with a different number of propagating modes, which is determined by the surface topography. Asymmetry in the number of ballistic modes leads to a nonzero Hall voltage.

We employ the circular-polarization-resolved magnetophotoluminescence technique to probe the spin character of electron and hole states in a GaAs/AlGaAs strongly coupled double-quantum-well system. The photoluminescence (PL) intensities of the lines associated with symmetric and antisymmetric electron states present clear out-of-phase oscillations between integer values of the filling factor $\ensuremath{\nu}$ and are caused by magnetic-field-induced changes in the population of occupied Landau levels near to the Fermi level of the system. Moreover, the degree of circular polarization of these emissions also exhibits the oscillatory behavior with increasing magnetic field. Both quantum oscillations observed in the PL intensities and in the degree of polarizations may be understood in terms of a simple single-particle approach model. The $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ method was used to calculate the photoluminescence peak energies and the degree of circular polarizations in the double-quantum-well structure as a function of the magnetic field. These calculations prove that the character of valence band states plays an important role in the determination of the degree of circular polarization and, thus, resulting in a magnetic-field-induced change of the polarization sign.

The electron-electron interaction quantum correction to the conductivity of the gated double well Al${}_{x}$Ga${}_{1\ensuremath{-}x}$As/GaAs structures is investigated experimentally. The analysis of the temperature and magnetic field dependences of the conductivity tensor allows us to obtain reliably the diffusion part of the interaction correction for the regimes when the structure is balanced (i.e., for equal electron concentrations in the wells) and when only one quantum well is occupied. The surprising result is that the interaction correction does not reveal resonant behavior: it is practically the same for both regimes.

Coupling of acoustic and optical phonons to excitons in single InGaAs/GaAs quantum dots is investigated in detail experimentally and theoretically as a function of temperature. For the theoretical description of the luminescence spectrum, including acoustic and optical phonon scattering, we used the exactly solvable independent boson model. Surprisingly, only GaAs bulk-type longitudinal-optical (LO) phonons are detected in experiment. A quantitatively correct theoretical description of the optical-phonon replica is obtained by including a limited lifetime of the phonons and the dispersion of the LO phonon energy. Similarly, a numerically correct description of the acoustic phonon wings is again based on GaAs bulk material parameters for the phonon dispersion and deformation coupling. In addition, the line shape of the calculated spectra agrees with experiment only when realistic wave functions (e.g., based on eight-band $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ theory) are used for the electron-phonon coupling matrix elements. Gaussian wave functions describing the ground state of a harmonic oscillator fail to describe high-energy tails. Thus, fundamental insights of importance for the correct prediction of properties of nonclassical light sources, based on semiconductor nanostructures, are obtained.

Experimental studies of magnetoresistance in high-mobility wide quantum wells reveal oscillations which appear with an increase in temperature to 10 K and whose period is close to that of Shubnikov-de Haas oscillations. The observed phenomenon is identified as magnetointersubband oscillations caused by the scattering of electrons between two occupied subbands and the third subband which becomes occupied as a result of thermal activation. These small-period oscillations are less sensitive to thermal suppression than the largeperiod magnetointersubband oscillations caused by the scattering between the first and the second subbands. Theoretical study, based on consideration of electron scattering near the edge of the third subband, gives a reasonable explanation of our experimental findings., 9 pages, 5 figures

We report on temperature-dependent magnetoresistance measurements in balanced double quantum wells exposed to microwave irradiation for various frequencies. We have found that the resistance oscillations are described by the microwave-induced modification of electron distribution function limited by inelastic scattering (inelastic mechanism), up to a temperature of T*~4 K. With increasing temperature, a strong deviation of the oscillation amplitudes from the behavior predicted by this mechanism is observed, presumably indicating a crossover to another mechanism of microwave photoresistance, with similar frequency dependence. Our analysis shows that this deviation cannot be fully understood in terms of contribution from the mechanisms discussed in theory., 7 pages, 4 figures

G. M. Gusev,1 S. Wiedmann,2,3 O. E. Raichev,4 A. K. Bakarov,5 and J. C. Portal2,3,6 1Instituto de Fisica, Universidade de Sao Paulo, CP 66318, CEP 05315-970 Sao Paulo, SP, Brazil 2LNCMI–CNRS, UPR 3228, BP 166, 38042 Grenoble Cedex 9, France 3INSA Toulouse, 31077 Toulouse Cedex 4, France 4Institute of Semiconductor Physics, NAS of Ukraine, Prospekt Nauki 41, 03028 Kiev, Ukraine 5Institute of Semiconductor Physics, Novosibirsk 630090, Russia 6Institut Universitaire de France, 75005 Paris, France Received 28 September 2009; published 13 October 2009

We study the evolution of low-temperature magnetoresistance in double quantum wells in the region below 1 Tesla as the applied current density increases. A flip of the magneto-intersubband oscillation peaks, which occurs as a result of the current-induced inversion of the quantum component of resistivity, is observed. We also see splitting of these peaks as another manifestation of nonlinear behavior, specific for the two-subband electron systems. The experimental results are quantitatively explained by the theory based on the kinetic equation for the isotropic non-equilibrium part of electron distribution function. The inelastic scattering time is determined from the dependence of the inversion magnetic field on the current., Comment: 20 pages, 10 figures

We report on the observation of microwave-induced resistance oscillations associated with the fractional ratio $n/m$ of the microwave irradiation frequency to the cyclotron frequency for $m$ up to 8 in a two-dimensional electron system with high electron density. The features are quenched at high microwave frequencies independent of the fractional order $m$. We analyze temperature, power, and frequency dependencies of the magnetoresistance oscillations and discuss them in connection with existing theories.

is the Landau index number, in the tilted magneticfield.At high filling factors, the Shubnikov-de Haas oscillations exhibit beating effects at certain tilt angles. Weattribute such behavior to oscillations of the tunneling gap due to Aharonov-Bohm interference effect betweencyclotron orbits in different layers. The interplay between quantum and quasiclassical regimes is established.DOI: 10.1103/PhysRevB.78.155320 PACS number s : 73.50.Rb, 73.40.Lq

We present experimental and theoretical studies of the magnetoresistance oscillations induced by resonance transitions of electrons between tunnel-coupled states in double quantum wells. The suppression of these oscillations with increasing temperature is irrelevant to the thermal broadening of the Fermi distribution and reflects the temperature dependence of the quantum lifetime of electrons. The gate control of the period and amplitude of the oscillations is demonstrated., Comment: 5 pages 4 figures, to be published in the Physical Review B

We have studied the quantum Hall effect in parabolic ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ and square GaAs quantum wells with two occupied subbands in magnetic fields $B$ tilted by an angle $\ensuremath{\Theta}$ with respect to the normal to the sample. We built the density--magnetic field ${n}_{s}\text{\ensuremath{-}}B$ and angle-magnetic field $\ensuremath{\Theta}\text{\ensuremath{-}}B$ topological diagrams for the longitudinal resistivity ${\ensuremath{\rho}}_{xx}$ and observed that the latter shows a similarity with a simulation based on a single-particle harmonic potential approximation.

Linear ac $(888\phantom{\rule{0.3em}{0ex}}\mathrm{Hz})$ resistance of highly mobile two-dimensional electrons in GaAs heavily doped quantum wells is studied at different magnitudes of dc and ac $(10\phantom{\rule{0.3em}{0ex}}\mathrm{KHz}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}20\phantom{\rule{0.3em}{0ex}}\mathrm{GHz})$ excitations. In the dc excitation regime the differential resistance oscillates with the dc current and external magnetic field similar to that observed earlier in $\mathrm{Al}\mathrm{Ga}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ heterostructures [C. L. Yang et al., Phys. Rev. Lett. 89, 076801 (2002)]. At external ac excitations the resistance is also found to be oscillating with the magnetic field. However the form of the oscillations is considerably different from the dc case. We show that at frequency below $100\phantom{\rule{0.3em}{0ex}}\mathrm{KHz}$ the difference is the result of a specific average of the dc differential resistance during the period of the external ac excitations.

We study the Hall effect in wide AlcGac−1As parabolic wells in the presence of the tilted magnetic field. The Hall resistance is described by equations Rxy / cos Q=−B /ens at B,4 T, and Rxy / cos Q=−A3 sB−B0d /ens at B.4 T, where ns is the electron density, B0=2–2.6 T, A is the temperature dependent coefficient, and Q is the angle between the magnetic field and the normal to the well plane. The effective g factor in such materials depends on the Al composition and changes the sign along the well width. In the presence of the strong tilted magnetic field electron moving along the z direction acquires a spin flip process, which is strongly suppressed at low temperatures, and leads to the change of the Hall effect slope.

We present an experimental study of $\mathrm{In}\mathrm{As}∕\mathrm{Al}\mathrm{As}(\mathrm{Ga}\mathrm{As})$ periodical structures with InAs and AlAs quantum dots by means of Raman spectroscopy. Experiments on the asymmetric $\mathrm{Ga}\mathrm{As}∕\mathrm{In}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ quantum dot structures allowed us to investigate the interface phonons localized in the vicinity of corrugated dot/matrix interface and planar interface between the matrix and wetting layer. The interface phonon frequencies in the quantum dot structures determined from the experiment are compared to those calculated in the framework of the dielectric continuum model. A good agreement is obtained, especially if the preferential shape of the quantum dots determined from transmission electron microscopy is taken into account.

We have observed features in the magnetoresistance of a wide parabolic quantum well in the presence of the in-plane magnetic field at field three times larger than fundamental field corresponding to the depopulation of the last Landau level. The magnetoresistance structures shift with a specific sample parameter, such as potential width. We suggested the formation of correlated states of a three-dimensional electron gas at Landau filling factor 1/3, in analogy with a two-dimensional fractional quantum Hall effect.

Shubnikov–de Haas oscillations are measured in wide parabolic quantum wells with five to eight subbands in a tilted magnetic field. We find two types of oscillations. The oscillations at low magnetic fields are shifted toward higher field with the tilt angle increasing and can be attributed to two-dimensional Landau states. The position of the oscillations of the second type does not change with increasing the tilt angle which points to a three-dimensional character of these Landau states. We calculate the level broadening due to the elastic scattering rate G5\/2t , where t is the quantum time, and the energy separation between two-dimensional subbands, D i j5E j2Ei , in a parabolic well. For all levels we obtain G j;D i j , which means that the levels overlap, supporting the observation of three-dimensional Landau states. Surprisingly, we find that the lowest subband, which has a smaller energy separation from the higher level, does not overlap with these subbands and forms a two-dimensional state.

Vertical longitudinal magnetoresistance (VLMR) caused by the peculiar shape of the Fermi surface of a superlattice has been observed in $\mathrm{GaAs}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ superlattices. This VLMR occurs when the electrons occupy the open Fermi surface and their motion in the plane of the layers is quantized by a magnetic field. It was shown that there exists a critical magnetic field that cancels the contribution of the electrons occupying the open Fermi surface to the vertical conductivity in the case when the chemical potential exceeds the width of the miniband, thus resulting in the observed VLMR. This effect produces the conditions necessary to observe the quantized Hall effect in the three-dimensional electron system of a superlattice.