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Magneto-intersubband oscillations in a highly mobile two-subband electron system with one-dimensional periodic modulation of the potential under the conditions of overlapping Landau bands have been experimentally investigated. A significant modification of magneto-intersubband oscillations relative to the amplitude and phase is found: the amplitude decreases and the reversal of magneto-intersubband oscillations occurs in some ranges of magnetic fields. The obtained experimental data can be explained by a two-humped structure of the electron energy spectrum in Landau bands.

A physical mechanism has been proposed to explain the appearance of the multichannel electron transport regime in trench quantum point contacts. It has been shown that the redistribution of electrons in a two-dimensional electron gas, which is due to the Coulomb interaction between them and with charged donors and X-valley electrons, results in the experimentally observed multichannel regime. The numerical simulation of the redistribution of electrons and its comparison with the measurements of the conductance of trench quantum point contacts based on a GaAs/AlGaAs heterostructure demonstrate the formation of a multiwell confining potential.

The use of low-temperature (LT) GaAs layers as dislocation filters in GaAs/Si heterostructures (HSs) was investigated in this study. The effects of intermediate LT-GaAs layers and of the post-growth and cyclic in situ annealing on the structural properties of GaAs/LT-GaAs/GaAs/Si(001) HSs were studied. It was found that the introduction of LT-GaAs layers, in combination with post-growth cyclic annealing, reduced the threading dislocation density down to 5 × 106 cm−2, the root-mean-square roughness of the GaAs surface down to 1.1 nm, and the concentration of non-radiative recombination centers in the near-surface GaAs/Si regions down to the homoepitaxial GaAs level. Possible reasons for the improvement in the quality of near-surface GaAs layers are discussed. On the one hand, the presence of elastic deformations in the GaAs/LT-GaAs system led to dislocation line bending. On the other hand, gallium vacancies, formed in the LT-GaAs layers, diffused into the overlying GaAs layers and led to an increase in the dislocation glide rate. It was demonstrated that the GaAs/Si HSs obtained with these techniques are suitable for growing high-quality light-emitting HSs with self-assembled quantum dots.

Features of the molecular beam epitaxy growth of the AlSb/InAs heterostructures with a high-mobility two-dimensional electron gas for microwave transistors with ultralow power consumption are described. The main stages of fabrication of the transistors based on the AlSb/InAs heterostructures are outlined. The drain and drain–gate characteristics of the transistors are reported and discussed.

In this article, we present GaAs millimeter-wave pseudomorphic high-electron-mobility transistor (pHEMT) using sophisticated AlGaAs-InGaAs–GaAs heterostructure with an In0.22Ga0.78As quantum well located inside an external quantum well formed by space charge regions in AlGaAs layers. In a heterostructure with this design, high mobility [5800 cm2/( $\text {V}\cdot \text {s}$ )] and high density ( ${4.7}\times {10}^{{12}}$ cm $^{-{2}}$ ) of two-dimensional electron gas at 300 K were obtained. The localization of hot electrons in an external quantum well leads to an increase in their drift velocity and to enhance the dc and RF performances of the device. A field-effect transistor based on such a heterostructure with a T-gate of $0.14~\mu \text{m}$ in length shows a specific current density of about 0.7 A/mm, transconductance of about 250 mS/mm, gate–drain breakdown voltage in the range 23–31 V, which corresponds to more than 2 W/mm specific output RF power according to a simple estimate. The transistor demonstrates impressive RF characteristics, the maximum stable gain (MSG) of more than 15 dB at 40 GHz and more than 10 dB at 67 GHz, ${f}_{t} = {45}$ GHz and ${f} _{\text {max}} = {250}$ GHz. The MSG at 40 GHz increases with increasing distance between the gates in the drain and is almost constant in the range 25–55 GHz.

The conductance of a suspended quantum point contact fabricated on the basis of GaAs/AlGaAs heterostructures with a two-dimensional electron gas and equipped with side gates separated from the constriction using lithographical trenches is investigated. The gate dependences of the conductance of such structures correspond to the unusual double-channel mode with independent quantization of the conductance of two channels, and the conductance of separate channels can be independently controlled using two side gates. The electrostatic-formation mechanism of a two-channel structure inside a single constriction associated with the lateral redistribution of low-mobility X-valley electrons containing superlattice heterostructure layers, which leads to the formation of a potential barrier separating the conductivity electrons into two channels symmetrically shifted towards the lithographic trenches, specifying the nanostructure geometry, is considered.

A new type of AlGaAs/InGaAs/GaAs heterostructures with energy barriers formed in the AlGaAs transistor layers adjacent to the InGaAs channel, modulation-doped with donors and acceptors is presented. The height of the barriers associated with the potential of the space charge region in AlGaAs layers reaches 0.8 eV, which makes it possible to double the concentration of electrons in the channel, prevent the transition of hot electrons heated by the electric field into surrounding layers, and increase their saturation drift velocity by around $$1.2{-}1.3$$ times. As a result, microwave power output density of the transistor exceeded the world level by more than 50 $$\%$$ .

The electronic analog of the Poiseuille flow is the transport in a narrow channel with disordered edges that scatter electrons in a diffuse way. In the hydrodynamic regime, the resistivity decreases with temperature, referred to as the Gurzhi effect, distinct from conventional Ohmic behaviour. We studied experimentally an electronic analog of the Stokes flow around a disc immersed in a two-dimensional viscous liquid. The circle obstacle results in an additive contribution to resistivity. If specular boundary conditions apply, it is no longer possible to detect Poiseuille type flow and the Gurzhi effect. However, in flow through a channel with a circular obstacle, the resistivity decreases with temperature. By tuning the temperature, we observed the transport signatures of the ballistic and hydrodynamic regimes on the length scale of disc size. Our experimental results confirm theoretical predictions., Comment: 10 pages, 5 figures

We employed the reflective probing of linearly polarized light to explore the dependence of electron spin dynamics, in a high mobility dense two-dimensional electron gas, on the external magnetic field, excitation power and sample temperature using the time-resolved Kerr rotation. Owing to the complex layered structure, the dynamics of spin polarization in the studied sample enclosed information about the spin signal corresponding to the different populations of electrons. Fit to the data revealed multiperiodic spin precession, with distinct g-factors that modulate the decay of the TRKR envelope. Additionally, the spin precession in our structure was seen to be thermally robust, persisting up to 250 K. The dynamics of optically induced spins, from different electron populations, was monitored as a function of different experimental parameters.

A photodetector heterostructure based on AlInSb/InSb was grown by molecular-beam epitaxy. Mesostructures of different diameters were fabricated. The temperature dependence of the dark current was measured. It was demonstrated that the built-in barrier blocks the flow of majority charge carriers, thus reducing the dark-current density (relative to that for a pin structure based on InSb). The measured dependence of the dark current on the mesostructure size revealed that the bulk current component prevails over the surface one.

The diffusion of photo-generated holes is studied in a high-mobility mesoscopic GaAs\ channel where electrons exhibit hydrodynamic properties. It is shown that the injection of holes into such an electron system leads to the formation of a hydrodynamic three-component mixture consisted of electrons and photo-generated heavy and light holes. The obtained results are analyzed within the framework of ambipolar diffusion, which reveals characteristics of a viscous flow. Both hole types exhibit similar hydrodynamic characteristics. In such a way the diffusion lengths, ambipolar diffusion coefficient and the effective viscosity of the electron-hole system are determined., 6 pages, 4 figures

The recombination dynamics are studied in viscous electron–hole plasma, consisting of electrons and photo-generated heavy and light holes, formed in the high-mobility mesoscopic GaAs channel. It is shown that an increase in the pump power reduces the concentration and mobility of background electrons, which, in turn, slows down their recombination with photogenerated holes. At a critical pump power, the recombination time begins to decrease, which is a consequence of the transition of a viscous electron–hole plasma from the hydrodynamic regime to the Drude diffusive regime. The observed transition occurs when the scattering of electrons with disorder begins to dominate over electron–electron scattering, which leads to the transformation of an inhomogeneous Poiseuille charge flow into a homogeneous diffusion flow. As a result, an optical analogue of the Gurzhi effect has been found.

Magneto-intersubband resistance oscillations (MISOs) of highly mobile 2D electrons in symmetric GaAs quantum wells with two populated subbands are studied in magnetic fields [Formula: see text] tilted from the normal to the 2D electron layer at different temperatures [Formula: see text]. The in-plane component ([Formula: see text]) of the field [Formula: see text] induces magnetic entanglement between subbands, leading to beating in oscillating density of states (DOS) and to MISO suppression. Model of the MISO suppression is proposed. Within the model, a comparison of MISO amplitude in the entangled and disentangled ([Formula: see text]) 2D systems yields both difference frequency of DOS oscillations, [Formula: see text], and strength of the electron–electron interaction, described by parameter [Formula: see text], in the 2D system. These properties are analyzed using two methods, yielding consistent but not identical results for both [Formula: see text] and [Formula: see text]. The analysis reveals an additional angular dependent factor of MISO suppression. The factor is related to spin splitting of quantum levels in magnetic fields.

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

Low-temperature magnetotransport in a quasi-two-dimensional electron system based on a selectively doped GaAs quantum well with two occupied quantum-confinement subbands with one-dimensional periodic modulation of a potential is investigated. It is shown that commensurability oscillations of the resistance in this electron system coexist with magneto-intersubband oscillations. It is found that, in addition to commen-surability oscillations, the one-dimensional periodic potential in the two-subband electron system leads to the modulation of the magneto-intersubband oscillation amplitude. The experimental results are explained by the formation of Landau bands in the quasi-two-dimensional electron system with one-dimensional periodic modulation of the potential.

Electron transport in single GaAs quantum wells of widths from 22 to 46 nm with two populated quantum-confinement subbands ES and EAS is investigated at a temperature of T = 4.2 K in tilted magnetic fields B 72° and α > 85°, respectively. The origin of this unexpected behavior of magnetointersubband oscillations in tilted magnetic fields is discussed.

The on-chip piezoelectric actuation is experimentally shown to be a suitable method for driving the resonant vibrations of AlGaAs/GaAs-based nanomechanical resonators containing a two-dimensional electron gas. Both flexural and torsional vibrations of 166 nm-thick cantilevers and doubly clamped beams can be driven using this method at room temperature. At least two points should be addressed when reducing the size of the piezoelectrically driven resonators. First, as shown in the paper, the parasitic attenuation of the driving electrical signal becomes the main factor limiting the actuation efficiency at the eigenfrequencies increased to the megahertz range due to the size reduction. Second, thin and relatively long bridge-like AlGaAs/GaAs-based resonators are prone to the Euler buckling instability caused by the longitudinal compressive stress. It is demonstrated that the buckling does not hinder the actuation at near-critical compression. However, a large super-critical compression can lead to a complete suppression of the piezoelectrically driven vibrations. A method to avoid this suppression is proposed.

We observe a large negative magnetoresistance and a decrease of resistivity with increasing temperature, known as the Gurzhi effect, in a bilayer electron (BL) system formed by a wide GaAs quantum well. A hydrodynamic model for the single fluid transport parameters in narrow channels is employed and successfully describes our experimental findings. We find that the electron-electron scattering in the bilayer is more intensive in comparison with a single-band well (SW). The hydrodynamic assumption implies a strong dependence on boundary conditions, which can be characterized by slip length, describing the behavior of a liquid near the edge. Our results reveal that slip length in a BL is shorter than in a SW, and that the BL system goes deeper into the hydrodynamic regime. This is in agreement with the model proposed where the slip length is of the order of the electron-electron mean free path., 8 pages, 8 figures

The dependences of the transport scattering time {\tau}t, quantum lifetime {\tau}q, and their ratio {\tau}t/{\tau}q on the density ne of the electron gas in modulation-doped single GaAs quantum wells with AlAs/GaAs short-period super-lattice barriers are investigated. The experimental dependences are explained in terms of electron scattering by remote ionized donors with an effective two-dimensional concentration n*R and background impurities with a three-dimensional concentration nB. An expression for n*R(ne) is obtained including the contribution of X-valley electrons localized in AlAs layers to the suppression of scattering by the random potential of remote donors. It is shown that the experimentally observed abrupt increase in {\tau}t and {\tau}q with an increase in ne above a certain critical value nec is related to a decrease in n*R. It is established that the drop in {\tau}t/{\tau}q observed for electron densities ne > nec occurs because scattering by the random potential of background impurities in this two-dimensional system with a decrease in n*R limits an increase in {\tau}t more considerably than an increase in {\tau}q., Comment: 9 pages, 5 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 $25%$.

Spin relaxation was studied in a two-dimensional electron gas confined in a wide GaAs quantum well. Recently, the control of the spin relaxation anisotropy by diffusive motion was first shown in D. Iizasa et al., arXiv:2006.08253 (2020). Here, we demonstrate electrical control by drift transport in a system with two-subbands occupied. The combined effect of in-plane and gate voltages was investigated using time-resolved Kerr rotation. The measured relaxation time present strong anisotropy with respect to the transport direction. For an in-plane accelerating electric field along $\left[110\right]$, the lifetime was strongly suppressed irrespective of the applied gate voltage. Remarkably, for transport along $\left[1\bar{1}0\right]$, the data shows spin lifetime that was gate-dependent and longer than in the $\left[110\right]$ direction regardless of the in-plane voltage. In agreement, independent results of anisotropic spin precession frequencies are also presented. Nevertheless, the long spin lifetime, strong anisotropy and drift response seen in the data are beyond the existing models for spin drift and diffusion.

We develop a classical kinetic theory of magnetotransport of 2D electrons in narrow channels with partly diffusive boundary scattering and apply it to the description of magnetoresistance measured in the temperature interval 4.2--30 K in long mesoscopic bars fabricated from high-purity GaAs quantum well structures. Both experiment and theory demonstrate a number of characteristic features in the longitudinal and Hall resistances caused by the size effect in two dimensions owing to the high ballisticity of the transport. In addition to the features described previously, we also reveal a change in the slope of the first derivative of magnetoresistance when the cyclotron orbit diameter equals to half of the channel width. These features are suppressed with increasing temperature as a result of the electronic viscosity due to electron-electron interaction. By comparing theory and experiment, we determine the characteristic time of relaxation of angular distribution of electrons caused by electron-electron scattering.

The electron temperature of a two-dimensional electron gas heated by an electric field in DA-pHEMT heterostructures was determined by the Shubnikov – de Haas (SdH) oscillation method separately for each size-quantization subband. An analysis of the Fourier spectra of SdH oscillations showed that throughout the entire range of electron temperatures obtained, harmonics associated with different size-quantization subbands and transitions between the Landau levels of these subbands dominate in the spectra. To separate the SdH oscillations into individual harmonics, we used the approximation of the magnetic-field dependences of the oscillations by theoretical expressions using the ratio between the peak heights in the Fourier spectra. It is shown that this method allows one to determine the values of the electron temperature in case when the electron concentrations in the first and second subbands are close in magnitude. The obtained values of the electron temperature correspond to the literature data.

The atomic structure and energy spectrum of heterostructures formed in a system of InAs/AlAs binary compounds are studied. The InxAl1 –xSbyAs1 –y alloy, from which quantum wells are formed in InSb/AlAs structures, decomposes into two phases with different compositions. The characteristic dimensions of regions containing separate phases of the alloy in the structure plane are 5–7 nm. Spinodal decomposition of the alloy brings about the formation of coexisting indirect-gap regions with type-I and type-II energy spectra in quantum wells of the heterostructures.

This paper demonstrates the applicability of nanoindentation technique using atomic-force microscope cantilever for studying the elastic properties of suspended semiconductor structures on the basis of relatively thick GaAs/AlGaAs membranes in the case when their stiffness significantly exceeds that of the cantilever of atomic-force microscope, which is confirmed by the agreement between the experimentally determined values of both relative and absolute stiffness measured at different points of the investigated structure with theoretical predictions.

The effect of the silicon-atom distribution profile in donor δ-layers of AlGaAs/InGaAs/AlGaAs heterostructures with donor–acceptor doping on the mobility of the two-dimensional electron gas is studied. The parameters of the δ-layer profiles are determined using the normal approximation of the spatial distributions of silicon atoms, measured by secondary-ion mass spectroscopy. It is shown that the standard deviation σ of the δ-layer profile can be reduced from 3.4 to 2.5 nm by the proper selection of growth conditions. Measurements of the magnetic-field dependences of the Hall effect and conductivity show that such a decrease in σ allowed an increase in the mobility of the two-dimensional electron gas in heterostructures by 4000 cm2/(V s) at 77 K and 600 cm2/(V s) at 300 K. The mobility calculation taking into account filling of the first two size-quantization subbands shows that an increase in the mobility is well explained by a reduction in the Coulomb scattering at ionized donors due to an increase in the effective thickness of the spacer layer with decreasing σ of the δ-layer profile.

A standard way of determining the type of energy structure in heterostructures, based on analyzing the spectral shift of the band of steady-state photoluminescence (PL) as a function of the excitation power density, is developed with allowance for the effect of the fluctuation tails of the density of energy states. The technique is validated for InAs/AlAs, GaAs/AlAs, and AlSb y As1–y /AlAs heterostructures with quantum wells.

Low-dimensional quantum-well and nanoisland heterostructures formed in the InSb/AlAs system by molecular-beam epitaxy are studied by transmission electron microscopy and steady-state photoluminescence spectroscopy. The structures are grown under conditions of alternate In and Sb deposition (the socalled atomic-layer epitaxy mode) and the simultaneous deposition of materials (the traditional molecularbeam epitaxy mode). In both modes of growth, at a nominal amount of the deposited material in a single layer, large-sized (200 nm–1 μm) imperfect islands arranged on the In x Al1 – x Sb y As1–y quantum-well layer are formed. In the heterostructures grown under conditions of atomic layer epitaxy, the islands are surrounded by ring-shaped arrays of much smaller (~10 nm), coherently strained islands consisting of the In x Al1 – x Sb y As1 – y alloy as well. The composition of the alloy is defined by the intermixing of Group-V materials in the stage of InSb deposition and by the intermixing of materials because of the segregation of In and Sb atoms during overgrowth of the InSb layer by an AlAs layer.

The MBE method has been applied to grow InSb layers on InSb substrates. These layers have served as a basis for fabricating mid-wave IR photodetector arrays. The characteristics of photodetector arrays on epitaxial InSb layers have been compared with those of series-produced single-crystal InSb arrays.

The DC and AC conductivities of the n-GaAs/AlAs heterostructure with two filled size quantization levels are studied within a wide magnetic field range. The electron spectrum of such heterostructure is characterized by two subbands (symmetric $S$ and antisymmetric $AS$), separated by the band gap $��_{12}=15.5$ meV. It is shown that, in the linear regime at the applied magnetic field $B >3$ T, the system exhibits oscillations corresponding to the integer quantum Hall effect. A quite complicated pattern of such oscillations is well interpreted in terms of transitions between Landau levels related to different subbands. At $B, 7 pages, 8 figures

Field dependences of the electron-drift velocity in typical pseudomorphic high-electron-mobility transistor (pHEMT) heteroepitaxial structures (HESs) and in those with donor–acceptor doped (DApHEMT) heterostructures with quantum-well (QW) depth increased by 0.8–0.9 eV with the aid of acceptor layers have been studied by a pulsed technique. It is established that the saturated electron-drift velocity in DA-pHEMT-HESs is 1.2–1.3 times greater than that in the usual pHEMT-HESs. The electroluminescence (EL) spectra of DA-pHEMT-HESs do not contain emission bands related to the recombination in widebandgap layers (QW barriers). The EL intensity in these HESs is not saturated with increasing electric field. This is indicative of a suppressed real-space transfer of hot electrons from QW to barrier layers, which accounts for the observed increase in the saturated electron-drift velocity.

The direct-current (dc) $\sigma_{xx}^{dc}$ and alternating-current (ac) $\sigma_{xx}^{ac}=\sigma_1-i\sigma_2$ conductivities of a wide (46 nm) GaAs quantum well with the bilayer electron density distribution are measured. It is found that the magnetic field dependence of $\sigma_{xx}$ exhibits three sets of oscillations related to the transitions between Landau levels in symmetric and antisymmetric subbands and with the transitions occurring owing to the Zeeman splitting of these subbands. The analysis of the frequency dependence of the ac conductivity and the $\sigma_1 / \sigma_2$ ratio demonstrates that the conductivity at the minima of oscillations is determined by the hopping mechanism., Comment: 6 pages, 6 figures

The structural, transport and optical properties of DA-pHEMT heterostructures are considered. Despite the high 2DEG density (~4×1012 cm–2), parallel conductance is almost lacking in DA-pHEMT heterostructures: the electron density in the parallel conducting layer does not exceed 11% of the 2DEG density, and the electron mobility is lower than the 2DEG mobility by a factor of more than 2.5. It was determined that the beryllium concentration in the undoped InGaAs QW does not exceed 2×016 cm−3 in spite of the Be segregation from the p+-doped barriers during the epitaxy growth. The results of the development of powerful DA-pHEMT are presented. Transistors with a gate length of 0.4-0.5 μm and a total gate width of 0.8 mm at a frequency of 10 GHz have a gain of more than 9 dB, an output power density of more than 1.6 W/mm, a power added efficiency up to 50%.

The features of electrostatic actuation are experimentally studied in nanomechanical resonators based on AlGaAs/GaAs heterostructures with a two-dimensional electron gas. First, it is found that, when the gate voltage is varied, the oscillation amplitude reaches its minimum and the resonant frequency reaches its maximum at different non-zero voltages. Using numerical modeling, it is shown that this feature can be explained by the influence of the bound charge induced due to the piezoelectric effect in the resonator performing flexural oscillations. Second, the amplitude as a function of the gate voltage does not reach zero at its minimum. A short illumination makes the minimal amplitude much closer to zero. These facts can be explained by the influence of the poorly conducting edges of a two-dimensional electron gas. The obtained results show that the measurement of the specific gate voltages corresponding to the minimal amplitude and maximal resonant frequency can be used for sensing the electrical charge induced by the illumination.

The (001) surface of InSb is the most common growth surface, forming a number of surface reconstructions depending on the both ratio of group III and V species presented on surface and substrate temperature. In the present work surface structures were studied using reflection high energy electron diffraction (RHEED). The c(4×4)↔(1×3) reconstruction transition was investigated in details. The intensity of fractional spots of c(4×4) structure was measured during the variation of antimony flux at different substrate temperatures. At the substrate temperatures of T T >400 °C, hysteresis loop was not observed, that corresponds to continuous phase transition. It was shown that phase transition is analogous to the van der Waals transition. We developed a model to describe c(4×4)↔(1×3) transition in the framework of the lattice gas approximation. This model takes into account the complex nature of indirect interactions that result in the effective attraction between lattice gas cells forming surface reconstruction. The calculated surface state isotherms are in a good agreement with the experimental isotherms.

The nonequilibrium state of a two-dimensional electron gas in the quantum-Hall-effect regime is studied in Hall bars equipped with additional inner contacts situated within the bar. The magnetic-field dependence of the voltage drop between different contact pairs are studied at various temperatures. It was found that the voltage between the inner and outer contacts exhibits peaks of significant amplitude in narrow magnetic-field intervals near integer filling factors. Furthermore, the magnetic-field dependence of the voltage in these intervals exhibits a hysteresis, whereas the voltage between the outer contacts remains zero in the entire magnetic-field range. The appearance of the observed voltage peaks and their hysteretic behavior can be explained by an imbalance between the chemical potentials of edge and bulk states, resulting from nonequilibrium charge redistribution between the edge and bulk states when the magnetic field sweeps under conditions of the quantum Hall effect. The results of the study significantly complement the conventional picture of the quantum Hall effect, explicitly indicating the existence of a significant imbalance at the edge of the two-dimensional electron gas: the experimentally observed difference between the electrochemical potentials of the edge and bulk exceeds the distance between Landau levels by tens of times.

The crystal structure of new self-assembled InSb/AlAs and AlSb/AlAs quantum dots grown by molecularbeam epitaxy has been investigated by transmission electron microscopy. The theoretical calculations of the energy spectrum of the quantum dots have been supplemented by the experimental data on the steady-state and time-resolved photoluminescence spectroscopy. Deposition of 1.5 ML of InSb or AlSb on the AlAs surface carried out in the regime of atomic-layer epitaxy leads to the formation of pseudomorphically strained quantum dots composed of InAlSbAs and AlSbAs alloys, respectively. The quantum dots can have the type-I and type-II energy spectra depending on the composition of the alloy. The ground hole state in the quantum dot belongs to the heavy-hole band and the localization energy of holes is much higher than that of electrons. The ground electron state in the type-I quantum dots belongs to the indirect X XY valley of the conduction band of the alloy. The ground electron state in the type-II quantum dots belongs to the indirect X valley of the conduction band of the AlAs matrix.

The conductance of a single GaAs quantum point contact (QPC) with in-plane side gates separated from a channel by lithographic trenches is experimentally studied. It is shown that, in such a trench-type QPC, the conducting channel splits into two channels, each demonstrating a conductance quantization. A phase diagram of the 1D subband population in separate channels as a function of the voltages at side gates is experimentally obtained. The obtained data show that the electron transport regime undergoes the transition from the case of two independent parallel 1D conductors to the case of strongly correlated double-row transport. The transition occurs with the increase in the number of populated 1D subbands in the channels and manifests itself in the phase diagram by the change of behavior of the lines separating the regions corresponding to different numbers. The crossings of these lines at small numbers turn into anticrossings at large numbers. The anticrossings are explained by the splitting of degenerate levels in a double-well confinement potential formed inside the QPC constriction.

Low-temperature dissipation of mechanical energy is studied in AlGaAs/GaAs-based nanomechanical resonators with a two-dimensional electron gas. It is experimentally shown that the temperature dependence of dissipation demonstrates a peak near 30 K. A short illumination leads to a persistent change in the quality factor, which can be compared with the persistent photoconductivity effect. In particular, the illumination persistently suppresses the dissipation peak. This suppression shows that the nature of the peak is not related to the thermoelastic and Akhiezer damping. A hypothesis associating the peak with DX-centers or similar low-symmetry and light-sensitive centers is proposed. The observed effects should be taken into account when studying the low-temperature dissipation using optical methods.

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

The photoluminescence of AlGaAs/InGaAs/GaAs pseudomorphic high-electron mobility transistor heterostructures with donor-acceptor-doped AlGaAs barriers is studied. It is found that the introduction of additional p +-doped AlGaAs layers into the design brings about the appearance of new bands in the photoluminescence spectra. These bands are identified as resulting from transitions (i) in donor-acceptor pairs in doped AlGaAs layers and (ii) between the conduction subband and acceptor levels in the undoped InGaAs quantum well.