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We focused on the transverse AC magneto-conductance of a high mobility $p$-GaAs/AlGaAs quantum well ($p=1.2\times 10^{11}$~cm$^{-2}$) in the vicinity of two values of the Landau level filling factor $\nu$: $\nu =1$ (integer quantum Hall effect) and $\nu =1/3$ (fractional quantum Hall effect). The complex transverse AC conductance, $\sigma_{xx}^{AC} (\omega)$, was found from simultaneous measurements of attenuation and velocity of surface acoustic waves (SAWs) propagating along the interface between a piezoelectric crystal and the two-dimensional hole system under investigation. We analyzed both the real and imaginary parts of the hole conductance and compared the similarities and differences between the results for filling factor 1 and filling factor 1/3. Both to the left and to the right of these values maxima of a specific shape, "wings", arose in the $\sigma (\nu)$ dependences at those two $\nu$. Analysis of the results of our acoustic measurements at different temperatures and surface acoustic wave frequencies allowed us to attribute these wings to the formation of collective localized states, namely the domains of a pinned Wigner crystal, i.e., a Wigner solid. While the Wigner solid has been observed in 2D hole systems previously, we were able to detect 20 it at the highest hole density and, therefore, the lowest hole-hole interaction reported., Comment: 9 pages, 6 figures

Contactless acoustic methods were used to determine electrical parameters - electrical conductivity, carrier mobility and their concentration - in single-layer graphene deposited on the surface of lithium niobate., Comment: 7 pages, 9 figures

The effect of spin-orbit interaction was studied in a high-quality $p$-AlGaAs/GaAs/AlGaAs structure with a square quantum well using acoustic methods. The structure grown on a GaAs (100) substrate was symmetrically doped with carbon on both sides of the quantum well. Shubnikov-de Haas-type oscillations of the ac conductance of two-dimensional holes were measured. At a low magnetic field $B <$2 T conductance oscillations undergo beating induced by a spin-orbit interaction. Analysis of the beating character made it possible to separate the conductance contributions from the two heavy holes subbands split by the spin-orbit interaction. For each of the subbands the values of the effective masses and quantum relaxation times have been determined, and then the energy of the spin-orbit interaction was obtained. The quantum well profile, as well as the small magnitude of the spin-orbit interaction, allowed us to conclude that the spin-orbit splitting is governed by the Dresselhaus mechanism., Comment: 6 pages, 7 figures

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 $\Delta_{12}=15.5$ 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 $B <1$ T, magneto-intersubband resistance oscillations (MISOs) are observed. An increase in the conductivity with the electric current flowing across the sample or in the intensity of the surface acoustic wave (SAW) in the regime of the integer quantum Hall effect is determined by an increase in the electron gas temperature. In the case of intersubband transitions, it is found that nonlinearity cannot be explained by heating. At the same time, the decrease in the AC conductivity with increasing SAW electric field is independent of frequency, but the corresponding behavior does not coincide with that corresponding to the dependence of the DC conductivity on the Hall voltage $E_y$., Comment: 7 pages, 8 figures

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

Using acoustic method we study dependences of transverse AC conductance, $\sigma (\omega)$, on magnetic field, temperature and the amplitude of AC electric field in a wide (75 nm) quantum well (QW) structure focusing on the vicinity of the filling factor $\nu =1/2$. Measurements are performed in the frequency domain 30-307 MHz and in the temperature domain 20-500 mK. Usually, in wide QW structures closely to $\nu =1/2$ the fractional quantum Hall effect (FQHE) regime is realized at some parameters of the sample. However, in our structure, at $\nu =1/2$ it is a compressible state corresponding to gas of composite fermions which is observed. This is confirmed by apparent frequency independence and weakly decreasing temperature dependence of $\mathrm{Re}\, \sigma(\omega)$. Comparing the dependences of this quantity on temperature and power of the acoustic wave we conclude that the observed nonlinear behavior of the conductance is compatible with heating of the composite fermions by the acoustic wave. For comparison, we also study the vicinity of $\nu = 3/2$ where the FQHE regime is clearly observed., Comment: 6 pages, 6 figures

The hopping ac conductance, which is realized at the transverse conductance minima in the regime of the integer Hall effect, has been measured using a combination of acoustic and microwave methods. Measurements have been made in the p-GeSi/Ge/GeSi structures with quantum wells in a wide frequency range (30-1200 MHz). The experimental frequency dependences of the real part of ac conductance $\sigma_1$ have been interpreted on the basis of the model presuming hops between localized electronic states belonging to isolated clusters. At high frequencies, dominating clusters are pairs of close states; upon a decrease in frequency, large clusters that merge into an infinite percolation cluster as the frequency tends to zero become important. In this case, the frequency dependences of the ac conductance can be represented by a universal curve. The scaling parameters and their magnetic-field dependence have been determined., Comment: 9 pages, 12 figures, 1 table

Oscillations of the real component of AC conductivity $\sigma_1$ in a magnetic field were measured in the n-AlGaAs/GaAs structure with a wide (75 nm) quantum well by contactless acoustic methods at $T$=(20-500)~mK. In a wide quantum well, the electronic band structure is associated with the two-subband electron spectrum, namely the symmetric (S) and antisymmetric (AS) subbands formed due to electrostatic repulsion of electrons. A change of the oscillations amplitude in tilted magnetic field observed in the experiments occurs due to crossings of Landau levels of different subbands (S and AS) at the Fermi level. The theory developed in this work shows that these crossings are caused by the difference in the cyclotron energies in the S and AS subbands induced by the in-plane magnetic field., Comment: 7 pages, 5 figures

The effective g-factor of 2D holes in modulation doped \mbox{p-SiGe/Ge/SiGe} structures was studied. The AC conductivity of samples with hole densities from $3.9 \times 10^{11}$~to $6.2 \times 10^{11}~\text{cm}^{-2}$ was measured in perpendicular magnetic fields up to $8~\text{T}$ using a contactless acoustic method. From the analysis of the temperature dependence of conductivity oscillations, the $\mathrm{g}_{\perp}$-factor of each sample was determined. The $\mathrm{g}_{\perp}$-factor was found to be decreasing approximately linearly with hole density. This effect is attributed to non-parabolicity of the valence band., Comment: 4 pages, 5 figures

Slow magnetooscilations of the conductivity are observed in a 75 nm wide quantum well at heating of the two-dimensional electrons by a high-intensity surface acoustic wave. These magnetooscillations are caused by intersubband elastic scattering between the symmetric and asymmetric subbands formed due to an electrostatic barrier in the center of the quantum well. The tunneling splitting between these subbands as well as the intersubband scattering rate are determined., Comment: 5 pages, 5 figures

Using acoustic methods the complex high-frequency conductance of high-mobility $n$-GaAs/AlGaAs heterostructures was determined in magnetic fields 12$\div$18~T. Based on the observed frequency and temperature dependences we conclude that in the investigated magnetic field range and at sufficiently low temperatures, $T \lesssim 200$~mK, the electron system forms a Wigner crystal deformed due to pinning by disorder. At some temperature, which depends on the electron filling factor, the temperature dependences of both components of the complex conductance get substantially changed. We have ascribed this rapid change of the conduction mechanism to melting of the Wigner crystal and study the dependence of the so-defined melting temperature on the electron filling factor., Comment: 6 pages, 8 figures. arXiv admin note: text overlap with arXiv:1511.05376

We have measured magnetic field dependences of the attenuation and velocity of surface acoustic waves in a high-mobility $n$-GaAs/AlGaAs structure with a wide quantum well. The results allowed us to find the complex conductance, $\sigma(\omega)$, of the heterostructure for different frequencies, temperatures and magnetic fields near filling factors $\nu=1, 2$. Observed behavior of $\sigma(\omega)$ versus magnetic field outside close vicinities of integer fillings reveals an oscillation pattern indicative of the rich fractional quantum Hall effect. Our result is that in very close vicinities of integer filling factors the AC response of a high-mobility two-dimensional structures behaves as that of a two-dimensional system of localized electrons. Namely, both real and imaginary parts of the complex AC conductance at low temperatures agree with the predictions for the two-site model for a two-dimensional hopping system. Another result is the specific temperature dependences of $\sigma(\omega)$, which are extremely sensitive to the filling factor value. These dependences indicate a sharp crossover between the localized modes and a pinned Wigner crystal., Comment: 6 pages, 5 figures

High frequency (ac) conductivity in the single quantum well AlGaAs/GaAs/AlGaAs with high mobility was investigated by contactless acoustic methods in the fractional quantum Hall effect regime in perpendicular and tilted magnetic fields. We studied the dependence of ac conductivity $\sigma^{ac}=\sigma_1 - i\sigma_2$ on both the temperature and magnetic field tilt angle. Tilting the magnetic field relative to the sample surface enabled us to change the position of the conductivity oscillation minimum at $\nu$=2/3. We measured the temperature dependence of ac conductivity for each tilt angle and for the 2/3 state we calculated the activation energy $\Delta E$ which was derived by constructing the Arrhenius plot ln $\sigma_1$ against 1/$T$. Analyzing behavior of the activation energy in total magnetic field for the filling factor 2/3 we observed a distinct minimum which can be interpreted as the spin unpolarized-polarized phase transition., Comment: 4 pages, 4 figures

By simultaneous measurements of the attenuation and velocity of surface acoustic waves propagating in proximity to a high-quality GaAs quantum well we study the complex AC conductance of the two-dimensional electron system. Focusing on the vicinity of the filling factor $\nu=1/5$ we confirm that the insulating states formed closely to this value of $\nu$ are pinned Wigner crystals., Comment: 6 pages, 6 figures

The high-frequency (ac) conductivity of a high quality modulation doped GeSi/Ge/GeSi single quantum well structure with hole density $p$=6$\times$10$^{11}$cm$^{-2}$ was measured by the surface acoustic wave (SAW) technique at frequencies of 30 and 85~MHz and magnetic fields $B$ of up to 18 T in the temperature range of 0.3 -- 5.8 K. The acoustic effects were also measured as a function of the tilt angle of the magnetic field with respect to the normal of the two-dimensional channel at $T$=0.3 K. It is shown, that at the minima of the conductivity oscillations, holes are localized on the Fermi level, and that there is a temperature domain in which the high-frequency conductivity in the bulk of the quantum well is of the activation nature. The analysis of the temperature dependence of the conductivity at odd filling factors enables us to determine the effective $g_z$ factor. It is shown that the in-plane component of the magnetic field leads to an increase of the cyclotron mass and to a reduction of the $g_z$ factor. We developed a microscopic theory of these effects for the heavy-hole states of the complex valence band in quantum wells which describes well the experimental findings., Comment: 9 pages, 10 figures

We report a procedure to determine the frequency-dependent conductance of quantum Hall structures in a broad frequency domain. The procedure is based on the combination of two known probeless methods -- acoustic spectroscopy and microwave spectroscopy. By using the acoustic spectroscopy, we study the low-frequency attenuation and phase shift of a surface acoustic wave in a piezoelectric crystal in the vicinity of the electron (hole) layer. The electronic contribution is resolved using its dependence on a transverse magnetic field. At high frequencies, we study the attenuation of an electromagnetic wave in a coplanar waveguide. To quantitatively calibrate these data, we use the fact that in the quantum-Hall-effect regime the conductance at the maxima of its magnetic field dependence is determined by extended states. Therefore, it should be frequency independent in a broad frequency domain. The procedure is verified by studies of a well-characterized $p$-SiGe/Ge/SiGe heterostructure., Comment: 7 pages, 8 figures; to be submitted to JAP

The contactless Surface Acoustic Wave (SAW) technique was implemented to probe the high-frequency (ac) conductivity in a high-mobility $p$-SiGe/Ge/SiGe structure in the integer quantum Hall (IQHE) regime. The structure was grown by low-energy plasma-enhanced chemical vapor deposition and comprised a two-dimensional channel formed in a compressively strained Ge layer. It was investigated at temperatures of 0.3 - 5.8 K and magnetic fields up to 18 T at various SAW intensities. In the IQHE regime, in minima of the conductivity oscillations with small filling factors, holes are localized. The ac conductivity is of the hopping nature and can be described within the "two-site" model. Furthermore, the dependence of the ac conductivity on the electric field of the SAW was determined. The manifestation of non-linear effects is interpreted in terms of nonlinear percolation-based conductivity., Comment: 6 pages, 10 figures

The magnetoresistance components $\rho_{xx}$ and $\rho_{xy}$ were measured in two p-Si/SiGe/Si quantum wells that have an anisotropic g-factor in a tilted magnetic field as a function of temperature, field and tilt angle. Activation energy measurements demonstrate the existence of a ferromagnetic-paramagnetic (F-P) transition for a sample with a hole density of $p$=2$\times10^{11}$\,cm$^{-2}$. This transition is due to crossing of the 0$\uparrow$ and 1$\downarrow$ Landau levels. However, in another sample, with $p$=7.2$\times10^{10}$\,cm$^{-2}$, the 0$\uparrow$ and 1$\downarrow$ Landau levels coincide for angles $\Theta$=0-70$^{\text{o}}$. Only for $\Theta$ > 70$^{\text{o}}$ do the levels start to diverge which, in turn, results in the energy gap opening., Comment: 5 pages, 6 figures

Using acoustic methods we have measured nonlinear AC conductance in 2D arrays of Ge-in-Si quantum dots. The combination of experimental results and modeling of AC conductance of a dense lattice of localized states leads us to the conclusion that the main mechanism of AC conduction in hopping systems with large localization length is due to the charge transfer within large clusters, while the main mechanism behind its non-Ohmic behavior is charge heating by absorbed power., Comment: 5 pages, 7 figures

In case of a of the heterostructure n-GaAs/AlGaAs with sheet density $n=2 \times 10^{11}$cm$^{-2}$ and mobility $\mu \approx 2 \times 10^6$ cm$^2$/V$\cdot$s with integer and fractional quantum Hall effect (IQHE and FQHE, respectively) we demonstrate the wide applicability of acoustic methods for determining the general conduction parameters of a two dimensional electron gas. We also examine the mechanisms of low-temperature conductivity in the minima of oscillations of high frequency conductivity in the IQHE and FQHE regimes. In the magnetic field region where electrons are delocalized, the parameters determined by the acoustic technique do not differ from those determined by a direct current. However, the acoustic measurements do not require Hall bars and electrical contacts to be fabricated. In the minima of IQHE and FQHE oscillations electrons are localized, and ac conductivity turns to be via hopping. An analysis of the high frequency conductivity in the QHE regime has been carried out within a "two site" model. Furthermore, measurements of acoustoelectric effects in a tilted magnetic field provided the dependence of the activation energy on magnetic field in the fractional quantum Hall effect regime at $\nu$=2/3., Comment: 10 pages, 14 figures. Figures ordering fixed

Magnetoresistivity $\rho _{xx}$ and $\rho _{xy}$ and the acoustoelectronic effects are measured in $p$-Si/SiGe/Si with an impurity concentration $p$ = 2 $\times $ 10$^{11}$ cm$^{-2}$ in the temperature range 0.3-2.0 K and an tilted magnetic field up to 18 T. The dependence of the effective $g$-factor on the angle of magnetic field tilt $\theta $ to the normal to the plane of a two dimensional $p$-Si/SiGe/Si channel is determined. A first order ferromagnet-paramagnet phase transition is observed in the magnetic fields corresponding to a filling factor $\nu $ = 2 at $\theta \approx $ 59$^\textrm{o}$-60$^\textrm{o}$., Comment: 8 pages, 13 figures

We report the results of an experimental study of the magnetoresistance $\rho_{xx}$ and $\rho_{xy}$ in two samples of $p$-Si/SiGe with low carrier concentrations $p$=8.2$\times10^{10}$ cm$^{-2}$ and $p$=2$\times10^{11}$ cm$^{-2}$. The research was performed in the temperature range of 0.3-2 K and in the magnetic fields of up to 18 T, parallel or tilted with respect to the two-dimensional (2D) channel plane. The large in-plane magnetoresistance can be explained by the influence of the \textit{in-plane} magnetic field on the orbital motion of the charge carriers in the quasi-2D system. The measurements of $\rho_{xx}$ and $\rho_{xy}$ in the tilted magnetic field showed that the anomaly in $\rho_{xx}$, observed at filling factor $\nu$=3/2 is practically nonexistent in the conductivity $\sigma_{xx}$. The anomaly in $\sigma_{xx}$ at $\nu$=2 might be explained by overlapping of the levels with different spins 0$\uparrow$ and 1$\downarrow$ when the tilt angle of the applied magnetic field is changed. The dependence of g-factor $g^*(\Theta)/g^*(0^0)$ on the tilt angle $\Theta$ was determined., Comment: 4 pages, 6 figs, to appear in Annalen der Physik

Measurement results of the acoustoelectric effects (surface acoustic waves (SAW) attenuation and velocity) in a high-mobility $p$-SiGe/Ge/SiGe structure are presented. The structure was LEPECVD grown with a two dimensional (2D) channel buried in the strained Ge layer. The measurements were performed as a function of temperature (1.5 - 4.2 K) and magnetic field (up to 8.4 T) at different SAW intensities at frequencies 28 and 87 MHz. Shubnikov-de Haas-like oscillations of both SAW attenuation and the velocity change have been observed. Hole density and mobility, effective mass, quantum and transport relaxation times, as well as the Dingle temperature were measured with a method free of electric contacts. The effect of heating of the 2D hole gas by the electric field of the SAW was investigated. Energy relaxation time $\tau_{\varepsilon}$ and the deformation potential constant determined., Comment: 5 pages, 7 figures, 1 table

We report the results of an experimental study of the magnetoresistance $\rho_{xx}$ in two samples of $p$-Si/SiGe/Si with low carrier concentrations $p$=8.2$\times10^{10}$ cm$^{-2}$ and $p$=2$\times10^{11}$ cm$^{-2}$. The research was performed in the temperature range of 0.3-2 K in the magnetic fields of up to 18 T, parallel to the two-dimensional (2D) channel plane at two orientations of the in-plane magnetic field $B_{\parallel}$ against the current $I$: $B_{\parallel} \perp I$ and $B_{\parallel} \parallel I$. In the sample with the lowest density in the magnetic field range of 0-7.2 T the temperature dependence of $\rho_{xx}$ demonstrates the metallic characteristics ($d \rho_{xx}/dT>$0). However, at $B_{\parallel}$ =7.2 T the derivative $d \rho_{xx}/dT$ reverses the sign. Moreover, the resistance depends on the current orientation with respect to the in-plane magnetic field. At $B_{\parallel} \cong$ 13 T there is a transition from the dependence $\ln(\Delta\rho_{xx} / \rho_{0})\propto B_{\parallel}^2$ to the dependence $\ln(\Delta\rho_{xx} / \rho_{0})\propto B_{\parallel}$. The observed effects can be explained by the influence of the in-plane magnetic field on the orbital motion of the charge carriers in the quasi-2D system., Comment: revised: included AC conductivity experiments to study the role of localized state in transport; total 6 pages, 7 figures. Accepted PRB; to appear vol.79, Issue 20

We study DC and AC transport in low-density $p-$Si/SiGe heterostructures at low temperatures and in a broad domain of magnetic fields up to 18 T. Complex AC conductance is determined from simultaneous measurement of velocity and attenuation of a surface acoustic wave propagating in close vicinity of the 2D hole layer. The observed behaviors of DC and AC conductance are interpreted as an evolution from metallic conductance at B=0 through hopping between localized states in intermediate magnetic fields (close to the plateau of the integer quantum Hall effect corresponding to the Landau-level filling factor $\nu$=1) to formation of the Wigner glass in the extreme quantum limit ($B\gtrsim 14$, $T \lesssim 0.8$ K)., Comment: 11 pages, 14 figures

Complex AC-conductance, $\sigma^{AC}$, in the systems with dense Ge$_{0.7}$Si$_{0.3}$ quantum dot (QD) arrays in Si has been determined from simultaneous measurements of attenuation, $\Delta\Gamma=\Gamma(H)-\Gamma(0)$, and velocity, $\Delta V /V=(V(H)-V(0)) / V(0)$, of surface acoustic waves (SAW) with frequencies $f$ = 30-300 MHz as functions of transverse magnetic field $H \leq$ 18 T in the temperature range $T$ = 1-20 K. It has been shown that in the sample with dopant (B) concentration 8.2$ \times 10^{11}$ cm$^{-2}$ at temperatures $T \leq$4 K the AC conductivity is dominated by hopping between states localized in different QDs. The observed power-law temperature dependence, $\sigma_1(H=0)\propto T^{2.4}$, and weak frequency dependence, $\sigma_1(H=0)\propto \omega^0$, of the AC conductivity are consistent with predictions of the two-site model for AC hopping conductivity for the case of $\omega \tau_0 \gg $1, where $\omega=2\pi f$ is the SAW angular frequency and $\tau_0$ is the typical population relaxation time. At $T >$ 7 K the AC conductivity is due to thermal activation of the carriers (holes) to the mobility edge. In intermediate temperature region 4$ < T<$ 7 K, where AC conductivity is due to a combination of hops between QDs and diffusion on the mobility edge, one succeeded to separate both contributions. Temperature dependence of hopping contribution to the conductivity above $T^*\sim$ 4.5 K saturates, evidencing crossover to the regime where $\omega \tau_0 < $1. From crossover condition, $\omega \tau_0(T^*)$ = 1, the typical value, $\tau_0$, of the relaxation time has been determined., Comment: revtex, 3 pages, 6 figures

The interaction of surface acoustic waves (SAW) with $p$-type Si/Si$_{0.87}$Ge$_{0.13}$ heterostructures has been studied for SAW frequencies of 30-300 MHz. For temperatures in the range 0.7$

The surface acoustic wave (SAWs) attenuation coefficient $\Gamma$ and the velocity change $\Delta V /V$ were measured for $p$-type Si/SiGe heterostructures in the temperature range 0.7 - 1.6 K as a function of external magnetic field $H$ up to 7 T and in the frequency range 30-300 MHz in the hole Si/SiGe heterostructures. Oscillations of $\Gamma$ (H) and $\Delta V /V$ (H) in a magnetic field were observed. Both real $\sigma_1$ (H) and imaginary $\sigma_2$ (H) components of the high-frequency conductivity have been determined. Analysis of the $\sigma_1$ to $\sigma_2$ ratio allows the carrier localization to be followed as a function of temperature and magnetic field. At T=0.7 K the variation of $\Gamma$, $\Delta V /V$ and $\sigma_1$ with SAW intensity have been studied and could be attributed to 2DHG heating by the SAW electric field. The energy relaxation time is found to be dominated by scattering at the deformation potential of the acoustic phonons with weak screening., Comment: RevTeX4; 4 figs

Using simultaneous measurements of the attenuation and velocity of surface acoustic waves propagating along GaAs/Al$_{0.3}$Ga$_{0.7}$As heterostructures, complex ac conductance of the latters has been determined. In the magnetic fields corresponding to the middles of the Hall plateaus both the ac conductance, $\sigma (\omega)$, and the sheet electron density, $n_s$, in the two-dimensional conducting layer turn out to be dependent on the samples' cooling rate. As a result, the sample ``remembers'' the cooling conditions. The complex conductance is strongly dependent on an infrared illumination which also changes both $\sigma (\omega)$ and $n_s$. Remarkably, the correlation between $\sigma (\omega)$ and $n_s$ is \emph{universal}, i.e. it is independent of the way to change these quantities. The results are attributed to two-electron defects (so-called $DX^-$ centers) located in the Si doped layer., Comment: Revtex 4, 7 pages, 7 fugres

The absorption coefficient for surface acoustic wave $\Gamma$ and variation in the wave velocity $\Delta V/V$ were measured in GaAs/AlGaAs heterostructures; the above quantities are related to interaction of the wave with two-dimensional electron gas and depend nonlinearly on the power of the wave. Measurements were performed under conditions of the integer quantum Hall effect (IQHE), in which case the two-dimensional electron gas was localized in a random fluctuation potential of impurities. The dependences of the components $\sigma_1(E)$ and $\sigma_2(E)$ of high-frequency conductivity $\sigma=\sigma_1 - i\sigma_2$ on the electric field of the surface wave were determined. In the range of the conductivity obeying the Arrhenius law ($\sigma_1 \gg \sigma_2$), the results obtained are interpreted in terms of the Shklovskii theory of nonlinear percolation-based conductivity, which makes it possible to estimate the magnitude of the fluctuation potential of impurities. The dependences $\sigma_1(E)$ and $\sigma_2(E)$ in the range of high-frequency hopping electrical conductivity, in which case ($\sigma_1 \ll \sigma_2$) and the theory of nonlinearities has not been yet developed, are reported., Comment: RevTeX; 7 pages + 5 eps figs

The high-frequency conductivity of Si delta-doped GaAs/AlGaAs heterostructures is studied in the integer quantum Hall effect (QHE) regime, using acoustic methods. Both the real and the imaginary parts of the complex conductivity are determined from the experimentally observed magnetic field and temperature dependences of the velocity and the attenuation of a surface acoustic wave. It is demonstrated that in the structures studied the mechanism of low-temperature conductance near the QHE plateau centers is hopping. It is also shown that at magnetic fields corresponding to filling factors 2 and 4, the doped Si delta- layer efficiently shunts the conductance in the two-dimensional electron gas (2DEG) channel. A method to separate the two contributions to the real part of the conductivity is developed, and the localization length in the 2DEG channel is estimated., Comment: 8pages, 9 figures

The absorption coefficient for surface acoustic waves in a piezoelectric insulator in contact with a GaAs/AlGaAs heterostructure (with two-dimensional electron mobility $\mu= 1.3\times 10^5 cm^2/V\cdot s)$ at T=4.2K) via a small gap has been investigated experimentally as a function of the frequency of the wave, the width of the vacuum gap, the magnetic field, and the temperature. The magnetic field and frequency dependencies of the high-frequency conductivity (in the region 30-210 MHz) are calculated and analyzed. The experimental results can be explained if it assumed that there exists a fluctuation potential in which current carrier localization occurs. The absorption of the surface acoustic waves in an interaction with two-dimensional electrons localized in the energy "tails" of Landau levels is discussed., Comment: RevTeX 6 pages+6 EPS pics

The absorption and variation of the velocity of a surface acoustic wave of frequency $f$= 30 MHz interacting with two-dimensional electrons are investigated in GaAs/AlGaAs heterostructures with an electron density $n=(1.3 - 2.8) \times 10^{11} cm^{-2}$ at $T$=1.5 - 4.2 K in magnetic fields up to 7 T. Characteristic features associated with spin splitting of the Landau level are observed. The effective g factor and the width of the spin-split Landau bands are determined: $g^* \simeq 5$ and $A$=0.6 meV. The greater width of the orbital-split Landau bands (2 meV) relative to the spin-split bands is attributed to different shielding of the random fluctuation potential of charged impurities by 2D electrons. The mechanisms of the nonlinearities manifested in the dependence of the absorption and the velocity increment of the SAW on the SAW power in the presence of spin splitting of the Landau levels are investigated., Comment: Revtex 5 pages + 5 EPS Figures, v.2 - minor corrections in text and pics

High frequency (hf) conductivity in the form $\sigma^{hf} = \sigma_1^{hf} - i\sigma_2^{hf}$ was obtained from the measurement of Surface Acoustic Waves (SAW) attenuation and velocity (f=30 MHz) in GaAs/AlGaAs heterostructures ($n=1.3-7\cdot 10^{11}cm^{-2}$). It has been shown that in the Integer Quantum Hall Effect (IQHE) regime for all the samples at magnetic fields corresponding to the middle of the Hall plateaus and T=1.5 K, $\sigma_1 / \sigma_2 =0.14 \pm 0.03$. The ratio $\sigma_1 / \sigma_2=0.15$ points the case when the high-frequency hopping conductivity mechanism (electronic transition between the localized states formed by "tight" pairs) is valid \cite{1}. Dependencies of $\sigma_1$ and $\sigma_2$ on temperature and magnetic field is analyzed width of the Landau band broadened by the impurity random potential is determined., Comment: RevTeX 2 pages, 3 eps-figures; some corrections in cites

The heating of a two-dimensional electron gas by an rf electric field generated by a surface acoustic wave, which can be described by an electron temperature $T_e$, has been investigated. It is shown that the energy balance of the electron gas is determined by electron scattering by the piezoelectric potential of the acoustic phonons with $T_e$ determined from measurements at frequencies $f$= 30 and 150 MHz. The experimental curves of the energy loss $Q$ versus $T_e$ at different SAW frequencies depend on the value of $\omega \bar{\tau}_{\epsilon}$, compared to 1, where $ \bar {\tau}_{\epsilon}$ is the relaxation time of the average electron energy. Theoretical calculations of the heating of a two-dimensional electron gas by the electric field of the surface acoustic wave are presented for the case of thermal electrons ($\Delta T \ll T$). The calculations show that for the same energy losses $Q$ the degree of heating of the two-dimensional electrons (i.e., the ratio $T_e/T$) for $\omega \bar{\tau}_{\epsilon}>1$ ($f$= 150 MHz) is less than for $\omega \bar{\tau}_{\epsilon}<1$ ($f$=30 MHz). Experimental results confirming this calculation are presented., Comment: some technical corrections for pictures were made

High-frequency (HF) conductivity ($\sigma_{hf}$) measured by an acoustical method has been studied in GaAs/AlGaAs heterostructures in a linear and nonlinear regime on acoustic power. It has been shown that in the quantum Hall regime at magnetic fields corresponding to the middle of the Hall plateaus the HF conductivity is determined by the sum of the conductivity of 2-dimensional electrons in the high-mobility channel and the hopping conductivity of the electrons in the doped thick AlGaAs layer. The dependence of these conductivities on a temperature is analyzed. The width of the Landau level broadened by the impurity random potential is determined., Comment: RevTeX, 6 pages, 4 PS-figures, submitted to Phys.St.Sol.(Replaced with PS-figures technical corrections)

The parameters of two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure were determined by an acoustical (contactless) method in the delocalized electrons region ($B\le$2.5T). Nonlinear effects in Surface Acoustic Wave (SAW) absorption by 2DEG are determined by the electron heating in the electric field of SAW, which may be described in terms of electron temperature $T_e$. The energy relaxation time $\tau_{\epsilon}$ is determined by the scattering at piezoelectric potential of acoustic phonons with strong screening. At different SAW frequencies the heating depends on the relationship between $\omega\tau_{\epsilon}$ and 1 and is determined either by the instantaneously changing wave field ($\omega\tau_{\epsilon}$$<1$), or by the average wave power ($\omega\tau_{\epsilon}$$>1$)., Comment: RevTeX, 5 pages, 3 PS-figures, submitted to Physica Status Sol.(Technical corrections in PS-figs)