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Yu. M. Galperin

Publications and source records attributed to Yu. M. Galperin.

12 recordsLinked to original sources
arXiv2026

AC Conductance in n-InSb Structures with Quantum Well. Acoustic Studies

We studied the ac conductance of an $n$-InSb quantum well structure using acoustic methods in magnetic fields up to 18 T and at temperatures ranging from 20 to 500 mK. We attribute the unusual magnetic field dependences of surface acoustic wave (SAW) attenuation and velocity observed in the experiment to the presence of a conducting layer parallel to the quantum well in the sample. We successfully separated the contributions from both the quantum well and the shunting layer, enabling the identification of their distinct conduction mechanisms. Furthermore, by employing the coincidence technique, we determined the electron g-factor in the quantum well and investigated its dependence on the degree of spin polarization.

cond-mat.mes-hall↗
arXiv2020

Nonlinear AC and DC Conductivities in a Two-Subband n-GaAs/AlAs Heterostructure

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 $Δ_{12}=15.5$ 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 $E_y$.

cond-mat.mes-hall↗
arXiv2018

Universal Frequency Dependence of the Hopping AC Conductance in p-Ge/GeSi Structures in the Integer Quantum Hall Effect Regime

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 $σ_1$ 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.

cond-mat.mes-hall↗
arXiv2015

Quantization of entropy in a quasi-two-dimensional electron gas

We demonstrate that the partial entropy of a two-dimensional electron gas (2DEG) exhibits quantized peaks at resonances between the chemical potential and electron levels of size quantization. In the limit of no scattering, the peaks depend only on the subband quantization number and are independent on material parameters, shape of the confining potential, electron effective mass and temperature. The quantization of partial entropy is a signature of a topological phase transition in a 2DEG. In the presence of stationary disorder, the magnitude of peaks decreases. Its deviation from the quantized values is a direct measure of the disorder induced smearing of the electronic density of states.

cond-mat.mes-hall↗
arXiv2005

Magneto-optical imaging of magnetic flux patterns in superconducting films with antidots

Superconducting YBaCuO thin films were equipped with a special arrangement of antidots (holes) of 1 micron radius in order to guide the stream of magnetic flux moving in (or out of) the sample. The flux distribution and its dynamics were visualized using real-time magneto-optical imaging. It is clearly demonstrated that one-dimensional antidot arrays strongly facilitate propagation of magnetic flux. We also demonstrate a possibility to alter the direction of flux motion in a controlled way by special arrangement of intercepting antidot arrays. Our resolution was sufficient for observation of flux in particular antidots, which allows a more detailed dynamic analysis of such systems.

cond-mat.supr-con↗
arXiv2000

Acoustoelectric effect in a finite-length ballistic quantum channel

The dc current induced by a coherent surface acoustic wave (SAW) of wave vector q in a ballistic channel of length L is calculated. The current contains two contributions, even and odd in q. The even current exists only in a asymmetric channel, when the electron reflection coefficients r_1 and r_2 at both channel ends are different. The direction of the even current does not depend on the direction of the SAW propagation, but is reversed upon interchanging r_1 and r_2. The direction of the odd current is correlated with the direction of the SAW propagation, but is insensitive to the interchange of r_1 and r_2. It is shown that both contributions to the current are non zero only when the electron reflection coefficients at the channel ends are energy dependent. The current exhibits geometric oscillations as function of qL. These oscillations are the hallmark of the coherence of the SAW and are completely washed out when the current is induced by a flux of non-coherent phonons. The results are compared with those obtained previously by different methods and under different assumptions.

cond-mat.mes-hall↗
arXiv1998

Population Inversion Induced by Resonant States in Semiconductors

We present a theoretical prediction of a new mechanism for carrier population inversion in semiconductors under an applied electric field. The mechanism is originated from a coherent capture-emission type inelastic scattering of resonant states. We support our theory with concrete calculations for shallow acceptor resonant states in strained p-Ge where a lasing in THz frequency region has been recently observed.

cond-mat.mtrl-sci↗
arXiv1997

Nonlinear acoustic and microwave absorption in glasses

A theory of weakly-nonlinear low-temperature relaxational absorption of acoustic and electromagnetic waves in dielectric and metallic glasses is developed. Basing upon the model of two-level tunneling systems we show that the nonlinear contribution to the absorption can be anomalously large. This is the case at low enough frequencies, where freqeuency times the minimal relaxation time for the two-level system are much less than one. In dielectric glasses, the lowest-order nonlinear contribution is proportional to the wave's intensity. It is negative and exhibits anomalous frequency and temperature dependencies. In metallic glasses, the nonlinear contribution is also negative, and it is proportional to the square root of the wave's intensity and to the frequency. Numerical estimates show that the predicted nonlinear contribution can be measured experimentally.

cond-mat.dis-nn↗
arXiv1996

Magnetization of a Diffusive Ring: Beyond the Perturbation Theory

Average persistent current over a set of diffusive metallic rings with fixed number of electrons is considered. We study the the case where the phase breaking time is much greater than an inverse average inter-level distance. In such a situation, many return events for an electron have to be taken into account. As a result, one arrives at a non-perturbative problem for a fixed by an external magnetic field Cooperon mode. This multi-Cooperon problem has been considered previously by Altland et al., Europhys. Lett., 2, 155 (1992) and in several following papers within the framework of supersymmetric approach. Such an approach involves very tedious calculations which were performed using computer algebra package. Here we solve the problem in question with the help of replica trick. It is demonstrated that the replica trick in combination with a proper analytical continuation in the replica space allows one to obtain the results in much more explicit way.

cond-mat↗
arXiv1995

Excess Noise in Biased Superconducting Weak Links

Non-equilibrium excess noise of a short quasi one-dimensional constriction between two superconductors is considered. A general expression for the current-current correlation function valid for arbitrary temperatures and bias voltages is derived. This formalism is applied to a current-carrying quantum channel with perfect transparency. Contrary to a transparent channel separating two normal conductors, a weak link between two superconductors exhibits a finite level of noise. The source of noise is fractional Andreev scattering of quasiparticles with energies $|E|$ greater than the half-width $Δ$ of the superconducting gap. For high bias voltages, $V \gg Δ/e$, the relation between the zero-frequency limit of the noise spectrum, $S(0)$, and the excess current $I_{\text{exc}}$ reads $S(0)=(1/5)|e|I_{\text{exc}}$. As $Δ\rightarrow 0$ both the excess noise and the excess current vanish linearly in $Δ$, %$\propto Δ$, their ratio being constant.

cond-mat↗
arXiv1994

Non-Equilibrium Magnetization in a Ballistic Quantum Dot

We show that Aharonov-Bohm (AB) oscillations in the magnetic moment of an integrable ballistic quantum dot can be destroyed by a time dependent magnetic flux. The effect is due to a nonequilibrium population of perfectly coherent electronic states. For real ballistic systems the equilibrization process, which involves a special type of inelastic electron backscattering, can be so ineffective, that AB oscillations are suppressed when the flux varies with frequency $ω\sim$ 10$^7$-10$^8$ s$^{-1}$. The effect can be used to measure relaxation times for inelastic backscattering.

cond-mat↗