arXiv subjects

A. M. Diakonov

Publications and source records attributed to A. M. Diakonov.

14 recordsLinked to original sources

arXiv2014

Contactless Measurement of AC Conductance in Quantum Hall Structures

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.

cond-mat.mes-hall↗

arXiv2012

Nonlinear high-frequency hopping conduction in two-dimensional arrays of Ge-in-Si quantum dots: Acoustic methods

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.

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arXiv2009

Acoustoelectric effects in very high-mobility $p$-SiGe/Ge/SiGe heterostructure

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 $τ_{\varepsilon}$ and the deformation potential constant determined.

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arXiv2005

AC Conductance in Dense Array of the Ge$_{0.7}$Si$_{0.3}$ Quantum Dots in Si

Complex AC-conductance, $σ^{AC}$, 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, $ΔΓ=Γ(H)-Γ(0)$, and velocity, $ΔV /V=(V(H)-V(0)) / V(0)$, 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, $σ_1(H=0)\propto T^{2.4}$, and weak frequency dependence, $σ_1(H=0)\propto ω^0$, of the AC conductivity are consistent with predictions of the two-site model for AC hopping conductivity for the case of $ωτ_0 \gg $1, where $ω=2πf$ is the SAW angular frequency and $τ_0$ 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 $ωτ_0 < $1. From crossover condition, $ωτ_0(T^*)$ = 1, the typical value, $τ_0$, of the relaxation time has been determined.

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arXiv2004

High-frequency transport in $p$-type Si/Si$_{0.87}$Ge$_{0.13}$ heterostructures studied with surface acoustic waves in the quantum Hall regime

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$<T<$1.6 K and magnetic fields up to 7 T, the SAW attenuation coefficient $Γ$ and velocity change $ΔV /V$ were found to oscillate with filling factor. Both the real $σ_1$ and imaginary $σ_2$ components of the high-frequency conductivity have been determined and compared with quasi-dc magnetoresistance measurements at temperatures down to 33 mK. By analyzing the ratio of $σ_1$ to $σ_2$, carrier localization can be followed as a function of temperature and magnetic field. At $T$=0.7 K, the variations of $Γ$, $ΔV /V$ and $σ_1$ with SAW intensity have been studied and can be explained by heating of the two dimensional hole gas by the SAW electric field. Energy relaxation is found to be dominated by acoustic phonon deformation potential scattering with weak screening.

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arXiv2004

Surface Acoustic Waves probe of the p-type Si/SiGe heterostructures

The surface acoustic wave (SAWs) attenuation coefficient $Γ$ and the velocity change $ΔV /V$ 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 $Γ$ (H) and $ΔV /V$ (H) in a magnetic field were observed. Both real $σ_1$ (H) and imaginary $σ_2$ (H) components of the high-frequency conductivity have been determined. Analysis of the $σ_1$ to $σ_2$ ratio allows the carrier localization to be followed as a function of temperature and magnetic field. At T=0.7 K the variation of $Γ$, $ΔV /V$ and $σ_1$ 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.

cond-mat.mes-hall↗

arXiv2002

Memory effects in ac hopping conductance in the quantum Hall effect regime: Possible manifestation of DX$^-$ centers

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, $σ(ω)$, 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 $σ(ω)$ and $n_s$. Remarkably, the correlation between $σ(ω)$ 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.

cond-mat.mes-hall↗

arXiv2000

Nonlinearity of Acoustic Effects and High-Frequency Electrical Conductivity in GaAs/AlGaAs Heterostructures under Conditions of the Integer Quantum Hall Effect

The absorption coefficient for surface acoustic wave $Γ$ and variation in the wave velocity $ΔV/V$ 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 $σ_1(E)$ and $σ_2(E)$ of high-frequency conductivity $σ=σ_1 - iσ_2$ on the electric field of the surface wave were determined. In the range of the conductivity obeying the Arrhenius law ($σ_1 \gg σ_2$), 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 $σ_1(E)$ and $σ_2(E)$ in the range of high-frequency hopping electrical conductivity, in which case ($σ_1 \ll σ_2$) and the theory of nonlinearities has not been yet developed, are reported.

cond-mat.mes-hall↗

arXiv2000

High-frequency hopping conductivity in the quantum Hall effect regime: Acoustical studies

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.

cond-mat.mes-hall↗

arXiv1999

Electron localization in sound absorption oscillations in the quantum Hall effect regime

The absorption coefficient for surface acoustic waves in a piezoelectric insulator in contact with a GaAs/AlGaAs heterostructure (with two-dimensional electron mobility $μ= 1.3\times 10^5 cm^2/V\cdot s)$ 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.

cond-mat.mes-hall↗

arXiv1999

Interaction of surface acoustic waves with a two-dimensional electron gas in the presence of spin splitting of the Landau bands

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.

cond-mat.mes-hall↗

arXiv1999

High-Frequency Hopping conductivity of Disordered 2D-system in the IQHE Regime

High frequency (hf) conductivity in the form $σ^{hf} = σ_1^{hf} - iσ_2^{hf}$ 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, $σ_1 / σ_2 =0.14 \pm 0.03$. The ratio $σ_1 / σ_2=0.15$ 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 $σ_1$ and $σ_2$ on temperature and magnetic field is analyzed width of the Landau band broadened by the impurity random potential is determined.

cond-mat.mes-hall↗

arXiv1997

The investigation of the high frequency hopping conductivity in two- and three-dimensional electron gas by an acoustic method

High-frequency (HF) conductivity ($σ_{hf}$) 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.

cond-mat.mes-hall↗

arXiv1997

The nonlinear effects in 2DEG conductivity investigation by an acoustic method

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 $τ_ε$ 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 $ωτ_ε$ and 1 and is determined either by the instantaneously changing wave field ($ωτ_ε$$<1$), or by the average wave power ($ωτ_ε$$>1$).

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