arXiv subjects

V. I. Kozub

Publications and source records attributed to V. I. Kozub.

At least 37 records · Page 2Linked to original sources

arXiv2007

Evidence of the virtual Anderson transition in a narrow impurity band of p-GaAs/AlGaAs quantum wells: $ε_4$ conductivity and electric breakdown at low temperatures

In highly doped uncompensated p-type layers within the central part of GaAs/AlGaAs quantum wells at low temperatures we observed an activated behavior of the conductivity with low activation energies (1-3) meV which can not be ascribed to standard mechanisms. We attribute this behavior to the delocalization of hole states near the maximum of the narrow impurity band in the sense of the Anderson transition. Low temperature conduction $ε_4$ is supported by an activation of minority carriers - electrons (resulting from a weak compensation by back-ground defects) - from the Fermi level to the band of delocalized states mentioned above. The corresponding behavior can be specified as virtual Anderson transition. Low temperature transport ($<4$ K) exhibits also strong nonlinearity of a breakdown type characterized in particular by S-shaped I-V curve. The nonlinearity is observed in unexpectedly low fields ($<10$ V/cm). Such a behavior can be explained by a simple model implying an impact ionization of the localized states of the minority carriers mentioned above to the band of Anderson-delocalized states.

cond-mat.dis-nn↗

arXiv2006

Point contact spectroscopy of hopping transport: effects of a magnetic field

The conductance of a point contact between two hopping insulators is expected to be dominated by the individual localized states in its vicinity. Here we study the additional effects due to an external magnetic field. Combined with the measured conductance, the measured magnetoresistance provides detailed information on these states (e.g. their localization length, the energy difference and the hopping distance between them). We also calculate the statistics of this magnetoresistance, which can be collected by changing the gate voltage in a single device. Since the conductance is dominated by the quantum interference of particular mesoscopic structures near the point contact, it is predicted to exhibit Aharonov-Bohm oscillations, which yield information on the geometry of these structures. These oscillations also depend on local spin accumulation and correlations, which can be modified by the external field. Finally, we also estimate the mesoscopic Hall voltage due to these structures.

cond-mat.mes-hall↗

arXiv2006

Non-Gaussian conductance noise in disordered electronic systems due to a non-linear mechanism

We present results of conductance-noise experiments on disordered films of crystalline indium oxide with lateral dimensions 2microns to 1mm. The power-spectrum of the noise has the usual 1/f form, and its magnitude increases with inverse sample-volume down to sample size of 2microns, a behavior consistent with un-correlated fluctuators. A colored second spectrum is only occasionally encountered (in samples smaller than 40microns), and the lack of systematic dependence of non-Gaussianity on sample parameters persisted down to the smallest samples studied (2microns). Moreover, it turns out that the degree of non-Gaussianity exhibits a non-trivial dependence on the bias V used in the measurements; it initially increases with V then, when the bias is deeper into the non-linear transport regime it decreases with V. We describe a model that reproduces the main observed features and argue that such a behavior arises from a non-linear effect inherent to electronic transport in a hopping system and should be observed whether or not the system is glassy.

cond-mat.dis-nn↗

arXiv2005

1/f noise in hopping conduction: Role of multi-site aggregates

We propose a mechanism for 1/f-type noise in hopping insulators based on the multi-electron charge redistribution within the specific aggregates of the localized states located in the vicinity of the critical resistors. We predict that the noise with 1/f-type spectrum extends down to practically arbitrarily low frequencies.

cond-mat.mes-hall↗

arXiv2005

Charge transfer between a superconductor and a hopping insulator

We develop a theory of the low-temperature charge transfer between a superconductor and a hopping insulator. We show that the charge transfer is governed by the coherent two-electron -- Cooper pair conversion process, time reversal reflection, where electrons tunnel into superconductor from the localized states in the hopping insulator located near the interface, and calculate the corresponding interface resistance. This process is an analog to conventional Andreev reflection process. We show that the time reversal interface resistance is accessible experimentally, and that in mesoscopic structures it can exceed the bulk hopping resistance.

cond-mat.mes-hall↗

arXiv2005

Thermoelectric effect in superconducting nanostructures

We study thermoelectric effects in superconducting nanobridges and demonstrate that the magnitude of these effects can be comparable or even larger than that for a macroscopic superconducting circuit. The reason is related to a possibility to have very large gradients of electron temperature within the nanobridge. The corresponding heat conductivity problems are considered. It is shown that the nanoscale devices allow one to get rid of masking effects related to spurious magnetic fields.

cond-mat.supr-con↗

arXiv2005

Nonmonotonic Temperature Dependence of the Hall Resistance for 2D Electron System in Si

Weak field Hall resistance Rxy(T) of the 2D electron system in Si was measured over the range of temperatures 1-35 K and densities, where the diagonal resistivity exhibits a ``metallic'' behavior. The Rxy(T) dependence was found to be non-monotonic with a maximum at temperatures Tm~0.16Tf. The Rxy(T) variations in the low-temperature domain (T Tm), the Rxy(T) dependence can be qualitatively explained in terms of either a semiclassical T-dependence of a transport time, or a thermal activation of carries from a localized band.

cond-mat.str-el↗

arXiv2005

Orbital ac spin-Hall effect in the hopping regime

The Rashba and Dresselhaus spin-orbit interactions are both shown to yield the low temperature spin-Hall effect for strongly localized electrons coupled to phonons. A frequency-dependent electric field ${\bf E}(ω)$ generates a spin-polarization current, normal to ${\bf E}$, due to interference of hopping paths. At zero temperature the corresponding spin-Hall conductivity is real and is proportional to $ω^{2}$. At non-zero temperatures the coupling to the phonons yields an imaginary term proportional to $ω$. The interference also yields persistent spin currents at thermal equilibrium, at ${\bf E}=0$. The contributions from the Dresselhaus and Rashba interactions to the interference oppose each other.

cond-mat.mes-hall↗

arXiv2005

Effective description of hopping transport in granular metals

We develop a theory of a variable range hopping transport in granular conductors based on the sequential electron tunnelling through many grains in the presence of the strong Coulomb interaction. The processes of quantum tunnelling of real electrons are represented as trajectories (world lines) of charged classical particles in d+1 dimensions. We apply the developed technique to investigate the hopping conductivity of granular systems in the regime of small tunneling conductances between the grains g << 1.

cond-mat.mes-hall↗

arXiv2004

Current-induced magnetization changes in a spin valve due to incoherent emission of non-equilibrium magnons

We describe spin transfer in a ferromagnet/normal metal/ferromagnet spin-valve point contact. Spin is transferred from the spin-polarized device current to the magnetization of the free layer by the mechanism of incoherent magnon emission by electrons. Our approach is based on the rate equation for the magnon occupation, using Fermi's golden rule for magnon emission and absorption and the non-equilibrium electron distribution for a biased spin valve. The magnon emission reduces the magnetization of the free layer. For anti-parallel alignment of the magnetizations of the layers and at a critical bias a magnon avalanche occurs, characterized by a diverging effective magnon temperature. This critical behavior can result in magnetization reversal and consequently to suppression of magnon emission. However, magnon-magnon scattering can lead to saturation of the magnon concentration at a large but finite value. The further behavior depends on the parameters of the system. In particular, gradual evolution of the magnon concentration followed by a magnetization reversal is possible. Another scenario corresponds to a step-like increase of the magnon concentration followed by a slow decrease. In the latter case a spike in the differential resistance is expected due to a contribution of electron-magnon scattering. A comparison of the obtained results to existing experimental data and theoretical approches is given.

cond-mat.mes-hall↗

arXiv2004

Spontaneous current generation in gated nanostructures

We have observed an unusual dc current spontaneously generated in the conducting channel of a short-gated GaAs transistor. The magnitude and direction of this current critically depend upon the voltage applied to the gate. We propose that it is initiated by the injection of hot electrons from the gate that relax via phonon emission. The phonons then excite secondary electrons from asymmetrically distributed impurities in the channel, which leads to the observed current.

cond-mat.mes-hall↗

arXiv2004

Magnetoresistance of p-GaAs/AlGaAs structures in the vicinity of metal-insulator transition: Effect of superconducting leads

Experimental and theoretical studies on transport in semiconductor samples with superconducting electrodes are reported. We focus on the samples close to metal-insulator transition. In metallic samples, a peak of negative magnetoresistance at fields lower than critical magnetic field of the leads was observed. This peak is attributed to restoration of a single-particle tunneling emerging with suppression of superconductivity. The experimental results allow us to estimate tunneling transparency of the boundary between superconductor and metal. In contrast, for the insulating samples no such a peak was observed. We explain this behavior as related to properties of transport through the contact between superconductor and hopping conductor. This effect can be used to discriminate between weak localization and strong localization regimes.

cond-mat.dis-nn↗

arXiv2004

Low-frequency noise in tunneling through a single spin

We propose measurements of low-frequency noise in the tunneling current through a single molecule with a spin as an experimental probe for identifying a mechanism of the spin-dependent tunneling. A specific tail near the zero frequency in the noise spectrum is predicted; the amplitude and the width of being of the same order of magnitude as the recently reported peak in the noise spectrum at the spin Larmor frequency. The ratio of the spectrum amplitudes at zero- and Larmor frequencies is shown to be a convenient tool for testing theoretical predictions.

cond-mat.mes-hall↗

arXiv2002

Josephson transport through a Hubbard impurity center

We investigate the Josephson transport through a thin semiconductor barrier containing impurity centers with the on-site Hubbard interaction $u$ of an arbitrary sign and strength. We find that in the case of the repulsive interaction the Josephson current changes sign with the temperature increase if the energy of the impurity level $ε$ (measured from the Fermi energy of superconductors) falls in the interval $(-u,0)$. We predict strong temporal fluctuations of the current if only a few centers present within the junction. In the case of the attractive impurity potential ($u<0$) and at low temperatures, the model is reduced to the effective two level Hamiltonian allowing thus a simple description of the nonstationary Josephson effect in terms of pair tunneling processes.

cond-mat.supr-con↗

arXiv2002

Is weak temperature dependence of electron dephasing possible?

The first-principle theory of electron dephasing by disorder-induced two state fluctuators is developed. There exist two mechanisms of dephasing. First, dephasing occurs due to direct transitions between the defect levels caused by inelastic electron-defect scattering. The second mechanism is due to violation of the time reversal symmetry caused by time-dependent fluctuations of the scattering potential. These fluctuations originate from an interaction between the dynamic defects and conduction electrons forming a thermal bath. The first contribution to the dephasing rate saturates as temperature decreases. The second contribution does not saturate, although its temperature dependence is rather weak, $\propto T^{1/3}$. The quantitative estimates based on the experimental data show that these mechanisms considered can explain the weak temperature dependence of the dephasing rate in some temperature interval. However, below some temperature dependent on the model of dynamic defects the dephasing rate tends rapidly to zero. The relation to earlier studies of the dephasing caused by the dynamical defects is discussed.

cond-mat.mes-hall↗

arXiv2001

On the theory of thermoelectric phenomena in superconductors

The theory of thermoelectric effects in superconductors is discussed in connection to the recent publication by Marinescu and Overhauser, Phys. Rev. B 55, 11637 (1997). We argue that the charge on-conservation arguments by Marinescu and Overhauser do not require any revision of the Boltzmann transport equation in superconductors, and show that their ``electron-conserving transport theory'' contradicts thermodynamics. Possible mechanisms responsible for the discrepancy between some experimental data and the theory by Galperin, Gurevich, and Kozub, JETP Lett. 17, 476 (1973) are discussed.

cond-mat.supr-con↗

arXiv2001

Metal-insulator transition in 2D: a role of the upper Habbard band

To explain the main features of the metal-insulator transition (MIT) in 2D we suggest a simple model taking into account strongly localized states in the band tail of 2D conductivity band with a specific emphasize of a role of doubly-occupied states (upper Hubbard band). The metallic behavior of resistance is explained as result of activation of localized electrons to conductance band leading to a suppression of non-linear screening of the disorder potential. The magnetoresistance (MR) in the critical region is related to depopulation of double occupied localized states also leading to partial suppression of the nonlinear screening. The most informative data are related to nearly activated temperature dependence of MR in strongly insulating limit (which can be in particular reached from the metallic state in high enough fields). According to our model this behavior originates due to a lowering of a position of chemical potential in the upper Hubbard band due to Zeeman splitting. We compare the theoretical predictions to the existing experimental data and demonstrate that the model explains such features of the 2D MIT as scaling behavior in the critical region, saturation of MR and H/T scaling of MR in the insulating limit. The quantitative analysis of MR in strongly insulating limit based on the model suggested leads to the values of g-factors being in good agreement with known values for localized states in corresponding materials.

cond-mat.dis-nn↗

arXiv1999

A possible role of D^- band in hopping conductivity and metal-insulator transition in 2D structures

A simple two-band model is suggested explaining recently reported unusual features for hopping magnetoresistance and the metal-insulator transition in 2D structures. The model implies that the conductivity is dominated by the upper Hubbard band (D^- band). Experimental studies of hopping magnetoresistance for Si delta doped GaAs/AlGaAs heterostructure give additional evidences for the model.

cond-mat.mes-hall↗