arXiv subjects
Publications and source records attributed to Y. M. Galperin.
The relevance of tunneling two-level systems (TLS) for electron dephasing in metals is analyzed. We demonstrate that if the concentration of TLS is sufficient to cause the observed dephasing rate, one also should expect quite substantial effects in the specific heat and ultrasound attenuation. In both cases TLS contribution should dominate the electronic one at low enough temperatures.
The possibility for a two-channel Kondo ($2CK$) non Fermi liquid state to appear in a metal as a result of the interaction between electrons and movable structural defects is revisited. As usual, the defect is modeled by a heavy particle moving in an almost symmetric double-well potential (DWP). Taking into account only the two lowest states in DWP is known to lead to a Kondo-like Hamiltonian with rather low Kondo temperature, $T_K$. We prove that, in contrast to previous believes, the contribution of higher excited states in DWP does not enhance $T_K$. On the contrary, $T_K$ is reduced by three orders of magnitude as compared with the two-level model: the prefactor in $T_K$ is determined by the spacing between the second and the third levels in DWP rather than by the electron Fermi energy. Moreover, $T_K$, turns out to be parametrically smaller than the splitting between the two lowest levels. Therefore, there is no microscopic model of movable defects which may justify non-Fermi liquid $2CK$ phenomenology.
Nonlinear hopping absorption of ultrasound and electromagnetic waves in amorphous and doped semiconductors is considered. It is shown that even at low amplitudes of the electric (or acoustic) field the nonlinear corrections to the relaxational absorption appear anomalously large. The physical reason for such behavior is that the nonlinear contribution is dominated by a small group of close impurity pairs having one electron per pair. Since the group is small, it is strongly influenced by the field. An external magnetic field strongly influences the absorption by changing the overlap between the pair components' wave functions. It is important that the influence is substantially different for the linear and nonlinear contributions. This property provides an additional tool to extract nonlinear effects.
Two commonly accepted approaches for simulations of thermally-activated vortex motion in superconductors are compared. These are (i) the so-called flux creep approach based on the expression E=vB relating the electric field E to the velocity v of the thermally-activated flux motion and the local flux density B, and (ii) the approach employing a phenomenological nonlinear current-voltage curve, E(j). Our results show that the two approaches give similar but also distinctly different behaviors for the distributions of current and flux density in both a long slab and thin strip geometry. The differences are most pronounced where the local B is small. Magneto-optical imaging of a YBaCuO thin film carrying a transport current was performed to compare the simulations with experimental behavior. It is shown that the flux creep approach describes the experiments far better than simulations based on the E(j) approach.
A study is presented concerning the influence of flicker noise in the junction transparency on coherent transport in Andreev states. The amount of dephasing is estimated for a microwave-activated quantum interferometer. Possibilities of experimentally investigating the coupling between a superconducting quantum point contact and its electromagnetic environment are discussed.
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.
Magnetization hysteresis loops and the ac susceptibility of a superconducting thin disk are calculated in the critical-state model assuming a field-dependent critical current density, Jc(B). The results are obtained by solving numerically the set of coupled integral equations for the flux and current distributions [PRB 60(17) p.13112 (1999)] for a disk placed in a perpendicular applied field Ba. From the magnetization curves the range of fields where the vertical width of the loop relates directly to Jc(Ba) is determined. Comparing our results on susceptibility with experimental data for parametric plot Im(chi) vs. Re(chi) shows that by taking the B-dependence of Jc into account the agreement improves dramatically. We show that the asymptotic behavior for large ac-field amplitudes Bam changes from Re(chi) ~ 1/Bam^{3/2} and Im(chi) ~ 1/Bam for the Bean model, to Re(chi) ~ 1/Bam^3 and Im(chi) ~ 1/Bam^2 for Jc decreasing with |B| as 1/|B| or faster. For small Bam the behavior can always be described by an effective Bean model with a renormalized Jc. We also find that in the Im(chi) vs. Re(chi) plot the peak of Im(chi) increases in magnitude and shifts to lower |Re(chi)|. This allows an easy experimental discrimination between a Bean model behavior, one with Jc(B), and one where flux creep is an ingredient.
The critical state in a superconducting thin circular disk with an arbitrary magnetic field dependence of the critical sheet current, Jc(B), is analyzed. With an applied field Ba perpendicular to the disk, a set of coupled integral equations for the flux and current distributions is derived. The equations are solved numerically, and flux and current profiles are presented graphically for several commonly used Jc(B) dependences. It is shown that for small Ba the flux penetration depth can be described by an effective Bean model with a renormalized Jc entering the leading term. We argue that these results are qualitatively correct for thin superconductors of any shape. The results contrast the parallel geometry behavior, where at small Ba the B-dependence of the critical current can be ignored.
Magnetic flux distribution across a high-temperature superconductor strip is measured using magneto-optical imaging at 15 K. Both the current-carrying state and the remanent state after transport current are studied up to the currents 0.97 Ic where Ic is the critical current. To avoid overheating of the sample current pulses with duration 50 ms were employed. The results are compared with predictions of the Bean model for the thin strip geometry. In the current-carrying state, reasonable agreement is found. However, there is a systematic deviation -- the flux penetration is deeper than theoretically predicted. A much better agreement is achieved by accounting for flux creep as shown by our computer simulations. In the remanent state the Bean model fails to explain the experimental results. The results for the currents I < 0.7 Ic can be understood within the framework of our flux creep simulations. However, after the currents I > 0.7 Ic the total flux trapped in a strip is substantially less than predicted by the simulations. Furthermore, it decreases with increasing current. Excessive dissipation of power in the annihilation zone formed in the remanent state is believed to be the source of this unexpected behavior.
Exact analytical results are obtained for the magnetization of a superconducting thin strip with a general behavior J_c(B) of the critical current density. We show that within the critical-state model the magnetization as function of applied field, B_a, has an extremum located exactly at B_a=0. This result is in excellent agreement with presented experimental data for a YBCO thin film. After introducing granularity by patterning the film, the central peak becomes shifted to positive fields on the descending field branch of the loop. Our results show that a positive peak position is a definite signature of granularity in superconductors.
Spatially-resolved studies of a YBaCuO thin film bridge using electron probe microanalysis (EPMA), low-temperature scanning electron microscopy (LTSEM), and magneto-optical flux visualization (MO) have been carried out. Variations in chemical composition along the bridge were measured by EPMA with 3 microns resolution. Using LTSEM the spatial distributions of the critical temperature, Tc, and of the local transition width, dTc, were determined with 5 microns resolution. Distributions of magnetic flux over the bridge in an applied magnetic field have been measured at 15 and 50 K by magneto-optical technique. The critical current density Jc as a function of coordinate along the bridge was extracted from the measured distributions by a new specially developed method. Significant correlations between Jc, Tc, dTc and cation composition have been revealed. It is shown that in low magnetic fields deviation from the stoichiometric composition leads to a decrease in both Tc and Jc. The profile of Jc follows the Tc-profile on large length scales and has an additional fine structure on short scales. The profile of Jc along the bridge normalized to its value at any point is almost independent of temperature.
We calculate the rate of acoustic phonon generation by a current-carrying, ballistic quantum channel, defined in a two-dimensional electron gas by a split-gate. Both uniform and nonuniform channels are considered. The generation rate of acoustic phonons of a particular frequency and direction of propagation is a steplike function of the applied bias voltage, with threshold voltages that are calculated in the paper. The emitted phonons have a characteristic angular distribution which changes significantly at the thresholds. The voltage dependence of the generation rate is shown to be sensitive to the shape of the channel. Thus the spectral and spatial distributions of the emitted phonons bear information both on electron-phonon coupling in the vicinity of the device and on characteristics of the electron spectrum.