arXiv subjects

M. E. Gershenson

Publications and source records attributed to M. E. Gershenson.

44 records · Page 3Linked to original sources

arXiv1999

Giant 1/f noise in perovskite manganites: evidence of the percolation threshold

We discovered an unprecedented magnitude of the 1/f noise near the Curie temperature Tc in low-Tc manganites. The scaling behavior of the 1/f noise and the resistance provides strong evidence of the percolation nature of the ferromagnetic transition in the polycrystalline samples. The step-like changes of the resistance with temperature, observed for single crystals, suggest that the size of the ferromagnetic domains depends on the size of crystallites.

cond-mat.str-el↗

arXiv1999

Electron-Assisted Hopping in Two Dimensions

We have studied the non-ohmic effects in the conductivity of a two-dimensional system which undergoes the crossover from weak to strong localization with decreasing electron concentration. When the electrons are removed from equilibrium with phonons, the hopping conductivity depends only on the electron temperature. This indicates that the hopping transport in a system with a large localization length is assisted by electron-electron interactions rather than by the phonons.

cond-mat.mes-hall↗

arXiv1998

Comment on "Quantum Decoherence in Disordered Mesoscopic Systems"

In a recent paper, Phys. Rev. Lett. 81, 1074 (1998), Golubev and Zaikin (GZ) found that ``zero-point fluctuations of electrons'' contribute to the dephasing rate extracted from the magnetoresistance. As a result, the dephasing rate remains finite at zero temperature. GZ claimed that their results ``agree well with the experimental data''. We point out that the GZ results are incompatible with (i) conventional perturbation theory of the effects of interaction on weak localization (WL), and (ii) with the available experimental data. More detailed criticism of GZ findings can be found in cond-mat/9808053.

cond-mat.dis-nn↗

arXiv1998

Interaction effects and phase relaxation in disordered systems

This paper is intended to demonstrate that there is no need to revise the existing theory of the transport properties of disordered conductors in the so-called weak localization regime. In particular, we demonstrate explicitly that recent attempts to justify theoretically that the dephasing rate (extracted from the magnetoresistance) remains finite at zero temperature are based on the profoundly incorrect calculation. This demonstration is based on a straightforward evaluation of the effect of the electron-electron interaction on the weak localization correction to the conductivity of disordered metals. Using well-controlled perturbation theory with the inverse conductance $g$ as the small parameter, we show that this effect consists of two contributions. First contribution comes from the processes with energy transfer smaller than the temperature. This contribution is responsible for setting the energy scale for the magnetoresistance. The second contribution originates from the virtual processes with energy transfer larger than the temperature. It is shown that the latter processes have nothing to do with the dephasing, but rather manifest the second order (in $1/g$) correction to the conductance. This correction is calculated for the first time. The paper also contains a brief review of the existing experiments on the dephasing of electrons in disordered conductors and an extended qualitative discussion of the quantum corrections to the conductivity and to the density of electronic states in the weak localization regime.

cond-mat.dis-nn↗

arXiv1998

Decoherence and the Thouless Crossover in One-Dimensional Conductors

The temperature and magnetic-field dependences of the resistance of one-dimensional (1D) conductors have been studied in the vicinity of the Thouless crossover. We find that on the weak localization (WL) side of the crossover, these dependences are consistent with the theory of quantum corrections to the resistance, and the phase breaking is due to the quasi-elastic electron-electron interactions (the Nyquist noise). The temperature dependence of the phase coherence time does not saturate, and the quasiparticle states remain well defined over the whole WL temperature range. This fact, as well as observation of the Thouless crossover in 1D samples, argues against the idea of intrinsic decoherence by zero-point fluctuations of the electrons (Mohanty et al., Phys.Rev.Lett. 78, 3366 (1997)). We believe that frequently observed saturation of the phase coherence time is caused by the external microwave noise.

cond-mat.dis-nn↗

arXiv1998

Strong localization of electrons in quasi-one-dimensional conductors

We report on the experimental study of electron transport in sub-micron-wide ''wires'' fabricated from Si $δ$-doped GaAs. These quasi-one-dimensional (Q1D) conductors demonstrate the crossover from weak to strong localization with decreasing the temperature. On the insulating side of the crossover, the resistance has been measured as a function of temperature, magnetic field, and applied voltage for different values of the electron concentration, which was varied by applying the gate voltage. The activation temperature dependence of the resistance has been observed with the activation energy close to the mean energy spacing of electron states within the localization domain. The study of non-linearity of the current-voltage characteristics provides information on the distance between the critical hops which govern the resistance of Q1D conductors in the strong localization (SL) regime. We observe the exponentially strong negative magnetoresistance; this orbital magnetoresistance is due to the universal magnetic-field dependence of the localization length in Q1D conductors. The method of measuring of the single-particle density of states (DoS) in the SL regime has been suggested. Our data indicate that there is a minimum of DoS at the Fermi level due to the long-range Coulomb interaction.

cond-mat.dis-nn↗

arXiv1998

Phase Relaxation of Electrons in Disordered Conductors

Conduction electrons in disordered metals and heavily doped semiconductors at low temperatures preserve their phase coherence for a long time: phase relaxation time $τ_ϕ$ can be orders of magnitude longer than the momentum relaxation time. The large difference in these time scales gives rise to well known effects of weak localization, such as anomalous magnetoresistance. Among other interesting characteristics, study of these effects provide quantitative information on the dephasing rate $1/τ_ϕ$. This parameter is of fundamental interest: the relation between $\hbar/τ_ϕ$ and the temperature $T$ (a typical energy scale of an electron) determines how well a single electron state is defined. We will discuss the basic physical meaning of $1/τ_ϕ$ in different situations and its difference from the energy relaxation rate. At low temperatures, the phase relaxation rate is governed by collisions between electrons. We will review existing theories of dephasing by these collisions or (which is the same) by electric noise inside the sample. We also discuss recent experiments on the magnetoresistance of 1D systems: some of them show saturation of $1/τ_ϕ$ at low temperatures, the other do not. To resolve this contradiction we discuss dephasing by an external microwave field and by nonequilibrium electric noise.

cond-mat.dis-nn↗

arXiv1996

An Experimentally Realizable Weiss Model for Disorder-Free Glassiness

We summarize recent work on a frustrated periodic long-range Josephson array in a parameter regime where its dynamical behavior is identical to that of the $p=4$ disordered spherical model. We also discuss the physical requirements imposed by the theory on the experimental realization of this superconducting network.

cond-mat.dis-nn↗