Effects of many-electron jumps in relaxation and conductivity of Coulomb glasses

A numerical study of the energy relaxation and conductivity of the Coulomb glass is presented. The role of many-electron transitions is studied by two complementary methods: a kinetic Monte Carlo algorithm and a master equation in configuration space method. A calculation of the transition rate for two-electron transitions is presented, and the proper extension of this to multi-electron transitions is discussed. It is shown that two-electron transitions are important in bypassing energy barriers which effectively block sequential one-electron transitions. The effect of two-electron transitions is also discussed.Comment: 8 pages, 6 figure

Asymmetric metal-insulator transition in disordered ferromagnetic films

We present experimental data and a theoretical interpretation on the conductance near the metal-insulator transition in thin ferromagnetic Gd films of thickness b approximately 2-10 nm. A large phase relaxation rate caused by scattering of quasiparticles off spin wave excitations renders the dephasing length L_phi < b in the range of sheet resistances considered, so that the effective dimension is d = 3. The observed approximate fractional temperature power law of the conductivity is ascribed to the scaling regime near the transition. The conductivity data as a function of temperature and disorder strength collapse on to two scaling curves for the metallic and insulating regimes. The best fit is obtained for a dynamical exponent z approximately 2.5 and a correlation length critical exponent \nu' approximately 1.4 on the metallic side and a localization length exponent \nu approximately 0.8 on the insulating side.Comment: 4 pages, 4 figure

Logarithmic relaxation and stress aging in the electron glass

Slow relaxation and aging of the conductance are experimental features of a range of materials, which are collectively known as electron glasses. We report dynamic Monte Carlo simulations of the standard electron glass lattice model. In a non-equilibrium state, the electrons will often form a Fermi distribution with an effective electron temperature higher than the phonon bath temperature. We study the effective temperature as a function of time in three different situations: relaxation after a quench from an initial random state, during driving by an external electric field and during relaxation after such driving. We observe logarithmic relaxation of the effective temperature after a quench from a random initial state as well as after driving the system for some time $t_w$ with a strong electric field. For not too strong electric field and not too long $t_w$ we observe that data for the effective temperature at different waiting times collapse when plotted as functions of $t/t_w$ -- the so-called simple aging. During the driving period we study how the effective temperature is established, separating the contributions from the sites involved in jumps from those that were not involved. It is found that the heating mainly affects the sites involved in jumps, but at strong driving, also the remaining sites are heated

Out of equilibrium electronic transport properties of a misfit cobaltite thin film

We report on transport measurements in a thin film of the 2D misfit Cobaltite $Ca_{3}Co_{4}O_{9}$. Dc magnetoresistance measurements obey the modified variable range hopping law expected for a soft Coulomb gap. When the sample is cooled down, we observe large telegraphic-like fluctuations. At low temperature, these slow fluctuations have non Gaussian statistics, and are stable under a large magnetic field. These results suggest that the low temperature state is a glassy electronic state. Resistance relaxation and memory effects of pure magnetic origin are also observed, but without aging phenomena. This indicates that these magnetic effects are not glassy-like and are not directly coupled to the electronic part.Comment: accepted in Phys Rev B, Brief report

Technique for Magnetic Susceptibility Determination in the High Doped Semiconductors by Electron Spin Resonance

Method for determining the magnetic susceptibility in the high doped semiconductors is considered. A procedure that is based on double integration of the positive part of the derivative of the absorption line having a Dyson shape and takes into account the depth of the skin layer is described. Analysis is made for the example of arsenic doped germanium samples at a rather high concentration corresponding to the insulator metal phase transition.Comment: Pages 13, figures 9, references 1

Theory of hopping conduction in arrays of doped semiconductor nanocrystals

The resistivity of a dense crystalline array of semiconductor nanocrystals (NCs) depends in a sensitive way on the level of doping as well as on the NC size and spacing. The choice of these parameters determines whether electron conduction through the array will be characterized by activated nearest-neighbor hopping or variable-range hopping (VRH). Thus far, no general theory exists to explain how these different behaviors arise at different doping levels and for different types of NCs. In this paper we examine a simple theoretical model of an array of doped semiconductor NCs that can explain the transition from activated transport to VRH. We show that in sufficiently small NCs, the fluctuations in donor number from one NC to another provide sufficient disorder to produce charging of some NCs, as electrons are driven to vacate higher shells of the quantum confinement energy spectrum. This confinement-driven charging produces a disordered Coulomb landscape throughout the array and leads to VRH at low temperature. We use a simple computer simulation to identify different regimes of conduction in the space of temperature, doping level, and NC diameter. We also discuss the implications of our results for large NCs with external impurity charges and for NCs that are gated electrochemically.Comment: 14 pages, 10 figures; extra schematic figures added; revised introductio

The glass transition and the Coulomb gap in electron glasses

We establish the connection between the presence of a glass phase and the appearance of a Coulomb gap in disordered materials with strongly interacting electrons. Treating multiparticle correlations in a systematic way, we show that in the case of strong disorder a continuous glass transition takes place whose Landau expansion is identical to that of the Sherrington-Kirkpatrick spin glass. We show that the marginal stability of the glass phase controls the physics of these systems: it results in slow dynamics and leads to the formation of a Coulomb gap

Electronic structure and light-induced conductivity in a transparent refractory oxide

Combined first-principles and experimental investigations reveal the underlying mechanism responsible for a drastic change of the conductivity (by 10 orders of magnitude) following hydrogen annealing and UV-irradiation in a transparent oxide, 12CaO.7Al2O3, found by Hayashi et al. The charge transport associated with photo-excitation of an electron from H, occurs by electron hopping. We identify the atoms participating in the hops, determine the exact paths for the carrier migration, estimate the temperature behavior of the hopping transport and predict a way to enhance the conductivity by specific doping.Comment: 4 pages including 4 figure