Low frequency excitations in the Coulomb Glass: numerical analysis using the avalanche method

We develop a numerical method, which allows to find pairs of metastable states differing by transition of several electrons. We show that at low temperature these pairs can be treated as local metastable systems that determine low-frequency properties of Coulomb Glass with low external disorder. The contribution of these pairs to low-frequency properties is suppressed when the strength of the external disorder becomes comparable with interaction between neighboring electrons.Comment: Full version. Shorter version is published in "Solid State Communications

Spin excitations in systems with hopping electron transport and strong position disorder in a large magnetic field

We discuss the spin excitations in systems with hopping electron conduction and strong position disorder. We focus on the problem in a strong magnetic field when the spin Hamiltonian can be reduced to the effective single-particle Hamiltonian and treated with conventional numerical technics. It is shown that in a 3D system with Heisenberg exchange interaction the spin excitations have a delocalized part of the spectrum even in the limit of strong disorder, thus leading to the possibility of the coherent spin transport. The spin transport provided by the delocalized excitations can be described by a diffusion coefficient. Non-homogenous magnetic fields lead to the Anderson localization of spin excitations while anisotropy of the exchange interaction results in the Lifshitz localization of excitations. We discuss the possible effect of the additional exchange-driven spin diffusion on the organic spin-valve devices.Comment: To be published in Journal of Physics: Condensed Matte

Kinetic equations for the hopping transport and spin relaxation in random magnetic field

We derive the kinetic equations for the hopping transport that take into account electron spin and the possibility of double occupation. In the Ohmic regime the equations are reduced to the generalized Miller-Abrahams resistor network. We apply these equations to the problem of the magnetic moment relaxation due to the interaction with the random hyperfine fields. It is shown that in a wide range of parameters the relaxation rate is governed by the hops with the similar rates as spin precession frequency. It is demonstrated that at the large time scale spin relaxation is non-exponential. We argue that the non-exponential relaxation of the magnetic moment is related to the spin of electrons in the slow-relaxing traps. Interestingly the traps can significantly influence the spin relaxation in the infinite conducting cluster at large times

Giant magnetoresistance for ensembles of ferromagnetic granules in variable range hopping conductivity regime

We study an effect of moderate magnetic field on variable range hopping conductivity in arrays of ferromagnetic granules separated by tunnel barriers. It is shown that the resulting magnetoresistance can be significantly larger than the standard "giant" magnetoresistance in Fe-N-Fe-N... multilayers. The effect is related to a gain in densities of states available for the virtual processes within the intermediate granules due to magnetic-field induced alignment of the granule magnetizations

Magnetoresistance in organic spintronic devices: the role of nonlinear effects

We derive kinetic equations describing injection and transport of spin polarized carriers in organic semiconductors with hopping conductivity via an impurity level. The model predicts a strongly voltage dependent magnetoresistance, defined as resistance variation between devices with parallel and antiparallel electrode magnetizations (spin valve effect). The voltage dependence of the magnetoresistance splits into three distinct regimes. The first regime matches well known inorganic spintronic regimes, corresponding to barrier controlled spin injection or the well known conductivity mismatch case. The second regime at intermediate voltages corresponds to strongly suppressed magnetoresistance. The third regime develops at higher voltages and accounts for a novel paradigm. It is promoted by the strong non-linearity in the charge transport which strength is characterized by the dimensionless parameter $eU/k_BT$. This nonlinearity, depending on device conditions, can lead to both significant enhancement or to exponential suppression of the spin valve effect in organic devices. We believe that these predictions are valid beyond the case of organic semiconductors and should be considered for any material characterized by strongly non-linear charge transport.Comment: 7 pages, 5 figure

Oscillations of Echo Amplitude in Glasses in a Magnetic Field Induced by Nuclear Dipole-Dipole Interaction

The effect of a magnetic field on the dipole echo amplitude in glasses (at temperatures of about 10 mK) induced by the dipole-dipole interaction of nuclear spins has been theoretically studied. It has been shown that a change in the positions of nuclear spins as a result of tunneling and their interaction with the external magnetic field E_H lead to a nonmonotonic magnetic field dependence of the dipole echo amplitude. The approximation that the nuclear dipole-dipole interaction energy E_d is much smaller than the Zeeman energy E_H has been found to be valid in the experimentally important cases. It has been shown that the dipole echo amplitude in this approximation may be described by a simple universal analytic function independent of the microscopic structure of the two-level systems. An excellent agreement of the theory with the experimental data has been obtained without fitting parameters (except for the unknown echo amplitude)Comment: 5 pages, 1 figur

The system of correlation kinetic equations and the generalized equivalent circuit for hopping transport

We derive the system of equations that allows to include non-equilibrium correlations of filling numbers into the theory of the hopping transport. The system includes the correlations of arbitrary order in a universal way and can be cut at any place relevant to a specific problem to achieve the balance between rigor and computation possibilities. In the linear-response approximation, it can be represented as an equivalent electric circuit that generalizes the Miller-Abrahams resistor network. With our approach, we show that non-equilibrium correlations are essential to calculate conductivity and distribution of currents in certain disordered systems. Different types of disorder affect the correlations in different applied fields. The effect of energy disorder is most important at weak electric fields while the position disorder by itself leads to non-zero correlations only in strong fields

Magnetoresistance in organic semiconductors: including pair correlations in the kinetic equations for hopping transport

We derive the kinetic equations for polaron hopping in organics that explicitly take into account the double occupation possibility and pair intersite correlations. The equations include simplified phenomenological spin dynamics and provide a self-consistent framework for the description of the bipolaron mechanism of the organic magnetoresistance. At low applied voltages the equations can be reduced to effective resistor network that generalizes the Miller-Abrahams network and includes the effect of spin relaxation on the system resistivity. Our theory discloses the close relationship between the organic magnetoresistance and the intersite correlations. Moreover, in the absence of correlations, as in ordered system with zero Hubbard energy, the magnetoresistance vanishes.Comment: 17 pages 10 figures accepted for publication in Physical Review

Spin-related phenomena in two-dimensional hopping regime in magnetic field

The spin relaxation time of localized charge carriers is few orders of magnitude larger than that of free electrons and holes. Therefore mutual conversion of spin polarization, charge current and spin current turns out to be underlined in the hopping conductivity regime. We reveal different regimes of the coupled spin and charge dynamics depending on the relation between spin relaxation time and the characteristic hopping time. We derive kinetic equations to describe electrical spin orientation, dc spin-Hall effect, and spin galvanic effect in the transverse magnetic field. The generalized macroscopic conductivities describing these effects are calculated using percolation theory supported by numerical simulation. The conductivities change the sign at least once as functions of magnetic field for all values of the spin relaxation time.Comment: 15 pages, 6 figure

Spin dynamics of hopping electrons in quantum wires: algebraic decay and noise

We study theoretically spin decoherence and intrinsic spin noise in semiconductor quantum wires caused by an interplay of electron hopping between the localized states and the hyperfine interaction of electron and nuclear spins. At a sufficiently low density of localization sites the hopping rates have an exponentially broad distribution. It allows the description of the spin dynamics in terms of closely-situated "pairs" of sites and single "reaching" states, from which the series of hops result in the electron localized inside a "pair". The developed analytical model and numerical simulations demonstrate disorder-dependent algebraic tails in the spin decay and power-law singularity-like features in the low-frequency part of the spin noise spectrum.Comment: 5 pages, 3 figures + supplementary materia