We present an efficient numerical methodology to study the effect of off-diagonal randomness on superconductivity. This is a real space approach using Bogoliubov-de-Gennes (BdG) method coupled with augmented space vector recursion (ASVR) technique for disorder averaging. We find in the presence of off-diagonal randomness s-wave superconductivity is favoured by greater hopping integral strength between similar species as compared to that between dissimilar ones and d-wave superconductivity is suppressed by off-diagonal randomness much like diagonal randomness. Combined diagonal and off-diagonal randomness leads to asymmetry in both the normal as well as the superconducting densities of states.

A real space approach to study the effect of off-diagonal disorder on superconductivity

The general theory of disorder in superconducting alloys was discussed We use standard Wannier function representation and allow for diagonal and ofi-diagonal disorder We generalized the coherent potential approximation in the version able to deal with ofi-diagonal disorder in normal systems to treat superconducting state As an illustration we calculate the quasiparticle density of states and gap function of a d-wave superconductor We show inter alia that the rate at which superconductivity disappears depends on the kind of disorder and for ofi-diagonal disorder it depends on the details of its realization

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CPA Theory of Superconducting Alloys with Diagonal and Off-Diagonal Disorder

This work deals with the determination of the characteristics of the Coulomb gap (CG) in the density of states (DOS) in the vicinity of the Fermi level, from the analysis of the temperature dependence of dc conductivity in the temperature range (1.5 K–300 K) in germanium, disordered by large fluences of fast reactor neutrons. From application, the percolation theory of conductivity and the variable range hopping (VRH) under the effect of the Coulomb interaction of localized carriers, the localization radius is found to be 49.2 A. The dielectric constants of the samples are determined. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The characteristics of the Coulomb gap in the density of states of germanium doped irradiation defects

I address here the question of the mutual interplay of strong correlations and disorder in the system. I consider random version of the Hubbard model. Diagonal randomness is introduced {\it via} random on-site energies and treated by the coherent potential approximation. Strong, short ranged, electron - electron interactions are described by the slave boson technique and found to induce additional disorder in the system. As an example I calculate the density of states of the random interacting binary alloy and compare it with that for non-interacting system.

Interplay of Disorder and Correlations

The metal-insulator transition induced by sufficiently strong short range repulsive electron-electron interaction U is called Mott-Hubbard transition. In this work we study the effect of disorder on the critical value of the repulsion U c . The Hubbard model with random site energies to describe the correlated and disordered system has been used. The Kotliar-Ruckenstein slave-boson technique to treat correlations and coherent potential approximation (CPA) to deal with disorder have been applied. We have found that the low temperature conductivity is an increasing or decreasing function of U depending on the carrier concentration. The critical value U c for metal-insulator transition is decreasing function of the disorder strength.

Charge transport and metal–insulator transition in correlated and disordered systems

1. IntroductionThe study of disordered superconductors has a long history. On experimentalside it started with early work of Shalnikov [1], Buckel [2], and others, whiletheoretical ideas have been developed by Anderson [3], Abrikosov and Gorkov[4, 5] and many others [6{10]. Nowadays the reaction of superconductors againstthe impurities intentionally introduced into them is the ﬂrst indication of the non

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M. Michalik

Papers

Charge transport and metal–insulator transition in correlated and disordered systems

CPA Theory of Superconducting Alloys with Diagonal and Off-Diagonal Disorder

Off-Diagonal Nonmagnetic Disorder in s- and d-Wave Superconductors