The role of density of states fluctuations in the normal state properties of highTcsuperconductors

Abstract

During the last decade a lot of efforts have been undertaken to explain the unusual normal state properties of high temperature superconductors (HTS) in the framework of unconventional theories based on strongly interacting electrons, pre-formed Cooper pairs, polaron mechanism of super conductivity etc. A different approach to this problem would be to develop the perturbation theory for interacting electrons in the normal phase of strongly anisotropic superconductors without specifying the origin of this interaction. The Cooper channel of interelectron interaction is equivalent to the superconducting fluctuations which are unusually strong in HTS. We show that the peculiarities of such systems not only lead to the increase of the magnitude but are also frequently responsible for the change of the hierarchy of different fluctuation effects and even of the sign of the total corrections. As a result the fluctuation contributions can manifest themselves in very unusual forms. The first and well known result is that that now one has the 'pre-formed Cooper pairs' automatically, from ab initio calculations: taking into account thermal fluctuations leads to the appearance of a non-zero density of fluctuating Cooper pairs (with finite lifetime) within layers without the establishment of long range order in the system. The fluctuation Cooper pair density decreases with temperature very slowly(∼ln T c/(T - T c) in the 2D case). The formation of these pairs of normal electrons leads to the decrease of the density of one-electron states (DOS renormalization) at the Fermi level, and this turns out to be the key effect in our discussion. The DOS contribution to the most of characteristics is negligible for traditional superconducting materials. However, it becomes dominant when highly anisotropic materials are discussed, and therefore is very important in HTS, especially when transport along the c-axis is considered. We analyse the role of the DOS fluctuations in the properties of HTS and show how, taking into account this effect, many puzzling and long debated properties of HTS materials (such as the steep increase of the electrical resistivity along the c-axis just above T c, the anomalous magnetoresistance, effects of the magnetic field on the resistive transition along the c-axis, the c-axis far infrared absorption spectrum, NMR characteristics around the critical temperature etc.) can be understood leading to a simple, consistent description in terms of the fluctuation theory.