The Role of Density of States Fluctuations on the C-Axis Resistivity of High T _c Superconductors

Abstract

Among all unconventional normal state properties of the high temperature super- conductors, the transport properties are the most puzzling. While the in-plane resistivity is almost linear with temperature in clean samples the transverse resis-tivity generally shows a peak just above T _c . Such behavior was observed in all high T _c compounds [1, 2, 3, 4] and even in conventional layered superconductors [5]. As noted by Anderson [6], the difference in the temperature dependence between the transverse and in-plane resistivity is very difficult to explain in a conventional Fermi liquid theory. It turns out that this peak is directly connected with the anisotropy of the sample, being very pronounced in highly anisotropic samples (BSCCO, YBCO annealed in reducing atmosphere) but almost absent in samples with low anisotropy (fully oxygenated BSCCO or YBCO). We identify here at least one source of this difference: the suppression of the positive paraconductivity along c-direction by the square of the interlayer transparency together with the growth of the normal resistance related to the fluctuation depression of the density of states at the Fermi level.