How normal is the "normal" state of superconducting cuprates?

Abstract

High magnetic field studies of the cuprate superconductors revealed a non-BCS temperature dependence of the upper critical field $H_{c2}(T)$ determined resistively by several groups. These determinations caused some doubts on the grounds of the contrasting effect of the magnetic field on the in-plane, ${\rho }_{ab}$, and out-of-plane, ${\rho }_c$ resistances reported for large sample of Bi2212. Here we present careful measurements of both ${\rho }_{ab}(B)$ and ${\rho }_c(B)$ of tiny Bi2212 crystals in magnetic fields up to 50 Tesla. None of our measurements revealed a situation when on field increase ${\rho }_c$ reaches its maximum while ${\rho }_{ab}$ remains very small if not zero. The resistive $H_{c2}(T)$ estimated from ${\rho }_{ab}(B)$ and ${\rho }_c(B)$ are approximately the same. We also present a simple explanation of the unusual Nernst signal in superconducting cuprates as a normal state phenomenon. Our results support any theory of cuprates, which describes the state above the resistive phase transition as perfectly 'normal' with a zero off-diagonal order parameter.