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, $ρ_{ab}$, and out-of-plane, $ρ_{c}$ resistances reported for large sample of Bi2212. Here we present careful measurements of both $ρ_{ab}(B)$ and $ρ_{c}(B)$ of tiny Bi2212 crystals in magnetic fields up to 50 Tesla. None of our measurements revealed a situation when on field increase $ρ_c$ reaches its maximum while $ρ_{ab}$ remains very small if not zero. The resistive $H_{c2}(T)$ estimated from $ρ_{ab}(B)$ and $ρ_{c}(B)$ 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.