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

TL;DR

This study provides detailed measurements of resistances in Bi2212 cuprates under high magnetic fields, showing consistent upper critical fields and supporting the view that the normal state above the superconducting transition is truly normal without off-diagonal order.

Contribution

It offers comprehensive resistivity measurements in high magnetic fields, clarifying the behavior of the upper critical field and challenging previous doubts about the normal state of cuprates.

Findings

Resistances $ ho_{ab}$ and $ ho_{c}$ do not show maximum separation under high fields.

Estimated $H_{c2}(T)$ from both resistances are consistent.

Supports the view of a normal state with zero off-diagonal order parameter.

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.