The "normal" state of superconducting cuprates might really be normal after all

TL;DR

This study investigates the normal state of cuprate superconductors under high magnetic fields, finding results consistent with a truly normal state above the resistive transition, challenging previous notions of a pseudogap or fluctuating superconductivity.

Contribution

The paper provides experimental evidence that the resistive upper critical fields in Bi2212 are consistent across different measurement directions, supporting the view of a normal state above the transition without off-diagonal order.

Findings

Resistive measurements show no maximum in out-of-plane resistivity with small in-plane resistivity.

Upper critical fields from different directions are approximately equal.

Results support a normal state interpretation for the pseudogap and Nernst signals.

Abstract

High magnetic field studies of 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 both the contrasting effect of the magnetic field on the in-plane and out-of-plane resistances reported for large Bi2212 sample and the large Nernst signal \emph{well above} $T_{c}$. Here we present 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 the field increase $\rho_c$ reaches its maximum while $\rho_{ab}$ remains very small if not zero. The resistive %upper critical fields estimated from the in-plane and out-of-plane $H_{c2}(T)$ estimated from $\rho_{ab}(B)$ and $\rho_{c}(B)$ are approximately the same. Our results support any theory of cuprates that describes the state above the resistive phase transition as perfectly normal with a zero off-diagonal order parameter. In particular, the anomalous Nernst effect above the resistive phase transition in high-$T_{c}$ cuprates can be described quantitatively as a normal state phenomenon in a model with itinerant and localised fermions and/or charged bosons.