Upper critical field from normal state fluctuations in Bi$_2$Sr$_2$CuO$_{6+\delta}$

TL;DR

This study investigates the in-plane magnetoresistance of Bi$_2$Sr$_2$CuO$_{6+eta}$ thin films above $T_c$, using orbital magnetoconductance to determine the upper critical field and its implications for superconducting fluctuations.

Contribution

It introduces a method to extract the upper critical field from normal state fluctuations, showing consistency with below-$T_c$ measurements and revealing insights into the resistive transition.

Findings

Upper critical field $H_{c2}^*(T)$ matches resistivity-based $H_{c2}$.

$H_{c2}^*(T)$ exhibits upward curvature.

Results suggest a critical correlation length influences the transition.

Abstract

The in-plane magnetoresistance of an epitaxial Bi$_2$Sr$_2$CuO$_{6+\delta}$ thin film was systematically investigated as a function of doping, above $T_c$. The orbital magnetoconductance is used to extract the crossover field line $H_{c2}^*(T)$ in the fluctuation regime. This field is found in good agreement with the upper critical field obtained from resistivity data below $T_c$, and exhibits a similar upward curvature, thus pointing toward the existence of a critical correlation length. The consequences regarding the nature of the resistive transition are discussed.