Direct measurement of the upper critical field in a cuprate superconductor

TL;DR

This study directly measures the upper critical field in cuprate superconductors, revealing doping-dependent peaks and the influence of competing phases on superconductivity suppression.

Contribution

It provides the first direct measurement of Hc2 in cuprates using thermal conductivity, clarifying its doping dependence and the impact of Fermi surface transformations.

Findings

Hc2 exhibits two peaks at critical doping points.

Condensation energy collapses 20-fold below the higher critical point.

No vortex liquid phase exists at T=0 in studied cuprates.

Abstract

The upper critical field Hc2 is a fundamental measure of the pairing strength, yet there is no agreement on its magnitude and doping dependence in cuprate superconductors. We have used thermal conductivity as a direct probe of Hc2 in the cuprates YBa2Cu3Oy and YBa2Cu4O8 to show that there is no vortex liquid at T = 0, allowing us to use high-field resistivity measurements to map out the doping dependence of Hc2 across the phase diagram. Hc2(p) exhibits two peaks, each located at a critical point where the Fermi surface undergoes a transformation. The condensation energy obtained directly from Hc2, and previous Hc1 data, undergoes a 20-fold collapse below the higher critical point. These data provide quantitative information on the impact of competing phases in suppressing superconductivity in cuprates.