Universal upper critical field of unconventional superconductors

TL;DR

This paper demonstrates a universal nonlinear temperature dependence of the upper critical field in various unconventional superconductors, attributed to Bose-Einstein condensation of preformed pairs, and resolves measurement controversies.

Contribution

It introduces a universal scaling law for Hc2(T) across different materials based on the charged Bose-gas model, unifying diverse experimental observations.

Findings

Hc2(T) follows a universal nonlinear temperature dependence near Tc.

Exceeding the Pauli limit is explained by the model.

Measurement discrepancies are resolved within the Bose-gas framework.

Abstract

The resistive upper critical field, Hc2(T) of cuprates, superconducting spin-ladders, and organic (TMTSF)2X systems is shown to follow a universal nonlinear temperature dependence in a wide range near Tc, while its low-temperature behaviour depends on the chemical formula and sample quality. Hc2(T) is ascribed to the Bose-Einstein condensation field of preformed pairs. The universality originates from the scaling arguments. Exceeding the Pauli paramagnetic limit is explained. Controversy in the determination of Hc2(T) from the kinetic and thermodynamic measurements is resolved in the framework of the charged Bose-gas model with impurity scattering.