Extreme High-Field Superconductivity in Thin Re Films

TL;DR

This study reveals that thin disordered Re films exhibit extraordinarily high upper critical magnetic fields, surpassing the Pauli limit, despite having typical superconducting properties, indicating potential for high-field superconductor applications.

Contribution

The paper demonstrates that ultra-thin disordered Re films can achieve extremely high upper critical fields, exceeding the Pauli limit, which is a novel finding for elemental superconductors.

Findings

Zero-temperature $H_{c2}$ exceeds twice the Pauli limit.

Estimated $H_{c2}/T_c$ ratio among the highest for elemental superconductors.

$H_{c2}$ approaches the theoretical limit for strongly disordered superconductors.

Abstract

We report the high-field superconducting properties of thin, disordered Re films via magneto-transport and tunneling density of states measurements. Films with thicknesses in the range of 9 nm to 3 nm had normal state sheet resistances of $\sim$0.2 k$\Omega$ to $\sim$1 k$\Omega$ and corresponding transition temperatures in the range of 6 K to 3 K. Tunneling spectra were consistent with those of a moderate coupling BCS superconductor. Notwithstanding these unremarkable superconducting properties, the films exhibited an extraordinarily high upper critical field. We estimate their zero-temperature $H_{c2}$ to be more than twice the Pauli limit. Indeed, in 6 nm samples the estimated reduced critical field $H_{c2}/T_c\sim$ 5.6 T/K is among the highest reported for any elemental superconductor. Although the sheet resistances of the films were well below the quantum resistance $R_Q=h/4e^2$, their $H_{c2}$'s approached the theoretical upper limit of a strongly disordered superconductor for which $k_F\ell\sim1$.