Superconducting critical field far above the Pauli limit in one-dimensional Li$_{0.9}$Mo$_6$O$_{17}$

TL;DR

This study reveals that the upper critical magnetic field in Li$_{0.9}$Mo$_6$O$_{17}$ exceeds the Pauli limit significantly, indicating unconventional superconductivity potentially involving spin-triplet pairing.

Contribution

The paper provides experimental evidence of an exceptionally high critical field in a one-dimensional superconductor, challenging existing theories and suggesting a novel pairing mechanism.

Findings

$H_{c2}$ exceeds the Pauli limit by five times.

Anisotropic $H_{c2}$ aligns with resistivity anisotropy.

Inadequacy of existing theories to explain the high $H_{c2}$.

Abstract

The upper critical field $H_{c2}$ of purple bronze Li$_{0.9}$Mo$_6$O$_{17}$ is found to exhibit a large anisotropy, in quantitative agreement with that expected from the observed electrical resistivity anisotropy. With the field aligned along the most conducting axis, $H_{c2}$ increases monotonically with decreasing temperature to a value five times larger than the estimated paramagnetic pair-breaking field. Theories for the enhancement of $H_{c2}$ invoking spin-orbit scattering or strong-coupling superconductivity are shown to be inadequate in explaining the observed behavior, suggesting that the pairing state in Li$_{0.9}$Mo$_6$O$_{17}$ is unconventional and possibly spin-triplet.