Uncommonly High Upper Critical Field in the Superconducting KOs$_2$O$_6$ Pyrochlore

TL;DR

This study reveals an exceptionally high and linearly temperature-dependent upper critical field in KOs$_2$O$_6$, driven by orbital effects and s-wave pairing on small Fermi surfaces, challenging conventional limits.

Contribution

The paper provides the first detailed temperature dependence of $H_{c2}$ in KOs$_2$O$_6$, showing an unusually high value and linear behavior, supported by ab initio calculations.

Findings

$H_{c2}(0)$ exceeds the simple paramagnetic limit.

$H_{c2}(T)$ is linearly dependent on temperature below $T_c$.

Superconductivity is consistent with s-wave pairing on small Fermi surfaces.

Abstract

The entire temperature dependence of the upper critical field $H_{\rm c2}$ in the $\beta$-pyrochlore KOs$_2$O$_6$ is obtained from high-field resistivity and magnetic measurements. Both techniques identically give $H_{\rm c2}(T \simeq 0 {\rm K})$ not only surprisingly high ($\sim 33$ T), but also the approach to it unusually temperature-\emph{linear} all the way below $T_{\rm c}$ (= 9.6 K). We show that, while $H_{\rm c2}(0)$ exceeds a simple spin-singlet paramagnetic limit $H_{\rm P}$, it is well below an $H_{\rm P}$ enhanced due to the missing spatial inversion symmetry reported recently in KOs$_2$O$_6$, ensuring that the pair-breaking here is executed by orbital degrees. {\it Ab initio} calculations of orbital $H_{\rm c2}$ show that the unusual temperature dependence is reproduced if dominant s-wave superconductivity resides on the smaller closed Fermi surfaces.