Possible singlet to triplet pairing transition in NaxCoO2 H2O

TL;DR

This paper reports on unusual behavior of the critical magnetic field in NaxCoO2 H2O, suggesting a possible transition from singlet to triplet superconductivity driven by magnetic field effects.

Contribution

It provides the first precise measurements of Hc2 in NaxCoO2 H2O and proposes a theoretical explanation for the observed anomalous temperature dependence.

Findings

Unusual temperature dependence of Hc2 with a slope change

Evidence supporting a field-induced singlet to triplet transition

Theoretical modeling using Gor'kov equations on a triangular lattice

Abstract

We present precise measurements of the upper critical field (Hc2) in the recently discovered cobalt oxide superconductor. We have found that the critical field has an unusual temperature dependence; namely, there is an abrupt change of the slope of Hc2(T) in a weak field regime. In order to explain this result we have derived and solved Gor'kov equations on a triangular lattice. Our experimental results may be interpreted in terms of the field-induced transition from singlet to triplet superconductivity.