Muon Spin Relaxation Measurements in Na_xCoO_2*yH_2O

TL;DR

This study uses muon spin relaxation to measure the magnetic penetration depth in Na_xCoO_2·yH_2O, revealing a nodal order parameter, a large effective mass of superconducting carriers, and adherence to the Uemura relation.

Contribution

It provides the first measurements of the penetration depth in this material, indicating unconventional superconductivity with a large effective mass and nodal order parameter.

Findings

The penetration depth is very small and near the sensitivity limit.

The order parameter has nodes, suggesting unconventional pairing symmetry.

The effective mass of superconducting carriers is about 70 times the electron mass.

Abstract

Using the transverse field muon spin relaxation technique we measure the temperature dependence of the magnetic field penetration depth $\lambda$, in the Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O system. We find that $\lambda,$ which is determined by superfluid density $n_{s}$ and the effective mass $m^{\ast}$, is very small and on the edge of the TF-$\mu$SR sensitivity. Nevertheless, the results indicate that the order parameter in this system has nodes and that it obeys the Uemura relation. By comparing $\lambda$ with the normal state electron density we conclude that $m^{\ast}$ of the superconductivity carrier is 70 times larger than the mass of bare electrons.