Muon Spin Relaxation Measurements in Na_xCoO_2*yH_2O

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.