Evidence of nodal superconductivity in Na0.35CoO2 center dot 1.3 H2O: A specific-heat study

Abstract

Comprehensive low-temperature specific-heat data C(T,H) of Na0.35CoO2.1.3 H2O with temperature T down to 0.6 K and the magnetic field H up to 8 T are presented. In the normal state, the values of gamma(n)=13.94 mJ/mol K-2 and Debye temperature Theta(D)=362 K are reported. At zero field, a very sharp superconducting anomaly was observed at T-c=4.5 K with DeltaC/gamma(ns)T(c)=1.45 if the specific-heat jump is normalized to the superconducting volume fraction, which is estimated to be 47.4% based on the consideration of entropy balance at T-c for the second-order superconducting phase transition. In the superconducting state, the electronic contribution C-es at H=0 can be well described by the model of the line nodal order parameter. In low H, deltagamma(H)proportional toH(1/2), which is also a manifestation of the line nodes. The behaviors of both T-c(H) and gamma(H) suggest the anisotropy of H-c2, or possible crossovers or transitions occurring in the mixed state.