Thermodynamic properties in the normal and superconducting states of NaxCoO2·yH2O powder measured by heat capacity experiments

The heat capacity of superconducting NaxCoO2 DT yH2O was measured and the data are discussed based on two different models: The BCS theory and a model including the effects of line nodes in the superconducting gap function. The electronic heat capacity is separated from the lattice contribution in a thermodynamically consistent way maintaining the entropy balance of superconducting and normal states at the critical temperature. It is shown that for a fully gapped superconductor the data can only be explained by a reduced (~50%) superconducting volume fraction. The data are compatible with 100% superconductivity in the case where line nodes are present in the superconducting gap function.