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

The heat capacity of superconducting NaxCoO2 . yH(2)O 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 (approximate to50%) superconducting volume fraction. The data are compatible with 100% superconductivity in the case where line nodes are present in the superconducting gap function. (C) 2003 Elsevier B.V. All rights reserved.