Dependence of the specific heat of NaxCoO2•yH2O/D2O on sodium and water concentrations -: art. no. 060512

We report specific heat measurements down to 0.4 K on the layered oxide NaxCoO2 center dot yH(2)O/D2O with 0 <= x <= 0.74 and y=0 and 1.4. For the nonhydrated system (y=0), the electronic specific heat coefficient gamma(N) and the Debye temperature Theta(D) vary nonmonotonically with x, both displaying minima when x is close to 0.5. This indicates a systematic change of the electronic and vibrational structures with Na content. For both hydrated and deuterated systems (x=0.35 and y=1.4), the specific heat reveals a sharp peak with Delta C-p similar to 45.5 mJ/mol K at T (mid)(c)similar to 4.7 K and an anomaly at T-x similar to 0.8 K. While the origin of the latter is not clear, the former corresponds to the superconducting transition. With the application of magnetic fields up to 14 Tesla, T-c decreases gradually but T-x remains more or less unchanged. The implication of these results is discussed.