Variation of Specific Heat with x and y in NaxCoO2-yH2O/D2O
  We report specific heat measurements down to 0.4 K on the layered oxide Na$_x$CoO$_2\cdot\it{y}$H$_2$O/D$_2$O with 0 $\le$ $\it{x}$ $\le$ 0.74 and $\it{y}$ = 0 and 1.4. For the nonhydrated system ($\it{y}$ = 0), the electronic specific heat coefficient $\gamma_N$ and the Debye temperature $\Theta_D$ vary nonmonotonically with $\it{x}$, both displaying minima when $\it{x}$ 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 ($\it{x}$ = 0.35 and $\it{y}$ = 1.4), the specific heat reveals a sharp peak with $\Delta$C$_p \sim$ 45.5 mJ/mol-K at T$_c^{mid} \sim$ 4.7 K and an anomaly at T$_x \sim$ 0.8 K. While the origin of the later is unknown, the former corresponds to the superconducting transition. Interestingly, the electronic specific heat, after subtracting lattice and Schottky contributions, exhibits roughly T$^2$ behavior between 0.2T$_c$ and T$_c$. This can be explained by assuming an unconventional superconducting symmetry with line nodes. The results obtained under applied magnetic field further support this scenario. 
R. Jin, B.C. Sales, S. Li, D. Mandrus