Variation of Specific Heat with x and y in NaxCoO2-yH2O/D2O

Abstract

We report specific heat measurements down to 0.4 K on the layered oxide Na${}_x$CoO${}_2\cdot y$H${}_2$O/D${}_2$O with 0 $\le$ $x$ $\le$ 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${}_p\sim$ 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.

4 page, 3 figures, 1 table