Evidence of Nodal Superconductivity in Na0.35CoO2$\cdot$1.3H2O:A Specific Heat Study

Comprehensive low-temperature specific heat data $C(T$,$H)$ of Na$_{0.35}$CoO$_{2}$·$1.3$H$_{2}$O with temperature $T$ down to 0.6 K and the magnetic field $H$ up to 8 T are presented. A very sharp superconducting anomaly was observed at $T_{c}$=4.5 and \textit{$\gamma $}$_{n}$=13.94 mJ/mol K$^{2}$ and $\Theta _{D}$=362 K are derived. In the superconducting state, the electronic contribution $C_{es}$ at$ H$=0 can be well described by the model of the line nodal order parameter. In low $H$, $\delta $\textit{$\gamma $}($H)\propto H^{1/2}$ which is also a manifestation of the line nodes. The behaviors of both $T_{c}(H)$ and \textit{$\gamma $}($H)$ suggest the anisotropy of $H_{c2}$ or possible crossovers or transitions occurring in the mixed state. Collaborated with NCTU$^{\ast }$, TAIWAN and NIMS$^{\# }$, JAPAN