Anomalous specific heat of NaxCoO2-yH2O: implications for a novel superconductor
Comprehensive magnetic field (H) dependence of the low-temperature specific heat C(T,H) in NaxCoO2-yH2O is presented. At H=0, a very sharp anomaly was observed at T=4.7 K indicating the existence of bulk superconductivity. There exists the (T2 term in C(T,H=0)) in the superconducting state manifesting the line nodal superconducting order parameter. The superconducting volume fraction is estimated to be 26.6 % based on the consideration of entropy conservation at Tc for the second-order superconducting phase transition. However, the violation of entropy conservation at Tc was observed for the superconducting transition in H. Possible scenarios such as the multiple phase transitions in the mixed state and superconductivity with the magnetic correlation are discussed.