Specific heat of unconventional superconductivity in NaxCoO2-yH2O

Abstract

Comprehensive studies of the low-temperature specific heat C(T,H) in NaxCoO2-yH2O are presented. At H=0, a very sharp anomaly was observed at T=4.7 K indicating the existence of bulk superconductivity. There exists the T^2 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. An abrupt change of the slope in Tc(H) was observed. Possible scenarios such as the multiple phase transitions in the mixed state are discussed.

arXiv comment10 pages, 4 figures; the statement of entropy imbalance in the previous version was withdrawn