Heat Capacity of Na0.3CoO2‧1.3H2O, a New Two-Gap Superconductor: Comparison with the Heat Capacity of MgB2

N. Oeschler ； R. A. Fisher ； N. E. Phillips ； J. E. Gordon ； M. L. Foo ； R. J. Cava

《Chinese Journal of Physics》 43卷3S期 (2005/06) Pp. 574-582

並列摘要

The superconducting-state heat capacity of Na0.3CoO2‧1.3H2O shows unusual, marked deviations from BCS theory, at all temperatures. At low temperatures the heat capacity has the T^2 dependence characteristic of line nodes in the energy gap, rather than the exponential temperature dependence of a fully gapped, conventional superconductor. At temperatures of the order of one fifth of the critical temperature and above, the deviations are strikingly similar to those of MgB2, which are known to be a consequence of the existence of substantially different energy gaps on different sheets of the Fermi surface. A two-gap fit to the Na0.3CoO2‧1.3H2O data gives gap amplitudes of 45% and 125% of the BCS value, on parts of the Fermi surface that contribute, respectively, 45% and 55% to the normal-state density of states. The temperature of the onset of the transition to the vortex state is independent of magnetic field, which shows the presence of unusually strong fluctuations.