Heat Capacity of Na$_{0.3}$CoO$_{2}$$\cdot$1.3H$_{2}$O, a New Two-Gap Superconductor: Comparison with the Heat Capacity of MgB$_2$

Abstract

The superconducting-state heat capacity of Na$_{0.3}$CoO$_{2}$$\cdot$1.3H$_{2}$O shows unusual, marked deviations from BCS theory, at all temperatures. At low temperatures the heat capacity has the $T^2$ dependence characteristic of line nodes in the energy gap, rather than the exponential temperature dependence of a fully gapped, conventional superconductor. At temperatures of the order of one fifth of the critical temperature and above, the deviations are strikingly similar to those of MgB$_2$, which are known to be a consequence of the existence of substantially different energy gaps on different sheets of the Fermi surface. A two-gap fit to the Na$_{0.3}$CoO$_{2}$$\cdot$1.3H${_2}$O 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.