Volume dependence of the rattling in the caged superconductor and thermoelectric materials by high-pressure specific-heat measurement

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The caged compound KOs_2O_6 exhibits a superconducting transition at T_c=9.6 K, and a first-order transition at T_p=7.5K, which manifested itself in the sharp peak of the specific heat. In order to examine the relationship between these anomalous properties and low-energy anharmonic vibration of K ions, we measured specific heat under pressure with a high-precision AC calorimeter using a Bridgeman anvil cell. The peak in the specific heat at Tp disappeared at a low pressure of 0.02GPa. Furthermore, Tc increases from 9.65K at P=0 to 9.80K at P=0.02 GPa. This finding suggests that the superconducting state is stabilized by the suppression of the first-order transition at T_p.