(P, T) Phase Diagram of Clathrate Superconductor Ba24Ge100

Precise electrical resistivity and ac calorimetry measurements under pressure shave revealed (P, T) phase diagram of superconducting clathrate Ba24Ge100. Following conventional superconducting theory, the pressure dependence of the density of state at Fermi energy, N(E-F), estimated by the electronic specific heat C/T has been discussed with the T-c(P). Up to P=2.1 GPa, where T-c (P) starts to saturate, C/T increases with T-c in agreement with the theory. On further compression, C/T is strongly suppressed while T-c slightly decreases. Across P-c similar to 2.9 GPa, where structural deforming transitions at T-s1(s2) are suppressed, pressure variation of C/T becomes smaller. In addition to the enhancement of the N(E-F), other changes in crystal and/or electrical structure may play important roles to pressure dependence of T-c in Ba24Ge100.