(P, T) phase diagram of clathrate superconductor Ba₂₄Ge ₁₀₀

Precise electrical resistivity and ac calorimetry measurements under pressures have 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 ∼ 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.