Superconducting properties of Ca₃T₄Sn₁₃ (T = Co, Rh, and Ir)

We present the results of the superconducting properties of R ₃Ti₄Sn₁₃ (R = La, Sr, and Ca; T = Co, Rh, and Ir) with quasi skutterudite structure by means of magnetic susceptibility, electrical resistivity, and specific heat measurements. It is shown that the superconductivity is of conventional s-wave type and lies in the extremely strong-coupling regime. Electron-phonon coupling constants λ_ep were estimated to be almost 1 for R₃Ti₄Sn₁₃, respectively. The result can be categorized as a strong-coupling superconductor; this categorization is in good agreement with the results of specific heat measurements. The λ_ep and the electronic density of states at the Fermi level (N(E_F)) increases with the superconducting transition temperature. However, superconducting transition temperature of Co-compound cannot be explained by the difference of N(E_F). From these results, we concluded that T_c of R₃Ti₄Sn₁₃ can be described by the strength of the λ_ep.