Superconducting state in the ternary stannide superconductors R ₃T₄Sn₁₃ (R = La, Sr; T = Rh, Ir) with a quasiskutterudite structure

Single crystals of R₃T₄Sn₁₃ (R = La, Sr; T = Rh, Ir) are synthesized by a Sn-flux method. Superconducting transition temperatures T_c are determined on the basis of magnetic susceptibility and electrical resistivity measurements. For La₃T ₄Sn₁₃, T_c = 3.1 and 2.5 K when T = Rh and Ir, respectively; similarly, for Sr₃T₄Sn₁₃, T _c = 4.2 and 5.0 K. Specific heat measurements indicate that superconducting gaps of these compounds are isotropic and that these superconductors can be categorized as strong-coupling superconductors on the basis of the values of ΔC_e/γT_c and 2Δ(0)/k_BT_c. The upper critical field shows a clear enhancement with respect to the Werthamer-Helfand-Hohenberg prediction; this enhancement is consistent with the presence of a strong electron-phonon coupling. We summarized the normal and superconducting parameters of R ₃T₄Sn₁₃ (R = La, Sr; T = Rh, Ir) in Table.