Superconducting state in the ternary stannide superconductors R3T4Sn13 (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 La3T4Sn13, Tc = 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 Delta C-e/gamma T-c and 2 Delta(0)/k(B)T(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 R3T4Sn13 (R = La, Sr; T = Rh, Ir) in Table I.