Superconducting properties of R3T4Sn13 (R = La, Sr, Ca, T = Rh, Ir) with a quasi-skutterudite structure

We measured the superconducting and normal state by using the single crystals of R3T4Sn13 (R = La, Sr, Ca, T = Rh, Ir). The compounds crystallize in a Yb3T4Sn13-type structure with a cubic space group of Pm3-n (No. 223). Superconducting state of R3T4Sn13 was investigated by magnetic susceptibility, electrical resistivity, and specific heat measurements. From the electrical resistivity measurements under several magnetic fields, the upper critical field Hc2(0) was estimated. From the results, we discovered that the Hc2(T) curves cannot be described as the Wertharmer-Helfand-Hohenberg (WHH) prediction. Specific heat measurements indicated that these compounds were typical BCS superconductors, and categorized as strong-coupling superconductors.