The effective increase in atomic scale disorder by doping and superconductivity in Ca3Rh4Sn13

A comprehensive study of the electronic structure, thermodynamic and electrical transport properties reveals the existence of inhomogeneous superconductivity due to structural disorder in Ca3Rh4Sn13 doped with La (Ca3−x La x Rh4Sn13) or Ce (Ca3−x Ce x Rh4Sn13) with superconducting critical temperatures T*c higher than those (T c ) observed in the parent compounds. The T − x diagrams and the entropy S(x) T isotherms document well the relation between the degree of atomic disorder and separation of the high-temperature T*c and T c -bulk phases. In these dirty superconductors, with the mean free path much smaller than the coherence length, the Werthamer–Helfand–Hohenber theoretical model does not fit well the H c2(T) data. We demonstrate that this discrepancy can result from the presence of strong inhomogeneity or from two-band superconductivity in these systems. Both the approaches very well describe the H − T dependencies, but the present results as well as our previous studies give stronger arguments for the scenario based on the presence of nanoscopic inhomogeneity of the superconducting state. A comparative study of La-doped and Ce-doped Ca3Rh4Sn13 showed that in the disordered Ca3−x Ce x Rh4Sn13 alloys the presence of spin-glass effects is the cause of the additional increase of T*c in respect to the critical temperatures of disordered Ca3−x La x Rh4Sn13. We also revisited the nature of structural phase transition at T*~130÷170 K and documented that there might be another precursor transition at higher temperatures. Raman spectroscopy and thermodynamic properties suggest that this structural transition may be associated with a CDW-type instability.