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

Abstract

Acomprehensive 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−xLaxRh4Sn13) or Ce (Ca3−xCexRh4Sn13) with superconducting critical temperatures Tc  higher than those (Tc) 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 Tc  and Tc-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 theHc2(T) data.Wedemonstrate 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 theH−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−xCexRh4Sn13 alloys the presence of spin-glass effects is the cause of the additional increase of Tc  in respect to the critical temperatures of disordered Ca3−xLaxRh4Sn13.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.