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

Abstract

A comprehensive study of the electronic structure, thermodynamic and electrical transport properties reveals the existence of inhomogeneous superconductivity due to structural disorder in Ca _3 Rh _4 Sn _13 doped with La (Ca _3− _x La _x Rh _4 Sn _13 ) or Ce (Ca _3− _x Ce _x Rh _4 Sn _13 ) with superconducting critical temperatures ${T}_{c}^{\star }$ 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 ${T}_{c}^{\star }$ 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 _c _2 ( 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 Ca _3 Rh _4 Sn _13 showed that in the disordered Ca _3− _x Ce _x Rh _4 Sn _13 alloys the presence of spin-glass effects is the cause of the additional increase of ${T}_{c}^{\star }$ in respect to the critical temperatures of disordered Ca _3− _x La _x Rh _4 Sn _13 . We also revisited the nature of structural phase transition at ${T}^{\star }\sim 130\div170$ 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.

Keywords

• superconductivity
• atomic disorder
• electronic band structure
• thermodynamic and transport properties