Nodeless superconductivity in the cage-type superconductor Sc₅Ru₆Sn₁₈ with preserved time-reversal symmetry

We report the single-crystal synthesis and detailed investigations of the cage-type superconductor Sc₅Ru₆Sn₁₈, using powder x-ray diffraction (XRD), magnetization, specific-heat and muon-spin relaxation (µSR) measurements. Sc₅Ru₆Sn₁₈ crystallizes in a tetragonal structure (space group I4₁/acd) with lattice parameters a  =  1.387(3) nm and c  =  2.641(5) nm. Both DC and AC magnetization measurements prove the type-II superconductivity in Sc₅Ru₆Sn₁₈ with T _c  ≈  3.5(1) K, a lower critical field   =  157(9) Oe and an upper critical field,   =  26(1) kOe. The zero-field electronic specific-heat data are well fitted using a single-gap BCS model, with   =  0.64(1) meV. The Sommerfeld constant γ varies linearly with the applied magnetic field, indicating s-wave superconductivity in Sc₅Ru₆Sn₁₈. Specific-heat and transverse-field (TF) µSR measurements reveal that Sc₅Ru₆Sn₁₈ is a superconductor with strong electron–phonon coupling, with TF-µSR also suggesting a single-gap s-wave character of the superconductivity. Furthermore, zero-field µSR measurements do not detect spontaneous magnetic fields below T _c, hence implying that time-reversal symmetry is preserved in Sc₅Ru₆Sn₁₈. (paper)