Unconventional superconductivity in the cage-type compound Sc5Rh6Sn18

Abstract

We have examined the superconducting ground state properties of the caged type compound Sc5Rh6Sn18 using magnetization, heat capacity, and muon-spin relaxation or rotation (µSR) measurements. Magnetization measurements indicate type-II superconductivity with an upper critical field µ0Hc2(0) = 7.24 T. The zero-field cooled and field cooled susceptibility measurements unveil an onset of diamagnetic signal below Tc = 4.4 K. The interpretation of the heat capacity results below Tc using the α−BCS model unveils the value of α = 2.65, which gives the dimensionless ratio 2∆(0)/kBTc = 5.3, intimating that Sc5Rh6Sn18 is a strong-coupling BCS superconductor. The zero-field µSR measurements in the longitudinal geometry exhibit a signature of a spontaneous appearance of the internal magnetic field below the superconducting transition temperature, indicating that the superconducting state is characterized by the broken time-reversal symmetry (TRS). We have compared the results of broken TRS in Sc5Rh6Sn18 with that observed in R5Rh6Sn18 (R = Lu and Y).

Conventional isotropic s -wave superconductivity with strong electron-phonon coupling in Sc5Rh6Sn18

“…Here it should be noted that the description of the low-temperature phonons is a typical problem of 5:6:18 stannides. To model c ph for Sc 5 Ru 6 Sn 18 Kumar et al [21] fitted c p to γ T + βT 3 + δT 5 above T c = 3.5 K. A comparison of the value of γ = 51.1 mJ mol −1 K −2 obtained in [15] for Sc 5 Rh 6 Sn 18 with Fig. 2(a) in this work hints at its underestimation.…”

“…Recently performed zero-field muon-spin rotation (μSR) measurements in longitudinal geometry revealed for Sc 5 Rh 6 Sn 18 the spontaneous appearance of an internal magnetic field below T c . This would imply a breaking of TRS in the sc state and thus unconventional superconductivity with either a singlet d + id state with a line node or, alternatively, nonunitary triplet pairing with point nodes [15].…”

Recent progress on superconductors with time-reversal symmetry breaking

“…Here the tetragonal structure with space group I4 1 /acd corresponds to a distortion of a cubic structure, where the periodicity of the lattice doubles along the c-axis. The signatures of TRS breaking were found for all three compounds from zero-field µSR measurements [157][158][159] , as manifested in an increase of λ ZF below T c . On the basis of a symmetry analysis, two potential unconventional TRS breaking states were proposed, a d + id singlet state and a non-unitary triplet state 157 .…”

Nodeless superconductivity in Lu5−xRh6Sn18+x with broken time reversal symmetry

“…In this context, it is of particular interest to examine the superconductivity of weakly correlated centrosymmetric superconductors exhibiting TRS breaking. One such family of superconductors are the caged compounds R 5 Rh 6 Sn 18 (R = Sc, Y, Lu), which crystallize with the tetragonal space group I4 1 /acd [23], and evidence for TRS breaking is found from muon spin relaxation (μSR) measurements [24][25][26] (although TRS breaking signatures were not observed in another study for R = Sc [27]). Resistivity measurements of all three compounds show semiconducting behavior in the normal state, before becoming superconducting below 5.0, 3.0, and 4.0 K for R = Sc, Y, and Lu, respectively [28,29].…”