Nodeless time-reversal symmetry breaking in the centrosymmetric superconductor Sc${}_5$Co${}_4$Si${}_{10}$ probed by muon-spin spectroscopy

Abstract

We investigate the superconducting properties of Sc${}_5$Co${}_4$Si${}_{10}$ using low-temperature resistivity, magnetization, heat capacity, and muon-spin rotation and relaxation ($\mu$SR) measurements. We find that Sc${}_5$Co${}_4$Si${}_{10}$ {exhibits type-II} superconductivity with a superconducting transition temperature $T_{\mathrm{C}}=3.5(1)$\,K. The temperature dependence of the superfluid density obtained from transverse-field $\mu$SR spectra is best modeled using an isotropic Bardeen-Cooper-Schrieffer type $s$-wave gap symmetry with $2\Delta /k_{\mathrm{B}}{T}_{\mathrm{C}}=2.84(2)$. However, the zero-field muon-spin relaxation asymmetry reveals the appearance of a spontaneous magnetic field below $T_{\mathrm{C}}$, indicating that time-reversal symmetry (TRS) is broken in the superconducting state. Although this behavior is commonly associated with non-unitary or mixed singlet-triplet pairing, our group-theoretical analysis of the Ginzburg-Landau free energy alongside density functional theory calculations indicates that unconventional mechanisms are pretty unlikely. Therefore, we have hypothesized that TRS breaking may occur via a conventional electron-phonon process.

9 pages, 6 figures

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