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

TL;DR

This study reveals that the centrosymmetric superconductor Sc$_5$Co$_4$Si$_{10}$ exhibits nodeless superconductivity with broken time-reversal symmetry, likely due to a conventional electron-phonon mechanism, as shown by muon-spin spectroscopy and theoretical analysis.

Contribution

It demonstrates TRS breaking in a centrosymmetric superconductor without unconventional pairing, challenging previous assumptions about TRS breaking mechanisms.

Findings

Superconducting transition temperature T_C = 3.5 K.

Isotropic s-wave superconducting gap with 2Δ/k_B T_C ≈ 2.84.

Observation of spontaneous magnetic fields below T_C indicating TRS breaking.

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.