Isotropic s -wave superconductivity in the noncentrosymmetric charge density wave superconductor SrPt 2 As 2

${\mathrm{SrPt}}_{2}{\mathrm{As}}_{2}$ undergoes a structural transition around 470 K (from a high-temperature tetragonal to a low-temperature orthorhombic phase) accompanied by a charge density wave transition and followed by a superconducting transition at much lower temperature (5 K). The superconducting gap symmetry of this noncentrosymmetric compound was investigated in detail by means of muon-spin rotation or relaxation $(\ensuremath{\mu}\mathrm{SR})$ experiments in transverse-field (TF) configuration. The temperature-dependent magnetic penetration depth obtained from the $\mathrm{TF}\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{SR}$ spectra suggests an isotropic $s$-wave-type superconducting gap. No observable differences are seen in the asymmetry time spectra collected in zero field above and below the superconducting transition temperature ${T}_{c}$. This strongly suggests the absence of spontaneous magnetic fields in the superconducting state, hence pointing to a preserved time-reversal symmetry below ${T}_{c}$.