Time reversal symmetry breaking and s-wave superconductivity in CaPd2Ge2: A $\mu$SR study

$\mathrm{C}\mathrm{a}\mathrm{P}{\mathrm{d}}_2\mathrm{G}{\mathrm{e}}_2$ which crystallizes in $\mathrm{T}\mathrm{h}\mathrm{C}{\mathrm{r}}_2\mathrm{S}{\mathrm{i}}_2$-type body-centered tetragonal structure exhibits superconductivity below the critical temperature $T_{\mathrm{c}}=1.69$~K@. We have investigated the superconducting gap structure and time reversal symmetry of the ground state in $\mathrm{C}\mathrm{a}\mathrm{P}{\mathrm{d}}_2\mathrm{G}{\mathrm{e}}_2$ by means of muon spin relaxation and rotation ($\mu$SR) measurements. Our analysis of $\mu$SR data collected in transverse magnetic field reveals BCS superconductivity with a single-band $s$-wave singlet pairing and an isotropic energy gap having the value $2\mathrm{\Delta }(0)\mathrm{/}{k}_{\mathrm{B}}{T}_{\mathrm{c}}=3.50(1)$. Further, an increased relaxation rate in zero field $\mu$SR asymmetry spectra below $T_{\mathrm{c}}$ provides evidence for the presence of a spontaneous magnetic field in the superconducting state revealing that the time-reversal symmetry is broken in $\mathrm{C}\mathrm{a}\mathrm{P}{\mathrm{d}}_2\mathrm{G}{\mathrm{e}}_2$.