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

${\rm CaPd_2Ge_2}$ which crystallizes in ${\rm ThCr_2Si_2}$-type body-centered tetragonal structure exhibits superconductivity below the critical temperature $T_{\rm c} = 1.69$~K\@. We have investigated the superconducting gap structure and time reversal symmetry of the ground state in ${\rm CaPd_2Ge_2}$ 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 $2\Delta(0)/k_{\rm B}T_{\rm c} = 3.50(1)$. Further, an increased relaxation rate in zero field $\mu$SR asymmetry spectra below $T_{\rm c}$ provides evidence for the presence of a spontaneous magnetic field in the superconducting state revealing that the time-reversal symmetry is broken in ${\rm CaPd_2Ge_2}$.