Time-reversal Symmetry Breaking and S-Wave Superconductivity in CaPd2Ge2: A SR Study

CaPd2Ge2 which crystallizes in ThCr2Si2-type body-centered tetragonal structure exhibits superconductivity below the critical temperature Tc = 1.69 K. We have investigated the superconducting gap structure and timereversal symmetry of the ground state in CaPd2Ge2 by means of muon spin relaxation and rotation (mu SR) measurements. Our analysis of mu SR data collected in a transverse magnetic field reveals BCS superconductivity with a single-band s-wave singlet pairing and an isotropic energy gap having the value 2 ?(0)/kBTc = 3.50(1). Further, an increased relaxation rate in zero-field mu SR asymmetry spectra below Tc provides evidence for the presence of a spontaneous magnetic field in the superconducting state, revealing that the time-reversal symmetry is broken in CaPd2Ge2.