Time-reversal symmetry breaking in the s -wave superconductor CaPd₂As₂ probed by μsR

CaPd2As2 is known to superconduct below Tc=1.27(3) K. In this work we examine whether the time-reversal symmetry (TRS) is preserved or broken upon entering the superconducting state in CaPd2As2, and probe its superconducting gap structure using muon spin relaxation and rotation (μSR) measurements in zero field (ZF) and transverse field (TF), respectively. The analysis of ZF-μSR spectra reveals an increase in muon spin relaxation rate λZF in the superconducting (SC) state attesting the presence of a weak magnetic field. The appearance of spontaneous magnetic field indicates the breaking of time-reversal symmetry state upon entering the SC state of CaPd2As2. The analysis of TF-μSR spectra provides information about the superconducting gap structure of CaPd2As2 which is found to have a single-band isotropic s-wave symmetry. We find an isotropic energy gap Δ(0)=0.160(1) meV corresponding to 2Δ(0)/kBTc=3.37(3) which is close to the weak-coupling BCS value of 3.53. The analysis of TF μSR also provides an estimate of effective penetration depth which is found to be λeff(0)=415(16) nm. Our μSR data thus find evidence for TRS breaking in CaPd2As2 with an s-wave singlet paring which is not expected for a conventional superconductor.