Superconductivity with broken time reversal symmetry in ion irradiated Ba\(_{0.27}\)K\(_{0.73}\)Fe\(_2\)As\(_2\) single crystals

Abstract

Over the last years a lot of theoretical and experimental efforts have been made to find states with broken time reversal symmetry (BTRS) in multi-band superconductors. In particular, it was theoretically proposed that in the Ba\(_{1-x}\)K\(_{x}\)Fe\(_2\)As\(_2\) system either an \(s+is\) or an \(s+id\) BTRS state may exist at high doping levels in a narrow region of the phase diagram. Here we report the observation of an enhanced zero field muon spin relaxation rate below the superconducting transition temperature for a high quality crystalline sample with \(x \approx\) 0.73. This indicates that indeed the time reversal symmetry is broken in superconducting Ba\(_{1-x}\)K\(_{x}\)Fe\(_2\)As\(_2\) at this doping level.