Superconductivity with broken time-reversal symmetry in ion irradiated Ba_0.27K_0.73Fe₂As₂ single crystals

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−xK_xFe₂As₂ 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 ≈ 0.73. This indicates that indeed the time reversal symmetry is broken in superconducting Ba_1−xK_xFe₂As₂ at this doping level.