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.

5 pages, 3 figures, and supplementary materials

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