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

## TL;DR

This study provides experimental evidence of broken time reversal symmetry in high-doped Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ superconductors, supporting theoretical predictions of exotic superconducting states in this material.

## Contribution

First experimental observation of BTRS in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ at high doping levels, confirming theoretical models.

## Key findings

- Enhanced muon spin relaxation rate below Tc
- Indication of BTRS in superconducting state
- Supports existence of exotic pairing states

## 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.