s+is’ state and BCS-BEC effects in Fe-based superconductors

An important characteristic of many iron-based superconductors is the relative smallness of the Fermi surface sizes and doping-induced Lifshitz transitions. In some cases, the Fermi energies of some of the bands are of the order of the superconducting gaps, suggesting the possible relevance of BCS-BEC crossover effects. I have shown analytically, that they are important in the case of competing superconducting instabilities present; namely the region where the system breaks time-reversal symmetry due to coexistence (s+is’ state) is significantly extended (compare the black dashed line and the colored region in the figure) and found always to occur close to a Lifshitz transition (see inset). I have also analyzed the qualitative signatures of the TRSB state that can be observed in scanning tunneling microscopy experiments. The existence of TRSB state near the Lifshitz transition has been shown experimentally in Ba_1-xK_x Fe₂As₂ and I have contributed to the interpretation of the thermodynamic data in that work.