Abstract
Bogoliubov Fermi surfaces are contours of zero-energy excitations that are protected in the superconducting state. Here we show that multiband superconductors with dominant spin singlet, intraband pairing of spin-1/2 electrons can undergo a transition to a state with Bogoliubov Fermi surfaces if spin-orbit coupling, interband pairing and time reversal symmetry breaking are also present. These latter effects may be small, but drive the transition to the topological state for appropriate nodal structure of the intra-band pair. Such a state should display nonzero zero-bias density of states and corresponding residual Sommerfeld coefficient as for a disordered nodal superconductor, but occurring even in the pure case. We present a model appropriate for iron-based superconductors where the topological transition associated with creation of a Bogoliubov Fermi surface can be studied. The model gives results that strongly resemble experiments on FeSe1−xSx across the nematic transition, where this ultranodal behavior may already have been observed.
Highlights
Bogoliubov Fermi surfaces are contours of zero-energy excitations that are protected in the superconducting state
There has been an ongoing discussion about the possibility of time-reversal symmetry breaking (TRSB) states in the Fe-based materials
It has been pointed out that such terms can be generated by spin-orbit coupling in iron based systems[3,4]. Since both spin-orbit effects and time reversal symmetry breaking can individually lead to deviations from the s± paradigm, it is interesting to investigate their interplay, in the context of unusual phenomena observed in the Fe chalcogenides
Summary
Bogoliubov Fermi surfaces are contours of zero-energy excitations that are protected in the superconducting state. We show that multiband superconductors with dominant spin singlet, intraband pairing of spin-1/2 electrons can undergo a transition to a state with Bogoliubov Fermi surfaces if spin-orbit coupling, interband pairing and time reversal symmetry breaking are present. These latter effects may be small, but drive the transition to the topological state for appropriate nodal structure of the intra-band pair. Since both spin-orbit effects and time reversal symmetry breaking can individually lead to deviations from the s± paradigm, it is interesting to investigate their interplay, in the context of unusual phenomena observed in the Fe chalcogenides
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