# Topological Ultranodal pair states in iron-based superconductors

## TL;DR

This paper predicts a topological transition to a state with Bogoliubov Fermi surfaces in multiband superconductors, explaining experimental observations in FeSe$_{1-x}$S$_x$ and revealing a new class of ultranodal superconducting states.

## Contribution

It introduces a theoretical model showing how spin-orbit coupling, interband pairing, and time reversal symmetry breaking induce topological ultranodal states in iron-based superconductors.

## Key findings

- Model reproduces experimental features in FeSe$_{1-x}$S$_x$
- Predicts residual density of states in pure superconductors
- Identifies conditions for topological transition to ultranodal state

## 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 FeSe$_{1-x}$S$_x$ across the nematic transition, where this ultranodal behavior may already have been observed.