Stability of j=3/2 Bogoliubov Fermi surfaces under external fields

Abstract Stimulated by recent debates on the ultranodal state in FeSe, we examine the stability of Bogoliubov Fermi Surface (BFS) under Zeeman fields. As a simple model, we consider a spherical j = 3/2 model introduced by Agterberg et al. [Phys. Rev. Lett. 118 , 127001 (2017)]. By solving the BCS gap equation, we present the H − T phase diagram, showing a first-order transition in the high-field regime. Furthermore, we discuss the evolution of the BFS under Zeeman fields. Since the BFS does not necessarily consist of symmetry-protected nodes, its evolution is nontrivial; BFSs become larger in Fermi surfaces with significant spin splitting under Zeeman fields. Such BFS evolution can be observed using angle-resolved photoemission spectroscopy.