It is commonly believed that, in the absence of disorder or an external magnetic field, there are three possible types of superconducting excitation gaps: The gap is nodeless, it has point nodes, or it has line nodes. Here, we show that, for an even-parity nodal superconducting state which spontaneously breaks time-reversal symmetry, the low-energy excitation spectrum generally does not belong to any of these categories; instead, it has extended Bogoliubov Fermi surfaces [1,2]. These Fermi surfaces are topologically protected from being gapped by a non-trivial Z2 invariant. In this talk, I will discuss the physical origin, topological protection, and energetic stability of these Bogoliubov Fermi surfaces, using superconductivity in j=3/2 fermions as a representative example.
1. D.F. Agterberg, P.M.R. Brydon, and C. Timm, Phys. Rev. Lett. 118, 127001 (2017) .
2. P. M. R. Brydon, D. F. Agterberg, H. Menke, and C. Timm, arXiv:1806.03773.