The fourth superconducting gap: intrinsic Bogoliubov Fermi surfaces

The superconductivity of cubic semimetals such as YPtBi represents a new paradigm for the interplay of Cooper pairing with strong spin-orbit coupling. The j=3/2 intrinsic angular momentum of the low-energy fermions leads to qualitative differences from the conventional theory of pairing between j=1/2 states. In particular, this permits Cooper pairs with quintet or septet total angular momentum, in addition to singlet and triplet states. Purely on-site interactions can generate s-wave quintet time-reversal symmetry-breaking states which mimic chiral d-wave gaps [1]; remarkably, however, the expected point and line nodes are replaced by intrinsic Fermi surfaces of Bogoliubov quasiparticles [2]. This effect arises from the strong spin-orbit coupling, which produces interband pairing potentials; these in turn generate an effective magnetic-field-like term, which “inflates” the point or line nodes into spheroids or tori, respectively. These Fermi surfaces can be energetically stable, and are topologically protected by particle-hole and inversion symmetries; “accidental” Fermi surfaces can also persist when inversion is weakly broken [3]. Our theory can be readily generalized to other systems where j=3/2 fermions are not present, e.g. URu₂Si₂ and SrPtAs, and so we expect these Fermi surfaces to be a ubiquitous feature of even-parity time-reversal symmetry-breaking superconductivity.

[1] P. M. R. Brydon, L. M. Wang, M. Weinert, and D. F. Agterberg, Phys. Rev. Lett. 116, 177001 (2016).
[2] D. F. Agterberg, P. M. R. Brydon, and C. Timm, Phys. Rev. Lett. 118, 127001 (2017).
[3] C. Timm, A. P. Schnyder, D. F. Agterberg, and P. M. R. Brydon, Phys. Rev. B 96, 096526 (2017).