title: Bogoliubov Fermi Surfaces in Superconductors with Broken Time-Reversal Symmetry.
abstract: 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. These Fermi surfaces can be visualized as two-dimensional surfaces generated by "inflating" point or line nodes into spheroids or tori, respectively. These inflated nodes are topologically protected from being gapped by a Z_{2} invariant, which we give in terms of a Pfaffian. We also show that superconducting states possessing these Fermi surfaces can be energetically stable. A crucial ingredient in our theory is that more than one band is involved in the pairing; since all candidate materials for even-parity superconductivity with broken time-reversal symmetry are multiband systems, we expect these Z_{2}-protected Fermi surfaces to be ubiquitous.
authors.affiliation: Department of Physics, University of Wisconsin, Milwaukee, Wisconsin 53201, USA.
authors.forename: D F
authors.initials: DF
authors.lastname: Agterberg
authors.affiliation: Department of Physics, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand.
authors.forename: P M R
authors.initials: PM
authors.lastname: Brydon
authors.affiliation: Institute of Theoretical Physics, Technische Universität Dresden, 01062 Dresden, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Timm