Bogoliubov Fermi Surfaces in Superconductors with Broken Time-Reversal Symmetry

D. F. Agterberg; P. M. R. Brydon; C. Timm

doi:10.1103/PhysRevLett.118.127001

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Bogoliubov Fermi Surfaces in Superconductors with Broken Time-Reversal Symmetry

D. F. Agterberg, P. M. R. Brydon, C. Timm

DOI 10.1103/PhysRevLett.118.127001 · Physical Review Letters

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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 Z2 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 Z2-protected Fermi surfaces to be ubiquitous.

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