Description
It is commonly believed that superconductors fall into one of
three classes: They can have a full energy gap, a gap with
point nodes, or a gap with line nodes. We have shown in [1]
that multiband, even-parity, nodal superconducting states
that break time-reversal symmetry do not belong to these
classes. Instead, they generically possess two-dimensional
Fermi surfaces. These Fermi surfaces are generated by ?
inflating? point and line nodes into spheroidal and toroidal
pockets, respectively, by a pseudomagnetic field resulting
from interband pairing. Such states can be energetically
stable in spite of the extended Fermi surfaces. The inflated
nodes are topologically protected by a Z2 invariant, which we
give in terms of a Pfaffian [1]. In addition, inflated point
nodes retain a nontrivial Chern number, while inflated line
nodes are characterized by a second Pfaffian. Finally, I will
discuss the surface states and their interplay with the inflated
nodes in multiband superconductors.
[1] D. F. Agterberg, P. M. R. Brydon, and C. Timm, Phys. Rev.
Lett. 118, 127001 (2017).