title: Spin-Peierls Instability of Three-Dimensional Spin Liquids with Majorana Fermi Surfaces.
abstract: Three-dimensional (3D) variants of the Kitaev model can harbor gapless spin liquids with a Majorana Fermi surface on certain tricoordinated lattice structures such as the recently introduced hyperoctagon lattice. Here, we investigate Fermi surface instabilities arising from additional spin exchange terms (such as a Heisenberg coupling) which introduce interactions between the emergent Majorana fermion degrees of freedom. We show that independent of the sign and structure of the interactions, the Majorana surface is always unstable. Generically, the system spontaneously doubles its unit cell at exponentially small temperatures and forms a spin liquid with line nodes. Depending on the microscopics, further symmetries of the system can be broken at this transition. These spin-Peierls instabilities of a 3D spin liquid are closely related to BCS instabilities of fermions.
authors.affiliation: Institute for Theoretical Physics, University of Cologne, 50937 Cologne, Germany.
authors.forename: Maria
authors.initials: M
authors.lastname: Hermanns
authors.affiliation: Institute for Theoretical Physics, University of Cologne, 50937 Cologne, Germany.
authors.forename: Simon
authors.initials: S
authors.lastname: Trebst
authors.affiliation: Institute for Theoretical Physics, University of Cologne, 50937 Cologne, Germany.
authors.forename: Achim
authors.initials: A
authors.lastname: Rosch