Abstract

Frustration in spin systems can lead to quantum spin liquid ground states, devoid of spontaneous symmetry breaking, supporting fractionalized excitations and topological order. With the Kitaev honeycomb model as a exactly solvable and realistic model for such a quantum spin liquid at hand, it is of particular interest to study what novel phases may arise when coupling additional degrees of freedom to such strongly frustrated systems.
In my talk, I will introduce the Kitaev Kondo lattice, where conduction electrons couple via a Kondo interaction to the local moments in the Kitaev model and argue that at small Kondo couplings a fractionalized Fermi liquid is realized, a stable non-Fermi liquid where electronic quasiparticles coexist with the deconfined excitations of the spin liquid. Utilizing Majorana-fermion mean-field theory to map out the phase diagram, we remarkably find nematic triplet superconducting phases which mask the quantum phase transition between fractionalized and conventional Fermi liquid phases. Their pairing structure is inherited from the Kitaev spin liquid, i.e., superconductivity is driven by Majorana glue.
Moreover, I will consider bilayer Kitaev systems, where two Kitaev honeycomb models are coupled via an interlayer Heisenberg interaction. Varying interlayer coupling and Kitaev coupling anisotropy, we find both direct transitions from the spin liquid to a trivial dimer paramagnet as well as intermediate "macrospin" phases, which can be studied by mappings to effective transverse-field Ising models. Further, we find a novel interlayer coherent pi-flux phase.