Magnetic field effects on the Kitaev model coupled to environment

Abstract

Open quantum systems display unusual phenomena not seen in closed systems, such as new topological phases and unconventional phase transitions. An interesting example was studied for a quantum spin liquid in the Kitaev model [Phys. Rev. Lett. 126, 077201 (2021)]; an effective non-Hermitian Kitaev model, which incorporates dissipation effects, was shown to give rise to a gapless spin liquid state with exceptional points in the Majorana dispersions. Given that an external magnetic field induces a gapped Majorana topological state in the Hermitian case, the exceptional points may bring about intriguing quantum phenomena under a magnetic field. Here we investigate the non-Hermitian Kitaev model perturbed by a magnetic field. We show that the exceptional points remain gapless up to a nonzero critical magnetic field, in stark contrast with the Hermitian case where an infinitesimal field opens a gap. The gapless state is stable over a wide range of the magnetic field for some particular parameter sets, and, in special cases, undergoes topological transitions to another gapless state with different winding number around the exceptional points without opening a gap. In addition, in the system with edges, we find that the non-Hermitian skin effect is induced by the magnetic field, even for the parameters where the skin effect is absent at zero field. The chirality of edge states is switched through the exceptional points, similarly to the surface Fermi arcs connected by the Weyl points in three-dimensional Weyl semimetals. Our results provide a new possible route to stabilize topological gapless quantum spin liquids under the magnetic field in the presence of dissipation.