Near-zero modes in condensate phases of the Dirac theory on the honeycomb lattice

Abstract

We investigate a number of fermionic condensate phases on the honeycomb lattice to determine whether topological defects (vortices and edges) in these phases can support bound states with zero energy. We argue that topological zero modes bound to vortices and at edges are not only connected, but should in fact be identified. Recently, it has been shown that the simplest $s$-wave superconducting state for the Dirac fermion approximation of the honeycomb lattice at precisely half filling, supports zero modes inside the cores of vortices [P. Ghaemi and F. Wilczek, arXiv:0709.2626 (unpublished)]. We find that within the continuum Dirac theory the zero modes are not unique, either to this phase or to half filling. In addition, we find the exact wave functions for vortex bound zero modes, as well as the complete edge state spectrum of the phases we discuss. The zero modes in all the phases we examine have even-numbered degeneracy, and as such pairs of any Majorana modes are simply equivalent to one ordinary fermion. As a result, contrary to bound-state zero modes in ${p}_{x}+i{p}_{y}$ superconductors, vortices here do not exhibit non-Abelian exchange statistics. The zero modes in the pure Dirac theory are seemingly topologically protected by the effective low-energy symmetry of the theory, yet on the original honeycomb lattice model these zero modes are split, by explicit breaking of the effective low-energy symmetry.