Majorana fermions on a disordered triangular lattice
  Vortices of several condensed matter systems are predicted to have zero-energy core excitations which are Majorana fermions. These exotic quasi-particles are neutral, massless, and expected to have non-Abelian statistics. Furthermore, they make the ground state of the system highly degenerate. For a large density of vortices, an Abrikosov lattice is formed, and tunneling of Majorana fermions between vortices removes the energy degeneracy. In particular the spectrum of Majorana fermions in a triangular lattice is gapped, and the Hamiltonian which describes such a system is antisymmetric under time-reversal. We consider Majorana fermions on a disordered triangular lattice. We find that even for very weak disorder in the location of the vortices localized sub-gap modes appear. As the disorder becomes strong, a percolation phase transition takes place, and the gap is fully closed by extended states. The mechanism that underlies these phenomena is domain walls between two time-reversed phases, which are created by flipping the sign of the tunneling matrix elements. The density of states in the disordered lattice seems to diverge at zero energy. 
Yaacov E. Kraus and Ady Stern
cond-mat.str-el cond-mat.other