Gapless Hamiltonians for the toric code using the projected entangled pair state formalism

Phys Rev Lett. 2012 Dec 28;109(26):260401. doi: 10.1103/PhysRevLett.109.260401. Epub 2012 Dec 26.

Abstract

We study Hamiltonians which have Kitaev's toric code as a ground state, and show how to construct a Hamiltonian which shares the ground space of the toric code, but which has gapless excitations with a continuous spectrum in the thermodynamic limit. Our construction is based on the framework of projected entangled pair states, and can be applied to a large class of two-dimensional systems to obtain gapless "uncle Hamiltonians."