The cubic code
http://arxiv.org/abs/1112.3252

A  general category theoretic description of domain walls in Levin-Wen models
http://arxiv.org/abs/1104.5047

A lower bound on the energy barrier of a Hamiltonian satisfying no strings
http://arxiv.org/abs/1105.4159

A no-go theorem on self correction in two-dimensons
http://arxiv.org/abs/0810.1983