Spacetime as a topological insulator: Mechanism for the origin of the   fermion generations
  We suggest a mechanism whereby the three generations of quarks and leptons correspond to surface modes in a five-dimensional theory. These modes arise from a nonlinear fermion dispersion relation in the extra dimension, much in the same manner as fermion surface modes in a topological insulator or lattice implementation of domain wall fermions. We also show that the topological properties can persist in a deconstructed version of the model in four dimensions. 
David B. Kaplan and Sichun Sun
hep-ph cond-mat.mes-hall hep-th