Princeton University

School of Engineering & Applied Science

Exotic quantum phase transitions of 2+1d Dirac fermions, and connections to 2d and 3d topological insulators

Speaker: 
Kevin Slagle, Dept. of Physicis, UC Santa Barbara
Location: 
E-Quad B205
Date/Time: 
Monday, October 19, 2015 - 1:30pm

Abstract: Using determinant quantum Monte Carlo simulations, we demonstrate that an extended Hubbard model on a bilayer honeycomb lattice has two novel quantum phase transitions, each with connections to symmetry protected topological states (SPT). 1) The first is a continuous phase transition between the weakly interacting gapless Dirac fermion phase and a strongly interacting fully gapped and symmetric trivial phase. Because there is no spontaneous symmetry breaking, this transition cannot be described by the standard Gross-Neveu model. We argue that this phase transition is related to the Z_16 classification of the topological superconductor 3He-B phase with interactions. 2) The second is a quantum critical point between a quantum spin Hall insulator with spin S^z conservation and the previously mentioned strongly interacting gapped trivial phase. This transition can also be viewed as a direct transition between a bosonic SPT and a trivial state. At the critical point the single particle excitations remain gapped, while spin and charge gaps close. We argue that this transition is described by a bosonic O(4) nonlinear sigma model field theory with a topological Theta-term.