Landau-forbidden quantum phase transitions between bosonic Z2 symmetry-protected topological phases in 2+1D

We perform a Quantum Monte Carlo study of quantum phase transitions between different classes of Z2 bosonic symmetry-protected topological (SPT) phases in 2+1D. This is made possible by an advantageous choice of basis which takes care of the sign problem that one might naively expect. Depending on the classes of SPTs considered, we find a variety of unusual intermediate symmetry-broken phases, as well as evidence for a quantum critical direct transition.