Symmetry-preserving boundary of (2+1)D fractional quantum Hall states

Abstract

We investigate symmetry-preserving gapped boundary of (2+1)D topological phases with global symmetry, which can be either bosonic or fermionic. We develop a general algebraic description for gapped boundary condition for symmetry-enriched or fermionic topological phases, extending the framework of Lagrangian algebra anyon for bosonic phases without symmetry. We then focus on application to the case with U(1) symmetry. We derive new obstructions to symmetry-preserving gapped boundary for U(1)$^f$-symmetric (2+1)D fermionic topological phases, which are beyond chiral central charge $c_-$ and electric Hall conductivity $\sigma_H$. These obstructions are given by a simple Gauss-Milgram type formula valid for super-modular category, and regarded as a higher version of $c_-$ and $\sigma_H$.