  Symmetry protected topological (SPT) phases in free fermion and interacting
bosonic systems have been classified, but the physical phenomena of interacting
fermionic SPT phases have not been fully explored. Here, employing large-scale
quantum Monte Carlo simulation, we investigate the edge physics of a bilayer
Kane-Mele-Hubbard model with zigzag ribbon geometry. Our unbiased numerical
results show that the fermion edge modes are gapped out by interaction, while
the bosonic edge modes remain gapless at the $(1+1)d$ boundary, before the bulk
quantum phase transition to a topologically trivial phase. Therefore, finite
fermion gaps both in the bulk and on the edge, together with the robust gapless
bosonic edge modes, prove that our system becomes an emergent bosonic SPT phase
at low energy, which is, for the first time, directly observed in an
interacting fermion lattice model.
