  In this paper, based on a non-Hermitian toric-code model, we surprisingly
find that the degeneracy of ground states can be changed by a local
non-Hermitian perturbation (even in thermodynamic limit). We call it
non-Hermitian avalanche effect. As the physics consequences of the
non-Hermitian avalanche effect, a correspondence between bulk quasi-particles
and topologically protected degenerate ground states for Z2 topological order
is borken down. In addition, the PT symmetry breaking transition of the
topologically degenerate ground states subspace can be observed by fidelity
susceptibility.
