Resonant-amplified and invisible Bragg scattering based on spin coalescing modes

Unlike a real magnetic field, which separates the energy levels of particle with opposite spin polarization, a complex field can lead to a special kind of spectral degeneracy, known as exceptional point (EP), at which two spin eigenmodes coalesce. It allows an EP impurity to be an invisible scattering center for a fermion with the resonant spin polarization, but an amplifying emitter for opposite polarization. We show that a pair of conjugate EP modes supports resonant mutual stimulation, acting as a resonant amplifier based on the underlying mechanism of positive-feedback loop. Together with other Hermitian eigenmodes, a fermion with EP polarization exhibits some exclusive dynamics, referred to as EP dynamics. We construct several typical superlattices, which are built up by embedding EP-impurity arrays in a Hermitian two-dimensional square lattice. Numerical simulations are performed to demonstrate resonant amplification and invisibility of Bragg scattering.