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PT-symmetry breaking with divergent potentials: lattice and continuum cases

We investigate the parity- and time-reversal ($\mathcal{PT}$)-symmetry breaking in lattice models in the presence of long-ranged, non-hermitian, $\mathcal{PT}$-symmetric potentials that remain finite or become divergent in the continuum limit. By scaling analysis of the fragile $\mathcal{PT}$ threshold for an open finite lattice, we show that continuum loss-gain potentials $V_α(x)\propto i |x|^α\mathrm{sign}(x)$ have a positive $\mathcal{PT}$-breaking threshold for $α>-2$, and a zero threshold for $α\leq -2$. When $α<0$ localized states with complex (conjugate) energies in the continuum energy-band occur at higher loss-gain strengths. We investigate the signatures of $\mathcal{PT}$-symmetry breaking in coupled waveguides, and show that the emergence of localized states dramatically shortens the relevant time-scale in the $\mathcal{PT}$-symmetry broken region.