Emergent localized states in twofold PT-symmetric ladder lattice.

We have investigated the localized states at the interface of intertwined PT-symmetric systems which are robust against external pumping and damping. The intertwinement induces an additional PT-symmetry and interface exceptional points (EP) emerge and play the role of new critical points of non-Hermitian phenomena with bulk exceptional point. There are two distinct regimes possessing solidly localized states even though balanced gain and loss is increased. We obtain analytic expressions for the localization length and exceptional points in terms of tight-binding formalism. This theory is demonstrated by numerical calculation of eigenenergies and wave functions on quasi 1D ladder lattice and 2D bi-layered square lattice.