Supercharging exceptional points: full-spectrum pairwise coalescence in non-Hermitian systems

Abstract

We consider non-Hermitian tight-binding one-dimensional Hamiltonians and show that imposing a certain symmetry causes all eigenvalues to pair up and the corresponding eigenstates to coalesce in pairs. This pairwise coalescence (PC) is an enhanced version of an exceptional point-the complete spectrum pairs up, not just one pair of eigenstates. The symmetry is that of reflection excluding the central two sites, and allowing flipping of nonreciprocal hoppings ("generalized off-center reflection symmetry"). Two simple examples of PC exist in the literature-our construction encompasses these examples and extends them to a vast class of Hamiltonians. We study several families of such Hamiltonians, extend to cases of full-spectrum higher-order coalescences, and show how the PC point corresponds to amplified nonorthogonality of the eigenstates and enhanced loss of norm in time evolution.