Green's Functions of Non‐Hermitian Systems at Exceptional Deficiency

ABSTRACT

Exceptional deficiency (ED) is a newly discovered broadband non‐Hermitian condition, at which the system's spectrum is entirely composed of exceptional points and all eigenvectors pairwise coalesce. Here, we analyze the steady‐state and time‐domain responses of non‐Hermitian lattices at ED by considering their frequency‐ and time‐domain Green's functions (GFs). We show that the frequency‐domain GFs are characterized by the emergence of second‐order poles across the entire spectrum, producing broadband second‐order super‐Lorentzian line shapes, enhanced magnitude, and sequential 2π phase shifts across each resonance. These second‐order poles also underpin unconventional responses to global dissipation, by which we uncover a loss‐revival of defective skin effect (LRDSE): the skin modes that are “missing” due to the system's defectiveness at ED can re‐emerge in the responses. In the time‐domain responses, a linear‐in‐time amplification factor is identified in the evolution kernel, which can enhance skin‐effect dynamics that would otherwise be suppressed by global loss. The amplification strength scales with the degree of defectiveness of the system. Our work establishes a theoretical framework for analyzing steady‐state and dynamic signatures of ED, and opens new routes for dissipation‐controlled broadband exceptional responses.