Enhanced eigenvector sensitivity and algebraic classification of sublattice-symmetric exceptional points

TL;DR

This paper investigates higher-order exceptional points in fermionic systems with sublattice symmetry, revealing the existence of odd-order EPs and their enhanced sensitivity, along with an algebraic framework to classify these degeneracies.

Contribution

It introduces an algebraic method to identify and classify odd-order exceptional points in sublattice-symmetric systems, expanding understanding of EP topology and sensitivity.

Findings

Odd-order EPs can exist in sublattice-symmetric fermionic systems.

Enhanced eigenvector sensitivity occurs near odd-order EPs.

New algebraic conditions for the existence of higher-order EPs are provided.

Abstract

Exceptional points (EPs) are degeneracy of non-Hermitian Hamiltonians, at which the eigenvalues, along with their eigenvectors, coalesce. Their orders are given by the Jordan decomposition. Here, we focus on higher-order EPs arising in fermionic systems with a sublattice symmetry, which restricts the eigenvalues of the Hamitlonian to appear in pairs of $\lbrace E, -E\rbrace $. Thus, a naive prediction might lead to only even-order EPs at zero energy. However, we show that odd-order EPs can exist and exhibit enhanced sensitivity in the behaviour of eigenvector-coalescence in their neighbourhood, depending on how we approach the degenerate point. The odd-order EPs can be understood as a mixture of higher- and lower-valued even-order EPs. Such an anomalous behaviour is related to the irregular topology of the EPs as the subspace of the Hamiltonians in question, which is a unique feature of the Jordan blocks. The enhanced eigenvector sensitivity can be described by observing how the quantum distance to the target eigenvector converges to zero. In order to capture the eigenvector-coalescence, we provide an algebraic method to describe the conditions for the existence of these EPs. This complements previous studies based on resultants and discriminants, and unveils heretofore unexplored structures of higher-order exceptional degeneracy.