Spectral Topology and Delocalization in Disordered Hatano-Nelson Chains

TL;DR

This paper explores how disorder affects spectral topology and localization in non-Hermitian Hatano-Nelson chains, revealing topological transitions and delocalized states linked to spectral winding numbers.

Contribution

It uncovers the relationship between spectral topology, disorder strength, and localization properties in disordered Hatano-Nelson chains, highlighting topological transitions and delocalized states.

Findings

Spectral spectrum transitions from a single loop to two loops at a critical disorder.

Eigenstates are subexponentially localized with analytically varying localization length.

Two delocalized states emerge at weak and critical disorder, linked to spectral winding number.

Abstract

The unidirectional Hatano-Nelson chain serves as the fundamental non-Hermitian building block of the Su-Schrieffer-Heeger (SSH) model. We investigate its Anderson localization properties under diagonal binary disorder. For weak disorder, the complex eigenvalue spectrum forms a single closed loop, which bifurcates into two distinct loops at a critical disorder threshold. Correspondingly, the spectral winding number {\nu} undergoes a transition from {\nu} = 1 in the weak-disorder regime, through {\nu} = 1/2 at the critical point, to {\nu} = 0 in the strong-disorder limit. We show that the eigenstates are subexponentially localized, with a localization length that varies analytically as a function of the momentum-like quantum number q. Notably, at weak and critical disorder, the spectrum hosts two completely delocalized states with diverging localization lengths. This divergence is directly correlated with the non-trivial spectral winding number. These findings remain robust under various boundary conditions, with the exception of strictly open boundaries.