Boundary-dependent dynamical instability of bosonic Green's function: Dissipative Bogoliubov-de Gennes Hamiltonian and its application to non-Hermitian skin effect

TL;DR

This paper explores boundary-dependent dynamical instability in bosonic systems, revealing how non-Hermitian effects from both bosonic BdG structure and open quantum systems influence spectral behavior, especially under the non-Hermitian skin effect.

Contribution

It introduces a boundary-dependent dynamical instability model combining bosonic BdG Hamiltonian and dissipative effects, highlighting the interplay of two non-Hermitian sources.

Findings

Imaginary spectrum parts can be positive without amplification.

Boundary conditions strongly affect the spectrum due to the non-Hermitian skin effect.

The model demonstrates how non-Hermitian effects lead to dynamical instability.

Abstract

The energy spectrum of bosonic excitations from a condensate is given by the spectrum of a non-Hermitian Hamiltonian constructed from a bosonic Bogoliubov-de Gennes (BdG) Hamiltonian in general even though the system is essentially Hermitian. In other words, two types of non-Hermiticity can coexist: one from the bosonic BdG nature and the other from the open quantum nature. In this paper, we propose boundary-dependent dynamical instability. We first define the bosonic dissipative BdG Hamiltonian in terms of Green's function in Nambu space and discuss the correct particle-hole symmetry of the corresponding non-Hermitian Hamiltonian. We then construct a model of the boundary-dependent dynamical instability so that it satisfies the correct particle-hole symmetry. In this model, an anomalous term that breaks the particle number conservation represents the non-Hermiticity of the BdG nature, while a normal term is given by a dissipative Hatano-Nelson model. Thanks to the competition between the two types of non-Hermiticity, the imaginary part of the spectrum can be positive without the help of the amplification of the normal part and the particle-hole band touching that causes the Landau instability. This leads to the boundary-dependent dynamical instability under the non-Hermitian skin effect, -strong dependence of spectra on boundary conditions for non-Hermitian Hamiltonians-, of the Bogoliubov spectrum.