Many-body excitations in trapped Bose gas: A non-Hermitian view

TL;DR

This paper introduces a non-Hermitian framework for analyzing many-body excitations in a trapped Bose gas, deriving excitation spectra and eigenstates through a novel operator approach inspired by earlier theoretical work.

Contribution

It develops a non-Hermitian method using a Riccati equation for pair-excitation kernels to analyze excited states in a dilute Bose gas, connecting to classical excitation theories.

Findings

Derived a nonlocal equation for low-lying excitations.

Recovered Fetter's excitation spectrum.

Established an existence theory for the Riccati operator equation.

Abstract

We provide the analysis of a physically inspired model for a trapped dilute Bose gas with repulsive pairwise atomic interactions at zero temperature. Our goal is to describe aspects of the excited many-body quantum states by accounting for the scattering of atoms in pairs from the macroscopic state (condensate). We formally construct a many-body Hamiltonian, $\mathcal{H}_{\text{app}}$, that is quadratic in the Boson field operators for noncondensate atoms. This $\mathcal{H}_{\text{app}}$ conserves the total number of atoms. Inspired by Wu (J. Math. Phys., 2:105-123, 1961), we apply a non-unitary transformation to $\mathcal{H}_{\text{app}}$. Key in this non-Hermitian view is the pair-excitation kernel, which in operator form obeys a Riccati equation. In the stationary case, we develop an existence theory for solutions to this operator equation by a variational approach. We connect this theory to the one-particle excitation wave functions heuristically derived by Fetter (Ann. Phys., 70:67-101, 1972). These functions solve an eigenvalue problem for a $J$-self-adjoint operator. From the non-Hermitian Hamiltonian, we derive a one-particle nonlocal equation for low-lying excitations, describe its solutions, and recover Fetter's excitation spectrum. Our approach leads to a description of the excited eigenstates of the reduced Hamiltonian in the $N$-particle sector of Fock space.