Many-body effects in the excitations and dynamics of trapped Bose-Einstein condensates

TL;DR

This review discusses many-body effects in the excitations and dynamics of trapped Bose-Einstein condensates, highlighting the importance of beyond mean-field methods like MCTDHB and LR-MCTDHB for accurate results.

Contribution

It introduces and benchmarks the linear-response theory atop MCTDHB for calculating excitation spectra, revealing significant many-body effects beyond mean-field approximations.

Findings

Many-body effects significantly alter excitation spectra.

Dynamics such as tunneling and vortices are inherently many-body.

Bosons with angular momentum are more sensitive to many-body effects.

Abstract

This review explores the dynamics and the low-energy excitation spectra of Bose-Einstein condensates (BECs) of interacting bosons in external potential traps putting particular emphasis on the emerging many-body effects beyond mean-field descriptions. To do so, methods have to be used that, in principle, can provide numerically exact results for both the dynamics and the excitation spectra in a systematic manner. Numerically exact results for the dynamics are presented employing the well-established multicongurational time-dependent Hartree for bosons (MCTDHB) method. The respective excitation spectra are calculated utilizing the more recently introduced linear-response theory atop it (LR-MCTDHB). The latter theory gives rise to an, in general, non-hermitian eigenvalue problem. The theory and its newly developed implementation are described in detail and benchmarked towards the exactly-solvable harmonic-interaction model. Several applications to BECs in one- and two-dimensional potential traps are discussed. With respect to dynamics, it is shown that both the out-of-equilibrium tunneling dynamics and the dynamics of trapped vortices are of many-body nature. Furthermore, many-body effects in the excitation spectra are presented for BECs in different trap geometries. It is demonstrated that even for essentially-condensed systems, the spectrum of the lowest-in-energy excitations computed at the many-body level can differ substantially from the standard mean-field description. In general, it is shown that bosons carrying angular momentum are more sensitive to many-body effects than bosons without. These effects are present in both the dynamics and the excitation spectrum.