Collective and single-particle excitations in 2D dipolar Bose gases

TL;DR

This study investigates the temperature and coupling strength effects on excitation spectra in 2D dipolar Bose gases, revealing sharp resonances in superfluid phases and damping in normal phases, thus extending zero-temperature analyses.

Contribution

It provides a detailed analysis of density and single-particle excitation spectra at finite temperatures, using advanced reconstruction methods, and discusses the hybridization and damping phenomena in different phases.

Findings

Sharp energy resonances in superfluid phase

Damped density modes in normal phase

Generalization of zero-temperature excitation spectra

Abstract

The Berezinskii-Kosterlitz-Thouless transition in 2D dipolar systems has been studied recently by path integral Monte Carlo (PIMC) simulations [A. Filinov et al., PRL 105, 070401 (2010)]. Here, we complement this analysis and study temperature-coupling strength dependence of the density (particle-hole) and single-particle (SP) excitation spectra both in superfluid and normal phases. The dynamic structure factor, S(q,omega), of the longitudinal excitations is rigorously reconstructed with full information on damping. The SP spectral function, A(q,omega), is worked out from the one-particle Matsubara Green's function. A stochastic optimization method is applied for reconstruction from imaginary times. In the superfluid regime sharp energy resonances are observed both in the density and SP excitations. The involved hybridization of both spectra is discussed. In contrast, in the normal phase, when there is no coupling, the density modes, beyond acoustic phonons, are significantly damped. Our results generalize previous zero temperature analyses based on variational many-body wavefunctions [F. Mazzanti et al., PRL 102, 110405 (2009), D. Hufnagl et al., PRL 107, 065303 (2011)], where the underlying physics of the excitation spectrum and the role of the condensate has not been addressed.