Collective modes across the soliton-droplet crossover in binary Bose mixtures

TL;DR

This paper investigates the collective excitations in binary Bose mixtures during the transition from soliton to droplet states, revealing distinct behaviors and emission mechanisms across regimes, with implications for experimental observations.

Contribution

It introduces a beyond-mean-field model with a variational approach to analyze collective modes across the soliton-droplet crossover in binary Bose gases.

Findings

Soliton regime shows smooth mode variation with particle number.

Droplet regime exhibits inhibited modes due to particle emission.

Monopole frequency may be observable in a specific particle number and interaction window.

Abstract

We study the collective modes of a binary Bose mixture across the soliton to droplet crossover in a quasi one dimensional waveguide with a beyond-mean-field equation of state and a variational Gaussian ansatz for the scalar bosonic field of the corresponding effective action. We observe a sharp difference in the collective modes in the two regimes. Within the soliton regime modes vary smoothly upon the variation of particle number or interaction strength. On the droplet side collective modes are inhibited by the emission of particles. This mechanism turns out to be dominant for a wide range of particle numbers and interactions. In a small window of particle number range and for intermediate interactions we find that monopole frequency is likely to be observed. In the last part we focus on the spin-dipole modes for the case of equal intraspecies interactions and equal equilibrium particle numbers in the presence of a weak longitudinal confinement. We found that such modes might be unobservable in the real-time dynamics close to the equilibrium as their frequency is higher than the particle emission spectrum by at least one order of magnitude in the droplet phase. Our results are relevant for experiments with two-component BECs for which we provide realistic parameters.