Collective modes of two-species Bose-Einstein condensates in a Josephson junction barrier

TL;DR

This paper investigates how a Josephson barrier affects the collective excitation spectra of two-species Bose-Einstein condensates, revealing additional Goldstone modes and the impact of interspecies correlations.

Contribution

It provides a theoretical analysis of the influence of barriers on collective modes in two-species BECs, highlighting new Goldstone modes and the role of interspecies interactions.

Findings

Barrier induces additional Goldstone modes in two-species BECs.

Anharmonic potential increases critical barrier strength for mode softening.

Distinct collective responses observed in miscible and immiscible states.

Abstract

The ultracold atoms are an ideal platform to implement atomtronics and Josephson junctions analogous to superconducting circuits. The collective modes of a Bose gas split by a potential barrier have been known. However, the role of barriers on the collective excitation spectra of ultracold atomic mixtures has not been examined. Here, we examine the low-lying collective modes of (an)harmonically trapped quasi-one-dimensional Bose-Einstein condensates in a Josephson barrier by employing the variational approach and Bogoliubov theory. We first show that the anharmonicity of the external potential leads to an increase in the critical barrier strength of mode softening in a single-species condensate. The Josephson barrier drives the softening of in-phase and out-of-phase dipole modes of two-species Bose-Einstein condensates, and consequently leads to two additional zero-energy Goldstone modes in the miscible phase, in agreement with the variational approach. Furthermore, the sandwich immiscible state results in an additional Goldstone mode due to the barrier, in contrast to the spatially symmetry-broken side-by-side profile. Our results unveil the distinct collective response of the Josephson barrier in binary mixtures owing to interspecies atomic correlations.