Competing interactions in population-imbalanced two-component Bose-Einstein condensates

TL;DR

This paper investigates the effects of spin-orbit interactions and thermal fluctuations on two-component Bose-Einstein condensates, revealing multiple phases and phase transitions, including a Kosterlitz-Thouless transition and various modulated states.

Contribution

It provides a comprehensive analysis of competing interactions in population-imbalanced two-component BECs, including the impact of spin-orbit coupling and thermal fluctuations on phase behavior.

Findings

Identification of three distinct phases depending on interaction strengths.

Confirmation of a Kosterlitz-Thouless universality class transition.

Observation of phase modulation and amplitude modulation transitions.

Abstract

We consider a two-component Bose-Einstein condensate with and without synthetic "spin-orbit" interactions in two dimensions. Density- and phase-fluctuations of the condensate are included, allowing us to study the impact of thermal fluctuations and density-density interactions on the physics originating with spin-orbit interactions. In the absence of spin-orbit interactions, we find that inter-component density interactions deplete the minority condensate. The thermally driven phase transition is driven by coupled density and phase-fluctuations, but is nevertheless shown to be a phase-transition in the Kosterlitz-Thouless universality class with close to universal amplitude ratios irrespective of whether both the minority- and majority condensates exist in the ground state, or only one condensate exists. In the presence of spin-orbit interactions we observe three separate phases, depending on the strength of the spin-orbit coupling and inter-component density-density interactions: a phase-modulated phase with uniform amplitudes for small intercomponent interactions, a completely imbalanced, effectively single-component, condensate for intermediate spin-orbit coupling strength and suficciently large inter-component interactions, and a phase-modulated \textit{and} amplitude-modulated phase for sufficiently large values of both the spin-orbit coupling and the inter-component density-density interactions. The phase which is modulated by a single $\bv q$-vector only is observed to transition into an isoptropic liquid through a strong de-pinning transition with periodic boundary conditions, which weakens with open boundaries.