Phase diagram of a Bose gas near a wide Feshbach resonance

TL;DR

This paper maps the phase diagram of a homogeneous Bose gas near a wide Feshbach resonance at zero temperature, revealing stability regimes and the nature of atomic and molecular condensates.

Contribution

It provides a detailed analysis of the stability and phase transitions of atomic and molecular BEC states near a wide Feshbach resonance, including critical scattering lengths.

Findings

Atomic BEC becomes metastable beyond a critical scattering length.

Molecular BEC state can be unstable as a coherent mixture.

Many-body effects reduce the binding energy of diatomic molecules.

Abstract

In this paper, we study the phase diagram of a homogeneous Bose gas with a repulsive interaction near a wide Feshbach resonance at zero temperature. The Bose-Einstein-condensation (BEC) state of atoms is a metastable state. When the scattering length $a$ exceeds a critical value depending on the atom density $n$, $na^3>0.035$, the molecular excitation energy is imaginary and the atomic BEC state is dynamically unstable against molecule formation. The BEC state of diatomic molecules has lower energy, where the atomic excitation is gapped and the molecular excitation is gapless. However when the scattering length is above another critical value, $na^3>0.0164$, the molecular BEC state becomes a unstable coherent mixture of atoms and molecules. In both BEC states, the binding energy of diatomic molecules is reduced due to the many-body effect.