Joint first-order superfluid transition of a strongly ferromagnetic spin-1 Bose gas

The strongly ferromagnetic spin-1 Bose-Einstein condensate (BEC) has recently been realized with atomic ⁷Li [1]. It was earlier predicted that the ferromagnetic interaction, if sufficiently strong relativet to the spin-independent part of the potential, can drive the normal gas into a magnetized phase at a temperature above the superfluid transition [2]. In light of our recent negative result in the context of an unconventional superconductor [3], we re-examine this theoretical proposal and conclude that there exists no stable normal, magnetized phase for a dilute ferromagnetic Bose gas. For ⁷Li, we predict that the normal gas undergoes a joint first order transition and jump directly into a state with finite superfluid density. We estimate the size of the first order jump, and examine how a non-zero initial magnetic moment of the gas affects the first order transition.

[1] Huh, Kim, Kwon, and Choi. 2020, Physical Review Research 2 (3): 033471.

[2] Natu and Mueller 2011,  Physical Review. A 84 (5): 053625.

[3] How and Yip. 2023. Physical Review. B, Condensed Matter 107 (10): 104514.