Ultra-cold fermions in real or fictitious magnetic fields: The BCS-BEC evolution and the type-I--type-II transition

TL;DR

This paper investigates ultra-cold fermion superfluids under magnetic fields, revealing a transition from type-I to type-II superfluidity in charged systems and highlighting the robustness of neutral superfluids like lithium and potassium in the BCS-BEC crossover.

Contribution

It demonstrates the magnetic phase transition in charged superfluids and characterizes the extreme type-II nature and robustness of neutral fermion superfluids across the BCS-BEC crossover.

Findings

Charged superfluids transition from type-I to type-II with increasing interaction.

Neutral superfluids like $^6$Li and $^{40}$K are extreme type-II superfluids.

Critical fictitious magnetic field peaks near unitarity in the BCS-BEC crossover.

Abstract

We study ultra-cold neutral fermion superfluids in the presence of fictitious magnetic fields, as well as charged fermion superfluids in the presence of real magnetic fields. Charged fermion superfluids undergo a phase transition from type-I to type-II superfluidity, where the magnetic properties of the superfluid change from being a perfect diamagnet without vortices to a partial diamagnet with the emergence of the Abrikosov vortex lattice. The transition from type-I to type-II superfluidity is tunned by changing the scattering parameter (interaction) for fixed density. We also find that neutral fermion superfluids such as $^6$Li and $^{40}$K are extreme type-II superfluids, and that they are more robust to the penetration of a fictitious magnetic field in the BCS-BEC crossover region near unitarity, where the critical fictitious magnetic field reaches a maximum as a function of the scattering parameter (interaction).