Phase Transitions and Pairing Signature in Strongly Attractive Fermi Atomic Gases

TL;DR

This paper analytically studies phase transitions, pairing, and magnetization in a one-dimensional attractive Fermi gas, revealing a phase diagram with singlet, ferromagnetic, and mixed phases, akin to type II superconductors.

Contribution

It provides an exact thermodynamic Bethe ansatz analysis of phase transitions and pairing signatures in a 1D attractive Fermi gas under external fields, detailing the phase diagram and critical phenomena.

Findings

Bound pairs form a singlet ground state at low fields.

Ferromagnetic phase occurs at high external fields.

Mixed phase with coexisting pairs and unpaired atoms exists between critical fields.

Abstract

We investigate pairing and quantum phase transitions in the one-dimensional two-component Fermi atomic gas in an external field. The phase diagram, critical fields, magnetization and local pairing correlation are obtained analytically via the exact thermodynamic Bethe ansatz solution. At zero temperature, bound pairs of fermions with opposite spin states form a singlet ground state when the external field $H < H_{c1}$. A completely ferromagnetic phase without pairing occurs when the external field $H > H_{c2}$. In the region $H_{c1} < H < H_{c2}$ we observe a mixed phase of matter in which paired and unpaired atoms coexist. The phase diagram is reminiscent of that of type II superconductors. For temperatures below the degenerate temperature and in the absence of an external field, the bound pairs of fermions form hard-core bosons obeying generalized exclusion statistics.