Bose-Fermi Mapping in Hubbard Models at Imaginary Chemical Potential and Phase-Induced Fermionization

TL;DR

This paper establishes a mapping between attractive Fermi-Hubbard and repulsive Bose-Hubbard models at imaginary chemical potential, revealing a thermodynamic mechanism for fermionization without infinite repulsion.

Contribution

It introduces a novel phase-twisting approach to induce fermion-like behavior in bosonic systems via thermodynamics, expanding understanding of crossovers in lattice models.

Findings

Partition functions relate through a simple shift in imaginary chemical potential.

Special angles define boundaries of a universal thermal window.

Fermionization can occur at finite interactions via thermodynamic effects.

Abstract

We find a mapping between the attractive Fermi-Hubbard model and the repulsive Bose-Hubbard model at finite temperature and at imaginary chemical potential $\mu =i\theta$. We show, by using a large $N$-expansion, that the partition functions of the two models are related by a simple shift $\theta \to \theta + \pi$. This condition maps the BCS--BEC crossover of attractive fermions to a Bose--Fermi crossover (fermion-like occupation) of repulsive bosons. Central feature of this correspondence plays the thermal kernel $g(\beta E,\phi),$ whose analytic continuation $g_B(\beta E,\phi) = g_F(\beta E,\phi+\pi)$ governs the bosonic and fermionic sectors. Interestingly, we are able to find that the special angles $\phi = 2\pi/3,4\pi/3$ for fermions correspond to $\phi = \pi/3,5\pi/3$ for bosons, marking the boundaries of a universal thermal window. We further argue that the present mechanism shows that fermionization can occur at finite interaction strength through a thermodynamic effect induced by the imaginary chemical potential. This suggests that it is a new way of fermionization (not a change in statistics but a fermion-like behaviour) unlike the Tonks--Girardeau limit, where fermionization arises from an infinite repulsive interaction and anyonic or Floquet-engineered systems where transmutation emerges from modified statistics or dynamics. Essentially, the phase $\phi$ is a statistical parameter; by twisting the thermal phase, it generates fermion-like behaviour without hard-core constraints or infinite repulsion but only by using thermodynamics. We derive the gap equation and number equation for the bosonic model, highlighting the role of the imaginary chemical potential as a statistical regulator. Our results provide a unified framework for understanding crossovers in interacting lattice systems.