Anomalous pseudogap in population imbalanced Fermi superfluids

TL;DR

This paper investigates the unusual spectral behaviors and pseudogap phenomena in population imbalanced Fermi superfluids, revealing complex temperature and imbalance-dependent features beyond mean field theory.

Contribution

It uncovers anomalous pseudogap behaviors in imbalanced Fermi gases using Monte Carlo simulations, extending understanding beyond traditional mean field approaches.

Findings

Weak imbalance leads to non-monotonic spectral features.

Pseudogap behavior varies with temperature and imbalance.

FFLO states exhibit pseudogap characteristics.

Abstract

In a Fermi superfluid increasing population imbalance leads initially to reduction of the transition temperature, then the appearance of modulated Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, and finally the suppression of pairing itself. For interaction strength such that the `balanced' system has a normal state pseudogap, increasing imbalance reveals anomalous spectral behavior. At a fixed weak imbalance (small polarization) the stable homogeneous superfluid occurs only above a certain temperature. The density of states has a minimum at the Fermi level, then a weak peak {\it within the gap}, and then the large, gap edge, coherence features. On heating, this non monotonic energy dependence changes to a more conventional fluctuation driven pseudogap, with a monotonic energy dependence. At large imbalance the ground state is FFLO and `pseudogapped' due to the modulated order. It changes to a gapless normal state on heating, and then shows a pseudogap again at a higher temperature. These weak imbalance and strong imbalance features both involve effects well beyond mean field theory. We establish them by using a Monte Carlo technique on large lattices, motivate the results in terms of the pairing field distribution, and compare them to spectroscopic results in the imbalanced unitary Fermi gas.