Single-particle properties and pseudogap effects in the BCS-BEC crossover regime of an ultracold Fermi gas above Tc

TL;DR

This paper studies the evolution of the pseudogap in ultracold Fermi gases across the BCS-BEC crossover above the critical temperature, revealing how the pseudogap develops and transforms into a full gap.

Contribution

It provides a detailed analysis of the pseudogap phenomena and characteristic temperatures in the BCS-BEC crossover using the $T$-matrix approximation, clarifying the phase diagram.

Findings

Pseudogap appears in the DOS above $T_c$ and evolves into a full gap in the BEC regime.

The pseudogap temperature $T^*$ and the double peak disappearance temperature $T^{**}$ differ and vary across regimes.

The pseudogap region is mapped in the phase diagram of ultracold Fermi gases.

Abstract

We investigate strong-coupling effects on normal state properties of an ultracold Fermi gas. Within the framework of $T$-matrix approximation in terms of pairing fluctuations, we calculate the single-particle density of states (DOS), as well as the spectral weight, over the entire BCS-BEC crossover region above the superfluid phase transition temperature $T_{\rm c}$. Starting from the weak-coupling BCS regime, we show that the so-called pseudogap develops in DOS above $T_{\rm c}$, which becomes remarkable in the crossover region. The pseudogap structure continuously changes into a fully gapped one in the strong-coupling BEC regime, where the gap energy is directly related to the binding energy of tightly bound molecules. We determine the pseudogap temperature $T^*$ where the dip structure in DOS vanishes. The value of $T^*$ is shown to be very different from another characteristic temperature $T^{**}$ where a BCS-type double peak structure disappears in the spectral weight. While one finds $T^*>T^{**}$ in the BCS regime, $T^{**}$ becomes higher than $T^*$ in the crossover region and BEC regime. Including this, we determine the pseudogap region in the phase diagram of ultracold Fermi gases. Our results would be useful in the search for the pseudogap region in ultracold $^6$Li and $^{40}$K Fermi gases.