Spectral study of the pseudogap in unitary Fermi gases

Abstract

The existence of a pseudogap in unitary Fermi gases has recently been established and measured experimentally [Li et al., Nature 626, 288 (2024)]. This lends strong support for the pairing origin as the mechanism of the pseudogap in Fermi superfluids. Here we present a spectral study of unitary Fermi gases, and show how the data can be understood quantitatively, when compared with theoretically calculated momentum-resolved rf or microwave spectra, and the pseudogap extracted from the spectra. We use an iterative treatment of the fermion self energy and hence the spectral function, beyond previous pseudogap approximation, based on a pairing fluctuation theory that incorporates both particle-particle and particle-hole T matrices, with self-consistent self energy feedback. Our results not only provide a microscopic explanation of the experimental data but also strengthen the support for both the pairing-induced pseudogap physics and the pairing fluctuation theory of Fermi superfluidity.

Figures (5)

• Figure 1: (a) Real and (b) imaginary parts of the retarded self-energy $\Sigma^{\text{R}}(k_{\mu},\omega)$ for a unitary Fermi gas at $|\mathbf{k}|=k_{\mu}$ and $T_{\text{c}}$. Red dashed and green dash-dotted lines show the Bose and Fermi components of total self-energy (blue solid lines), respectively, along with the average Hartree energy $\bar{E}_{\text{Hartree}}$ (cyan dotted line).
• Figure 2: Contour plot of $\mathbf{k}^2 A(\mathbf{k},\omega)$ at (a) $T/T_\text{c}=0.77$, (b) $1$, (c) $1.11$, and (d) $1.51$, with $T_\text{c}/T_\text{F} = 0.2$, showing quasiparticle dispersions evolving from BCS-like gapped branches to a single S-shaped branch with a decreasing $\Delta$.
• Figure 3: Overlay of fitted dispersions on top of the spectral intensity map of $\mathbf{k}^{2}A(\mathbf{k},\omega)$ for (a) $T/T_\text{c}=0.9$ and (b) $1.11$. Red dots indicate $E_\text{max}(k)$, with orange and blue lines corresponding to $E_k^\pm$, respectively. Comparison between theory (blue squares) and experiment (orange triangles) for (c) $\Delta$ and (d) $U$, as well as $m^*$ (inset), at different $T/T_\text{c}$. The error bars represent one standard deviation.
• Figure 4: EDCs at (a) $T/T_\text{c}=1$ and (b) $T/T_\text{c}=1.11$ for various $k/k_\text{F}$ near $k_\mu$, showing the quasiparticle peak evolution.
• Figure 5: (a) Normalized EDCs of $A(\mathbf{k},\omega)$ from our calculations (blue) at $k=0.93k_\text{F}$ for different $T/T_\text{c}$, as labeled, (b) numerical $\Delta$ (blue) (c) $\Gamma_0$ and $\Gamma_1$ (inset) from the EDC fit. For comparison, the corresponding experimental data are also shown (orange). The theoretical $\Gamma_0$ data are also fitted with an exponentially activated behavior (blue line) in (c). The error bars represent one standard deviation.