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Modern Physics Letters B

SPECTRAL FUNCTION AND DOS OF ULTRACOLD FERMI GASES IN THE NORMAL PHASE INCLUDING THE EFFECT OF INDUCED INTERACTION

Hao Ran Gong, Xiao-Xia Ruan, Long Du, Weimin Sun, Hong-Shi Zong

Published Nov 1, 2013• 4 citations
Publisher

Abstract

In this paper, taking into account the effect of the induced interaction, we calculate the spectral function and hence density of states (DOS) of ultracold Fermi gases within the framework of non-self-consistent T-matrix approximation (nTMA) in the normal phase. The corresponding equations are calculated on the real-frequency axis directly with high accuracy. Our result shows that pseudogap phenomenon still exists in the spectral function and DOS.

DOI
10.1142/s021798491350214x

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In this paper, taking into account the effect of the induced interaction, we calculate the spectral function and hence density of states (DOS) of ultracold Fermi gases within the framework of non-self-consistent T-matrix approximation (nTMA) in the normal phase. The corresponding equations are calculated on the real-frequency axis directly with high accuracy. Our result shows that pseudogap phenomenon still exists in the spectral function and DOS.
Abstract In the normal state of a superfluid condensate, like the Bardeen–Cooper–Schrieffer (BCS) superconducting state, we do not expect a gapped feature in the single-particle spectral function. Here we calculate the spectral function and single-particle density of states (DOS) of ultracold Fermi gases at different temperatures in the normal state. The calculation is conducted based on the non-self-consistent T -matrix approximation (nTMA) with particle–hole fluctuation. It is found that a dip structure emerges in the DOS above the superfluid phase-transition temperature T c . We attribute this dip structure to the pseudogap, and its temperature is determined. From the temperature dependence of the spectral function, we also find another characteristic temperature where a two-peak structure disappears. The results provide deep insight into the superfluid condensate formed by ultracold fermions.
In this paper, taking into account the effect of the induced interaction, we calculate the energy of ultracold Fermi gases at unitarity in the framework of non-self-consistent T-matrix approximation (nTMA) above the critical temperature and compare the result with the experimental data and other theoretical calculation without induced interaction. Our calculated chemical potential is higher than the experimental data, but our calculated energy obtains a good agreement with Tokyo experiment for temperature range between [Formula: see text] and [Formula: see text].
Taking into account the effect of the generalized Gor'kov and Melik—Barkhudarov (GMB) approximation, we calculate the radio-frequency spectra of balanced and homogeneous ultracold Fermi gases within the framework of the non-self-consistent T-matrix approximation at unitarity in the normal phase. The corresponding equations are numerically calculated on the real frequency axis directly. It is found that our results agree well with the experimental result of the radio-frequency spectroscopy [Phys. Rev. Lett. 101 (2008) 140403].
We calculate nonperturbative self-consistent fermionic and bosonic spectral functions of ultracold Fermi gases with the spectral functional approach. This approach allows for a direct real-time computation of nonperturbative correlation functions, and in the present work we use spectral Dyson-Schwinger equations. We focus on the normal phase of the spin-balanced Fermi gas and provide numerical results for the full fermionic and bosonic spectral functions. The spectral functions are then used for the determination of the equation of state, the Tan contact, and ejection rf spectra at unitarity. These results are compared to experimental data, the self-consistent TTT-matrix approach, and lattice results. Our approach offers a wide range of applications, including the ab initio calculation of transport and spectral properties of the superfluid phase in the BCS-BEC crossover.
We investigate strong-coupling effects on normal-state properties of an ultracold Fermi gas. Within the framework of TTT-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--Bose-Einstein condensate (BEC) crossover region above the superfluid phase-transition temperature Tc{T}_{\text{c}}Tc​. Starting from the weak-coupling BCS regime, we show that the so-called pseudogap develops in DOS above Tc{T}_{\text{c}}Tc​, 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 ${T}^{\ensuremath{\ast}}$ where the dip structure in DOS vanishes. The value of ${T}^{\ensuremath{\ast}}$ is shown to be very different from another characteristic temperature ${T}^{\ensuremath{\ast}\ensuremath{\ast}}$ where a BCS-type double-peak structure disappears in the spectral weight. While one finds ${T}^{\ensuremath{\ast}}>{T}^{\ensuremath{\ast}\ensuremath{\ast}}$ in the BCS regime, ${T}^{\ensuremath{\ast}\ensuremath{\ast}}$ becomes higher than ${T}^{\ensuremath{\ast}}$ 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 6Li^{6}\text{L}\text{i}6Li and 40K^{40}\text{K}40K Fermi gases.
We investigate fluctuation effects in a single-component ultracold Fermi gas with a p-wave interaction. Within the framework of a strong-coupling T-matrix theory, we calculate the single-particle density of states (DOS) at the superfluid phase transition temperature Tc. Starting from the weak-coupling regime, we show that the so-called pseudogap structure develops in DOS, with increasing the strength of a pairing interaction. However, in contrast to the case of s-wave interaction (where the pseudogap size in DOS monotonically increases with increasing the interaction strength), the pseudogap structure is found to gradually disappear in the strong-coupling regime. We point out that this non-monotonic behavior of pseudogap originates from the momentum dependence of p-wave interaction. Our results clarify how the symmetry of pairing interaction affects pseudogap phenomena in strongly correlated fermion systems.