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Modern Physics Letters B

MOMENTUM RESOLVED RADIO FREQUENCY SPECTROSCOPY OF A UNITARY FERMI GAS WITH EXTENDED GMB APPROXIMATION

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

Published Jan 16, 2014• 2 citations
Publisher

Abstract

In this paper, the momentum-resolved radio-frequency spectra of a unitary Fermi gas in the normal phase are presented and analyzed on the basis of the framework of the non-self-consistent T-matrix approximation with the extended Gorkov and Melik-Barkhudarov (GMB) approximation. We report the calculated occupied spectral intensity and energy distribution curves for different temperatures above T c . Our results indicate the existence of pseudogap phenomenon which would be helpful for further understanding of the pseudogap in high-temperature superconductivity.

DOI
10.1142/s0217984914500286

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In this paper, the momentum-resolved radio-frequency spectra of a unitary Fermi gas in the normal phase are presented and analyzed on the basis of the framework of the non-self-consistent T-matrix approximation with the extended Gorkov and Melik-Barkhudarov (GMB) approximation. We report the calculated occupied spectral intensity and energy distribution curves for different temperatures above T c . Our results indicate the existence of pseudogap phenomenon which would be helpful for further understanding of the pseudogap in high-temperature superconductivity.
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].
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.
We investigate strong-coupling corrections to single-particle excitations in the normal state of a spin-polarized unitary Fermi gas. Within the framework of an extended TTT-matrix approximation, we calculate the single-particle density of states, as well as the single-particle spectral weight, to show that the so-called pseudogap phenomenon gradually disappears with increasing magnitude of an effective magnetic field. In the highly spin-polarized regime, the calculated spin polarization as a function of the effective magnetic field agrees well with the recent experiment on a 6{}^{6}6Li Fermi gas. Although this experiment has been considered to be incompatible with the existence of the pseudogap in an unpolarized Fermi gas, our result clarifies that the observed spin polarization in the highly spin-polarized regime and the pseudogap in the unpolarized limit can be explained in a consistent manner when one correctly includes effects of population imbalance on single-particle excitations. Since it is a crucial issue to clarify whether the pseudogap exists or not in the BCS-BEC crossover regime of an ultracold Fermi gas, our results would be useful for the understanding of this strongly interacting fermion system.
In this paper we present an overview of radio frequency (RF) spectroscopy in the atomic Fermi superfluids. An ultimate goal is to suggest new directions in the cold gas research agenda from the condensed matter perspective.Our focus is on the experimental and theoretical literature of cold gases and photoemission spectroscopy of the cuprates particularly as it pertains to areas of overlap. This paper contains a systematic overview of the theory of RF spectroscopy, both momentum integrated and momentum resolved. We discuss the effects of traps, population imbalance, final state interactions over the entire range of temperatures and compare theory and experiment. We show that this broad range of phenomena can be accomodated within the BCS-Leggett description of BCS-BEC crossover and that this scheme also captures some of the central observations in photoemission experiments in the cuprates. In this last context, we note that the key themes which have emerged in cuprate photoemission studies involve characterization of the fermionic self energy, of the pseudogap and of the effects of superconducting coherence (in passing from above to below the superfluid transition temperature, TcT_cTc​).These issues have a counterpart in the cold Fermi gases and it would be most useful in future to use these atomic systems to address these and the more sweeping question of how to describe that anomalous superfluid phase which forms in the presence of a normal state excitation gap.
It is of central importance to probe the local spectral function $A(\mathbf{k},\ensuremath{\omega})$ of a strongly interacting Fermi gas in a trap. Momentum-resolved rf spectroscopy has been demonstrated to be able to probe the trap-averaged $A(\mathbf{k},\ensuremath{\omega})$. However, the usefulness of this technique was limited by the trap inhomogeneity. Independent of a specific theory, here we propose that by studying the momentum-resolved rf spectra of the minority fermions of a phase-separated, population-imbalanced Fermi gas at low temperature, one can effectively extract $A(\mathbf{k},\ensuremath{\omega})$ of a homogeneous superfluid Fermi gas (at the trap center). In support, we present calculated spectral functions and spectral intensity maps for various cases from BCS through Bose-Einstein condensation regimes using different theories.