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arXiv:1102.1243 · Physical Review A

Spectroscopic Bogoliubov features near the unitary limit

T. Domański

Published Aug 25, 2011• 6 citations
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Abstract

We analyze the single-particle excitation spectrum of the ultracold fermion atom system close to the unitary limit where there has been found experimental evidence for the Bogoliubov quasiparticles below as well as above the transition temperature ${T}_{c}$. We consider the short-range correlations originating from the preformed pairs and try to discuss the experimental data adapting phenomenological self-energy previously used for description of the antinodal spectra of the underdoped cuprate superconductors. We show that this ansatz qualitatively accounts for the momentum-resolved rf spectroscopic data obtained for ${}^{40}$K atoms.

DOI
10.1103/physreva.84.023634

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Single-particle spectra are calculated in the superconducting state for a fermionic system with an\nattractive interaction, as functions of temperature and coupling strength from weak to strong. The\nfermionic system is described by a single-particle self-energy that includes pairing-fluctuation effects\nin the superconducting state. The theory reduces to the ordinary BCS approximation in weak coupling\nand to the Bogoliubov approximation for the composite bosons in strong coupling. Several features of\nthe single-particle spectral function are shown to compare favorably with experimental data for cuprate\nsuperconductors.

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We analyze the single-particle excitation spectrum of the ultracold fermion atom system close to the unitary limit where there has been found experimental evidence for the Bogoliubov quasiparticles below as well as above the transition temperature Tc{T}_{c}Tc​. We consider the short-range correlations originating from the preformed pairs and try to discuss the experimental data adapting phenomenological self-energy previously used for description of the antinodal spectra of the underdoped cuprate superconductors. We show that this ansatz qualitatively accounts for the momentum-resolved rf spectroscopic data obtained for 40{}^{40}40K atoms.
We present a calculation of the spectral functions and the associated rf response of ultracold fermionic atoms near a Feshbach resonance. The single-particle spectra are peaked at energies that can be modeled by a modified BCS dispersion. However, even at very low temperatures their width is comparable to their energy except for a small region around the dispersion minimum. The structure of the excitation spectrum of the unitary gas at infinite scattering length agrees with recent momentum-resolved rf spectra near the critical temperature. A detailed comparison is made with momentum integrated, locally resolved rf spectra of the unitary gas at arbitrary temperatures and shows very good agreement between theory and experiment. The pair size defined from the width of these spectra is found to coincide with that obtained from the leading gradient corrections to the effective-field theory of the superfluid.
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.
Single-particle spectra are calculated in the superconducting state for a fermionic system with an attractive interaction, as functions of temperature and coupling strength from weak to strong. The fermionic system is described by a single-particle self-energy that includes pairing-fluctuation effects in the superconducting state. The theory reduces to the ordinary BCS approximation in weak coupling and to the Bogoliubov approximation for the composite bosons in strong coupling. Several features of the single-particle spectral function are shown to compare favorably with experimental data for cuprate superconductors.
Ultracold atomic gases are a powerful tool to experimentally study strongly correlated quantum many-body systems. In particular, ultracold Fermi gases with tunable interactions have allowed to realize the famous BEC-BCS crossover from a Bose-Einstein condensate (BEC) of molecules to a Bardeen-Cooper-Schrieffer (BCS) superfluid of weakly bound Cooper pairs. However, large parts of the excitation spectrum of fermionic superfluids in the BEC-BCS crossover are still unexplored. In this work, we use Bragg spectroscopy to measure the full momentum-resolved low-energy excitation spectrum of strongly interacting ultracold Fermi gases. This enables us to directly observe the smooth transformation from a bosonic to a fermionic superfluid that takes place in the BEC-BCS crossover. We also use our spectra to determine the evolution of the superfluid gap and find excellent agreement with previous experiments and self-consistent T-matrix calculations both in the BEC and crossover regime. However, toward the BCS regime a calculation that includes the effects of particle-hole correlations shows better agreement with our data.
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.
Single-particle spectra are calculated in the superconducting state for a fermionic system with an\nattractive interaction, as functions of temperature and coupling strength from weak to strong. The\nfermionic system is described by a single-particle self-energy that includes pairing-fluctuation effects\nin the superconducting state. The theory reduces to the ordinary BCS approximation in weak coupling\nand to the Bogoliubov approximation for the composite bosons in strong coupling. Several features of\nthe single-particle spectral function are shown to compare favorably with experimental data for cuprate\nsuperconductors.