Pseudogap and preformed pairs in the imbalanced Fermi gas in two dimensions

Abstract

The physics of the pseudogap state is intimately linked with the pairing mechanism that gives rise to superfluidity in quantum gases and to superconductivity in high-Tc cuprates, and therefore, both in quantum gases and superconductors, the pseudogap state and preformed pairs have been under intensive experimental scrutiny. Here, we develop a path integral treatment that provides a divergence-free description of the paired state in two-dimensional Fermi gases. Within this formalism, we derive the pseudogap temperature and the pair fluctuation spectral function, and compare these results with the recent experimental measument of the pairing in the two-dimensional Fermi gas. The removal of the infrared divergence in the number equations is shown both numerically and analytically, through a study of the long-wavelength and low-energy limit of the pair fluctuation density. Besides the pseudogap temperature, also the pair formation temperature and the critical temperature for superfluidity are derived. The latter corresponds to the Berezinski-Kosterlitz-Thouless (BKT) temperature. The pseudogap temperature, which coincides with the pair formation temperature in mean field, is found to be suppressed with respect to the pair formation temperature by fluctuations. This suppression is strongest for large binding energies of the pairs. Finally, we investigate how the pair formation temperature, the pseudogap temperature and the BKT temperature behave as a function of both binding energy and imbalance between the pairing partners in the Fermi gas. This allows to set up phase diagrams for the two-dimensional Fermi gas, in which the superfluid phase, the phase-fluctuating quasicondensate, and the normal state can be identified.

17 pages, 6 figures

References in corpus (13)

• Many-Body Physics with Ultracold Gases
• Observation of pseudogap behavior in a strongly interacting Fermi gas
• Quantum Fluctuations in the Superfluid State of the BCS-BEC Crossover
• Equation of state of a superfluid Fermi gas in the BCS-BEC crossover
• Evolution of the Normal State of a Strongly Interacting Fermi Gas from a Pseudogap Phase to a Molecular Bose Gas
• BCS-BEC crossover in a system of microcavity polaritons
• Pairing fluctuations and the superfluid density through the BCS-BEC crossover
• Temperature of a trapped unitary Fermi gas at finite entropy
• The effect of population imbalance on the Berezinskii-Kosterlitz-Thouless phase transition in a superfluid Fermi gas
• Imbalanced Fermi superfluid in a one-dimensional optical potential
• Comparison between a diagrammatic theory for the BCS-BEC crossover and Quantum Monte Carlo results
• Phase separation in imbalanced fermion superfluids beyond mean-field
• Imbalanced d-wave superfluids in the BCS-BEC crossover regime at finite temperatures

Cited by in corpus (13)

• Spin-Imbalanced Quasi-Two-Dimensional Fermi Gases
• Composite bosons in the 2D BCS-BEC crossover from Gaussian fluctuations
• Shell-shaped atomic gases
• Quantum Lifshitz points and fluctuation-induced first-order phase transitions in imbalanced Fermi mixtures
• Mobile impurity probing a two-dimensional superfluid phase transition
• Pseudogap phenomena in a two-dimensional ultracold Fermi gas near the Berezinskii-Kosterlitz-Thouless transition
• Low-Dimensional Fluctuations and Pseudogap in Gaudin-Yang Fermi Gases
• Kosterlitz-Thouless transition and vortex-antivortex lattice melting in two-dimensional Fermi gases with $p$- or $d$-wave pairing
• Scattering length of composite bosons in the 3D BCS-BEC crossover
• Unitary Fermi superfluid near the critical temperature: thermodynamics and sound modes from elementary excitations
• Defining a critical temperature of a crossover from BEC to the normal phase in anisotropic quantum magnets
• Phase correlations and quasicondensate in a two-dimensional ultracold Fermi gas
• Effect of Population Imbalance on Vortex Mass in Superfluid Fermi Gases