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Quantum Electronics

Crossover from an atomic Fermi gas to a molecular Bose gas in a 2D system

Vasiliy Makhalov, Andrei V. Turlapov

Published May 7, 2018• 2 citations
Publisher

Abstract

The results of experimental and theoretical studies of a smooth crossover from a kinematically 2D Fermi gas of ultracold atoms into a Bose gas of molecular dimers are compared. The main attention is paid to the measurements and calculations in the zero-temperature approximation. The discrepancies between the results are discussed along with the questions that remain still open.

DOI
10.1070/qel16658

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The results of experimental and theoretical studies of a smooth crossover from a kinematically 2D Fermi gas of ultracold atoms into a Bose gas of molecular dimers are compared. The main attention is paid to the measurements and calculations in the zero-temperature approximation. The discrepancies between the results are discussed along with the questions that remain still open.
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The paper gives an overview of the recent developments in the field of ultracold fermionic quantum gases. In particular, results obtained on an ultracold spin-mixture of /sup 6/Li atoms in a series of experiments performed in Innsbruck is presented . Bose-Einstein condensation of strongly bound atom pairs (molecules) , the reversible crossover to a degenerate Fermi gas, measurements of collective excitation modes, and spectroscopic observation of pairing in a strongly interacting Fermi gas of atoms are also discussed. The experimental results provide strong evidence for superfluidity.
We report the experimental measurement of the equation of state of a two-dimensional Fermi gas with attractive s-wave interactions throughout the crossover from a weakly coupled Fermi gas to a Bose gas of tightly bound dimers as the interaction strength is varied. We demonstrate that interactions lead to a renormalization of the density of the Fermi gas by several orders of magnitude. We compare our data near the ground state and at finite temperature with predictions for both fermions and bosons from quantum Monte Carlo simulations and Luttinger-Ward theory. Our results serve as input for investigations of close-to-equilibrium dynamics and transport in the two-dimensional system.
Ultracold atomic Fermi gases have been a popular topic of research, with attention being paid recently to two-dimensional (2D) gases. In this work, we perform T=0T=0T=0 ab initio diffusion Monte Carlo calculations for a strongly interacting two-component Fermi gas confined to two dimensions. We first go over finite-size systems and the connection to the thermodynamic limit. After that, we illustrate pertinent 2D scattering physics and properties of the wave function. We then show energy results for the strong-coupling crossover, in between the Bose-Einstein condensation (BEC) and Bardeen-Cooper-Schrieffer (BCS) regimes. Our energy results for the BEC-BCS crossover are parametrized to produce an equation of state, which is used to determine Tan's contact. We carry out a detailed comparison with other microscopic results. Finally, we calculate the pairing gap for a range of interaction strengths in the strong coupling regime, following from variationally optimized many-body wave functions.
We investigate the Bose-Einstein condensation of fermionic pairs in a uniform two-component Fermi gas, obtaining an explicit formula for the condensate density as a function of the chemical potential and the energy gap. We analyze the condensate fraction in the crossover from the Bardeen-Cooper-Schrieffer (BCS) state of weakly interacting Cooper pairs to the Bose-Einstein condensate of molecular dimers. By using the local-density approximation we study confined Fermi vapors of alkali-metal atoms for which there is experimental evidence of condensation also on the BCS side of the Feshbach resonance. Our theoretical results are in agreement with these experimental data and give the behavior of the condensate on both sides of the Feshbach resonance at zero temperature.
We investigate the crossover from Bardeen-Cooper-Schrieffer (BCS) superfluidity to Bose-Einstein condensation (BEC) in a two-dimensional Fermi gas at T = 0 using the fixed-node diffusion Monte Carlo method. We calculate the equation of state and the gap parameter as a function of the interaction strength, observing large deviations compared to mean-field predictions. In the BEC regime our results show the important role of dimer-dimer and atom-dimer interaction effects that are completely neglected in the mean-field picture. We also consider the highly polarized gas and the competition between a polaronic and a molecular picture.