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Unitary Fermi Gas and BCS-BECCrossover

Soumik Mitra

Published May 1, 2015• 0 citations
OpenAlex

Abstract

In this thesis,we investigate the Unitary Fermi gas and its thermodynamic properties. We look into the problem using the Lowest Order Constrained Variational method,using the Jastrow correlation function and determine the thermodynamic quantities of interest around unitarity.From this variational calculation we calculate quantities like energy per particle and chemical potential and study their variation as a function of the s-wave scattering length. We find there are two energy branches which suggests that the way to approach unitarity is not unique.We compare our results with those found from toy model for unitary fermi gas,a single particle in a spherical box interacting with a fixed scatterer at the center of the cavity interactions being modelled by the fermi pseudo potential.We find a surprising qualitative agreement between the results obtained. We also study the BCS-BEC crossover in three dimension using mean field analysis, and evaluate quantities like gap parameter and chemical potential across the whole range of the crossover.

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In this thesis,we investigate the Unitary Fermi gas and its thermodynamic properties.
We look into the problem using the Lowest Order Constrained Variational
method,using the Jastrow correlation function and determine the thermodynamic
quantities of interest around unitarity.From this variational calculation we calculate
quantities like energy per particle and chemical potential and study their
variation as a function of the s-wave scattering length. We find there are two energy
branches which suggests that the way to approach unitarity is not unique.We
compare our results with those found from toy model for unitary fermi gas,a single
particle in a spherical box interacting with a fixed scatterer at the center of the
cavity interactions being modelled by the fermi pseudo potential.We find a surprising
qualitative agreement between the results obtained.
We also study the BCS-BEC crossover in three dimension using mean field analysis,
and evaluate quantities like gap parameter and chemical potential across the
whole range of the crossover.
Properties of the ground state of an unpolarized ultracold Fermi gas along the BCS-BEC crossover are investigated by the variational and diffusion Monte Carlo methods. We apply the Wigner-Bethe-Peierls boundary condition in our calculations to avoid any bias from using an interatomic potential with finite effective range. Properties for several values of the scattering length are studied in the range $\ensuremath{-}8\ensuremath{\le}1/a{k}_{F}\ensuremath{\le}4$, including the unitary limit. The contact parameters as a function of scattering length are obtained by fitting the pair distribution functions for particles with different spins. The energies and contact parameters are in very good agreement with experimental data reported in the literature.
We extend a recently introduced separable interaction for the unitary trapped Fermi gas to all values of the scattering length. We derive closed expressions for the interaction matrix elements and the two-particle eigenvectors. At unitarity we analytically demonstrate the convergence of this interaction to the zero-range two-body pseudopotential for sss-wave scattering. We apply this effective interaction to the three- and four-particle systems along the BEC-BCS crossover and find that their low-lying energies exhibit convergence in the regularization parameter that is much faster than for the conventional renormalized contact interaction. We find similar convergence properties of the three-particle free energy at unitarity.
We calculate Bardeen-Cooper-Schrieffer (BCS) state of a unitary Fermi gas of atoms interacting with the finite-ranged Jost-Kohn potential which has been recently shown to account for the resonant interactions [2019 {\rm J. Phys. B: At. Mol. Opt. Phys.} {\bf 52}, 165004]. Using exact scattering solution of the potential, we derive two-body T{\mathbf T}T-matrix element which is employed to construct the BCS Hamiltonian in momentum space. We present results on the energy- and range-dependence of the pairing gap and superfluid density and the range-dependence of the chemical potential for a wide variation of the scattering length including the unitarity regime. In the zero range limit our calculated gap at the Fermi energy is found to be nearly equal to that calculated with contact potential. The mean gap averaged over the full width at half maximum of the gap function in the zero range and unitarity limits is found to be 0.42EF0.42 E_F0.42EF​ which is quite close to the recent result of the quantum Monte Carlo simulation [2018 {\rm Phys. Rev.A} {\bf 97}, 013601]. The chemical potential in the zero range limit also agrees well with that for the contact potential.
The thermodynamic potential is calculated for a uniform superfluid gas of fermi atoms from the mean field BCS equations including corrections from induced interactions, Hartree-Fock energies and quasiparticle selfenergies. The entropy, specific heat and sound modes are calculated as function of temperature, density and interaction strength from the BCS to the unitarity limit and around the BCS-BEC crossover. The second sound speed is of particular interest as it is a clear signal of a superfluid component and it determines the critical temperature.
On the basis of quantum hydrodynamical equations we derive a unitarity Schrödinger equation of a finite trapped superfluid Fermi gas valid in the whole interaction regime from BCS superfluid to BEC. This equation is just the Ginzburg–Laudau-type equation for the fermionic Cooper pairs in the BCS side, the Gross–Pitaevskii-type equation for the bosonic dimers in the BEC side, and a unitarity equation for a strongly interacting Fermi superfluid in the unitarity limit. By taking a modified Gauss-like trial wave function, we solve the unitarity Schrödinger equation, calculate the energy, chemical potential, sizes and profiles of the ground-state condensate, and discuss the properties of the ground state in the entire BCS-BEC crossover regimes.
The ground-state properties of a two-component Fermi gas with attractive short-range interactions are calculated using the fixed-node diffusion Monte Carlo method. The interaction strength is varied over a wide range by tuning the value a of the s-wave scattering length of the two-body potential. We calculate the energy per particle, the one- and two-body density matrix as a function of the interaction strength. Results for the momentum distribution of the atoms, as obtained from the Fourier transform of the one-body density matrix, are reported as a function of the interaction strength. Off-diagonal long-range order in the system is investigated through the asymptotic behavior of the two-body density matrix. The condensate fraction of pairs is calculated in the unitary limit and on both sides of the BCS-BEC crossover.