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arXiv:2309.06766 · Physical Review A

Harmonically trapped fermions in one dimension: A finite-temperature lattice Monte Carlo study

Felipe Attanasio, Marc Bauer, Renzo Kapust, Jan Martin Pawlowski

Published Mar 5, 2024• 2 citations
arXivHTMLPDF

Abstract

We study a one-dimensional two-component Fermi gas in a harmonic trapping potential using finite-temperature lattice quantum Monte Carlo methods. We are able to compute observables in the canonical ensemble via an efficient projective approach. Results for density profiles, correlations, as well as energy-related observables are presented for systems with up to 80 particles and various temperatures. Our simulations reproduce known numerical results and compare well against available experimental data close to the ground state, while at higher temperature they are benchmarked against the exact solution of the two-particle system. This provides an indication that a standard lattice discretization is sufficient to capture the physics of the trapped system. In the special case of a spin-imbalanced gas, we find no sign problem in the studied parameter ranges, allowing access without the need of specialized methods. This includes simulations close to the ground state and at large population imbalance, where we present results for density correlations, indicating pairing at finite total momentum.

DOI
10.1103/physreva.109.033305

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We study a one-dimensional two-component Fermi gas in a harmonic trapping potential using finite-temperature lattice quantum Monte Carlo methods. We are able to compute observables in the canonical ensemble via an efficient projective approach. Results for density profiles, correlations, as well as energy-related observables are presented for systems with up to 80 particles and various temperatures. Our simulations reproduce known numerical results and compare well against available experimental data close to the ground state, while at higher temperature they are benchmarked against the exact solution of the two-particle system. This provides an indication that a standard lattice discretization is sufficient to capture the physics of the trapped system. In the special case of a spin-imbalanced gas, we find no sign problem in the studied parameter ranges, allowing access without the need of specialized methods. This includes simulations close to the ground state and at large population imbalance, where we present results for density correlations, indicating pairing at finite total momentum.
Using exact continuous quantum Monte Carlo techniques, we study the zero- and finite-temperature properties of a system of harmonically trapped one-dimensional spin-12\frac{1}{2}21​ fermions with short-range interactions. Motivated by experimental searches for modulated Fulde-Ferrel-Larkin-Ovchinikov states, we systematically examine the impact of a spin imbalance on the density profiles. We quantify the accuracy of the Thomas-Fermi approximation, finding that for sufficiently large particle numbers $(N\ensuremath{\gtrsim}100)$ it quantitatively reproduces most features of the exact density profile. The Thomas-Fermi approximation fails to capture small Friedel-like spin and density oscillations and overestimates the size of the fully paired region in the outer shell of the trap. Based on our results, we suggest a range of experimentally tunable parameters to maximize the visibility of the double-shell structure of the system and the Fulde-Ferrel-Larkin-Ovchinikov state in the one-dimensional harmonic trap. Furthermore, we analyze the fingerprints of the attractive contact interactions in the features of the momentum and pair momentum distributions.
The system of interacting, trapped fermions in one dimension has been of interest in both the theoretical and experimental communities. This system is realizable experimentally using ultracold atoms in traps, where the interactions can be tuned to simulate a number of important situations in nuclear theory, condensed matter, quantum information, and QCD. Theoretically, however, this system remains a challenge to treat, and no known benchmarks exist for the ground state energy, Tan's contact, or density profiles for the few- to many-body regime. This project implements a lattice Monte Carlo (LMC) method to solve for these quantities. The method blends hybrid Monte Carlo (HMC) - a pillar of lattice quantum chromodynamics (lattice QCD) - with a non-uniform lattice defined using Gauss-Hermite quadrature points and weights. This coordinate basis is the natural one for the harmonic oscillator trapping potential, and can be generalized to traps of other shapes. Using this method, we determine the ground-state energy and Tan's contact of attractively interacting few-fermion systems in a one-dimensional harmonic trap, for a range of couplings and particle numbers. Complementing those results, we show the corresponding density profiles. We present results for N=4,...,20N = 4,...,20N=4,...,20 particles - and the method is capable of extending beyond that. The method is the first lattice calculation of its kind, and is exact up to statistical and systematic uncertainties, which we account for. Our results are therefore a benchmark for other methods and a prediction for ultracold-atom experiments.
A one-dimensional world is very unusual as there is an interplay between quantum statistics and geometry, and a strong short-range repulsion between atoms mimics Fermi exclusion principle, fermionizing the system. Instead, a system with a large number of components with a single atom in each, on the opposite acquires many bosonic properties. We study the ground-state properties of a multicomponent repulsive Fermi gas trapped in a harmonic trap by a fixed-node diffusion Monte Carlo method. The interaction between all components is considered to be the same. We investigate how the energetic properties (energy, contact) and correlation functions (density profile and momentum distribution) evolve as the number of components is changed. It is shown that the system fermionizes in the limit of strong interactions. Analytical expressions are derived in the limit of weak interactions within the local density approximation for an arbitrary number of components and for one plus one particle using an exact solution.
A one-dimensional world is very unusual as there is an interplay between quantum statistics and geometry, and a strong short-range repulsion between atoms mimics Fermi exclusion principle, fermionizing the system. Instead, a system with a large number of components with a single atom in each, on the opposite acquires many bosonic properties. We study the ground-state properties of a multicomponent repulsive Fermi gas trapped in a harmonic trap by a fixed-node diffusion Monte Carlo method. The interaction between all components is considered to be the same. We investigate how the energetic properties (energy, contact) and correlation functions (density profile and momentum distribution) evolve as the number of components is changed. It is shown that the system fermionizes in the limit of strong interactions. Analytical expressions are derived in the limit of weak interactions within the local density approximation for an arbitrary number of components and for one plus one particle using an exact solution.
Pairing in a population-imbalanced Fermi system in a two-dimensional optical lattice is studied using determinant quantum Monte Carlo simulations and mean-field calculations. The approximation-free numerical results show a wide range of stability of the Fulde-Ferrell-Larkin-Ovchinnikov phase. Contrary to claims of fragility with increased dimensionality, we find that this phase is stable across a wide range of values for the polarization, temperature, and interaction strength. Both homogeneous and harmonically trapped systems display pairing with finite center-of-mass momentum, with clear signatures either in momentum space or real space, which could be observed in cold-atomic gases loaded in an optical lattice. We also use the harmonic level basis in the confined system and find that pairs can form between particles occupying different levels, which can be seen as the analog of the finite center-of-mass momentum pairing in the translationally invariant case. Finally, we perform mean-field calculations for the uniform and confined systems and show the results to be in good agreement with quantum Monte Carlo. This leads to a simple picture of the different pairing mechanisms, depending on the filling and confining potential.
We determine the ground-state energy and Tan's contact of attractively interacting few-fermion systems in a one-dimensional harmonic trap, for a range of couplings and particle numbers. Complementing those results, we show the corresponding density profiles. The calculations were performed with a lattice Monte Carlo approach based on a nonuniform discretization of space, defined via Gauss-Hermite quadrature points and weights. This particular coordinate basis is natural for systems in harmonic traps, and can be generalized to traps of other shapes. In all cases, it yields a position-dependent coupling and a corresponding nonuniform Hubbard-Stratonovich transformation. The resulting path integral is performed with hybrid Monte Carlo as a proof of principle for calculations at finite temperature and in higher dimensions. We present results for N=4,...,20N=4,...,20N=4,...,20 particles (although the method can be extended beyond that) to cover the range from few- to many-particle systems. This method is exact up to statistical and systematic uncertainties, which we account for---and thus also represents an ab initio calculation of this system, providing a benchmark for other methods and a prediction for ultracold-atom experiments.