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arXiv:cond-mat/0611699 · Physical Review A

Emergence of Wigner molecules in one-dimensional systems of repulsive fermions under harmonic confinement

Saeed H. Abedinpour, Marco Polini, Gao Xianlong, M. P. Tosi

Published Jan 9, 2007• 30 citations
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Abstract

A Bethe-ansatz spin-density functional approach is developed to evaluate the ground-state density profile in a system of repulsively interacting spin-$1∕2$ fermions inside a quasi-one-dimensional harmonic well. The approach allows for the formation of antiferromagnetic quasiorder with increasing coupling strength and reproduces with high accuracy the exact solution that is available for the two-fermion system.

DOI
10.1103/physreva.75.015602

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A Bethe-ansatz spin-density functional approach is developed to evaluate the ground-state density profile in a system of repulsively interacting spin-1∕21∕21∕2 fermions inside a quasi-one-dimensional harmonic well. The approach allows for the formation of antiferromagnetic quasiorder with increasing coupling strength and reproduces with high accuracy the exact solution that is available for the two-fermion system.
According to the exact analytical Bethe-ansatz solution of the one-dimensional spin-1/2 XXZ model, we perform a numerical study of one-dimensional spinless fermions in an optical lattice in the presence of harmonic potential by using a local density approximation. We study the density profile, and obtain the -u phase diagram (here =NfV2/t is the characteristic dimensionless density and u=V/t is the interaction strength scaled in units of the hopping parameter t). With the increases of particle density and nearest-neighbour interaction, the system undergoes five different quantum phases. Through calculating the thermodynamic stiffness S, we find that it can be used as an order parameter. Its singular points can measure the quantum phase transitions in such a confined system.
We study, by means of exact-diagonalization techniques, the ground state of a few-fermion system with strong short-range repulsive interactions trapped by a harmonic potential in one spatial dimension. Even when the ground-state density profile displays, at strong coupling, very well pronounced Wigner oscillations with a 4kF4{k}_{\mathrm{F}}4kF​ periodicity, the pair-correlation function does not show any signature of Wigner-molecule-type correlations. For the sake of comparison, we present also numerical results for few-electron systems with Coulomb interactions, demonstrating that their ground state at strong coupling is, on the contrary, a Wigner molecule.
Density oscillations of confined one-dimensional Fermi gases of contact repulsive interactions in a continuous space are discussed within Bethe-ansatz-based spin-density-functional theory. The results are compared against the exact analytical and the exact diagonalization method. For an unpolarized system, the number of peaks in the density profiles is doubled, signaling the crossover of the 2kF2{k}_{F}2kF​-Friedel to 4kF4{k}_{F}4kF​-Wigner oscillations (with kF{k}_{\mathrm{F}}kF​ being the Fermi wave vector). For both unpolarized and polarized systems, a threshold of the short-range interaction strength can be found where Nf{N}_{f}Nf​-peak Wigner oscillations of a 4kF4{k}_{F}4kF​ wave vector appear in the density profile (Nf{N}_{f}Nf​ is the total particle number).
We propose the density-functional theory for one-dimensional harmonically trapped spin-1 bosons in the ground state with repulsive density-density interaction and antiferromagnetic spin-exchange interaction. The density distributions of spin-singlet-paired bosons and polarized bosons with different total polarization for various interaction parameters are obtained by solving the Kohn-Sham equations, which are derived based on the local density approximation and the Bethe ansatz exact results for a homogeneous system. Nonmonotonicity of the central densities is attributed to the competition between the density interaction and spin-exchange. The results reveal the phase separation of the paired and polarized bosons, the density profiles of which respectively approach the Tonks-Girardeau gases of Bose-Bose pairs and scalar bosons in the case of strong interaction. We give the radius-polarization phase diagram at strong interaction and find the critical polarization, which paves the way to directly observe the exotic singlet pairing in spinor gas experimentally.
Quasi-one-dimensional two-component Fermi gases with effectively attractive and repulsive interactions are characterized for arbitrary interaction strength. The ground-state properties of the gas confined in highly elongated harmonic traps are determined within the local density approximation. For strong attractive effective interactions the existence of a molecular Tonks-Girardeau gas is predicted. The frequency of the lowest breathing mode is calculated as a function of the coupling strength for both attractive and repulsive interactions.
We present an extensive numerical study of the ground-state properties of confined repulsively interacting fermions in one-dimensional optical lattices. Detailed predictions for the atom-density profiles are obtained from parallel Kohn-Sham density-functional calculations and quantum Monte Carlo simulations. The density-functional calculations employ a Bethe ansatz based local-density approximation for the correlation energy that accounts for Luttinger-liquid and Mott-insulator physics. Semianalytical and fully numerical formulations of this approximation are compared with each other and with a cruder Thomas-Fermi-type local-density approximation for the total energy. Precise quantum Monte Carlo simulations are used to assess the reliability of the various local-density approximations, and in conjunction with these provide a detailed microscopic picture of the consequences of the interplay between particle-particle interactions and confinement in one-dimensional systems of strongly correlated fermions.