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Journal of Physics B Atomic Molecular and Optical Physics

Friedel oscillations in phase space: Wigner function of trapped interacting fermions

F. Gleisberg, W. P. Schleich, W. Wonneberger

Published Nov 23, 2001• 2 citations
Publisher

Abstract

The Wigner function W ( z , k ) for a model of interacting one-dimensional fermions confined to a harmonic trap is evaluated at zero temperature. The model considers two hyperfine states of the same fermionic species and treats the dominant interactions between the two using the bosonization method. Interactions substantially modify the shape of the Wigner function. Irrespective of the sign of the coupling constant, the Friedel oscillations inherent in the Wigner function are enhanced in the k -direction and suppressed in the z -direction.

DOI
10.1088/0953-4075/34/23/309

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The Wigner function W ( z , k ) for a model of interacting one-dimensional fermions confined to a harmonic trap is evaluated at zero temperature. The model considers two hyperfine states of the same fermionic species and treats the dominant interactions between the two using the bosonization method. Interactions substantially modify the shape of the Wigner function. Irrespective of the sign of the coupling constant, the Friedel oscillations inherent in the Wigner function are enhanced in the k -direction and suppressed in the z -direction.
A model of interacting one-dimensional fermions confined to a harmonic trap is proposed. The model is treated analytically to all orders of the coupling constant by a method analogous to that used for the Luttinger model. As a first application, the particle density is evaluated and the behavior of Friedel oscillations under the influence of interactions is studied. It is found that attractive interactions tend to suppress the Friedel oscillations, while strong repulsive interactions enhance the Friedel oscillations significantly. The momentum distribution function and the relation of the model interaction to realistic pair interactions are also discussed.
Using an asymptotic phase representation of the particle density operator in the one-dimensional harmonic trap, the part which describes the Friedel oscillations is extracted. The expectation value with respect to the interacting ground state requires the calculation of the mean square average of a properly defined phase operator. This calculation is performed analytically for the Tomonaga–Luttinger model with harmonic confinement. It is found that the envelope of the Friedel oscillations at zero temperature decays with the boundary exponent ν = ( K +1)/2 away from the classical boundaries. This value differs from that known for open boundary conditions or strong pinning impurities. The soft boundary in the present case thus modifies the decay of Friedel oscillations. The case of two components is also discussed.
We assess whether a confined Wigner molecule constituted by 2N2N2N fermions behaves as NNN bosons or 2N2N2N fermions. Following the work by Law [Phys. Rev. A 71, 034306 (2005)] and Chudzicki et al. [Phys. Rev. Lett. 104, 070402 (2010)] we discuss the physical meaning and the reason why a large amount of entanglement is needed in order to ensure a bosonic composite behavior. By applying a composite boson ansatz, we found that a Wigner molecule confined in a two-dimensional trap presents a bosonic behavior induced by symmetry. The two-particle Wigner molecule ground state required by the composite boson ansatz was obtained within the harmonic approximation in the strong interacting regime. Our approach allows us to address few-particle states (widely studied within a variety of theoretical and numerical techniques) as well as a large number of particles (difficult to address due to computational costs). For a large number of particles, we found strong fermionic correlations exposed by the suppression of particle fluctuations. For a small number of particles, we show that the wave function calculated within the composite boson ansatz captures the Friedel-Wigner transition. The latter is shown in a regime in which strong correlations due to the Pauli exclusion principle arise; therefore, we conclude that the coboson ansatz reproduces the many-particle physics of a confined Wigner molecule, even in the presence of strong deviations of the ideal bosonic behavior due to fermionic correlations.
An asymptotically exact many-body theory for spin-polarized interacting fermions in a one-dimensional harmonic atom trap is developed using the bosonization method and including backward scattering. In contrast to the Luttinger model, backscattering in the trap generates one-particle potentials which must be diagonalized simultaneously with the two-body interactions. Inclusion of backscattering becomes necessary because backscattering is a dominant interaction process between confined identical one-dimensional fermions. The bosonization method is applied to the calculation of one-particle matrix elements at zero temperature. A detailed discussion of the validity of the results from bosonization is given, including a comparison with direct numerical diagonalization in fermionic Hilbert space. A model for the interaction coefficients is developed along the lines of the Luttinger model with only one coupling constant K. With these results, particle densities, the Wigner function, and the central pair-correlation function are calculated and displayed for large fermion numbers. It is shown how interactions modify these quantities. The anomalous dimension of the pair-correlation function in the center of the trap is also discussed and found to be in accord with the Luttinger model.
A many-body theory for a two-component system of spin-polarized interacting fermions in a one-dimensional harmonic trap is developed. The model considers two different states of the same fermionic species and treats the dominant interactions between the two using the bosonization method for forward scattering. Asymptotically exact results for the one-particle matrix elements at zero temperature are given. Using them, occupation probabilities of oscillator states are discussed. Particle and momentum densities are calculated and displayed. It is demonstrated how interactions modify all these quantities. An asymptotic connection with Luttinger liquids is suggested. The relation of the coupling constant of the theory to the dipole-dipole interaction is also discussed.
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).