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arXiv:2109.03493 · Physical Review A

Statistical properties of the momentum occupation numbers of the Tonks-Girardeau gas in a harmonic trap

P. Devillard, A. Benzahi, Patrizia Vignolo, Mathias Albert

Published Nov 8, 2021• 5 citations
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Abstract

We compute the fluctuations of the number of bosons with a given momentum for the Tonks-Girardeau gas at zero and finite temperature in a harmonic trap. We show that correlations between opposite momentum states $p$, which is an important fingerprint of long-range order in weakly interacting Bose systems, are suppressed. Nontrivial correlations, including negative correlations, are observed for momenta smaller than or of the order of the inverse radius of the gas. The full distribution of the number of bosons with momentum $p$ exhibits an interesting crossover from a nontrivial distribution at zero momentum to an exponential distribution. The distribution of the quasicondensate occupation is also studied. The experimental relevance of our findings for recent cold-atom experiments is discussed.

DOI
10.1103/physreva.104.053306

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We compute the fluctuations of the number of bosons with a given momentum for the Tonks-Girardeau gas at zero and finite temperature in a harmonic trap. We show that correlations between opposite momentum states ppp, which is an important fingerprint of long-range order in weakly interacting Bose systems, are suppressed. Nontrivial correlations, including negative correlations, are observed for momenta smaller than or of the order of the inverse radius of the gas. The full distribution of the number of bosons with momentum ppp exhibits an interesting crossover from a nontrivial distribution at zero momentum to an exponential distribution. The distribution of the quasicondensate occupation is also studied. The experimental relevance of our findings for recent cold-atom experiments is discussed.
We compute the fluctuations of the number of bosons with a given momentum for the Tonks-Girardeau gas at zero temperature. We show that correlations between opposite momentum states, which are an important identifying characteristic of long-range order in weakly interacting Bose systems, are suppressed and that the full distribution of the number of bosons with nonzero momentum is exponential. The distribution of the quasicondensate is however quasi-Gaussian. The experimental relevance of our findings for recent cold-atom experiments is discussed.
We determine the exact many-body properties of a bosonic Tonks-Girardeau gas confined in a harmonic potential with a tunable $\ensuremath{\delta}$-function barrier at the trap center. This is done by calculating the reduced single-particle density matrix, the pair-distribution function, and the momentum distribution of the gas as a function of barrier strength and particle number. With increasing barrier height we find that the ground-state occupation in a diagonal basis diverges from the N\sqrt{N}N​ behavior that is expected for the case of a simple harmonic trap. In fact, the scaling of the occupation number depends on whether one has an even or odd number of particles. Since this quantity is a measure of the coherence of our sample we show how the odd-even effect manifests itself in both the momentum distribution of the Bose gas and interference fringe visibility during free temporal evolution.
In the present paper, we investigate the Tonks–Girardeau (TG) gas confined in a harmonic trap at finite temperature with thermal Bose–Fermi mapping method. The pair distribution, density distribution, reduced one-body density matrix (ROBDM), the first-order correlation function, the occupations number of natural orbitals and momentum distribution are evaluated. In the whole temperature regime, the pair distribution and density distribution exhibit the same properties as those of spin-polarized fermions because both of them depend on the modulus of wavefunction rather than wavefunction. While the ROBDM, the first-order correlation function, the natural orbital occupation, momentum distribution, which depend on wavefunction, of Tonks gas displays Bose properties different from spin-polarized fermions at low temperature. At high temperature we cannot distinguish Tonks gas from the spin-polarized Fermi gas qualitatively by all properties.
We consider a strongly interacting one-dimensional (1D) Bose-Fermi mixture confined in a harmonic trap. It consists of a Tonks-Girardeau (TG) gas (1D Bose gas with repulsive hard-core interactions) and of a noninteracting Fermi gas (1D spin-aligned Fermi gas), both species interacting through hard-core repulsive interactions. Using a generalized Bose-Fermi mapping, we determine the one-body density matrices, exact particle density profiles, momentum distributions, and behavior of the mixture under 1D expansion when opening the trap. In real space, bosons and fermions do not display any phase separation: the respective density profiles extend over the same region and they both present a number of peaks equal to the total number of particles in the trap. In momentum space the bosonic component has the typical narrow TG profile, while the fermionic component shows a broad distribution with fermionic oscillations at small momenta. Due to the large boson-fermion repulsive interactions, both the bosonic and the fermionic momentum distributions decay as $C{p}^{\ensuremath{-}4}$ at large momenta, like in the case of a pure bosonic TG gas. The coefficient CCC is related to the two-body density matrix and to the bosonic concentration in the mixture. When opening the trap, both momentum distributions ``fermionize'' under expansion and turn into that of a Fermi gas with a particle number equal to the total number of particles in the mixture.
We present a simulation study of the properties of the Tonks-Girardeau gas, a system of NNN hard-core bosons confined in a one-dimensional harmonic trap. For this system the ground-state wave function is exactly known based on a Bose-Fermi mapping theorem. We employ Nelsons' interpretation of quantum mechanics in which this NNN-body wave function gives rise to a set of stochastic differential equations for the positions of the NNN particles, which can be simulated by standard methods. In particular, real space densities and momentum distributions can simply be determined by time averages along particle trajectories. Employing this approach we are able to significantly extend the range of particle numbers NNN treated numerically compared to earlier approaches, while reproducing all exactly known results for this model. We also show that for the bosons in a harmonic trap, contrary to what has been assumed so far, the momentum distribution reflects the system size scaling of the occupation numbers of the natural orbitals; that is, it can be used to decide on the presence of a Bose-Einstein condensate.
We analyze the two-body momentum correlation function for a uniform, weakly interacting, one-dimensional Bose gas. We show that the strong positive correlation between opposite momenta, expected in a Bose-Einstein condensate with a true long-range order, almost vanishes in a phase-fluctuating quasicondensate where the long-range order is destroyed. Using the Luttinger liquid approach, we derive an analytic expression for the momentum correlation function in the quasicondensate regime, showing (i) the reduction and broadening of the opposite-momentum correlations (compared to the singular behavior in a true condensate) and (ii) an emergence of anticorrelations at small momenta. We also numerically investigate the momentum correlations in the crossover between the quasicondensate and the ideal Bose-gas regimes using a classical field approach and show how the anticorrelations gradually disappear in the ideal-gas limit.