Quasicondensation in a two-dimensional interacting Bose gas

Kagan, Yu.; Kashurnikov, V. A.; Krasavin, А. V.; Prokof’ev, Nikolay; Svistunov, Boris

doi:10.1103/physreva.61.043608

Cited by 74 publications

(51 citation statements)

References 16 publications

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“…In higher dimensions, F a takes constant values at large distances in the SF phase, which is a manifestation of the Bose-Einstein condensation. This was confirmed by QMC calculations for two-dimensional systems in discrete [360,363] and continuum [175] models.…”

Section: One-body Density Matrixsupporting

confidence: 58%

Ultracold bosons with short-range interaction in regular optical lattices

2016

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During the last decade, many exciting phenomena have been experimentally observed and theoretically predicted for ultracold atoms in optical lattices. This paper reviews these rapid developments concentrating mainly on the theory. Different types of the bosonic systems in homogeneous lattices of different dimensions as well as in the presence of harmonic traps are considered. An overview of the theoretical methods used for these investigations as well as of the obtained results is given. Available experimental techniques are presented and discussed in connection with theoretical considerations. Eigenstates of the interacting bosons in homogeneous lattices and in the presence of harmonic confinement are analyzed. Their knowledge is essential for understanding of quantum phase transitions at zero and finite temperature.

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Section: One-body Density Matrixsupporting

confidence: 58%

Ultracold bosons with short-range interaction in regular optical lattices

2016

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“…Another interesting observation was in two dimensional spin polarized atomic hydrogen on liquid 4 He [8] where a reduction in three-body dipolar recombination (which is usually associated with condensation) was observed well above the BKT transition temperature. This observation results from a reduction of density fluctuations, which corresponds to quasi-condensation [9] [10].In this letter, we present evidence of transitions in a quasi-2D Bose gas from thermal (normal gas), to quasicondensate without superfluidity, to superfluid quasicondensate (BKT transition). We explicitly identify the theoretically expected non-superfluid quasi-condensate, a feature not clearly seen in other experiments on a 2D trapped Bose gases [4,5].…”

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confidence: 83%

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confidence: 99%

Observation of a 2D Bose Gas: From Thermal to Quasicondensate to Superfluid

et al. 2009

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We present experimental results on a Bose gas in a quasi-2D geometry near the Berezinskii, Kosterlitz and Thouless (BKT) transition temperature. By measuring the density profile, in situ and after time of flight, and the coherence length, we identify different states of the gas. In particular, we observe that the gas develops a bimodal distribution without long range order. In this state, the gas presents a longer coherence length than the thermal cloud; it is quasi-condensed but is not superfluid. Experimental evidence indicates that we observe the superfluid transition (BKT transition).PACS numbers: 03.75. Lm, 64.70.Tg One of the most fascinating aspects of a Bose gas in the degenerate regime is the role of dimensionality. A 2D interacting Bose gas is superfluid at low enough temperature [1,2]. However, by contrast to the 3D case, there is no long range coherence and the coherence decays as a power law [1,2]. At temperatures above the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature, the gas is not superfluid. Due to proliferation of free vortices, the quasi-condensate (QC) is fractured into small regions of nearly uniform phase, whose size, which corresponds to the typical length of the exponential decay of the coherence, is larger than the thermal de Broglie wavelength (λ = 2π 2 /M k B T , where T is the temperature and M the atomic mass). For higher T, this size becomes smaller and approaches λ, as the gas crosses over to the thermal phase.Experiments on 2D bosonic systems, such as twodimensional 4 He films [3] and trapped Bose gases [4,5], are able to show signatures of the BKT transition. Other systems, such as the superconducting transition in arrays of Josephson junctions [6] and a two dimensional lattice of (3D) Bose-Einstein condensates [7], also exhibit a similar transition. Another interesting observation was in two dimensional spin polarized atomic hydrogen on liquid 4 He [8] where a reduction in three-body dipolar recombination (which is usually associated with condensation) was observed well above the BKT transition temperature. This observation results from a reduction of density fluctuations, which corresponds to quasi-condensation [9] [10].In this letter, we present evidence of transitions in a quasi-2D Bose gas from thermal (normal gas), to quasicondensate without superfluidity, to superfluid quasicondensate (BKT transition). We explicitly identify the theoretically expected non-superfluid quasi-condensate, a feature not clearly seen in other experiments on a 2D trapped Bose gases [4,5]. We use an interferometric method to study the coherence of the gas down to distances smaller than the thermal de Broglie wavelength. Our results can be understood using the local density approximation (LDA) on a model [11] of a homogeneous system. More recently, calculations for a trapped system have been carried out using classical-quantum field methods [12] and quantum Monte Carlo methods [13].The BKT transition occurs at a universal value of the superfluid density n s = 4/λ 2 . However, the ...

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“…As a consequence, there is no Bose condensate in the homogeneous limit. Still, a so-called quasi-condensate can be identified where long-range coherence manifests itself in the suppression of density fluctuations, while the phase is correlated only over distances smaller than the system size [3][4][5][6][7][8]. The situation is similar to a "fragmented" condensate where several low-energy modes appear with comparable weight [9].…”

Section: Introductionmentioning

confidence: 99%

“…It is not entirely clear, however, how to obtain a gapless excitation spectrum for the system in the homogeneous limit, as required by the HugenholtzPines theorem [6,7,[30][31][32][33][34]. A modified mean field theory for low-dimensional quasi-condensates was developed by Stoof's group and one of the present co-authors [6,7], building on previous path-integral approaches pioneered by Popov [3,5]. In this "modified Popov theory", the infrared divergences due to phase fluctuations are systematically removed, leading to a gapless, convergent and computationally convenient scheme that applies in all dimensions, for homogeneous and trapped systems.…”

Section: Introductionmentioning

confidence: 99%

Comparison between microscopic methods for finite-temperature Bose gases

et al. 2011

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We analyze the equilibrium properties of a weakly interacting, trapped quasi-one-dimensional Bose gas at finite temperatures and compare different theoretical approaches. We focus in particular on two stochastic theories: a number-conserving Bogoliubov (ncB) approach and a stochastic GrossPitaevskii equation (sGPe) that have been extensively used in numerical simulations. Equilibrium properties like density profiles, correlation functions, and the condensate statistics are compared to predictions based upon a number of alternative theories. We find that due to thermal phase fluctuations, and the corresponding condensate depletion, the ncB approach loses its validity at relatively low temperatures. This can be attributed to the change in the Bogoliubov spectrum, as the condensate gets thermally depleted, and to large fluctuations beyond perturbation theory. Although the two stochastic theories are built on different thermodynamic ensembles (ncB: canonical, sGPe: grandcanonical), they yield the correct condensate statistics in a large BEC (strong enough particle interactions). For smaller systems, the sGPe results are prone to anomalously large number fluctuations, well-known for the grand-canonical, ideal Bose gas. Based on the comparison of the above theories to the modified Popov approach, we propose a simple procedure for approximately extracting the Penrose-Onsager condensate from first-and second-order correlation functions that is computationally convenient. This also clarifies the link between condensate and quasi-condensate in the Popov theory of low-dimensional systems.

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Quasicondensation in a two-dimensional interacting Bose gas

Cited by 74 publications

References 16 publications

Ultracold bosons with short-range interaction in regular optical lattices

Ultracold bosons with short-range interaction in regular optical lattices

Observation of a 2D Bose Gas: From Thermal to Quasicondensate to Superfluid

Comparison between microscopic methods for finite-temperature Bose gases

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