Efimov-driven phase transitions of the unitary Bose gas

Piatecki, Swann; Krauth, Werner

doi:10.1038/ncomms4503

Cited by 52 publications

(80 citation statements)

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“…The microscopic structure of the first-discovered quantum fluid, superfluid liquid helium, is difficult to access due to limited experimental probes. While an ultracold atomic Bose gas with tunable interactions (characterized by its scattering length, a) had been proposed as an alternative strongly interacting Bose system [1][2][3][4][5][6][7][8] , experimental progress [9][10][11][12] has been limited by its short lifetime. Here we present time-resolved measurements of the momentum distribution of a Bose-condensed gas that is suddenly jumped to unitarity, i.e.…”

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confidence: 99%

Universal dynamics of a degenerate unitary Bose gas

et al. 2014

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From neutron stars to high-temperature superconductors, strongly interacting many-body systems at or near quantum degeneracy are a rich source of intriguing phenomena. The microscopic structure of the first-discovered quantum fluid, superfluid liquid helium, is difficult to access due to limited experimental probes. While an ultracold atomic Bose gas with tunable interactions (characterized by its scattering length, a) had been proposed as an alternative strongly interacting Bose system [1][2][3][4][5][6][7][8] , experimental progress [9][10][11][12] has been limited by its short lifetime. Here we present time-resolved measurements of the momentum distribution of a Bose-condensed gas that is suddenly jumped to unitarity, i.e. to a = ∞. Contrary to expectation, we observe that the gas lives long enough to permit the momentum to evolve to a quasi-steady-state distribution, consistent with universality, while remaining degenerate. Investigations of the time evolution of this unitary Bose gas may lead to a deeper understanding of quantum many-body physics.A powerful feature of atom gas experiments that provides access to these new regimes is the ability to change the interaction strength using a magnetic-field Feshbach resonance [13]. In particular, at the resonance location, a is infinite. For atomic Fermi gases [14][15][16][17][18][19][20], accessing this regime by adiabatically changing a led to the achievement of superfluids of paired fermions and enabled investigation of the crossover from superfluidity of weakly bound pairs, analogous to the Bardeen-Cooper-Schrieffer (BCS) theory of superconductors, to Bose-Einstein condensation (BEC) of tightly bound molecules [16,17]. For bosonic atoms, however, this route to strong interactions is stymied by the fact that three-body inelastic collisions increase as a to the fourth power [21][22][23]. This circumstance has limited experimental investigation of Bose gases with increasing interaction strength to studying either non-quantum-degenerate gases [24,25] or BECs with modest interaction strengths (na 3 < 0.008, where n is the atom number density) [9][10][11][12].The problem is that the loss rate scales as n 2 a 4 while the equilibration rate scales as na 2 v, where v is the average velocity. Thus, it would seem that the losses will always dominate as a is increased to ∞. Even if we were to forsake thermal equilibrium and suddenly change a in order to project a weakly interacting BEC onto strong interactions [12,[26][27][28], one might expect that three-body losses would still dominate the ensuing dynamics for large a. In this work, however, we use this approach to take a BEC to the unitary gas regime, and we observe dynamics that in fact saturate on a timescale shorter than that set by three-body losses and that exhibit universal scaling with density.

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Universal dynamics of a degenerate unitary Bose gas

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“…We include zero-range unitary interactions between two bosons through the exact two-body propagator [24,25], and treat them with a highly efficient direct-sampling approach [15]. The many-body density matrix is then built via the pair-product approximation.…”

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Momentum Distribution in the Unitary Bose Gas from First Principles

Comparin

Krauth

2016

Phys. Rev. Lett.

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We consider a realistic bosonic N -particle model with unitary interactions relevant for Efimov physics. Using quantum Monte Carlo methods, we find that the critical temperature for BoseEinstein condensation is decreased with respect to the ideal Bose gas. We also determine the full momentum distribution of the gas, including its universal asymptotic behavior, and compare this crucial observable to recent experimental data. Similar to the experiments with different atomic species, differentiated solely by a three-body length scale, our model only depends on a single parameter. We establish a weak influence of this parameter on physical observables. In current experiments, the thermodynamic instability of our model from the atomic gas towards an Efimov liquid could be masked by the dynamical instability due to three-body losses.First predicted in 1970[1], the Efimov effect describes the behavior of three strongly interacting bosons when any two of them cannot bind. At unitarity, when the scattering length diverges, the three-body bound states are scale invariant and they form a sequence up to vanishing binding energy and infinite spatial extension. Efimov trimers had been intensely discussed in nuclear physics, but it was in an ultracold gas of caesium atoms that they were finally discovered [2]. To observe Efimov trimers, experiments in atomic physics rely on Feshbach resonances [3], which permits to instantly switch a gas between weak interactions and the unitary limit. Such a control of interactions lacks in nuclear physics or condensed matter experiments, and singular interactions can be probed there only in the presence of accidental fine tuning [4]. Beyond the original system[2], Efimov trimers have now been observed for several multi-component systems, including bosonic, fermionic and Bose-Fermi mixtures [5][6][7]. These experimental findings are interpreted in terms of the theory of few-body strongly interacting quantum systems. For three identical bosons in three dimensions, a complete universal theory is available, on and off unitarity [4]. Further theoretical work is aimed at understanding bound states for more than three bosons, mixtures, and the effects of dimensionality.Near-unitary interparticle interactions also impact the thermodynamics of the atomic gas, the description of which presents a challenge beyond the traditional theory of the Efimov effect. In addition, mean-field theory does not apply to infinite interactions [8], and the virial expansion [9] fails to describe the low-temperature state. Moreover, in atomic-physics experiments, strong interactions enhance the three-body loss rate, making the gas of bosons unstable. A characterization of the universal dynamics of these losses has been recently achieved [10][11][12]. On the other hand, a single breakthrough experiment [13] has addressed the low-temperature thermodynamics for a unitary bosonic gas, coming to the conclusion that equilibrium was approached faster than the system life-time. The importance of this system stems from its univer...

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“…We now demonstrate that the outlined sampling scheme provides a reliable description of Bose systems at unitarity, which have attracted a great deal of attention recently experimentally and theoretically [43][44][45][46][47][48][49][50]. While the properties of unitary Fermi systems with zero-range interactions are fully determined by the s-wave scattering length [12,14,[51][52][53][54] those of Bose systems additionally depend on a three-body parameter [15,17].…”

Section: Three Dimensional Testsmentioning

confidence: 99%

“…The move described here is related to the compression-dilation move introduced in Ref. [43]. Few details were given in Ref.…”

Section: Three Dimensional Testsmentioning

confidence: 99%

“…Our simulations pursue an alternative approach, in which the scattering states of the system are discretized in such a way that the relative ground state energy E cluster of the Nbody cluster is much larger than the energy scale introduced by the discretization. We utilize a spherically symmetric harmonic trap and adjust the trapping frequency such that |E cluster | ≫ ω. Simulations are then performed at a temperature where the Bose droplet is in the ground-state dominated liquid-phase [43,56], where the finite temperature introduces center-of-mass excitations but not excitations of the relative degrees of freedom. The temperature T tr at which excitations of the relative degrees of freedom become relevant can be estimated using the "combined model" introduced in Ref.…”

Section: Three Dimensional Testsmentioning

confidence: 99%

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Incorporating exact two-body propagators for zero-range interactions intoN-body Monte Carlo simulations

Yan¹,

Blume²

2015

Phys. Rev. A

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Ultracold atomic gases are, to a very good approximation, described by pairwise zero-range interactions. This paper demonstrates that N -body systems with two-body zero-range interactions can be treated reliably and efficiently by the finite temperature and ground state path integral Monte Carlo approaches, using the exact two-body propagator for zero-range interactions in the pair product approximation. Harmonically trapped one-and three-dimensional systems are considered. A new propagator for the harmonically trapped two-body system with infinitely strong zero-range interaction, which may also have applications in real time evolution schemes, is presented.

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Efimov-driven phase transitions of the unitary Bose gas

Cited by 52 publications

References 42 publications

Universal dynamics of a degenerate unitary Bose gas

Universal dynamics of a degenerate unitary Bose gas

Momentum Distribution in the Unitary Bose Gas from First Principles

Incorporating exact two-body propagators for zero-range interactions intoN-body Monte Carlo simulations

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