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arXiv:1401.5798 · Physical Review A

Confinement-induced interlayer molecules: A route to strong interatomic interactions

Márton Kanász-Nagy, Eugene Demler, Gergely Zaránd

Published Mar 16, 2015• 4 citations
arXivHTMLPDF

Abstract

We study theoretically the interaction between two species of ultracold atoms confined in two layers of a finite separation and demonstrate the existence of confinement-induced interlayer bound and quasibound molecules: These excitonlike interlayer molecules appear for both positive and negative scattering lengths and exist even for layer separations many times larger than the interspecies scattering length. The lifetime of the quasibound molecules grows exponentially with increasing layer separation and the molecules can therefore be observed in simple shaking experiments, as we demonstrate through detailed many-body calculations. These quasibound molecules can also give rise to interspecies Feshbach resonances, enabling one to control geometrically the interaction between the two species by changing the layer separation. Rather counterintuitively, the species can be made strongly interacting, by actually increasing their spatial separation. The separation-induced interlayer resonances provide a powerful tool for the experimental control of interspecies interactions and enables one to realize novel quantum phases of multicomponent quantum gases.

DOI
10.1103/physreva.91.032704

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We study theoretically the interaction between two species of ultracold atoms confined in two layers of a finite separation and demonstrate the existence of confinement-induced interlayer bound and quasibound molecules: These excitonlike interlayer molecules appear for both positive and negative scattering lengths and exist even for layer separations many times larger than the interspecies scattering length. The lifetime of the quasibound molecules grows exponentially with increasing layer separation and the molecules can therefore be observed in simple shaking experiments, as we demonstrate through detailed many-body calculations. These quasibound molecules can also give rise to interspecies Feshbach resonances, enabling one to control geometrically the interaction between the two species by changing the layer separation. Rather counterintuitively, the species can be made strongly interacting, by actually increasing their spatial separation. The separation-induced interlayer resonances provide a powerful tool for the experimental control of interspecies interactions and enables one to realize novel quantum phases of multicomponent quantum gases.
We study theoretically the interaction between two species of bosons confined parabolically in one transverse direction but with a finite separation between the centers of the confining potentials. We demonstrate the existence of new types of confinement-induced Feshbach resonances that can be tuned by changing separation between the layers. They are much sharper than usual two-dimensional confinement-induced resonances, and can be observed in the shaking spectrum of the two-dimensional layers, as shown by detailed many-body calculations for a thermal Bose gas.
We investigate a two-species Fermi gas in which one species is confined in two parallel layers and interacts with the other species in the three-dimensional space by a tunable short-range interaction. Based on the controlled weak coupling analysis and the exact three-body calculation, we show that the system has a rich phase diagram in the plane of the effective scattering length and the layer separation. Resulting phases include an interlayer sss-wave pairing, an intralayer ppp-wave pairing, a dimer Bose-Einstein condensation, and a Fermi gas of stable Efimov-like trimers. Our system provides a widely applicable scheme to induce long-range interlayer correlations in ultracold atoms.
Using a two-channel model, we investigate theoretically the binding energy of confinement-induced Feshbach molecules in two- and one-dimensional ultracold atomic systems near a Feshbach resonance. We show that the two-channel prediction will evidently deviate from the simple single-channel theory as the width of Feshbach resonances decreases. For the one-dimensional system, we perform a full two-channel calculation, with the inclusion of bare interatomic interactions in the open channel. Away from the resonance, we find a sizable correction to the binding energy, if we neglect the bare interatomic interactions as in the previous work [D. B. M. Dickerscheid and H. T. C. Stoof, Phys. Rev. A 72, 053625 (2005)]. We compare our theoretical results with the existing experimental data and present predictions for narrow Feshbach resonances that could be tested in future experiments.
A detailed study of the anharmonicity-induced resonances caused by the coupling of center-of-mass and relative motion is presented for a system of two ultracold atoms in single-well potentials. As has been confirmed experimentally, these inelastic confinement-induced resonances are of interest since they can lead to coherent molecule formation, losses, and heating in ultracold atomic gases. A perturbative model is introduced to describe the resonance positions and the coupling strengths. The validity of the model and the behavior of the resonances for different confinement geometries are analyzed in comparison with exact numerical ab initio calculations. While such resonances have so far only been detected for large positive values of the s-waves\text{-wave}s-wave scattering length, it is found that they are present also for negative s-waves\text{-wave}s-wave scattering lengths, i.e., for attractive interactions. The possibility to coherently tune the resonances by a variation of the external confinement geometry might pave the way for coherent molecule association where magnetic Feshbach resonances are inaccessible.
We studied the scattering problem of two distinguishable atoms with unequal mass, where one atom (atom $\ensuremath{\alpha}$) is trapped in a quasi-one-dimensional (quasi-1D) tube and the other one (atom $\ensuremath{\beta}$) is localized by a 3D harmonic trap. We show that in such a system if atom $\ensuremath{\alpha}$ is much heavier than $\ensuremath{\beta}$, confinement-induced resonance (CIR) can appear when the 3D sss-wave scattering length as{a}_{s}as​ of these two atoms is much smaller than the characteristic lengths (CLs) of the confinements, for either as>0{a}_{s}>0as​>0 or as<0{a}_{s}<0as​<0. This is quite different from the usual CIRs which occur only when as{a}_{s}as​ is comparable with the CL of confinement. Moreover, the CIRs we find are broad enough to serve as a tool for the control of effective interatomic interaction. We further show the mechanism of these CIRs via the Born-Oppenheimer approximation. Our results can be used for the realization of strongly interacting systems with ultracold atoms with weak 3D background interaction (i.e., small as{a}_{s}as​), e.g., the realization of ultracold gases with strong spin-dependent interaction at zero magnetic fields.
The confinement induced resonance provides an indispensable tool for the realization of the low-dimensional strongly interacting quantum system. Here, we investigate the confinement induced resonance in spin-orbit coupled cold atoms with Raman coupling. We find that the quasi-bound levels induced by the spin-orbit coupling and Raman coupling result in the Feshbach-type resonances. For sufficiently large Raman coupling, the bound states in one dimension exist only for sufficiently strong attractive interaction. Furthermore, the bound states in quasi-one dimension exist only for sufficient large ratio of the length scale of confinement to three dimensional s-wave scattering length. The Raman coupling substantially changes the confinement-induced resonance position. We give a proposal to realize confinement induced resonance through increasing Raman coupling strength in experiments.