Radial and Angular Rotons in Trapped Dipolar Gases

Ronen, Shai; Bortolotti, Daniele C. E.; Bohn, John L.

doi:10.1103/physrevlett.98.030406

Cited by 237 publications

(455 citation statements)

References 29 publications

(31 reference statements)

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“…The large discrepancy between the variational and the numerical predications on stability boundaries stems from the fact that a simple variational ansatz is generally incapable of capturing the local collapse. Similar discrepancy between the variational and numerical results also exists for dipolar Bose-Einstein condensates [19,20,21].…”

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confidence: 87%

Stability and free expansion of a dipolar Fermi gas

Zhang

et al. 2008

Phys. Rev. A

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We investigate the stability and the free expansion of a trapped dipolar Fermi gas. We show that stabilizing the system relying on tuning the trap geometry is generally inefficient. We further show that the expanded density profile always gets stretched along the attractive direction of dipolar interaction. We also point out that by switching off the dipolar interaction simultaneously with the trapping potential, the deformation of momentum distribution can be directly observed.

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confidence: 87%

Stability and free expansion of a dipolar Fermi gas

Zhang

et al. 2008

Phys. Rev. A

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“…Thus, experiments with polar molecules go beyond quantum simulation of effective theories motivated by electronic systems and aim at exploring a genuinely new domain of many-body quantum behavior, unique to dipolar interactions. Dipolar interactions can be utilized to generate long-range interactions of arbitrary shape using microwave fields [11], simulate exotic spin Hamiltonians [12,13] and are theoretically predicted to give rise to numerous interesting collective phenomena such as roton softening [14][15][16] Despite the theoretical prediction of numerous exotic quantum many-body phenomena in polar molecules, realization and observation of many of these novel predictions are still an experimental challenge. At the time this paper was written, the coldest gas of fermionic polar molecules has been realized with KRb molecules at a temperature of 1.4 T F [6-10], where T F is the Fermi temperature.…”

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Collective phenomena in a quasi-two-dimensional system of fermionic polar molecules: Band renormalization and excitons

Babadi

Demler

2011

Phys. Rev. A

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We theoretically analyze a quasi-two-dimensional system of fermionic polar molecules in a harmonic transverse confining potential. The renormalized energy bands are calculated by solving the Hartree-Fock equation numerically for various trap and dipolar interaction strengths. The inter-subband excitations of the system are studied in the conserving time-dependent Hartree-Fock (TDHF) approximation from the perspective of lattice modulation spectroscopy experiments. We find that the excitation spectrum consists of both intersubband particle-hole excitation continuums and anti-bound excitons, arising from the anisotropic nature of dipolar interactions. The excitonic modes capture the majority of the spectral weight. We also evaluate the inter-subband transition rates in order to investigate the nature of the excitonic modes and find that they are antibound states formed from particle-hole excitations arising from several subbands. Our results indicate that the excitonic effects are present for interaction strengths and temperatures accessible in current experiments with polar molecules. I. INTRODUCTIONIn the past decade, much of the experimental and theoretical progress in the field of ultracold atoms [1][2][3] are motivated by the prospect of realizing novel strongly correlated many-body states of matter. In particular, experimental realization of quantum degenerate gases of fermionic polar molecules has witnessed a very rapid progress. By association of atoms via a Feshbach resonance to form deeply bound ultracold molecules [4,5], a nearly degenerate gas of KRb polar molecules in their rotational and vibrational ground state has been recently realized [6][7][8][9][10]. The molecules can be polarized by applying a d.c. electric field, resulting in strong dipole-dipole inter-molecular interactions.A strikingly new feature of systems of fermionic polar molecules is the anisotropy of dipolar interactions, making them unparalleled among the traditional condensed matter systems. Thus, experiments with polar molecules go beyond quantum simulation of effective theories motivated by electronic systems and aim at exploring a genuinely new domain of many-body quantum behavior, unique to dipolar interactions. Dipolar interactions can be utilized to generate long-range interactions of arbitrary shape using microwave fields [11], simulate exotic spin Hamiltonians [12,13] and are theoretically predicted to give rise to numerous interesting collective phenomena such as roton softening [14][15][16] Despite the theoretical prediction of numerous exotic quantum many-body phenomena in polar molecules, realization and observation of many of these novel predictions are still an experimental challenge. At the time this paper was written, the coldest gas of fermionic polar molecules has been realized with KRb molecules at a temperature of 1.4 T F [6-10], where T F is the Fermi temperature. The majority of the mentioned quantum phenomena require strong suppression of thermal fluctuations, i.e. strong degeneracy condition (T T F ).A major...

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“…Another remarkable property of a dipolar BEC in a pancake-shaped trap is the existence of a roton minimum in its Bogoliubov spectrum [18]. Furthermore, close to the collapse threshold, the existence of structured ground states is predicted [28,29], a precursor for the supersolid phase [30] that is expected to appear in dipolar BECs in three dimensional optical lattices. Finally, a field that has gained increasing interest in the recent past is the study of unusual vortex lattice patterns in rotating dipolar BECs [31].…”

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Stabilization of a purely dipolar quantum gas against collapse

et al. 2008

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We report on the experimental observation of the dipolar collapse of a quantum gas which sets in when we reduce the contact interaction below some critical value using a Feshbach resonance. Due to the anisotropy of the dipole-dipole interaction, the stability of a dipolar Bose-Einstein condensate depends not only on the strength of the contact interaction, but also on the trapping geometry. We investigate the stability diagram and find good agreement with a universal stability threshold arising from a simple theoretical model. Using a pancake-shaped trap with the dipoles oriented along the short axis of the trap, we are able to tune the scattering length to zero, stabilizing a purely dipolar quantum gas.Interactions between atoms dominate most of the properties of quantum degenerate gases [1]. In the ultracold regime these interactions are usually well described by an effective isotropic zero-range potential. The strength and sign of this contact interaction is determined by a single parameter, the scattering length a. The contact interaction is responsible for a variety of striking properties of quantum gases. Strongly influencing the excitation spectrum of the condensate it gives rise to e.g. the superfluidity of Bose-Einstein condensates (BEC) or the existence of vortex lattices. The contact interaction also plays a crucial role in the physics of strongly correlated systems like in the BEC-BCS crossover [2] or in quantum phase transitions like the Mott insulator transition [3].Another fundamental topic is the question of the existence of a stable ground state depending on the modulus and sign of the contact interaction. In the homogeneous case repulsive contact interaction (a > 0) is necessary for the stability of the BEC. In contrast, if the contact interaction is attractive (a < 0), the BEC is unstable. This instability can be prevented by an external trapping potential. The tendency to shrink towards the center of the trap is in that case counteracted by the repulsive quantum pressure arising from the Heisenberg uncertainty relation. Detailed analysis [4] yields that a condensate is stable as long as the number of atoms N in the condensate stays below a critical value N crit given bywhere a ho is the harmonic oscillator length and k is a constant on the order of 1/2. This scaling, as well as the collapse dynamics for N > N crit , have been studied experimentally with condensates of 7 Li [5, 6] and 85 Rb [7,8]. In [9, 10] the atom number dependance of the collapse of mixtures of bosonic 87 Rb and fermionic 40 K quantum gases has been investigated. Being anisotropic and long-range, the dipole-dipole interaction (DDI) differs fundamentally from the contact interaction. Besides many other properties, the stability condition therefore changes in a system with a DDI present. Considering the case of a purely dipolar condensate with homogeneous density polarized by an external field, one finds that due to the anisotropy of the DDI, the BEC is unstable, independent of how small the dipole moment is [11]. As in the ...

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Radial and Angular Rotons in Trapped Dipolar Gases

Cited by 237 publications

References 29 publications

Stability and free expansion of a dipolar Fermi gas

Stability and free expansion of a dipolar Fermi gas

Collective phenomena in a quasi-two-dimensional system of fermionic polar molecules: Band renormalization and excitons

Stabilization of a purely dipolar quantum gas against collapse

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