Bulletin of the American Physical Society
47th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 61, Number 8
Monday–Friday, May 23–27, 2016; Providence, Rhode Island
Session N7: Long-range or Anisotropic Interactions in Cold Gases |
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Chair: Christopher Ticknor, Los Alamos National Laboratory Room: 553AB |
Thursday, May 26, 2016 10:30AM - 10:42AM |
N7.00001: Slow dynamics in many-body quantum systems with long range interactions Lea Santos, Francisco Perez-Bernal In recent experiments with ion traps the range of the interactions between spins-1/2 can be controlled. In the limit of infinite-range interaction the system may be described by the Lipkin model, which exhibits an excited state quantum phase transition (ESQPT). The latter corresponds to a singularity in the spectrum that occurs at the ground state and propagates to higher energies as the control parameter increases beyond the ground state critical point. We show that the evolution of an initial state with energy close to the ESQPT critical point may be extremely slow. This result is surprising, since the dynamics is usually expected to be very fast in systems with long-range interactions. This behavior is justified with the analysis of the structures of the eigenstates. [Preview Abstract] |
Thursday, May 26, 2016 10:42AM - 10:54AM |
N7.00002: Master equation with quantized atomic motion including dipole-dipole interactions Fran\c{c}ois Damanet, Daniel Braun, John Martin We derive a markovian master equation for the internal dynamics of an ensemble of two-level atoms including all effects related to the quantization of their motion [1]. Our equation provides a unifying picture of the consequences of recoil and indistinguishability of atoms beyond the Lamb-Dicke regime on both their dissipative and conservative dynamics, and is relevant for experiments with ultracold trapped atoms. We give general expressions for the decay rates and the dipole-dipole shifts for any motional states, and we find analytical formulas for a number of relevant states (Gaussian states, Fock states and thermal states). In particular, we show that the dipole-dipole interactions and cooperative photon emission can be modulated through the external state of motion. The effects predicted should be experimentally observable with Rydberg atoms [2]. \\[5pt] {[1] F.\ Damanet, D.\ Braun, and J.\ Martin, arXiv:1512.06676v2.} \\ {[2] K.\ Afrousheh, P.\ Bohlouli-Zanjani, D.\ Vagale, A.\ Mugford, M.\ Fedorov, and J.\ D.\ D.\ Martin, Phys.\ Rev.\ Lett.\ \textbf{93}, 233001 (2004).} [Preview Abstract] |
Thursday, May 26, 2016 10:54AM - 11:06AM |
N7.00003: Dynamics of nematic order in ultracold dipolar gases Ulrich Ebling, Masahito Ueda We study dynamcial properties of ultracold atoms with strong dipole-dipole interactions, such as rare-earth atoms like Erbium or Dysprosium. Dipole-dipole interactions are anisotropic and can lead to the appearance of two types of nematic order in such quantum gases. Orbital nematic order is related to spatial anisotropies such as the deformation of a Fermi surface of an ultracold dipolar Fermi gas. Spin nematic order is present only in systems with spin larger than 1/2 as a higher moment of the spin operators. We study the case of a not fully polarized dipolar gas, such that the intrinsic coupling of spin and orbital degrees of freedom can lead to an interplay between orbital and spin nematic order. We investigate how this interplay can lead to a transfer between orbital and spin nematicity, similar to the transfer of spin into orbital angular momentum predicted for dipolar gases. [Preview Abstract] |
Thursday, May 26, 2016 11:06AM - 11:18AM |
N7.00004: Synthetic p-wave interaction and topological superfluids in s-wave quantum gases Han Pu, Bin Wang, Zhen Zheng, Xubo Zou, Guangcan Guo $P$-wave interaction in cold atoms may give rise to exotic topological superfluids. However, realization of $p$-wave interaction in cold atom system is experimentally challenging. Here we propose a simple scheme to synthesize effective $p$-wave interaction in conventional $s$-wave interacting quantum gases. The key idea is to load atoms into spin-dependent optical lattice potential. Using two concrete examples involving spin-1/2 fermions, we show how the original system can be mapped into a model describing spinless fermions with nearest neighbor $p$-wave interaction, whose ground state can be a topological superfluid that supports Majorana fermions under proper conditions. Our proposal has the advantage that it does not require spin-orbit coupling or loading atoms onto higher orbitals, which is the key in earlier proposals to synthesize effective $p$-wave interaction in $s$-wave quantum gases, and may provide a completely new route for realizing $p$-wave topological superfluids. [Preview Abstract] |
Thursday, May 26, 2016 11:18AM - 11:30AM |
N7.00005: Controllable non-local bright solitons in dipolar condensates Matthew Edmonds, Thomas Bland, Nick Parker The recent achievement of condensation of atoms possessing significant magnetic dipole moments affords a new opportunity to explore the interplay of magnetic effects with the coherent nature of the condensate. The dipolar interaction introduces a non-local potential into the mean-field equation of motion for the condensate, leading to novel phenomena. The ability to precisely control the condensate's dimensionality and interactions by tuning the scattering length allows the creation and probing of long-lived bright solitary wave structures. In a recent work (1) dark solitary wave excitations were explored for repulsively interacting dipolar condensates. Here, we extend this analysis to cover the case of attractive atomic interactions, leading instead to bright soliton solutions. We numerically analyse the role the relative phase and velocity play in multiple soliton collisions in conjunction with the dipole-dipole polarization and interaction strength, observing novel bound states whose character depends dramatically on the relative phase between solitons.\\ \\ (1) T. Bland, M. J. Edmonds, N. P. Proukakis, A. M. Martin, D. H. J. O'Dell, and N. G. Parker, {\it Phys. Rev. A} {\bf 92}, 063601 (2015). [Preview Abstract] |
Thursday, May 26, 2016 11:30AM - 11:42AM |
N7.00006: Observation of a Rosensweig Instability and Stable Quantum Droplets in a Dipolar Bose Gas Tilman Pfau, Igor Ferrier Barbut, Holger Kadau, Matthias Schmitt, Matthias Wenzel Ferrofluids show unusual hydrodynamic effects due to the magnetic nature of their constituents. For increasing magnetization a classical ferrofluid undergoes a Rosensweig instability and creates self-organized ordered surface structures or droplet crystals. We observe a related instability in a Bose-Einstein condensate with strong dipolar interactions resulting in surprisingly stable droplet crystals. We find that quantum fluctuations which are the origin of genuine quantum many-body effects cannot be neglected and provide a stabilizing mechanism. We study experimentally individual stable quantum droplets containing about 800 atoms which are expected to collapse at the mean-field level due to the essentially attractive interaction. By systematic measurements on individual droplets we demonstrate quantitatively that quantum fluctuations stabilize them against the mean-field collapse. We observe in addition interference of several droplets indicating that this stable many-body state is phase coherent. [Preview Abstract] |
Thursday, May 26, 2016 11:42AM - 11:54AM |
N7.00007: Crystallization of a dilute atomic dipolar condensate Russell Bisset, Blair Blakie A recent experiment found that a dilute BEC of highly-magnetic dysprosium atoms may spontaneously break up into a crystal of droplets, a process reminiscent of the Rosensweig instability [ArXiv:1508.05007]. We dynamically simulate this scenario and find that the standard dipolar Gross-Pitaevskii equation (GPE) cannot explain such a droplet crystal. Indeed, the GPE predicts too much heating during the violent droplet formation, and a droplet lifetime that is much shorter than observed in the experiment. We investigate the requisite properties of the unknown stabilization mechanism, and find that an effective repulsive interaction with a higher order density dependence than the usual two-body interactions is required to quantitatively reproduce the experimental results. [Preview Abstract] |
Thursday, May 26, 2016 11:54AM - 12:06PM |
N7.00008: Properties of a dipolar condensate with three-body interactions Peter Blakie We discuss the properties of a harmonically trapped dilute dipolar condensate with a short ranged conservative three-body interaction. We show that this system supports two distinct fluid states: a usual condensate state and a self-cohering droplet state. We develop a simple model to quantify the energetics of these states, which we verify with full numerical calculations. Based on our simple model we develop a phase diagram showing that there is a first order phase transition between the states. Using dynamical simulations we explore the phase transition dynamics, revealing that the droplet crystal observed in previous work is an excited state that arises from heating as the system crosses the phase transition. Utilising our phase diagram we show it is feasible to produce a single droplet by dynamically adjusting the confining potential. [Preview Abstract] |
Thursday, May 26, 2016 12:06PM - 12:18PM |
N7.00009: Emergent Weyl excitations in systems of polar particles Sergey Syzranov, Michael Wall, Bihui Zhu, Victor Gurarie, Ana Maria Rey Systems with Weyl quasiparticle dispersion have been predicted to display a plethora of novel fascinating phenomena: chiral anomaly, topologically protected Fermi arcs on the surfaces, non-Anderson disorder-driven transitions, etc. Over the last several years enormous research efforts have been directed at finding new Weyl semimetals in solid-state systems and ways to realise them in ultracold atomic gases. We demonstrate that excitations with Weyl dispersion generically exist in three-dimensional systems of polar particles in the presence of magnetic field. They emerge due to the dipolar-interaction-induced transitions between the $J=0$ and $J=1$ angular-momentum states of the particles. Also, we calculate the quasiparticle spectra microscopically for systems of alkaline-earth atoms that can be realised experimentally and suggest a Ramsey-spectroscopy protocol for observing Weyl excitations in them. [Preview Abstract] |
Thursday, May 26, 2016 12:18PM - 12:30PM |
N7.00010: Dipolar Vortices and Dark Solitons in Quantum Ferrofluids Nick Parker, Thomas Bland, Matthew Edmonds, Nick Proukakis, Andrew Martin, Duncan O'Dell The experimental achievement of Bose-condensed gases of atoms with large magnetic dipole moments has realized a quantum ferrofluid, which combines both superfluid and ferrofluid properties (for a review, see T. Lahaye {\it et al.}, Rep. Prog. Phys. 72, 126401 (2009)). Here the conventional isotropic and short-range atom-atom interactions become supplemented by long-range and anisotropic dipolar interactions, enriching the physical properties of the system. Here we discuss how the dipolar interactions modify quantized vortices, the fundamental nonlinear excitations of superfluids in two and three dimensions. As well as distorting the vortex profile, the dipolar interactions cause each vortex to approximate a macroscopic dipole; the vortex-vortex interaction then develops a novel anisotropic and long-range contribution (B. C. Mulkerin {\it et al.}, Phys. Rev. Lett. 111, 170402 (2013)). This is shown to significantly modify the two-vortex dynamics, and has implications for multi-vortex states. We also extend our analysis to dark solitons, the one-dimensional analogs of vortices, where dipolar interactions support unconventional dark soliton bound states (T. Bland {\it et al.}, Phys. Rev. A 92, 063601 (2015)). [Preview Abstract] |
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