Bulletin of the American Physical Society
46th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 60, Number 7
Monday–Friday, June 8–12, 2015; Columbus, Ohio
Session M6: Long-range Interactions in Cold Gases |
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Chair: Eite Tiesinga, NIST Room: Delaware AB |
Thursday, June 11, 2015 8:00AM - 8:12AM |
M6.00001: Measurement and simulation of scattering properties of dysprosium Yijun Tang, Nathaniel Burdick, Benjamin Lev, Andrew Sykes, John Bohn Ultracold collisions can often be characterized by a single parameter, the \textit{s}-wave scattering length \textit{a}, but despite the simplicity of this model, the scattering length a often must be determined experimentally, even for alkali atoms. For highly magnetic lanthanide atoms such as dysprosium (Dy, 10 $\mu_B$), the dipolar interaction may strongly affect the scattering properties and must also be taken into account. We have characterized the elastic cross-section for scattering between ultracold Dy atoms by measuring the rethermalization rate in a Dy clouds driven out of equilibrium. The experimental data agree well with numerical simulations based on Boltzmann equations that include the dipolar interaction contribution. Our recent work on observations of inelastic dipolar scattering will also be briefly discussed. \\[4pt] [1] N. Burdick, K. Baumann, Y. Tang, M. Lu, and B. L. Lev, Phys Rev Lett \textbf{114}, 023201 (2015). [Preview Abstract] |
Thursday, June 11, 2015 8:12AM - 8:24AM |
M6.00002: The effect of a rotary echo on the correlation function of interacting Rydberg atoms Nithiwadee Thaicharoen, Andrew Schwarzkopf, Georg Raithel We use a direct spatial imaging technique to study rotary-echo effects on the pair correlation function of interacting rubidium Rydberg atoms. The echo is achieved by inverting the phase of the Rydberg-atom excitation pulse at selected times during the pulse. The resultant rotary excitation echo depends on the interplay between atom-field detuning and van-der-Waals interactions of Rydberg-atom pairs. In the on-resonant case, the rotary echo enhances the pair correlation function at distances near the blockade radius. As predicted previously [1], this is because un-paired Rydberg excitations are de-excited back to a ground state due to the echo, leaving pair-excitations, whose energies are shifted by the van-der-Waals interaction, to be detected. In the case of off-resonant excitation, we have identified a complementary case in which simultaneously excited Rydberg-atom pairs undergo the echo, leaving un-paired Rydberg excitations to be detected.\\[4pt] [1] Phys. Rev. A 81, 023406 (2010) [Preview Abstract] |
Thursday, June 11, 2015 8:24AM - 8:36AM |
M6.00003: Observation of Rydberg blockade effects at very high $n$, $n\sim300$, using strontium $n$$ ^{1}F_{3}$ states Xinyue Zhang, F.B. Dunning, Shuhei Yoshida, Joachim Burgd\"{o}rfer Rydberg blockade at very high $n$, $n\sim300$, is examined using strontium $n$$ ^{1}F_{3}$ Rydberg atoms excited in a small volume defined by two tightly-focused crossed laser beams. Measurements of the number distribution of Rydberg atoms created show deviations from a Poisson distribution revealing sizeable blockade effects. The statistics of the number distribution are studied using a Monte Carlo method in which the interaction between strontium Rydberg atoms is evaluated by solving the Schr\"{o}dinger equation within a two-active-electron model. The strength of blockade is analyzed in detail with respect to the alignment of two atoms relative to the laser polarizations. With careful control of the experimental parameters the probability for creating one, and only one, Rydberg atom, $P(1)$, in the excitation volume can be sufficiently large, $P(1)>0.6$, as to enable detailed studies of strongly-coupled Rydberg atom pairs. [Preview Abstract] |
Thursday, June 11, 2015 8:36AM - 8:48AM |
M6.00004: The role of two body interaction on the broadening of a F\"{o}rster resonance Jorge Kondo, Luis Goncalves, Jonathan Tallant, Donald Booth, James Shaffer, Luis Marcassa Since the early days of ultracold Rydberg atom physics, many-body effects in ultracold trapped alkali gases has been of central interest. The first experiments in this field involved the study of F\"{o}rster resonances as a function of atomic density. We present a study of a dc electric field tuned F\"{o}rster resonance involving 37D state Rb atoms in a high density atomic sample held in an optical dipole trap. Our results show that as the atomic density increases, the resonance linewidth increases until the resonance peaks merge. Simultaneously, we measure the 39P state population which is produced through interactions between 37D atoms. It is shown that the 39P population depends quadratically on the total Rydberg 37D atomic population. A theoretical model that takes into account the multilevel character of the interaction and Rydberg atom blockade process using only pair interactions was implemented to explain the results. The comparison between the experimental data and the model is very good, suggesting that the F\"{o}ster resonance process is dominate by two-body interaction. This work was supported by Fapesp, NSF and INCT-IQ. [Preview Abstract] |
Thursday, June 11, 2015 8:48AM - 9:00AM |
M6.00005: A survey of near threshold resonances for $1/R^n$ long-range potentials Di Shu, Ionel Simbotin, Robin C\^ot\'e We present a systematic study of threshold behavior properties for scattering problems with attractive potentials with an asymptotic tail of the type $V(R)\approx-C_n/R^n$. We show detailed numerical results for partial waves $\ell=0,\;1,\;2$, which we analyze in terms of the Jost function. Although the energy dependence of phaseshifts and cross sections is strongly affected by the presence of resonances, the Jost function has a rather smooth behavior. Thus, using very simple parametrizations for the Jost function, one can easily account for near threshold resonances, while also including fully the contribution of the long range tail of the potential. [Preview Abstract] |
Thursday, June 11, 2015 9:00AM - 9:12AM |
M6.00006: On mechanisms of BEC stability and fermions instability for electric dipolar quantum gases with the exchange part of dipole-dipole interaction Pavel Andreev In spite of the long-range nature of the dipole-dipole interaction, the self-consistent field part of the dipole-dipole interaction in BECs equals to zero. Hence the dipole-dipole interaction is related to the exchange part of the dipole-dipole interaction in BECs. However the exchange part of the dipole-dipole interaction in BECs coincides with the result of the formal application of the self-consistent field to dipolar BECs. Considering the electric dipole-dipole interaction in accordance with the Maxwell equations we obtain the positive and stable contribution of dipoles in the Bogoliubov spectrum. We obtain a different picture at the study of dipolar degenerate fermions, where there are both parts of the dipole-dipole interaction. The self-consistent field part gives the anisotropic positive contribution and the exchange part gives the negative isotropic contribution. The sing of the full contribution of dipoles depends on the direction of wave propagation. Hence the dipolar part of the spectrum of fermions brings the instability at large enough dipole moment, when the dipolar part overcomes the Fermi pressure. Strong dependence of the electric dipole-dipole interaction on the spin polarization is described as well. [Preview Abstract] |
Thursday, June 11, 2015 9:12AM - 9:24AM |
M6.00007: Quantum Chaos and Quantum Magnetism with 4f-Submerged-Shell Atoms Svetlana Kotochigova, Constantinos Makrides, Alexander Petrov, Eite Tiesinga We report on a theoretical investigation of the quantum level statistics of ultra-cold gases of open-4f-shell Er and Dy atoms based on a recently-developed computational model that can evaluate their weakly-bound molecular structure. A large interaction anisotropy between these atoms due to the large orbital angular momentum of their electrons creates a rich electronic structure. We find that this structure and their relatively large mass generates an extremely dense spectrum of rotational and vibrational levels near the dissociation limits for magnetic field strengths up to 100 Gauss. We analyzed these bound-state energy spectra and Feshbach resonance locations for signatures of chaos. For example, we find that in contrast to many other atomic systems these weakly-bound molecules already have a chaotic level distribution even in the absence of a magnetic field. We also report on the feasibility to detect quantum magnetism in a system where pairs of erbium or dysprosium are trapped in sites of an optical lattice. We predict the existence of spinor oscillations, where the population of magnetic sub levels oscillates in time due to the presence of anisotropic atomic interactions. Their periods can be used to characterize these interactions at zero and small magnetic field. [Preview Abstract] |
Thursday, June 11, 2015 9:24AM - 9:36AM |
M6.00008: The molecular quantum rotor in cold reactions at the Langevin universal limit Yuval Shagam, Ayelet Klein, Wojciech Skomorowski, Renjie Yun, Vitali Averbukh, Christiane Koch, Edvardas Narevicius Fast chemical reactions have been predicted to be solely governed by long-range interactions as was established by Langevin in 1905. The theory has become central to astrochemistry, where fast chemical processes dominate, giving rise to collision energy scaling laws of reaction rates, such as E$^{\mathrm{1/6}}$ for the van der Waals interaction. Importantly, for molecular reactants, the presence of additional anisotropic long-range interactions, such as quadrupole-quadrupole, is predicted to surface only when the molecule is rotationally excited, changing the scaling law to E$^{\mathrm{1/10}}$. Although molecular reactions with near unit probability have been observed at ultra-cold temperatures, these scaling laws and the role of the rotational state remain unconfirmed experimentally. We report the direct observation of universal scaling laws in chemi-ionization reactions of H$_{\mathrm{2}}$ and HD by He(2$^{\mathrm{3}}$P$_{\mathrm{2}})$ extending over three orders of magnitude in collision energies. For rotationally ground-state HD molecules the rate follows the E$^{\mathrm{1/6}}$ scaling, while for H$_{\mathrm{2}}$, where the majority of the molecules are rotationally excited, the scaling changes to E$^{\mathrm{1/10}}$ at low collision energies only. At the lowest collision energies the Wigner threshold laws start governing the reactions as the classical Langevin theory breaks down. [Preview Abstract] |
Thursday, June 11, 2015 9:36AM - 9:48AM |
M6.00009: On the emergence of quantum chaos from anisotropic interactions between ultracold Lanthanide atoms Constantinos Makrides, Alexander Petrov, Eite Tiesinga, Svetlana Kotochigova Ongoing experiments in the trapping and cooling of magnetic Lanthanide atoms have enabled investigations into the effects of their complex internal electronic structure on collision dynamics. Conceptually, complex-structure atoms are a middle ground between simple atomic system, such as alkali metals, and molecules. Gaining an understanding into the nature of the interactions between these atoms will in the future aid in modeling of the even-more complex molecular collision physics. Recently, we found that the collision between two Erbium atoms contains a dense set of Feshbach resonances over a modest magnetic field range. Furthermore, we found that the distribution of spacings between the resonances resembles a Wigner-Dyson distribution indicating the existence of quantum chaotic behavior. What was not understood, however, is to what degree the long-range atom-atom interactions play a role in the emergence of chaos. We show in this presentation that as a result of the complex internal structure of these atoms it is the anisotropic component of the interactions that leads to the emergence of quantum chaos. We base this observation on the exact calculations of the near-threshold bound-states of both Erbium and Dysprosium. [Preview Abstract] |
Thursday, June 11, 2015 9:48AM - 10:00AM |
M6.00010: Degeneracy and Inversion of Band Structure for Wigner Crystals on a Helix Peter Schmelcher, Alexandra Zampetaki, Jan Stockhofe We explore the formation of Wigner crystals for charged particles on a toroidal helix. Focusing on certain commensurate cases we show that the ground state undergoes a pitchfork bifurcation from the totally symmetric polygonic to a zig-zag-like configuration with increasing radius of the helix. Remarkably, we find that for a specific value of the helix radius, below the bifurcation point, the vibrational frequency spectrum collapses to a single frequency. This allows for an essentially independent small-amplitude motion of the individual particles and consequently localized excitations can propagate in time without significant spreading. Increasing the radius beyond the degeneracy point, the band structure is inverted, with the out-of-phase oscillation mode becoming lower in frequency than the mode corresponding to the center of mass motion. [Preview Abstract] |
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