Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T45: Focus Session: Magnetic and Spin Ordering in Atomic and Optical Systems |
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Sponsoring Units: GMAG DAMOP Chair: Erich Mueller, Cornell University Room: A310 |
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T45.00001: Macroscopic quantum phenomenon in a spin-orbit coupled Bose-Einstein condensate Shizhong Zhang, Tin-Lun Ho It is well-known in electron physics in semiconductors that the spin-orbit coupling gives rise to many exciting new physics, for example, the topological insulator that is now being actively studied. With the advent of artificial gauge field that can be generated using Raman lasers for neutral bosonic atoms, we can now study the corresponding effects in the boson system. In this talk, I shall discuss the structure of the spinor condensate with spin-orbit coupling. In particular, we show that the system develops stripe structure in each spin component, as a result of the fact that the ground state consists of two dressed states carrying different momentum. We also work out the phase diagram of the system, which compares well with the recent experiment. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T45.00002: Spin-orbit coupled spinor Bose-Einstein condensate Hui Zhai, Chunji Wang, Chao Gao, Chao-Ming Jian An effective spin-orbit coupling can be generated in cold atom system by engineering atom-light interactions. We study spin-1/2 and spin-1 Bose-Einstein condensates with Rashba spin-orbit coupling, and find that the condensate wave function will develop non-trivial structures. From numerical simulation we have identified two different phases. In one phase the ground state is a single plane wave, and often we find the system splits into domains and an array of vortices plays the role as domain wall. In this phase, time-reversal symmetry is broken. In the other phase the condensate wave function is a standing wave and it forms spin stripe. The transition between them is driven by interactions between bosons. We also provide an analytical understanding of these results and determines the transition point between the two phases. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T45.00003: Phase transitions of spin-orbit coupled Bose-Einstein Condensate in an external trap potential Xiangfa Zhou, Ian Mondragon-Shem, Congjun Wu Recently, the experimental realization of artificial magnetic fields using laser beams in a Rb87 Bose-Einstein condensate provides a valuable opportunity to investigate the rich physics of atomic gases in the presence of external Abelian and Non-Abelian gauge fields. We investigate the ground state properties of two-component BECs with Rashba spin-orbit coupling in the presence of external trapping potential. In the presence of density-density interaction between particles, the competitions among interaction, spin-orbit coupling and trap potential results in phase transitions of the ground states from a spiral spin-density wave state to a skyrmion type spin texture with rotational symmetry. We numerically solve the Gross-Pitaevskii equation and plot the phase diagram. The corresponding physics with asymmetrical Rashba coupling is also discussed. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:42PM |
T45.00004: Artificial Staggered Magnetic Field for Ultracold Atoms in Optical Lattices Invited Speaker: Uniform magnetic fields are ubiquitous in nature, but this is not the case for staggered magnetic fields. In this talk, I will discuss an experimental set-up for cold atoms recently proposed by us [1], which allows for the realization of a ``staggered gauge field'' in a 2D square optical lattice. If the lattice is loaded with bosons, it may be described by an effective Bose-Hubbard Hamiltonian, with complex and anisotropic hopping coefficients. A very rich phase diagram emerges: besides the usual Mott-insulator and zero-momentum condensate, a new phase with a finite momentum condensate becomes the ground-state at strong gauge fields [2]. By using the technique of Feshbach resonance, the dynamics of a coherent superposition of a vortex-carrying atomic condensate and a conventional zero-momentum molecular condensate can also be studied within the same scheme [3]. On the other hand, if the lattice is loaded with fermions, a highly tunable, graphene-like band structure can be realized, without requiring the honeycomb lattice symmetry [2]. When the system is loaded with a mixture of bosons and two-species fermions, several features of the high-Tc phase diagram can be reproduced. A dome-shaped unconventional superconducting region arises, surrounded by a non-Fermi liquid and a Fermi liquid at low and high doping, respectively [4].\\[4pt] [1] A. Hemmerich and C. Morais Smith, Phys. Rev. Lett. 99, 113002 (2007).\\[0pt] [2] Lih-King Lim, A. Hemmerich, and C. Morais Smith, Phys. Rev. Lett. 100, 130402 (2008), Phys. Rev. A 81, 023404 (2010).\\[0pt] [3] Lih-King Lim, T. Troppenz, and C. Morais Smith, arXiv:1009.1471.\\[0pt] [4] Lih-King Lim, A. Lazarides, A. Hemmerich, and C. Morais Smith, EPL 88, 36001 (2009) and Phys. Rev. A 82, 013616 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T45.00005: Itinerant ferromagnetism in a Fermi gas with contact interaction: Magnetic properties in a dilute Hubbard model Chia-Chen Chang, Shiwei Zhang, David M. Ceperley Motivated by recent experiments addressing the issue of itinerant ferromagnetism in a dilute ultra-cold Fermi gas with contact interaction, we examine ground state properties of the repulsive Hubbard model on a cubic lattice [1] by means of a very accurate auxiliary-field quantum Monte Carlo method [2]. We focus on low-density systems with varying on-site interaction $U/t$, in the range relevant to the experiments. Twist-averaged boundary conditions are used to eliminate open-shell effects and large lattice sizes are studied to reduce finite-size effects. The sign problem is controlled by a generalized constrained path approximation [2]. We find no ferromagnetic phase transition in this model. The ground-state correlations are consistent with those of a paramagnetic Fermi liquid. \\[4pt] [1] Chia-Chen Chang, Shiwei Zhang, and David M. Ceperley, arXiv:1009.1409\\[0pt] [2] Chia-Chen Chang and Shiwei Zhang, Phys. Rev. B 78, 165101 (2008). [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T45.00006: Magnetic instabilities in spin imbalanced ultracold Fermi gases Inti Sodemann Villadiego, Dmytro Pesin, Allan MacDonald We study the possibility of preparing magnetic states of spin imbalanced ultracold Fermi gases near a broad Feshbach resonance by analyzing the unstable collective magnetization modes developed when the system is placed on the BEC side. Within the approximation of momentum independent interatomic scattering, transverse magnetization instabilities appear at lower critical interaction strengths than those corresponding to the longitudinal instabilities, suggesting that the former ones are primarily responsible for driving the system into a textured state with inhomogeneous magnetization direction. The critical interaction for the onset of transverse instabilities increases with polarization. However the system already has ferromagnetic character below these interaction strengths because of a change in sign of the spin stiffness which occurs close to the Stoner transition of the corresponding unpolarized gas. We also discuss the behavior expected beyond the momentum independent scattering approximation across the resonance for, both, the superfluid and ferromagnetic instabilities present in the system and the implication of these results for experiments. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T45.00007: Energy decay constant in sodium spinor condensates Jie Jiang, Yingmei Liu, Eduardo Gomez, D.A. Quinones , P.D. Lett Spinor condensates of F=1 sodium atoms display rich spin dynamics due to the antiferromagnetic nature of the interactions in this system. Damped spin oscillations are observed in sodium spinor condensates, which eventually lead to the mean-field ground state. In recent experiments we have been able to track and observe the time evolution of atom number fluctuations, which enables the first quantitative measure of energy dissipation in the spinor condensate. We also develop a method to extract the energy in spinor dynamics from experimental data, and characterize the energy dissipation with a decay constant. This decay constant appears to follow a power-law dependence with the energy of spinor condensates. This power-law dependence has been experimentally checked for a wide range of the spinor energy, by varying the applied magnetic field strength, the magnetization and the density of the spinor condensate. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T45.00008: Tunneling properties of collective spin wave excitations in the supercurrent state of a spin-1 spinor BEC Shohei Watabe, Yusuke Kato, Yoji Ohashi We theoretically investigate tunneling properties of spin wave excitations through a barrier in the supercurrent state of a spin-1 BEC. In the ferromagnetic phase, we show that the transverse spin wave always exhibits perfect transmission, when the spin- wave momentum $p$ coincides with the momentum of supercurrent $q$. This is quite different from the case of the Bogoliubov mode, where the so-called anomalous tunneling phenomenon always occurs when $p=0$, unless the system is in the critical current state ($q=q_c$). In the polar phase, spin wave modes always exhibit perfect transmission when $p=0$, as in the case of the Bogoliubov mode. However, this anomalous tunneling behaviors of spin wave modes are shown to still hold even in the critical current state, in contrast to the breakdown of the perfect transmission of the Bogoliubov mode at $q_c$. Only when the Gross-Pitaevskii equation for the spin-1 BEC is integrable, perfect transmission of the spin wave is absent at $q_c$. Using a simple $delta$-functional barrier, we also discuss similarity between the condensate wave function in the supercurrent state and the wave functions of spin wave excitations when perfect transmission occurs. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T45.00009: Quantum rotor theory of spinor condensates in tight traps Ryan Barnett, Hoi-Yin Hui, Chien-Hung Lin, Jay D. Sau, S. Das Sarma In this talk, we theoretically construct exact mappings of many-particle bosonic systems onto quantum rotor models. In particular, we analyze the rotor representation of spinor Bose-Einstein condensates. There is an exact mapping of a spin-one condensate of fixed particle number with quadratic Zeeman interaction onto a quantum rotor model. We use the rotor mapping to describe the different dynamical regimes recently observed in $^{23}$Na condensates. We also suggest a way to experimentally observe quantum mechanical effects (collapse and revival) in spinor condensates. We classify three distinct physical limits of the rotor model: the Rabi, Josephson, and Fock regimes. The last regime corresponds to a fragmented condensate and is thus not captured by the Bogoliubov theory. The semiclassical limit of the rotor problem is discussed and connections with the quantum wave functions are made through use of the Husimi distribution function. Finally, we describe how to extend the analysis to higher-spin systems and derive a rotor model for the spin-two condensate. This work was supported by the NSF JQI Physics Frontier Center. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T45.00010: Spin susceptibility of spin-1/2 fermions with dipole interactions Benjamin M. Fregoso, Eduardo Fradkin The general form of the spin susceptibility is found for spin- 1/2 fermions with dipole interactions. In the paramagnetic phase and partially magnetized phase (ferro-nematic) the spin susceptibility is explicitly computed in the Random Phase Approximation. Important modifications to the static susceptibility are discussed which are relevant for future experiments. Unconventional collective modes in the paramagnetic and ferro-nematic phases are also discussed. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T45.00011: Quasi-two-dimensional fermionic dipolar gases in the Hartree-Fock approximation: band renormalization, inter-subband excitons and the liquid-solid phase diagram Mehrtash Babadi, Eugene Demler We study quasi-two-dimensional systems of fermionic dipolar gases in the Hartree-Fock approximation for various trap frequencies and dipolar interactions at finite temperatures and evaluate the energy dispersions of the renormalized subbands. We also study the inter-subband excitation spectrum of the system in the Time-Dependent Hartree-Fock approximation and predict the energy absorption rates in lattice modulation spectroscopy experiments. It is shown that the spectrum consists of inter-subband particle-hole excitation continuums as well as excitonic modes, and that their observation is highly likely in current experiments. Finally, we calculate the liquid-solid phase diagram of the system and find novel features such as multiple crystalline orders and re-entrant crystallization. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T45.00012: Probing non-Abelian statistics of Majorana fermions in ultracold atomic superfluid Shi-Liang Zhu, L.B. Shao, Z.D. Wang, Lu-Ming Duan We propose an experiment to directly probe the non-Abelian statistics of Majorana fermions by braiding them in an s-wave superfluid of ultracold atoms. We show different orders of braiding operations give orthogonal output states that can be distinguished through Raman spectroscopy. Realization of Majorana bound states in an s-wave superfluid requires strong spin-orbital coupling and a controllable Zeeman field in the perpendicular direction. We present a simple laser configuration to generate the artificial spin-orbital coupling and the required Zeeman field in the dark state subspace. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T45.00013: Magnetism and Cooper pairing in one-dimensional large spin fermions with repulsive interactions Hsiang-hsuan Hung, Yupeng Wang, Congjun Wu The recent experimental realization of ultracold large-spin fermionic systems provides a new opportunity to investigate exotic magnetism and Cooper pairing physics. By means of exact diagonalizaton and the density matrix renormalization group, we systematically study the magnetic properties of the Mott-insulating state of the simplest large-spin systems with hyperfine spin $F=3/2$ in one-dimension and at quarter filling. Such a system is characterized by an exact $SO(5)$ symmetry. The ground state shows various profiles at various $\theta=\tan^{-1}J_0/J_2$, where $J_0/J_2$ is the ratio of exchange strengths of the singlet ($S_T=0$) and quintet ($S_T=2$) channels. As $\theta> 45^{\circ}$ the ground state is a gapped state with dimerization patterns whereas as $\theta \le 45^{\circ}$ it is a gapless Luttinger liquid state. Furthermore, we found that in the Luttinger liquid phase the static correlation functions show power-law decays with a four-site periodicity, which is similar to an SU(4) chain. We also study the spin-$3/2$ model with doping. In the regime of $\theta> 45^{\circ}$ and at moderate doping, the singlet pairing correlations indicate power-law decays whereas the quintet pairing correlations have exponential decays. On the other hand, in the regime of $\theta \le 45^{\circ}$ the quintet pairing correlations are more robust than the singlet pairing correlations. [Preview Abstract] |
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