Bulletin of the American Physical Society
2013 Joint Meeting of the APS Division of Atomic, Molecular & Optical Physics and the CAP Division of Atomic, Molecular & Optical Physics, Canada
Volume 58, Number 6
Monday–Friday, June 3–7, 2013; Quebec City, Canada
Session H1: Ultracold Atoms in the Presence of Artificial Magnetic and Spin-orbit Fields |
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Chair: Han Pu, Rice University Room: 200A |
Wednesday, June 5, 2013 10:30AM - 11:00AM |
H1.00001: Who is the Lord of the Rings in the Zeeman-spin-orbit Saga: Majorana, Dirac or Lifshitz? Invited Speaker: Carlos Sa de Melo I discuss the simultaneous effects of Zeeman and spin-orbit fields during the evolution from BCS to BEC superfluidity for ultra-cold fermions. It has been recently demonstrated experimentally that Zeeman or spin-orbit fields and interactions can be tuned in the context of ultra-cold atoms and allow for the visitation of several different phases. For systems with zero Zeeman field, the evolution from BCS to BEC superfluidity in the presence of spin-orbit effects is only a crossover [1] as the system remains fully gapped, even though a triplet component of the order parameter emerges. In contrast, for finite Zeeman fields, spin-orbit coupling induces a triplet component in the order parameter that produces nodes in the quasiparticle excitation spectrum leading to bulk topological phase transitions of the Lifshitz type [2]. Additionally, a fully gapped phase exists, where a crossover from indirect to direct gap occurs. For spin-orbit couplings with equal Rashba and Dresselhaus strengths the nodal quasi-particles are Dirac fermions that live at and in the vicinity of rings of nodes. Transitions from and to nodal phases can occur via the emergence of zero-mode Majorana fermions at phase boundaries, where rings of nodes of Dirac fermions annihilate [3,4]. Lastly, I characterize different phases via spectroscopic and thermodynamic properties and conclude that Lifshitz is the ``Lord of the Rings.''\\[4pt] [1] Li Han, C. A. R. S\'a de Melo, ``Evolution from BCS to BEC superfluidity in the presence of spin-orbit coupling,'' Phys. Rev. A 85, 011606(R) (2012), see also arXiv:1106.3613v1. \\[0pt] [2] Kangjun Seo, Li Han and C. A. R. S\'a de Melo, ``Topological phase transitions in ultra-cold Fermi superfluids: the evolution from BCS to BEC under arificial spin-orbit fields,'' Phys. Rev. A 85, 033601 (2012), see also arXiv:1108.4068v2.\\[0pt] [3] Kangjun Seo, Li Han and C. A. R. S\'a de Melo, ``Artificial spin-orbit coupling in ultra-cold Fermi superfluids,'' arXiv:1110.6364v1.\\[0pt] [4] Kangjun Seo, Li Han, and C. A. R. S\'a de Melo, ``Emergence of Majorana and Dirac Particles in Ultracold Fermions via Tunable Interactions, Spin-Orbit Effects, and Zeeman Fields,'' Phys. Rev. Lett. 109, 105303 (2012), see also arXiv:1201.0177v1. [Preview Abstract] |
Wednesday, June 5, 2013 11:00AM - 11:30AM |
H1.00002: Spin-orbit Coupled Fermi Gases and Heavy Solitons in Fermionic Superfluids Invited Speaker: Lawrence Cheuk The coupling of the spin of electrons to their motional state lies at the heart of topological phases of matter. We have created and detected spin-orbit coupling in an atomic Fermi gas via spin-injection spectroscopy, which characterizes the energy-momentum dispersion and spin composition of the quantum states. For energies within the spin-orbit gap, the system acts as a spin diode. To fully inhibit transport, we open an additional spin gap with radio-frequency coupling, thereby creating a spin-orbit coupled lattice whose spinful band structure we probe. In the presence of s-wave interactions, spin-orbit coupled fermion systems should display induced p-wave pairing and consequently topological superfluidity. Such systems can be described by a relativistic Dirac theory with a mass term that can be made to vary spatially. Topologically protected edge states are expected to occur whenever the mass term changes sign. A system that similarly supports edges states is the strongly interacting atomic Fermi gas near a Feshbach resonance. Topological excitations, such as vortices - line defects - or solitons - planar defects - have been described theoretically for decades in many different physical contexts. In superconductivity and superfluidity they represent a defect in the order parameter and give rise to localized bound states. We have created and directly observed solitons in a fermionic superfluid by imprinting a phase step into the superfluid wavefunction. These are found to be stable for many seconds, allowing us to track their oscillatory motion in the trapped superfluid. Their trapping period increases dramatically as the interactions are tuned from the BEC to the BCS regime. At the Feshbach resonance, their period is an order of magnitude larger than expectations from mean-field Bogoliubov-de Gennes theory, signaling strong effects of bosonic quantum fluctuations and possible filling of Andreev bound states. Our work opens the study of fermionic edge states in ultracold gases. In the presence of spin imbalance, the solitons created here represent one limit of the long-sought Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state of mobile Cooper pairs.\\[4pt] Additional Authors: Ariel Sommer, Massachusetts Institute of Technology; Mark Ku, Massachusetts Institute of Technology; Wenjie Jie, Massachusetts Institute of Technology; Zoran Hadzibabic, University of Cambridge; Tarik Yefsah, Massachusetts Institute of Technology; Waseem Bakr, Massachusetts Institute of Technology; Martin Zwierlein, Massachusetts Institute of Technology [Preview Abstract] |
Wednesday, June 5, 2013 11:30AM - 11:42AM |
H1.00003: How can we probe the atom mass currents induced by synthetic gauge fields? Arun Paramekanti, Matthew Killi, Stefan Trotzky Ultracold atomic fermions and bosons in an optical lattice can have quantum ground states which support equilibrium currents in the presence of synthetic magnetic fields or spin orbit coupling. As a tool to uncover these mass currents, we propose using an anisotropic quantum quench of the optical lattice which dynamically converts the current patterns into measurable density patterns. Using analytical calculations and numerical simulations, we show that this scheme can probe diverse equilibrium bulk current patterns in Bose superfluids and Fermi fluids induced by synthetic magnetic fields, as well as detect the chiral edge currents in topological states of atomic matter such as quantum Hall and quantum spin Hall insulators. [Preview Abstract] |
Wednesday, June 5, 2013 11:42AM - 11:54AM |
H1.00004: Simulating an interacting gauge theory with ultracold Bose gases Matthew Edmonds, Manuel Valiente, Gediminas Juzeliunas, Luis Santos, Patrik Ohberg Here, we will discuss how one can create artificial gauge fields for an ensemble of interacting ultracold bosonic atoms using the interacting dressed states of the light-matter coupling. Until now, all experimental gauge potentials have been static. We will show how to induce a U(1) interacting gauge field, such that there is an effective back-action between the emergent gauge potential and the matter field. By performing the appropriate transformation, the gauge field appearing in the quasi one-dimensional many-body equation of motion can be shown to be equivalent with a current operator. The resulting non-linear equation of motion can be solved exactly to yield chiral solitons as well as critical particle numbers required for the onset of rotation of a condensate in a ring geometry Finally, we will discuss the conditions relevant for observation of the above effects in terms of scattering lengths and the two-photon Rabi frequency. [Preview Abstract] |
Wednesday, June 5, 2013 11:54AM - 12:06PM |
H1.00005: Synthetic gauge fields in synthetic dimensions Gediminas Juzeliunas, Julius Ruseckas, Ian Spielman, Alessio Celi, Pietro Massignan, Maciej Lewenstein Recently a general strategy has been put forward to extend the dimension of optical lattices by employing atomic internal degrees of freedom acting as an extra dimension [1]. Here we demonstrate that by employing atoms in a standard 1D optical lattice and including an ``extra dimension'' obtained by laser-assisted transitions between the atomic sub-levels in the ground state manifold, one can effectively engineer an extended 2D lattice with a non-trivial magnetic flux. The flux is generated by a combination of the ordinary tunneling in the real space and the laser-assisted tunneling in the extra dimension, the latter being characterized by the complex amplitudes. A distinctive feature of the proposed scheme is a formation of the sharp boundaries in the extra dimension, a feature which is difficult to implement in the real-space tunneling between the atoms in optical lattices. The boundaries of the extra dimension can be closed down using additional laser-assisted transitions. This leads to the realization of the fractional (Hofstadter butterfly-type) spectrum in a remarkably simple manner. \\[4pt] [1] O. Boada, A. Celi, J. I. Latorre, and M. Lewenstein, Phys. Rev. Lett. 108, 133001 (2012). [Preview Abstract] |
Wednesday, June 5, 2013 12:06PM - 12:18PM |
H1.00006: Artificial gauge fields in a triangular optical lattice allow for an Ising-XY phase transition Juliette Simonet, Julian Struck, Malte Weinberg, Christoph Oelschlaeger, Robert Hoeppner, Ludwig Mathey, Patrick Windpassinger, Klaus Sengstock, Andre Eckardt, Philipp Hauke, Maciej Lewenstein The emulation of synthetic gauge fields for ultracold atomic systems is crucial in order to access the rich physics arising when condensed matter is placed into magnetic fields. We report here on the experimental realization of gauge-invariant staggered magnetic fluxes on a periodically driven triangular lattice. The phase distribution of a superfluid submitted to $\pi $ staggered fluxes obeys both the discrete Ising (Z2) and the continuous U(1) global phase symmetry. The interplay of these symmetries naturally raises the question of coupled order parameters and new universality classes of phase transitions. We analyze the behavior of the discrete and continuous order parameters measured for this two-dimensional spin-chirality coupled system. The strength of the staggered artificial gauge field is used to control the Z2 symmetry breaking, by lifting the degeneracy between the Ising states in analogy to a longitudinal homogeneous magnetic field in the standard Ising-Spin model. We observe a thermally driven Ising-type phase transition from an ordered, ferromagnetic to an unordered, paramagnetic state. [Preview Abstract] |
Wednesday, June 5, 2013 12:18PM - 12:30PM |
H1.00007: Universal Impurity-Induced Bound State in Topological Superfluids Lei Jiang, Hui Hu, Xia-Ji Liu, Han Pu, Yan Chen We predict a universal midgap bound state in topological superfluids, induced by either nonmagnetic or magnetic impurities in the strong scattering limit. This universal state is similar to the lowest-energy Caroli-de Gennes-Martricon bound state in a vortex core, but is bound to localized impurities. We argue that the observation of such a universal bound state can be a clear signature for identifying topological superfluids. We theoretically examine our argument for a spin-orbit coupled ultracold atomic Fermi gas trapped in a two-dimensional harmonic potential by performing extensive self-consistent calculations within the mean-field Bogoliubov-de Gennes theory. A realistic scenario for observing a universal bound state in ultracold $^{40}$K atoms is proposed. [Preview Abstract] |
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