Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session K50: Spin-Orbit Coupling and Artificial Gauge Fields |
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Sponsoring Units: DAMOP Chair: Lauren Aycock, Joint Quantum Institute, University of Maryland Room: Hilton Baltimore Holiday Ballroom 1 |
Wednesday, March 16, 2016 8:00AM - 8:12AM |
K50.00001: Magnetic phases of spin-1 spin-orbit coupled Bose gases Daniel Campbell, Ryan Price, Andika Putra, Ana Vald\'{e}s-Curiel, Dimitrios Trypogeorgos, Ian Spielman We experimentally explore the magnetic phases present in a near-zero temperature spin-1 spin-orbit coupled atomic Bose gas. We observe ferromagnetic and unpolarized phases which are stabilized by the spin-orbit couplingâ€™s explicit locking between spin and motion. In the limit of weak spin-orbit coupling, these phases are separated by a critical curve of 1st order quantum phase transitions, with an observed width as small as $h \times 4 \text{ Hz}$. These phase transitions give rise to long-lived metastable states. [Preview Abstract] |
Wednesday, March 16, 2016 8:12AM - 8:24AM |
K50.00002: Quantum double-exchange physics with ultracold atoms and synthetic gauge potentials Johannes Schachenmayer, Leonid Isaev, Ana Maria Rey We study an interplay between local spin exchange and N\'eel antiferromagnetism in a two-band optical lattice. The lowest narrow band is half-filled and implements the magnetic background, while a higher band contains mobile atoms. When the local spins are locked in a N\'eel state, the motion of itinerant atoms is hindered by exchange energy barriers and the system is a flat-band insulator. As we show, this picture breaks down when exchange interaction between local and mobile spins is comparable to an energy scale of the N\'eel state. In this regime, formation of singlets between local and itinerant spins gives rise to a metallic phase of mobile atoms dressed by the spin fluctuations. This state is characterized by coupled spin-charge excitations whose spin is transverse to the N\'eel vector. Our predictions can be realized with ultracold alkaline-earth fermionic atoms coupled to a laser-induced staggered magnetic field, which stabilizes the N\'eel order and controls the amount of quantum fluctuations of local spins. By tuning the strength of this laser coupling relative to the exchange interaction, one can either adiabatically drive the crossover between the flat-band insulator and correlated metal phases, or explore non-equilibrium spin-charge dynamics in quench experiments. [Preview Abstract] |
Wednesday, March 16, 2016 8:24AM - 8:36AM |
K50.00003: Interaction effects in cold gases in synthetic gauge fields Thomas Bilitewski, Nigel Cooper \setlength{\parindent}{15 pt}There has been a long-standing goal to find ways to cause neutral atoms to experience synthetic gauge fields, extending the capabilities of ultracold gases as simulators of quantum many-body systems. Such gauge fields can mimic the effect of magnetic fields and generate topological energy bands. Recent proposals to generate synthetic gauge fields rely on time-dependent periodic forcing of the quantum system. \par Interactions are of particular interest in these systems, as the interplay of time dependence and interactions can lead to inelastic scattering and the combined effect of synthetic gauge fields and strong correlations could lead to a variety of novel many body phases of degenerate fermionic or bosonic atoms. \par In the framework of Floquet Theory we study the effects of inelastic scattering induced by the intrinsic time dependence of the eigenstates and the elastic two-body interactions [1]. Specifically, we discuss this mechanism as a potential explanation of heating and band population dynamics in current experimental setups. \\ $[1]$ T. Bilitewski and N. R. Cooper, \textbf{Phys. Rev. A} 91, 033601 (2015) \& \textbf{Phys. Rev. A} 91, 063611 (2015) [Preview Abstract] |
Wednesday, March 16, 2016 8:36AM - 8:48AM |
K50.00004: Spin-Orbit Coupled Fermions in Harmonic Trap Doga Murat Kurkcuoglu After the realization of artificial spin-orbit coupling in ultracold atoms experimentally, there is an interest in spin-orbit coupled systems in ultracold atoms. In this abstract, I will discuss the emergence of two-body bound states between two Fermi atoms in the presence of spin-orbit coupling and Zeeman fields. The fermions are assumed to have only two internal states and to have attractive contact (zero-ranged) interactions. We also add an isotropic three-dimensional harmonic trap to the system, since it is the experimentally relevant case. For such a system, I will describe the few-body solutions and the effective masses of the bound-states as a function of spin-orbit and Zeeman fields. [Preview Abstract] |
Wednesday, March 16, 2016 8:48AM - 9:00AM |
K50.00005: Thermodynamics of interacting cold atomic Fermi gases with spin-orbit coupling Scott Jensen, Yoram Alhassid, Christopher Gilbreth New physics is suggested with the prediction of novel phases in cold atom systems when a synthetic spin-orbit coupling is introduced. In particular, recent studies show that a new type of Bose-Einstein condensate, termed Rashbon-BEC, is formed when a generalized Rashba spin-orbit term is present [1]. The Rashbon-BEC phase can be obtained by tuning the spin-orbit coupling strength even in the case of finite negative scattering length. This stands in contrast to the BCS-BEC crossover in the absence of spin-orbit coupling where a negative scattering length is associated with BCS physics, and its divergence signals the crossover. In our work we apply finite-temperature quantum Monte Carlo methods to a spherical Rashba spin-orbit coupled two-species Fermi gas with contact s-wave interaction in three dimensions. We will discuss the phase diagram for this system, and its crossover behavior from BCS to Rashbon-BEC.\\ \noindent [1] See for example in J. P. Vyasanakere, S. Zhang and V. B. Shenoy, Phys. Rev. B 84, 14512 (2011). [Preview Abstract] |
Wednesday, March 16, 2016 9:00AM - 9:12AM |
K50.00006: Spin-orbit coupling in the strongly interacting Fermi gas: an exact quantum Monte Carlo study Peter Rosenberg, Hao Shi, Simone Chiesa, Shiwei Zhang Spin-orbit coupling (SOC) plays an essential role in a variety of intriguing condensed matter phenomena, including the quantum Hall effect, and topological insulators and superconductors. The recent experimental realization of spin-orbit coupled Fermi gases provides a unique opportunity to study the effects of SOC in a tunable, disorder-free system. Motivated by this experimental progress, we present here the first exact numerical results on the two-dimensional, unpolarized, uniform Fermi gas with attractive interactions and Rashba SOC. Using auxiliary-field quantum Monte Carlo and incorporating recent algorithmic advances, we carry out exact calculations on sufficiently large system sizes to provide accurate results systematically as a function of experimental parameters. We obtain the equation of state, study the spin behavior and momentum distribution, and examine the interplay of SOC and pairing in real and momentum space. Our results help illuminate the rich pairing structure induced by SOC, and provide important guidance to future experimental efforts. [Preview Abstract] |
Wednesday, March 16, 2016 9:12AM - 9:24AM |
K50.00007: Melting of phase-stripes in Bose-Einstein condensates with synthetic spin-orbit coupling Asle Sudbo, Peder Galteland, Egor Babaev We study a two-component, density imbalanced Bose-Einstein condensate with density-density interactions and synthetic spin-orbit coupling, focusing on the impact of thermal fluctuations and density-density interactions on spin-orbit induced effects. We find that for intermediate density imbalance and small intercomponent density-density interactions, the ground state is non-uniform, represented by a striped state of modulated phases of the individual complex order parameter components. By using mean-field stability arguments, we calculate a critical value for the intercomponent density-density interaction, above which the non-uniform ground state collapses into a uniform single-component state. This is reproduced in Monte-Carlo simulations for intermediate values of the spin-orbit coupling. We also find that the non-uniform ground state is disordered by thermal fluctuations when heated, through a Berizinskii-Kosterlitz-Thouless unbinding of disclocation pairs. We argue that, to lowest order, the spin-orbit coupling can be seen as an effective Josephson-type locking of the phase difference $\theta_1-\theta_2$ while simultaneously allowing the system to gain energy by modulating the phase sum $\theta_1+\theta_2$. [Preview Abstract] |
Wednesday, March 16, 2016 9:24AM - 9:36AM |
K50.00008: Pairing of fermions with unequal charges in an artificial magnetic field Nur Unal, M. O. Oktel Artificial magnetic fields (AMFs) created for ultra cold systems depend sensitively on the internal structure of the atoms. In a mixture, each component couples to the AMF with a different charge. This enables the study of Bardeen-Cooper-Schrieffer pairing of fermions with unequal charges. We investigate the superconducting (SC) transition of a system formed by such pairs as a function of the field strength. We consider a homogeneous two-component Fermi gas of unequal charges but equal densities with attractive interactions. We find that the phase diagram is altered drastically compared to the usual equal charge case. First, for some AMFs there is no SC transition and isolated SC phases are formed, reflecting the discrete Landau level (LL) structure. SC phases become reentrant both in AMF and temperature. For extremely high fields where both components are confined to their lowest LLs, the effect of the charge imbalance is suppressed. Charge asymmetry reduces the critical temperature even in the low-field semiclassical regime. We discuss a pair breaking mechanism due to the unequal Lorentz forces acting on the components of the Cooper pairs to identify the underlying physics. [Preview Abstract] |
Wednesday, March 16, 2016 9:36AM - 9:48AM |
K50.00009: SPECTRUM OF THE RASHBA SPIN-ORBIT COUPLED HAMILTONIAN WITH SPIN-DEPENDENT CONTACT INTERACTION IN DIMENSION THREE Rytis Jursenas The presentation provides functional analytic interpretation for the spectrum of the Rashba spin-orbit coupled Hamiltonian considered in the presence of the out-of-plane magnetic field. The impurity scattering is treated by means of a spin-dependent contact interaction. The research was inspired by a recently proposed technique [1, 2, 3] for producing the Rashba-type spin-orbit coupling for a three-dimensional ultracold atom. The analysis of the resolvent formula shows that, for nonzero spin-orbit coupling, the eigenvalues solve the transcendental equation. For small spin-orbit-coupling strength $\alpha $, the eigenvalues are derived analytically with the accuracy up to $O(\alpha^{4})$. It is shown that there are no eigenvalues above the threshold no matter the form of a nonzero coupling parameter of contact interaction. When the lower branch of dispersion relation attains two minima, the eigenvalues are situated only below the threshold or above the minimum of the upper branch of dispersion relation; the upper bound of discrete states is also obtained. [1] B. M. Anderson et al, Phys. Rev. Lett. 111 (2013), 125301. [2] D. L. Campbell et al, Phys. Rev. A 84 (2011), 025602. [3] F. Jendrzejewski et al, Nature Physics 8 (2012), 398. [Preview Abstract] |
Wednesday, March 16, 2016 9:48AM - 10:00AM |
K50.00010: Vortex line of spin-orbit coupled Fermi superfluid through BCS to BEC Crossover Juan Yao, Shizhong Zhang Superfluid Fermi gases with spin-orbit interaction provides a unique opportunity to investigate possible effects of strong interaction in a topological superfluid. It has been suggested that with addition of Rashba-type spin-orbit coupling, a two-component Fermi gas with strong s-wave interaction can become a topological superfluid with zero-energy bound state at the core of the vortex. In this talk, I discuss the evolution of vortex structure in a spin-orbit coupled Fermi gas through the BCS-BEC crossover within Bogoliubov-de Genne formalism. We find that the largest critical current occurs in the BEC side of the resonance, in contradiction to the usual crossover without spin-orbit coupling where it occurs at unitarity. Furthermore, we discuss the core structure of the vortex by calculating the spin and density distribution around the vortex. [Preview Abstract] |
Wednesday, March 16, 2016 10:00AM - 10:12AM |
K50.00011: Stripe phase and double-roton excitations in interacting spin-orbit-coupled spin-1 Bose-Einstein condensates Kuei Sun, Chunlei Qu, Yong Xu, Yongping Zhang, Chuanwei Zhang Spin-orbit (SO) coupling plays a major role in many important phenomena in condensed matter physics. However, the SO coupling physics in high-spin systems, especially with superfluids, has not been well explored because of the spin half of electrons in solids. In this context, the recent experimental realization of spin-orbit coupling in spin-1 Bose-Einstein condensates (BECs) has opened a completely new avenue for exploring SO-coupled high-spin superfluids. Nevertheless, the experiment has only revealed the single-particle physics of the system. Here, we study the effects of interactions between atoms on the ground states and collective excitations of SO-coupled spin-1 BECs in the presence of a spin-tensor potential. We find that ferromagnetic interaction between atoms can induce a stripe phase exhibiting two modulating patterns. We characterize the phase transitions between different phases using the spin-tensor density as well as the collective dipole motion of the BEC. We show that there exists a new type of double maxon-roton structure in the Bogoliubov-excitation spectrum, attributing to the three band minima of the SO-coupled spin-1 BEC. Our work could motivate further theoretical and experimental study along this direction. [Preview Abstract] |
Wednesday, March 16, 2016 10:12AM - 10:24AM |
K50.00012: Finite temperature theory of spin-orbit coupled fermions in three dimensions in the presence of external Zeeman fields and tunable s-wave interactions Philip Powell, Gordon Baym, Carlos Sa de Melo We develop a finite temperature theory of ultracold three-dimensional Fermi gases in the presence of artificial spin-orbit coupling, Zeeman fields, and tunable s-wave interactions. With the inclusion of quadratic fluctuations, we compute both the critical temperature for superfluidity and the population of bound and unbound fermions throughout the evolution from the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensate (BEC) regimes. In particular, we show that in the BEC regime, spin-orbit coupling is capable of increasing the critical temperature relative to the no-field case, by inducing a triplet component to the superfluid order parameter, while decreasing the many-body effective mass. We also derive the time-dependent Ginzburg-Landau equation to sixth-order in the superfluid order parameter, and obtain explicit expressions for the coefficients of the effective theory valid across the entire evolution from BCS to BEC superfluidity. [Preview Abstract] |
Wednesday, March 16, 2016 10:24AM - 10:36AM |
K50.00013: Entanglement of Vortex Lattices for Ultracold Bose Gases in a Non-Abelian Gauge Potential Szu-Cheng Cheng, T. F. Jiang, Shih-Da Jheng We develop a theory, referred to as the von Neumann lattice in a higher Landau level, for vortex lattices labelled by an integral number of flux quantums per unit cell in a higher Landau level. Using this lattice theory, we study the vortex lattice states of a pseudospin-1/2 ultracold Bose gas with contact interactions in a non-Abelian gauge potential. In addition to a uniform magnetic field, the Bose gas is also subjected to a non-Abelian gauge field, which creates an effect of the spin-orbit coupling to lift the spin degeneracy of the Landau levels. Because of interactions from the spin-orbit coupling, there are new degenerate points of the single particle spectrum due to the crossings of two Landau levels at certain coupling strengths. We show that interactions from the spin-orbit coupling force the nature and structure of the vortex lattice changing dramatically if the strength of the non-Abelian gauge field is increasing. We also find that the ground state of the vortex lattice at a degenerate point exhibits strong correlation and entanglement involving vortex lattices from different Landau levels. This entangled state builds the connection between two phases of vortex lattices during the first order phase transition of the adiabatic evolution. [Preview Abstract] |
Wednesday, March 16, 2016 10:36AM - 10:48AM |
K50.00014: Baryon squishing in synthetic dimensions by effective $SU(M)$ gauge fields Sudeep Kumar Ghosh, Umesh K. Yadav, Vijay B. Shenoy We investigate the physics of $SU(M)$ symmetric interactions in the ``synthetic dimensions'' (Celi et al., PRL 112, 043001 (2014)) that provides a cold atom realization of the Hofstadter model. We show that this system is equivalent to particles (with $SU(M)$ symmetric interactions) experiencing an $SU(M)$ Zeeman field at each lattice site {\em and} a non-Abelian $SU(M)$ gauge potential that affects their hopping. This equivalence brings out the possibility of generating {\em non-local} interactions between particles at different sites of the optical lattice. In addition, the gauge field induces a {\em flavor-orbital coupling}, which mitigates the ``baryon breaking'' effect of the Zeeman field. For $M$ particles, concomitantly, the $SU(M)$ singlet baryon which is site localized in the usual 1d optical lattice, is deformed to a non-local object (``squished baryon''). We conclusively demonstrate this effect by analytical arguments and exact (numerical) diagonalization studies. Our study promises a rich many-body phase diagram for this system. It also uncovers the possibility of using the synthetic dimension system to laboratory realize condensed matter models such as the $SU(M)$ random flux model, inconceivable in conventional experimental systems. Reference: arXiv:1503.02301 [Preview Abstract] |
Wednesday, March 16, 2016 10:48AM - 11:00AM |
K50.00015: Cavity-Assisted Spin Orbit Coupling Chuanzhou Zhu, Lin Dong, Han Pu We consider a single ultracold atom trapped inside a single-mode optical cavity, where a two-photon Raman process induces an effective coupling between atom's pseudo-spin and external center-of-mass (COM) motion. Without the COM motion, this system is described by the Jaynes-Cummings (JC) model. We show how the atomic COM motion dramatically modifies the predictions based on the JC model. We also investigated the situation when cavity pumping and decay are taken into account. We take a quantum Master equation approach to study this open system and again show how the cavity-induced spin-orbit coupling affects the properties of the system. [Preview Abstract] |
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