Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T16: Fermions in Optical Lattices I |
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Sponsoring Units: DAMOP Chair: Henning Moritz, ETH Zurich Room: 317 |
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T16.00001: Probing quasiparticle dispersion and order parameter symmetry of ultracold fermionic superfluids and density waves via lattice David Pekker, Rajdeep Sensarma, Eugene Demler We propose a pump-probe experiment for studying the properties of condensed states of ultracold fermionic atoms in optical lattices. The pump consists of periodic modulations of the optical lattice intensity in time and the probe of measuring either the momentum distribution function or the density-density correlation functions. We apply our scheme to probing d-wave superfluids and d-density waves. In both cases we show that the dispersion relation of quasi-particles can be extracted from the momentum distribution function, and the order-parameter symmetry can be extracted from the pattern of peaks and dips that form due to the interference of the excited quasiparticles in the appropriate density-density correlation function. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T16.00002: Relaxation of Double Occupancies in large U Hubbard Model Eugene Demler, Rajdeep Sensarma, David Pekker We study the relaxation rates of double occupancies in large U Fermionic Hubbard model both in the Mott insulating state and in the compressible state with holes. We find that the relaxation rate $\sim t exp(-U2/t2)$ in the insulating state and $\sim t exp(-U/t)$ in the compressible state. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T16.00003: Modulation of Optical Lattice Potential in a Fermionic Mott Insulator Rajdeep Sensarma, David Pekker, Eugene Demler We analyze the double occupancies produced in a Fermionic Mott insulator near half-filling by modulating the optical lattice potential. We relate the rate of production of doublons to the spectral function of a hole and a doublon in the background of the spins. In the high temperature (spin disordered) state, the hole (doublon) is completely incoherent and the rate of production of doublons is peaked around $\omega=U$ and decreases montonically upto twice the bandwidth on either side. We also derive a sum-rule for energy integrated response in this limit. At low temperatures (anti-ferromagnetically ordered state), the spin ordering leads to a coherent peak in the hole spectral function along with other broad features corresponding to shake-off of spin-waves. This shows up in the doublon production rate as a sharp edge at the lower end of the spectrum and oscillations as a function of perturbing frequency. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T16.00004: Equation of state and magnetic properties of the three-dimensional repulsive Hubbard model Chia-Chen Chang, Shiwei Zhang Motivated by recent advances in fermionic optical-lattice experiments, we report results from numerical simulations of the ground state properties of the three-dimensional single-band Hubbard model (with nearest-neighbor hopping and repulsive $s$-save onsite interaction). We focus on intermediate interaction strengths, $U/t<\sim 10$. The constrained-path auxiliary-field quantum Monte Carlo method is used, with a phaseless approximation to control the sign/phase problem. One-body finite size effects and shell effects are eliminated by implementing twist-averaged boundary conditions. The equation of state is determined accurately for several values of $U/t$. We study the nature of the ground state away from half-filling by examining the spin-spin and other correlation functions as a function of doping. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T16.00005: Phase diagram and Neel temperature of fermions in a three-dimensional optical lattice Charles Mathy, David Huse One of the most exciting prospects in the field of ultracold atoms is the experimental realization of an antiferromagnetic Mott state, in a system of fermions in a three-dimensional optical lattice. Experimentalists are currently wrestling with achieving the requisite ordering temperatures. We address the question of which regions of parameter space one should explore to find the highest Neel temperatures. To this end, we perform Hartree-Fock calculations and map out the magnetic phase diagram of two component fermions in a three-dimensional simple cubic lattice. We find that the superexchange and Neel temperature are maximized in a regime of intermediate coupling, where the system is no longer well described by a one-band Hubbard model. We also perform a perturbative expansion in a Wannier basis, and study the corrections to the Hubbard model in this region. We find that the largest correction is a Hund's rule ferromagnetic coupling. Finally, our calculations suggest that the Mott plateau would be large in the intermediate coupling regime, and therefore experimentally accessible. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T16.00006: Collisional cooling of ultra-cold atom ensembles using Feshbach resonances Ludwig Mathey, Eite Tiesinga, Paul Julienne, Charles Clark We propose a new type of cooling mechanism for ultra-cold fermionic atom ensembles, which capitalizes on the energy dependence of inelastic collisions in the presence of a Feshbach resonance. We first discuss the case of a single magnetic resonance, and find that the final temperature and the cooling rate is limited by the width of the resonance. A concrete example, based on a $p$-wave resonance of $^{40}$K, is given. We then improve upon this setup by using both a very sharp optical resonance and a very broad magnetic resonance and show that one can reach temperatures competitive to those created by current technologies. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T16.00007: Fulde-Ferrell-Larkin-Ovchinnikov-like pairing of attractively interacting fermions on a two-leg ladder geometry Fabian Heidrich-Meisner, Adrian Feiguin Recent experiments on spin-imbalanced ultracold Fermi gases at MIT and Rice have stimulated an active search for conditions that would allow for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state to be realized in ultracold atomic gases. Theoretical work indicates that FFLO-like pairing is favored in low-dimensions (see [1] and references therein). Indeed, in one-dimensional (1D) optical lattices, the FFLO pairing mechanism is a dominating feature and survives in the presence of a confining potential. Here we extend our previous study [1] to the case of a two-leg ladder geometry [2]. Experimentally, ladders can be realized as arrays of double wells. Using a numerically exact approach, the density matrix renormalization group method, we show that FFLO-like pairing is found in a large part of the respective phase diagram, with an order parameter with a much richer structure than in the strict 1D case. We further shed light on the effect of a harmonic potential as present in optical lattices and establish the emergence of two-dimensional physics and a novel phase separation scenario not encountered in 1D chains. \\[0pt] [1] A. E. Feiguin and F. Heidrich-Meisner, Phys. Rev. B 76, 220508(R) (2007)\\[0pt] [2] A. E. Feiguin and F. Heidrich-Meisner, preprint arXiv:0809.1539. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T16.00008: Complete Phase Diagram of the Attractive Hubbard Model with a Zeeman Field Yen Lee Loh, Nandini Trivedi We study the attractive Hubbard model on square and cubic lattices, using a variational mean-field theory in which the interaction is decoupled in six channels (spin, charge, and pairing). We present the phase diagram as a function of attraction U, chemical potential mu, and Zeeman (spin-exchange) field h, and also as a function of the numbers of up and down spins. We test our hypothesis that FFLO states have a larger range of stability in a lattice than in the continuum, especially in lower dimensions. We discuss the results in the context of ultracold fermions in optical lattices, as well as the implications for thin film superconductors in a parallel magnetic field. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T16.00009: Orbital Analogue of the Quantum Anomalous Hall Effect in p-Band cold fermion Systems Congjun Wu We investigate the topological insulating states of the p-band systems in optical lattices induced by the on site orbital angular momentum polarization, which exhibit gapless edge modes in the absence of Landau levels. This effect arises from the energy-level splitting between the on site px+ipy and px-ipy orbitals by rotating each optical lattice site around its own center. At large rotation angular velocities, this model naturally reduces to two copies of Haldane's quantum Hall model. The distribution of the Berry curvature in momentum space and the quantized Chern numbers are calculated. The experimental realization of this state is feasible. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T16.00010: Anomalous expansion of attractively interacting Fermions in an optical lattice Takuya Kitagawa, Lucia Hackerm\"uller, Ulrich Schneider, Mar\'ia Moreno-Cardoner, Thorsten Best, Sebastian Will, Eugene Demler, Immanuel Bloch, Bel\'en Paredes We consider a two component Fermi mixture of ultracold atoms with attractive interactions in an optical lattice and in the presence of a parabolic potential. Using a high temperature expansion, we analyze the behavior of the system size when adiabatically increasing the interaction strength. We show that entropy conservation leads to an anomalous radius increase for large values of the interaction. We also find that the competition between entropy and the Hartree part of the attractive interaction makes the system reach a minimum size at a nonzero value of the interaction. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T16.00011: Strongly correlated Fermions in optical lattices: static and dynamic properties Ulrich Schneider, Lucia Hackerm\"uller, Thorsten Best, Sebastian Will, Simon Braun, Immanuel Bloch Fermionic atoms in optical lattices can serve as a model system for condensed matter physics: They implement the Hubbard model with high experimental control of the relevant parameters. We study static and dynamic properties of ultracold fermions in different regimes, varying from a metal to a band insulator in the non-interacting system and including complex metals and the Fermionic Mott Insulator for strongly repulsive systems. In the experiment, spin mixtures of fermionic $^{40}$K are loaded into a combination of a blue detuned three dimensional optical lattice and a red detuned dipole trap. This combination of optical potentials allows an independent control of lattice depth and harmonic confinement, thus enabling us to explore different regimes. In addition to the static properties we present measurements of the dynamic response of the system to changes of the external parameters. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T16.00012: Attractively Interacting Fermions in an Optical Lattice Lucia Hackermueller, Ulrich Schneider, Maria Moreno-Cardoner, Takuya Kitagawa, Thorsten Best, Sebastian Will, Simon Braun, Eugene Demler, Belen Paredes, Immanuel Bloch Mixtures of ultracold fermionic species in optical lattices can serve as a tool to test condensed matter physics models, a prominent example being the Fermi-Hubbard-Hamiltonian. We study a balanced spin mixture of $^{40}$K in $|F,m_F \rangle=|\frac{9}{2},-\frac{9}{2} \rangle$ and $|\frac{9}{2},-\frac{7}{2}\rangle$ in a three dimensional blue detuned optical lattice, where the interaction between the spin states can be tuned via a Feshbach resonance. Changing the scattering length allows us to go from non-interacting to attractive states and finally to a strongly paired system. For small attractive interactions, the cloud size shrinks upon increasing the interactions, but surprisingly we find a minimal cloud size for medium attractive interactions. For stronger interactions the cloud size increases again. This anomalous increase can be understood with a straightforward entropy argument. We compare the experimental data with the prediction of an exact calculation in the zero-tunneling limit and with a high temperature expansion theory. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T16.00013: Spin and Charge Dynamics in Atomic Fermions Loaded on Optical Lattice Masahiko Okumura, Hiroaki Onishi, Susumu Yamada, Masahiko Machida We study spin and charge dynamics of trapped two-component fermions loaded on an optical lattice by using the time dependent density matrix renormalization group (TDDMRG) method. The present target issue is dynamics of spin and charge in Mott state recently realized experimentally by [1]. Firstly, we simply shake a trapped potential superposed onto an optical lattice and observe the charge dynamics on the Mott state by using TDDMRG. Secondly, we do the same thing on a trapped potential which works only on a pseudo-spin species and observe the spin density dynamics. These results are compared with non-trapped case with an open boundary condition. Also, we compare one-dimensional chain like cases with those of n-legs square and triangular ladder systems. References [1] U. Schneider, L. Hackermuller, S. Will, Th. Best, I. Bloch, T. A. Costi, R. W. Helmes, D. Rasch, A. Rosch, arXiv:0809.1464. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T16.00014: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T16.00015: Multiband superfluidity and superfluid-insulator transition of strongly interacting fermions in an optical lattice Anton Burkov, Arun Paramekanti We study the multiband superfluid phase and superfluid-insulator transition of strongly interacting fermionic cold atoms in an optical lattice at a filling of two fermions per lattice site. Our mean-field approach includes both Hartree and pairing correlations and thus differs from previous approaches to this problem. We point out a very significant discrepancy between the mean-field theory result for the critical lattice depth of the superfluid-insulator transition and its experimental value. We show that this discrepancy is due to a very small superfluid stiffness of the paired fermion superfluid in a deep optical lattice. We also present new experimentally testable results on the modulated components of the superfluid order parameter, quasiparticle gap,and band occupation as a function of the lattice depth. [Preview Abstract] |
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