Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session S14: Fermions in Optical Lattices |
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Sponsoring Units: DAMOP Chair: Ana Maria Rey, Harvard University Room: Morial Convention Center 205 |
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S14.00001: Strongly interacting Fermi gases in an optical lattice Niels Strohmaier, Robert Joerdens, Kenneth Guenter, Yosuke Takasu, Michael Koehl, Henning Moritz, Tilman Esslinger When cold fermionic atoms are placed in the periodic potential of an optical lattice, they behave similarly to electrons in a crystal. However, the properties of this synthetic material can be changed at will. Here, we report on the experimental realization and investigation of strongly interacting Fermi gases with tunable interactions. By changing the interaction strength we are able to control the transport properties: while dipole oscillations are observed for a non-interacting gas, the atomic cloud relaxes very slowly to its equilibrium position for strong attractive interactions. We suggest an interpretation in the framework of the Hubbard model including external confinement: local fermionic pairing occurs, leading to a drastically reduced tunneling rate. Furthermore, experimental results on the behavior of repulsively interacting Fermi gases will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S14.00002: Sqeezing out the entropy of Fermions in an optical lattice. Qi Zhou, Tin-Lun Ho We point out a new scheme for achieving the strongly correlated system in an optical lattice. By turning the bulk of the trapped fermions into a band insulator, the entropy of the system is expelled to the surface and removed by various means. Our scheme also illustrates a general principle of cooling in a many body system. That is, one can use a gapped state to squeeze out the entropy and then turn it into the desired state after the entropy is removed. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S14.00003: Superfluid-insulator transitions of the Fermi gas with near-unitary interactions in a periodic potential Eun-Gook Moon, Predrag Nikolic, Subir Sachdev We consider a gas of spin-1/2 fermions with interactions near the unitary limit. In an applied periodic potential, and with a density of an even integer number of fermions per unit cell, there is a second-order quantum phase transition between superfluid and insulating ground states at a critical amplitude of the lattice potential. We map out the universal phase diagram at $N=\infty$ in a model with Sp($2N$) spin symmetry, and compute the universal ratio between the critical lattice amplitude and molecule recoil energy. As the interactions between fermions are varied, the insulator evolves smoothly between a band insulator of fermions and a Mott insulator of fermion pairs. We discuss implications for recent ultra-cold atom experiments. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S14.00004: Pattern formation in mixtures of different mass ultracold atoms in optical lattices: an inhomogeneous DMFT study James Freericks Dynamical mean-field theory (DMFT) is generalized to include an inhomogeneous trap and applied to the problem of different mass spin-polarized fermionic atoms that have an interspecies interaction $U$. Such a system is described by the spinless Falicov-Kimball model in a harmonic trap (in the limit where the more massive atom is localized on the optical lattice); we examine atoms moving on a 51X51 two-dimensional square lattice. When the temperature is low enough, the system exhibits pattern formation with different types of order, ranging from checkerboard phases and phase separation to an analog of the viscous fingering seen in immiscible liquids. The temperature evolution of these ordered patterns are interesting as well, as the system can form rings of ordered phases surrounding disordered phases, which expand in size as $T$ is lowered. These patterns can be detected with noise-correlation spectroscopy or Bragg scattering, and their evolution with $T$ could by employed for thermometry. The inhomogeneous DMFT algorithm parallelizes well and is quite efficient. The main difference with exact numerical solutions or the local density approximation is that the temperature scales for the ordering are significantly higher in the IDMFT solution. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S14.00005: Fermion mixtures on an optical lattice Shan-Wen Tsai, Tun Wang We investigate mixtures of two species of fermionic ultracold atoms loaded on an optical lattice. We consider fermions with unequal masses and also fermions with unequal spin populations. We discretize the two Fermi surfaces and employ a functional renormalization-group (RG) approach to calculate the flows of effective interaction vertices in order to identify the instabilities of the system. Without any interactions produced via tuning of Feshbach resonances, a question arises as to whether there can be BCS pairing from repulsive interactions (produced by the optical trap) combined with lattice effects. For spin-independent bare contact interaction, the RG flows for unequal spin populations generate effective spin-dependent interactions. We investigate cases with different interactions, both attractive and repulsive, and different shapes of the Fermi surfaces, in particular the case when one of the fermion species has a nested Fermi surface and the other one does not. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S14.00006: Effects of frozen double occupancies on fermions in optical lattices Rajdeep Sensarma, Eugene Demler, Ehud Altman We study the effects of ``frozen'' double occupancies in metastable states of 3D fermionic Hubbard model in the strongly interacting regime. Such long lived states can be created with ultracold fermions in an optical lattice, as the lattice potential is ramped up to access strongly correlated regimes. We shall discuss how the presence of the double occupancies affect anti-ferromagnetism in these systems. We will also discuss possible ``charge'' orders in these systems. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S14.00007: Dynamical instabilities of paired fermion superfluids in optical lattices Ganesh Ramachandran, Anton Burkov, Arun Paramekanti We report on a study of dynamical instabilities in cold atom fermionic superfluids in optical lattices. Of particular interest to us are the properties of such superfluids at fermion densities corresponding to noninteger average number of pairs per lattice site. In this case superfluidity competes with charge-density-wave ordered states, which has a profound effect on how such superfluids respond to flow. In particular, by varying the pairing momentum, we find a complex interplay between dynamical, pair breaking and Landau (phonon excitation) instabilities at different fillings and different magnitudes of the pairing interaction. Using insights from this study, we construct the ``dynamical phase diagrams'' of fermion superfluids, and discuss the experimental observability of the proposed effects. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S14.00008: BCS-BEC Crossover of a Quasi-two-dimensional Fermi Gas: the Significance of Dressed Molecules Wei Zhang, Guin-Dar Lin, Luming Duan We study the crossover of a quasi-two-dimensional Fermi gas trapped in the radial plane from the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) regime by crossing a Feshbach resonance. Using an effective two-dimensional Hamiltonian with renormalized interaction between atoms and dressed molecules, we calculate the zero temperature cloud size and number density distribution and conclude that the results are consistent with the picture of BCS-BEC crossover. These results are in clear contrast to the predictions of an effective two-dimensional Hamiltonian with renormalized atom-atom interaction, where a constant cloud size and identical density profile are expected for arbitrary detunings. This inconsistence indicates that the inclusion of dressed molecules is essential to describe the two-dimensional Fermi systems, especially on the BEC side of the Feshbach resonance. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S14.00009: Itinerant Ferromagnetism in an Atom Trap Ilya Berdnikov, Piers Coleman, Steve Simon Interest in ferromagnetism has prompted the development of many theoretical techinques to study the phonomenon. However, even the most sophisticated schemes, though well motivated physically, remain intrinsically uncontrolled. This is a clear signal that more experimental input is needed, and the rapidly developing field of ultra-cold atomic gases affords just such an investigation. We propose an experiment to explore the magnetic phase transition in interacting fermionic systems, and establish signatures of ferromagnetic correlations in the observed ground states. We find, that for large trap radii ($R > 4$, in units of coherence length $\xi$), ground states are topological in nature, a ``skyrmion'' in 2D, and a ``hedgehog'' in 3D. Finally, we describe how to obtain the ferromagnetic phase diagram of itinerant electron systems from these experiments. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S14.00010: The expansion of strongly interacting fermions after the release from a trap Fabian Heidrich-Meisner, Marcos Rigol, Alejandro Muramatsu, Adrian Feiguin, Elbio Dagotto Both the recent experimental progress in cold atom gas realizations and developments in computational techniques has fueled interest in nonequilibrium properties of strongly correlated systems. Here we study the expansion of fermions in a one-dimensional lattice after released from a trap. Using the time-dependent density matrix renormalization group method, we analyze properties of the one-particle density matrix as well as the evolution of spin and density correlations. A comparison of particles escaping from a metallic region as compared to a Mott-insulating one shows that some memory on the initial state is preserved during the expansion. We further address the question to what extent the correlations measured during the expansion and thus in a non-equilibrium situation resemble those of appropriately chosen reference systems in equilibrium. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S14.00011: Quantum Antiferromagnetism of Fermion in Optical Lattices with Half-filled p-band Hui Zhai, Kai Wu We study Fermi gases in a three-dimensional optical lattice with five fermions per site, i.e. the s-band is completely filled and the $p$-band with three-fold degeneracy is half filled. We show that, for repulsive interaction between fermions, the system will exhibit spin-$3/2$ antiferromagnetic order at low temperature. This conclusion is obtained in strong interaction regime by strong coupling expansion which yields an isotropic spin-$3/2$ Heisenberg model, and also in weak interaction regime, by Hatree-Fock mean-field theory and analysis of Fermi surface nesting. We show that the critical temperature for this antiferromagnetism of a $p$-band Mott insulator is about two orders of magnitudes higher than that of an $s$-band Mott insulator, which is close to the lowest temperature attainable nowadays. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S14.00012: Stripe Formation and Superfluiditiy Correlation on Two-dimensional Optical Lattice: DMRG Studies for $n$-leg repulsive Hubbard model Masahiko Machida, Masahiko Okumura, Susumu Yamada In order to predict strongly-correlated behaviors on two-dimensional (2-D) optical lattice, we employ the parallel density matrix renormalization group (p-DMRG) method and examine 2-D repulsive (square-lattice) Hubbard model. In the presentation, we firstly suggest that box shape trap enables to observe intrinsic properties of the Hubbard model in a fixed doping in contrast to the harmonic trap bringing about wide spatial variations of atom density profiles. Next, we show atomic density profile on 4-leg repulsive Hubbard model with the open boundary condition under the box trap. The variation parameters in the simulation are the doping rate below the half-filling and the repulsive interaction $U/t$. As a result, we find that stripe formations are universal in a low hole doping range and the stripe sensitively changes its structure with variations of $U/t$ and the doping rate. A striking feature is that a stripe by a hole pair turns to one by a bi-hole pair when entering a limited strong $U/t$ range. Furthermore, a systematic calculation reveals that the Hubbard model shows a change from the stripe to the Friedel oscillation with increasing the doping rate. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S14.00013: Superfluidity of fermions with repulsive on-site interaction in an anisotropic optical lattice near a Feshbach resonance Bin Wang, Luming Duan We present numerical analysis of ground state properties of the one-dimensional general Hubbard model (GHM) with particle assisted tunnelling rates and repulsive on-site interaction (positive-U), which describes fermionic atoms in an anisotropic optical lattice near a wide Feshbach resonance. Our calculation uses the time evolving block decimation algorithm, which is an extension of the density matrix renormalization group and provides a well controlled method for one-dimensional systems. We show that the positive-U GHM, when hole doped from half-filling, shows up a phase with coexistence of quasi-long-range superfluid and charge-density-wave orders. This feature is different from the property of the conventional Hubbard model with positive-U, indicting the particle assisted tunneling in the GHM could bring in qualitatively new physics. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S14.00014: Predicted itinerant ferromagnetism with cold fermions in optical lattices Shizhong Zhang, Congjun Wu Itinerant ferromagnetism is one of the central topics in condensed matter physics. Ferromagnetism is intrinsically strong coupling physics which does not have a weak coupling limit, i.e., spontaneous spin polarization requires strong interactions to overcome the kinetic energy cost. In spite of its importance, ferromagnetism has not received enough attention in the cold atom community because the system is unstable to molecular formation if the interaction is tuned close to resonance from the positive side of the Feshbach Resonance. To overcome this difficulty, we instead propose to realize the ferromagnetic state in the $p$-orbital honeycomb lattice by taking advantage of its flat band structure. Due to the divergent density of states, even weak repulsions can drive the ferromagnetic transition while ensure the stability of the system. This will open up a new opportunity to investigate ferromagnetism with precise controllability and to realize spin transport and even spintronics applications with cold atoms. [Preview Abstract] |
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