Bulletin of the American Physical Society
48th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 62, Number 8
Monday–Friday, June 5–9, 2017; Sacramento, California
Session M9: Quantum Gases in Low Dimensions |
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Chair: Joseph Thywissen, University of Toronto Room: 315 |
Thursday, June 8, 2017 8:00AM - 8:12AM |
M9.00001: FFLO superfluidity in a spin imbalanced Fermi gas Anna L. Marchant, Jacob A. Fry, Yi Jin, Melissa C. Revelle, Randall G. Hulet Ultracold atomic gases confined in optical lattices have proven to be highly versatile, tunable systems, capable of emulating condensed matter systems. Using the lowest two hyperfine states of $^{6}$Li to create a pseudo-spin-1/2 system we can engineer a spin imbalance in the gas, analogous to applying a magnetic field to a superconductor. Using a 2D optical lattice to create an array of 1D tubes the tunneling between tubes can be precisely controlled whilst a Feshbach resonance is used to tune the atomic interactions. Previously we identified a universal crossover regime\footnote{M. C. Revelle et al., Phys. Rev. Lett. 117, 235301 (2016)} from 1D- to 3D-like behavior in the phase separation of this spin-imbalanced Fermi gas when varying the lattice tunneling. This crossover region is expected to be a promising regime in which to observe the elusive polarized superfluid FFLO where magnetism is accommodated by the formation of pairs with finite momentum. Here we present our progress towards the observation of this exotic superfluid state. By compensating the optical potential along the weak axial direction of the lattice we can carry out 1D time-of-flight expansion to study the momentum distribution of the gas and thus search for experimental signatures of the FFLO phase. [Preview Abstract] |
Thursday, June 8, 2017 8:12AM - 8:24AM |
M9.00002: Dynamics of small trapped one-dimensional Fermi gas under oscillating magnetic fields X. Y. Yin, Yangqian Yan, D. Hudson Smith Deterministic preparation of an ultracold harmonically trapped one-dimensional Fermi gas consisting of a few fermions has been realized by the Heidelberg group. Using Floquet formalism, we study the time dynamics of two- and three-fermion systems in a harmonic trap under an oscillating magnetic field. The oscillating magnetic field produces a time-dependent interaction strength through a Feshbach resonance. We explore the dependence of these dynamics on the frequency of the oscillating magnetic field for non-interacting, weakly interacting, and strongly interacting systems. We identify the regimes where the system can be described by an effective two-state model and an effective three-state model. We find an unbounded coupling to all excited states at the infinitely strong interaction limit and several simple relations that characterize the dynamics. Based on our findings, we propose a technique for driving transition from the ground state to the excited states using an oscillating magnetic field. [Preview Abstract] |
Thursday, June 8, 2017 8:24AM - 8:36AM |
M9.00003: A Homogeneous 2D Fermi Gas Niclas Luick, Klaus Hueck, Lennart Sobirey, Jonas Siegl, Thomas Lompe, Henning Moritz Ultracold 2D Fermi gases allow to precisely characterize the interplay of reduced dimensionality and strong interactions in a quantum many-body system. So far, ultracold 2D Fermi gases have been studied in harmonic trapping potentials, which gives rise to an inhomogeneous density distribution. This complicates the interpretation of non-local quantities like correlation functions and the momentum distribution, which can only be extracted as trap-averaged quantities. In addition, the inhomogeneous density distribution reduces the chance of creating quantum phases which are predicted to exist in only small regions of the phase diagram. \\ Here, we present our realization of an ultracold 2D Fermi gas trapped in a homogeneous disk-shaped potential. The radial confinement is realized by a ring-shaped blue-detuned beam with steep walls. Additionally, a digital micro mirror device can be used to remove residual inhomogeneities and to imprint arbitrary repulsive potentials onto the system. [Preview Abstract] |
Thursday, June 8, 2017 8:36AM - 8:48AM |
M9.00004: The normal state of a strongly interacting two-dimensional Fermi gas Puneet A Murthy, Igor Boettcher, Ralf Klemt, Marvin Holten, Gerhard Z\"{u}rn, Tilman Enss, Mathias Neidig, Philipp Preiss, Selim Jochim We explore the normal phase of a strongly interacting two-dimensional Fermi gas in the BEC-BCS crossover. We use spatially resolved RF spectroscopy to measure the homogeneous response of the system for a wide range of temperatures and interaction strengths. We observe the formation of two-body dimers at high temperatures. At lower temperatures close to the superfluid critical temperatures, we find that the RF spectra show large density-dependent deviations from the dimer binding energy. Whereas pairing on the BEC side can explained by two-body physics, we observe that the pair formation in the strongly interacting crossover regime is a many-body phenomena. Surprisingly, our analysis of the density-denpendent shifts suggests that the physics across the entire crossover can be captured in a mean-field picture. [Preview Abstract] |
Thursday, June 8, 2017 8:48AM - 9:00AM |
M9.00005: Collective oscillations of an interacting quasi-2D Fermi gas. Paul Dyke, Tyson Peppler, Marta Zamorano, Sascha Hoinka, Chris Vale Ultracold gases have become an important paradigm for studying many-body quantum phenomena. One example is a two-component 2D Fermi gas with tunable interactions that allows the study of the BCS to BKT superfluid crossover. We produce a 2D Fermi gas in a highly oblate trapping potential between the antinodes of a cylindrically focused, blue detuned, TEM01 mode laser beam. A weak magnetic field curvature provides a highly harmonic confinement in the two radial directions. We investigate the collective oscillations, in particular the breathing mode frequency, of a 2D Fermi gas of Li-6 atoms throughout the 2D to 3D crossover. The breathing mode frequency provides insight into the thermodynamic equation of state which displays different limiting behaviors in 2D and 3D. We observe in the 3D regime a breathing mode frequency approaching $\sqrt 3\omega_r$, where $\omega_r$ is the radial trap frequency. In the weakly and non-interacting 2D regime we see that the breathing frequency approaches $2\omega_r$, consistent with the prediction of a classical scale invariance. However, for stronger interactions our results display an increase in the breathing mode frequency above $2\omega_r$, by around $4\%$, indicating that this classical scale invariance is broken by a quantum anomaly. [Preview Abstract] |
Thursday, June 8, 2017 9:00AM - 9:12AM |
M9.00006: Exact nonequilibrium dynamics of finite-temperature Tonks-Girardeau gases in arbitrary trapping potentials. Yasar Atas, Dimitri Gangardt, Isabelle Bouchoule, Karen Kheruntsyan We develop an exact approach for calculating the out-of-equilibrium dynamics of finite-temperature Tonks-Giradeau gases in arbitrary trapping potentials. Using the Fredholm determinant approach and the Bose-Fermi mapping we show how the problem can be reduced to a single-particle basis, wherein the finite-temperature effects enter the solution via an effective ``dressing'' of the single-particle wavefunctions by the Fermi-Dirac occupation factors. We demonstrate the utility and computational efficiency of the approach in two nontrivial out-of-equilibrium scenarios: collective breathing-mode oscillations in a harmonic trap and collisional dynamics in the Newton's cradle setting involving real-time evolution in a periodic Bragg potential. [Preview Abstract] |
Thursday, June 8, 2017 9:12AM - 9:24AM |
M9.00007: Quantum effects in cold-atom breathers Vladimir Yurovsky, Maxim Olshanii, Boris Melamed The one-dimensional (1D) Gross-Pitaevskii equation (GPE) has exact oscillatory solutions --- breathers --- predicted by the inverse scattering transform. In the mean-field approximation, they are formed from bright solitons by four-fold quench of the attractive interaction strength. Here, we address quantum counterparts of the breather states, applying the same quench to the Lieb-Liniger-McGuire model (the quantum version of the 1D GPE). Using exact Bethe-ansatz solutions for up to $N=20$ atoms, we find that the quench leads to formation of all multi-soliton (multi-string) states, possible for $N=20$, while two-soliton pairs dominate. The calculated fidelity exhibits a damped oscillatory dynamics. One of the decay mechanisms --- dephasing due to non-equidistant energy spectrum of the two-soliton states --- is expected to be suppressed at large $N$. In addition, the dephasing due to the solitons' kinetic energy spread leads to decay of the fidelity oscillation amplitude by half in the course of $\sim 2.5$ classical soliton periods for $4\le N \le 20$. The results suggest a possibility of observation of macroscopic quantum effects. [Preview Abstract] |
Thursday, June 8, 2017 9:24AM - 9:36AM |
M9.00008: Observing Spin-Charge Separation in a 1D Fermi Gas Andrew Marcum, Arif Mawardi Ismail, Francisco Fonta, Kenneth O'Hara The low energy excitations of an interacting Fermi gas in one dimension are collective sound-like modes which independently govern spin transport and density transport in such systems. In general, these spin- and density-waves travel with different interaction-dependent sound velocities. In electronic systems, this phenomenon -- referred to as spin-charge separation as density transport implies charge transport -- has been observed indirectly in tunneling experiments. Here, we aim to directly observe spin-``charge'' (i.e. density) separation in an ultracold two-component Fermi gas of atoms. Absorption imaging allows for the direct observation in real space of the dynamics of spin-density and ``charge''-density waves excited in an ultracold gas of spin-1/2 fermions confined in an array of 1D optical waveguides (formed by a 2D optical lattice). To excite spin waves in a two-component mixture of $^{\mathrm{6}}$Li atoms with minimal heating we employ a two-photon Raman transition acting only on one of the internal states to either (1) excite a spin-dipole mode or (2) locally eject one spin state from the trap. In the first approach, spin-charge separation manifests as a strong dependence of the spin-dipole mode frequency on interaction strength in contrast to a weak dependence for the density-dipole and density-quadrupole mode frequencies. In the second approach, spin-density and ``charge''-density wavepackets are excited directly and propagate at different velocities in the interacting system. [Preview Abstract] |
Thursday, June 8, 2017 9:36AM - 9:48AM |
M9.00009: Characterizing spin-charge separation in ultracold atoms confined to 1D Tsung-Lin Yang, Ya-Ting Chang, ZhengHao Zhao, Chung-You Shih, Randall Hulet One dimensional systems of fermions are predicted by Luttinger liquid theory to have different dispersion relations for spin and charge excitations. In the past, evidence of spin-charge separation has been seen in quantum wire tunneling experiments \footnote{O. M. Auslaender et al., Science 308, 88 (2005).}$^,$\footnote{Y. Jompol et al., Science 325, 597 (2009).}. However, independent measurements for spin and charge dispersion were not realized. Ultracold atoms, however, provide a highly tunable system to directly observe this phenomenon using Bragg spectroscopy\footnote{S. Hoinka et al., Phys. Rev. Lett. 109 , 050403 (2012)}. We realized such a system with fermionic $^6$Li in a 2-D optical lattice. By measuring the momentum transfer from a Raman transition while varying the relative detuning of the two-photon transition, we can measure the dispersion relation $\omega(k)$. The two ``spin" states are different hyperfine levels of the atom, and by appropriate choice of detuning, it may be possible to independently measure the spin and charge excitations. Using the tunability of interactions via a Feshbach resonance, we have measured the Bragg spectrum for the charge mode for a range of interaction strengths from the non-interacting Fermi gas to a strongly interacting one. [Preview Abstract] |
Thursday, June 8, 2017 9:48AM - 10:00AM |
M9.00010: One-Body Density Matrix and Momentum Distribution of Strongly Interacting One-Dimensional Spinor Quantum Gases Li Yang, Han Pu The one-body density matrix (OBDM) of a strongly interacting spinor quantum gas in one dimension can be written as a summation of products of spatial and spin parts. We find that there is a remarkable connection between the spatial part and the OBDM of a spinless hard-core anyon gas. This connection allows us to efficiently calculate the OBDM of the spinor system with particle numbers much larger than what was previously possible. Given the OBDM, we can easily calculate the momentum distribution of the spinor system, which again is related to the momentum distribution of the hard-core ayone gas. [Preview Abstract] |
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