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 U7: Quantum Degenerate Gases |
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Chair: Ken O'Hara, Pennsylvania State University Room: 313 |
Friday, June 9, 2017 10:30AM - 10:42AM |
U7.00001: The contact of a homogeneous unitary Fermi gas Biswaroop Mukherjee, Parth Patel, Zhenjie Yan, Richard Fletcher, Julian Struck, Martin Zwierlein The contact is a fundamental quantity that measures the strength of short-range correlations in quantum gases. As one of its most important implications, it provides a link between the microscopic two-particle correlation function at small distance and the macroscopic thermodynamic properties of the gas. In particular, pairing and superfluidity in a unitary Fermi gas can be expected to leave its mark in behavior of the contact. Here we present measurements on the temperature dependence of the contact of a unitary Fermi gas across the superfluid transition. By trapping ultracold $^6$Li atoms in a potential that is homogeneous in two directions and harmonic in the third\footnote{Mukherjee et. al. arXiv:1610.10100 (2016)}, we obtain radiofrequency spectra of the homogeneous gas at a high signal-to-noise ratio. We compare our data to existing, but often mutually excluding theoretical calculations for the strongly interacting Fermi gas. [Preview Abstract] |
Friday, June 9, 2017 10:42AM - 10:54AM |
U7.00002: Collective modes of fermionic alkaline earth atoms with SU($N$) spin symmetry Sayan Choudhury, Erich Mueller, Thomas Killian, Kaden Hazzard Alkaline earth atoms have a large spin degeneracy (controllable from $N=1,\ldots, 10$) and an enhanced interaction symmetry that can enhance quantum fluctuations. Collective modes, excited by quickly changing the trap frequencies in a trapped gas, can be used to investigate properties of the excitations that emerge from these interactions. In particular, the collective mode frequency and damping time reflect properties of the gas's quasiparticles like their lifetime and typical interaction energy. We calculate the frequencies and damping rate of the breathing and quadrupole modes for fermionic alkaline earth atoms confined by a quasi-2D harmonic trap. We find a significant interaction dependent shift in the collective mode frequencies. For an isotropic trap, the breathing mode does not exhibit damping. However, the quadrupole mode, a crossover occurs from the collisionless to the hydrodynamic regime as the interaction strength increases. For the experimentally relevant case of an anisotropic trap, the breathing and quadrupole modes couple and both of these modes exhibits damping. The most important physical consequence of the large $N$ in this system is to give the ability to parametrically tune the ratio of the typical interaction strength to collisional damping. [Preview Abstract] |
Friday, June 9, 2017 10:54AM - 11:06AM |
U7.00003: Repulsive Fermi polarons in the universal mass-balanced broad-resonance case Francesco Scazza, Giacomo Valtolina, Matteo Zaccanti, Giacomo Roati, Andrea Amico, Alessia Burchianti, Chiara Fort, Massimo Inguscio, Pietro Massignan, Alessio Recati The Fermi polaron represents a fundamental problem in many-body physics. In particular, repulsive Fermi polarons are centrally important for understanding the whole phase diagram of the repulsive Fermi gas and for realizing repulsive many-body states. We employ radio-frequency spectroscopy to investigate spin-mixtures of ultracold Li-6 atoms with tunable polarization in the vicinity of a broad Feshbach resonance. We report on the observation of well-defined coherent quasiparticles up to unitarity-limited interactions. We characterize the many-body system by extracting the key properties of repulsive Fermi polarons: the energy E$_{\mathrm{+}}$, the effective mass m$^{\mathrm{\ast }}$, the residue $Z$ and the decay rate $\Gamma $. Above a critical interaction, we find E$_{\mathrm{+}}$ to exceed the Fermi energy of the bath while m$^{\mathrm{\ast }}$ diverges and even turns negative, revealing an instability of the repulsive Fermi liquid. [Preview Abstract] |
Friday, June 9, 2017 11:06AM - 11:18AM |
U7.00004: Quantum Turbulence in Fermionic Superfluids Michael Forbes, Aurel Bulgac, Gabriel Wlazlowski Fermionic superfluids provide a new realization of quantum turbulence, accessible to both experiment and theory, yet relevant to phenomena from both cold atoms to nuclear astrophysics. In this talk I will underscore several unique properties such as the high vortex line density of the unitary Fermi gas, which allows for quantum turbulence in small systems that can be studied experimentally and with a time-dependent density functional theory (TDDFT) that quantitatively captures their behaviour. Applications range from explaining cold atom experiments to resolving the 40-year old mystery of pulsar glitches in neutron stars. [Preview Abstract] |
Friday, June 9, 2017 11:18AM - 11:30AM |
U7.00005: Equation of State of Fermi Polarons Zhenjie Yan, Biswaroop Mukherjee, Parth Patel, Richard Fletcher, Julian Struck, Martin Zwierlein Fermi polarons are spin impurities dressed by interactions with a fermionic bath: an extension of the classic polaron problem where an electron is coupled to a sea of phonons. Here we present thermodynamic measurements of strongly interacting Fermi gases in the polaronic regime. We trap spin-imbalanced $^6$Li gases in a hybrid potential that is harmonic in one dimension and uniform in the other two, allowing us to extract local thermodynamic quantities with high signal to noise. The density and compressibility of the majority spin component are observed to deviate from the ideal Fermi gas. In addition, we report progress towards the gradual “undressing” of the Fermi polarons as the temperature is increased. [Preview Abstract] |
Friday, June 9, 2017 11:30AM - 11:42AM |
U7.00006: Abstract Withdrawn
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Friday, June 9, 2017 11:42AM - 11:54AM |
U7.00007: Two-dimensional Fermi gases at a p-wave resonance Shaojian Jiang, Fei Zhou We study the possibility of $p$-wave superfluid of two-dimensional Fermi gases at a $p$-wave resonance using a two-channel model. Supplemented by an $\epsilon$-expansion near two dimensions, a systematic analysis is carried out at the broad-resonance limit when the interchannel coupling is strong. We show that a homogeneous p-wave pairing expected at the mean-field level is actually unstable due to fluctuation effects, in contrast to the previously predicted $p+ip$ superfluid at the narrow-resonance limit. This implies an onset of instability when the interchannel coupling is increased. [Preview Abstract] |
Friday, June 9, 2017 11:54AM - 12:06PM |
U7.00008: Time-dependent restricted-active-space self-consistent-field theory for bosonic many-body systems Camille Leveque, Lars Bojer Madsen We have developed an \textit{ab-initio} time-dependent wavefunction based theory for the description of many-body systems of bosons. The theory is based on a configurational interaction \textit{Ansatz} for the many-body wavefunction with time-dependent self-consistent-field orbitals. The active space of the orbital excitations is subject to restrictions to be specified based on the physical situation at hand. The restrictions on the active space allow the theory to be evaluated under conditions where other wavefunction based methods, due to exponential scaling in the numerical efforts, cannot. The restrictions also allow us to clearly identify the excitations that are important for an accurate description, significantly beyond the mean-field approach. We first apply this theory to compute the ground-state energy of tens of trapped bosons, and second to simulate the dynamics following an instantaneous quenching of a non-contact interaction. The method provides accurate results and its computational cost is largely reduced compared with other wavefunction based many-body methods thanks to the restriction of the active orbital space. The important excitations are clearly identified and the method provides a new way to gain insight in correlation effects. [Preview Abstract] |
Friday, June 9, 2017 12:06PM - 12:18PM |
U7.00009: Direct visualization of strong atom--atom interactions with colliding BECs Rachel Wooten, Mackillo Kira Macroscopic quantum properties of matter can hardly become more tangible than in the 1997 experiment\footnote{M. R. Andrews, {\it et al.}, Science \textbf{275}, 637 (1997).} where an interference pattern was literally seen by imaging the collision of two BECs comprised of weakly interacting atoms. Extending such a study to strong interactions is more challenging, but feasible, following an experimental success\footnote{P. Makotyn, {\it et al.}, Nat. Phys. \textbf{10}, 116--119 (2014).} in rapidly quenching a BEC from weak to strong atom--atom interactions. A recently developed cluster-expansion approach\footnote{M.~Kira, Ann.~Phys.~{\bf 356}, 185--243 (2015).} yields a nonperturbative description of strongly interacting BECs, and it has been demonstrated to quantitatively explain\footnote{M. Kira, Nat. Commun. \textbf{6} 6624 (2015).} experiments. Here, we generalize this method to describe collision of two BECs and a simultaneous quench of atom--atom interactions. We will present how the resulting quantum many-body interactions enhances spatial bunching of the atoms which can be literally seen as dramatic, macroscopically-visible changes in the interference pattern. Consequently, future experiments should easily access many-body correlations via such an imaging. [Preview Abstract] |
Friday, June 9, 2017 12:18PM - 12:30PM |
U7.00010: Probing density and spin correlations in two-dimensional Hubbard model with ultracold fermions Chun Fai Chan, Jan Henning Drewes, Marcell Gall, Nicola Wurz, Eugenio Cocchi, Luke Miller, Daniel Pertot, Ferdinand Brennecke, Michael Koehl Quantum gases of interacting fermionic atoms in optical lattices is a promising candidate to study strongly correlated quantum phases of the Hubbard model such as the Mott-insulator, spin-ordered phases, or in particular d-wave superconductivity. We experimentally realise the two-dimensional Hubbard model by loading a quantum degenerate Fermi gas of $^{40}K$ atoms into a three-dimensional optical lattice geometry. High-resolution absorption imaging in combination with radiofrequency spectroscopy is applied to spatially resolve the atomic distribution in a single 2D layer. We investigate in local measurements of spatial correlations in both the density and spin sector as a function of filling, temperature and interaction strength. In the density sector, we compare the local density fluctuations and the global thermodynamic quantities, and in the spin sector, we observe the onset of non-local spin correlation, signalling the emergence of the anti-ferromagnetic phase. We would report our recent experimental endeavours to investigate further down in temperature in the spin sector. [Preview Abstract] |
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