Bulletin of the American Physical Society
47th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 61, Number 8
Monday–Friday, May 23–27, 2016; Providence, Rhode Island
Session N6: Strongly Interacting Fermions |
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Chair: Kaden Hazzard, Rice University Room: 552AB |
Thursday, May 26, 2016 10:30AM - 10:42AM |
N6.00001: Theory of two-dimensional polarized Fermi gases Brandon Anderson, Chien-Te Wu, Rufus Boyack, K. Levin In this talk we apply a version of BCS-BEC theory (which is explicitly compatible with the Mermin-Wagner theorem) to study the signatures of quasi-condensation in two-dimensional polarized Fermi superfluids, with and without traps. The approach we use is well calibrated, as it captured [1] the important features of recent experiments [2,3] in unpolarized 2D Fermi gases. Here, in contrast to 3D gases there is no condensate. Nevertheless for a quasi-condensed state we show: how and where phase separation occurs, where balanced core phases (as reported in the literature) emerge, and where states with finite net momentum pairing can possibly occur. [1] Phys. Rev. Lett. 115, 240401 (2015) [2] Phys. Rev. Lett. 114, 230401 (2015) [3] Phys. Rev. Lett. 115, 010401 (2015) [Preview Abstract] |
Thursday, May 26, 2016 10:42AM - 10:54AM |
N6.00002: Measuring the spectral function of a strongly interacting Fermi gas Rabin Paudel, Roman Chapurin, Tara Drake, Yoav Sagi, Deborah Jin We present a novel measurement of the single-particle spectral function for a homogeneous Fermi gas above the critical temperature for superfluidity throughout the BCS-BEC crossover. The data show signatures of both Fermi liquid behavior and pairing. We find that the data fit well to a two-component function that includes fermionic quasiparticles and an ``incoherent background'' that is modeled using the dispersion of thermal molecules. As the strength of interactions is increased, the quasiparticle spectral weight vanishes, which signals the breakdown of a Fermi liquid description. [Preview Abstract] |
Thursday, May 26, 2016 10:54AM - 11:06AM |
N6.00003: Dissociation of Cooper pairs in the BCS Limit using an Oscillating Magnetic Field Abhishek Mohapatra, D. Hudson Smith, Eric Braaten In a gas of ultracold fermionic atoms with two spin states, the scattering length can be controlled by using a Feschbach resonance. In the BCS limit, the interaction between a pair of atoms is weakly attractive and supports the formation of Cooper pairs. An oscillating magnetic field with frequency near the pairing gap can dissociate Cooper pairs into pairs of atoms. We calculate the transition rate for the dissociation process using a recently developed formalism that takes into account many-body effects through a transition matrix element of the contact operator. Our results can be used to make the first direct measurement of the pairing gap. [Preview Abstract] |
Thursday, May 26, 2016 11:06AM - 11:18AM |
N6.00004: Path integral Monte Carlo determination of the fourth-order virial coefficient for unitary two-component Fermi gas with zero-range interactions Yangqian Yan, D. Blume The unitary equal-mass Fermi gas with zero-range interactions constitutes a paradigmatic model system that is relevant to atomic, condensed matter, nuclear, particle, and astro physics. This work determines the fourth-order virial coefficient $b_4$ of such a strongly-interacting Fermi gas using a customized \textit{ab inito} path integral Monte Carlo (PIMC) algorithm. In contrast to earlier theoretical results, which disagreed on the sign and magnitude of $b_4$, our $b_4$ agrees with the experimentally determined value, thereby resolving an ongoing literature debate. Utilizing a trap regulator, our PIMC approach determines the fourth-order virial coefficient by directly sampling the partition function. An on-the-fly anti-symmetrization avoids the Thomas collapse and, combined with the use of the exact two-body zero-range propagator, establishes an efficient general means to treat small Fermi systems with zero-range interactions. [Preview Abstract] |
Thursday, May 26, 2016 11:18AM - 11:30AM |
N6.00005: Low-lying excitations in a strongly interacting Fermi gas Christopher Vale, Sascha Hoinka, Paul Dyke, Marcus Lingham We present measurements of the low-lying excitation spectrum of a strongly interacting Fermi gas across the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensate (BEC) crossover using Bragg spectroscopy. By focussing the Bragg lasers onto the central volume of the cloud we can probe atoms at near-uniform density allowing measurement of the homogeneous density-density response function. The Bragg wavevector is set to be approximately half of the Fermi wavevector to probe the collective response. Below the superfluid transition temperature the Bragg spectra dominated by the Bogoliubov-Anderson phonon mode. Single particle excitations become visible at energies greater than twice the pairing gap. As interactions are tuned from the BCS to BEC regime the phonon and single particle modes separate apart and both the pairing gap and speed of sound can be directly read off in certain regions of the crossover. Single particle pair-breaking excitations become heavily suppressed as interactions are tuned from the BCS to BEC regimes. [Preview Abstract] |
Thursday, May 26, 2016 11:30AM - 11:42AM |
N6.00006: RF Spectroscopy on a Homogeneous Fermi Gas Zhenjie Yan, Biswaroop Mukherjee, Parth Patel, Julian Struck, Martin Zwierlein Over the last two decades RF spectroscopy has been established as an indispensable tool to probe a large variety of fundamental properties of strongly interacting Fermi gases. This ranges from measurement of the pairing gap over tan's contact to the quasi-particle weight of Fermi polarons. So far, most RF spectroscopy experiments have been performed in harmonic traps, resulting in an averaged response over different densities. We have realized an optical uniform potential for ultracold Fermi gases of $^6$Li atoms, which allows us to avoid the usual problems connected to inhomogeneous systems. Here we present recent results on RF spectroscopy of these homogeneous samples with a high signal to noise ratio. In addition, we report progress on measuring the contact of a unitary Fermi gas across the normal to superfluid transition. [Preview Abstract] |
Thursday, May 26, 2016 11:42AM - 11:54AM |
N6.00007: Probing Strongly Interacting Fermi Gases in Homogeneous Potentials Parth Patel, Zhenjie Yan, Biswaroop Mukherjee, Julian Struck, Martin Zwierlein We have realized a homogeneous optical potential for quantum degenerate Fermionic atoms ($^6$Li), and characterized the flatness using in-situ tomography. This is an ideal testbed for a variety of bulk condensed matter systems. In addition, this trap can be converted to a hybrid potential, which is flat along two of the directions and harmonic along the third. This is an ideal trapping geometry to study the density response of a quantum gas to varying external potentials. Here we present our recent measurements on strongly interacting Fermions in homogeneous potentials. [Preview Abstract] |
Thursday, May 26, 2016 11:54AM - 12:06PM |
N6.00008: Vortex lattices in strongly interacting Fermi gas with crossed-beam dipole trap Yuping Wu, Xingcan Yao, Haoze Chen, Xiangpei Liu, Xiaoqiong Wang We have built an experiment system to explore the dynamic and vortex in quantum degenerate Li6 gas. By using UV MOT and crossed-beam dipole trap, we obtained BEC of 2*10\textasciicircum 5 molecules. With a tightly focused 532nm laser beam as rotating bucket wall, We observed vortex formation in strongly interacting fermi superfluid. At suitable stirring frequency we produced the condensate of fermi pairs for which up to 10 vortices were simultaneously present. We produced vortex lattices in different magnetic fields (from BEC side to BCS side). Also we measured the lifetime of vortex lattices in different interaction region. [Preview Abstract] |
Thursday, May 26, 2016 12:06PM - 12:18PM |
N6.00009: Observation of a ferromagnetic instability in repulsively interacting Fermi gases of ${}^6$Li Francesco Scazza, Giacomo Valtolina, Andrea Amico, Alessia Burchianti, Chiara Fort, Matteo Zaccanti, Massimo Inguscio, Giacomo Roati The fine control of interaction strengths and optical trapping potentials in ultracold atomic ensembles provide unique opportunities to explore strongly correlated fermion phenomena, such as superfluidity and magnetism. In our setup, we produce ${}^6$Li quantum gases in the vicinity of a broad Feshbach resonance and we subsequently superimpose to the samples a thin optical barrier to engineer either a Josephson weak link or a ferromagnetic domain wall. This technique recently enabled the experimental study of the Josephson dynamics of superfluid Fermi gases flowing through an insulating barrier, spanning a wide range of interaction strengths across the BEC-BCS crossover. On the other hand, by preparing adjacent and fully spin-polarized domains, we are able to experimentally address the upper branch of a repulsively interacting Fermi gas and its magnetic properties. Here, we report on the investigation of the onset and the stability of the ferromagnetic state. Measurements of spin diffusion dynamics in the system reveal a total suppression of spin conductance above a critical interaction strength, accompanied by a softening of the collective spin-dipole mode, indicating the existence of a ferromagnetic instability. [Preview Abstract] |
Thursday, May 26, 2016 12:18PM - 12:30PM |
N6.00010: Evidence for universal relations describing a gas with p-wave interactions S. Smale, C. Luciuk, S. Trotzky, Zhenhua Yu, Shizhong Zhang, J. H. Thywissen A remarkable set of universal relations is known to directly connect thermodynamic and microscopic properties of interacting Fermi gases. So far, these contact relations have been established only for interactions with s-wave symmetry, i.e., with zero relative angular momentum. We report measurements of two new physical quantities, the p-wave contacts, and present evidence [1] that they encode the universal aspects of p-wave interactions through recently proposed relations [2,3]. Our experiments use a spin-polarized ultracold Fermi gas of $^{40}$K, in which s-wave interactions are suppressed, while p-wave interactions are enhanced near a Feshbach resonance. Using time-resolved spectroscopy and momentum distribution measurements, we study how correlations in the system develop after quenching the atoms into an interacting state. Combining quasi-steady-state measurements with new contact relations, we infer an attractive p-wave interaction energy as large as the Fermi energy. Our results reveal new ways to understand and characterize the properties of resonantly interacting p-wave quantum gases. [1] C. Luciuk et al., Nature Phys. to appear (2016); [2] S. M. Yoshida, M. Ueda, PRL 115, 135303 (2015); [3] Zhenhua Yu, J. H. Thywissen, Shizhong Zhang, PRL 115, 135304 (2015). [Preview Abstract] |
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