Bulletin of the American Physical Society
52nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 66, Number 6
Monday–Friday, May 31–June 4 2021; Virtual; Time Zone: Central Daylight Time, USA
Session U07: Quantum Simulation with Degenerate Fermi GasesLive
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Chair: Randy Hulet, Rice |
Thursday, June 3, 2021 2:00PM - 2:12PM Live |
U07.00001: Tunneling transport of strongly interacting Fermi gases across the superfluid transition Giulia Del Pace, Woo Jin Kwon, Francesco Scazza, Giacomo Roati, Massimo Inguscio, Matteo Zaccanti Tunneling transport measurements provide a powerful tool to unveil both the coherence properties of a many-body system, through the celebrated Josephson effect, and the role of excitations in its conduction dynamics.Here I present our results in probing condensation of strongly interacting fermionic superfluids, by injecting controlled currents in an ultracold atom two-reservoirs system weakly-coupled through a tunable tunneling barrier. In the absence of any applied chemical potential difference, we directly measure Josephson supercurrents, that depend sinusoidally on the relative phase. By comparing the measured Josephson critical current throughout the BEC-BCS crossover with an analytic model, we extract the condensed fraction of fermionic superfluids.Moreover, we characterize the operation of an atomic junction at unitarity across the superfluid transition. We find Josephson supercurrents to vanish when approaching the critical temperature due to condensate depletion. Remarkably, we observe the condensate to contribute also to resistive currents. In contrast with superconducting junctions, we detect a large anomalous normal conductance at low temperature, arising from the coherent coupling between the condensate and Bogoliubov-Anderson phonons.Our work highlights the key role of transport measurements to disclose the nature of quantum materials. |
Thursday, June 3, 2021 2:12PM - 2:24PM Live |
U07.00002: Direct Observation of Heat Transport in Strongly Interacting Fermi Gases Zhenjie Yan, Parth B Patel, Biswaroop Mukherjee, Richard Fletcher, Martin W Zwierlein The transport of heat is one of the most fundamental features of any material. It can distinguish states of matter and offers powerful insights into the underlying microscopic mechanisms of transport. In general, any measure sensitive to the energy distribution of the sample can be used as a thermometer, such as light emitted from a black body. As we demonstrate here in a strongly interacting Fermi gas, radio-frequency (rf) spectra of interacting atomic gases can serve as a direct in situ thermometer. The rf pulse transfers atoms in an energy-selective way into a non-interacting state, essentially providing a local map of quasi-particle excitations, in our case the number of broken fermion pairs. Using this technique for the unitary Fermi gas, we observe two distinctive modes for the transfer of heat: In a normal, non-superfluid gas heat propagates diffusively, while below the superfluid transition temperature heat propagates ballistically as second sound. From the damping time of heat diffusion we obtain the thermal conductivity, while the speed and damping of second sound yields the superfluid density and the second sound diffusivity. The response functions we measure are well represented by the two-fluid expressions by Hohenberg and Martin. The results inform theories of transport of strongly interacting fermionic matter, from strongly correlated superconductors to neutron stars and quark matter. |
Thursday, June 3, 2021 2:24PM - 2:36PM Live |
U07.00003: Microscopy of Continuum Fermi Gases Philipp M Preiss, Luca Bayha, Marvin Holten, Keerthan Subramanian, Ralf Klemt, Sandra Brandstetter, Carl Heintze, Selim Jochim Ultracold atomic gases are exceptionally well suited to study the physics of strongly interacting Fermi gases. Tunable interactions and engineered geometries have enabled insights into the origin of fermionic pairing, superfluidity, and collective excitations in normal and degenerate Fermi gases. The next step in this endeavor will be to gain access to single-particle observables for a microscopic comparison between experiment and theory. |
Thursday, June 3, 2021 2:36PM - 2:48PM Live |
U07.00004: On the Stability of the Repulsive Fermi Gas with Short-Range Interactions Yunpeng Ji, Gabriel Assumpcao, Jianyi Chen, Jere Makinen, Grant Schumacher, Philip Tuckman, Franklin Vivanco, Nir Navon We investigate the stability of a uniform spin-balanced repulsive Fermi gas with short-range interactions. When the atom loss is dominated by background gas collisions, we observe isoenthalpic heating, the quantum Joule-Thomson effect of a nearly ideal Fermi gas. At stronger interactions, we observe the threshold law for three-body recombination with two indistinguishable fermions, i.e. the linear dependence of the three-body loss coefficient, K3, on kinetic energy. We also measure the scaling relation between K3 and the two-body scattering length, K3 ∝ a6 across more than three decades. |
Thursday, June 3, 2021 2:48PM - 3:00PM Live |
U07.00005: Observation of spin-charge separation in a 1D 6Li Fermi gas Danyel Cavazos-Cavazos, Ruwan Senaratne, Ya-Ting Chang, Randall G Hulet Interacting fermions that are confined to 1D can only support collective excitations and are thus governed by the Tomonaga-Luttinger liquid (TLL) theory, in which collective excitations decouple into charge and spin modes instead of particle-like excitations. Previously, we characterized the dynamic response for the low energy charge excitations of a 1D spin-1/2 gas of atomic fermions using Bragg spectroscopy, and found good agreement with TLL theory [1]. Spontaneous emission induced by the near-detuned Bragg probe, however, prevented the observation of spin excitations. In this work, we use the relatively narrow 2S-3P transition in 6Li in order to minimize the rate of spontaneous emission. A pseudospin-1/2 system is realized with the lowest- and third-to-lowest, |1>-|3>, hyperfine sublevels of 6Li. The atoms are loaded into a 2D optical lattice, which creates an array of quasi-1D tubes. We tune the inter-species interactions via a magnetic Feshbach resonance and use Bragg spectroscopy with a momentum transfer of 0.2 kF to obtain the low-energy excitation spectra for both modes. We compare the measured dynamical structure factor with the Tomonaga-Luttinger liquid theory, thus realizing the first direct measurement of the speeds of the spin and charge excitations with tunable interactions. |
Thursday, June 3, 2021 3:00PM - 3:12PM Live |
U07.00006: Pauli blocking of atomic spontaneous decay Lingfeng Yan, Christian Sanner, Lindsay Sonderhouse, Ross Hutson, William R Milner, Jun Ye Observation of Pauli blocking on atomic spontaneous decay has been a long-standing goal in the atomic physics community as it represents a fundamental aspect of quantum state engineering to influence quantum decoherence. By angular and spatially resolved measurement of the fluorescence from a weakly driven 87Sr degenerate Fermi gas we demonstrate suppressed incoherent photon scattering due to Pauli blocking. Our measurement takes place over a range of Fermi degeneracy and the ratio of recoil and Fermi momentum. |
Thursday, June 3, 2021 3:12PM - 3:24PM Live |
U07.00007: Spin Susceptibility Above the Superuid Onset in Ultracold Fermi Gases Feng Xiong, Colin V Parker, Yun Long The high-Tc cuprates display a suppression of the density of states at the transition temperature, known as pseudogap, which has a manifestation in reducing spin susceptibility with temperature. Motivated by such effect and possible analogies in cold gases, we want to study the spin susceptibility of an ultracold fermionic system in the BEC-BCS crossover. Towards this end, we prepared a quantum degenerate Fermi gas in a mixture of its second lowest two hyperfine states, using an all-optical method. We developed and deployed a novel spin susceptibility measurement method using radiofrequency (RF) dressing. By exposing the Fermi gas mixtures to RF radiation along with a magnetic gradient, we succeeded in mapping the mixture to an RF-dressed basis where we can reach thermal equilibrium with a small chemical potential difference between the spin states. By measuring the spin number difference, we are able to extract the spin susceptibility and perform analysis. In this talk, I will elaborate on the details of the method we adopted to measure spin susceptibility and discuss the results of such measurements over the interaction strength-temperature phase diagram. I will compare our result for gases in the strongly interacting regions, to a mean-field model (which is suprisingly accurate), to the ideal Fermi gas model, and to experimental results from several other publications. Certain highlights on the design of our apparatus will also be mentioned. |
Thursday, June 3, 2021 3:24PM - 3:36PM Live |
U07.00008: Compressing ultracold Fermi gases to new regimes Yair Margalit, Furkan Top, Wolfgang Ketterle We report on the highest densities achieved with ultracold Fermi gases, by combining sympathetic cooling with tight compression of lithium-6 atoms in an optical potential. In contrast to the usual notion of non-interacting spin-polarized fermions, we observe elastic and inelastic p-wave collisions far away from any Feshbach resonance. We realize p-wave evaporation that reaches a temperature of T/TF =0.42, and also obtain the first experimental measurement of a p-wave background scattering volume, determined to be Vp=(39 a0)3. The record Fermi energy obtained, combined with the high optical densities, will allow to study light scattering in new regimes, where one can expect to observe superradiance and Pauli blocking of light scattering. |
Thursday, June 3, 2021 3:36PM - 3:48PM Live |
U07.00009: Observation of a Smooth Polaron-Molecule Transition in a Degenerate Fermi Gas Oriana K Diessel We study experimentally and theoretically the fate of the first-order polaron-to-molecule transition in a Fermi gas at finite temperature and impurity density. Our experiments are performed with a spin-imbalanced ultracold Fermi gas with tunable interactions. Utilizing a novel Raman spectroscopy combined with a high-sensitivity fluorescence detection technique, we isolate the quasiparticle contribution and extract the polaron energy, spectral weight, and contact parameter. As the interaction strength is increased, we observe a continuous variation of all observables, in particular a smooth reduction of the quasiparticle weight as it goes to zero beyond the transition point. Our observation is in good agreement with a theoretical model where polaron and molecule quasiparticle states are thermally occupied according to their quantum statistics. The emerging physical picture is thus that of a smooth transition between polarons and molecules and a coexistence of both in the region around the expected transition. |
Thursday, June 3, 2021 3:48PM - 4:00PM Live |
U07.00010: Search for the FFLO phase in the 1D-3D Crossover of a Spin-Imbalanced Fermi Gas Jacob A Fry, Bhagwan D Singh, Randall G Hulet The Fulde-Ferrell Larkin-Ovchinnikov (FFLO) type superconductor is an exotic form of matter known as a supersolid because it simultaneously supports superfluid and magnetic order. In both condensed matter and ultracold atomic gases, this phase has yet to be conclusively observed despite long interest in both theory and experiment. In one-dimension (1D), unlike expectations for higher dimensions, the FFLO phase is found in a large region of the phase diagram1. The effect of quantum and thermal fluctuations, however, are expected to be reduced in higher dimensions. These considerations motivated the proposal to search for FFLO near the 1D-3D dimensional crossover2, which we have identified and characterized3. We confine a spin-imbalanced Fermi gas of 6Li to an array of 1D tubes using a 2D optical lattice. We bring the system into the dimensional crossover by increasing the inter-tube tunneling rate and using a Feshbach resonance to tune interactions. We present our progress towards direct observation of the domain walls containing the excess unpaired fermions. The periodicity of these domain walls, which depends on the magnitude of the polarization, is a definitive signature of the FFLO phase. |
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