Bulletin of the American Physical Society
50th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics APS Meeting
Volume 64, Number 4
Monday–Friday, May 27–31, 2019; Milwaukee, Wisconsin
Session H06: Degenerate Fermi gases |
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Chair: Martin Zwierlein, MIT Room: Wisconsin Center 102DE |
Wednesday, May 29, 2019 8:00AM - 8:12AM |
H06.00001: FFLO Superfluidity in the 1D-3D Crossover of a Spin-imbalanced Fermi Gas Jacob A. Fry, Yi Jin, Eduardo Ibarra G.P., Randall G. Hulet Ultracold atomic gases provide an ideal platform to realize novel quantum many-body states due to their tunability and versatility. In particular, spin-polarized Fermi gases provide a promising environment for the search of exotic superfluids such as the Fulde--Ferrell--Larkin--Ovchinnikov (FFLO) phase. We create a pseudo-spin-1/2 system using the lowest two hyperfine states of fermionic lithium that are polarized with a high degree of control. The atoms are confined in an array of 1D tubes with variable tunneling generated with a 2D optical lattice, while interactions are tuned via an s-wave Feshbach resonance. Previous work identified the crossover from 1D to 3D as the most likely region to stabilize the FFLO superfluid\footnote{M M. Parish et al., Phys. Rev. Lett. 99, 250403 (2007).}$^{,}$\footnote{M. C. Revelle et al., Phys. Rev. Lett. 117, 235301 (2016).}, therefore we bring the system to the dimensional crossover by tuning the inter-tube tunneling rate and interaction strength. 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. [Preview Abstract] |
Wednesday, May 29, 2019 8:12AM - 8:24AM |
H06.00002: Bragg spectroscopy of a strongly interacting Fermi gas in the 3D to 2D crossover Carlos Noschang Kuhn, Paul Dyke, Ivan Herrera, Sascha Hoinka, Christopher Vale Strongly correlated gases of ultracold atoms offer a versatile platform for quantitative studies of many-body physics. Dimensionality plays a key role in setting the properties of the interparticle interactions, for example, in general, the coupling constant in two dimensional (2D) systems depends on the collision energy, unlike 1D and 3D geometries. Here we use two-photon Bragg spectroscopy with large transferred momentum to measure the dynamic and static structure factors of strongly interacting Fermi gases as the dimensionality is tuned from 3D to 2D. From the static structure factor, we determine Tan's contact parameter, which quantifies the likelihood of finding two particles with opposite spins in close proximity to each other. Furthermore, we investigate pair correlations as the 3D scattering length is tuned via a Feshbach resonance to gain insight into how the dimensionality effects pairing. [Preview Abstract] |
Wednesday, May 29, 2019 8:24AM - 8:36AM |
H06.00003: Deeply degenerate Fermi-Fermi mixture of dysprosium and potassium atoms Cornelis Ravensbergen, Vincent Corre, Elisa Soave, Marian Kreyer, Emil Kirilov, Rudolf Grimm We report on the first realization of a mixture of fermionic $^{161}$Dy and fermionic $^{40}$K where both species are deep in the quantum-degenerate regime [1]. Both components are spin polarized in their absolute ground states, and the low temperatures are achieved by means of evaporative cooling of the dipolar dysprosium atoms together with sympathetic cooling of the potassium atoms. We describe the trapping and cooling methods, in particular the final evaporation stage, which leads to Fermi degeneracy of both species. By analyzing the cross-species thermalization we obtain an estimate of the magnitude of the inter species $s$-wave scattering length at low magnetic field. We have also carried our first Feshbach scans. The preliminary results show three magnetic field regions with very fast thermalization, which we assosiate with broad inter-species Feshbach resonances. The $^{161}$Dy-$^{40}$K system therefore features all ingredients needed to realize a strongly interacting mass-imbalanced fermion mixture. \newline \newline [1] C. Ravensbergen et al. Phys. Rev. A. \textbf{98}, 063624 (2018). [Preview Abstract] |
Wednesday, May 29, 2019 8:36AM - 8:48AM |
H06.00004: Fermion-mediated interactions in a quantum degenerate Bose-Fermi mixture Krutik Patel, B.J. DeSalvo, Geyue Cai, Cheng Chin When a Bose-Einstein condensate (BEC) is immersed in a degenerate Fermi gas (DFG), the Fermi sea can significantly modify the properties of the condensate. It alters the confinement felt by the condensate, and introduces an effective interaction between bosons mediated by fermionic excitations near the Fermi surface (spinless analog of the Ruderman-Kittel-Kasuya-Yosida mechanism). These interactions are expected to be long-ranged, with a functional form determined by Fermi statistics. We observe experimentally that the presence of degenerate fermions induces an attractive interaction between bosons. Our system is based on a quantum degenerate mixture of $^{133}$Cs and $^6$Li, where the large mass imbalance between the two species allows the Cs BEC to be fully embedded in the much larger Li DFG. Our measurements of the strength of this fermion-mediated interaction are in fair agreement with theoretical predictions. Furthermore, we demonstrate that these interactions can cause qualitative changes to the bosonic ground state, leading to collapse and soliton formation in an otherwise stable BEC. [Preview Abstract] |
Wednesday, May 29, 2019 8:48AM - 9:00AM |
H06.00005: Two-field Optical Vernier to Probe Energy-Dependent Feshbach resonances Arunkumar Jagannathan, Nithya Arunkumar, John Thomas Optical control of two-body interactions in ultracold gases enables new high resolution probes of energy-dependent narrow Feshbach resonances, which are strongly dependent on the relative momentum of colliding atom pairs. We demonstrate a two-field optical vernier using electromagnetically induced transparency, which expands kHz (mG) magnetic field detunings into MHz optical field detunings, providing new insights into the momentum-dependent two body interactions. Two-photon atom loss spectra are measured near the narrow Feshbach resonance in $^6$Li atoms, revealing rich structure in good agreement with our theoretical model. [Preview Abstract] |
Wednesday, May 29, 2019 9:00AM - 9:12AM |
H06.00006: Spin-Energy Correlation in Degenerate Weakly-Interacting Fermi Gases Saeed Pegahan, Ilya Arakelyan, John Thomas Very weakly interacting Fermi gases offer a new paradigm for exploring the interplay between spin, motion, Fermi statistics, and interactions in many-body systems. We study the formation of spin-energy correlations in a very weakly interacting Fermi gas of $^6$Li contained in an optical trap with a spin-dependent potential. Measuring time dependent spin density profiles for coherently prepared, quantum degenerate clouds of $^6$Li, we find that the profiles are in very good agreement with a one-dimensional mean field description for small s-wave scattering lengths, where the energy changing collision rate is negligible. For temperatures in the classical Boltzmann regime, we observe a modified spin density patterns arising from the energy dependence of the scattering length and measure the zero crossing shift in the scattering length. [Preview Abstract] |
Wednesday, May 29, 2019 9:12AM - 9:24AM |
H06.00007: Measurement of bulk viscosity of ultracold Fermi gas in the unitary regime Takahiro Tsumori, Munekazu Horikoshi, Keisuke Fujii, Yusuke Nishida, Junji Yumoto, Makoto Kuwata-Gonokami To understand the transport properties of a strongly correlated Fermi gas, it is important to measure the transport coefficient of ultracold Fermi gas in the unitary regime. In this study, we measure the bulk viscosity of ultracold Fermi gas in a wide range of temperatures. We apply a new theory which pays attention to the effects of the spacetime-dependent scattering length by employing a hydrodynamic description, and show that the spacetime-dependence of the scattering length is reflected as a correction term in the divergence of the velocity. This suggests that the bulk viscosity coefficient can be measured by applying a time modulation to the scattering length. The entropy of the system increases from the bulk viscosity, and so, the bulk viscosity coefficient can be measured as an energy increase. Through experiment, we measure the bulk viscosity coefficient of ultracold $Li^6$, and compare results with that predicted in the literature. [Preview Abstract] |
Wednesday, May 29, 2019 9:24AM - 9:36AM |
H06.00008: Temperature dependence of low-lying excitiations in a unitary Fermi gas Christopher Vale, Carlos Kuhn, Sascha Hoinka, Paul Dyke, Ivan Herrera, Jami Kinnunen, Georg Bruun Low-lying excitations are central to understanding the physical properties of many-body quantum systems. Here, we use two-photon Bragg spectroscopy to probe the elementary excitations in a unitary Fermi gas as a function of the temperature. At a momentum of approximately half the Fermi momentum, the Bragg spectra show dramatic changes across the superfluid to normal fluid transition. Below the transition temperature $T_c$, the dominant excitation is the Bogoliubov-Anderson phonon whose amplitude and width have a temperature dependence consistent with a theoretical model based on the quasi-particle random phase approximation. This indicates the dominant damping mechanism for phonons in the temperature range $0.5 < T/T_c < 1$ is via collisions with fermionic quasi-particles. Above the superfluid transition, the excitation spectra become much broader lying in the cross-over between zero and first sound. [Preview Abstract] |
Wednesday, May 29, 2019 9:36AM - 9:48AM |
H06.00009: Persistent Flow in Fermionic Superfluid Rings Kevin Wright, Yanping Cai, Daniel Allman, Parth Sabharwal We will report on our efforts to create persistent currents and Josephson junctions in a ring of dilute fermionic superfluid, which requires a somewhat different technical approach than previous experiments with bosonic superfluids. Our goal is to test predictions of quantum many-body theory for superflows in this setting. After validating these methods in the well-understood BEC-BCS regime, we will apply them to studying transport phenomena in unconvenional fermionic superfluids and other exotic quantum phases of matter. [Preview Abstract] |
Wednesday, May 29, 2019 9:48AM - 10:00AM |
H06.00010: Quantum Turbulence from Cold Atoms to Neutron Stars Michael Forbes Quantum turbulence, resulting from the motion and interaction of vortices and other superfluid defects, lies at the heart of several different fields. In this talk I will discuss multidisciplinary aspects of quantum turblence emerging from the workshop "Quantum Turbulence: Cold Atoms, Heavy Ions, and Neutron Stars" hosted at the national Institute for Nuclear Theory (INT), connecting experimental phenomena in helium and cold atoms with applications to neutron stars and heavy ion collisions. [Preview Abstract] |
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