Bulletin of the American Physical Society
46th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 60, Number 7
Monday–Friday, June 8–12, 2015; Columbus, Ohio
Session M7: New Developments in Degenerate Fermi Gases |
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Chair: Ken O'Hara, Pennsylvania State University Room: Delaware CD |
Thursday, June 11, 2015 8:00AM - 8:12AM |
M7.00001: Observation of the p-wave contact C. Luciuk, S. Smale, S. Trotzky, N. Zuber, Shizhong Zhang, J.H. Thywissen The contact, i.e. the normalization of the many-body wave function at short range, has been studied extensively for s-wave interactions. It is well understood that the functional form of the contact describes universal thermodynamics near a broad s-wave scattering resonance. Here we present the first measurements of the contact for a system with near-resonant p-wave interactions. We tune a spin-polarized degenerate Fermi gas of $^{40}$K using the p-wave Fano-Feshbach resonances near 199 G. Time-resolved radio-frequency (rf) spectroscopy enables us to study the gas even though it decays within a millisecond. We observe that near either p-wave resonance, the spin-flip rate to a weakly interacting probe state scales as $\delta^{-1/2}$ for large positive $\delta$, where $\delta$ is the detuning from the single-particle transition. Such a scaling is equivalent to a $1/k^2$ scaling of the momentum distribution, as opposed to the $1/k^4$ scaling of the pair wave function for s-waves, where $k$ is relative momentum. [Preview Abstract] |
Thursday, June 11, 2015 8:12AM - 8:24AM |
M7.00002: Universal high-momentum behaviors and thermodynamic relations in a spinless Fermi gas with a resonant p-wave interaction Shuhei M. Yoshida, Masahito Ueda A series of universal relations, which include high-momentum or short-range behaviors of correlation functions and thermodynamic relations, have attracted great attention, especially in studies of the unitary regime of the BCS-BEC crossover. So far, most studies of the universal relations have been conducted within the regime in which a contact interaction model and a local effective field theoretical approach are available. What remains elusive is a spinless Fermi gas with a resonant p-wave interaction, in which a strong singularity due to the centrifugal barrier precludes a contact interaction description. We study high-momentum or short-range behaviors in such a gas and show several relations which are insensitive to its short-range details. We find universal asymptotes in the momentum distribution and the density correlation function, which originate from the two-body collisions. We also find a common coefficient on them which we call a p-wave contact and discuss its physical interpretation. We show that the p-wave contact is proportional to the number of closed-channel molecules, and derive an adiabatic sweep theorem, which states that the p-wave contact is the adiabatic derivative of the energy with respect to the scattering volume. [Preview Abstract] |
Thursday, June 11, 2015 8:24AM - 8:36AM |
M7.00003: The spectral function of the normal phase in the BCS-BEC crossover Tara Drake, Yoav Sagi, Rabin Paudel, Roman Chapurin, Deborah Jin The nature of the normal phase of an ultracold Fermi gas in the BCS-BEC crossover regime is an interesting and unresolved question. While the many-body ground state remains a condensate of paired fermions, the normal state must evolve from a Fermi liquid to a Bose gas of molecules as interactions increase. We explore how this occurs with measurements of the distribution of single-particle energies and momenta in a nearly homogeneous gas above Tc. We find that the data fit well to a two-part function that includes a peak corresponding to fermionic quasiparticles and an ``incoherent background'' that can be modeled using the dispersion of thermal molecules. The data show signatures of both Fermi liquid behavior and pairing. As the strength of interactions is increased, the quasiparticle spectral weight vanishes, which signals the breakdown of a Fermi liquid description. [Preview Abstract] |
Thursday, June 11, 2015 8:36AM - 8:48AM |
M7.00004: Dissociation of Cooper pairs in the BCS Limit using an Oscillating Magnetic Field Abhishek Mohapatra, D.Hudson Smith, Eric Braaten In a gas of ultra cold fermionic atoms with two spin states, the effective interactions between the atoms can be controlled by using a Feshbach resonance to control the scattering length. In the BCS limit, the effective interaction between the atoms is weakly attractive and supports the formation of Cooper pairs. An oscillating magnetic field with frequency near the gap energy 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. [Preview Abstract] |
Thursday, June 11, 2015 8:48AM - 9:00AM |
M7.00005: Superfluidity in Strongly Interacting Spin-Polarized Fermi Gases Ben A. Olsen, Melissa C. Revelle, Jacob A. Fry, Randall G. Hulet, Daniel E. Sheehy We report measurements of the phase boundaries of a harmonically trapped, spin polarized two-component Fermi gas. The interactions in the gas are varied using a magnetically-tuned Feshbach resonance between the weakly-interacting BCS and strongly-interacting BEC regimes. Using spin-selective imaging, we measure the density profiles for the two lowest hyperfine levels of $^6$Li, with the superfluid phase being indicated by an unpolarized central core. We determine phase boundaries between the unpolarized superfluid, partially polarized, and ferromagnetic normal phases as functions of interactions and polarization. We find results that are consistent with earlier experimental results\footnote{M. Zwierlein et al., Science {\bf 311}, 492 (2006); N. Navon et al., Science {\bf 328}, 729 (2010)} as well as Quantum Monte Carlo (QMC) simulations\footnote{G. Bertaina and S. Giorgini, PRA {\bf 79}, 013616 (2009)} in the crossover regime. We explore the deep BCS regime, where few theoretical predictions are available, and also explore the BEC side of resonance, where we observe a superfluid core at higher polarization than predicted by QMC; we discuss the relative contributions of beyond-mean-field and temperature effects to this disparity. [Preview Abstract] |
Thursday, June 11, 2015 9:00AM - 9:12AM |
M7.00006: Structured Weyl Points in Fulde-Ferrell Superfluids Yong Xu, Fan Zhang, Chuanwei Zhang We demonstrate that a Weyl point, widely examined in 3D Weyl semimetals and superfluids, can develop a pair of non-degenerate gapless spheres. Such a bouquet of two spheres is characterized by three distinct topological invariants of manifolds with full energy gaps, i.e., the Chern number of a 0D point inside one developed sphere, the winding number of a 1D loop around the original Weyl point, and the Chern number of a 2D surface enclosing the whole bouquet. We show that such structured Weyl points can be realized in the Fulde-Ferrell superfluid quasiparticle spectrum of a 3D degenerate Fermi gas subject to spin-orbit couplings and Zeeman fields. [Preview Abstract] |
Thursday, June 11, 2015 9:12AM - 9:24AM |
M7.00007: Optical control of two-body scattering in Fermi gases Nithya Arunkumar, Arunkumar Jagannathan, James Joseph, John Thomas Traditionally, collisional interactions in ultra-cold gases have been tuned by an external magnetic field near a Feshbach resonance. Optical tuning techniques permit much finer temporal and spatial control of two-body scattering, enabling new studies of non-equilibrium phenomena. Optical methods are of particular interest in strongly interacting Fermi gases, which are stable near resonance. Unfortunately, single field optical control methods suffer from spontaneous scattering, which limits their utility. We are developing two-field optical control methods that employ quantum interference to greatly suppress spontaneous scattering. We will describe experiments using this method for broad and narrow Feshbach resonances in $^6$Li. [Preview Abstract] |
Thursday, June 11, 2015 9:24AM - 9:36AM |
M7.00008: A Fermi gas in a homogeneous box potential Biswaroop Mukherjee, Mark Ku, Zhenjie Yan, Parth Patel, Elmer Guardado-Sanchez, Tarik Yefsah, Julian Struck, Martin Zwierlein Traditionally, bulk quantum gas experiments take place in inhomogeneous optical and/or magnetic traps. The properties of the homogeneous gas are in many cases masked by line-of-sight integration over the inhomogeneous sample. We report on the trapping of strongly interacting fermionic atoms ($^6$Li) in a quasi-homogenous all-optical potential. We characterize the potential flatness through in-trap imaging, and discuss progress towards directly observing the momentum distribution of the fermions in a box, with the prospect to test predictions from Fermi liquid theory for interacting gases. In contrast to inhomogeneous traps, box potentials prepare a system in one particular point of the phase diagram, giving access to the properties of bulk matter with a high signal-to-noise ratio. This sets a new direction for the exploration of strongly interacting Fermi gases at finite temperature and in the presence of spin imbalance. [Preview Abstract] |
Thursday, June 11, 2015 9:36AM - 9:48AM |
M7.00009: Proposal to directly observe the Kondo effect through enhanced photo-induced scattering of cold fermionic and bosonic atoms Bhuvanesh Sundar, Erich Mueller We propose an experimental protocol to directly observe the Kondo effect by scattering ultracold atoms with spin-dependent interactions. The Kondo effect is a transport anomaly which occurs when conduction electrons interact with magnetic impurities. We consider an ultracold system consisting of a gas of fermionic $^6$Li atoms and a gas of bosonic $^{87}$Rb atoms, where $^6$Li atoms play the role of conduction electrons and $^{87}$Rb atoms play the role of magnetic impurities. We propose a method to engineer Kondo-like interactions between them. To measure the Kondo effect, we imagine launching the $^{87}$Rb gas into the $^6$Li gas, and calculate the momentum transferred to the $^6$Li gas. We show that the temperature dependence of this momentum is logarithmic at low temperatures and has a minimum, characteristic of the Kondo effect and analogous to the behavior of electrical resistance of magnetic alloys. Experimental implementation of our proposal will give a new perspective on an iconic problem. [Preview Abstract] |
Thursday, June 11, 2015 9:48AM - 10:00AM |
M7.00010: Double superfluidity of Bose-Fermi mixtures Fr\'ed\'eric Chevy, Marion Delehaye, Igor Ferrier-Barbut, S\'ebastien Laurent, Christophe Salmon Since the discovery of superfluid $^3$He in 1972, the realization of a doubly-superfluid Bose-Fermi mixture has been one the major goals in the field of quantum liquids. However, due to strong repulsive interactions between helium atoms, the fraction of $^3$He inside $^4$He cannot exceed 6\%. This high dilution of the fermionic species reduces dramatically its critical temperature from 2.5 mK for pure $^3$He to a predicted value of 40 $\mu$K in the mixture. Despite decades of efforts, this range of temperature is still inaccessible to experimental investigation and has prevented the observation of a dual superfluid phase in liquid helium. In cold atoms however, Feshbach resonances make it possible to control the strength of interatomic interactions and realize stable Bose-Fermi mixtures. In my talk I will discuss the physical properties of weakly-coupled superfluid mixtures of $^6$Li and $^7$Li. Superfluidity was revealed by the existence of a critical velocity below which the relative motion of the two species is undamped and the energy transfer between the two gases is coherent. We could interpret this critical velocity using a generalized Landau mechanism in which excitations are shed in both superfluids. [Preview Abstract] |
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