Bulletin of the American Physical Society
43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 57, Number 5
Monday–Friday, June 4–8, 2012; Orange County, California
Session J1: Focus Session: Two-Dimensional Fermi Gas |
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Chair: Kaden Hazzard, JILA and University of Colorado Room: Grand Ballroom BCD |
Wednesday, June 6, 2012 2:00PM - 2:30PM |
J1.00001: Two-dimensional Fermi Gases Invited Speaker: Michael Koehl Pairing of fermions is ubiquitous in nature and it is responsible for a large variety of fascinating phenomena like superconductivity, superfluidity of 3He, the anomalous rotation of neutron stars, and the BEC-BCS crossover in strongly interacting Fermi gases. When confined to two dimensions, interacting many-body systems bear even more subtle effects, many of which lack understanding at a fundamental level. Most striking is the, yet unexplained, effect of high-temperature superconductivity in cuprates, which is intimately related to the two-dimensional geometry of the crystal structure. In particular, the questions how many-body pairing is established at high temperature and whether it precedes superconductivity are crucial to be answered. We report on the observation of pairing in trapped two-dimensional atomic Fermi gas in the regime of strong coupling. We perform momentum-resolved photoemission spectroscopy to measure the spectral function of the gas and we detect a many-body pairing gap above the superfluid transition temperature. Moreover, using the same technique, we investigate spin-imbalanced Fermi gases and find evidence for the formation of polarons and their crossover to a dimer state in two dimensions. Our observations mark a significant step in the emulation of layered two-dimensional strongly correlated superconductors using ultracold atomic gases. [Preview Abstract] |
Wednesday, June 6, 2012 2:30PM - 2:42PM |
J1.00002: Clock shift in a strongly interacting two-dimensional Fermi gas Christian Langmack, Marcus Barth, Wilhelm Zwerger, Eric Braaten We derive universal relations for the radio-frequency (rf) spectroscopy of a two-dimensional Fermi gas consisting of two spin states interacting through an S-wave scattering length. The rf transition rate has a high-frequency tail that is proportional to the contact and displays logarithmic scaling violations, decreasing asymptotically like $1/(\omega^2 \ln^2 \omega)$. Its coefficient is proportional to $\ln^2(a_{2D}'/a_{2D})$, where $a_{2D}$ and $a_{2D}'$ are the 2-dimensional scattering lengths associated with initial-state and final-state interactions. The clock shift is proportional to the contact and to $\ln(a_{2D}'/a_{2D})$. If $|\ln(a_{2D}'/a_{2D})| \gg 1$, the clock shift arises as a cancellation between much larger contributions proportional to $\ln^2(a_{2D}'/a_{2D})$ from bound-bound and bound-free rf transitions. [Preview Abstract] |
Wednesday, June 6, 2012 2:42PM - 2:54PM |
J1.00003: Scale invariance and viscosity of a two-dimensional Fermi gas Enrico Vogt, Michael Feld, Bernd Fr\"{o}hlich, Daniel Pertot, Marko Koschorreck, Michael K\"{o}hl We investigate the collective excitations of a harmonically trapped two-dimensional Fermi gas from the collisionless to the hydrodynamic regime. In the experiment we create two-dimensional Fermi gases of $^{40}$K atoms by using an optical lattice. Interactions are tuned by applying a magnetic field close to the Feshbach resonance. We observe the existence of a breathing mode at twice the trap frequency, which is invariant against interaction strength, amplitude of the excitation, and temperature. Moreover, this breathing mode is undamped as compared to the dipole mode, which provides evidence for a SO(2,1) scaling symmetry of the two-dimensional Fermi gas. In addition, we investigate the quadrupole mode to measure the shear viscosity of the two-dimensional gas and study its temperature dependence. [Preview Abstract] |
Wednesday, June 6, 2012 2:54PM - 3:24PM |
J1.00004: Evolution of Fermion Pairing from Three to Two Dimensions Invited Speaker: Martin Zwierlein The behavior of interacting fermions in two dimensions has long been of great interest. Unconventional superconductivity in high-transition-temperature superconductors arises in the two-dimensional cooper-oxide planes, with only weak intralayer coupling. Layered organic conductors and certain heavy-fermion superconductors also feature a quasi-2D structure, with strongly anisotropic conductivity. In two dimensions, the role of thermal and quantum fluctuations is enhanced, destroying long-range order and leading to algebraic decay of the order parameter. On the other hand, in quantum mechanics, two particles in vacuum with arbitrarily weak interactions may still bind in two dimensions, while binding of weakly interacting fermions in three dimensions requires a many-body effect, Cooper pairing. It is thus interesting to ask whether superconductivity or superfluidity is enhanced somewhere in between two and three dimensions. In recent years, experiments on ultracold gases of fermionic atoms in three dimensions have allowed access to the crossover from Bose-Einstein condensation (BEC) of tightly-bound fermion pairs to Bardeen-Cooper-Schrieffer (BCS) superfluidity of long-range Cooper pairs. Such a fermion pair superfluid loaded into a periodic potential should form stacks of two-dimensional superfluids with tunable interlayer coupling, an ideal model for Josephson-coupled quasi-2D superconductors. For deep potentials in the regime of uncoupled 2D layers, increasing the temperature of the gas is expected to destroy superfluidity through the Berezinskii-Kosterlitz-Thouless mechanism, while more exotic multi-plane vortex loop excitations are predicted for a 3D-anisotropic BCS superfluid near the critical point. In this talk I will present our recent experiments, where we follow the evolution of fermion pairing in the dimensional crossover from 3D to 2D as a strongly interacting Fermi gas of $^6$Li atoms becomes confined to a stack of two-dimensional layers formed by a one-dimensional optical lattice. Decreasing the dimensionality leads to the opening of a gap in radio-frequency spectra, even on the Bardeen-Cooper-Schrieffer side of a Feshbach resonance. The measured binding energy of fermion pairs closely follows the theoretical prediction for the binding of two particles in isolation. In the two-dimensional limit, it is in surprising agreement with zero-temperature mean-field BEC-BCS crossover theory that predicts the energy threshold for radio-frequency dissociation to lie at the two-body binding energy. [Preview Abstract] |
Wednesday, June 6, 2012 3:24PM - 3:36PM |
J1.00005: Two-dimensional attractive Fermi gases' excitations and radio-frequency spectra across the BEC/BCS crossover Kaden Hazzard We calculate the radio-frequency spectra of two-dimensional attractive Fermi gases, including final state interactions, motivated by recent measurements by the groups of Koehl, Thomas, and Zwierlein. The calculation includes coherent excitations generated by the radio-frequency probe on top of the mean field solution. We find that although the gap is identical to the two particle theory, spectral shapes are modified both by many-body effects and by final state interactions. We compare these shapes to experimental measurements. [Preview Abstract] |
Wednesday, June 6, 2012 3:36PM - 3:48PM |
J1.00006: Observation of polaron-to-polaron transitions in the radio-frequency spectra of a quasi-two-dimensional Fermi gas Yingyi Zhang, Willie Ong, Ilya Arakelyan, John Thomas We measure radio-frequency spectra for a two-component mixture of a $^6$Li atomic Fermi gas in a quasi-two-dimensional trapping potential. We study the many-body regime, where the Fermi energy is comparable to the energy level spacing in the tightly confined direction. BCS theory predicts that the spectra should be determined by dimer transitions. Well below the Feshbach resonance, we observe spectra due to molecular dimers. However, near the Feshbach resonance, we find that the observed resonances do not correspond to the predicted transitions between confinement-induced dimers. Instead, the spectra appear to be well-described by transitions between noninteracting polaron states in two dimensions. [Preview Abstract] |
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