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Search Results: 1 - 10 of 571982 matches for " K. M. O'Hara "
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The nonlinear Dirac equation: Preparation and stability of relativistic vortices in Bose-Einstein condensates
L. H. Haddad,K. M. O'Hara,Lincoln D. Carr
Physics , 2012, DOI: 10.1103/PhysRevA.91.043609
Abstract: We propose a detailed experimental procedure for preparing relativistic vortices, governed by the nonlinear Dirac equation, in a two-dimensional Bose-Einstein condensate (BEC) in a honeycomb optical lattice. Our setup contains Dirac points, in direct analogy to graphene. We determine a range of practical values for all relevant physical parameters needed to realize relativistic vortices in a BEC of $^{87}\mathrm{Rb}$ atoms. Seven distinct vortex types, including Anderson-Toulouse and Mermin-Ho skyrmion textures and half-quantum vortices, are obtained, and their discrete spectra and stability properties are calculated in a weak harmonic trap. We predict that most vortices are stable with a lifetime between $1$ and $10$ seconds.
Unitarity-Limited Elastic Collision Rate in a Harmonically-Trapped Fermi Gas
M. E. Gehm,S. L. Hemmer,K. M. O'Hara,J. E. Thomas
Physics , 2003, DOI: 10.1103/PhysRevA.68.011603
Abstract: We derive the elastic collision rate for a harmonically-trapped Fermi gas in the extreme unitarity limit where the s-wave scattering cross section is $\sigma (k) =4\pi/k^2$, with $\hbar k$ the relative momentum. The collision rate is given in the form $\Gamma=\gamma I(T/T_F)$--the product of a universal collision rate $\gamma =k_B T_F/(6 \pi \hbar)$ and a dimensionless function of the ratio of the temperature $T$ to the Fermi temperature $T_F$. We find $I$ has a peak value of $\simeq4.6$ at $T/T_F\simeq0.4$, $I\simeq 82 (T/T_F)^2$ for $T/T_F\leq 0.15$, and $I\simeq 2(T_F/T)^2$ for $T/T_F>1.5$. We estimate the collision rate for recent experiments on a strongly-interacting degenerate Fermi gas of atoms.
Mechanical Stability of a Strongly-Interacting Fermi Gas of Atoms
M. E. Gehm,S. L. Hemmer,S. R. Granade,K. M. O'Hara,J. E. Thomas
Physics , 2002, DOI: 10.1103/PhysRevA.68.011401
Abstract: A strongly-attractive, two-component Fermi gas of atoms exhibits universal behavior and should be mechanically stable as a consequence of the quantum mechanical requirement of unitarity. This requirement limits the maximum attractive force to a value smaller than that of the outward Fermi pressure. To experimentally demonstrate this stability, we use all-optical methods to produce a highly degenerate, two-component gas of $^6$Li atoms in an applied magnetic field near a Feshbach resonance, where strong interactions are observed. We find that the gas is stable at densities far exceeding that predicted previously for the onset of mechanical instability. Further, we provide a temperature-corrected measurement of an important, universal, many-body parameter which determines the stability--the mean field contribution to the chemical potential in units of the local Fermi energy.
Observation of a Strongly-Interacting Degenerate Fermi Gas of Atoms
K. M. O'Hara,S. L. Hemmer,M. E. Gehm,S. R. Granade,J. E. Thomas
Physics , 2002, DOI: 10.1126/science.1079107
Abstract: We report on the observation of a highly-degenerate, strongly-interacting Fermi gas of atoms. Fermionic $^6$Li atoms in an optical trap are evaporatively cooled to degeneracy using a magnetic field to induce strong, resonant interactions. Upon abruptly releasing the cloud from the trap, the gas is observed to expand rapidly in the transverse direction while remaining nearly stationary in the axial. We interpret the expansion dynamics in terms of collisionless superfluid and collisional hydrodynamics. For the data taken at the longest evaporation times, we find that collisional hydrodynamics does not provide a satisfactory explanation, while superfluidity is plausible.
Stable, Strongly Attractive, Two-State Mixture of Lithium Fermions in an Optical Trap
K. M. O'Hara,M. E. Gehm,S. R. Granade,S. Bali,J. E. Thomas
Physics , 2000, DOI: 10.1103/PhysRevLett.85.2092
Abstract: We use an all-optical trap to confine a strongly attractive two-state mixture of lithium fermions. By measuring the rate of evaporation from the trap, we determine the effective elastic scattering cross section (4 PI a^2) to show that the magnitude of the scattering length |a| is very large, in agreement with predictions. We show that the mixture is stable against inelastic decay provided that a small bias magnetic field is applied. For this system, the s-wave interaction is widely tunable at low magnetic field, and can be turned on and off rapidly via a Raman PI pulse. Hence, this mixture is well suited for fundamental studies of an interacting Fermi gas.
All-Optical Production of a Degenerate Fermi Gas
S. R. Granade,M. E. Gehm,K. M. O'Hara,J. E. Thomas
Physics , 2001, DOI: 10.1103/PhysRevLett.88.120405
Abstract: We achieve degeneracy in a mixture of the two lowest hyperfine states of $^6$Li by direct evaporation in a CO$_2$ laser trap, yielding the first all-optically produced degenerate Fermi gas. More than $10^5$ atoms are confined at temperatures below $4 \mu$K at full trap depth, where the Fermi temperature for each state is $8 \mu$K. This degenerate two-component mixture is ideal for exploring mechanisms of superconductivity ranging from Cooper pairing to Bose condensation of strongly bound pairs.
Preparing a highly degenerate Fermi gas in an optical lattice
J. R. Williams,J. H. Huckans,R. W. Stites,E. L. Hazlett,K. M. O'Hara
Physics , 2008,
Abstract: We propose a method to prepare a sample of fermionic atoms in a three-dimensional (3D) optical lattice at unprecedentedly low temperatures and uniform filling factors. The process involves adiabatic loading of atoms into multiple energy bands of an optical lattice followed by a filtering stage whereby atoms from all but the ground band are removed. Of critical importance is the use of a non-harmonic trapping potential, taken here to be the radial profile of a high-order Laguerre-Gaussian laser beam, to provide external confinement for the atoms. For realistic experimental parameters, this procedure should produce samples with temperatures $\sim10^{-3}$ of the Fermi temperature. This would allow the investigation of the low-temperature phase diagram of the Fermi-Hubbard model as well as the initialization of a high-fidelity quantum register.
Three-body recombination in a three-state Fermi gas with widely tunable interactions
J. H. Huckans,J. R. Williams,E. L. Hazlett,R. W. Stites,K. M. O'Hara
Physics , 2008, DOI: 10.1103/PhysRevLett.102.165302
Abstract: We investigate the stability of a three spin state mixture of ultracold fermionic $^6$Li atoms over a range of magnetic fields encompassing three Feshbach resonances. For most field values, we attribute decay of the atomic population to three-body processes involving one atom from each spin state and find that the three-body loss coefficient varies by over four orders of magnitude. We observe high stability when at least two of the three scattering lengths are small, rapid loss near the Feshbach resonances, and two unexpected resonant loss features. At our highest fields, where all pairwise scattering lengths are approaching $a_t = -2140 a_0$, we measure a three-body loss coefficient $L_3 \simeq 5\times 10^{-22} \mathrm{cm}^6/\mathrm{s}$ and a trend toward lower decay rates for higher fields indicating that future studies of color superfluidity and trion formation in a SU(3) symmetric Fermi gas may be feasible.
Realization of a Resonant Fermi Gas with a Large Effective Range
E. L. Hazlett,Y. Zhang,R. W. Stites,K. M. O'Hara
Physics , 2011, DOI: 10.1103/PhysRevLett.108.045304
Abstract: We have measured the interaction energy and three-body recombination rate for a two-component Fermi gas near a narrow Feshbach resonance and found both to be strongly energy dependent. Even for deBroglie wavelengths greatly exceeding the van der Waals length scale, the behavior of the interaction energy as a function of temperature cannot be described by atoms interacting via a contact potential. Rather, energy-dependent corrections beyond the scattering length approximation are required, indicating a resonance with an anomalously large effective range. For fields where the molecular state is above threshold, the rate of three-body recombination is enhanced by a sharp, two-body resonance arising from the closed-channel molecular state which can be magnetically tuned through the continuum. This narrow resonance can be used to study strongly correlated Fermi gases that simultaneously have a sizeable effective range and a large scattering length.
Toxicokinetics of mercury in blood compartments and hair of fish-fed sled dogs
Camilla L Lieske, Sara K Moses, Judith M Castellini, Jessica Klejka, Karsten Hueffer, Todd M O'Hara
Acta Veterinaria Scandinavica , 2011, DOI: 10.1186/1751-0147-53-66
Abstract: Eight sled dogs (Canis lupus familiaris) were fed either a fish and kibble diet (n = 4), or a fish-free control diet (n = 4) for 12 weeks. Concentrations of Hg were monitored throughout the exposure period, and for 10 weeks post exposure, until Hg concentrations in all blood compartments of one of the exposed dogs dropped below detection limit. Additionally, foreleg hair was sampled during acclimation and weeks 0 and 12.Hg was detected primarily in whole blood and packed cells, although it was sporadically detected at low concentrations in plasma and serum in two of the fish fed dogs. Dogs ingested an estimated average of 13.4 ± 0.58 μg Hg per kg body weight per day. Hg was detectable in whole blood and packed cells within a week of exposure. Detected concentrations continued to rise until plateauing at approximately 3-6 weeks of exposure at a mean of 9.2 ± 1.97 ng/g (ppb) in whole blood. Hg concentration decreased post exposure following 1st order elimination. The mean half-life (t1/2) in whole blood for Hg was 7 weeks. Mean Hg in hair for the fish-fed dogs at week 12 was 540 ± 111 ppb and was significantly greater (about 7-fold) than the Hg hair concentration for the control dogs. The hair to blood ratio for Hg in fish-fed dogs was 59.0 ± 7.6:1.This study found the sled dog model to be an effective method for investigating and characterizing blood Hg distribution (whole blood, serum, plasma, packed cells) and toxicokinetics associated with a piscivorous diet, especially for Hg-exposed fur bearing mammals (such as polar bears). Although hair excretion and hair to blood Hg ratios were not similar to human concentrations and ratios, the sled dog toxicokinetics of Hg in blood, was more similar to that of humans than traditional laboratory animals (such as the rat).Conflicting studies have been published either extolling the benefits of a fish diet or cautioning against the risks of mercury (Hg) and other contaminants exposure [1-6]. Because of the importance of a mari
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