arXiv subjects

K. M. O'Hara

Publications and source records attributed to K. M. O'Hara.

14 recordsLinked to original sources

arXiv2015

The nonlinear Dirac equation: Preparation and stability of relativistic vortices in Bose-Einstein condensates

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.

cond-mat.quant-gas↗

arXiv2011

Realization of a Resonant Fermi Gas with a Large Effective Range

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.

cond-mat.quant-gas↗

arXiv2009

Evidence for Ground- and Excited-State Efimov Trimers in a Three-State Fermi Gas

We observe enhanced three-body recombination in an ultracold three-component $^6$Li Fermi gas with large but unequal scattering lengths attributable to an excited Efimov trimer state near the three-atom scattering threshold. We find excellent agreement between the measured three-body recombination rate and the recombination rate calculated in the zero-range approximation where the only free parameters are the Efimov parameters $κ_*$ and $η_*$. The value of $κ_*$ determined by the location of the Efimov resonance we observe at 895 G also predicts the locations of loss features previously observed near 130 and 500 G \cite{Jochim08,OHara09} suggesting that all three features are associated with universal Efimov trimer states. We also report on the first realization of a quantum degenerate three-state Fermi gas with approximate SU(3) symmetry.

quant-ph↗

arXiv2009

Three-body recombination in a three-state Fermi gas with widely tunable interactions

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.

physics.atom-ph↗

arXiv2008

Preparing a highly degenerate Fermi gas in an optical lattice

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.

cond-mat.stat-mech↗

arXiv2004

Strongly inhibited transport of a 1D Bose gas in a lattice

We report the observation of strongly damped dipole oscillations of a quantum degenerate 1D atomic Bose gas in a combined harmonic and optical lattice potential. Damping is significant for very shallow axial lattices (0.25 photon recoil energies), and increases dramatically with increasing lattice depth, such that the gas becomes nearly immobile for times an order of magnitude longer than the single-particle tunneling time. Surprisingly, we see no broadening of the atomic quasimomentum distribution after damped motion. Recent theoretical work suggests that quantum fluctuations can strongly damp dipole oscillations of 1D atomic Bose gas, providing a possible explanation for our observations.

cond-mat.other↗

arXiv2003

Observation of Reduced Three-Body Recombination in a Fermionized 1D Bose Gas

We investigate correlation properties of a one-dimensional interacting Bose gas by loading a magnetically trapped 87-Rb Bose-Einstein condensate into a deep two-dimensional optical lattice. We measure the three-body recombination rate for both the BEC in the magnetic trap and the BEC loaded into the optical lattice. The recombination rate coefficient is a factor of seven smaller in the lattice, which we interpret as a reduction in the local three-body correlation function in the 1D case. This is a signature of correlation intermediate between that of the uncorrelated phase coherent 1D mean-field regime and the strongly correlated Tonks-Girardeau regime.

cond-mat.soft↗

arXiv2003

Unitarity-Limited Elastic Collision Rate in a Harmonically-Trapped Fermi Gas

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 $σ(k) =4π/k^2$, with $\hbar k$ the relative momentum. The collision rate is given in the form $Γ=γI(T/T_F)$--the product of a universal collision rate $γ=k_B T_F/(6 π\hbar)$ 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.

cond-mat.stat-mech↗

arXiv2003

Mechanical Stability of a Strongly-Interacting Fermi Gas of Atoms

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.

cond-mat.supr-con↗

arXiv2002

Observation of a Strongly-Interacting Degenerate Fermi Gas of Atoms

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.

cond-mat.supr-con↗

arXiv2002

Measurement of the Zero Crossing in a Feshbach Resonance of Fermionic 6-Li

We measure a zero crossing in the scattering length of a mixture of the two lowest hyperfine states of 6-Li. To locate the zero crossing, we monitor the decrease in temperature and atom number arising from evaporation in a CO2 laser trap as a function of magnetic field B. The temperature decrease and atom loss are minimized for B=528(4) G, consistent with no evaporation. We also present preliminary calculations using potentials that have been constrained by the measured zero crossing and locate a broad Feshbach resonance at approximately 860 G, in agreement with previous theoretical predictions. In addition, our theoretical model predicts a second and much narrower Feshbach resonance near 550 G.

cond-mat.soft↗

arXiv2002

All-Optical Production of a Degenerate Fermi Gas

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 $4 μ$K at full trap depth, where the Fermi temperature for each state is $8 μ$K. This degenerate two-component mixture is ideal for exploring mechanisms of superconductivity ranging from Cooper pairing to Bose condensation of strongly bound pairs.

cond-mat.soft↗

arXiv2000

Stable, Strongly Attractive, Two-State Mixture of Lithium Fermions in an Optical Trap

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.

physics.atom-ph↗

arXiv2000

Ultrastable CO2 Laser Trapping of Lithium Fermions

We demonstrate an ultrastable CO2 laser trap that provides tight confinement of neutral atoms with negligible optical scattering and minimal laser-noise- induced heating. Using this method, fermionic 6Li atoms are stored in a 0.4 mK deep well with a 1/e trap lifetime of 300 sec, consistent with a background pressure of 10^(-11) Torr. To our knowledge, this is the longest storage time ever achieved with an all-optical trap, comparable to the best reported magnetic traps.

physics.atom-ph↗