Browsing by Author "Sackett, C.A."
Now showing 1 - 11 of 11
Results Per Page
Sort Options
Item Analysis of in situ images of Bose-Einstein condensates of lithium(American Physical Society, 1997) Bradley, C.C.; Sackett, C.A.; Hulet, R.G.; Rice Quantum InstituteThe initial evidence for Bose-Einstein condensation (BEC) of Li7 consisted of image distortions that suddenly appeared as the gas was evaporatively cooled to near the transition temperature. We present results of a model of the imaging system used to produce these images. The model confirms that the distortions are a sensitive indicator of BEC, but shows that they make quantitative analysis difficult. We find that the distortions resulted from light scattered by the condensate, in conjunction with aberrations in the imaging lens.Item Bose-Einstein Condensation of Lithium: Observation of Limited Condensate Number(American Physical Society, 1997) Bradley, C.C.; Sackett, C.A.; Hulet, R.G.; Rice Quantum InstituteBose-Einstein condensation of 7Li has been studied in a magnetically trapped gas. Because of the effectively attractive interactions between 7Li atoms, many-body quantum theory predicts that the occupation number of the condensate is limited to about 1400 atoms. We observe the condensate number to be limited to a maximum value between 650 and 1300 atoms. The measurements were made using a versatile phase-contrast imaging technique.Item Dipolar relaxation collisions in magnetically trapped 7Li(American Physical Society, 1999) Gerton, J.M.; Sackett, C.A.; Frew, B.J.; Hulet, R.G.; Rice Quantum InstituteWe report the measurement of the rate constant for dipolar relaxation from the (F=2, mF=2) hyperfine state of 7Li. The atoms are confined in a permanent magnet trap at a field of 103 G. The measured value of (1.05±0.10)×10−14 cm3/s agrees well with theory. Additionally, we determine an upper bound for the three-body molecular recombination rate constant of 10−27 cm6/s, which is also consistent with predictions.Item Evidence of Bose-Einstein Condensation in an Atomic Gas with Attractive Interactions(American Physical Society, 1995) Bradley, C.C.; Sackett, C.A.; Tollett, J.J.; Hulet, R.G.; Rice Quantum InstituteEvidence for Bose-Einstein condensation of a gas of spin-polarized 7Li atoms is reported. Atoms confined to a permanent-magnet trap are laser cooled to 200 μK and are then evaporatively cooled to lower temperatures. Phase-space densities consistent with quantum degeneracy are measured for temperatures in the range of 100 to 400 nK. At these high phase-space densities, diffraction of a probe laser beam is observed. Modeling shows that this diffraction is a sensitive indicator of the presence of a spatially localized condensate. Although measurements of the number of condensate atoms have not been performed, the measured phase-space densities are consistent with a majority of the atoms being in the condensate, for total trap numbers as high as 2×10˄5 atoms. For 7Li, the spin-triplet s-wave scattering length is known to be negative, corresponding to an attractive interatomic interaction. Previously, Bose-Einstein condensation was predicted not to occur in such a system.Item Growth and Collapse of a Bose-Einstein Condensate with Attractive Interactions(American Physical Society, 1998) Sackett, C.A.; Stoof, H.T.C.; Hulet, R.G.; Rice Quantum InstituteWe consider the dynamics of a quantum degenerate trapped gas of 7Li atoms. Because the atoms have a negative s-wave scattering length, a Bose condensate of 7Li becomes mechanically unstable when the number of condensate atoms approaches a maximum value. We calculate the dynamics of the collapse that occurs when the unstable point is reached. In addition, we use the quantum Boltzmann equation to investigate the nonequilibrium kinetics of the atomic distribution during and after evaporative cooling. The condensate is found to undergo many cycles of growth and collapse before a stationary state is reached.Item Measurements of Collective Collapse in a Bose-Einstein Condensate with Attractive Interactions(American Physical Society, 1999) Sackett, C.A.; Gerton, J.M.; Welling, M.; Hulet, R.G.The occupation number of a magnetically trapped Bose-Einstein condensate is limited for atoms with attractive interactions. It has been predicted that, as this limit is approached, the condensate will collapse by a collective process. The measured spread in condensate number for samples of Li7 atoms undergoing thermal equilibration is consistent with the occurrence of such collapses.Item Optical detection of a Bardeen-Cooper-Schrieffer phase transition in a trapped gas of fermionic atoms(American Physical Society, 1999) Zhang, Weiping; Sackett, C.A.; Hulet, R.G.; Rice Quantum InstituteA theoretical treatment of light scattering from a degenerate Fermi gas of trapped ultracold 6Li atoms is presented. We find that the scattered light contains information that directly reflects the quantum pair correlation due to the formation of atomic Cooper pairs resulting from a Bardeen-Cooper-Schrieffer phase transition to a superfluid state. Evidence for pairing should be observable in both the space and time domains.Item Optimization of evaporative cooling(American Physical Society, 1997) Sackett, C.A.; Bradley, C.C.; Hulet, R.G.Recent experiments have used forced evaporative cooling to produce Bose-Einstein condensation in dilute gases. The evaporative cooling process can be optimized to provide the maximum phase-space density with a specified number of atoms remaining. We show that this global optimization is approximately achieved by locally optimizing the cooling efficiency at each instant. We discuss how this method can be implemented, and present the results for our Li7 trap. The predicted behavior of the gas is found to agree well with experiment.Item Permanent magnet trap for cold atoms(American Physical Society, 1995) Tollett, J.J.; Bradley, C.C.; Sackett, C.A.; Hulet, R.G.; Rice Quantum InstituteWe report the trapping of neutral atoms in a permanent magnet trap. Approximately 1×10˄8 ground-state lithium atoms have been confined in a nonzero magnetic-field minimum produced by six permanent magnets in an Ioffe configuration. These atoms have a kinetic temperature of 1.1 mK and a peak density of approximately 3×10˄9 cm˄−3. The trapped-atom lifetime is 240 s, limited by collisions with background gas. This trap provides an environment in which quantum-statistical effects, atomic collisions, and other ultralow-temperature phenomena can be studied.Item Spectroscopic Determination of the s-Wave Scattering Length of Lithium(American Physical Society, 1995) Abraham, E.R.I.; McAlexander, W.I.; Sackett, C.A.; Hulet, Randall G.; Rice Quantum InstituteTwo-photon photoassociation of colliding ultracold 7Li atoms is used to probe the Σu+3(a) ground state of 7Li2. The binding energy of the least-bound state of this triplet potential, υ=10, is found to be 12.47±0.04 GHz. This spectroscopic information establishes that the s-wave scattering length for 7Li atoms in the F=2, mF=2 state is (−27.3±0.8)a0. The negative sign of the scattering length has important implications as to whether atoms at this doubly spin-polarized state can undergo a Bose-Einstein condensation.Item Superfluid state of atomic 6Li in a magnetic trap(American Physical Society, 1997) Houbiers, M.; Ferwerda, R.; Stoof, H.T.C.; McAlexander, W.I.; Sackett, C.A.; Hulet, R.G.; Rice Quantum InstituteWe report on a study of the superfluid state of spin-polarized atomic 6Li confined in a magnetic trap. Density profiles of this degenerate Fermi gas and the spatial distribution of the BCS order parameter are calculated in the local-density approximation. The critical temperature is determined as a function of the number of particles in the trap. Furthermore, we consider the mechanical stability of an interacting two-component Fermi gas, in the case of both attractive and repulsive interatomic interactions. For spin-polarized 6Li we also calculate the decay rate of the gas and show that within the mechanically stable regime of phase space, the lifetime is long enough to perform experiments on the gas below and above the critical temperature if a bias magnetic field of about 5 T is applied. Moreover, we propose that a measurement of the decay rate of the system might signal the presence of the superfluid state.