Nonlinear Scattering of a Bose-Einstein Condensate on a Rectangular Barrier

We consider the nonlinear scattering and transmission of an atom laser, or Bose-Einstein condensate (BEC) on a finite rectangular potential barrier. The nonlinearity inherent in this problem leads to several new physical features beyond the well-known picture from single-particle quantum mechanics. We find numerical evidence for a denumerably infinite string of bifurcations in the transmission resonances as a function of nonlinearity and chemical potential, when the potential barrier is wide compared to the wavelength of oscillations in the condensate. Near the bifurcations, we observe extended regions of near-perfect resonance, in which the barrier is effectively invisible to the BEC. Unlike in the linear case, it is mainly the barrier width, not the height, that controls the transmission behavior. We show that the potential barrier can be used to create and localize a dark soliton or dark soliton train from a phonon-like standing wave.Comment: 15 pages, 15 figures, new version includes clarification of definition of transmission coefficient in general nonlinear vs. linear cas

International Chemical Congresses.

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Crossover from Fermi liquid to Wigner molecule behavior in quantum dots

The crossover from weak to strong correlations in parabolic quantum dots at zero magnetic field is studied by numerically exact path-integral Monte Carlo simulations for up to eight electrons. By the use of a multilevel blocking algorithm, the simulations are carried out free of the fermion sign problem. We obtain a universal crossover only governed by the density parameter $r_s$. For $r_s>r_c$, the data are consistent with a Wigner molecule description, while for $r_s<r_c$, Fermi liquid behavior is recovered. The crossover value $r_c \approx 4$ is surprisingly small.Comment: 4 pages RevTeX, 3 figures, corrected Tabl

Antioxidant vitamin intakes assessed using a food-frequency questionnaire: correlation with biochemical status in smokers and non-smokers

The increasing interest in the possible role of antioxidant vitamins in many disease states means that methods of assessing vitamin intakes which are suitable for large-scale investigations are now required. The suitability of the food-frequency questionnaire, which was developed by the Medical Research Council - Cardiff Group, for determining dietary intake of antioxidant vitamins in epidemiological studies was investigated in 196 Scottish men. The validity of the dietary data was assessed by comparison with serum vitamin concentrations, and separate analyses were performed for current smokers and non-smokers. The results showed that total energy intake and the percentage of energy derived from sugar were higher in smokers, and that both dietary and serum values of vitamin C, β-carotene and vitamin E were lower in smokers than non-smokers. After adjustment for serum lipids, energy intake and body mass index, correlation coefficients between dietary and serum vitamins C and E were similar for smokers (r 0.555 and 0.25 respectively) and non-smokers (r 0.58 and 0.32 respectively). Correlation between dietary and serum carotenes was reduced from 0.28 in non-smokers to 0.09 in smokers and correlations for retinol and total vitamin A were weakly significant only for non-smokers. The food-frequency questionnaire assigned > 70% of subjects correctly into the upper or lower plus adjacent tertiles of serum vitamin values, with the exception of β-carotene and total vitamin A for smokers. Thus, the food-frequency questionnaire appeared to be an adequate tool for assigning individuals into tertiles of serum antioxidant vitamins with the main exception of β-carotene for smokers. Marked differences do occur between the vitamins and between the smoking groups which may reflect reduced accuracy of reporting on the food-frequency questionnaire or differential absorption and metabolism of the vitamin

Two ground-state modifications of quantum-dot beryllium

Exact electronic properties of a system of four Coulomb-interacting two-dimensional electrons in a parabolic confinement are reported. We show that degenerate ground states of this system are characterized by qualitatively different internal electron-electron correlations, and that the formation of Wigner molecule in the strong-interaction regime is going on in essentially different ways in these ground states.Comment: 5 pages, incl 5 Figures and 2 Table

Results on Plasma Focusing of High Energy Density Electron and Positron Beams

We present results from the SLAC E-150 experiment on plasma focusing of high energy density electron and, for the first time, positron beams. We also discuss measurements on plasma lens-induced synchrotron radiation, longitudinal dynamics of plasma focusing, and laser- and beam-plasma interactions.Comment: LINAC 2000 paper No. THC13, Monterey, CA. Aug.21-25,2000, 3 pages, 2 figure

Observation of Plasma Focusing of a 28.5 GeV Positron Beam

The observation of plasma focusing of a 28.5 GeV positron beam is reported. The plasma was formed by ionizing a nitrogen jet only 3 mm thick. Simultaneous focusing in both transverse dimensions was observed with effective focusing strengths of order Tesla per micron. The minimum area of the beam spot was reduced by a factor of 2.0 +/- 0.3 by the plasma. The longitudinal beam envelope was measured and compared with numerical calculations