Table of contents

In this letter we show that the super 6 – j (s6 – j) symbols for the superalgebra osp(1|2) possess, in addition to the usual tetrahedral symmetry, a Regge symmetry. However, for an arbitrary s6 – j symbol, this additional symmetry is smaller than in the case of the rotation 6 – j symbols. Only in some particular cases does the s6 – j symbol possess the full Regge symmetry.

The infra-red behaviour of the type-II closed thermal superstring is examined for the dimensionally regularized one-loop cosmological constant within the framework of the thermofield dynamics.

About 10⁴ triplet metastable helium atoms have been trapped in a magneto-optical trap, based on the 2³S₁–2³P₂ transition at 1.083 μm (volume 0.1 mm³, temperature 1 mK). The losses of the trap are analysed by monitoring the ion current due to Penning collisions either with background gases, or between trapped atoms. The rate coefficient for these collisions between trapped metastable helium atoms has been found to be unusually large, and the crucial role of the quasi-resonant laser light has been experimentally demonstrated. The rate coefficient of the dominant process (He*(2³S₁) + He*(2³P₂) Penning collisions) has been measured by two different methods, and found to be K_sp ≃ 10^-7 cm³ s^-1 within an estimated factor of 4. The corresponding cross-section is 6 · 10⁶ Å².

We report on the observation of a new laser trap for neutral atoms. It uses three orthogonal pairs of counterpropagating laser beams having linear polarizations at 45° and a quadrupole magnetic field. 10⁸ cesium atoms were thus confined in a 0.15 mm³ volume at a temperature of 60 μK, a factor of 2 below the Doppler cooling limit. We interpret this trapping as being due to the new magneto-optical force recently observed by Grimm et al. and which is essentially a dipole force rectified by the magnetic field. This trap opens new possibilities for increasing the phase-space density of laser-cooled atoms.

Considering the atmospheric turbulence consisting of acoustic-gravity vortices, we estimate both the density fluctuation spectrum and the effective diffusion coefficient.

From a comparison between calculated STM images of Xe on Cu(110) and experimental images of Xe on Ni(110), tip apex structures are proposed for the Eigler atomic switch in the "on" and "off" states. After the voltage pulse and before a scan, the Xe is found to be on the top site of the apex. A scan induces a migration of the Xe on the apex facet opposite to the Cu(110) step edge. The calculated "on/off" current intensity ratio of the switch is in good agreement with the experimental one.

Very accurate ultrasonic experiments have been performed to check the elastic isotropy of Al-Mn-Pd quasi-crystals. The velocities of ultrasonic waves propagating along a twofold or a fivefold axis have been found equal to (6521 ± 10) m · s^-1 for longitudinal waves and (3292 ± 3) m · s^-1 for shear waves. The accuracy of these measurements is one order of magnitude better than previous results. In addition, when the polarization of the shear waves propagating along a twofold axis is changed, the velocity is constant within an experimental uncertainty of 10^-4. In this paper, we also report the first experimental evidence for anisotropic ultrasonic attenuation. Such an anisotropy is expected to occur for instance if phasons and phonons are coupled in the crystal.

The response of the interface between liquid and solid ⁴He is investigated when the helium crystal is exposed to uniaxial stress. It is observed that part of the crystal melts, the change in height of the interface being roughly proportional to the square of the stress. Beyond a critical value the interface becomes unstable and develops corrugations with a wave vector of the order of the inverse capillary length, which is associated with the so-called Grinfeld instability. These results confirm recent predictions by Balibar, Edwards and Saam.

In this letter, we propose a new quasi-electron trial wave function for N interacting electrons in two dimensions moving in a strong magnetic field in a disk geometry. Requiring that the trial wave function exhibits the correct filling factor of a quasi-electron wave function, we obtain N + 1 angular-momentum eigenfunctions. The expectation values of the energy are calculated and compared with the data of an exact numerical diagonalization.

Structures in the conductance which are periodic in bias voltage have been observed in very weakly coupled Bi₂Sr₂CaCu₂O_x/W point-contact tunneling junctions. These reproducible features appear to result from geometrical resonances of the order parameter, as would be found in superconductor-normal multilayers, rather than from Coulomb-staircase or multiple-gap effects. The data and its analysis strongly suggest the validity of treating even microscopic properties of Bi₂Sr₂CaCu₂O_x using multilayer models with geometrical parameters inferred from the placement and thicknesses of the CuO₂ conductive sheets within the unit cell.

The morphological evolution of the surfaces of gold deposits grown from the vapour on smooth glass under nonequilibrium conditions and incident angle near substrate normal is studied at the nanometer level by scanning tunnelling microscopy. For an average film thickness equal to or greater than 500 nm, the interface thickness (ξ) reaches a steady state. Under these conditions, ξ depends on the scan length (L) as ξ ∝ L^α with α = 0.35 ± 0.05 for L > d_s, where d_s is the columnar size, and α = 0.89 ± 0.05 for L < d_s. These results indicate that the growing surface spontaneously reaches a steady state and it can be described as a self-affine fractal. The value of α for L > d_s agrees with the prediction of ballistic deposition models without restructuring, whereas that for L < d_s exceeds the prediction of ballistic models including restructuring.

We study the learning of a time-dependent linearly separable rule in a neural network. The rule is represented by an N-vector performing a random walk. A single-layer perceptron is trained on-line using a Hebb-like algorithm with an additional weight decay. The evolution of the generalization error is calculated exactly in the thermodynamic limit N → ∞. We consider both, training examples which are drawn randomly and using a query strategy. The rule is never learnt perfectly, but can be tracked within a certain error level. Simulations confirm the analytic results.