Table of contents

On the basis of our assumption that the Tersoff-Bayer averaging is a consequence of the nonlocal correlations inherent in the quantum potential model of Bohm we describe a situation where this potential can be "switched" on and off giving rise to Bose-Einstein and Boltzmann statistics, respectively. This would imply a change in the fringe visibility in interference experiments.

We calculate the contribution to the anomalous (g - 2) of the muon at the one-loop level from a spin-two particle which appears in the strongly coupled standard model of Abbot and Farhi. We find that this contribution is significant when compared to the standard-model result, and is a clear signature of the strongly coupled model.

It is shown that the lifetime of the hyperfine level giving rise to the 21 cm hydrogen line might be affected by the so-called Zeno effect, and hence that the amount of neutral hydrogen in our galaxy may be larger than previously deduced.

Two-photon ionization of Na₂ has been performed in the presence of a d.c. electric field F. Using the A¹Σ_u⁺ (υ' = 0, J') state as an intermediate level, it is possible to excite only the (υ⁺ = 0, N⁺ ≃ J' ± 1) states of the ion. At low J' values, the field F is found to shift the ionization threshold following a - √F law. At higher J' values, this is not the case, and our results show that the electric field induces rotational autoionization, so that the threshold corresponds to a level N⁺ ≪ J'.

The master equation for optical bistability in the bad cavity limit is solved directly when the number of atoms is small. A necessary condition for bistability is given. In contrast to the semi-classical analysis, there is no sense in which there is bistability for 4 atoms; rather the order of 12 atoms is needed.

Second harmonic generation (SHG) was observed from Z-type Langmuir-Blodgett multilayers of 4-[4-(N-n-dodecyl-N-methylamino)phenylazo]-3-nitrobenzoic acid (DPNA) on irradiation at 1.06 μm (Nd³⁺:YAG laser). Even a single LB layer gave a SH signal significantly above background; further layers increased the intensity although the increase did not obey the expected square law. As the incidence angle was varied, all the signals showed a pattern of fringes due to dispersion of the glass substrate. The χ⁽²⁾ of film deposited in the presence of Cd²⁺ ions was calculated to be (2.5 ± 0.5) · 10^-20 C³ J^-2, a value significantly higher than the best known crystalline materials and comparable with other LB materials.

By high-resolution numerical integration of two-dimensional Navier-Stokes equations we show that the turbulent flow at high Reynolds number is dominated by a simple and weakly unstable Hamiltonian system of pointlike vortices. The large instabilities, typical of the turbulent flow, are found uniquely outside vortices, in the wide dissipative region which results to be only a small perturbation of the vortex system. Moreover, the statistical distribution of vortex sizes determines the slope of the energy spectrum, which is steeper than that predicted by phenomenological theories.

The variation of superconductivity and electrical resistivity of CeCu₂Si₂ thin films as a function of lattice disorder introduced by low-temperature irradiation with fast ions is reported. Lattice defects lead to a reduction of the temperature range where Δρ ∝ T² is valid and to a shift of the position of the low-temperature maximum in ρ(T) to lower temperatures. The most striking result is the extreme sensitivity of T_c to lattice defects, resulting in a complete suppression of T_c (< 6 mK) at an estimated concentration of displaced atoms of about 1%.

From the temperature dependence of the elastic constants in CeCu₆ magnetoelastic and Grüneisen parameter effects are identified. A quantitative interpretation of the magnetoelastic effects determines the parameters of the crystal field Hamiltonian, which account also for the single-ion magnetic susceptibility and the Schottky specific heat. The longitudinal c₁₁ and c₃₃-modes show additional softening at low temperatures. From this we deduce a Grüneisen parameter Ω ∼ 80. A new magnetoacoustic effect is also presented.

The absence of any observed lock-in transition in some incommensurate materials (ThBr₄, biphenyl III) down to 4.2 K is at first surprising. An analytic proof is given that the Janssen-Tjon model system exhibiting incommensurate phases does not lock onto a simple commensurate (super)lattice in its ground state for some values of the parameters. The same is shown for the incommensurate model with two different interacting components. This behaviour contrasts with that of the ANNNI model which always has a lock-in transition, the modulated phases being entropy stabilized at higher temperatures. The mineral mullite appears to present a further mechanism different from the one considered here.

We suggest that the previously observed jump in third-sound velocity as a function of temperature, should be associated with self-trapping of the ³He "impurities" on Kosterlitz-Thouless vortices.

We discuss Antonov's rule for the Ising model on a half-infinite lattice and establish the existence of the wetting- and a layering transition in this system.

Time-resolved stimulated two-photon emission is introduced as a powerful technique to investigate electronic relaxation processes in solids. As an example the decay of the 1S exciton in Cu₂O is investigated after quadrupole excitation. The cross relaxation rate between the components of the threefold degenerate 1S exciton is deduced to be larger than 10¹⁰ s^-1, which contradicts a previous experimental value of 2.5 · 10⁸ s^-1.

The Balmer series of hydrogen was studied in the reflexion of hydrogen atoms from clean and cesiated nickel. We observe an enhancement of the line intensity with the decrease of the work function induced by the Cs over layer up to n = 7. For higher main quantum numbers n the series is depressed in comparison to the excitation from the clean Ni surface.

Erratum for Europhys. Lett.3 (3) 361-366 (1987)