Table of contents

We study discretized random paths with a curvature-dependent action. The scaling limits of the corresponding statistical mechanical models can be constructed explicitly and are either usual Brownian motion or a theory where the correlations of tangents are nonzero and described by diffusion on the unit sphere. In the latter case the two-point function has an anomalous dimension η = 1.

We study spin systems–ferromagnets, spin glasses, graph bipartitioning–on random lattices with a finite connectivity. New features of spin glass theory are pointed out, such as the influence of the shape of the distribution of exchange couplings. For the bipartitioning of random graphs with fixed connectivity, the results of the replica symmetric solution give a good first approximation of the ground-state energy.

A Lagrangian consisting of a linear combination of dimensionally continued Euler forms leads to field equations with at most second derivatives of the metric tensor. Excluding a cosmological constant, it is shown that such a Lagrangian has to contain the Einstein-Hilbert (curvature scalar) term, in order to yield a well-behaved set of field equations. In particular, this rules out the Lanczos Lagrangian recently proposed.

A study is made of the properties of a susy preonic model. Constraints such as the 't Hooft anomaly matching condition, supersymmetry, conservation of electric charge and the assumption that only a single left-handed and a single right-handed fermion doublet is present are imposed. Applications to β-decay, Cabibbo-allowed and Cabibbo-suppressed processes are considered. Finally predictions for changes in the proton and neutron x-distribution are given.

The momentum distribution of nuclear matter n(k) is evaluated by using the cluster expansion for the occupation probability. The gap in n(k) at the Fermi surface is found to be about 0.96 compared with 1.0 for the noninteracting Fermi gas model. It is shown that the high-momentum tail of n(k) is very much smaller than it has been thought.

The dissipative reactions of ³²S on ²⁴Mg at incident energies of 140 and 152 MeV have been studied by measuring inclusive fragment distributions and fragment-fragment coincidences. The experiment employed two multiparameter ion chambers. Fragment-fragment coincidences have been used to determine the production of fragments of different charge and mass, and related Q-value distributions.

We study the instabilities of p-bonded systems with a particular emphasis on the structure of group-V and -VI elements. We show that the trivalency of As (respectively divalency of Se and Te) is due to the Peierls instability of a half-filled band (respectively two-third-filled band). A moment calculation shows that the periodicity is not a necessary condition for the Peierls instability. Consequently, the trivalency of As (respectively, divalency of Se, Te) also holds in disordered structures (liquid, amorphous structures), as has been shown experimentally. We also study the pressure dependence of the instability: a bifurcation point is observed for a given pressure, above which no distortion appears.

We have measured the specific heat of the icosahedral i-Al₈₄Mn₁₆ quasi-crystal and also of three crystalline compounds with related composition: hexagonal h-Al₄Mn and h-Al₇₁Fe₁₉Si₁₀, and cubic c-Al₅₀Mn₁₂Si₇. The electron density of states at the Fermi level is high in both i-Al₈₄Mn₁₆ and h-Al₄Mn alloys. This result is consistent with recent predictions for icosahedral Al clusters by McHenry et al. The Debye temperature θ_D is lower in the nonperiodic i-phase, thus revealing a softening of elastic constants.

Structural phase transitions in Si_1-xGe_x alloys are studied by energy dispersive X-ray diffraction at room temperature and pressures up to 42 GPa. The comparison with the behaviour of the pure elements shows a very strong nonlinearity in the β-Sn to p.h. phase transition pressure with respect to the alloys concentration x. No new phase is observed in the alloys.

We present a comparison between theoretical calculation and experimental data for the thermal attenuation of helium diffraction peaks for a beam scattered from Cu(100). The calculations take into account 2-phonon exchange exactly and multiphonon exchange with a good approximation up to the highest temperatures. It is shown that there is good agreement up to T = 450 K with the model of crystal surface atom thermal displacements developed by Jayanthi et al. Above, there is a significant enhancement of these displacements which indicates either an enhancement of anharmonicity in the surface or the onset of a thermal structural roughening.

ARUPS investigations have been performed along on the Ta(100) face covered with Na atoms. For any Na coverage up to the monolayer, the overlayers are disordered. Upon Na adsorption, the high-lying surface-resonance band of this face exhibits a continuous bending. At one monolayer this band is parabolic for k_|| < 0.6 Å^-1 and its bottom reaches - 2 eV. Such a dispersion occurring on disordered overlayers shows that the chemical bond involves the hibridization of d-like surface resonance states with Na 3s-derived states. This covalent bonding exists over the monolayer range.

New experimental data on the specific heat c_p(T) of superconducting UBe₁₃ between 0.065 and 0.18 K are presented. They are compared with calculations of c_p(T) which include the influence of resonant impurity scattering. The comparison supports the claim that the superconducting state of UBe₁₃ is of an axial type.

The complex susceptibility of five amorphous alloys of the series Dy_xGd_1-xNi has been measured at low temperature. The observed variations of χ' and χ" are fitted to a simple constrained dynamical model. In the limit T → 0, the imaginary susceptibility vanishes, whereas the real component is proportional to (J/D)^3.9±0.15, where J and D represent local exchange and anisotropy energies. This result strongly suggests that the lower critical dimensionality of random anisotropy systems is equal to four.

By means of antiferromagnetic resonance in the linear-chain antiferromagnet (CH₃)₄NMnCl₃ (TMMC) we have studied the effect of sine-Gordon solitons on the q* = 0 magnon. Like in ferromagnetic chains the phase coupling between solitons and magnons results in a broadening of the magnon line width which is proportional to the soliton density and shows a drastic dependence on temperature and on the external magnetic field.

First results are presented on the study by X-ray photoemission spectroscopy (XPS) of the effect of chemical cleaning on the near surface composition of vapour phase epitaxial Ga_0.47In_0.53As. In most cases the surface is extremely perturbed. Ga depletion is very reproducibly observed on the surfaces we studied. Acid and basic solutions differ by their action on arsenic. The basic solution was found to remove partly the excess arsenic which is present on the initial surface. Oxides are greatly reduced by all the treatments. None of the solutions give a correct stoichiometry.

We have recently started a systematic study of short-period (periods less than 60 min) variations in the cosmic-ray intensity. By making use of the MEM, we have found intensity oscillations with periods varying from 16 to 27 minutes during the GLE on February 16, 1984.