Table of contents

We study the distribution of ground-state energies of directed polymers on disordered hierarchical lattices. The problem can be reduced to finding the stable distribution when one combines random variables in a nonlinear way (for example, e = min (e₁ + e₂, e₃ + e₄)). The ground-state energy fluctuations of a polymer of length L increase like L^ω. We calculate ω numerically for a whole family of hierarchical lattices. In a limiting case, we present a perturbation theory for the ground-state energy and the exponent ω, which agrees well with the numerical results.

Microscopic calculations of energies, structure and transition probabilities of low-lying 2⁺ states in ⁵⁶Fe have been performed in the framework of the quasi-particle phonon nuclear model. The interaction of the phonons appears to play an important role in determination of the isotopic character of low-lying quadrupole states.

Based on a geometric consideration, applying the multisource model the E_T-distributions in the mid-rapidity and the target fragmentation regions in high-energy heavy-ion collisions are calculated. For different projectiles and targets the same set of parameters is used to fit the new data of CERN experiments.

For a chain of particles with competing and anharmonic interactions the number of metastable states (at T = 0 K) is investigated as a function of pressure p. For p < p_max the number M(N,p) of metastable configurations varies exponentially with N, i.e. M(N,p) ∼ exp[ν(p)·N], where N is the number of particles. ν(p) exhibits a staircaselike behaviour with an infinite number of plateaus and it vanishes for p ⩾ p_max. The heights of the plateaus do not depend on the model parameters. For p near p_max it is shown that ν(p) ∼ (ln|ln(p_max - p)|)/|ln(p_max - p)| implying that ν(p) goes to zero with infinite slope.

Experiments of Rayleigh-Bénard convection in intermediate geometries show that very small wavelengths can be stabilized at very high values of the Rayleigh number when the fluid layer is quasi one-dimensional. A new collective oscillating instability is then observed; this instability may be a first step toward spatio-temporal chaos.

Langmuir-probe-based measurements of the time-resolved electron energy distribution function, EEDF, in a low-frequency (100 kHz) r.f. glow discharge are presented. These high-resolution measurements show the contrasting nature of the EEDF in a molecular (N₂) and atomic (Ar) plasma. The effect of the interaction of the electrons with excited species in the plasma is observed and supports previously untested theoretical calculations. This interaction results in a highly non-Maxwellian EEDF for electron energies above a few eV and has strong implications for the understanding of the chemistry of plasma reactors, and the basic physics of r.f. glow discharges.

We present an investigation of the atomic structure of Ni₄₀Ti₆₀ glass by computer simulation, using a tight-binding Hamiltonian to describe effective pair interactions. In addition to a good agreement with the various experimental structural data, the results establish a connection between the microscopic description of the chemical bond and the chemical short-range order.

A one-dimensional system with anharmonic energy of the Landau type is considered. The motion of the system is governed by Newton's equations. It is shown that unexpectedly a dispersive mode describes the dynamics of this system near the mean-field transition temperature. The universal functions which describe the correlation in time and space are obtained by numerical simulations. The result applies in particular to the description of the 2k_F mode softening (CDW formation) in quasi-one-dimensional compounds.

Incoherent inelastic neutron spectroscopy combined with Brillouin scattering data are used to estimate the absolute density of states of fractal silica aerogels over the entire frequency range. The results indicate an extended fracton regime, and allow the thermal properties to be calculated in agreement with experiments.

A new description of the coupling between translational and rotational degrees of freedom in plastic crystals is presented. Using the lattice-gas variable {t_i} for translations and the q-state Potts variable {r_i} for rotations a tetralinear coupling t_ir_it_jr_j is introduced between nearest neighbours. Moreover, ionic crystals and molecular crystals are found to be described by different Hamiltonians. Random fields which couple solely to the {r_i} are involved in the molecular case. The corresponding phase diagrams are obtained. The results are discussed with respect to experiments.

Using high-pressure thermal quenching, we have prepared a homogeneous Al_0.94Si_0.06 solid solution. Specific heat measured between 0.12 K and 7 K has revealed a softening of the transverse acoustic modes of phonons and well below T_c ≃ 3 K, a contribution to the specific heat with a linear temperature dependence. These effects appear as experimentally due to the metallic state of silicon in an f.c.c. matrix which is a highly unstable state. The large decrease of the shear modulus (Δμ/μ ≃ 20%) in this nonequilibrium solid solution associated with a quasi-linear contribution to the specific heat is analysed as a sign of a lattice instability.

A new type of magnetoresistance oscillation periodic in 1/B is observed when the carrier density N_s of a two-dimensional electron gas is weakly modulated with a period smaller than the mean free path of the electrons. Experiments with high mobility AlGaAs-GaAs heterojunctions where N_s is modulated by holographic illumination at T ⩽ 4.2 K show that the period of the additional quantum oscillation is determined by the separation a of the interference fringes. This period corresponds to Shubnikov-de Haas oscillations where only the electrons within the first reduced Brillouin zone with |k| < π/a contribute.

We present fully self-consistent calculations on the image-plane position of metal surfaces, including pseudopotentials averaged over the directions parallel to the surface. Our results show that the distance of the image plane to the first atomic layer depends only weakly on the crystallographic orientation of the surface. We think that this weak dependence explains why the energies of the image states present a relative low dependence on surface orientation.

The temperature dependence of the nuclear spin-lattice relaxation rates of ²⁰⁵Tl has been studied for thallium atoms in the large lattice channels in a Pauli paramagnet, Tl_xV₆S₈ and an exchange-enhanced Pauli paramagnet with antiferromagnetic spin fluctuations, Tl_xV₅S₈. The thallium atoms are found to be not in the ionic states but in the metallic states together with vanadium atoms to form rather three-dimensional band structures of the compounds. The relaxation rate of ²⁰⁵Tl in the high-T_c Tl-Ba-Ca-Cu-O system is compared with the above results.

The temperature-dependent electronic structure of 1T-TaS₂ near the Fermi level, determined by means of angle-resolved photoemission with high-energy resolution of 35 meV, exhibits a distinct energy gap associated with a metal-semiconductor transition at 180 K on cooling, whereas on warming up the retransformation occurs in the electronic structure at 280 K. The complex phase transformation behaviour including the resistivity can be consistently explained between 400 K and 120 K. For the metal-semiconductor transition Mott-Hubbard localization of the d conduction electrons driven by critical interlayer coupling seems most likely.

The local dynamics of lipid molecules (fully hydrated) ordered in multilayers of dipalmitoylphosphatidylcholine (DPPC) and DPPC-d₆₂ are studied by IQENS. In the smectic A(L_α) state the diffusion coefficient of the lipid rotational diffusion is D_rot = 2 · 10^-9 s^-1 and of the lateral Brownian motion D_lat = 2 · 10^-7 cm² s^-1. The lipids also exhibit an out-of-plane vibrational motion (= dynamic surface roughness) the frequency of which depends on the amplitude, b (τ^-1 = 1 · 10⁸ s^-1 for b = 0.2 nm; τ^-1 = 2.3 · 10⁷ s^-1 for b = 0.6 nm). In the solid state (L_β) the choline headgroups exhibit rotational motion about an average axis forming an angle of 55 degrees with the normal, while the chain ends (the last 5 CH₂-groups) exhibit a hindered rotation (correlation time 2 · 10⁹ s^-1).