Table of contents

The quantum Langevin force of a parametrically driven damped harmonic oscillator is determined from the condition of the preservation of the canonical commutation relations. The arguments are independent of the microscopic modelling of the heat reservoir. For damping with and without retardation the Langevin force is found to be independent of the time-dependent parametric driving force and determined by the damping rate alone.

The difficulties in formulating a natural and simple operator description of the phase of a quantum oscillator or single-mode electromagnetic field have been known for some time. We present a unitary phase operator whose eigenstates are well-defined phase states and whose properties coincide with those normally associated with a phase. The corresponding phase eigenvalues form only a dense subset of the real numbers. A natural extension to the definition of a time-measurement operator yields a corresponding countable infinity of eigenvalues.

We study the chiral Schwinger model shown to be unitary under certain conditions by Jackiw and Rajaraman. We show that, whenever the model is unitary, it is also equivalent to a manifestly gauge-invariant model containing a Dirac fermion and a decoupled chiral fermion, considered in a specific gauge.

Discrete transitions have been observed in ¹⁵⁶Dy up to spin 53^- in the lowest-energy odd-spin negative-parity sequence. Three other negative-parity level sequences and the lowest positive-parity sequence have each been observed to spins between 41 and 46. Doppler shift attenuation data show that these structures remain collective up to the highest spins. Irregularities in the γ-ray energy sequences from the decays of these highest spin levels are not characteristic of quasi-particle alignment governed by the pairing interaction, but may be explained in terms of the crossing of unpaired bands.

The possibility to perform optical experiments with only a few impurity ions (e.g., 1 to 10) fixed in a crystal is demonstrated.

The stability of stationary patterns and travelling waves against arbitrary fluctuations in spatially extended systems are investigated for the first time near the codimension-2 point. An important result is that travelling waves (and also standing waves) lose stability in some range via Benjamin-Feir resonance if the stationary pattern bifurcates inversely, in qualitative agreement with measurements on binary fluid mixtures. The results presented could stimulate further investigations.

The tube model of polymer melt dynamics is extended to examine multiply branched molecules. The case of the regular tree exhibits a hierarchy of relaxation times, each associated with a different tube diameter. A log-quadratic relaxation curve is predicted, and compared to relaxation of low-density polyethylene. At the longest times the polymer behaves as if it were not entangled. The geometric mean of the disentanglement timescale and the monomeric timescale is of the order of the relaxation times of the free ends.

Constraints for quasi-crystals arising from the energetical stability assumption are obtained. For grid models the constraints are expressed in a simple algebraic form (SI-conditions) and applied to planar tilings. SI-conditions allow only for quadratic irrationalities a + b √D (a, b and D are rational numbers). n-fold rotational symmetric quasi-crystals are allowed only if n = 5, 8, 10, 12 (pentagonal, octagonal, decagonal and dodecagonal symmetries).

An acoustic hole burning experiment in a tetrasil glass is presented. A sharp discrepancy between experimental data and theoretical expectations based on the standard Bloch equations appears as soon as high input powers are involved. On the contrary, good agreement is obtained if the Bloch equations as modified in the Yamanoi and Eberly theory are used. The results of the experiment are the transverse relaxation time T₂⁰ and the correlation time τ_c of the random field due to spin-spin interactions.

I speculate about the ingredients needed for a theory of the liquid → glass transition. Experimental phenomenology suggests that there is an underlying equilibrium thermodynamic transition. Because of a breakdown of hyperscaling, the free-energy barriers to relaxation diverge at the critical point leading to a Vogel-Fulcher-like divergence of the characteristic relaxation times. The glass is thus a material trapped just above its critical point.

The local jump diffusion of trapped H in two Nb(OH)_x samples was investigated between 10 and 160 K by neutron spectroscopy. The jump rates vary by less than a factor of 7. Below ∼ 70 K, they exhibit Kondo's T^2K-1 temperature power law for a diffusion process that is controlled by nonadiabatic coupling to conduction electrons. Above ∼ 70 K, the diffusion is dominated by the interaction with phonons.

A mechanically driven graphite fiber microbalance is used to investigate the wetting behavior of argon on graphite. We found evidence of complete wetting above the bulk triple point T₃ and incomplete wetting below. Below T₃ the thickness d of the saturated adsorbed film is found to obey the power law d ∝ (t)^x, with x = -1/3, in the reduced temperature range 10^-5 < t = (T₃ - T)/T₃ < 10^-2. Above T₃ the film thickness diverges as d ∝ [ln (P₀/P)]^x, with x = -1/3.

We propose a new approach for the evaluation of the conductivity as given by the Kubo-Greenwood formula. Our method is based on a procedure similar to that of the recursion method and expresses the conductivity at every frequency as the sum of a series which is expected to converge better when the mean free path is short. The first term of the series is the expression of the conductivity within the random phase approximation. A first application is given.

Interface formation between MBa₂Cu₃O_7-δ (M = Y, Sm) and the monovalent metals Ag and Rb was studied by photoemission using synchrotron radiation. The interfaces with Ag are found to be nonreactive, while those with Rb are characterized by strong chemical reactions. These include oxidation of the Rb overlayer as well as a metal-semiconductor transition induced by oxygen deficiency of the substrate layer adjacent to the interface. The observed quenching of the 12.5 eV two-hole final-state satellite in the valence-band spectra indicates a reduction of Cu(II) to Cu(I) at the Rb interfaces.

We performed a numerical simulation of the three-dimensional nearest-neighbour Ising spin glass with J = ±1, using the microcanonical ensemble method. We found that, at very low temperature, and for very long times (as long as we could measure) the system behaves as if ergodicity was broken.

In this paper the DLVO picture is used to make a novel prediction about the phase diagram of charged colloidal suspensions. Competition between the short-range Hamaker attraction and long-range screened charge repulsion can cause the critical point between the low density disperse and the high density floc phases to be preempted by a finite wave vector critical point. The resulting phase diagram also contains intermediate density, macrocrystal, incommensurate, b.c.c. and layered phases, reminiscent of layered phases found in magnetic and amphiphilic systems. These phases should occur elsewhere in the theory of liquids.