Table of contents

Volume 26

Number 8, June 1994

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00 GENERAL

561

The difference ΔE between the two lowest energy levels of a one-spin system with classically degenerate ground state is calculated as a function of an external magnetic field B. Using standard quantum-mechanical perturbation theory, it is shown that the tunnel splitting ΔE vanishes for specific discrete values of the field B. Consequently, the quenching derived previously by a semi-classical evaluation of path integrals for large spin is seen to persist for arbitrary spin lengths. Further, the structure of the full quantum-mechanical energy spectrum is obtained.

565

and

We consider the behavior of a dilute phase of bilayer surfactant vesicles. We show that the free energy of a single vesicle increases logarithmically with vesicle size, due to a previously unappreciated finite-size contribution to the fluctuation free energy, in contrast to previous studies, which assumed a logarithmic decrease of the free energy due to thermal softening of the bending rigidity. This result leads to qualitatively different predictions for the behavior of an equilibrium vesicle phase.

571

, and

A periodically driven bistable system is considered in the region of validity of a two-state hopping approximation. In the limit of weak noise the hopping process becomes discrete not only in space but also in time so that an explicit evaluation of the autocorrelation function becomes possible. This function turns out to be a non-analytic function of time leading to a rather peculiar structure of the power spectrum. Results are compared with numerical and experimental data.

577

By using a local-chaos hypothesis, we obtain the static mean-field equations giving the distribution for the fixed points of asymmetric random neural networks in the presence of an external field. These equations are the SK solutions for the Sherrington-Kirkpatrick model. We then study the critical value for the destabilization of the network, vs. the external field, and find it follows the De Almeida-Thouless line. We numerically compute the critical value for the entry into the chaotic regime and observe that, for increasing size, the destabilization line and the chaotic line collapse onto the AT line.

30 ATOMIC, MOLECULAR AND OPTICAL PHYSICS

583

and

We present a new scheme to extract numerically "optimal" interatomic potentials from large amounts of data produced by first-principles calculations. The method is based on fitting the potential to ab initio atomic forces of many atomic configurations, including surfaces, clusters, liquids and crystals at finite temperature. The extensive data set overcomes the difficulties encountered by traditional fitting approaches when using rich and complex analytic forms, allowing to construct potentials with a degree of accuracy comparable to that obtained by ab initio methods. A glue potential for aluminium obtained with this method is presented and discussed.

60 CONDENSED MATTER: STRUCTURAL, MECHANICAL AND THERMAL PROPERTIES

589

, and

Two kinds of two-colour Penrose quasi-lattices are proposed, which are constructed by stacking the two-colour Penrose tiling (P) in the sequence of Pbar PPbar P... (bar P is P with inversion colour) and in the sequence of PPP..., respectively. They are new three-dimensional quasi-lattices which are quasi-periodic in two dimensions and periodic in the third, perpendicular direction. The space groups of the two-colour Penrose quasi-lattices are P105/mmc in the first case and P10m2 (= P5/mm2) in the second case. The application of the idea of two-colour lattice to other quasi-periodic lattices is discussed.

595

, , and

The phase transition of ferroelastic lead phosphate has already been extensively studied. Because many questions concerning this "textbook" compound remained unsolved, a careful examination of the reciprocal space has been performed using synchrotron radiation (high-intensity precession camera photographs). A new type of diffuse scattering is observed showing the existence of a kind of local ordering not detected in previous experiments: the phase transition appears much more complex than originally thought. The present results raise new questions about the transition order parameter(s) and suggest new experiments to be performed.

601

, , and

We give a theoretical argument which indicates that the combined effect of substrate inhomogeneity and grain boundaries can be very important in determining the growth morphology of thin metal films on non-wetting substrates. An elongated metal island on a smooth non-wetting substrate, with a single grain boundary at its center, is shown to be in a state of unstable equilibrium. Any substrate inhomogeneity is then able to pin the island in a metastable state. An estimate of the depinning temperature is also given.

70 CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC AND OPTICAL PROPERTIES

607

, , , and

We present the results of a calculation for the bulk electronic structure of gallium nitride in the zincblende phase. We determine the equilibrium lattice constant, the cohesive energy and the bulk modulus in the Density Functional approach within the Local Density Approximation (DFT-LDA). The one-particle eigenvalues of the DFT Kohn-Sham equation do in principle not agree with the experimental band structure. Therefore, we calculate the quasi-particle energies by including self-energy corrections to the DFT-LDA exchange correlation potential, with the GW approximation for the electron self-energy. We use norm-conserving pseudopotentials and a large plane-wave basis set (100 Ry cut-off) for a converged calculation in the DFT-LDA. The LDA band gap turns out to be very sensitive to the crystal volume. We find that GW corrections to the LDA band gap are significant. A detailed comparison with other DFT-LDA results and approximate GW calculations and with existing experimental data is given.

613

and

Recent model calculations for thin-film antiferromagnetic superstructure show that the magnetic-anisotropy energy (MAE) and the orbital magnetic moment can be anomalously large although it is problematic if the corresponding structures are stable. For this reason it is interesting to consider superstructures that might be prepared in thin bimetallic films. In this paper we study the hypothetical c(2 × 2) FeCo and CoNi alloy (001) monolayers within a tight-binding d-band model. The MAE is in both cases above 1 meV/A-B pair and prefers the in-plane (perpendicular) direction of magnetization for FeCo (CoNi). The Co atoms in CoNi have a considerable magnetic orbital moment (∼ 0.5μB).

619

, , and

Electron spin resonance measurements on a disordered conducting polymer were performed from 4 K up to 400 K for the first time in order to investigate the nature of the so-called "Pauli-like" regime. The usual departure of the susceptibility from a Curie law, a shift in the g-factor value and a transition from a line broadening regime to a line narrowing one were observed for T > 200 K. We argue that all of these high-temperature ESR properties are due to thermal excitation of bipolarons from the singlet to the triplet state. The appearance of Pauli paramagnetism is ruled out for this system. A detailed discussion of the susceptibility, spin relaxation and g-factor is given.

625

, , and

The luminescence and Raman spectra of porous silicon have been investigated at room temperature as a function of hydrostatic pressure up to 15 GPa. In the high-pressure metallic phase the luminescence is still observed showing that it is not related to the diamond phase of the silicon and, therefore, not originating from simple quantum size effects.

80 CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

631

and

A model is set up which is believed to contain the essential physics of foam drainage. Mathematically, the model leads to a non-linear partial differential equation, which we were able to solve exactly by analytical methods for the physical boundary conditions relevant for recent experiments in which a dry foam is wetted by a steady addition of liquid. The solution represents a density wave of constant profile (or solitary wave) in qualitative accord with these experiments. The observed dependences of liquid fraction and wet/dry foam boundary velocity on input flow are reproduced by the model.

COMMENT

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