The Theory of angular momentum waves in pure ortho-hydrogen is presented in the Bloch spin wave approximation. Since there is a large energy gap between the ground state and the excitation this approximation can be justified. Quadrupole-quadrupole interaction of nearest neighbours is used. We have derived some dispersion curves and the spectrum.

We present detailed Monte Carlo results for a two-dimensional grain boundary model on a lattice. The effective Hamiltonian of the system results from the microscopic interaction of grains with orientations described by spins of unit length, and leads to a nearest-neighbour interaction proportional to the absolute value of the angle between the grains. Our analysis of the correlation length xi and susceptibility chi in the high-temperature phase favour a Kosterlitz-Thouless-like (KT) singularity over a second-order phase transition. Unconstrained KT fits of chi and xi confirm the predicted value for the critical exponent nu, while the values of eta deviate from the theoretical prediction. Additionally we apply finite-size scaling theory and investigate the question of multiplicative logarithmic corrections to a KT transition. As for the critical exponents our results are similar to data obtained from the XY model, so that both models probably lie in the same universality class.Comment: 13 pages, Latex, 7 figures, to appear in Physica

Es werden die Translations‐ und Rotationsschwingungen der als starr betrachteten Moleküle eines Molekülkristalls untersucht. Die Eigenschwingungen des Kristalls sind ebene Wellen, die teilweise Translations‐ und teilweise Rotationscharakter besitzen („Phononen”︁ und „Torsonen”︁). Für den Fall, daß die Verrückungen der Atome als Translationen und Rotationen der Moleküle beschrieben werden, werden die harmonischen Kopplungsparameter und die zwischen ihnen bestehenden Symmetriebeziehungen angegeben. Zur Illustration wird ein einfaches Modell einer Molekülkette behandelt.

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We present an efficient method which provides a binary rule to locally identify either f.c.c.-like or non-f.c.c.-like clusters directly in a configuration of atoms as it was simulated, without any artifice which involves moving around the atoms a posteriori like, e.g., Voronoi edge reduction combined with the steepest-descent method. The method makes possible a detailed analysis of the structural aspects of melting and solidification of systems with f.c.c. symmetry from a local perspective. We present first results on the nucleation and growth of an f.c.c. phase in a melt of Lennard-Jones atoms.