Thermodynamics of the harmonic oscillator using coherent states

The ongoing discussion whether thermodynamic properties can be extracted from a (possibly approximate) quantum mechanical time evolution using time averages is fed with an instructive example. It is shown for the harmonic oscillator how the Hilbert space or an appropriately defined phase space must be populated in terms of coherent states in order to obtain the quantum result respectively the classical one.Comment: 6 pages, 2 postscript figures, uses 'epsfig.sty'. Submitted to Europhysics Letters. Introduction changed and references added for the revised version. More information available at http://obelix.physik.uni-osnabrueck.de/~schnack

Magnetic response of magnetic molecules with non-collinear local d-tensors

Investigations of molecular magnets are driven both by prospective applications in future storage technology or quantum computing as well as by fundamental questions. Nowadays numerical simulation techniques and computer capabilities make it possible to investigate spin Hamiltonians with realistic arrangements of local anisotropy tensors. In this contribution I will discuss the magnetic response of a small spin system with special emphasis on non-collinear alignments of the local anisotropy axes.Comment: 8 pages, 18 figures, accepted for a special issue of Condensed Matter Physics (CMP), typos correcte

Unexpected properties of the first excited state of non-bipartite Heisenberg spin rings

Systematic properties of the first excited state are presented for various ring sizes and spin quantum numbers which are only partly covered by the theorem of Lieb, Schultz and Mattis. For odd ring sizes the first excited energy eigenvalue shows unexpected degeneracy and related shift quantum numbers. As a byproduct the ground state energy as well as the energy of the first excited state of infinite chains are calculated by extrapolating the properties of only a few, finite, antiferromagnetically coupled Heisenberg rings using the powerful Levin sequence acceleration method.Comment: 9 pages, 3 figures, uses 'epsfig.sty'. Submitted to Phys. Rev.

Sampling the two-dimensional density of states g(E,M) of a giant magnetic molecule using the Wang-Landau method

The Wang-Landau method is used to study the magnetic properties of the giant paramagnetic molecule Mo_72Fe_30 in which 30 Fe3+ ions are coupled via antiferromagnetic exchange. The two-dimensional density of states g(E,M) in energy and magnetization space is calculated using a self-adaptive version of the Wang-Landau method. From g(E,M) the magnetization and magnetic susceptibility can be calculated for any temperature and external field.Comment: 5 pages, 9 figures, submitted to Phys. Rev.

Solitary waves on finite-size antiferromagnetic quantum Heisenberg spin rings

Motivated by the successful synthesis of several molecular quantum spin rings we are investigating whether such systems can host magnetic solitary waves. The small size of these spin systems forbids the application of a classical or continuum limit. We therefore investigate whether the time-dependent Schroedinger equation itself permits solitary waves. Example solutions are obtained via complete diagonalization of the underlying Heisenberg Hamiltonian.Comment: 6 pages, 7 figures, to appear in J. Magn. Magn. Mate