The T = 0 dynamics of the one-dimensional s = 1 2 planar anti ferromagnet is studied by an approach which consists of exact analytic calculations in the Bethe formalism and numerical finite-chain calculations on rings up to 10 spins. Our method makes use of well known critical exponents for the correlation functions and of exact sum rules. We obtain approximate analytic expressions for both the out-of-plane and the inplane dynamic structure factors, and for related quantities such as integrated intensities, susceptibilities and autocorrelation functions. The results are discussed in relation to possible experiments on quasi-1D magnetic compounds at low T . Our calculations make clear that the T = 0 dynamic structure factors are dominated by two-parameter continua of excitations rather than by single branches of spin-waves as predicted by classical spin-wave theory. By varying the planar anisotropy the autocorrelation functions display interesting features in their long-time asymptotic behaviour, such as a crossover from a uniform power-law decay to an oscillatory decay and a crossover between oscillatory decays with different frequencies. We conjecture a possibility of approaching the classical limit s = ∞ starting from the quantum limit s = 1 2 . This provides a qualitative, and in some aspects even quantitative, understanding of the dynamical behaviour of s > 1 2 systems in terms of a quantum approach.

By assuming the dominance of electrostatic terms in the Hamiltonia, Hubbard has recently formulated a ", dimer gas" model for tetracyanoquinodimethane which is closely related to a strongly anisotropic Heisenberg-Ising spin chain. We show here that in the anisotropic limit, the exact Bethe-ansatz equations describing the spin chain are very simple, and a full analysis of the lowest-state energy and elementary excitation spectrum for any magnetization is, given. We also find the excitation spectrum for the anisotropy and electron density thought to be correct for tetrathiafulvalene-tetracyanoquinodimethane, and the results are very similar to the limiting case.

We have studied two models for anisotropic quantum spin chains. (i) XY-chain with a field in the plane: The magnetization of the ferromagnet behaves as h1/3 for small fields, in agreement with scaling laws. The antiferromagnet shows a critical field at which the ground state is a simple Néel state and which separates power law from exponential decay of spatial correlations. (ii) Anisotropic XY-chain: The dynamic z-component spin correlation function can be decomposed into a spin wave and a soliton contribution. The nature of quantum soliton excitations is studied and their form compared to soliton solutions of classical equations of motion.

Integral and fractional order beam low-energy electron-diffraction intensity-voltage (1-V) data have been taken on a bombardment-annealed GaP(111)-(2&2),surface. We have compared these data with calculated I-V curves using a dynamical multiple scattering theory and found very good agreement for the following model: One out of every four Ga surface atoms is missing and the surface Ga-P bilayer is almost coplanar.Surface and deeper-layer atoms undergo vertical and lateral displacements from bulk positions. Similar results in other systems suggest that the vacancy model applies on the (111) face of many compound semiconductors.

No abstract