Studies of stucture and lattice dynamics of the strontium-copper-chloride(001) surface by helium-atom scattering.

摘要

This thesis presents the results of elastic and inelastic helium-atom scattering (HAS) measurements from the insulating Sr2CuO2 Cl2(001) surface. Elastic HAS provided diffraction patterns which contain information about the surface periodicity and the topology of the surface charge density. Inelastic HAS, implemented via time-of-flight techniques, determined the corresponding surface phonon dispersion curves and was used to characterize surface vibrational modes.; Elastic HAS revealed a surface periodicity consistent with the bulk termination, which suggests that the surface is non-polar and stable. This seems to favor one of the two surface candidates that may be exposed by the process of in-situ surface cleaving, namely SrCl and not CuO2.; In order to interpret the experimental phonon dispersion data, lattice dynamical calculations based on the shell model were carried out for the bulk and the two surface terminations of SrCl and CuO2. From the bulk calculations, appropriate model parameters were derived that were subsequently extended to a surface dynamical model via a slab method. Comparison between the experimental data and theoretical results for two different slabs with SrCl and CuO2 terminations showed that the experimental data conform exclusively with the SrCl surface modes.; Finally I report on a theoretical investigation of the anomalous magnetic properties of the CoO(001) surface. Recent experimental studies of the temperature-dependent antiferromagnetic ordering of the CoO(001) surface, using metastable He-atom scattering, revealed an anomalous enhancement in the sublattice magnetization with increasing temperature that straddles the bulk Neel temperature ( TbN ) of 290 K. I developed a Bogolyubov variational mean-field model based on a model Hamiltonian for the CoO(001) system by taking exchange, crystal field and magnetoelastic interactions into account. This model revealed that the enhancement in magnetization is closely related to the low-lying spin excited states on the CoO surface, and reproduced the observed anomalies.