R. A. Cowley

The phase transition of strontium titanate

The structural phase transition of strontium titanate has played a crucial role in the development of our understanding of structural phase transitions. It was the first system for which experiments showed a soft mode, non-classical exponents, two time scales and two length scales for the fluctuations above Tc These experiments are reviewed. The first two of these results are now well understood but the last two are in contradiction with the accepted scaling theory of phase transitions. Various suggestions have been put forward and these are reviewed for accounting for these results in terms of defects and imperfections but they are not completely convincing. Further work is needed either to confirm that these suggestions are correct or to develop new explanations.

Neutron scattering study of the magnetic structure of Cs2CuCl4

The magnetic structure in the ordered phase of the nearly one-dimensional Heisenberg antiferromagnet has been measured using elastic neutron scattering. crystallizes in the orthorhombic Pnma space group with spin chains running along the crystallographic b-direction. Below the ordering temperature the magnetic structure is incommensurate along the chain direction with a temperature-independent ordering wavevector q = (0, 0.472, 0) (rlu}). The occurrence of an incommensurate structure is shown to be the consequence of frustration on the spins induced by the exchange interaction between chains. Group theory is used to determine the possible magnetic structures compatible with the symmetry of the crystal. The results show that at T = 0.3 K the spin ordering is cycloidal with spins rotating in a plane that contains the propagation direction b. A mean-field calculation of the magnetic ground-state energy including exchange anisotropy effects is used to study the stability of the observed structure. Values for the interchain exchange constants that are consistent with the features of the magnetic structure are proposed.

Magnetic structures and interactions in erbium

The magnetic structures in a single crystal of isotopically enriched erbium have been studied using a high-resolution neutron diffraction technique at the HFIR reactor at Oak Ridge National Laboratory. In addition, these structures have been examined by mean-field calculations, in which the primary interactions were derived from magnetization and spin-wave measurements. The agreement between theory and experiment is generally very good. At intermediate temperatures, the structure is based on an elliptical cycloid in the a-c plane, and blocks of moments with alternating positive and negative components in the c-direction give rise to a series of commensurable structures in the manner proposed by Gibbs and co-workers (1986). These structures are, however, distorted by two-ion couplings of trigonal symmetry, which reflect the different orientation of the two HCP sublattices and have a magnitude which is a substantial fraction of the isotropic exchange interaction. The result is a wobbling cycloid, in which there is an oscillating moment in the b-direction whose period differs from that of the basic cycloidal structure. In the low-temperature cone phase, the moments bunch around alternating a-directions in a pattern with trigonal rather than hexagonal symmetry. Some further consequences of the trigonal couplings on the low-symmetry magnetic structures in erbium and holmium are discussed.

Quasi-Elastic Scattering, Random Fields and phonon-coupling effects in PbMg1/3Nb2/3O3

The low energy part of the vibration spectrum in PbMg1/3Nb2/3O3 (PMN) relaxor ferroelectric has been studied by means of neutron scattering above and below the Burns temperature, Td. The transverse acoustic and the lowest transverse optic phonons are strongly coupled and we have obtained a model for this coupling. We observe that the lowest optic branch is always underdamped. A resolution-limited central peak and quasi-elastic scattering appear in the vicinity of the Burns temperature. It is shown that it is unlikely that the quasi-elastic scattering originates from the combined effects of coupling between TA and TO phonons with an increase of the damping of the TO phonon below Td. The quasi-elastic scattering has a peak as a function of temperature close to the peak in the dielectric constant while the intensity of the central peak scattering increases strongly below this temperature. These results are discussed in terms of a random field model for relaxors.

Two-Magnon Excitations Observed by Neutron Scattering in the Two-Dimensional Spin-5/2 Heisenberg Antiferromagnet Rb2MnF4

The low-temperature magnetic excitations of the two-dimensional spin-

X-ray scattering, dislocations and orthorhombic GaSb

\frac{5}{2}

X-ray scattering from dislocation arrays in GaSb

square-lattice Heisenberg antiferromagnet

Size-Dependent Shape and Tilt Transitions in In2O3 Nanoislands Grown on Cubic Y-Stabilized ZrO2(001) by Molecular Beam Epitaxy

{\mathrm{Rb}}_{2}\mathrm{Mn}{\mathrm{F}}_{4}

On the magnetic phase diagram of erbium in a c axis magnetic field

have been probed using pulsed inelastic neutron scattering. In addition to dominant sharp peaks identified with one-magnon excitations, a relatively weak continuum scattering is also observed at higher energies. This is attributed to neutron scattering by pairs of magnons and the observed intensities are consistent with predictions of spin wave theory.

Multiparticle states in the S = 1 chain system CsNiCl3.

The structure of two GaSb layers, 13 nm and 327 nm thick, grown by metal-organic vapour-phase epitaxy on a GaAs(001) substrate have been studied by means of high-resolution x-ray diffraction. The large lattice mismatch is largely relaxed by regular arrays of 90° dislocations. The results show that both layers have orthorhombic crystal structure, and provide detailed quantitative information about the distortions caused by the dislocations in both the GaSb layers and the GaAs substrate.