Magnetic Field Effects on Neutron Diffraction in the Antiferromagnetic Phase of UPt3UPt_3

We discuss possible magnetic structures in UPt$_3$ based on our analysis of elastic neutron-scattering experiments in high magnetic fields at temperatures $T<T_N$. The existing experimental data can be explained by a single-{\bf q} antiferromagnetic structure with three independent domains. For modest in-plane spin-orbit interactions, the Zeeman coupling between the antiferromagnetic order parameter and the magnetic field induces a rotation of the magnetic moments, but not an adjustment of the propagation vector of the magnetic order. A triple-{\bf q} magnetic structure is also consistent with neutron experiments, but in general leads to a non-uniform magnetization in the crystal. New experiments could decide between these structures.Comment: 5 figures included in the tex

Diffraction studies of disordered materials

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An Insitu X-Ray-Diffraction Method For The Structure Of Amorphous Thin-Films Using Shallow Angles Of Incidence

A technique for the structural characterization of thin amorphous films employing synchrotron radiation parallel beam x-ray optics at grazing angles of incidence is detailed. At incident angles near to the critical angle for total external reflection, sampling of specimens may be achieved via the evanescent mode. The parallel beam geometry allows the use of a technique in which a 2-theta detector, incorporating a parallel plate collimator, scans diffraction data for a given incident angle. For a specified wavelength, the incident angle chosen will determine the penetration of the radiation into the sample (approximately 10-1000 angstrom). The data must be corrected for significant peak shifting resulting from x-ray refraction, as well as for the effects associated with conventional theta:2-theta scans. Preliminary data resulting from the first application of this technique to amorphous hydrogenated silicon:carbon thin films, deposited onto crystalline silicon substrates, will be presented and discussed. Conventional theta:2-theta powder diffraction data will also be presented as a comparative standard

A High-Resolution Neutron-Diffraction Study Of The Structure Of Amorphous Hydrogenated Carbon, a-CH

Current structural models for amorphous hydrogenated carbon (a-C:H) are called into question on the basis of neutron-diffraction experimental work carried out al the ISIS pulsed neutron source (UK) on a-C:H. The nature of the neutron source allows the collection of data over an exceptionally wide dynamic range that ensures a real-space resolution sufficient to allow direct observation, for the first time, of contributions from the principal C-C bond types. The data also reveal details of the C-H correlations, and the presence of trapped molecular hydrogen

Orientational correlations in liquid carbon tetrabromide: A neutron diffraction and RMC study

Neutron diffraction measurements have been made for liquid carbon tetrabromide (CBr4) and converted to a composite pair correlation function. The results have been analysed using a Reverse Monte Carlo (RMC) technique to study the inter-relation of neighbouring molecules and the influence of molecular shape on the centres correlation function. It is found that the four closest neighbouring molecules have a preferred tetrahedral distribution and the orientation is not strongly ordered relative to the reference molecule although certain geometrical configurations can be identified. Other molecules in close proximity are less ordered and create a loosely-packed hexagonal structure for the broad distribution within the first co-ordination shell. In effect, the shape contours of the molecule are seen to have a significant influence on the position and orientation of adjacent molecules in an inner sub-shell but the longer range structure is dominated by overall packing effects in the liquid

The Structure Of Amorphous Hydrogenated Carbon By Neutron-Diffraction

Neutron diffraction data from a large, off-substrate sample of amorphous hydrogenated carbon (a-C:H) is presented and discussed. The material is prepared using a fast-atom deposition system using acetylene as the precursor gas. The experiments were performed on the ISIS pulsed neutron source (Rutherford Appleton Laboratory, UK) which is capable of yielding data over an exceptionally wide dynamic range; this ensures a real-space resolution sufficient to resolve directly, for the first time, contributions from the principle C-C bond types. Precise details on the C-H correlations are also revealed by the data, including the presence of molecular hydrogen trapped within distorted spheroidal cages. Quantitative complementary data on the vibrational states of the bonded hydrogen, derived from inelastic neutron scattering (INS) using a simple force-field model, is also presented. In particular, the INS data is used to provide a reliable estimate of the CH:CH2 ratio

An X-Ray-Scattering Study Of Amorphous Hydrogenated Carbon

The structure of “diamond-like” carbon, a-C:H has been studied using the Xray diffraction facilities available at the Synchrotron Radiation Source (Daresbury Lab, UK). The principle advantage of using the SRS is that high fluxes are available at short wavelengths (? ? 0.6Å): it is therefore possible to extract the “diffuse” scattering profile associated with all amorphous systems with considerable precision over a very wide K-range. A novel diffraction technique is also described in which the X-ray beam is incident on an as-deposited thin film at or near the critical angle. In this geometry we demonstrate that it is possible to gather diffraction data on -1?m films in-situ