Colin Carlile

The diffusion of protons in the superionic conductor CsHSO4 by quasielastic neutron scattering

The diffusion of protons in the superionic phase of CsHSO4 was investigated by quasielastic neutron scattering (QNS) on the IRIS spectrometer at the pulsed spallation neutron source ISIS, Rutherford Appleton Laboratory, UK. Two processes with different characteristic times could be distinguished: long-range translational diffusion with a diffusion coefficient D=(1.00-1.17)*10-7 cm2 s-1 (for a temperature range from 423 to 448 K) and jump rotation of HSO4 groups between two possible orientations, the corresponding nonequivalent proton sites being 2.3 AA apart.

Observation of a dynamic coupling of methyl groups in 4-methylpyridine

Abstract Structure indicative of a mechanical coupling between pairs of methyl groups is observed in the inelastic neutron scattering spectrum arising from methyl rotation in 4-methylpyridine. The structure has a quite different form from that due to a mutual scalar hindering potential. It agrees closely with the form expected from a dynamic interaction between the angular momenta of two methyl groups giving rise to a mutual gauge or vector potential. Detailed agreement is obtained when both types of interaction, scalar and vector, are assumed to occur. This is the first observation of the dynamic interaction.

Angular and time resolution of neutron time of flight spectrometers

Abstract We show that the angular resolution Δφ and time resolution Δt of neutron time-of-flight spectrometers can be determined accurately by measuring the scattering spectrum of superfluid 4He at low temperature. The method is used to determine Δφ and Δt of the IRIS spectrometer at ISIS (UK), yielding the values (FWHM) Δφ=0.118 rad (6.8°) and Δt=212 μs, and to reinterpret the results of Blagoveshchenskii et al., showing that their measured linewidths for superfluid 4He can be ascribed solely to the effect of the angular resolution of their time-of-flight spectrometer.

Two-dimensional caesium-ammonia solid solutions in C28Cs(NH3)x

Neutron time-of-flight patterns at a resolution of Δd/d = 2 × 10-3 and inelastic scattering spectra at a resolution of 15 µeV have been measured for the ternary intercalation compounds which are formed when ammonia gas is absorbed at reduced pressure by stage II caesium-graphite. The diffraction patterns reveal a new staging process giving high stage ternary compounds as the first step in the reaction followed by lower stage compounds and ultimately a true second-stage ternary compound C28Cs(NH3)2·66. The rotational tunnelling spectra at the lowest ammonia concentrations (x < 1·25 in C28Cs(NH3) x ) show transitions at 126 µeV and 640 µeV, whilst the higher concentrations so far measured show a single transition at 89 µeV up to x = 1·9 when the tunnelling spectrum collapses. The two-line spectrum at low concentrations shows that half of the (on average four) NH3 ligands to each caesium ion have a much weaker V 3 hindering potential than the others (about 10/B) compared to 20/B for the 126 µeV line showing ...

Anomalous rotational dynamics of NH4 ions in the paraelectric phase of K1-x(NH4)xH2PO4 mixed crystals

A study of the rotational dynamics of NH4 ions in the paraelectric phase of mixed crystals of K1-x(NH4)xH2PO4 by quasielectric neutron scattering is reported. For high NH4 concentrations (x=0.9) no anomalous behaviour was found and the model of NH4 ion jumps between equivalent orientations in the crystal gave a good description of the experimental data. In contrast the crystal with x=0.6, which forms a protonic dipolar glass state at low temperatures, revealed an unusual temperature dependence of the elastic incoherent structure factor far above the glass formation boundary. This temperature dependence is analysed in the framework of the model of dynamical cluster formation in the paraelectric phase. The order parameter for the gradually condensing NH4 groups is obtained. The results are compared with data obtained by other methods on related compounds.

PRISMA - a spectrometer for the measurement of coherent excitations on a pulsed spallation neutron source

Abstract The measurement of nuclear and magnetic excitation spectra from single crystal samples remains central to condensed matter physics. The requirements in terms of the range and resolution of the scattering vector Q and energy transfer h ω are reviewed and typical experiments with a well defined cross-section are chosen. The performance and limitations of existing instruments are reviewed. A design for a new spectrometer, PRISMA, to be installed on the UK spallation neutron source, ISIS, is presented. Its performance for chosen experiments is given in terms of the Q and h ω range covered in a single scan, the resolution and the count rate.

Persistence of helical magnetic order in dysprosium - holmium superlattices

The magnetic structures of two Dy/Ho superlattices have been studied by neutron scattering using a time-of-flight spectrometer. For temperatures below the Neel temperature of bulk Dy, , but above , the Dy blocks order in a helical magnetic structure which has long-range coherence across several superlattice repeats, despite the Ho atoms being paramagnetic. Below 140 K helical order also develops in the Ho blocks, and the ferromagnetic transition of Dy at lower temperatures is suppressed, so the helical phase is found at all temperatures down to 2 K.

Rotational tunnelling of ammonia in two-dimensional metal–ammonia solutions

Rotational tunnelling of ammonia in the two-dimensional metal–ammonia solid solutions formed by high-stage caesium graphite intercalates is reported and used to define the adsorption site structure in these systems. Different stages and stoichiometries CnCs(NH3)x have been measured for 1.5 < T/K < 90 using high-resolution neutron scattering to obtain the dependence on stage and temperature of the tunnelling energies. Prominent transitions at 89 µeV (high fillings) and at 89 and 126 µeV (low fillings) indicate sensitivity to the ammonia content with multiple adsorption sites and threefold symmetry for the hindering potential. The activation energy for quenching the 89 µeV transitions is 6 meV, corresponding to a major peak in the vibrational density of states but not the expected E01 torsional transition energy. The bearing of these results on recent theories for the temperature dependence of tunnelling is discussed.

Structural and Dynamic Aspects of the Tunnelling of Hydrogen in Caesium-intercalated Graphite

Abstract Simultaneous neutron diffraction and spectroscopy from hydrogen absorbed into the caesium-graphite intercalate compound (GIC) C28Cs(H2)x shows a spectacular change in tunnelling spectra with hydrogen content. Although the changes in lattice swelling appear to be gradual, there are subtle signs of the step-like behaviour observed in the ammonia system. These indicate a clear correlation between the lattice swelling and the tunnelling lines. Three distinct regions are identified.

The virtue of cold neutrons on pulsed sources

Abstract The widely-held view that pulsed sources and reactors are complementary, the former producing epithermal neutrons and the latter cold neutrons, has been shown in recent years to be an oversimplification. The concept of complementarity, if in fact it exists, is one of methods rather than of sources, frequently allowing similar scientific topics to be examined in refreshingly different ways. As more experience is gained in the still unfolding new methods of pulsed neutron scattering, it is clear that no area can sensibly be excluded from investigation using pulsed sources. It is also crucial to realize that the body of knowledge is changing rapidly with time. The boundaries are not constant and the horizon has not yet been seen.