W.G. Stirling

Magnetic excitations in plutonium monoantimonide

Neutron inelastic inelastic scattering studies of uranium compounds have illustrated the complexity of the interactions in this part of pqriodic table. Recently, large crystals of plutonium compounds have been grown at the Transuranium Institute and we report here the first neutron inelastic scattering on PuSb. A monodomain sample of ferromagnetic PuSb was prepared by cooling in a field with H || [0011]. We observe an almost dispersionless magnetic transition, assigned to arise within the Г8 ground state, with an energy of 4.3 THz. Surprisingly, the lowest frequency (3.5 THz) excitation occurs at the zone boundary with a polarization along the wavevector. Theoretical predictions give the spin-wave gap and Г8-Г7 crystal field splitting (>12 THz experimentally) but are presently unable to explain the lifting of the degeneracy of the longitudinal and transverse modes, and the minimum of the former at the zone boundary.

X-ray study of the critical magnetic scattering in NpAs

Magnetic critical scattering has been observed by resonant-enhanced X-ray diffraction at the transition to the incommensurate antiferromagnetic phase of neptunium arsenide. The observed critical behaviour is significantly different to that observed with conventional neutron scattering techniques. Our analysis suggests that the critical scattering is composed of two components, the sharper of which may be related to the surface or be a result of long-range order nucleated by impurities.

Neutron inelastic studies of UAs and USb

Abstract Experiments have been performed on single crystals of UAs and USb in the antiferromagnetic state with a three-axis neutron spectrometer at the High-Flux Reactor, Institut Laue-Langevin, Grenoble. The results from the two systems are briefly compared and some implications for our understanding of the UX compounds outlined.

The collective excitations of superfluid4He in aerogel glass

When liquid4He is condensed in porous aerogel glass (typical pore size ≈ 500 Å), many of the superfluid properties are significantly altered. Neutron inelastic scattering has been used to measure the collective phonon-roton excitations of liquid4He in this restricted geometry. Although at low temperature (1.3 K) the observed dispersion relation is identical to that of bulk4He, its temperature dependence is shown to be different. Previously unexplained superfluid fraction data are shown to be derivable from these microscopic measurements. An intrinsic broadening of the excitations associated with the restricted geometry is also present. Possible microscopic explanations for the unusual temperature variation of the collective excitation energies are discussed.

Neutron scattering from liquid3He at intermediate to large wavevectors

Neutron inelastic scattering experiments have been performed on liquid3He at saturated vapour pressure and T = 1.4 K covering a wide range of energy and momentum transfers (9 ⩽ Q⩽ 20 Å−1). Allowing for the very strong3He neutron absorption, the data is of excellent quality and it has been analysed to extract information on the scattering function widths, peak positions and single particle kinetic energies. It is found that there are no width oscillations with momentum transfer and that the average atomic kinetic energy is approximately 20% lower than theoretical predictions, in agreement with previous neutron scattering results. A new method of analysis aimed at extracting both the atomic momentum distribution and final state effects is also presented and the results discussed in the light of current theory.

Precision determination of S(Q, ω) of liquid 4He as a function of temperature

Abstract We report high-resolution high-precision measurements of S ( Q , ω ) of liquid 4 He at SVP at a range of temperatures above and below T λ , covering the wave vector range from 0.3 to 2.1A -1 . The temperature dependence is seen to depend sensitively on Q , the phonon remaining a well-defined excitation above T λ , while the maxon and roton excitations change abruptly at T λ , leaving no sharp peak at temperatures above.

Search for nuclear spin polarisation in praseodymium

Abstract Direct evidence for nuclear-spin order in dhcp praseodymium metal at millikelvin temperatures has been found from polarised neutron diffraction measurements.

The commensurate spin excitation in chromium: a polarised neutron investigation

Abstract A polarised neutron experiment with neutron energy analysis has been performed with a single- Q sample of chromium in a large magnetic field. The 4-meV “commensurate” mode is found to involve spin fluctuations parallel to the ordered chromium moments.

Breakdown of conventional dynamic scaling at the ferromagnetic curie point in EuO

Abstract The temperature and wave number dependence of the relaxation rate of critical fluctuations have been found to violate the dynamic scaling assumption. These results, together with a crossover from Lorentzian to critical lineshape with increasing wave number at T = T c , are consistent with recent mode coupling theories taking into account the dipolar interaction.

Magnetic excitations in monodomain ferromagnetic UTe

Abstract Neutron inelastic scattering experiments have been found performed with the IN 8 triple-axis spectrometer at the Institut Laue-Langevin, Grenoble, on a single domain crystal of the actinide ferromagnetic UTe. We find that the anisotropy of both the dispersion relation and the damping of the excitations depends on whether the propagation direction is parallel or perpendicular to the moment direction.