David N. Timms

Direct measurements and path integral Monte Carlo calculations of kinetic energies of solid neon

Direct measurements are reported of the kinetic energies of FCC neon at seven temperatures between 4.25 and 20.2 K. The measurements employed the electronvolt spectrometer which is the neutron scattering instrument at the ISIS neutron source, UK. Wavevector transfers Q from 306 to were used, far larger than previously, in a regime where possible final-state-effect corrections to the observed longitudinal neutron Compton profile J(y) are easily deal with by a simple correctional procedure. The results are of comparable precision with the best previous work, carried out using the HRMECS instrument at IPNS, Argonne. From those previous measurements, Peek et al give a ground-state kinetic energy of . Path-integral Monte Carlo calculations were performed using both HFD-C2 and Lennard-Jones pair potentials. At 4.13 K, 10.13 K, 15.69 K and 20.31 K the HFD results were , , and , respectively. These values are broadly in agreement with the current measurements. They are significantly lower than the published values obtained from an HFD-based Wigner - Kirkwood high-temperature expansion including terms up to .

Spin density in ferromagnetic nickel : a magnetic Compton scattering study

The magnetic Compton profiles (MCPs) measured in the [100], [110], [111] and [112] directions in single-crystal nickel with an incident photon beam of energy 224 keV are presented and discussed. The momentum resolution achieved, of 0.43 atomic units, improves on previous studies by almost a factor of two, and facilitates the interpretation of the MCPs in terms of the underlying spin-dependent momentum densities. Calculations have been performed using the linear muffin-tin orbital method, within both the local spin-density approximation (LSDA) and the generalized gradient approximation (GGA). Comparison with experiment reveals the limitations of the LSDA at low momentum, where the GGA is better able to reproduce the contribution of the s- and p-like electrons. All of the calculations overestimate the moment associated with the d-like electrons, for momenta corresponding to the first Brillouin zone. We also confirm the existence of the so-called Umklapp shoulders, which derive from the Fermi surface topology.

The kinetic energy of lithium-7 above and below the martensitic transformation

The atomic momentum distribution of lithium-7 in natural Li has been determined as a function of temperature from 20 to 300 K. The experiments were undertaken within the impulse approximation using the electronvolt spectrometer at ISIS. Final state effects were present above the statistical accuracy of our data and were accounted for by symmetrizing the neutron Compton profiles. The mean atomic kinetic energy of the Li nucleus was obtained directly from our measurements and was found to be approximately 10% higher than that predicted by a harmonic model incorporating a density of states derived from previous phonon measurements. A fit of the data to the Debye model gave a Debye temperature of 419+or-3 K.

Final-state effects in neutron Compton scattering measurements on zirconium deuteride and beryllium.

We report inelastic neutron scattering measurements of the neutron Compton profile, J(y), for Be and for D in polycrystalline [Formula: see text] over a range of momentum transfers, q between 27 and [Formula: see text]. The measurements were performed using the inverse geometry spectrometer eVS which is situated at the UK pulsed spallation neutron source ISIS. We have investigated deviations from impulse approximation (IA) scattering which are generically referred to as final-state effects (FSEs) using a method described by Sears. This method allows both the magnitude and the q dependence of the FSE to be studied. Analysis of the measured data was compared with analysis of numerical simulations based on the harmonic approximation and good agreement was found for both [Formula: see text] and Be. Finally we have shown how [Formula: see text], where V is the interatomic potential, can be extracted from the antisymmetric component of J(y).

Deep Inelastic Neutron Scattering in the Study of Atomic Momentum Distributions

Abstract The electron-volt spectrometer (EVS) at the pulsed neutron source facility (ISIS) is being developed for the study of atomic momentum distributions. Neutrons with energies in the range 1 to 100 eV are incident on the sample, and the time-of-flight (TOF) spectrum of the scattered beam is measured by an array of fixed detectors. A resonant foil difference technique is used to yield a set of TOF spectra for those neutrons scattered into a fixed energy and through fixed angles. Information on the momentum distribution of the target nuclei can be deduced within an impulse approximation in a procedure analogous to that in Compton scattering of electrons by photons. Crystalline compounds containing aligned hydrogen bonds and other hydrogenous compounds are of particular interest owing to the high cross-section of the proton at these neutron energies. With improved statistical accuracy of the data it is anticipated that deviations of the protons potential from a harmonic potential may be determined. Non-hydrogenous systems have also been investigated. A description is given of the basic theory and interpretive method. Data obtained on numerous systems are presented and discussed.

Temperature dependence of the magnetic Compton profile of ferrimagnetic HoFe2

Circularly polarised synchrotron radiation has been used to measure the spin-dependent magnetic Compton profile of the ferromagnet HoFe 2 at six temperatures from 10 K to 300 K. The magnetic Compton profiles have been analysed using appropriate free atom Compton profiles and a free electron Compton profile which has allowed the determination of the individual spin moments on the Ho and Fe sites and the diffuse moment; the last two are, to a first approximation, temperature independent. The Ho moment, which is antiferromagnetically coupled to them, reduces by a factor of two across the temperature range, leading to a spin compensation temperature of 210±10 K. The experiments were performed with 45.9 keV photons at the KEK ARNE-1 station of the Accumulation Ring.

An Investigation of the Cross-Section for Magnetic Compton Scattering

The existence of a term in the cross section for inelastic photon scattering of circularly polarised photons which is dependent on orbital magnetisation has been proposed and supported by evidence from one Compton scattering experiment carried out at 45 keV. The integrated intensity of the Compton scattering is studied as a function of the angle between the incident beam and the direction of magnetisation. Further measurements of a similar nature on the same samples, Fe, Co, HoFe 2 , which have produced negative results, are reported here. These latest results, which were obtained at two different synchrotron rings (the Daresbury Storage Ring and the KEK Accumulation Ring), throw doubt upon the importance of such effects in the Compton limit.

Optimal number of directions in reconstructing 3D momentum densities from Compton profiles of semiconductors

Abstract In the framework of the pseudopotential theory, the three-dimensional momentum densities, ϱ(q) of Si and Ge are calculated using the cubic harmonics expansion method. In these expansions, we have included the sixteen relevant cubic harmonic functions (up to l = 22). The converging properties ϱ(q), Comptons profiles Jz(qz), and B(r) functions are discussed in connection with the reconstruction procedure for the three dimensional momentum densities from the experimental Jzs of semiconductors. It is shown that (1) the qualitative features of the three-dimensional momentum densities are well reproduced by the first three terms in the cubic harmonics expansion, and (2) the first ten terms are enough to reproduce ϱ(q) and its anisotropic part to detailed features.

Spin-polarized electron momentum density distributions REMn2Ge2 (RE=Gd,La)

The spin-polarized electron momentum distributions (EMD) for ferrimagnetic GdMn2Ge2 and ferromagnetic LaMn2Ge2 have been measured using the magnetic Compton scattering technique. The spin-polarized EMDs were resolved along the c-axis at 15 K: In the Gd sample, the Gd and Mn spin sublattices are aligned antiparallel with spin moments of 6:870:1mB per formula unit (FU � 1 ) and 3:770:1mB FU � 1 ; respectively, with the Gd spin sublattice oriented parallel to the scattering vector. The ferromagnetic La system exhibits no 4f moment and the Mn moment is aligned parallel to the scattering vector, the ordered spin moment on the Mn being 2:470:1mB FU � 1 : The two samples measured are the extreme points of the phase diagram for Gd1� xLaxMn2Ge2: Our data give a direct indication of the sublattice reorientation at xE0:5; implied from previous magnetization measurements and indicate that at a low temperature the Mn sublattice is a canted ferromagnetic system. r 2002 Elsevier Science B.V. All rights reserved.

The spin-dependent Compton profile of gadolinium

Monochromated circularly polarised synchrotron radiation with an energy of 46.3 keV has been used to measure the spin-dependent Compton profile of polycrystalline gadolinium. The lineshape is in good agreement with the superposition of the profile of an atomic 4f-electron distribution (7 mu B), and a small (0.55 mu B) free-electron parabola.