Measurement of anisotropies of the kinetic energy and anharmonicity in pyrolytic graphite by neutron Compton scattering

A.L. Fielding; D.N. Timms; J. Mayers

doi:doi:10.1209/epl/i1998-00465-1

DOI: 10.1209/epl/i1998-00465-1

Europhys. Lett, 44 (2), pp. 255-260 (1998)

Measurement of anisotropies of the kinetic energy and
anharmonicity in pyrolytic graphite by neutron
Compton scattering

A. L. Fielding ¹, D. N. Timms ¹ and J. Mayers ²

¹ Division of Physics, Portsmouth University, St. Michael's Building
Portsmouth PO1 2DT, UK
² ISIS Facility, Rutherford Appleton Laboratory
Chilton, Didcot, OX11 0QX, UK

(received 2 February 1998; accepted in final form 31 August 1998)

PACS. 81.05 ${\rm -t}$ - Specific materials: fabrication, treatment, testing and analysis.
PACS. 32.80Cy - Atomic scattering, cross sections, and form factors; Compton scattering.
PACS. 78.70Nx - Neutron inelastic scattering.

Abstract:

Neutron Compton scattering (NCS) measurements of the anisotropy of the momentum distribution and the mean Laplacian of the interatomic potential $\langle\nabla^2 V\rangle$ have been performed using electron volt neutrons, with wave vector transfers between $\rm 24\;\AA^{-1}$ and $\rm 98\;\AA^{-1}$ . The measured momentum distribution of the atoms displays significantly more anisotropy than a calculation using a model density of states. We have observed anisotropies in $\langle\nabla^2 V\rangle$ for the first time. The results suggest that the atomic potential is harmonic within the graphite planes, but anharmonic for vibrations perpendicular to the planes.

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