Neutron spectroscopy of solids—part II

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Abstract
A survey is given of some current applications of neutron diffraction to the elucidation of a wide range of the properties of both magnetic and non-magnetic materials. Naturally, one must use quantum mechanics in obtaining the details necessary for the interpretation of experimental results, though some useful semiclassical visualization is possible. The analysis of the dynamics of atomic systems into elementary, nearly independent excitations started with Debye's analysis of vibrational modes in 1912, and this gave rise to the quantized unit of vibrational energy, the phonon. Likewise, magnetic spin-wave theory started with Bloch in 1932 and in its quantized form gives us the ‘magnon’ concept. In complex structures, in liquids, and near the critical point theoretical concepts are less clear and we may use the neutron experiments to help us decide on the most appropriate approximation to use in describing them. We now seem to have a tool of remarkable power to help in solving the complex problems of energy levels and the dynamics of crystals. This second part of the paper continues the survey of current applications of neutron scattering in elucidating the microscopic properties of both magnetic and non-magnetic materials. The techniques described in Part I measure the wave-vector and frequency (energy) of disturbances in the sample. Recent results on elastic waves (phonons) were also described in Part I. Here we describe magnetic spin-wave investigations, the properties of phase-change critical points, and the elastic scattering from non-magnetic and magnetic defects. Quantum mechanics provides the natural framework for the discussions, and with its help we may get unambiguous results on some of the main problems of energy levels and dynamics of crystals.