Neutron spectroscopy of solids—Part I

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Abstract
Neutrons are scattered weakly by all nuclei, and by any atomic magnetic moment associated with the valence electrons. Isotopic and nuclear spin disorder causes some uniformly diffuse scattering; the coherent average component of scattering leads to structural information. Energy changes of the neutron are easily measured and depend on the dynamics of nuclear or magnetic motion. The experimental techniques are simple in principle and are briefly outlined. Interatomic forces in insulators and metals are related to atomic polarizability and electrostatic screening. Magnetic interactions are analysed in terms of spin waves for Heisenberg ‘atomic-like’ models and for band theories of metals. The time-dependence of thermodynamic fluctuations is explored, with special reference to phase transformations. The spatial dependence of magnetization round impurity atoms in magnetic materials can be measured and interpreted in terms of electron-band theory. Structural defects and impurities in non-magnetic materials give smaller effects, which can also be studied. The measurement of the transfer of electron spin from a magnetic atom to a nominally non-magnetic chemical partner gives a new look to the theory of chemical covalency.