SCHWINGER SCATTERING OF NEUTRONS IN THE STUDY OF CONDENSED MATTER

In a noncentrosymmetric crystal, the angular velocity of an incident neutron relative to the electric charges of the atoms gives rise to a small polarization-dependent term in the scattering cross section (Schwinger scattering). The Schwinger scattering is proportional to the difference between the nuclear charge and the X-ray scattering factor and hence, at low angles, to just those electrons which take part in bonding. The lack of a centre of symmetry in the zinc blende structure, adopted by the important III-V semiconductors, has been exploited to obtain information about bonding electron density in a number of these compounds: GaAs, GaSb, GaP, InAs, InSb and InP. The results obtained for the polarization dependence of the neutron cross section for the 111, 311 and 331 reflections are generally lower than those calculated using neutral free atom form factors. The result for the 222 reflection, which is zero for a spherical atom model, is highly significant for GaAs and of the same order in all the compounds measured; it can be attributed to tetrahedral distortion of the atomic charge distributions brought about by bonding. These results are compared to previous estimates obtained by X-ray, gamma-ray and electron scattering.