On the theory of coherent pair production in crystals in presence of   acoustic waves
  The influence of hypersonic waves excited in a single crystal is investigated on the process of electron-positron pair creation by high-energy photons. The coherent part of the corresponding differential cross-section is derived as a function of the amplitude and wave number of the hypersound. The values of the parameters are specified for which the latter affects remarkably on the pair creation cross-section. It is shown that under certain conditions the presence of hypersonic waves can result in enhancement of the process cross-section. 
A.R. Mkrtchyan, A.A. Saharian, L.Sh. Grigoryan, B.V. Khachatryan