Morphic effects. V. Time reversal symmetry and the mode properties of long wavelength optical phonons

Abstract

For pt. IV see abstr. A40688 of 1972. The authors investigate the symmetry properties of q approximately=0 optical phonons in magnetically ordered crystals. The use of corepresentation theory enables us to establish the form of the infrared absorption and Raman scattering tensors in the magnetically ordered state and determine the effects which arise from the removal of the time reversal symmetry. It turns out that the polarization selection rules for these phonons may be modified, normally IR and/or Raman inactive phonons may become IR and/or Raman active, and doubly degenerate modes may split, when the crystal undergoes the transition from the paramagnetic to the magnetic state and the time reversal symmetry is destroyed. Specific examples are treated in detail.