Submillimetre spectroscopy of acoustic phonons in crystals with modulated structures

Abstract

I t is now well established that in periodically layered super lattices one can observe Raman activation of acoustic modes with non-zero wave vectors k in the direction perpendicular to layers, where n is integer and b is the period of the super lattice. These modes become effectively long-wavelength and therefore possibly Raman and infrared (IR) active due to the folding of the Brillouin zone (BZ). This was observed in semiconductor heterostructures, crystalline lamellar polymers and can be expected in modulated crystal phases as well although we are not aware of any work devoted systematically to this problem.