International
Tables for Crystallography Volume D Physical properties of crystals Edited by A. Authier © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. D, ch. 2.3, pp. 327-328
Section 2.3.7. Conclusions^{a}Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ-18221 Prague 8, Czech Republic |
In this overview of Raman scattering in crystals, we have almost exclusively based our considerations on a phenomenological, semi-classical viewpoint without going into details of the underlying microscopic theory. This is surely an appropriate approach to a discussion of the fundamental consequences of crystal symmetry on the selection rules governing the varied phenomena of inelastic light scattering and on the symmetry-restricted form of the corresponding tensorial quantities encountered in this vast and fruitful field. We have attempted to treat the most important symmetry aspects of the inelastic scattering of light by collective excitations in perfect crystals, concentrating on scattering by optical phonons – in the traditional sense of Raman scattering studies. Within a limited scope, we tried to give some insight into the nature of the phenomena relevant in connection with this topic. Our coverage is certainly not exhaustive (nor original); we have also deliberately omitted all scattering phenomena connected with purely electronic excitations, although the corresponding symmetry aspects can be analysed on the same footing. The essence of the truth is rather simple: As long as the excitations are characterized by irreducible representations of the symmetry group of the system, the well proven tools of the theory of representations are at hand to work out the consequences (if common sense does not readily provide the answer).
For an experimental physicist, symmetry analysis is invaluable in designing the experimental arrangement expected to give evidence of some particular phenomenon. The type of answer one may expect from such analysis is characteristic of the group-theoretical predictions. One can learn whether a particular effect is allowed or forbidden, and – if it is allowed – one can learn how many independent coefficients may be needed to describe it. Symmetry analysis alone cannot predict how large the effect is, or why it occurs, though it may well indicate the way to make it more easily observable. In order to understand or justify its actual magnitude, one has to analyse in greater detail the underlying microphysical mechanisms; however, the analysis must be consistent with restrictions dictated by the symmetry of the problem and, here again, symmetry arguments with powerful group-theoretical tools provide reliable guidance.
For the sake of brevity, citations have been largely suppressed in the present overview. Instead, the relevant sources are included in the list of references.
For detailed information about all the varied aspects of light scattering in solids and recent advances in this vast and fruitful field, the reader is referred to specialized monographs, e.g. Turrell (1972), Hayes & Loudon (1978), and to a comprehensive series edited by Cardona & Güntherodt (1975–).
References
Cardona, M. & Güntherodt, G. (1975–). Editors. Light scattering in solids, Vols. I–IV. Berlin, Heidelberg, New York, Tokyo: Springer.Hayes, W. & Loudon, R. (1978). Scattering of light by crystals. New York: John Wiley & Sons.
Turrell, G. (1972). Infrared and Raman spectra of crystals. London, New York: Academic Press.