An Introduction to Crossing Effects in Phonon Scattering

Abstract

Transitions between two electronic states of a magnetic ion can be induced by a phonon because of the oscillatory electric fields it produces through the motion of the vibrating ligands. The transition probability and hence the phonon scattering rate are both resonant when the phonon frequency υ is equal to the transition frequency υo so that phonons can be used for spectroscopy in a similar way to photons.Resonant phonon scattering has now been seen from many different types of ion or impurity although for convenience the discussion here will be largely restricted to magnetic ions. I shall also concentrate on crossing effects seen in recent work where the experimental conditions conformed more closely to those assumed in the theoretical analysis presented here. A more complete description which attempts to give the earlier work its proper role in the development of the subject will be given elsewhere (Challis 1985) although I must mention here the pioneering work of Berman et al (1963) who observed the first frequency crossing signals and Walton (1966, 1967) and Hetzler and Walton (1973) who stressed the use of a known ion as a probe to investigate another.