Bound State of Phonons and CEF-Excitations in CeAl₂

Abstract

The coupling of phonons to the crystalline electric field (CEF)-states of Rare Earth (RE) compounds with integer RE-valence leads to various phonon anomalies for temperatures smaller than the CEF-splitting (T ≲ 100 K). In the long wavelength limit, the T-dependece of sound velocities, i.e. elastic constants is due to this “magnetoelastic coupling” which has been shown by numerous ultrasonic experiments [1]. A similar mechanism has been found to explain temperature, magnetic field and RE-concentration dependence of optical phonons in RE-halides and their corresponding La-diluted alloys as observed in Raman scattering experiments [2]. More recently inelastic neutron scattering experiments [3,4] revealed that intermetallic CeAl₂ is an exceptionally striking example for the effects produced by a strong coupling of phonons to CEF-excitations.