Theory of the bound state between phonons and a CEF excitation in CeAl2

The coupling of lattice vibrations to the crystalline electric field (CEF) states of 4f electrons plays an important role in rare-earth compounds. It is shown that under favourable conditions this magnetoelastic interaction can lead to a 'bound state' between CEF excitations and lattice vibrations. The existence of this bound state in the intermetallic Laves phase CeAl2explains the results of inelastic neutron scattering investigations which have not been understood before. In this compound, a Gamma7- Gamma8CEF excitation and low-lying vibrational modes of the Ce sublattice are involved in bound-state formation. The properties of this new type of excitation and its consequences for thermodynamic quantities are investigated. In addition the phonon softening at low temperatures found in CeAl2is discussed.