MICROSCOPY THEORY OF VELOCITY OF SOUND IN L2-XMXCUO4 (L = ND, LA, PR,GD M = SR, CE)

In this paper the microscopic theory of the relative change in velocit y of sound with temperature of the cuprate systems in normal state is reported. The phonon Green function is calculated using the Green func tion technique of Zubarev in the limit of zero wave vector and small t emperature. The Fulde electronic Hamiltonian in the presence of the ph onon interaction with hybridisation between the conduction electrons a nd the f-electrons is used. The relative change in velocity of sound a t various temperatures is studied for different model parameters namel y the position of the f-level, the effective phonon coupling strength, staggered magnetic field and hybridisation strength. The phonon anoma lies observed experimentally at different temperatures are explained t heoretically. An abrupt change in velocity at Neel temperature (T-N) i s observed clearly. It is observed that different parameters influence the velocity of sound and Neel temperature substantially. (C) 1997 El sevier Science Ltd.