Affiliation: | (1) Department of Theoretical Physics, Institute for Atomic Physics, PB MG-35, Magurele-Bucharest MG-6, Romania;(2) Laboratoire Léon Brillouin, CEA-CNRS, C.E.N./Saclay, F-91191 Gif/Yvette, France;(3) Laboratoire de Physique des Solides, Bat 510, F-91405 Orsay, France |