1. a Central Research Institute for Physics , H-1525, Budapest, P.O.B. 49, Hungary
2. b Institute of Solid State Physics of the Academy of Sciences of the USSR , 142432, Chernogolovka, USSR
3. c Joint Institute for Nuclear Research , Dubna, 101 000, Moscow, P.O.B. 79, USSR
4. d Léon Brillouin Laboratory , CEN Saclay, 91191, Gif sur Yvette, Cedex, France