Physics Dept. and INFM-SUPERMAT Lab., University of Salerno, via S. Allende, 84081 Baronissi (SA), Italy;
Groupe de Physique des Solides, Univ. Paris 6-7, CNRS (UMR 7588), 75015 Paris, France;
Solid State Physics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland;