1. IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany, Departamento de Fisica, PUC-Rio, CP38071,
22452-970 Rio de Janeiro, Brazil, Institut für Materialphysik, Strudlhofgasse 4, A-1090 Wien, Austria, and
Institut Catala de Nanotecnologia and Centro Nacional de Microelectrónica, Campus Universitat Autónoma de
Barcelona, E-08193 Bellaterra, Spain