Thank for the answer. M.Tassi -- Dr. Maria Tassi Post-Doctoral Researcher National and Kapodistrian University of Athens, Faculty of Physics, Department of Solid State Physics, Panepistimiopolis, Zografos, GR-15784 Athens, Greece E-mail: mtassi at phys.uoa.gr Tel.: +30 210 7276758 Mobile: +30 6932131934 Fax : +30 210 7276711