[1] CEMES-CNRS, 29 rue J. Marvig, 31055 Toulouse, France
[2] LNMO, INSA, D̀�partement de Physique, 135 Avenue de Rangueil, 31077 Toulouse, France
[3] Institute of Microelectronics, NCSR Demokritos, 15310 Aghia Praskevi, Greece
[4] Ioffe Physico-Technical Institute, St. Petersburg 194021, Russia