1. Aalto University (previously Helsinki University of Technology); Department of Applied Physics; P.O. Box 15100 Espoo FIN-00076 Aalto, Finland
2. UMR5819-SPrAM (CEA/CNRS/University Joseph FOURIER-GRENOBLE I); CEA-Grenoble, Institut Nanosciences & Cryogénie (INAC); 17 Rue des Martyrs F-38 054 Grenoble Cedex 9 France