1. Institute of Solid State Physics, Friedrich Schiller University Jena, Helmholtzweg 5, 07743 Jena, Germany
2. Department of Materials Science, University of Milano-Bicocca, Via R. Cozzi 55, 20125 Milano, Italy
3. STMicroelectronics, Via Tolomeo 1, 20010 Cornaredo, Italy
4. ETSF and Dipartimento di Fisica “Aldo Pontremoli”, Università degli Studi di Milano, Via Celoria, 16, 20133 Milano, Italy