1. CNR-IMEM Institute, Parco Area delle Scienze 37/A, I-43100 Parma, Italy
2. L-Ness and Dipartimento di Scienza dei Materiali, Via Cozzi 53, I-20125, Milano, Italy
3. Laboratory for Solid State Physics, ETH Zürich, Schafmattstr. 16, CH-8093 Zürich, Switzerland
4. CNR-IFN, L -NESS, via Anzani 42, I-22100, Como, Italy
5. Centre Suisse d'Electronique et Microtechnique, Jaquet-Droz 1, CH-2002 Neuchatel, Switzerland