1. Department of Condensed Matter Physics, University of Geneva, CH-1211 Geneva 4, Switzerland;
2. Institut Català de Recerca i Estudis Avançats (ICREA), Barcelona 08010, Catalunya, Spain
3. Institut Català de Nanociència i Nanotecnologia (ICN2), CSIC-ICN, Campus Universitat Autonoma de Barcelona, Bellaterra 08193, Spain;
4. Ceramics Laboratory, Swiss Federal Institute of Technology (EPFL), Lausanne 1015, Switzerland;