1. Laboratory of Semiconductor Materials, Institute of Materials, Faculty of Engineering, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
2. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, BIST, Campus UAB, Bellaterra, Barcelona, Catalonia, Spain
3. Texas Center for Superconductivity and Department of Physics, University of Houston, Houston, Texas 77204-5002, USA
4. ICREA, Pg. Lluís Companys 23, Barcelona, Catalonia, Spain
5. Institute of Physics, Faculty of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland