1. Facultat de Física, Departament d’Estructura i Constituents de la Matèria, Universitat de Barcelona, Diagonal 647, E-08028 Barcelona, Catalonia, Spain
2. Departament de Física i Enginyeria Nuclear, ETSEIB, Universitat Politècnica de Catalunya, Diagonal 647 and Center for Research in NanoEngineering, Pascual i Vila, 15. Barcelona, Catalonia, Spain
3. Ames Laboratory, U.S. Department of Energy and Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011-3020, USA