1. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
2. Universitat Politèdcnica de Catalunya-BarcelonaTech, 08034 Barcelona, Spain
3. Institute for Bioengineering of Catalonia (IBEC), The Barcelona Institute for Science and Technology (BIST), 08028 Barcelona, Spain
4. Centre Internacional de Mètodes Numèrics en Enginyeria (CIMNE), 08034 Barcelona, Spain