1. Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, United Kingdom
2. Unité Mixte de Physique CNRS/Thales, 91767 Palaiseau, France, and University of Paris-Sud, 91405 Orsay, France
3. Helmholtz-Zentrum Berlin fur Materialien und Energie, 12489 Berlin, Germany
4. Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany