1. Engineering Laboratory, University of Cambridge 1 , Trumpington Street, Cambridge CB2 1PZ, United Kingdom
2. Fritz-Haber-Institut der Max-Planck-Gesellschaft 2 , Faradayweg 4-6, D-14195 Berlin, Germany
3. Center for Materials Physics and Technology, U. S. Naval Research Laboratory Code 6393 3 , 4555 Overlook Ave. SW, Maryland, Washington, DC 20375, USA