1. Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.
2. Helmholtz-Zentrum Berlin für Materialien und Energie, D-12489 Berlin, Germany
3. Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, U.K.
4. Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, 01314 Dresden, Germany
5. Fritz Haber Institute, D-14195 Berlin-Dahlem, Germany