1. Cavendish Laboratory, Cambridge University, JJ Thomson Avenue, Cambridge CB3 OHE, UK.
2. National High Magnetic Field Laboratory, Tallahassee, FL 32310, USA.
3. Department of Physics, University of Warwick, Coventry CV4 7AL, UK.
4. National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, NM 87504, USA.
5. Center for Computational Materials Science, Naval Research Laboratory, Washington, DC 20375, USA.