1. Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany
2. Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
3. Quantum
Matter Institute, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
4. Department
of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
5. Institut
für Theoretische Physik, Universität Bremen, 28359 Bremen, Germany
6. Institut
de Physique de la Matière Condensée, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland