1. Centre for the Physics of Materials and Department of Physics, McGill University, Montréal, Québec, Canada H3A 2T8
2. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China
3. Department of Chemistry and Institute of Optical Sciences, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6