1. a Wheatstone Physics Laboratory , King's College London , Strand , London , WC2R 2LS , England
2. b National Institute for Research in Inorganic Materials , 1-1 Namiki, Tsukuba , Ibaraki , 305 , Japan
3. c Diamond Research Laboratory , De Beers Industrial Diamond Division (Pty) Ltd , PO Box 916, Johannesburg , 2000 , South Africa