Institution: | a NEST-INFM and Scuola Normale Superiore, Piazza dei Cavalieri 7, Pisa I-56126, Italy b Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA c Bell laboratories, Lucent Technologies, Murray Hill, NJ 07974, USA d Department of Physics, Columbia University, New York, NY 10027, USA e Department of Physics, Exeter University, Exeter, UK |