[1] CNR, Ist Nanosci, I-56126 Pisa, Italy
[2] Scuola Normale Super Pisa, I-56126 Pisa, Italy
[3] Radboud Univ Nijmegen, Inst Mol & Mat, NL-6525 AJ Nijmegen, Netherlands
[4] Chinese Acad Sci, Kavli Inst Theoret Phys China, Beijing 100190, Peoples R China
[5] Univ Missouri, Dept Phys & Astron, Columbia, MO 65211 USA
[6] Columbia Univ, Dept Appl Phys & Appl Math, New York, NY USA
[7] Columbia Univ, Dept Phys, New York, NY USA
[8] Princeton Univ, Dept Elect Engn, Princeton, NJ 08544 USA