1. Department of Physics, University of California at San Diego, La Jolla, California 92093-0319, School of Physics and Astronomy, University of Southampton, SO17 1BJ, Southampton, United Kingdom, Dipartimento di Fisica, Universita’ “Tor Vergata” via della Ricerca Scientifica, 00133 Rome, Italy, and Materials Department, University of California at Santa Barbara, Santa Barbara, California 93106-5050