[1] Natl. High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, FL 32306, United States
[2] Department of Physics, Stt. Univ. of New York at Buffalo, Buffalo, NY 14260, United States
[3] Department Natuurkunde, Universiteit Antwerpen (UIA), B-2610 Antwerpen, Belgium
[4] Departamento de Física, Univ. Federal de São Carlo, 13565-905 São Carlos, SP, Brazil
[5] Natl. Laboratory for Superlattices, Microstructures Inst. Semiconductors, Chinese Academy of Science, Beijing, China
[6] Department of Electrical Engineering, Cornell University, Ithaca, NY 14853, United States