1. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4037
2. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4037
3. Department of Physics, UFMG, Belo Horizonte, MG 30123-970, Brazil
4. Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4037
5. Department of Physics, Tohoku University, Sendai, 980-8578, Japan