1. Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA
2. Instituto de Quimica, Universidade Estadual de Campinas, Campinas, Sao Paulo 13083-859, Brazil
3. Instituto de Fisica, Universidade Estadual de Campinas, Campinas, Sao Paulo 13083-859, Brazil
4. Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, USA