1. Department of Electrical Engineering and Computer Science, Department of Physics, and Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, MG, 30123-970 Brazil, Departamento de Física, Universidade Federal do Ceará, Fortaleza, CE, 60455-760, Brazil, and Department of Electronic Engineering, University of Electro-Communications, Tokyo, 182-8585 Japan