[1] Univ Sao Paulo, Inst Fis, POB 66318, BR-05315970 Sao Paulo, Brazil
[2] Columbia Univ, Dept Elect Engn, New York, NY 10027 USA
[3] Cornell Univ, Sch Elect & Comp Engn, Ithaca, NY 14853 USA
[4] Columbia Univ, Dept Appl Phys & Appl Math, New York, NY 10027 USA