[1] Columbia Univ, Dept Phys, New York, NY 10027 USA
[2] Columbia Univ, Dept Elect Engn, New York, NY 10027 USA
[3] Columbia Univ, Dept Chem, New York, NY 10027 USA
[4] Stanford Univ, Dept Appl Phys, Stanford, CA 94305 USA
[5] SLAC Natl Accelerator Lab, Menlo Pk, CA 94025 USA