[1] Univ Penn, Dept Phys & Astron, Philadelphia, PA 19104 USA
[2] Univ Penn, Dept Elect & Syst Engn, Philadelphia, PA 19104 USA
[3] Rensselaer Polytech Inst, Dept Phys Appl Phys & Astron, Troy, NY 12180 USA
[4] Oak Ridge Natl Lab, Ctr Nanophase Mat Sci, Oak Ridge, TN 37831 USA
[5] MIT, Dept Elect Engn & Comp Sci, Cambridge, MA 02139 USA