[1] Univ Colorado, Dept Phys, Boulder, CO 80309 USA
[2] Univ Colorado, JILA, Boulder, CO 80309 USA
[3] Univ Nebraska, Dept Elect & Comp Engn, Lincoln, NE 68588 USA
[4] Lund Univ, NanoLund, POB 118, S-22100 Lund, Sweden
[5] Univ Calif Los Angeles, Coll Letters & Sci, Phys Sci, Los Angeles, CA 90095 USA
[6] Max Planck Gesell, Fritz Haber Insitute, Faradayweg 4-6, D-14195 Berlin, Germany