[1] Univ Calif Berkeley, Dept Elect Engn & Comp Sci, Berkeley, CA 94720 USA
[2] Tech Univ Munich, Walter Schottky Inst, D-85748 Garching, Germany
[3] Tech Univ Berlin, Inst Solid State Phys, D-10623 Berlin, Germany
[4] Univ Wurzburg, D-97074 Wurzburg, Germany
[5] VERTILAS GmbH, D-85748 Garching, Germany