[1] Univ Leipzig, Inst Expt Phys 2, D-04103 Leipzig, Germany
[2] Linkoping Univ, Dept Phys & Measurement Technol, SW-58183 Linkoping, Sweden
[3] Cornell Univ, Dept Elect & Comp Engn, Ithaca, NY 14853 USA
[4] Univ Nebraska Licoln, Dept Elect Engn & Ctr Mat Res & Anal CMRA, Lincoln, NE USA