[1] Inst Tecnol & Nucl, P-2686953 Sacavem, Portugal
[2] Linkoping Univ, IFM, SE-58183 Linkoping, Sweden
[3] Univ Nebraska, Dept Elect Engn, Lincoln, NE 68588 USA
[4] Univ Nebraska, Nebraska Ctr Mat & Nanosci, Lincoln, NE 68588 USA
[5] Cornell Univ, Dept Elect & Comp Engn, Ithaca, NY 14853 USA