1. Division
of Solid-State Electronics, Department of Engineering Sciences, Uppsala University, SE-751 21 Uppsala, Sweden
2. Department
of Physics, University of Jyväskylä, P.O. Box 35, FI-40014, Jyvaskylä, Finland
3. IBM Thomas J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, United States