[1] Univ Illinois, Dept Elect & Comp Engn, Urbana, IL 61801 USA
[2] Univ Texas, Dept Elect & Comp Engn, Austin, TX 78712 USA
[3] Tech Univ Berlin, Inst Festkorperphys, D-10623 Berlin, Germany
[4] Univ Tokyo, Adv Sci & Technol Res Ctr, Tokyo 106, Japan