1. Institute for Solid State Physics, University of Tokyo, Roppongi 7–22–1, Minato-ku, 106 Tokyo, Japan
2. Hahn-Meitner-Institut Berlin, Postfach 390128, D-1000 Berlin 39, Germany
3. Los Alamos National Laboratory, LANSCE MS H805, Los Alamos, New Mexico 87545, U.S.A.