1. Department of Physics, University of Illinois at Urbana‐Champaign, Urbana, Illinois 61801
2. Department of Material Science and Engineering, University of Pennsylvania, Philadelphia, Pennslyvania 19104
3. Department of Physics, Royal Institute of Technology, Stockholm, Sweden
4. Bell Laboratories, Murray Hill, New Jersey 07974
5. Kernforschungzentrum, Karlsruhe, Federal Republic of Germany