1. Exxon Research and Engineering Co., Route 22 East, Annandale, New Jersey 08801; Physics Department, Bar Ilan University, Ramat Gan 52900, Israel; Physics Department, Northern Illinois University, DeKalb, Illinois 60115; Materials Sciences Division, Argonne National Laboratory, Argonne, Illinois 60439; and Physics Department, Brookhaven National Laboratory, Upton, New York 11973