1. T. L. Morkved, M. Lu, A. M. Urbas, E. E. Ehrichs, H. M. Jaeger, James Franck Institute and Department of Physics, University of Chicago, Chicago, IL 60637, USA.
2. P. Mansky and T. P. Russell, IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120, USA.