1. Center for Advanced Microstructures and Devices, Louisiana State University, 6980 Jefferson Highway, Baton Rouge, Louisiana 70806, Physikalisches Institut, University of Bonn, Nussallee 12, D-53115 Bonn, Germany, Department of Physics, Southern University and A&M College, Baton Rouge, Louisiana 70813, and Lawrence Livermore National Laboratory, Livermore, California 94550