1. Alabama Agricultural and Mechanical University, Department of Physics, 4900 Meridian St., P.O. Box 1268, Normal, Alabama 35762-1268, USA
2. A. F. Ioffe Physico-Technical Institute RAN, 26 Polytekhnicheskaya, St. Petersburg 194291, Russia
3. Department of Physics, University of Puerto-Rico at Mayaguez, Puerto-Rico 00680, USA
4. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433-7702, USA