1. Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455, USA.
2. Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA.
3. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
4. Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA.