1. Materials Science, Engineering, and Commercialization Program, Texas State University 1 , San Marcos, Texas 78666, USA
2. Department of Physics, Texas State University 2 , San Marcos, Texas 78666, USA
3. Department of Electrical and Computer Engineering, University of Texas at Austin 3 , Austin, Texas 78712, USA
4. Now at Sandia National Laboratory 4 , Sandia, New Mexico 87185, USA