1. Microelectronics Research Center and the Electrical and Computer Engineering Department, The University of Texas at Austin 1 , Austin, Texas 78758, USA
2. Physics Department, The University of Texas at Austin 2 , Austin, Texas 78758, USA
3. Physics Department, Texas State University 3 , San Marcos, Texas 78666, USA
4. Department of Electrical and Computer Engineering, Johns Hopkins University 4 , Baltimore, Maryland 21218, USA