1. Department of Physics and Engineering Physics, Tulane University, New Orleans, Louisiana 70118, United States
2. Center for Advanced Microstructures and Devices, Louisiana State University, 6980 Jefferson Highway, Baton Rouge, Louisiana 70806, United States
3. Department of Physics, Dyal Singh College, Lodhi Road, New Delhi-110003, India
4. Institute of Applied Physics, Vienna University of Technology, Vienna, Austria