1. Department of Physics, University of Texas at Austin, Austin,
Texas 78712, United States
2. Department of Electrical
Engineering, Kwangwoon University, Seoul, South Korea 139-701
3. Institute for Nanoscale Science
and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States
4. State Key Laboratory of Electronic
Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, People’s
Republic of China 610054