1. Department of Physics and Astronomy & Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, USA
2. Key Laboratory of Materials Physics, Institute of Solid-State Physics, HFIPS, Chinese Academy of Sciences, Hefei 230031, China
3. University of Science and Technology of China, Hefei 230026, China
4. Department of Physics, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, USA