1. Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742-3511, USA
2. Wuhan National High Magnetic Field Center, School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Luoyu Road 1037, Hongshan District, Wuhan 430074, China
3. Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, Riga LV-1063, Latvia