1. Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., LV‐1063 Riga, Latvia
2. Department of Physics, Wake Forest University, Winston‐Salem, NC 27109, USA
3. Nanotechnology Center, Wake Forest University, Winston‐Salem, NC 27109, USA
4. Institute of Solid State and Semiconductor Physics, National Academy of Belarus, Minsk, Belarus