1. Physikalisches Institut, Experimentelle Physik III, Universitat Wurzburg 1 , Am Hubland, Wurzburg 97074, Germany
2. Institute for Topological Insulators, Am Hubland 2 , Wurzburg 97074, Germany
3. B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine 3 , Kharkiv 61103, Ukraine