1. Experimental Physics 2, University of Dortmund, 44227 Dortmund, Germany
2. Ioffe Physico‐Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia
3. Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia
4. Physikalisches Institut der Universität Würzburg, 97074 Würzburg, Germany
5. Abteilung Halbleiterphysik, Universität Ulm, 89081 Ulm, Germany