1. University of Vienna, Faculty of Physics, Electronic Properties of Materials, Boltzmanngasse 5, A-l 090 Wien, Austria
2. Physikalisches Institut and Center for Quantum Science (CQ) in LISA+, Universität Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen, Germany
3. Johannes-Kepler-University Linz, Institute of Applied Physics, Altenbergerstrasse 69, A-4040 Linz, Austria
4. Al-Farabi Kazakh National University, Almaty, Kazakhstan