1. Center of Excellence for Advanced Materials and Sensing Devices, Institute of Physics, Bijenička cesta 46, HR-10000 Zagreb, Croatia
2. II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
3. Department Physik, Universität Siegen, Walter-Flex-Str. 3, 57068 Siegen, Germany