1. Department of Physics; West Virginia University; Morgantown West Virginia 26506-6315 USA
2. Evatec AG; Hauptstrasse 1a; Trübbach CH-9477 Switzerland
3. Wigner Research Centre for Physics; Institute for Solid State Physics and Optics; Hungarian Academy of Sciences; Konkoly-Thege Miklós str. 29-33 Budapest 1121 Hungary
4. Institute for Electrical Engineering; Ruhr-University Bochum; Bochum 44780 Germany