1. Physik-Department, Technische Universität München, D-85748 Garching, Germany
2. Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, D-85748 Garching, Germany
3. Max Planck Institute For Solid State Research, 70569 Stuttgart, Germany
4. Institute for Quantum Matter and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
5. Max Planck Society Outstation at the FRM II, 85748 Garching, Germany