1. Quantum Device Physics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-41296 Göteborg, Sweden.
2. Dipartimento di Fisica, Politecnico di Milano, I-20133 Milano, Italy.
3. Institut für Physik, BTU Cottbus-Senftenberg, D-03013 Cottbus, Germany.
4. ESRF, European Synchrotron, F-38043 Grenoble, France.
5. Dipartimento di Fisica, Università di Roma “La Sapienza,” I-00185 Roma, Italy.
6. CNR-ISC, I-00185 Roma, Italy.
7. Division of Subatomic, High-Energy and Plasma Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden.
8. CNR-SPIN, Dipartimento di Fisica, Politecnico di Milano, I-20133 Milano, Italy.