[1] Univ Zurich, Phys Inst, Zurich, Switzerland
[2] Int Sch Adv Studies SISSA ISAS, Dept Phys, Trieste, Italy
[3] Univ Florida, Dept Phys, Gainesville, FL 32611 USA
[4] Ruhr Univ Bochum, Fak Phys & Astron, Lehrstuhl Expt Phys 4, Bochum, Germany
[5] Karlsruhe Inst Technol, Inst Quantum Mat & Technol, Karlsruhe, Germany