1. Laboratory for Quantum Magnetism, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland.
2. Laboratory for Neutron Scattering, Paul Scherrer Institute, 5232 Villigen, Switzerland.
3. Helmholtz-Zentrum Berlin, 14109 Berlin Wannsee, Germany.
4. James Franck Institute and Department of Physics, University of Chicago, Chicago, IL 60637, USA.
5. London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, UK.
6. Oak Ridge National Laboratory, Spallation Neutron Source, Oak Ridge, TN 37831, USA.
7. Department of Chemistry and Biochemistry, University of Bern, 3000 Bern 9, Switzerland.