1. Department of Physics and Technology, University of Bergen, Allégaten 55, 5007 Bergen, Norway
2. Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
3. Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29634, USA
4. Donostia International Physics Center (DIPC), Paseo Manual de Lardizabal, 4, 20018 Donostia-San Sebastián, Spain