1. nanoplus Nanosystems and Technologies GmbH, Oberer Kirschberg 4, Gerbrunn D-97218, Germany
2. Technische Physik, Physikalisches Institut and Wilhelm Conrad Röntgen-Research Center for Complex Material Systems, Universität Würzburg, Am Hubland, Würzburg D-97074, Germany
3. School of Physics and Astronomy, North Haugh, St. Andrews KY16 9SS, United Kingdom