1. Technische Physik and Wilhelm Conrad Röntgen Research Center for Complex Material Systems, Physikalisches Institut, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
2. Departamento de Fisica, Universidade Federal de Sao Carlos, 13565-905 Sao Carlos, SP, Brazil
3. SUPA, School of Physics and Astronomy, University of St. Andrews, St. Andrews, KY16 9SS, United Kingdom