Affiliation: | a Centre for Theoretical Physics and Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands b GISC, Departamento de Física de Materiales, Universidad Complutense, E-28040 Madrid, Spain c Ioffe Physico-Technical Institute, 26 Politechnicheskaya str., 194021 St. Petersburg, Russia |