1. a Institute of Physics , Na Slovance 2, 18221 Prague 8, Czech Republic
2. b Department of Ceramics and Glass Engineering, CICECO , University of Aveiro , 3810-193 , Aveiro , Portugal
3. c Physics Department , University of Arkansas , Fayetteville , Arkansas , 72701 , USA
4. d A.I. Ioffe Physical-Technical Institute , Politekhnicheskaya 26, 194021 , St. Petersburg , Russian Federation
5. e Leibniz Institute for Solid State and Materials Research Dresden , Helmholtzstrasse 20, 01069 , Dresden , Germany