[1] CSIC, Natl Ctr Biotechnol CNB, Dept Macromol Struct, Madrid, Spain
[2] Univ Basel, Dept Phys, Basel, Switzerland
[3] Univ Autonoma Madrid, Dept Fis Teor Mat Condensada, Madrid, Spain
[4] Univ Autonoma Madrid, IFIMAC Condensed Matter Phys Ctr, Madrid, Spain