[1] Univ Cambridge, Dept Phys, Cavendish Lab, 19 JJ Thomson Ave, Cambridge, England
[2] Imperial Coll London, Dept Mat, London SW7 2AZ, England
[3] Imperial Coll London, Dept Phys, London SW7 2AZ, England
[4] Imperial Coll London, Thomas Young Ctr Theory & Simulat Mat, London SW7 2AZ, England
[5] Ecole Polytech Fed Lausanne, Theory & Simulat Mat THEOS, Lausanne, Switzerland
[6] Ecole Polytech Fed Lausanne, Natl Ctr Computat Design & Discovery Novel Mat MA, Lausanne, Switzerland
[7] Univ Oxford, Dept Mat, Parks Rd, Oxford OX1 3PH, England