[1] Univ Edinburgh, Sch Phys & Astron, James Clerk Maxwell Bldg, Edinburgh EH9 3FD, Scotland
[2] Univ Edinburgh, Inst Mat & Proc, Sch Engn, Sanderson Bldg, Edinburgh EH9 3FB, Scotland
[3] Heriot Watt Univ, Inst Biol Chem Biophys & Bioengn, Edinburgh EH14 4AS, Scotland
[4] Toronto Metropolitan Univ, Dept Phys, Toronto, ON M5B 2K3, Canada