[1] Queens Univ Belfast, Dept Phys, APS Div, Belfast BT7 1NN, Antrim, North Ireland
[2] Lawrence Livermore Natl Lab, Livermore, CA 94551 USA
[3] Sandia Natl Labs, Albuquerque, NM 37185 USA
[4] Univ Oxford, Dept Phys, Clarendon Lab, Oxford OX1 3PU, England