Affiliation: | a Université du Maine, Faculté des Sciences, UPRESA 6087 CNRS, 72085 Le Mans Cedex 09, France b Laboratoire d’Optique des Solides, Université de Paris VI, Tour 13, 75006 Paris, France c Department of Physics, University of the Witswatersrand, P.O. Wits, Johannesburg, South Africa d Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Bld 235, Gaithersburg MD 20899-0001, USA |