S. Demuynck, J. Meersschaut, J. Dekoster, B. Swinnen, R. Moons, A. Vantomme, S. Cottenier, M. Rots Physical Review Letters 81 (12), 2562-25651998A1Published while none of the authors were employed at the CMMDOI http://dx.doi.org/10.1103/PhysRevLett.81.2562