B. Swinnen, J. Meersschaut, J. Dekoster, G. Langouche, S. Cottenier, S. Demuynck, M. Rots Physical Review Letters 80 (7), 1569-15691998A1Published while none of the authors were employed at the CMMDOI http://dx.doi.org/10.1103/PhysRevLett.80.1569