B. Swinnen, J. Meersschaut, J. Dekoster, G. Langouche, S. Cottenier, S. Demuynck, M. Rots Physical Review Letters 78 (2), 362-3651997A1Published while none of the authors were employed at the CMMDOI http://dx.doi.org/10.1103/PhysRevLett.78.362