Surface Demixing in a AuSn Liquid Alloy*

We present results of X-ray reflectivity studies of the eutectic AuSn alloy liquid-vapor interface. The analysis shows that in common with the BiSn eutectic, there is surface demixing that extends to more than one monolayer. This is in contrast to a common presumption that the Gibbs adsorption predicts complete demixing only in the surface monolayer. The composition profiles can be explained by surface segregation theory for attractive interaction between Sn and Au atoms, similar to BiIn [1] and BiSn [2]. \newline [1] E. DiMasi, H. Tostmann, O. G. Shpyrko, P. Huber, B. M. Ocko, P. S. Pershan, M. Deutsch, and L. E. Berman, Phys. Rev. Lett. \textbf{86}, 1538 (2001) \newline [2] O. G. Shpyrko, A. Y. Grigoriev, R. Streitel, D. Pontoni, P. S. Pershan, M. Deutsch, and B. M. Ocko, Phys. Rev. Lett. \textbf{95}, 106103 (2005) \newline *Present address: Center for Nanoscale Materials, ANL