Long-range Ordered Surface Phase in Liquid AuSi$_{x}$Ge$_{1-x}$ Eutectic Alloys

Synchrotron Xray studies of the free surface of the liquid phase of the Au$_{82}$Si$_{18}$ eutectic show that it is coated with a 2D AuSi$_{2}$ crystalline monolayer[1]. In contrast the surfaces of comparable liquid phases of Au$_{73}$Ge$_{27}$ and Au$_{77}$Ge$_{14}$Si$_{9}$ resemble normal liquids without surface crystalline order. The differences are presumably caused by the fact that Ge has a lower surface tension than both Si and Au and as a result Si is displaced from the surface. The amount of Ge in a lower concentration alloy(Au$_{82}$Si$_{18-x}$Ge$_{x}$, x$\le $ 1 atm{\%}) is not sufficient to completely replace the surface Si and in this alloy and the 2D lattice structure order is identical to that of Au$_{82}$Si$_{18,}$ except for the $\sim $ 20K higher melting temperature of the 2D lattice and the fact that it is a bilayer, rather than a monolayer[1]. \newline [1] Shpyrko, O.G., et al., \textit{Surface crystallization in a liquid AuSi alloy}. Science, \textbf{313}.77(2006)