Composition and Order-Disorder Transition in the $Cu_3Au$ (001) Surface Layer Investigated with the Use of DAES and DEPES

The composition of the first atomic layer of $Cu_3Au$(001) crystal (about half-and-half copper and gold atoms) changes only slowly even at temperatures much higher than that of the order-disorder transition ($T_C$ = 663 K). Theoretical and experimental works show a general trend of these changes but they differ in quantitative findings. In the present work we used directional elastic peak electron spectroscopy and directional Auger electron spectroscopy to investigate changes of atomic order and composition in the first atomic layers of the $Cu_3Au$(001) crystal during the sample temperature increase. The height of central maximum in DEPES polar profile of the sample investigated was measured as a function of sample temperature. It was found that the measured dependence is linear, but the slope of this dependence changes abruptly around $T_C$. This change seems to be connected with disappearance of the atomic order in the first and second atomic layers. In DAES the height of the Auger peaks for copper and gold low energy transitions (MVV and NVV, respectively) were measured in the dN(E)/dE mode as a function of the primary electron beam incidence angle. The composition of the first, second, and third atomic layers was determined by fitting the ratio of calculated DAES polar profiles for copper and gold to such a ratio for the measured profiles