Estados eletrônicos de superfície na face (111) do Au, Cu3Au e sulfeto sobre Cu3Au por espectroscopia de fotoelétrons resolvida em ângulo (ARPES)

Abstract

This work seeks to deepen the knowledge about electronic states of surfaces (111) in noble metals such as Au and Cu3Au using, mainly, the ARPES technique (Angle-Resolved Photoelectron Spectroscopy) that allows to obtain the Energy versus Momentum dispersion for states close to the Fermi level directly. In addition to ARPES, XPS (X-ray Photoelectron Spectroscopy) was used to investigate the cleanliness and chemical composition of the samples and LEED (Low Energy Electron Diffraction) to verify the symmetry pattern of the sample surface.

On Cu3Au sample, an interesting result was obtained after heating processes, resulting in the formation of an ordered surface involving Copper, Gold and also Sulfur. Its surface state was also measured, revealing unprecedented and relevant results, and it may even have technological applications.

The experiments were performed on samples that undergo cleaning cycles via sputtering and annealing, and carried out in UHV (Ultra High Vacuum), at a base pressure of 1x10^{-10}Torr. The famous parabolic dispersion with resolution for the spin-split was observed for Gold, which proved the excellent calibration of the ARPES equipment. The parameters such as effective mass and Rashba coefficient obtained match with the literature. The parabolic dispersion was also obtained for the called ``disordered'' phase of Cu3Au clean. The parabolic surface states on the face (111) of noble metals have traditionally been described by the Shockley model in most of the literature, but they also have been related to the more recently discovered topological states. This conceptual issue is addressed in this work in order to bring clarity to a better description of the surface states.

The alloy formed on Cu3Au containing sulfur (with structure (\sqrt{7}x\sqrt{7})R19.1^{o}-S according to LEED and the literature) led to a linear dispersion, with a crossing point, which is very common in so-called Topological Insulators. This result brings an addition to scientific knowledge, as no ARPES study of this sulfide on Cu3Au was known, not even on Cu(111). The LEED pattern obtained is very similar to that shown in the literature for Sulfide over Cu(111). Combining this with the calculation of concentrations obtained by XPS of the Cu3Au sample, it was noticed that the sample has much more copper than the nominal. Therefore, for better understanding of the physical nature of the surface states and further deepening of the studies on this unprecedented result of the dispersion with crossing point for the sulfide over Cu3Au can be quite relevant for technological advances and new materials.