Photoinduced valence transition in gold complexesCs2Au2X6(X=Cland Br) probed by x-ray photoemission spectroscopy

Abstract

Perovskite-type gold complex ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{Br}}_{6}$ is known to show a photoinduced valence transition as well as a pressure-induced valence transition from the mixed-valence ${\mathrm{Au}}^{+}$ and ${\mathrm{Au}}^{3+}$ state to the single-valence ${\mathrm{Au}}^{2+}$ state under high pressure of 6.5 GPa. Since photoemission spectroscopy permits direct observation of electronic states at the solid surface, it is well suited as a probe to identify electronic changes due to photoinduced phase transitions (PIPT) at the surface. By means of photoemission spectroscopy, we have succeeded to observe the photoinduced valence transition at a surface of ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{Br}}_{6}$ even under vacuum conditions (0 GPa). This observation shows that PIPT at the surface of transition-metal compounds, which have yet to be explored, can be different from those in the bulk, suggesting new possibilities to develop optical devices using PIPT at a solid surface.