Conduction-electron screening in the bulk and at low-index surfaces of Ta metal

Abstract

High-resolution core-level photoemission spectra from Ta(100) have been measured. The well-resolved peak from the first atomic layer allows a separate assessment of bulk and surface-layer screening responses: singularity indices ${\mathrm{\ensuremath{\alpha}}}_{\mathit{B}}$=0.10\ifmmode\pm\else\textpm\fi{}0.01 and ${\mathrm{\ensuremath{\alpha}}}_{100}$=0.205\ifmmode\pm\else\textpm\fi{}0.025, respectively, are obtained. The measured surface-atom lifetime broadening of 70\ifmmode\pm\else\textpm\fi{}20 meV (compared to 37\ifmmode\pm\else\textpm\fi{}5 meV in the bulk) is consistent with published Auger--photoemission coincidence measurements. The result for the bulk singularity index has been applied in the analysis of previous data in order to extract the screening response in the first atomic layer of Ta(111) (${\mathrm{\ensuremath{\alpha}}}_{111}$=0.16\ifmmode\pm\else\textpm\fi{}0.01) and Ta(110) (${\mathrm{\ensuremath{\alpha}}}_{110}$=0.150\ifmmode\pm\else\textpm\fi{}0.015). The trend in surface \ensuremath{\alpha}'s implies an increasingly atomic character for the surface-layer atoms as one proceeds from Ta(110) to Ta(111) to Ta(100).