Are high-energy photoemission final states free-electron-like?

TL;DR

This paper demonstrates that high-energy photoemission final states are more complex than the free-electron model suggests, affecting the interpretation of 3D band structures in ARPES data across various materials.

Contribution

It reveals the multiband nature of photoemission final states at high energies, challenging the common free-electron approximation in ARPES analysis.

Findings

Final states can include multiple Bloch waves with different out-of-plane momenta.

Spectral peaks show complex structures and broadening due to multiband final states.

The phenomenon is observed in Ag, Si, and GaN, impacting band structure determination.

Abstract

Three-dimensional (3D) electronic band structure is fundamental for understanding a vast diversity of physical phenomena in solid-state systems, including topological phases, interlayer interactions in van der Waals materials, dimensionality-driven phase transitions, etc. Interpretation of ARPES data in terms of 3D electron dispersions is commonly based on the free-electron approximation for the photoemission final states. Our soft-X-ray ARPES data on Ag metal reveals, however, that even at high excitation energies the final states can be a way more complex, incorporating several Bloch waves with different out-of-plane momenta. Such multiband final states manifest themselves as a complex structure and excessive broadening of the spectral peaks from 3D electron states. We analyse the origins of this phenomenon, and trace it to other materials such as Si and GaN. Our findings are essential for accurate determination of the 3D band structure over a wide range of materials and excitation energies in the ARPES experiment.