Detection of Image Potential States above the vacuum level in GeTe

TL;DR

This study uncovers three image potential states above the vacuum level in GeTe(111), revealing unoccupied electronic states through advanced spectroscopy and theoretical analysis, which were previously unexplored.

Contribution

It is the first to observe and characterize IPS above the vacuum level in GeTe(111), combining experimental spectroscopy with theoretical calculations.

Findings

Three IPS observed up to 0.8 eV above vacuum level

Full parabolic dispersions of IPS resolved

Persistence of IPS linked to dipole transitions and electron reservoirs

Abstract

The ferroelectric semiconductor {\alpha}-GeTe(111) has attracted significant attention in the last decade due to its unique properties, with extensive studies focusing on its occupied electronic bandstructure. In contrast, its unoccupied states - particularly those near the conduction band minimum - remain largely unexplored. In an effort to characterize those states, we surprisingly observe three image potential states (IPS) in {\alpha}-GeTe(111) extending up to 0.8 eV above the vacuum level. Using time and angle-resolved photoemission spectroscopy, we resolve the full parabolic dispersions of the first three IPS and determine their binding energies. Our analysis, combined with Bloch spectral function calculations, reveals that the unexpected persistence of IPS above the vacuum level originates from strong dipole transitions and the presence of large electron reservoirs in GeTe.