Surface Screening Charge and Effective Charge

TL;DR

This paper investigates how surface atoms on Ag(001) respond to external electric fields by analyzing charge screening and effective charge, revealing how Xe atoms are influenced and how charge sign changes at high fields.

Contribution

It introduces a method to determine surface screening and effective charge from potential shifts, applying it to Xe on Ag(001) under various electric fields.

Findings

Xe has a negative effective charge of -0.093 e at low fields

Charge sign reverses at 2.3 V/Å, leading to outward forces at higher fields

Results relate to field desorption and the Eigler switch phenomena

Abstract

The charge on an atom at a metallic surface in an electric field is defined as the field-derivative of the force on the atom, and this is consistent with definitions of effective charge and screening charge. This charge can be found from the shift in the potential outside the surface when the atoms are moved. This is used to study forces and screening on surface atoms of Ag(001) c$(2\times 2)$ -- Xe as a function of external field. It is found that at low positive (outward) fields, the Xe with a negative effective charge of -0.093 $|{e}|$ is pushed into the surface. At a field of 2.3 V \AA$^{-1}$ the charge changes sign, and for fields greater than 4.1 V \AA$^{-1}$ the Xe experiences an outward force. Field desorption and the Eigler switch are discussed in terms of these results.