Dynamic screening of an ion in a degenerate electron gas within the second-order Born approximation

TL;DR

This paper derives a second-order Born approximation for the dynamic Friedel sum rule in a degenerate electron gas, providing improved screening parameters for ions at various velocities, especially at moderate and low speeds.

Contribution

It introduces a second-order Born approximation for dynamic screening, accounting for ion atomic number, and compares it with exact results, showing improved accuracy over first-order methods.

Findings

B2 approximation closely matches exact screening at moderate and low velocities.

B1 approximation deviates significantly at lower velocities.

Padé approximant further improves the perturbative results across all velocities.

Abstract

The dynamic Friedel sum rule (FSR) is derived within the second-order Born (B2) approximation for an ion that moves in a fully degenerate electron gas and for an arbitrary spherically-symmetric electron-ion interaction potential. This results in an implicit equation for the dynamic B2 screening parameter which depends on the ion atomic number $Z_{1}$ unlike the first-order Born (B1) dynamic screening parameter reported earlier by some authors. Furthermore, for typical metallic densities our analytical results for the Yukawa and hydrogenic potentials are compared, for both positive and negative ions, to the exact screening parameters calculated self-consistently by imposing the exact dynamic FSR requirement to the scattering phase shifts. The B1 and B2 screening parameters agree excellently with the exact values at large velocities, while at moderate and low velocities the B1 approximation deviates from the exact solution whereas the B2 approximation still remains close to it. In addition, a Pad\'{e} approximant to the Born series yields a further improvement of the perturbative approach, showing an excellent agreement on the whole velocity range in the case of antiprotons.