Spin-dependent dipole excitation in alkali-metal nanoparticles

TL;DR

This paper investigates spin-dependent electronic excitations in alkali-metal nanoparticles, focusing on the surface paramagnon and low-energy spin-dipole modes, revealing their non-collective nature and dependence on ground-state spin polarization.

Contribution

It provides a detailed analysis of spin-dipole excitations, including the surface paramagnon, using numerical and analytical methods, and clarifies their non-collective character.

Findings

The surface paramagnon frequency can be estimated from simple models.

Low-energy spin-dipole modes are linked to particle-hole excitations.

Increased response correlates with ground-state spin polarization in open-shell systems.

Abstract

We study the spin-dependent electronic excitations in alkali-metal nanoparticles. Using numerical and analytical approaches, we focus on the resonances in the response to spin-dependent dipole fields. In the spin-dipole absorption spectrum for closed-shell systems, we investigate in detail the lowest-energy excitation, the "surface paramagnon" predicted by L. Serra et al. [Phys. Rev. A 47, R1601 (1993)]. We estimate its frequency from simple assumptions for the dynamical magnetization density. In addition, we numerically determine the dynamical magnetization density for all low-energy spin-dipole modes in the spectrum. Those many-body excitations can be traced back to particle-hole excitations of the noninteracting system. Thus, we argue that the spin-dipole modes are not of collective nature. In open-shell systems, the spin-dipole response to an electrical dipole field is found to increase proportionally with the ground-state spin polarization.