Spin and electronic excitations in $4f$ atomic chains on Au(111) substrates

TL;DR

This study models the electronic and magnetic excitations of a one-dimensional Eu atomic chain on Au(111), revealing exchange interactions, spin excitation spectra, and the effects of magnetic fields relevant for quantum applications.

Contribution

It provides a detailed ab initio analysis of spin interactions and excitations in a rare-earth atomic chain on a metal surface, a system with sparse prior research.

Findings

Eu atoms have a net magnetic moment of ~3.5 μB on gold.

Exchange energies J1 ≈ -1.2 K and J2 ≈ 0.2 K were determined.

The system exhibits a highly degenerate ferromagnetic ground state with significant anisotropy.

Abstract

High spin systems, like those that incorporate rare-earth $4f$ elements (REEs), are increasingly relevant in many fields. Although research in such systems is sparse, the large Hilbert spaces they occupy are promising for many applications. In this work, we examine a one-dimensional linear array of europium (Eu) atoms on a Au(111) surface and study their electronic and magnetic excitations. Ab initio calculations using VASP with PBE+U are employed to study the structure. We find Eu atoms to have a net charge when on gold, consistent with a net magnetic momemt of $\simeq 3.5 \mu_B$. Examining various spin-projection configurations, we can evaluate first and second neighbor exchange energies in an isotropic Heisenberg model between spin-$\frac{7}{2}$ moments to obtain $J_1 \approx -1.2 \, \mathrm{K}$ and $J_2 \approx 0.2 \, \mathrm{K}$ for the relaxed-chain atomic separation of $a \approx 5$ $\mathrm{\dot{A}}$. These parameters are used to obtain the full spin excitation spectrum of a physically realizable four-atom chain. The large $|J_1|/J_2$ ratio results in a highly degenerate ferromagnetic ground state that is split by a significant easy plane single ion anisotropy of $0.6$ K. Spin-flip excitations are calculated to extract differential conductance profiles as those obtained by scanning tunneling microscopy techniques. We uncover interesting behavior of local spin excitations, especially as we track their dispersion with applied magnetic fields.