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

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 GGA+U, spin-orbit coupling, and the HSE hybrid functional are employed to study the structure.  We find Eu atoms to exhibit minimal charge transfer, indicating physisorption of neutral atoms with a net spin-7/2 when on gold.  Examining various spin-projection configurations, we can evaluate first and second neighbor exchange energies to obtain J₁ ≈ -0.947 K and J₂ ≈ 4.29 K for the relaxed-chain atomic separation of a ≈ 5 Å. These parameters are used in an isotropic Heisenberg model to obtain the full spin excitation spectrum of a physically realizable four-atom chain. The large J₂|J₁| ratio results in a singlet ground state with dimer-like antiferromagnetic correlations between second neighbors.  The seemingly small J₁ values significantly affect the spectra, underscoring the important role that nearest-neighbor coupling plays in this system, including possible Dzyaloshinskii-Moriya interactions. Spin-flip excitations are calculated to extract differential conductance profiles as those obtained by scanning tunneling microscopy techniques. We uncover interesting behavior for local higher spin excitations, especially as we track their dispersion with applied magnetic fields.