Valence fluctuations in a lattice of magnetic molecules: application to iron(II) phtalocyanine molecules on Au(111)

TL;DR

This paper models a lattice of FePc molecules on Au(111) using an effective Hubbard-Anderson model, revealing intermediate-valence behavior and providing insights into recent STM experiments.

Contribution

It introduces a generalized slave-boson mean-field approach that accurately describes both non-interacting and strongly-interacting regimes for the system.

Findings

Lattice of FePc molecules exhibits intermediate-valence regime.

The model reproduces the correct Kondo temperature for isolated molecules.

System behavior differs from the SU(4) Kondo regime of isolated molecules.

Abstract

We study theoretically a square lattice of the organometallic Kondo adsorbate iron(II) phtalocyanine (FePc) deposited on top of Au(111), motivated by recent scanning tunneling microscopy experiments. We describe the system by means of an effective Hubbard-Anderson model, where each molecule has degenerate effective $d-$orbitals with $xz$ and $yz$ symmetry, which we solve for arbitrary occupation and arbitrary on-site repulsion $U$. To that end, we introduce a generalized slave-boson mean-field approximation (SBMFA) which correctly describes both the non-interacting limit (NIL) $U=0$ and the strongly-interacting limit $U \rightarrow \infty$, where our formalism reproduces the correct value of the Kondo temperature for an isolated FePc molecule. Our results indicate that while the isolated molecule can be described by an SU(4) Anderson model in the Kondo regime, the case of the square lattice corresponds to the intermediate-valence regime, with a total occupation of nearly 1.65 holes in the FePc molecular orbitals. Our results have important implications for the physical interpretation of the experiment.