Many-particle effects in adsorbed magnetic atoms with easy-axis anisotropy: the case of Fe on CuN/Cu(100) surface

TL;DR

This paper models the magnetic interactions of Fe atoms on CuN/Cu(100) surfaces using an anisotropic Kondo model, comparing theoretical spectral functions with experimental STM data and exploring magnetic field effects and the Kondo phenomenon.

Contribution

It introduces a method for calculating impurity spectral functions and magnetic susceptibilities in NRG without spin symmetry, and provides DFT analysis of the adsorbates' electronic structure.

Findings

Spectral functions match experimental data for J up to half of cobalt's coupling constant.

High magnetic fields induce a crossing of impurity energy states, leading to a Kondo zero-bias resonance.

The approach enables detailed analysis of magnetic properties in adsorbed magnetic atoms.

Abstract

We study the effects of the exchange interaction between an adsorbed magnetic atom with easy-axis magnetic anisotropy and the conduction-band electrons from the substrate. We model the system using an anisotropic Kondo model and we compute the impurity spectral function which is related to the differential conductance (dI/dV) spectra measured using a scanning tunneling microscope. To make contact with the known experimental results for iron atoms on the CuN/Cu(100) surface [Hirjibehedin et al., Science {\bf 317}, 1199 (2007)], we calculated the spectral functions in the presence of an external magnetic field of varying strength applied along all three spatial directions. It is possible to establish an upper bound on the coupling constant J: in the range of the magnetic fields for which the experimental results are currently known (up to 7T), the low-energy features in the calculated spectra agree well with the measured dI/dV spectra if the exchange coupling constant J is at most half as large as that for cobalt atoms on the same surface. We show that for even higher magnetic field (between 8 and 9T) applied along the ``hollow direction'', the impurity energy states cross, giving rise to a Kondo effect which takes the form of a zero-bias resonance. The paper introduces an approach for calculating the expectation values of global spin operators and all components of the impurity magnetic susceptibility tensor in numerical renormalization group (NRG) calculations with no spin symmetry. An appendix contains a density-functional-theory (DFT) study of the Cu and Fe adsorbates on CuN/Cu(100) surface: we compare magnetic moments, as well as orbital energies, occupancies, centers, and spreads by calculating the maximally localized Wannier orbitals of the adsorbates.