Co adatoms on Cu surfaces: ballistic conductance and Kondo temperature

TL;DR

This study combines theoretical calculations and experiments to analyze how Co adatoms on Cu surfaces exhibit different Kondo behaviors and conductance features depending on surface orientation and tip proximity.

Contribution

It provides a detailed theoretical framework using DFT+NRG to understand the Kondo effect and conductance variations of Co adatoms on Cu(100) and Cu(111) surfaces, including symmetry considerations.

Findings

Kondo temperature varies with tip height on Cu(100) but not on Cu(111).

The S=1 Anderson model describes Co adatoms on both surfaces.

Fano lineshape is reproduced in tunneling but not contact regime for Cu(100).

Abstract

The Kondo zero bias anomaly of Co adatoms probed by scanning tunneling microscopy is known to depend on the height of the tip above the surface, and this dependence is different on different low index Cu surfaces. On the (100) surface, the Kondo temperature first decreases then increases as the tip approaches the adatom, while on the (111) surface it is virtually unaffected. These trends are captured by combined density functional theory and numerical renormalization group (DFT+NRG) calculations. The adatoms are found to be described by an S = 1 Anderson model on both surfaces, and ab initio calculations help identify the symmetry of the active d orbitals. We correctly reproduce the Fano lineshape of the zero bias anomaly for Co/Cu(100) in the tunneling regime but not in the contact regime, where it is probably dependent on the details of the tip and contact geometry. The lineshape for Co/Cu(111) is presumably affected by the presence of surface states, which are not included in our method. We also discuss the role of symmetry, which is preserved in our model scattering geometry but most likely broken in experimental conditions.