Kondo effect of an adsorbed cobalt phthalocyanine (CoPc) molecule: the role of quantum interference

TL;DR

This study models the Kondo effect in an adsorbed CoPc molecule, revealing how quantum interference and molecular interactions influence the emergence and strength of the Kondo resonance.

Contribution

It provides an exact solution of a model showing how molecular interactions and quantum interference control the Kondo effect in adsorbed molecules.

Findings

Quantum interference suppresses the Kondo effect in undistorted molecules.

Interaction among molecule lobes enables the Kondo resonance.

The Kondo resonance's strength depends on coupling and molecular interactions.

Abstract

A recent experimental study showed that, distorting a CoPc molecule adsorbed on a Au(111) surface, a Kondo effect is induced with a temperature higher than 200 K. We examine a model in which an atom with strong Coulomb repulsion (Co) is surrounded by four atoms on a square (molecule lobes), and two atoms above and below it representing the apex of the STM tip and an atom on the gold surface (all with a single, half-filled, atomic orbital). The Hamiltonian is solved exactly for the isolated cluster, and, after connecting the leads (STM tip and gold), the conductance is calculated by standard techniques. Quantum interference prevents the existence of the Kondo effect when the orbitals on the square do not interact (undistorted molecule); the Kondo resonance shows up after switching on that interaction. The weight of the Kondo resonance is controlled by the interplay of couplings to the STM tip and the gold surface, and between the molecule lobes.