Renormalization of spin excitations and Kondo effect in open shell nanographenes

TL;DR

This paper investigates how substrate coupling affects spin excitations and the Kondo effect in open-shell nanographenes, revealing renormalization, broadening, and Kondo peak emergence in spectral features.

Contribution

It introduces a detailed theoretical analysis of substrate-induced effects on spin excitations and Kondo phenomena in nanographenes using multi-orbital Anderson models.

Findings

Substrate coupling causes significant renormalization of spin excitation energies.

Spectral features are broadened due to hybridization with the substrate.

Zero bias Kondo peaks emerge for $S=1$ ground states.

Abstract

We study spin excitations and Kondo effect in open-shell nanographenes, motivated by recent scanning tunneling inelastic spectroscopy experiments. Specifically, we consider three systems, the triangulene, the extended triangulene with rocket shape, both with an $S=1$ ground state, and a triangulene dimer with $S=0$ on account of intermolecular exchange. We focus on the consequences of hybridization of the nanographene zero-modes with a conducting substrate on the $dI/dV$ lineshapes associated with spin excitations. The partially filled nanographene zero-modes coupled to the conduction electrons in the substrate constitute multi-orbital Anderson impurity models that we solve in the one-crossing approximation which treats the coupling to the substrate to infinite order. We find that the coupling to the substrate leads to (i) renormalization of the spin flip excitation energies of the bare molecule, (ii) broadening of the spectral features and (iii) the emergence of zero bias Kondo peaks for the $S=1$ ground states. The calculated substrate induced shift of the spin excitation energies is found to be significantly larger than their broadening, which implies that this effect has to be considered when comparing experimental results and theory.