Interfering tunneling paths through magnetic molecules on superconductors: Asymmetries of Kondo and Yu-Shiba-Rusinov resonances

TL;DR

This study investigates the bias-voltage asymmetries in Kondo and Yu-Shiba-Rusinov resonances observed in magnetic molecules on superconductors, revealing interference effects between tunneling paths as the underlying cause.

Contribution

It introduces a model explaining the asymmetries in Kondo and YSR resonances based on interfering tunneling paths involving molecular orbitals.

Findings

Correlated asymmetries observed across the molecule.

Symmetric resonances at nodal planes of the HOMO.

Interference effects explain bias asymmetries.

Abstract

Magnetic adsorbates on superconductors induce a Kondo resonance outside and Yu-Shiba-Rusinov (YSR) bound states inside the superconducting energy gap. When probed by scanning tunneling spectroscopy, the associated differential-conductance spectra frequently exhibit characteristic bias-voltage asymmetries. Here, we observe correlated variations of Kondo and YSR asymmetries across an Fe-porphyrin molecule adsorbed on Pb(111). We show that both asymmetries originate in interfering tunneling paths via a spin-carrying orbital and the highest occupied molecular orbital (HOMO). Strong evidence for this model comes from nodal planes of the HOMO, where tunneling reveals symmetric Kondo and YSR resonances. Our results establish an important mechanism for the asymmetries of Kondo and YSR lineshapes.