Electron and Cooper pair transport across a single magnetic molecule explored with a scanning tunneling microscope

TL;DR

This study uses a scanning tunneling microscope to investigate electron and Cooper pair transport across single magnetic molecules on a superconducting surface, revealing a transition from energy gaps to zero-energy resonances linked to Andreev and Yu-Shiba-Rusinov states.

Contribution

It demonstrates how electron and Cooper pair transport evolve at the atomic scale across magnetic molecules, combining experimental STM data with theoretical transport calculations.

Findings

Observation of a transition from energy gap to zero-energy resonance.

Identification of Yu-Shiba-Rusinov states and Andreev reflection contributions.

Evidence of Josephson current in superconductor-superconductor contacts.

Abstract

A scanning tunneling microscope is used to explore the evolution of electron and Cooper pair transport across single Mn-phthalocyanine molecules adsorbed on Pb(111) from tunneling to contact ranges. Normal-metal as well as superconducting tips give rise to a gradual transition of the Bardeen-Cooper-Schrieffer energy gap in the tunneling range into a zero-energy resonance close to and at contact. Supporting transport calculations show that in the normal-metal - superconductor junctions this resonance reflects the merging of in-gap Yu-Shiba-Rusinov states as well as the onset of Andreev reflection. For the superconductor - superconductor contacts the zero-energy resonance is rationalized in terms of a finite Josephson current that is carried by phase-dependent Andreev and Yu-Shiba-Rusinov levels.