Switchable molecular functionalization of an STM tip: from a Yu-Shiba-Rusinov tip to a Kondo tip

TL;DR

This paper demonstrates the reversible functionalization of a superconducting Nb STM tip with a TCNQ molecule, enabling switchable magnetic states between Yu-Shiba-Rusinov and Kondo regimes through controlled tip-sample interactions.

Contribution

It introduces a method to reversibly switch the magnetic coupling regime of a superconducting STM tip via molecular functionalization and distance tuning.

Findings

Reversible control of magnetic coupling regimes on the STM tip.

Observation of Yu-Shiba-Rusinov states and their transition to Kondo states.

Controlled manipulation of many-body effects in superconducting tunneling spectroscopy.

Abstract

In this work we fabricate and characterize a functionalized superconducting (SC) Nb tip of a scanning tunnelling microscope (STM). The tip is functionalized with a Tetracyanoquinodimethane molecule (TCNQ) that accepts charge from the tip and develops a magnetic moment. As a consequence, in scanning tunnelling spectroscopy (STS), sharp, bias symmetric sub-gap states identified as Yu-Shiba-Rusinov (YSR) bound states appear against the featureless density of states of a metallic gr/Ir(111) sample. Although the coupling regime of the magnetic impurity with the SC tip depends on the initial absorption configuration of the molecule, the interaction strength between the superconducting tip and the charged TCNQ molecule can be reversibly controlled by tuning the tip-sample distance. The controlled transition from one coupling regime to the other allows us to verify the relation between the energy scales of the two competing many-body effects for the functionalized tip. Quenching the SC state of the Nb tip with a magnetic field switches abruptly from a tip dominated by the YSR bound states to a Kondo tip.