Tuning the coupling of an individual magnetic impurity to a superconductor: quantum phase transition and transport

TL;DR

This study demonstrates how tuning the coupling between a magnetic impurity and a superconductor can induce a quantum phase transition, affecting the magnetic state and transport properties, using Fe-porphin molecules on Pb(111).

Contribution

It introduces a reversible method to tune the exchange coupling in a single magnetic molecule on a superconductor, revealing the quantum phase transition between screened and free-spin states.

Findings

Controlled tuning of the magnetic ground state via STM tip force.

Observation of the quantum phase transition through transport measurements.

Characterization of YSR states and Andreev reflections during tuning.

Abstract

The exchange scattering at magnetic adsorbates on superconductors gives rise to Yu-Shiba-Rusinov (YSR) bound states. Depending on the strength of the exchange coupling, the magnetic moment perturbs the Cooper pair condensate only weakly, resulting in a free-spin ground state, or binds a quasiparticle in its vicinity, leading to a (partially) screened spin state. Here, we use the flexibility of Fe-porphin molecules adsorbed on a Pb(111) surface to reversibly and continuously tune between these distinct ground states. We find that the FeP moment is screened in the pristine adsorption state. Approaching the tip of a scanning tunneling microscope, we exert a sufficiently strong attractive force to tune the molecule through the quantum phase transition into the free-spin state. We ascertain and characterize the transition by investigating the transport processes as function of tip-molecule distance, exciting the YSR states by single-electron tunneling as well as (multiple) Andreev reflections.