Quantum phase transition in magnetic nanographenes on a lead superconductor

TL;DR

This study demonstrates a quantum phase transition in magnetic nanographenes on a lead superconductor, revealing tunable magnetic bound states and potential for exploring Majorana states and quantum spin physics.

Contribution

It reports the fabrication of nanographenes with spin-1/2 on Pb(111) and observes a quantum phase transition by engineering exchange interactions, advancing understanding of quantum magnetism on superconductors.

Findings

Observation of quantum phase transition from singlet to doublet states

Detection of magnetic bound states coexisting with Kondo screening

Tunable magnetic interactions in graphene-based quantum systems

Abstract

Quantum spins, referred to the spin operator preserved by full SU(2) symmetry in the absence of the magnetic anistropy, have been proposed to host exotic interactions with superconductivity4. However, spin orbit coupling and crystal field splitting normally cause a significant magnetic anisotropy for d/f-shell spins on surfaces6,9, breaking SU(2) symmetry and fabricating the spins with Ising properties10. Recently, magnetic nanographenes have been proven to host intrinsic quantum magnetism due to their negligible spin orbital coupling and crystal field splitting. Here, we fabricate three atomically precise nanographenes with the same magnetic ground state of spin S=1/2 on Pb(111) through engineering sublattice imbalance in graphene honeycomb lattice. Scanning tunneling spectroscopy reveals the coexistence of magnetic bound states and Kondo screening in such hybridized system. Through engineering the magnetic exchange strength between the unpaired spin in nanographenes and cooper pairs, quantum phase transition from the singlet to the doublet state has been observed, in consistent with quantum models of spins on superconductors. Our work demonstrates delocalized graphene magnetism host highly tunable magnetic bound states with cooper pairs, which can be further developed to study the Majorana bound states and other rich quantum physics of low-dimensional quantum spins on superconductors.