Abstract:

In 1930, a resistance minimum observed in dilute magnetic alloys was the first experimental evidence for a new scattering mechanism of conduction electrons at magnetic impurities at low temperatures. More than 30 years later J. Kondo developed a theory that describes the effect as a consequence of the spin-flip scattering of conduction electrons at a localized magnetic impurity: The term “Kondo effect” was born for this phenomenon. About the same time P. W. Anderson developed his “single impurity model” where he calculated the implications of this scattering mechanism for the local density of states:

He found a strong singularity at the Fermi level, termed “Kondo resonance”. This prediction triggered numerous experimentalists to search and find this resonance in rare earth and transition metal alloys with techniques such as point contact measurements, photoelectron, and scanning tunneling spectroscopies (STS).

The latter technique enabled us to study the manifestations of the Kondo effect for individual atoms and molecules adsorbed on surfaces. An interesting case are the rare earth atoms. Their magnetic moment originates from electrons in the partially filled 4f orbitals which are well shielded by the outerlying orbitals leading to a very weak hybridization between them and the localized 4f electron. This is most probably the reason, why so far no Kondo resonance has been detected for a single Ce adatom on metallic and insulating surfaces. The Ce adatom remains spectroscopically “dark”. However, here we show that we can detect the magnetic moment of an individual Ce adatom adsorbed on a Cu2N ultra thin film on Cu(100) by using a sensor tip that has its apex functionalized with a Kondo screened spin system, a small Ce-cluster. We calibrate the sensor tip by deliberately coupling it to a well characterised Fe surface atom. Subsequently we use the splitting of the tip’s Kondo resonance when approaching a spectroscopically dark Ce atom to sense its magnetic moment [1]. Thus, the functionalized tip is used as a spin detector for single magnetic Ce-adatoms in the absence of an external magnetic field. This achievement indicates an alternative route to the study of magnetic nanostructures circumventing the application of spin-polarized STM tips.

When Ce-atoms self-assemble to create a superlattice on Ag(111) with an interatomic

lattice spacing of 3.2 nm [2], a Kondo lattice is formed. The differential conductance spectra

obtained on single Ce-adatoms within the superlattice reveal a considerably broadened Kondo

resonance as compared to the one of about 1 meV found for isolated Ce-clusters. This observation might indicate the presence of antiferromagnetic indirect exchange interactions (RKKY) in the superlattice. Depending on the distance between the Ce-adatoms in the superlattice the competition between Kondo singlet formation and RKKY interaction may lead to a coherent antiferromagnetic or ferromagnetic state when passing through a quantum critical point. Future studies at lower temperatures and with higher spectroscopic

resolution may provide new insights into the interplay between Kondo physics, localized spin-flip excitations, and the magnetic exchange interaction.

[1] M. Ternes, C. P. Lutz, A. J. Heinrich, and W.-D. Schneider, Phys. Rev. Lett. 124, 167202 (2020)

[2] F. Silly, M. Pivetta, M. Ternes, F. Patthey, J. P. Pelz, and W.-D. Schneider, Phys. Rev. Lett. 92, 016101 (2004)