The combination of electron spin resonance (ESR) and scanning tunneling microscopy (STM) enables to measure the magnetic coherence time of individual surface adsorbed atoms. Currently, there are about five laboratories Worldwide that are able to perform such measurements, the lab of the applicant being the first to reproduce the pioneering results by C. Lutz and A. J. Heinrich at IBM Almaden (Science 2015). Since then, the community has almost exclusively focused on two adsorbate/substrate systems that show an ESR signal in STM, namely Fe and hydrogenated Ti atoms adsorbed on a few atomic layer thick MgO(100) thin films grown on Ag(100).

While very interesting quantum physics has been demonstrated with these two systems, they have very low coherence times of the order of 100 ns. The present proposal intends to widen the horizon and to derive a more profound understanding of that field in i) exploring new adsorbate/substrate systems that display STM-ESR signals and in ii) developing new methods to coherently manipulate and measure the quantum states of surface adsorbed atoms.

i) Identifying adsorbate substrate combinations that might display ESR in the STM requires knowledge on the magnetic quantum levels in the crystal field of the adsorption site. There we have a competitive advantage with respect to other STM groups as we can perform synchrotron-based X-ray magnetic circular dichroism (XMCD) at our EPFL-PSI endstation XTreme. As adsorbates, we will focus on rare-earth atoms, as they have very long coherence times in diluted bulk crystals.

Further, we were pioneering the first single atom magnets (Science 2016), and all single atom magnets known since then are based on rare-earth atoms. These systems have astonishingly long spin-relaxation times of the order of T₁ ≈ 1 hour and can therefore have similarly long coherence times T₂.

A second ingredient for ESR-STM to work is the spin-polarization of the electronic levels contributing to the tunnel current. We have discovered systems with very large polarization of the 5d and 6s shells that is coupled to the localized and for the STM inaccessible 4f electrons (PRX 2020). There are many hitherto unexplored systems that might display detectable ESR-STM signals and exhibit long coherence times, rare-earth atoms being the most promising ones. The substrates, such as thin NaCl films, graphene, hexagonal boron nitride, or transition metal dichalcogenides, will be grown by molecular or atomic beam epitaxy, one of the key expertises of the applicant.

ii) The first new method for the investigation of magnetic quantum states of surface adsorbed atoms will be to combine non-contact atomic force microscopy with ESR. This is entirely new and opens up to address coherent quantum states in atoms and molecules adsorbed onto insulators. Based on diluted magnetic impurities in bulk samples, the coherence times expected on insulators are due to the reduced electron scattering by orders of magnitude longer than on metallic single crystals that have only a few monolayer thin insulating layers.

The second new method we propose is to combine optical excitation and read-out with atomic scale STM. Lasers and STM have been combined, e.g., for time-resolved STM measurements, but not with the goal to manipulate magnetic quantum states.