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Spins are the epitome of the quantum nature of matter and are, as such, at the root of many fascinating phenomena. In this talk, I will discuss the interaction of a spin first with a continuum formed by the Cooper pairs of a superconductor, and then with another discrete spin, where quantum coherent dynamics can be tuned and observed directly. Magnetic impurities on top of superconductors induce bound states inside the superconducting gap, called Yu-Shiba-Rusinov (YSR) states. These states are promising building blocks for topological superconductivity, and I will present two experiments that shed light on their fundamental properties. YSR states arise due to exchange coupling between the magnetic impurity and Cooper pairs of the substrate, leading to two possible ground states in which the impurity spin is either free or screened. By approaching the tip toward the Fe center of an Fe-porphine molecule, we can tune the exchange coupling strength and thereby drive the system through a quantum phase transition from a screened to a free-spin ground state [1]. In a second experiment, we investigate the formation of YSR bands in a kagome lattice, using molecular self-assembly to ensure both atomic precision and long-range order [2]. In the second part of my talk, I will present recent technological advances to access the free coherent dynamics of atomic spins. By carefully placing two Ti atoms on MgO/Ag(100) next to each other and adjusting the height of a spin-polarized STM tip above one of them, we can tune the level of entanglement between their two spins, which we observe by means of ESR-STM measurements. We then investigate the dynamics of the system using DC pump-probe spectroscopy. We find that at maximal entanglement, a spin excitation can be coherently swapped back and forth between the two atomic spins [3]. The frequency and characteristics of this coherent oscillation are solely dictated by the intrinsic properties of the system, providing us with a unique way to investigate the properties of quantum materials. We further expanded our investigations to the study of spin-flip propagation in 1D and quasi-1D networks, and lastly, resolved coherent dynamics between a nuclear and electron spin in a single atom [4]. These studies set the foundation for exciting future research in which I will combine the atomic-scale control of scanning tunneling microscopy with coherent driving to dynamically tune the properties of quantum materials. [1] Farinacci et al., Phys. Rev. Lett. 121, 196803 (2018) [2] Farinacci et al., arXiv:2307.09993 (2023) [3] Veldman, Farinacci et al., Science 372, 964 (2021) [4] Veldman, … Farinacci, Otte, arXiv:2309.03749 (2023)

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Laëtitia Farinacci, Uni. Stuttgart
Seminar Room of the Institute of Physics II
Contact: Erwann/Matteo