## Invited talk

**Overcoming the energy and time limitation of STM**

__Läetitia Farinacci__^{1,*}, L.M. Veldman^{1}, R. Rejali^{1}, R. Broekhoven^{1}, J. Gobeil^{1}, D. Coffey^{1}, M. Ternes^{2}, P. Willke^{3}, and S. Otte^{1}

*
1 Department of Quantum Nanoscience, Delft University of Technology, the Netherlands
2 RWTH Aachen University, Peter-Grünberg-Institute, Aachen, German
3 Physikalisches Institut, Karlsruhe Institute of Technology, Karlsruhe, Germany
* current affiliation: Freie Universität Berlin, Germany
*

The combination of electron spin resonance with scanning tunneling microscopy drastically widened the range of interactions that can be studied on the atomic scale [1]. Overcoming the energy resolution of typical scanning tunneling spectroscopy by 3 orders of magnitude, it has been used to characterize both dipolar and exchange interactions between single spins [2], tune their level of entanglement [3] and even resolve the hyperfine interaction of single atomic spins [4]. In this talk I will first discuss how studying the anisotropy of the hyperfine interaction can be used to determine the electronic ground state of TiH/MgO/Ag(100), a quantity that so far eluded experimentalists [5].

The second part of the talk will be devoted to the study of dynamics of atomic spins. We demonstrate that a DC-pump-pulse scheme can be used to study the free evolution of two coupled atomic spins: by tuning their level of entanglement, we find a tuning point in which a spin excitation is coherently swapped back and forth between them [6]. Finally, I will present our most recent results in which we study the free propagation of a spin excitation in chains and quasi-2D structures, as well as the coherent free dynamics between a nucleus and electron spin within an atom.

Figure 1: (a) Using ESR-STM we investigate the anisotropy of the hyperfine interaction of Ti/MgO/Ag(100) and develop a method to experimentally determine the ground state orbital [5]. (b) Using DC-pump-probe we can measure the free coherent evolution of a spin excitation over a pair of coupled atomic spins [6].

[1] Baumann et al., Science 350, 6259 (2015)

[2] Choi et al., Nat. Nano 12, (2017)

[3] Yang et al., Phys. Rev. Lett. 119, 227206 (2017)

[4] Willke et al., Science 362 (2018)

[5] Farinacci et al., Nano Letters 22, 8470 (2022)

[6] Veldman et al., Science 372, 6545 (2021)