In this talk I will present our recent progress in single atom electron spin resonance (ESR) realized in a scanning tunneling microscope (STM) [1,2].In the first part I will show that we can resolve and control the hyperfine interaction of individual atoms by using ESR-STM. Using atom manipulation to move single atoms we find that the hyperfine interaction strongly depends on the binding configuration of the atom as well as the proximity to other magnetic atoms. This allows the extraction of atom- and position-dependent information about the electronic ground state, the state mixing with neighboring atoms as well as properties of the nuclear spin. Thus, the hyperfine spectrum becomes a powerful probe of the chemical environment of individual atoms and nanostructures. In addition, I show that we are able to extend ESR-STM to perform magnetic resonance imaging (MRI) for the first time on individual atoms. Here, we use the magnetic field of the STM tip to shift the Zeeman energy of the atom on the surface and cause spatial variations in the resonance frequency. While this is similar to existing scanning field gradient methods[3,4], our approach exceeds their spatial resolution by one to two orders of magnitude, to achieve Ångström resolution.Hyperfine interaction of individual titanium atoms. ESR spectra for 48Ti (top, nuclear spin I = 0) 47Ti (middle, I = 5/2) and 49Ti (bottom, I = 7/2). Right panel: STM images of the individual Ti atoms on a MgO surface.

REFERENCES:
[1] Baumann et al., Science 350, 6259 (2015).
[2] Willke et al., Science Advances 4, eaaq1543 (2018).
[3] Rugar et al., Nature 430, 329-332 (2004).
[4] Balasubramanian et al., Nature 455, 648 (2008).