Single-atom electron paramagnetic resonance in a scanning tunneling microscope driven by a radiofrequency antenna at 4 K

Combining electron paramagnetic resonance (EPR) with scanning tunneling microscopy (STM) enables detailed insight into the interactions and magnetic properties of single atoms on surfaces [1]. A requirement for EPR-STM is the efficient coupling of a microwave excitation to the tunnel junction. Here, we present a coupling efficiency of the order of unity by using a radiofrequency (RF) antenna placed parallel to the STM tip [2]. This highly efficient coupling allows us to observe the EPR of individual atoms on an MgO surface routinely at 4 K. Using this technique, we perform a systematic study of the EPR of Fe and hydrogenated Ti atoms on MgO, comparing different tunneling parameters, frequency and magnetic field sweeps as well as amplitude and frequency modulation in order to maximize the EPR signal. We interpret the data based on density functional theory and charge transfer multiplet calculations, revealing the important role of the tip magnetic field in EPR-STM [3].

[1] S. Baumann, W. Paul, T. Choi, et al., Science 350, 417 (2015).
[2] T. S. Seifert, S. Kovarik, C. Nistor, et al., Phys. Rev. Research 2, 013032 (2020).
[3] T. S. Seifert, S. Kovarik, D. M. Juraschek, et al., Sci. Adv. 6, eabc5511 (2020).