Longitudinal and transverse electron paramagnetic resonance in a scanning tunneling microscope

Combining the sub-atomic resolution of scanning tunneling microscopy (STM) with the spectral resolution of electron-paramagnetic resonance (EPR) allows for sensitively probing magnetic interactions of single atoms on a surface [1]. However, the experimental requirements for driving the EPR transitions are still under debate. In-depth understanding of the EPR-STM driving is mandatory to explore novel material systems and optimize the sensitivity of this technique. Here, we acquire and model EPR spectra of single Fe and hydrogenated Ti atoms on bilayer MgO on Ag [2]. We systematically investigate the impact of RF excitation strength and tunneling parameters on the EPR signal and find strong evidence for a piezoelectric coupling mechanism [3]. In this mechanism, the surface atom oscillates at RF frequencies in the inhomogeneous tip magnetic field. Based on density functional theory and atomic-multiplet calculations, we reveal different driving mechanisms for single Fe and hydrogenated Ti atoms on the surface. Specifically, transverse magnetic field gradients drive the spin-1/2 hydrogenated Ti, whereas longitudinal magnetic field gradients drive the spin-2 Fe.
[1] S. Baumann et al., Science 350 (2015); [2] T.S. Seifert et al., PRR 2 (2020); [3] T.S. Seifert et al., Sci. Adv. 6 (2020)