Longitudinal and transverse electron paramagnetic resonance in a scanning tunneling microscope

Tom S Seifert; Stepan Kovarik; Dominik M Juraschek; Nicola A Spaldin; Pietro Gambardella; Sebastian Stepanow

doi:10.1126/sciadv.abc5511

Longitudinal and transverse electron paramagnetic resonance in a scanning tunneling microscope

Sci Adv. 2020 Sep 30;6(40):eabc5511. doi: 10.1126/sciadv.abc5511. Print 2020 Sep.

Authors

Tom S Seifert¹, Stepan Kovarik², Dominik M Juraschek^{2

3}, Nicola A Spaldin², Pietro Gambardella², Sebastian Stepanow¹

Affiliations

¹ Department of Materials, ETH Zürich, 8093 Zürich, Switzerland. tom.seifert@mat.ethz.ch sebastian.stepanow@mat.ethz.ch.
² Department of Materials, ETH Zürich, 8093 Zürich, Switzerland.
³ Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.

Abstract

Electron paramagnetic resonance (EPR) spectroscopy is widely used to characterize paramagnetic complexes. Recently, EPR combined with scanning tunneling microscopy (STM) achieved single-spin sensitivity with sub-angstrom spatial resolution. The excitation mechanism of EPR in STM, however, is broadly debated, raising concerns about widespread application of this technique. We present an extensive experimental study and modeling of EPR-STM of Fe and hydrogenated Ti atoms on a MgO surface. Our results support a piezoelectric coupling mechanism, in which the EPR species oscillate adiabatically in the inhomogeneous magnetic field of the STM tip. An analysis based on Bloch equations combined with atomic-multiplet calculations identifies different EPR driving forces. Specifically, transverse magnetic field gradients drive the spin-1/2 hydrogenated Ti, whereas longitudinal magnetic field gradients drive the spin-2 Fe. Also, our results highlight the potential of piezoelectric coupling to induce electric dipole moments, thereby broadening the scope of EPR-STM to nonpolar species and nonlinear excitation schemes.

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