Exchange-based noise spectroscopy of a single precessing spin with scanning tunnelling microscopy

Abstract

Electron spin resonance-scanning tunnelling microscopy is an emerging technique which is capable of detecting the precession of a single spin. We discuss a mechanism based on direct exchange coupling between the tunnelling electrons and the local precessing spin S. We claim that, since the number of tunnelling electrons in a single precessing period is small (about 20), one may expect a net temporary polarization within this period which will couple via exchange interaction to the localized spin. This coupling will modulate the tunnelling barrier with the Larmor frequency of the precessing spin ω_L. This modulation, although randomly changing from cycle to cycle, will produce an elevated noise in the current at ω_L. We find that for relevant values of parameters the signal-to-noise ratio in the spectral characteristic is 2–4 and is comparable with the values of the signal-to-noise ratio reported by Manassen and co-workers and by Durkan and Welland. The magnitude of the current fluctuation is a relatively weak increasing function of the dc and the magnetic field. The linewidth produced by the back action effect of tunnelling electrons on the precessing spin is also discussed.