ESR-STM of a single precessing spin: Detection of exchange-based spin noise

Electron Spin resonance scanning tunneling microscopy (ESR-STM) is an emerging technique which is capable of detecting the precession of a single spin. We discuss the mechanism of ESR-STM based on a direct exchange coupling between the tunneling electrons and the local precessing spin S. We claim that since the number of tunneling electrons in a single precessing period is small (∼20), one may expect a net temporary polarization within this period that will couple via exchange interaction to the localized spin. This coupling will randomly modulate the tunneling barrier and create a dispersion in the tunneling current which is a product of a Larmor frequency component due to the precession of the single spin and the dispersion of the spin of the tunneling electrons. This noise component is spread over the whole frequency range for random white noise spin polarization of electrons. In the opposite case where the power spectrum of the spins of the tunneling electrons has a peak at zero frequency an elevated noise in the current at ω_L will appear. We discuss the possible source of this spin polarization. We find that for relevant values of parameters the signal-to-noise ratio in the spectral characteristic is 2-4 and is comparable to the reported signal to noise ratio.^1,2 The magnitude of the current fluctuation is a relatively weak increaing function of the dc current and magnetic field. The linewidth produced by the back action effect of tunneling electrons on the precessing spin is also discussed.