Current shot noise in atomic contacts: Fe and FeH2 between Au electrodes

Single Fe atoms on Au(111) surfaces were hydrogenated and dehydrogenated with the Au tip of a low-temperature scanning tunneling microscope (STM). Fe and FeH2 were contacted with the tip of the microscope and show distinctly different evolutions of the conductance with the tip-substrate distance. The current shot noise of these contacts has been measured and indicates a single relevant conductance channel with the spin-polarized transmission. For FeH2, the spin polarization reaches values up to 80% for low conductances and is reduced if the tip-surface distance is decreased. These observations are partially reproduced using density functional theory (DFT)-based transport calculations. We suggest that the quantum motion of the hydrogen atoms, which is not taken into account in our DFT modeling, may have a significant effect on the results.