Imaging single electron spin in a molecule trapped within a nanocavity of tunable dimension

Control of magnetism at the nanoscale is shown by a reversible transfer of an electron to and from a single molecule within the tunable gap of a scanning tunneling microscope. The addition of an electron to magnesium porphine changes the molecule from the diamagnetic state to the paramagnetic state. The existence of the single unpaired electron in the molecule is confirmed by spectroscopy and spatial imaging of the many body Kondo state and inelastic spin excitation between the Zeeman levels at 600 mK and up to 9 Tesla magnetic field. Here, we show that the spin is delocalized in an extended molecular orbital, in contrast to the spatially confined d and f states in atoms and magnetic centers in molecules. Furthermore, by tuning the dimension of the tunneling gap and visualizing the spectroscopic images, the inelastic spin-flip scatterings are shown to underlie the formation of the Kondo state.