Nonlocality of the exchange interaction probed by scanning tunneling spectroscopy

The energy states of the tip induced quantum dot are investigated in a magnetic field as a function of the tip position with respect to the surface. To study the influence of potential perturbations on the exchange interaction in the quantum dot, emphasis is given on the Landau and spin levels probed at positive bias. Mapping of the spatial dependence of the peak energies and comparison with the surface potential map–reconstructed from the analysis of scattering states around ionized dopants–reveals that the fluctuating surface potential acts as a local perturbation on the quantum dot states. In contrast, the spin splitting is influenced nonlocally by the surface potential, which directly evidences the nonlocal character of the exchange interaction.