  Bodies in relative motion separated by a gap of a few nanometers can
experience a tiny friction force. This non-contact dissipation can have various
origins and can be successfully measured by a sensitive pendulum atomic force
microscope tip oscillating laterally above the surface. Here, we report on the
observation of dissipation peaks at selected voltage-dependent tip-surface
distances for oxygen-deficient strontium titanate (SrTiO_3) surface at low
temperatures (T = 5K). The observed dissipation peaks are attributed to
tip-induced charge and spin state transitions in quantum-dot-like entities
formed by single oxygen vacancies (and clusters thereof, possibly through a
collective mechanism) at the SrTiO_3 surface, which in view of technological
and fundamental research relevance of the material opens important avenues for
further studies and applications.
