  Negative differential resistance remains an attractive but elusive
functionality, so far only finding niche applications. Atom scale entities have
shown promising properties, but viability of device fabrication requires fuller
understanding of electron dynamics than has been possible to date. Using an
all-electronic time-resolved scanning tunneling microscopy technique and a
Green's function transport model, we study an isolated dangling bond on a
hydrogen terminated silicon surface. A robust negative differential resistance
feature is identified as a many body phenomenon related to occupation dependent
electron capture by a single atomic level. We measure all the time constants
involved in this process and present atomically resolved, nanosecond timescale
images to simultaneously capture the spatial and temporal variation of the
observed feature.
