  Orbital memory is defined by two stable valencies that can be electrically
switched and read-out. To explore the influence of an electric field on orbital
memory, we studied the distance-dependent influence of an atomic Cu donor on
the state favorability of an individual Co atom on black phosphorus. Using low
temperature scanning tunneling microscopy/spectroscopy, we characterized the
electronic properties of individual Cu donors, corroborating this behavior with
ab initio calculations based on density functional theory. We studied the
influence of an individual donor on the charging energy and stochastic behavior
of an individual Co atom. We found a strong impact on the state favorability in
the stochastic limit. These findings provide quantitative information about the
influence of local electric fields on atomic orbital memory.
