  It has been a long-standing puzzle why buckled dimers of the Si(001) surface
appeared symmetric below 20 K in scanning tunneling microscopy (STM)
experiments. Although such symmetric dimer images were concluded to be due to
an artifact induced by STM measurements, its underlying mechanism is still
veiled. Here,we demonstrate, based on a first-principles density-functional
theory calculation, that the symmetric dimer images are originated from the
flip-flop motion of buckled dimers, driven by quantum tunneling (QT). It is
revealed that at low temperature the tunneling-induced surface charging with
holes reduces the energy barrier for the flipping of buckled dimers, thereby
giving rise to a sizable QT-driven frequency of the flip-flop motion. However,
such a QT phenomenon becomes marginal in the tunneling-induced surface charging
with electrons. Our findings provide an explanation for low-temperature STM
data that exhibits apparent symmetric (buckled) dimer structure in the
filled-state (empty-state) images.
