Origin of Symmetric Dimer Images of Si(001) Observed in Low-Temperature STM

摘要

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