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

TL;DR

This study explains the symmetric dimer images of Si(001) surfaces observed in low-temperature STM as a result of quantum tunneling-driven flip-flop motion of buckled dimers, influenced by surface charging effects.

Contribution

It reveals that quantum tunneling causes the flip-flop motion of dimers, explaining low-temperature symmetric images in STM, a mechanism not previously understood.

Findings

Quantum tunneling induces flip-flop motion of dimers.

Surface charging with holes reduces energy barriers for flipping.

Symmetric images are due to tunneling-driven motion, not measurement artifacts.

Abstract

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.