利用傅立葉轉換掃描式穿隧電子顯微鏡研究金(111)表面上區域狀態密度的震盪

Abstract

在金(111)表面存在強Rashba效應自旋分裂的表面態，而Rashba效應與當今熱門題材拓撲絕緣體(Topological insulators)有所關連，這個表面態接近自由二維電子氣存在許多有趣的物理性質，其中之一是區域狀態密度的震盪行為，它產生於電子受散射物的散射而產生之量子干涉行為，亦可稱之為駐波或準粒子干涉。在我們的工作中，我們利用掃描式穿隧電子顯微鏡/能譜(STM/STS)在低溫下研究金(111)表面的結構狀態、電子特性和由點缺陷產生的駐波。藉由對不同能量下的dI/dV分布圖作傅利葉分析，可以從中發現新的區域狀態密度的震盪模式，不同於文獻上所提及的那些包含來自彈性碰撞的表面電子和塊材內部電子的貢獻。我們認為這個新的震盪模式來自於表面電子與具有未填滿能階之凹陷點缺陷的非彈性碰撞，藉由STS的量測，在一些凹陷點缺陷上我們量測到這個未填滿能階在0.22 eV處，我們利用凹陷點缺陷產生的量子井去解釋這個能階的存在。而且根據這個能階，我們可以建立一個非彈性碰撞的模型去解釋這些新的震盪模式，當一個高於費米能的表面電子入射到這個具有未填滿能階之凹陷點缺陷時，它可能會與附近位於費米面的電子發生碰撞，促使這個在費米面的電子躍遷到這個未填滿的能階，因此入射電子損失了0.22 eV，進而產生新的震盪模式，經過比較這個模型與我們的數據後，發現模型確實相當吻合我們的數據。

At the Au(111) surface, there exists a strong Rashba spin splitting surface state, which refers to a prevalent issue topological insulators. The surface state with two-dimensional nearly-free electron gas exhibits many interesting properties. One of them is the spatially oscillating local density of states (LDOS), which come from interference of electrons scattered by scatterers and also was named as the standing waves or quasi-particle interference (QPI). In this work, we have investigated the morphology, the electronic property and the standing wave induced by point defects on the Au(111) surface by scanning tunneling microscope and spectroscopy (STM/STS) at low temperature. By analyzing the Fourier-transform images from dI/dV images at different energies, we obtained additional spatially oscillating LDOS other than the contributions from elastically scattered surface electrons and bulk electrons. We contended that the new oscillations might result from the inelastic collision between surface electrons and concave point defects with an unoccupied state. By STS, the unoccupied state was found at 0.22 eV on some concave point defects. We used the quantum well formed by the concave point defect to explain the unoccupied state. According to this unoccupied state, we built an inelastic collision model to explain the new oscillations. The inelastic collision model is that the surface electrons with energy higher than Fermi energy incident into the defect collide with the electrons near the defect and make the electrons exited to the unoccupied state from the Fermi level. Hence, the incident electrons losing energy create the new oscillation modes. By comparison between the model and our data, we found the results from the model was consistent with our data.