探究Rashba效應在鍺銀表面合金的表面態能帶分裂

Study of Rashba effect on the surface-state band splitting of Ag2Ge surface alloy

楊立群(Li-Chuan Yang)

指導教授 ： 唐述中

國立清華大學/原子科學院/先進光源科技學位學程/碩士(2021年)

摘要

以銀(111)為基底鍍鍺至準條紋相的實驗過程中，我們發現在Γ¯M¯Ge 方向上的Γ 點附近有一個有趣的表面態能帶分裂，本論文以Rashba效應為理論模型對實驗數據的分裂能帶進行曲線擬合以獲取實驗參數，並理論推導出其 哈密頓量的能帶結構，發現異常於一般Rashbe效應的水平電場，更精確地計算且描繪出電子自旋結果與實驗數據吻合一致。同時我們也以穿隧式掃描電 子顯微鏡得到的準條紋相圖合理推論其水平電場源自於準條紋相特殊的表面原子排列結構。我們的研究成果充分合理地解釋此非一般型的Rashba效應。

關鍵字

鍺銀表面合金 ； Rashba

並列摘要

Using angle-resolved photoemission spectroscopy (ARPES), I studied the surface electronic structure of Ag2Ge surface alloy on Ag(111), finding that there is an interesting surface-state-band splitting centered at the zone center Γ¯ point. The splitting is anisotropic that it is largest in the symmetry direction Γ¯M¯Ag2Ge but negligible in another symmetry direction Γ¯K¯Ag2Ge. The corresponding constant energy contours measured present an inner hexagon enclosed by an outer 30 -rotated snow-flake contour. Using the Rashba-effect model including the typical first-order term and the second-order warping term, the energy bands as well as the constant energy contours derived from the corresponding Hamiltonian match the measured counterparts very well, revealing a negligible out-of-plane electric potential gradient and an exclusive in-plane electric potential gradient in the Ag2Ge surface alloy. Via inves- tigating the Ge coverage-dependent splitting of the surface state band and low-energy electron diffraction, we further realize the surface-state band split- ting actually occurs when striped-phase (SP) germanene coexists with Ag2Ge surface alloy. The measured topographic image by scanning tunneling microscopy (STM) shows a 2-dimentional triangle array, in which Ag2Ge surface alloy is enclosed by SP germanene for each triangle unit. We therefore propose a model that under the condition of the broken in-plane symmetry caused by the 3-fold triangles, the different charge distributions of Ag2Ge surface alloys and SP germanene induce an in-plane electric potential gradient that drives the unique Rashba effect as manifested by the observed anisotropic surface- state band splitting. The spin texture of the splitting surface-state band is also measured by the laser angle-and-spin-resolved photoemission, which is compared with that derived from the Hamiltonian for Rashba effects including the first- and second-order terms.

並列關鍵字

Ag2Ge ； Rashba

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