第一原理計算與掃描穿隧式電子顯微術研究狄拉克半金屬ZrXY (X=Si, Ge; Y=S, Se, Te) 之自旋霍爾、自旋能斯特效應、以及表面電子態

Surface States, Spin Hall, and Spin Nernst Effects in Dirac Semimetals ZrXY (X=Si, Ge; Y=S, Se, Te) : Ab Initio Calculations and Scanning Tunneling Microscopy

晏雲(Yun Yen)

指導教授 ： 郭光宇

共同指導教授 ： 莊天明(Tien-Ming Chuang)

國立臺灣大學/理學院/物理學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202000338

摘要

狄拉克半金屬 ZrXY (X = Si, Ge; Y = S, Se, Te) 具有被非點群對稱性所保護的三維狄拉克節線 (Dirac nodal line)、表面拓樸態、以及伴隨這些新穎電子結構而來的應用價值，因此在近年來受到很大的關注。
另一方面，被打開能隙的狄拉克節點或是節線可能具有巨大的自旋貝里曲率，由此而產生可觀的自旋霍爾效應 (spin Hall effect) 以及自旋能斯特效應 (spin Nernst effect)。利用這些效應我們可以生成純淨的自旋流，並且實現不需外加磁場的自旋電子 (spintronics) 以及自旋熱力電子 (spin caloritronics) 元件。本文以第一原理相對論性能帶計算研究ZrXY 中的自旋霍爾效應與自旋能斯特效應。我們發現其中某些材料具有相當大的自旋霍爾導電率以及自旋能斯特導電率，而 ZrSiTe 甚至具有比白金更大的自旋霍爾角 (spin Hall angle)。更甚者，在這些材料中的自旋霍爾效應與自旋能斯特效應可以藉由自旋流方向、溫度、或是費米能級來調控。藉由分析不同能帶的自旋貝里曲率，我們發現其中的物理根源正是來自於被自旋軌道耦合 (spin­orbit coupling) 打開能隙的狄拉克節線。
我們進一步利用掃描式穿隧電子顯微術 (scanning tunneling microscopy) 以及第一原理聯合態密度模擬 (joint density of states) 解析ZrGeTe 的表面電子結構。我們成功找到所有準粒子干涉 q 向量的物理根源，而其中在 X 的無能隙表面態更展現了拉什巴效應，且在近日被與我們合作的角分辨光電子能譜實驗組 (angle-resolved photoemission spectroscopy) 測量到。

關鍵字

自旋電子學 ； 自旋霍爾效應 ； 自旋能斯特效應 ； 狄拉克半金屬 ； 非點群對稱 ； 拉什巴效應 ； 第一原理計算 ； 準粒子干涉 ； 掃描式穿隧電子顯微術

並列摘要

Dirac semimetals ZrXY (X = Si, Ge; Y = S, Se, Te) have been attracting considerable interests in recent years due to their three dimensional Dirac line nodes protected by nonsymmorphic glide symmetry, nontrivial topology, and the potential functionalities.
On the other hand, gapped Dirac nodes can possess large spin Berry curvatures and thus give rise to large spin Hall effect (SHE) and spin Nernst effect (SNE), which can be utilized to generate pure spin current for spintronics and spin caloritronics devices without applied magnetic field. In this thesis we study both SHE and SNE in ZrXY based on ab initio relativistic band structure calculations. Our calculations reveal that some of these compounds exhibit large intrinsic spin Hall conductivity (SHC) and spin Nernst conductivity (SNC), where ZrSiTe has spin Hall angle even larger than that of platinum. Remarkably, we find that both the magnitude and sign of the SHE and SNE in these compounds can be significantly tuned with the spin current direction, temperature, and Fermi level. Analysis of the calculated band­ and k­resolved spin Berry curvatures show that the large SHE and SNE as well as their remarkable tunabilities originate from the presence of many slightly spin­orbit coupling­gapped Dirac nodal lines in these Dirac semimetals.
We further utilized scanning tunneling microscopy (STM) and first-­principle simulated joint density of states (JDOS) to resolve the surface electronic structure of ZrGeTe, which possesses the strongest spin­orbit coupling strength in ZrXY compounds. We identified the origins of the quasiparitcle scattering interference q­vectors in the simulation. The floating gapless surface states at X exhibit the Rashba splitting, which is confirmed by our cooperating angleresolved photoemission spectroscopy (ARPES) measurements

並列關鍵字

Spintronics ； spin Hall effect ； spin Nernst effect ； Dirac semimetals ； nonsymmorphic symmetry ； Rashba effect ； first­-principle calculation ； quasipartical interference (QPI) ； scanning tunneling microscopy (STM)

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