碲化銅中增強的波動狀電荷密度波序化促進電子的加速與輕化

Abstract

隨著量子材料的出現，越來越多有趣的性質開始被大家研究並檢視。近年來，擁有在對稱保護下表現線性色散能帶的拓樸半金屬引起了大家的注意，這樣特殊的能帶結構，會造成特別的電荷屏蔽效應。我們利用電荷響應函數(Charge Response Function)去計算電磁易感率(Susceptibility)特別是電荷密度波(CDW)存在於材料中的情況，可幫助我們了解拓樸量子材料中有趣的電子結構。在本論文中，我們利用動量解析電子能量損失光譜(q-EELS)以及古典電漿子(plasmon)色散關係去偵測並且計算材料中載子的有效質量和費米速度，我們發現在碲化銅(CuTe)中，載子的有效質量和費米速度隨著電荷密度波序化的增強而分別變輕和增快，這與一般擁有電荷密度波的材料所表現的行為相反，也是本論文想要探討的主要目標。 在第一章中，我們對於量子材料和電荷屏蔽效應進行簡單的介紹，我們藉由對稱性的觀點切入量子材料，分別由反對稱中心和時間反演對稱中心以及破壞對於狄拉克(Dirac)和外爾(Weyl)費米子進行探討；我們也在電荷屏蔽的章節裡，簡單介紹電荷密度波、電漿子以及德汝德-勞倫茲模型(Drude-Lorentz Model)。在第二章，我們詳細的闡述掃瞄式穿透電子顯微鏡以及動量解析電子能量損失光譜的原理和應用。在第三章中，我們討論了關於碲化銅在動量解析電子能量損失光譜的結果，在不同動量空間下的光譜中，我們可以利用電漿子的色散關係得到相關載子的有效質量和費米速度，在碲化銅中，主要載子來源於線性色散能帶的輕電子以及與其垂直方向的重電洞。 在第四章中，我們探討在不同溫度下的動量解析電子能量損失光譜的實驗結果，因為隨著溫度低於電荷密度波相轉變溫度，電荷密度波序化會隨著溫度降低而增強，我們可以觀察電荷密度波序化增強與載子的關係。在室溫時，線性色散能帶的電子，同時也是與電荷密度波序化相關的載子，其有效質量約為0.28倍的電子靜止質量(m0)，而其費米速度則約為光速的0.005倍。隨著溫度的降低和電荷密度波序化的增強，我們發現輕電子的有效質量變得更輕並且費米速度變得更快。在溫度到100 K時，載子的有效質量和費米速度相對於室溫時，變輕和增快約百分之20。我們推測，造成這樣的原因，是因為線性色散的能帶在低溫下進行能帶的重整化(Band Renormalization)，使得能帶變的更陡峭，進而導致載子的有效質量變輕，以其費米速度增快。 電荷密度波隨著不同溫度下的序化現象在材料中有著重要的地位，碲化銅是一個適合針對電荷密度波以及弱相關系統進行研究的材料，而在適當的條件下，動量解析電子能量損失光譜更可以助於我們瞭解材料內的物理現象以及計算重要的物理參數，我們期待在更多擁有豐富物理性質的材料上，利用動量解析電子能量損失光譜獲得更加有趣的實驗結果。

With the rapid advances discovery in various systems, quantum materials have aroused lots of attention in the field. Recently, topological semimetals featuring symmetry-protected crossing of linearly dispersing bands in the bulk electronic structure has gained growing attentions in the investigation of the electronic screening due to a finite density of states. Those unique material systems become interested with the concept of the matters susceptible to electronic ordering, where charge density waves (CDWs) are pervasive orders in the systems. The capability to probe the carrier density near the Fermi level inside the CDW systems in topological quantum matters allows a direct unveiling observation of the electronic structure. This Ph.D. thesis has been dedicated to the momentum-dependent electron energy loss spectroscopy (q-EELS) on probing the effective mass and the Fermi velocity without further experiment setup. The reduced effective mass and the enhanced Fermi velocity in our CuTe system with CDW order growing exhibits an inverse result to the usual CDW systems. A general introduction to the quantum materials and the charge response phenomena is presented in Chapter 1 and the experimental elucidation is addressed in Chapter 2. In Chapter 3, we show the q-EELS experimental result on the CuTe crystal, where we can obtain the effective mass and the Fermi velocity using the classical plasmon dispersion relation. We can simultaneously capture the effective mass and the Fermi velocity of the related, practically linearly dispersing electron and the counterpart of heavy-hole carrier. In Chapter 4, we show the temperature dependent q-EELS experiment across the transition temperature 335 K (TCDW, CDW transition temperature) to help to observe the change followed the CDW order growth. The effective mass of practically linearly dispersing electron relating to CDW gap opening is 0.28 m0 (m0, the electron rest mass) and the Fermi velocity is approximate 0.005 c (c, the speed of light) at room temperature. Following the CDW order growth, the electrons becomes lighter and moves faster by ~20% toward to 100 K. Thorough inspection below TCDW unveils the essential role of the increasing opening of the CDW gap. CuTe is a rich platform for the exploration of CDW and weak-coupled correlation physics with q-EELS as a useful tool for probing the associated fundamental properties.