利用穿透式電子顯微鏡研究稀土過渡金屬矽化物(Lu2Ir3Si5與Dy5Ir4Si10)電荷密度波之行為

Study of Charge Density Waves in Rare-Earth Transition-Metal Silicides Dy5Ir4Si10 and Lu2Ir3Si5 by Transmission Electron Microscopy

李明浩(Ming-Hao Lee)

指導教授 ： 陳正弦

國立臺灣大學/理學院/物理研究所/博士(2012年)

https://doi.org/10.6342/NTU.2012.02616

摘要

電荷密度波（CDW）是德國理論物理學家皮爾斯(Peierls)於1955 年所預測提出，假設一維的線性金屬系統由於電子與聲子相互作用而導致晶格不穩定，系統的基態不再如同正常晶格中原子等距分配，而是晶格中原子呈遠-近-遠-近的分配，使得電荷密度有週期性分佈。高度異向性(anisotropic)能帶結構是導致這些基態的重要性質。 CDW的基態也伴隨著在費米能階附近能隙的產生。由於能隙的形成使得此一維線性金屬成為趨向絕緣性。一般而言，電荷密度波多發現在低維度材料中，因為低維度材料擁有特殊異向的晶體結構和電子能帶結構，以及較強的電子–聲子相互作用。文獻上已經發表的稀土過渡金屬的三維晶體結構有電荷密度波（CDW）的相變，如的R5T4Si10和R2T3Si5類型，此類三元矽化物在CDW的相變研究中發現，磁性和電性…等巨觀性質，在ＣＤＷ相變溫度發生時有相當顯著的變化。在本篇博士論文中，我們利用穿透式電子顯微鏡（TEM）的各種功能：電子繞射和暗場成像技術…等，詳細討論了兩種不同的化合物三維晶體結構的材料：Dy5Ir4Si10 和Lu2Ir3Si5 CDW形成的主因，以及相變過程的詳細記錄。
研究稀土過渡金屬矽化物Dy5Ir4Si10時，發現將樣品冷卻到低於CDW的相變溫度以下，觀察電子繞射圖形時，發現了超晶格沿c軸方向的超晶格繞射點。此外，利用聚焦電子繞射（CBED）的觀察發現，室溫結構沿c軸方向對稱性會在低溫的新結構下消失。這種不尋常的行為也將被討論的詳細特性。
同樣的低溫技術之下，我們研究電荷密度波（CDW）在Lu2Ir3Si5超晶格繞射點和暗場成像的產生，確認CDW相變的存在。最有趣的是，在低溫的狀態下發現CDW相與不發生CDW的正常相（Normal State）會共存。有別所有典型CDW的系統此現象並不會發生，並且發現CDW相與正常相（Normal State）的面積比率的會隨著溫度而改變，似乎為典型一階相變的特性。

關鍵字

電荷密度波 ； 穿透式電子顯微鏡 ； 電子繞射 ； 暗場成像 ； 超晶格 ； 聚焦電子繞射

並列摘要

The charge-density wave (CDW) is a ground state with broken translational symmetry of metals, brought by electron-phonon interactions. It was recognized that highly anisotropic band structures are important in leading to these ground state. And the ground states are coherent superposition of electron-hole pairs, and result in a periodic space variation of charge density as name. The CDW ground state is also accompanied by the opening of energy gap at the Fermi surface. Since the energy gap forms within the former conduction band, a 1D metal would be expected to become insulating in CDW state. Rare-earth transition-metal ternary silicides with three-dimensional crystallographic structures, such as the R5T4Si10 and R2T3Si5 types, have been shown to exhibit CDW phase transitions with remarkable anomalies observable in the thermal and electrical transport measurements. In this thesis, I describe the experimental results of probing CDW formation in two different compounds of 3-D materials: Dy5Ir4Si10 and Lu2Ir3Si5.
The CDW study in Dy5Ir4Si10
The tetragonal rare-earth transition-metal silicide system, R5T4Si10, where R is Dy, Ho, Er, Tm, and Lu, and T = Ir and Rh, with seemingly three-dimensional crystallographic structure, has been shown to exhibit fascinating charge density wave (CDW) phase transitions, a phenomenon found largely in low-dimensional systems. In this study we report the investigations of CDW in Dy5Ir4Si10 at different temperatures using transmission electron microscopy (TEM) techniques including electron diffraction and dark-field imaging.
Superlattice diffraction spots along c-axis were observed in the electron diffraction pattern when the sample was cooled below the CDW transition temperature (TCDW ~ 200K), indicating the presence of incommensurate CDW state with the modulation wave vector of . CDW become commensurate with further cooling below ~ 160K. Configurations of CDW dislocations convincingly show that the CDW phase transition is accompanied by a concomitant cell-doubling crystallographic structural phase transition. Furthermore, symmetry breakdown along c-axis observed by convergent beam electron diffraction (CBED) gives rise to two different type of CDW domains. Detailed characteristics of this unusual behavior will also be discussed.
The CDW study in Lu2Ir3Si5
We report the investigation of charge density wave (CDW) in Lu2Ir3Si5 by electron diffraction and dark-field imaging using superlattice diffraction spots. The CDW state is confirmed by the presence of superlattice reflections. Most interestingly, the CDW state at low temperatures is found to be electronically phase-separated with the coexistence of CDW domains and low-temperature normal phase domains. Upon change of temperatures, unlike other typical incommensurate CDW systems in which commensurability varies with temperatures, we find that commensurability remains unchanged in the present case and the predominant change is in the redistribution of the area ratio of the two coexisted phases, which is clearly revealed in the dark-field images obtained from the CDW superlattice reflections. The electronic phase separation in the CDW state of Lu2Ir3Si5 is unprecedented in CDW systems, and its temperature dependence is also anomalous.

並列關鍵字

charge density wave ； transmission electron microscopy ； Dy5Ir4Si10 ； Lu2Ir3Si5 ； superlattice ； dark-field ； electron diffraction.

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