LaTe2-δ中电荷密度波及空位序调制结构扫描透射电子显微镜直接观察

Cs-corrected HAADF-STEM observations on the structural modulations caused by charge density wave and Te-vacancy ordering in LaTe_(2-δ)

高角环形暗场扫描透射电子显微技术(HAADF-STEM),不仅可以分辨晶体结构的空间原子分布,还可以直接显示晶体内原子种类的分布和缺陷结构;针对强关联材料,可以明确区别电荷密度波、空位序、离子序调制,对理解微观结构缺陷、局域对称性破缺具有重要意义.本文首先对稀土碲化物LaTe_(2-δ)体系结构调制进行了系统的透射电子显微镜分析,指出LaTe_(2-δ)中,除了q=(1/2)a*的公度电荷密度波调制,受缺陷掺杂调节的系列调制波矢为q=(1/2-α)a*的非公度电荷密度波调制,还存在调制波矢q_2=1/5(3a*+b*CDW)的超结构.高精度原子分辨HAADF-STEM实空间直接观察澄清,q_(CDW)和电荷密度波元激发伴随的单层Te原子层晶格畸变密切相关,q2调制结构的形成归结为单层Te原子层中的Te空位有序.HRTEM结果分析表明,LaTe_(2-δ)中,空位序的形成完全抑制了该体系的电荷密度波转变,两种调制结构以相分离的形式存在.

Microstructural features play a critical role for the understanding of the essential properties of novel functional materials and new devices. Atomic-resolution scanning transmission electron microscopy（STEM） is invaluable for determining the average atomic structure and local structural defects. For strongly correlated electron materials, STEM can be applied to distinguish the structural modulation caused by charge density wave（CDW）, chemical ordering（vacancy or impurity atoms）. RTe（2-δ）（R=La, Ce） compounds have attracted recent attention due to their effective low dimensionality. The materials play host to a CDW state above room temperature and can be described in terms of a modulated Cu2Sb-type structure（P4/nmm） based on alternating layers of square-planar Te sheets and a corrugated RTe slab. Furthermore, pressure-induced superconductivity in CeTe（1.82） with TC of 2.7 K has been reported, suggesting that the nonstoichiometric Te defects are correlated to superconductivity in this material system. Here, we report the study of the structural modulations in LaTe（2-δ） using STEM. The LaTe（2-δ） single crystal was grown by self-flux technique. The TEM samples used in the present study were prepared by crushing the wellcharacterized single crystal, and then the resultant suspensions were dispersed on a holey carbon-covered Cu grid. Electron diffraction experiments were performed in the FEI Tecnai F20 microscope, and HRTEM and high angle annular dark field（HAADF） STEM were performed in the JEOL ARM200F equipped with double aberration correctors and cold field emission gun at room temperature. The experiment result revealing the charge density wave in LaTe（2-δ） can be tuned by the Te content; the structural modulation correlated with charge density wave can be characterized by a modulation wave vector of q（CDW）=（1/2-α）a＊, where α is the incommensurate parameter determined by the chemical composition. Our experiment data demonstrate that the Cs-corrected HAADF-STEM image can directly reveal the atomic displacements in the Te plane due to electron-phonon coupling. Detailed analysis suggests that the Te atomic displacements adopt an incommensurate wave-pocket structure along each Te-chain with a long periodicity determined by the CDW incommensurability. In addition to the q（CDW）=（1/2-α）a＊ CDW modulation, a superstructure with the vector q2=1/5（3a＊＋b＊） has been also observed in some regions. In previous study, this modulation was proposed to be correlated with CDW instability, however, recent experimental and theoretical analysis on the electronic structure（FS） shows that there is no such nesting wave vector could be identified. In this paper, our Cs-STEM observations directly demonstrated that the q2 superstructure actually originates from an imperfect stoichiometry in this layered system. We proposed a 5-（1/2） ×5-（1/2） supercell associated with the Te vacancy ordering with the chemical composition of LaTe（1.85） based on detailed STEM data analysis. From HRTEM images, it is also noted that the CDW modulation（q（CDW）） totally disappears in the region with Te vacancy ordering, suggesting that the CDW can not coexist in the crystals with the Te vacancy ordering. The possible correlation between the ordering of the nonstoichiometric Te defects and the superconductivity in this material system needs to be further investigated.