利用剖面式掃描穿隧電子顯微鏡量測原子解析的不相稱電荷有序態

Abstract

高溫超導已經被發現超過三十年並至今吸引了相當多的研究興趣。然而，由於其本身的不均勻性及複雜的相態，這些材料仍有相當大的研究空間。而要更進一步理解銅氧超導的機制，實空間且高能量解析度的研究是不可或缺的。在近期的研究中，銅氧超導中的電荷有序態(charge order)被認為是主要與超導競爭的重要因子。釔鋇銅氧(YBa2Cu3O6+x)，作為高溫超導的里程碑，由於缺乏實空間直接量測電荷有序態的技術，仍有許多不了解之處。 在本研究工作中，藉由剖面式掃描穿隧電子顯微鏡，我們量測到釔鋇銅氧的銅氧面與銅氧鏈的原子解析的電子結構。並且，我們在銅氧的原子面上觀察到不相稱的電荷有序態。我們也同時觀察到電荷有序態區域與超導區域在b-c面上分別共存的現象。除此之外，我們直接觀察到沿c軸的電荷密度波，指出穩定的電子有序態存在於銅氧面與銅氧鍊，並在這兩層有一致的振幅、波長，及能態密度。同時，我們的觀察指出，電子在相鄰層的躍遷過程不只侷限在兩個銅氧面之間，並且也存在於銅氧面與銅氧鏈之間。我們的研究方法提供了探索電荷有序態與超導機制的關聯性的一個新方向。

High-temperature superconductors have been discovered for over 30 years and have drawn considerable research interest. However, due to their intrinsic inhomogeneity and the complex phase diagram, the widely accepted microscopic theory remains a mystery. To fully understand the cuprate superconductors, real-space studies with high energy resolution is strongly required. Recently, charge-ordering (CO) was identified as a key factor in competition with superconductivity in cuprate superconductors. YBa2Cu3O6+x, as a benchmark high-temperature superconductor, remains poorly understood due to the lack of the spatially-resolved technique probing the CO regime. In this thesis, we correlate the atomically-resolved electronic structure of CuO chains and CuO2 planes by using cross-sectional scanning tunneling microscopy. In addition, we observed the incommensurate CO on the Cu-O terminated surface. The separate coexistence of the CO domain and superconducting phase on the b-c plane in YBCO is mapped. Furthermore, our direct observation of CO states along the c direction indicates the formation of CO involves both chain and plane layers with comparable wave amplitude, wavelength, and local density of states. Meanwhile, our observation highlights the hopping process of electrons into the neighboring layers is not only through CuO2 bilayers, but also CuO chain layers. This approach potentially leads a new direction to explore CO correlations in the fundamental mechanism of superconductivity.