本論文主要利用軟X光散射來研究鉀鐵硒超導體的鐵空缺有序性。鉀鐵硒超導體為新發現的超導樣品，一些文獻也指出，超導現象與反鐵磁性鉀共存於鐵硒超導中。另一方面，鐵空缺的有序性與反鐵磁的有序性有密切關聯。因此，研究鐵空缺的有序性也是鉀鐵硒超導的重要課題。然而在眾多研究之中，軟X光散射的研究卻是少之又少。軟X光散射有著對倒晶格空間的高解析力。因此，我們用來探討鉀鐵硒超導的有序性。而在研究中，我們在原二環繞射系統上加了第三為度的旋轉以供不同的繞射平面可以同時量測，並也提供了高溫與低溫兩個不同的量測範圍。
在我們的研究中，我們藉著鐵L$_{2,3}$-edge的吸收結果得知樣品中的鐵帶二價正離子。也就是說，鐵原子是以的Fe$^{2+}$形態存在於K$_{2}$Fe$_{4}$Se$_{5}$樣品中。更重要的是，我們也藉著軟X光散射的研究，發現散射向量(3/5, 1/5, 2)強度在545K時開始減弱，直到553K時散射強度消失。同時也從散射強度的半高寬(HWHM)中，發現了在549K有最小值，表現出在高階相變的特性。所以鐵硒超導鐵空缺的相變溫度點在549K。另一方面，我們也在低溫的量測中，發現在35K有一個轉折點，呈現出多個有序參數在彼此競爭。因著此現象很產生的溫度很接近超導相變溫度(T$_{c}$)，我們認為此現象與超導的特性有很大的關聯。

K$_{2}$Fe$_{4}$Se$_{5}$ is a newly discovered compound with superconductivity. It was found that the antiferromagnetism coexist with the superconductivity and is also related to the iron vacancy order. This gives a great opportunity to study the mystery of superconductivity. However, among the researches of iron vacancy order, there is no study using soft X-ray diffraction on K$_{2}$Fe$_{4}$Se$_{5}$ with high q resolution.
In this thesis, we present the study of iron vacancy order of the K$_{2}$Fe$_{4}$Se$_{5}$ superconductor by using soft X-ray absorption and diffraction. Fe L$_{2,3}$-edge absorption data indicate that Fe atoms lose two electron and form Fe$^{2+}$ in K$_{2}$Fe$_{4}$Se$_{5}$. Furthermore, the diffraction results of momentum transfer (3/5, 1/5, 2) show that, as the temperature increases, the diffraction intensity starts to decrease at 545K and vanishes above 553K. In addition, the half width half maximum (HWHM) minimizes at 549K. In other words, the transition temperature of iron vacancy order is 549K. We also found a enhancement of iron vacancy order at 35K, a temperature close to the transition temperature(T$_{c}$) of superconductivity. That is, our results indicate competing orders in K$_{2}$Fe$_{4}$Se$_{5}$ superconductor.

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