高溫超導體之順磁邁斯納效應及微波元件特性研究

Abstract

本論文乃針對高溫超導體為研究主題, 其中包含利用溶液─凝膠法製備高 純度試片, 對電子結構, 磁性與微波特性的研究及設計製作微條式共振器 並量測之。立用X-光光子激發光譜儀對Bi0.8Pb0.2SrCaCu2Ox 的分析結果 顯示試片中(Cu+)/(Cu2+)的比例隨110K相的含量的增加而增加。根據這個 趨勢提出一個缺陷模型, 它能夠合理解釋國外研究群的一些實驗結果。譬 如, 降低氧分壓可促進110K相的形成, 及經由高解析度穿透式電子顯微鏡 的觀察可觀察到鉍系超導試片中同時存在的銅與氧的缺陷。此外, 氧位置 電洞雙量體化(dimeration)的機制亦被提出來解釋為何(Cu+)/(Cu2 +)的 比例隨著110K相的增加而增加的原因。論文中亦分別提出異方性與均方性 的理論模型來解釋磁性量測中所發現的順磁邁斯納效應。此二模型均基於 一個觀念, 就是此一異常效應是由於晶粒內的超導電流作用在晶粒間區域 所引起的。異方位模型適合於解釋有優選方向性晶粒組織的試片。而均方 位模型則適合於定量描述晶粒混亂排列的試片。此二模型合理的說明順磁 邁斯納效應是晶粒特性超導體(granular superconductors)的寄生現象。 因此, 如果想利用邁斯納訊號估算超導相的體積分率時, 此晶粒間效應應 被納入考慮, 尤其是在低磁場量測時。高溫超導體的微波特性亦在論文研 究。當超導模的模厚與穿透深度相當或甚至更小時, 其所量到的表面阻抗 並非本質表面阻抗而是有效表面阻抗。 This research was performed on the high-Tc superconductors ranging from the preparation of high-purity samples to the investigation their electronic structures, and magnetic properties then to the characterization of microwave pro- perties and final to the fabrication of microstrip resonators. The analysis of X-ray photoemission spectroscopy on the surface of Bi0.8Pb0.2SrCaCu2Ox revealed that the ratio of(cu +1)/(Cu+2) increased with the content of the 2223 phase (i.e., 110K phase). According to this trend, a defect model was proposed; The dimerization of holes in oxygen sites, being a possible mechanism for high-Tc superconducting phase, may be a response for the increase of the ratio of (Cu+1)/(Cu+2). The paramagnetic Meissner effect was interpreted theoretically by anisotropic and isotropic models, respectively. Both of these two models were conceptulized that the abnormal Meissner signals were induced by the action of intragranular supercurrents on the inteeergranular region. The anisotropic model was suitable to explained the textured samples, while the isotropic one could quantitatively analyze the sample with randomly grain-orientedstructure. The intergranular effect was demonstrated by these models to parasitically exist in granular superconductors. The microwave characteristics of high-Tc superconducting thin films were investigated in this work. The effective surface of superconducting film was investigated for the film thickness that is comparable with or even less than the field penetration depth.