臺灣博碩士論文加值系統

本實驗是以濺鍍方式在Si(100)基板上製備Nb單層膜、Nb/Co/Nb與Co/Nb/Co三層膜，並從它們在電阻性及磁化特性上的表現來探討其超導性質，主要使用的量測儀器包括低溫電阻量測系統及SQUID磁量儀。
我們觀察到Nb單層膜的超導溫度TC會隨其厚度dNb的增加而上升；而在dNb≧4000A時，樣品的TC值會逐漸趨近Nb塊材的TC值(9.2K)，此種現象與二維薄膜的尺寸效應有關。而對於Nb300A/CodCo/Nb300A三層膜，我們原想觀察其TC值隨dCo的增加而振盪的現象，以證明在此結構中是否存在著理論上所預測的π相位波函數，但因實驗條件(濺鍍順序不同)的影響，導致觀察上的困難。
另外，在Co20A/NbdNb/Co20A及Co100A/NbdNb/Co100A三層膜中，其TC隨dNb變化的趨勢與Nb單層膜類似，但在dNb較薄時，因為Co層對Cooper電子對的拆散效應在此時會較顯著，導致了樣品超導性的降低，因此其TC在dNb較薄時會往低溫處偏移。
而對於各組樣品在外加磁場下的磁化曲線圖，我們發現隨著dNb (/dCo)的增加，會產生由鐵磁(/超導)性為主的磁滯(/超導磁化)曲線，連續的轉變成由超導(/鐵磁)性為主的超導磁化(/磁滯)曲線。我們假設此種改變可視為超導層的Meissner效應(逆磁性)與鐵磁的鐵磁性磁矩個別產生的磁化量所疊加而成的結果。我們已試著用一種模擬疊合的方式，來驗證以上的假設是否合理及正確。模擬的結果似乎可以得到一些類似的曲線圖形來定性說明我們的假設之適切程度。

Single Nb films, Nb/Co/Nb and Co/Nb/Co trilayers were grown on Si(100) substrates at room temperature by dc sputtering method. We discussed their superconductivity from the resistive and magnetic properties. The main measuring instruments we used were low temperature resistance measuring system and SQUID magnetometer.
We observed that with increasing dNb, the value of superconducting transition temperature Tc increases monotonically, before eventually close to the terminal value of Tc~9.2K for pure bulk Nb as dNb≧4000A. This was related to the finite-size effect of 2D thin-films. For the Nb300A/CodCo/Nb300A trilayers, we wanted to verify whether there is oscillation in Tc with increasing dNb, a signature of the π-phase states in this structure predicted in theories. Unfortunately, fluctuation in Tc due to growth condition (sputtering sequence etc.) during deposition process made it difficult for our observation.
Otherwise, the dependence of Tc on dNb in Co20A/NbdNb/Co20A and Co100A/NbdNb/Co100A trilayers was similar to single Nb films. For thinner dNb, the Tc shifted to lower temperature because of the stronger Cooper pair-breaking effect of Co layers, which led to the reduction of superconductivity.
For the hysteresis loops in the whole series of samples, we found that when dNb (dCo) increased, the hysteresis loops changed gradually from ferromagnetic (superconductive) ones to superconductive (ferromagnetic) ones. We showed these transitions could be obtained from superposition of the magnetization of Meissner effect (diamagnetism) and the ferromagnetic moment in the superconducting layers and ferromagnetic layers, respectively. We have used a simulation method to confirm the superposition above was reasonable. From the results of our simulation, we could get some analogous hysteresis loops that could explain the applicableness of our superposition qualitatively.

誌謝·····························Ⅰ
中文摘要···························Ⅲ
Abstract···························Ⅳ
目錄·····························Ⅴ
圖列次··························· VII
表列次··························· XII
緒言····························· 1
簡介····························· 3
第一章 超導理論······················· 7
§1-1 Meissner效應·····················7
§1-2 Cooper電子對·····················9
§1-3 二流體模型····················· 11
§1-4 BCS理論·······················13
§1-5 超導量子干涉···················· 16
§1-6 第二類超導體···················· 20
§1-7 π相位振盪理論··················· 24
第二章 實驗原理·······················28
§2-1 薄膜形成機制···················· 28
§2-2 濺鍍原理······················ 30
§2-3 MOKE(磁光科爾效應) ·················32
§2-4 X-ray繞射原理····················34
第三章 實驗儀器·······················37
§3-1 探針式膜厚儀···················· 37
§3-2 濺鍍系統······················ 39
§3-3 四點量測系統···················· 43
§3-4 SQUID························48
第四章 實驗結果·······················50
§4-1 薄膜品質及結構之觀察與分析············· 50
§4-2 Nb單層膜之超導性·················· 55
§4-3 Co層在樣品中的磁性(室溫MOKE之量測)········· 60
第五章 Nb/Co/Nb三層膜之超導性的分析·············62
§5-1 電阻性上的表現 ···················62
§5-2 磁性上的表現···················· 73
第六章 Co/Nb/Co三層膜之超導性的分析·············81
§6-1 電阻性上的表現··················· 81
§6-2 磁性上的表現···················· 95
第七章 結論 ························ 109
附錄A (濺鍍流程) ······················111
附錄B (膜厚測量及石英膜厚測定器的校正) ···········113
參考文獻·························· 116

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