博碩士論文 942202008 詳細資訊


姓名 翁世宇(Shih-yu Weng)  查詢紙本館藏   畢業系所 物理學系
論文名稱 微粒間交互作用對奈米錫超導參數的影響
(The effects of interpartical interplay on superconductivity of Tin nanoparticles)
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摘要(中) 以熱蒸鍍法製備的兩組奈米錫微粒,經X光繞射鑑定粒徑為3 nm與6 nm。在1.8 K且零外加磁場環境下,測定3 nm與6 nm樣品之交流磁化率,結果分別顯現為正常態與超導態。利用壓合的方式以改變樣品微粒間距,發現3 nm錫樣品在微粒間距小於2 nm以下出現邁斯納效應;而在微粒間距趨近於零時,6 nm錫樣品的邁斯納效應為塊材樣品的15.1倍。邁斯納效應隨微粒間距的減小而增強,其可能之機制是以穿隧距離相關的直流約瑟夫遜效應為主。
我們定義微粒間距除上微粒直徑為相對間距,以探討超導臨界溫度的變化。兩組樣品的相對間距大於0.9時,3 nm微粒並未呈現超導特性;6 nm微粒可觀測到邁斯納效應與較塊材為高的超導臨界溫度。而相對間距介於0.9至0.2之間時,臨界溫度隨相對間距減小而降低,推測是聲子態密度受微粒間交互作用的影響。當微粒相對間距小於0.2以下,臨界溫度逐漸升高,可能與平均單電子能階分離能隙δ減小,安德森效應弱化有關。此外,兩組樣品皆具有電子自旋極化的現象。
摘要(英) Two tin nanoparticles samples were fabricated via thermal evaporation method and identified their particle diameters as 3 nm and 6 nm by X-ray diffraction scheme. AC magnetic susceptibility measurements reveal the normal and superconducting state of 3 and 6 nm Sn samples at 1.8 K with zero applied magnetic field respectively. However, the Meissner effect appears in 3 nm Sn while interparticle separation reduces to 2 nm. At the end, 15.1 times diamagnetism signal from susceptibility measurement is observed between 6 nm and bulk sample as interparticle separation is almost zero. The origin of enhanced Meissner effect may be mainly contributed by DC Josephson effect due to reduction of tunneling distance.
We define the relative separation as interparticle separation divided by particle diameter in order to study the variation on critical temperature of 3 nm and 6 nm Sn samples. As the relative separation is greater than 0.9, no superconducting signal was found in 3 nm Sn from susceptibility measurements; the TC of 6 nm Sn is higher than bulk one. While the relative separation lies between 0.9 and 0.2, the TC of two samples are depressed. It may be associated with the modification of phonon density of state. Nevertheless, the TC increases as the relative separation becomes below 0.2. It may be attributed to weakened Anderson effect. In addition, the polarization of electron spins exists in both samples.
關鍵字(中) ★ 錫
★ 超導
★ 奈米
★ 臨界溫度
關鍵字(英) ★ Tin
★ nanoparticles
★ critical temperature
★ superconductivity
論文目次 論文摘要…………………………………………………… I
Abstract…………………………………………………… II
致謝………………………………………………………… III
目錄………………………………………………………… IV
圖目與表目………………………………………………… VI
第一章 金屬錫超導性質概論……………………………………1
1-1 塊材錫的基本物理性質………………………………………1
1-2 已知錫超導臨界溫度研究結果………………………………2
1-3 實驗目的………………………………………………………5
第二章 實驗樣品與儀器介紹……………………………………6
2-1 奈米樣品製備…………………………………………………6
2-2 奈米微粒壓合模具……………………………………………8
2-3 交流磁化率實驗………………………………………………9
2-4 奈米粒徑分析………………………………………………10
2-5 樣品成份分析………………………………………………22
第三章 超導理論…………………………………………………29
3-1 奈米微粒平均間距估算……………………………………29
3-2 奈米微粒超導相變磁化率描述……………………………31
3-3 超導態的自由能與熵..……………………………………33
3-4 超導體的約瑟夫遜隧道……………………………………35
第四章 超導抗磁相變與臨界溫度…………………………………39
4-1 塊材樣品與Sn_A樣品交流磁化率實驗結果………………39
4-2 Sn_B樣品交流磁化率實驗結果……………………………51
4-3 交流磁化率實驗結果擬合分析………………………………61
4-4 超導抗磁磁化率變化探討……………………………………62
4-5 臨界溫度變化探討……………………………………………70
第五章 低溫電子自旋極化…………………………………………79
5-1 沿外加磁場排列的自旋極化磁矩……………………………79
5-2 自旋極矩磁化強度隨外加磁場變化關係……………………88
第六章 結論…………………………………………………………93
參考文獻………………………………………………………………95
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指導教授 李文献(Wen-Hsien Li) 審核日期 2007-7-16

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