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

翁世宇(Shih-yu Weng)

指導教授 ： 李文献

國立中央大學/理學院/物理學系/碩士(2007年)

摘要

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。

關鍵字

錫 ； 超導 ； 奈米 ； 臨界溫度

並列摘要

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.

並列關鍵字

Tin ； nanoparticles ； critical temperature ； superconductivity

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