Pb/PbO暨Sn奈米顆粒聚合體的超導及磁性探討

Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/59194

Title:	Pb/PbO暨Sn奈米顆粒聚合體的超導及磁性探討
Authors:	徐健綱;Hsu,Chien-Kang
Contributors:	物理學系
Keywords:	奈米顆粒;磁性;超導;magnetism;nanoparticles;superconductivity
Date:	2013-01-25
Issue Date:	2013-03-25 16:06:04 (UTC+8)
Publisher:	國立中央大學
Abstract:	本篇論文包含兩組實驗。對第一部分，我們使用熱蒸鍍冷凝法製作了Pb/PbO核殼結構奈米顆粒樣品，經由X光繞射譜圖以及AFM判定Pb核的直徑為5 nm，PbO殼層的厚度為1.2 nm。在Pb/PbO核殼結構奈米顆粒系統中，觀察到超導以及鐵磁自旋極化效應的共存。未經壓合的樣品的超導臨界磁場高達塊材鉛的30倍，推測可能不是奈米顆粒真實的超導臨界磁場，而是因為僅有一部份的磁力線可以穿透到奈米顆粒中，奈米顆粒實際上所感受到的磁場比塊材小很多。未經壓合的樣品的超導臨界溫度為6.60 K，超導臨界溫度隨著壓合密度上升而回升到塊材鉛的數值，推測是由於量子尺寸效應的影響減弱所致。奈米顆粒的磁矩隨著壓合密度上升整體呈現下降的趨勢，推測是由於顆粒間的交互作用造成顆粒表面的費米能階降低，使得表面與內部的電荷重新分布所造成。當壓合密度高於13%後，我們更換了壓合模具，樣品接觸到大氣而進一步氧化，氧化後經由X光繞射譜圖判定樣品的Pb核心的直徑為3.5 nm，PbO殼層的厚度為1.25 nm。由於氧化程度加重使得磁化強度上升，可能原因為表面氧化層的氧空缺增加，並且同時Pb核心的直徑降低所致。氧化後樣品的TC從7.01 K下降到6.74 K，臨界溫度下降了約3%，這是由於氧化程度加重後，Pb核心的粒徑由4.9 nm變為3.5 nm所致。在第二部份，我們製作了錫奈米顆粒，經由共同體機函數判定其平均粒徑為9 nm。超導臨界溫度為3.98 K，相比於塊材提高了約7 %，而超導臨界磁場為3.88 kOe，為塊材的13倍。在此9 nm錫奈米顆粒的系統中也觀察到了超導與鐵磁的共存。對此具有鐵磁性的9 nm錫奈米顆粒而言，布洛赫T3/2定律不再適用，飽和磁化強度隨T1.8變化。中子散射的實驗結果顯示9 nm錫奈米顆粒具有鐵磁性，原子的磁矩的方向是沿著a-c平面，與a軸的夾角為10度，6 K時的有效磁矩為0.38 ，而1.65 K時的有效磁矩為0.43 ，顯示有一額外的自發磁矩伴隨著超導相轉變而出現，我們認為這是由於在奈米顆粒的系統中，超導電子對可能以三重態耦合在一起，使得超導電子對具有淨磁矩所導致。There are two parts in this thesis. In part I, we fabricated Pb/PbO core-shell nanoparticles employing the gas-condensation method with 4.9 nm Pb core and 0.5 nm PbO shell(sample I). TC =6.60(4) K, which is 7% lower than the TC = 7.19 K of bulk Pb. A 30-fold increase in the critical magnetic field is also found. Ferromagnetism and superconductivity are found to coexist at T<TC. TC of sample I is enhanced which may be understood as the average single-electron level spacing decreases upon increasing f. A reduction in the mean particle moment results when the packing fraction of the assembly is increased. This maybe resulted from the smaller Fermi energy of surface atoms.The ratio of Pb:PbO becomes 22:76 at f=74%. The diameter of Pb core reduces to 3.5 nm, and the thickness of PbO shell increases to 1.25 nm (sample II). MS of sample II is significantly larger than MS of sample I, which may be linked to the smaller Pb core and/or thicker PbO shell of sample II. TC of sample II is lower than TC of sample I, which maybe a result of the smaller Pb core of sample II.In part II, we report on the observations of spontaneous magnetic moments in the normal as well as in the superconducting states of a 9 nm Sn nanoparticle assembly, through x-ray diffraction, magnetization, ac magnetic susceptibility and neutron diffraction measurements. TC =3.98 K, which is 7% higher than the TC = 3.72 K of bulk Sn. An unexpectedly large saturation magnetization of 1.04 emu/g was observed at 5 K. The MS(T) curve can be described by Bloch’s law, but with an exponent of ?=1.8. A magnetic moment of <?Z> = 0.38 ?B, lies in the a-c plane pointing 10o away from the a-axis, develops after cooling from 260 to 6 K. Surprisingly, an addition magnetic moment of <?Z> = 0.05 ?B develops upon entering the superconducting state.
Appears in Collections:	[Graduate Institute of Physics] Electronic Thesis & Dissertation

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