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研究生:李凡尼
研究生(外文):Agus Rifani
論文名稱:金銅合金金屬叢集(N=38)的磁性性質研究
論文名稱(外文):Magnetism in 38-atom gold-copper clusters
指導教授:賴山強
指導教授(外文):San-Kiong Lai
學位類別:碩士
校院名稱:國立中央大學
系所名稱:物理研究所
學門:自然科學學門
學類:物理學類
論文出版年:2012
畢業學年度:100
語文別:英文
論文頁數:26
中文關鍵詞:金屬叢集磁性
外文關鍵詞:metallic clusterMagnetic properties
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我們使用第一原理的方法研究AunCu38-n.的結構和電子性質.首先,我們利用 [P. J. Hsu and S. K. Lai, J. Chem. Phys. 124, 044711 (2006)] 發展的演算法去找出合金叢集在溫度為零的最低能量結構.這個演算法使用的是Gupta勢能搭配基因演算法併能量谷跳躍法. 雖然此演算法的可信度在文獻被證實上非常高,但因為Gupta勢能是一個使用經驗法則得到近似方法,無法提供電子方面的性質.所以我們接著把上述得到的最低能量結構放到第一原理的軟體中作電子結構的後續計算.在電子結構部份,我們使用的方法為密度泛函理論,選用的是高斯型的基底組.結果發現初始結構因為加上了電子的影響會有構形上的扭曲變形. [P.J. Ballester and W.G. Richards, J. Comput. Chem. 28, 1711 (2007)] 發展出的超快構形識別技術可以幫助我們有效的區分出初始結構和因為電子所造成扭曲變形後的差異.這個差異反映並幫助我們理解電子在構形扭曲過程中所扮演的錯綜複雜角色.另外,電荷以及自旋電荷在叢集中的分佈以及自旋態密度的分析可以幫助我們理解並解釋某些叢集產生不尋常的淨磁矩發生的背後機制.此外,化學的分子點群理論成功的解釋某些高度對稱的結構產生淨磁矩的原因.
We present first-principles theoretical calculations of the structural and electronic properties of bimetallic clusters AunCu38-n. For the former, we first appeal to the lowest energy configurations of AunCu38-n (for different n) that we determined previously from an accurate and reliable optimization algorithm [P. J. Hsu and S. K. Lai, J. Chem. Phys. 124, 044711 (2006)] which was used in conjunction with an empirical many-body potential, whereas for the latter we use a linear combination of Gaussian-type orbitals within the Kohn-Sham density functional theory. The above lowest energy structures are input as initial ionic configurations and employed in the spin unrestricted density functional theory calculations. A thorough comparison between the ionic structures obtained from the latter and those initial ones from the optimization algorithm is further effected by the ultra-fast shape recognition technique [P.J. Ballester and W.G. Richards, J. Comput. Chem. 28, 1711 (2007)] widely applied in chemistry for structural characterization. The disparity in cluster geometry between these two sets of ionic structures sheds light on the intricate role of valence electrons in their spatial distribution on the atomic sites in clusters. This information on charge and spin density dispersions together with spin-polarized density of states unveil the mystery of the net magnetic moments which are predicted uncommonly in some of the clusters of AunCu38-n. An explanation is offered of this unexpected magnetism in the context of the symmetry of ionic structures.
I. INTRODUCTION 1
II. THEORY 2
2.1. DENSITY FUNCTIONAL THEORY 2
III. RESULTS AND DISCUSSION 3
3.1. ATOMIC STRUCTURES 3
1.1. CHARGE DENSITY AND SPIN CHARGE DENSITY DISTRIBUTIONS 5
1.2. INTERPRETATION OF THE MAGNETIC MOMENTS 17
IV. CONCLUSION 24
ACKNOWLEDGMENTS 25
REFERENCES 25
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29 We should emphasize that the high structural similarity for the majority of clusters between the PTMBHPGA and DFTM is merely a reliability check of the Gupta potential and does not, however, imply that the relaxed structures of AunCu38-n in DFTM are global minima at the high-level of all-electron DFT calculations. This issue on the use of empirical potential has been discussed also in recent communications for AgnCu40-n [2] and for silver-copper clusters of a much larger size having anti-Mackay icosahedra of 45, 127, 279, 521,..,atoms which correspond to compositions Ag32Cu13, Ag72Cu55, Ag132Cu147, Ag212Cu309,..,respectively [8]. The high-level DFT calculations, in principle, can be carried out for pure metallic clusters [10-12], but is nonetheless a formidable task when the same strategy is applied to BCs of the size considered here.
30 B.I. Dunlap, Phys. Rev. A 41, 5691 (1990).
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32 F. Albert Cotton, Chemical Applications of Group Theory, 3rd ed., John Wiley & Sons, New York,1990.
33 The Amsterdam Density Functional (ADF) software is described in the web site: http://www.scm.com/Doc/Doc2010/Background/References/page4.html.
34 In the event that the point group theory can not identify the symmetry of the cluster structure, the ADF method would fail and we no longer be able to examine further the structure of the energy levels obtained in DFTM.
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