博碩士論文 992202015 詳細資訊




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姓名 林承風(Cheng-Feng Lin)  查詢紙本館藏   畢業系所 物理學系
論文名稱
(Spin transport calculation for thiol-ended single-molecule magnetic junction)
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摘要(中) 我們利用第一原理計算方法,先由以密度泛函理論為基礎的Quantum ESPRESSO 程序優化單一分子隧結 Ni/1,4-alkanedithiol(ADT)/Ni的結構,然後增加右邊Ni adatom與電極間的距離,進行結構優化,再重複以上步驟來模擬實驗上拉伸的過程,直到找到此單一分子隧結模型的斷裂點。我們接者再由應用了非平衡格林函數結合密度泛函理論與廣義梯度近似方法來計算在斷裂點時的projected density of states與transmission spectrum. 與應力相關的自旋極化穿隧頻譜可以透過Ni adatom的拓寬的自旋向上PDOS結合高低不平的自旋向下PDOS來了解。因為中間的以σ鍵為主的ADT分子是導電性較差的,以及由透過Ni adatom的直接穿隧主導穿隧機率。我們進一步計算了在斷裂點的電流-電壓特性以及磁阻率。令人驚訝的,在拉伸下對於導電性較差的以σ鍵為主的ADT單一分子接面的磁阻率值可達到至200%。
摘要(英) We employed the density-functional theory (DFT) within the generalized approximations (GGA) in the PBE form, to simulate the Ni/1,4-alkanedithiol(ADT)/Ni single-molecule magnetic junction under stretching process. The junction is stretched by increasing the distance between two Ni nanowires in small steps, optimize it again, and continue to do so, until the junction breaks down. The First-principle spin transport calculation is based on the non-equilibrium Green’s functions formalism and the DFT approach. The strain dependence of spin-polarized transmission spectra can be understood by the broad spin-up PDOS combined with the spiky spin-down PDOS of Ni adatom. This is because the central σ-saturated ADT molecule is less conductive and the transmission probability is dominated by the direct tunneling via the Ni adatom. We further calculated the I-V characteristic of breaking point and get the magnetoresisitance (MR). Surprisingly, the giant value about 200% and the bias-induced of MR can be found in this less conductive σ-saturated Ni/1,4-alkanedithiol(ADT)/Ni single molecular junction under the stretching.
關鍵字(中) ★ 單分子通道
★ 磁阻
★ 自旋傳輸
★ 第一原理計算
關鍵字(英) ★ single molecular junctions
★ magnetoresistance
★ spin transport
★ first-principles calculation
論文目次 Chapter 1 Introduction 1
Chapter 2 Theory 7
2.1 Density Function Theory 7
2.1.1 Born-Oppenheimer Approximation 8
2.1.2 The Hohenberg-Kohn Theorem 9
2.1.3 The Kohn-Sham Equation 11
2.1.4 Exchange-Correlation Energy Functionals 16
Local Density Approximation (LDA) 16
Generalized Gradient Approximation (GGA)18
2.1.5 Pseudopotential Method 19
2.1.6 Basis Functions 21
2.2 Non-Equilibrium Green’s Function Method 21
2.2.1 The NEGF formalism 22
Chapter 3 Computational Details 29
3.1 Structural Geometry 29
3.2 Parameters for Structural Relaxation Calculation 30
3.3 Parameters for Spin Transport Calculation 32
Chapter 4 Results and Discussion 34
4.1 Structural Relaxation during Stretching 37
4.2 Spin-Transport Properties 37
4.3 Spin-Polarized Current and Magnetoresistance 43
Chapter 5 Summary 52
References 53
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指導教授 唐毓慧(Yu-Hui Tang) 審核日期 2016-1-11
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