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石墨烯中自旋相关量子输运的研究

作　者: 张青天
导　师: 林子敬
学　校: 中国科学技术大学
专　业: 凝聚态物理学
关键词: graphene spin current spin polarization ferromagnetic proximity spinorbit interaction quantum pumping quantum transport
分类号: O469
类　型: 博士论文
年　份: 2013年
下　载: 71次
引　用: 0次
阅　读: 论文下载

内容摘要

Graphene is a strictly two-dimensional material of carbon atoms, which has attracted considerable research attention since its discovery in2004. Apart from its many peculiar and interesting electronic properties, graphene is also regarded as a promising candidate for spintronic devices. A number of experimental studies of graphene have proven it to be a material possessing characteristics suitable for spintronic applications, such as the long spin relaxation length and the gate tunable room temperature spin transport. This thesis discusses a numerical study of the manipulation of electron spin in graphene with the help of exchange interaction induced by ferromagnetic proximity effect or spin orbit interaction (SOI). It consists of the following seven chapters.Chapter one is an introduction to the research background. A brief introduction is given to the physical characteristics of mesoscopic systems, and we qualitatively characterize the quantum transport regimes. The electronic properties of graphene and its advantages for spintronic applications are briefly introduced. We also review the developments of spintronics, which helps us better understand the research background.In chapter two, we introduce the most popular theories for quantum transport. Scattering matrix, transfer matrix and Green’s function are all introduced with brief derivations, and the definitions and properties of these powerful approaches are also considered.In chapter three, we propose a method of generating spin currents in mono layer graphene through adiabatic quantum pumping by applying two periodic oscillating gate voltages to a monolayer graphene with exchange splitting induced by ferromagnetic proximity. The pumped charge and spin currents are sensitive functions of the Fermi energy and pure spin current and spin current with different degrees of polarization and large magnitudes are obtained in our scheme. We find that large spin currents can be obtained when the pumping amplitudes are increased for our spin-polarized pump.In chapter four, we study a method to generate pure spin current in monolayer graphene over a wide range of Fermi energy by adiabatic quantum pumping. The device consists of three gate electrodes and two ferromagnetic strips, which induce a spin-splitting in the graphene through the proximity effect. A pure spin current is generated by applying two periodic oscillating gate voltages. We find that the pumped pure spin current is a sensitive oscillatory function of the Fermi energy. Large spin currents can be found at Fermi energies where there are Fabry-Perot resonances in the barriers. Furthermore, we analyze the effects of the parameters of the system on the pumped currents.In chapter five, we studied spin-dependent transport in mono layer graphene with a spin-orbit barrier, a narrow strip in which the spin-orbit interaction is not zero. When the Fermi energy is between the two spin-split bands, the structure can be used to generate spin-polarized current. For a strong enough Rashba strength, a thick enough barrier or a low enough Fermi energy, highly spin-polarized current is generated (polarization-0.7-0.85). Under these conditions, the spin direction of the transmitted electron is approximately perpendicular to the direction of motion.In chapter six, we investigate spin dependent transport in monolayer graphene with a spatial modulation of the Rashba spin orbit interaction (RSOI). In this structure, spin polarized current can be generated with spin polarization being a sensitive oscillatory function of the Fermi energy. Rapid reversal of the spin polarization can be realized at some Fermi energies by slight changes in the Fermi energy. The magnitude of the spin polarization depends on the number of RSOI barriers.In chapter seven, we summarize the main results presented in this thesis and show our future work. We introduce our preliminary numerical results on the control of spins by Goos-Hanchen effect in the presence of double magnetic barriers. It is shown that an efficient spin beam splitter can be realized in graphene with double magnetic barriers.

全文目录

Abstract  5-7
Table of Content  7-10
List of Figures  10-18
List of Symbols and Abbreviations  18-20
Chapter 1 Introduction  20-42
  1.1 Introduction to Mesoscopic Transport  20-23
  1.2 Electronic Properties of Graphene  23-32
    1.2.1 Hybridizations of Carbon Atoms in Graphene  24-26
    1.2.2 Basic Properties of Graphene  26-29
    1.2.3 Klein Tunneling in Graphene  29-30
    1.2.4 Half Integer Quantum Hall Effect in Graphene  30-32
  1.3 Brief Introduction to Spintronics  32-36
    1.3.1 The Development of Spintronics  33-36
    1.3.2 Graphene's Advantage for Spintrortics  36
  1.4 Organization of This Thesis  36-38
  References  38-42
Chapter 2 Theoretical For malisms for Quantum Transport  42-59
  2.1 Scattering Matrix Theory  42-47
    2.1.1 Definition of The Scattering Matrix  42-44
    2.1.2 Combination of Scattering Matrices  44-45
    2.1.3 Transmission Probability and Landauer-ButtikerFormula  45-47
  2.2 Transfer Matrix  47-49
    2.2.1 Definition of Transfer Matrix  47-48
    2.2.2 Instability of Transfer Matrix  48-49
  2.3 Green's Function Formalism  49-57
    2.3.1 Two-Dimensional Tight-Binding Models  49-51
    2.3.2 Green's Function Formalism  51-54
    2.3.3 Application of Green's Function to Graphene Nanoribbons  54-57
  References  57-59
Chapter 3 Spin-Polarized Quantum Pumping in Monolayer Graphene  59-84
  3.1 Introduction to Quantum Pumping  59-62
  3.2 Theory for Quantum Pumping in Graphene  62-67
    3.2.1 General Formalism for Quantum Pumping  62-65
    3.2.2 Scattering Matrix in Graphene  65-67
  3.3 Theoretical Model for Spin-Polarized Quantum Pumping  67-68
  3.4 Spin Current Generation by Adiabatic Weak Pumping  68-71
  3.5 Generation of Large Spin Current in Graphene  71-80
  3.6 Conclusion  80-82
  References  82-84
Chapter 4 Pure Spin Current Generation by Quantum Pumping  84-99
  4.1 Introduction  84
  4.2 Theoretical Model  84-86
  4.3 Results and Discussion  86-96
  4.4 Conclusion  96-98
  References  98-99
Chapter 5 Spin Transport in Graphene Spin-Orbit Barrier Structure  99-117
  5.1 Introduction to Spin Orbit Interaction  99-102
    5.1.1 Spin Orbit Interaction in Conventional 2DEG  99-101
    5.1.2 Spin Orbit Interaction in Graphene  101-102
  5.2 Kane and Mele Model  102-104
  5.3 Energy Band Structure and Spin Direction  104-105
  5.4 Theoretical Model and Formalism  105-107
  5.5 Numerical Results and Discussion  107-114
  5.6 Conclusion  114-115
  References  115-117
Chapter 6 Spin Polarization Switching in Monolayer Graphene through aRashba Multi-Barrier Structure  117-134
  6.1 Introduction  117-118
  6.2 Model and Theory  118-123
    6.2.1 Theoretical Model  118-119
    6.2.2 The Scattering Matrix for Multi-RSOI Barrier Structure  119-123
  6.3 Results and Discussion  123-131
  6.4 Conclusion  131-133
  References  133-134
Chapter 7 Conclusion and Future Work  134-142
  7.1 Conclusion  134-136
  7.2 Future Work  136-141
    7.2.1 Introduction  136-137
    7.2.2 Theoretical Formalism  137-138
    7.2.3 Theoretical Model and Preliminary Results  138-141
  References  141-142
Acknowledgements  142-143
List of Publications  143-145
中文摘要  145-152

石墨烯中自旋相关量子输运的研究

内容摘要

全文目录

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