基于自旋和类自旋自由度的纳米器件设计
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摘要
电子是电荷与自旋的统一载体。在微电子学中已经对电子的电荷性质做了很多研究,并且基于电荷性质得到了很多实用的电学器件,使我们的生活日新月异。但是当微电子器件的尺寸小到一定尺度时就会有量子效应出现,器件的物理工作原理失效。因此,以电子的另一个性质——自旋作为信息载体的一个新兴的研究领域“自旋电子学”发展起来,成为凝聚态物理学中一个新的学科分支。自旋电子器件的操作需要产生、探测和操控自旋流。石墨烯是近年来纳米材料的另一个研究热点,是芯片产业替代硅的可能材料之一。本文第一章介绍了纳米电子学,自旋电子学和石墨烯材料。
文章的第二、三章是我们的主要工作。在第二章我们考虑了一个自旋轨道耦合调制下的量子点接触(QPC)器件。当自旋偏压施加到一个二端装置时,将会有电荷流产生。本章展示了这种电荷流的整流效应。我们发现当自旋偏压的极化方向(同时也是自旋的量子化轴)沿着QPC的横向时,两个自旋守恒的传输系数随入射能量的变化显著不同。这种差异对自旋偏压引起的电荷流整流效应而言是一个必要条件。将自旋偏压的作用从器件的一端改变到另一端时,电荷流可以从零改变到一个很显著的值。
石墨烯的能谱具有二重的谷简并。借鉴自旋电子学中对电子自旋的研究思路,这种谷自由度可用作信息载体。谷电子器件的运行需要两个谷的电子分布存在差异。目前关于谷过滤的设计方案都集中在对谷极化的全电学控制。本文第三章提出一种基于石墨烯块材料的谷过滤器件,该器件中的电子受到应力和磁场共同调制。对二端石墨烯器件,当狄拉克电子只受局域磁场或应力单独调制时,没有谷极化产生。当两种调制共同作用时石墨烯的两个谷在倒空间有不同大小的位移。相应地,输出电流是谷极化的。谷极化度可以用应力的强度和所施加的标量电势来调节。当局域磁场的方向改变时,谷极化的极性也随之改变。
Electrons are the carriers of both the charge and the spin. A great deal of efforts on the manipulation of the charge property have been made in microelectronics. Accordingly, various charge-based devices have been fabricated, which changes our lives day and day. When the size of microelectronic devices goes further to a certain small scale, the quantum effects would arise, which leads to the failure of the physical principle of the device. In recent years, a field dubbed spintronics has evolved as a new branch of condensed-matter physics, where the other property of electrons---spin is utilized as information carrier. The operation of a spintronic device requires the generation, detection, and manipulation of the spin current. As a possible candidate for the replacement of silicon in chip industry, graphene becomes a hot topic in nano materials. In Chapter 1, we give a brief introduction on these fields: nanoelectronics, spintronics, and graphene.
In Chapter 2, we consider a quantum point contact (QPC) modulated by a spin-orbit interaction. When a spin bias is applied to a two-terminal device, a charge current will be generated. We demonstrate the rectification of such a current in Chapter 2. When the polarization orientation of the spin bias (which is the spin-quantization axis) is along the transverse direction of the QPC, the two spin-conserved transmissions show a distinct variation with the incident energy. As a result, the charge current can turn from zero to a remarkable value by switching the spin bias from one lead to the other lead.
As a counterpart of electronic spin in spintronics, the two-fold valley degenaracy in graphene has been suggested to carry information. The operation of valley-based electronic devices requires the generation of an uneven valley distribution of electrons. All of previous proposals of valley filters focus on an all-electric control of valley polarization. In Chapter 3, we put forward a tunable valley filter based on bulk graphene under the modulations of both the substrate strains and local magnetic fields. When only one of the two modulations is present, no valley polarization can be generated. The combination of the two modulations results in a difference between the absolute shifts of the K and K' valleys, which could be utilized to generate a valley-polarized current. The degree of the valley polarization can be tuned by the strain strength and the inclusion of a scalar electric potential. The valley polarization changes its polarity as the local magnetic field switches its direction.
文章的第二、三章是我们的主要工作。在第二章我们考虑了一个自旋轨道耦合调制下的量子点接触(QPC)器件。当自旋偏压施加到一个二端装置时,将会有电荷流产生。本章展示了这种电荷流的整流效应。我们发现当自旋偏压的极化方向(同时也是自旋的量子化轴)沿着QPC的横向时,两个自旋守恒的传输系数随入射能量的变化显著不同。这种差异对自旋偏压引起的电荷流整流效应而言是一个必要条件。将自旋偏压的作用从器件的一端改变到另一端时,电荷流可以从零改变到一个很显著的值。
石墨烯的能谱具有二重的谷简并。借鉴自旋电子学中对电子自旋的研究思路,这种谷自由度可用作信息载体。谷电子器件的运行需要两个谷的电子分布存在差异。目前关于谷过滤的设计方案都集中在对谷极化的全电学控制。本文第三章提出一种基于石墨烯块材料的谷过滤器件,该器件中的电子受到应力和磁场共同调制。对二端石墨烯器件,当狄拉克电子只受局域磁场或应力单独调制时,没有谷极化产生。当两种调制共同作用时石墨烯的两个谷在倒空间有不同大小的位移。相应地,输出电流是谷极化的。谷极化度可以用应力的强度和所施加的标量电势来调节。当局域磁场的方向改变时,谷极化的极性也随之改变。
Electrons are the carriers of both the charge and the spin. A great deal of efforts on the manipulation of the charge property have been made in microelectronics. Accordingly, various charge-based devices have been fabricated, which changes our lives day and day. When the size of microelectronic devices goes further to a certain small scale, the quantum effects would arise, which leads to the failure of the physical principle of the device. In recent years, a field dubbed spintronics has evolved as a new branch of condensed-matter physics, where the other property of electrons---spin is utilized as information carrier. The operation of a spintronic device requires the generation, detection, and manipulation of the spin current. As a possible candidate for the replacement of silicon in chip industry, graphene becomes a hot topic in nano materials. In Chapter 1, we give a brief introduction on these fields: nanoelectronics, spintronics, and graphene.
In Chapter 2, we consider a quantum point contact (QPC) modulated by a spin-orbit interaction. When a spin bias is applied to a two-terminal device, a charge current will be generated. We demonstrate the rectification of such a current in Chapter 2. When the polarization orientation of the spin bias (which is the spin-quantization axis) is along the transverse direction of the QPC, the two spin-conserved transmissions show a distinct variation with the incident energy. As a result, the charge current can turn from zero to a remarkable value by switching the spin bias from one lead to the other lead.
As a counterpart of electronic spin in spintronics, the two-fold valley degenaracy in graphene has been suggested to carry information. The operation of valley-based electronic devices requires the generation of an uneven valley distribution of electrons. All of previous proposals of valley filters focus on an all-electric control of valley polarization. In Chapter 3, we put forward a tunable valley filter based on bulk graphene under the modulations of both the substrate strains and local magnetic fields. When only one of the two modulations is present, no valley polarization can be generated. The combination of the two modulations results in a difference between the absolute shifts of the K and K' valleys, which could be utilized to generate a valley-polarized current. The degree of the valley polarization can be tuned by the strain strength and the inclusion of a scalar electric potential. The valley polarization changes its polarity as the local magnetic field switches its direction.
引文
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[2]Baibich M N, Broto J M, Fert A, et al. Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices [J]. Phys. Rev. Lett.,1988,61(21):2472-2475.
[3]Hu C M, Nitta J, Jensen A, et al. Spin-polarized transport in a two-dimensional electron gas with interdigital-ferromagnetic contacts [J]. Phys. Rev. B,2001,63(12): 125333 (1)-125333 (4).
[4]Zhu H J, Ramsteiner M, Kostial, et al. Room-Temperature Spin Injection from Fe into GaAs [J]. Phys. Rev. Lett.,2001,87(1):016601 (1)-016601 (4).
[5]Schmidt G, Ferrand D, Molenkamp L M, et al. Fundamental obstacle for electrical spin injection from a ferromagnetic metal into a diffusive semiconductor [J]. Phys. Rev. B,2000,62 (8):R4790-R4793.
[6]Rashba E I. Theory of electrical spin injection:tunnel contacts as a solution of the conductivity mismatch problems [J]. Phys. Rev. B,2000,62(24):R16267.
[7]Fiederling R, Keim M, Reuscher G, et al. Injection and detection of a spin-polarized current in a light-emitting diode [J]. Nature (London),1999,402:787-790.
[8]Jonker B T, Park Y D, Bennett B R, et al. Robust electrical spin injection into a semiconductor heterostructure [J]. Phys. Rev. B,2000,62(12):8180-8183.
[9]Young D K, Johnston-Halperin E, Awschalom D D, et al. Anisotropic electrical spin injection in ferromagnetic semiconductor heterostructures [J]. Appl. Phys. Lett.,2002, 80(9):1598.
[10]Ohno Y, Young D K, Beschton B, et al. Electrical spin injection in a ferromagnetic semiconductor heterostructure [J]. Nature,1999,402(16):790.
[11]Upadhyay S K, Palanisami A, Louie R N et al. Probing Ferromagnets with Andreev Reflection [J]. Phys. Rev. Lett.,1998,81(15):3247-3250.
[12]常凯,杨文.半导体中自旋轨道耦合及自旋霍尔效应[J].物理学进展,2008,28(3)：236.
[13]张雅.一维量子波导中的电子自旋输运研究[D].大连：大连理工大学物理与光电工程学院,2009
[14]Datta S, Das B. Electronic analog of the electro-optic modulator [J]. Appl. Phys. Lett. 1990,56(7):665-667.
[15]Rashba E I. Sov. Phys. Solid State [J].1960,2:1109.
[16]Dresselhaus G. Spin-orbit coupling effects in zinc blende structure [J]. Phys. Rev, 1955,100(2):580-586.
[17]毕才华.自旋轨道耦合作用下磁调制2DEG的自旋输运[D].大连：大连理工大学物理与光电工程学院,2009
[18]Parkin S S P, Mauri D. Spin Engineering:Direct Determination of the Rudermarr-Kittel-Kasuya-Yosida far-field range function in ruthenium [J]. Phys. Rev. B,1991,44(13):7131-7134.
[19]Moodera J S, Kinder L R, Wong T M, et al. Large Magnetoresistance at Room Temperature in Ferromagnetic Thin Film Tunnel Junctions [J]. Phys. Rev. Lett.,1994,74(16): 3273-3276.
[20]Novoselov K S, Geim A K. Electric Field Effect in Atomically Thin Carbon Films [J]. Science,2004,306,666.
[21]Geim A K, Novoselov K S. The rise of graphene [J], Nature Mater.,2007,6,183-191.
[22]Theorique S D P, Merisiers 0 D. Effect of a Single Localized Impurity on the Local Density of States in Monolayer and Bilayer Graphene [J], Phys. Rev. Lett.,2008,100, 076601.
[23]Katsnelson M I, Novoselov K S, Geim A K. Chiral tunnelling and the Klein paradox in graphene [J], nat. phys.,2006,2,620-625.
[24]McCann E, Fal'ko V I. Landau-Level Degeneracy and Quantum Hall Effect in a Graphite Bilayer [J]. Phys. Rev. Lett.,2006,96(8):086805.
[25]Novoselov K S, McCann E, Morozov S V, et al. Unconventional quantum Hall effect and Berry's phase of 2π in bilayer graphene [J]. Nat Phys,2006,2(3):177-180.
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