有机聚合物中自旋注入及输运的动力学研究
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摘要
自旋电子学是现代凝聚态物理学极具研究潜力的领域之一。与传统的微电子学不同,自旋电子学将电子的自旋特性和电荷特性相结合,其核心内容是研究自旋极化电子的注入、输运和探测及自旋控制,其目的是将器件的输运特性、光特性和磁特性等组合在一起,实现新型的自旋电子器件。
在自旋电子学领域,磁性半导体和半金属是自旋注入半导体异质结的关键材料,而自旋极化的电子在磁性半导体、半金属、半导体异质结中的输运是自旋电子学研究的核心问题。目前,一些原型器件已被设计出来,如Datta和Das设计了第一个自旋晶体管;Zutic等人提出了磁场效应晶体管的模型等。器件的实际应用需要解决如何有效地将自旋极化电流注入半导体的问题,这要求理论与实验研究中准确描述自旋注入、演变以及界面效应等。目前研究发现,注入电流的自旋极化与两层材料的电阻之比密切相关,而电阻不匹配正是传统材料难以实现高自旋注入效率的原因所在。
相对于普通固体材料,柔软的有机半导体(OSEs:Organic semiconductors)可以和磁性层形成良好的接触。由于有机聚合物弱的自旋—轨道耦合和超精细相互作用,载流子的自旋扩散长度比较长,因而有机聚合物是实现自旋极化输运的理想候选材料。从小分子到高分子,人们对有机材料的电磁光等特性的认识越来越深入。其中,对于具有一维特征的导电聚合物的结构和物性,特别是它的电特性,人们已经有了一个比较准确的理解。无论从量子化学还是物理建模出发,都得到了与实验基本一致的理论结果。而对于这种材料中的自旋相关特性在近几年刚刚开始起步,成为一个专门的方向——有机自旋电子学。
目前国内外许多课题组都开始进行有机自旋电子学的研究。Dediu等人首次研究了半金属庞磁电阻钙钛矿材料La_(0.7)Sr_(0.3)MnO_3(LSMO)向有机材料齐分子聚合物六噻吩T_6(sexithienyl)的自旋注入和输运,表明有机体内的输运电流是自旋极化的;Xiong等人设计了有机薄膜自旋阀结构,11K温度下测得该自旋阀磁电阻达40%;有机分子自旋阀,有机纳米线自旋阀等结构的实验研究结果相继报道;Majumdar和Zhan等人还研究了界面效应对自旋极化注入的影响。除此之外,人们也开始使用铁磁性电极材料取代通常有机发光二极管(OLED)中的普通电极,利用自旋极化来控制器件的发光效率。
有机自旋电子学的研究刚刚起步,在此领域还有一些关键问题尚不清楚,这些问题包括:聚合物中的载流子性质分析、聚合物与铁磁层的界面耦合、界面电导等因素对自旋注入的影响,自旋载流子在聚合物内的自旋扩散,各种自旋相互作用对自旋输运的影响等等。与无机材料相比,有机材料有它的特殊性,如结构上的低维特征与“软”性,其载流子不再是通常的电子或空穴,而是带电自陷态,如孤子、极化子、双极化子等等。因此要真正实现具有可操作性的有机自旋电子器件,需要对有机材料内自旋注入和输运的基本机理进行研究。
目前对有机材料中自旋注入与输运的理论研究有两大类:一是基于宏观热力学模型;二是基于自旋-电荷耦合的微观模型。前者能够描述自旋载流子在聚合物内的自旋扩散情况,而后者却在了解自旋极化的微观机理方面具有优势。Xie等人结合有机材料中元激发的具体电荷-自旋关系,研究了CMR/有机聚合物系统的基态性质,特别指出了极化子或双极化子在有机自旋电子学中的重要作用理论。然而,上述模型仅仅描述了一种静态的图像,实际上自旋注入有机体和输运载流子的传输为非平衡过程。本论文中我们针对有机材料内自旋注入和输运过程,在一维紧束缚SSH模型和BK模型的基础上,通过非绝热动力学的方法,研究了有机共轭聚合物中的自旋注入和输运的动态特性。在输运过程中,自旋受到自旋相关散射的影响,并非是一守恒量,也是它不同于电荷输运的地方。本论文具体的研究内容和结果如下:
1.利用自组装单层膜调节有机器件中的自旋注入
目前有机自旋器件的实验和理论研究工作中,人们多数采用铁磁电极实现自旋注入。我们提出了一种利用自旋极化的自组装单层膜修饰金属电极与聚合物界面,从而实现自旋极化注入的方法,并研究了有机层内的载流子的自旋极化特征。
自组装单层膜通常用来调节有机电子器件中的电荷注入,而使用自旋极化的材料作为自组装单层膜会造成整个系统内自旋简并破缺。计算结果表明,自旋极化的自组装单层膜改变了金属电极与聚合物层之间的耦合强度,并且二者之间的耦合是自旋相关的。利用自旋极化的自组装单层膜调节金属电极与聚合物层之间的耦合强度可以调节自旋极化的注入。另外,自旋注入过程中受到的自旋无关反转散射会使注入效率降低,因此在实验中需要注意该效应的影响。
自旋极化的注入会使有机材料内部的载流子发生变化,利用非绝热动力学方法,我们给出了聚合物中的自旋载流子的图像。在较强的电子-晶格耦合作用下,注入的电荷形成自陷态。而在动力学输运过程中,由于电子的波动性,它会在整个系统内扩展。当考虑聚合物内载流子的特征时,我们发现了一种具有非整数电荷和净自旋的载流子,它不同于普通的极化子和双极化子。我们将其称为自旋准极化子。自旋准极化子的存在从理论上证实了自旋极化注入的可能性。
2.极化子自旋取向对激子产率的影响
目前在有机发光二极管研究中,人们采用铁磁电极替代普通金属电极,用于提高和控制单态激子的产率。两个铁磁电极平行和反平行排列时,单态激子的产率分别为0和50%。当铁磁电极极化方向夹角变化时,单态激子的产生效率也会发生相应的变化。由铁磁电极注入的极化子的自旋方向直接决定于铁磁电极的极化方向,因此我们研究了极化子自旋取向的变化对碰撞后形成激子产率的影响。
研究发现,不存在电子-电子相互作用时,总的激子态产率并不随极化子自旋取向的变化而变化。单、三态激子的产生效率均与极化子自旋取向夹角的余弦值cosφ呈线性关系。单、三态激子产率的比值反比于cosφ。电子-电子相互作用会使总的激子态产率也随极化子自旋取向的改变而改变。尤其单态激子的产率会受电子-电子相互作用的影响而降低。然而对于自旋反平行的两个极化子碰撞形成的单态激子产率并不受电子-电子相互作用的影响。
3.有机聚合物中极化子的自旋进动
对于有机自旋器件,除了研究自旋极化的注入效率,对自旋输运的操控也是有机自旋器件研究的基本问题之一。自旋极化的载流子注入有机层之后,由于自旋—轨道耦合等自旋相关的相互作用或外加磁场的影响,载流子在内部传输的过程中可能发生自旋翻转。在有机自旋器件中,目前有两种主要的自旋控制方法,一种是在自旋阀中利用两边铁磁电极的自旋极化取向来控制自旋注入的极化方向,另一种就是利用门电压或外加磁场对输运过程中载流子的极化方向进行调控。我们研究了门电压作用下极化子的自旋进动现象,并将其与磁场作用下的自旋进动现象作了对比,为将来有机自旋操控器件的制备提供了理论基础。
门电压作用下聚合物内的极化子自旋具有自旋进动特征。研究发现,载流子的自旋进动具有周期性特点,且自旋进动的周期长度与自旋轨道耦合强度成反比。在刚性半导体中自旋进动发生在整个导带中,即存在不同能级间的电子跃迁,称为能级间进动。与此不同,通过对极化子瞬时本征能级上电荷占据的分析发现,在有机材料中的自旋进动现象只发生在极化子能级,即能级内进动。由于门电压可以控制极化子的进动末态,因此有可能将该效应应用到自旋相关的有机电子器件中。通过调节载流子的自旋进动,改变其从中间层注出时的自旋取向。这样,在注出层铁磁电极的自旋极化方向不变时,我们能通过控制载流子的自旋进动实现对电流大小的控制。
除门电压控制方式之外,聚合物中运动的极化子在外加磁场作用下也会产生自旋进动现象。通过调节驱动电场的强度,我们可以控制极化子运动的速度。结果发现,外加磁场下极化子进动的周期时间与极化子运动速度无关,其进动的周期长度与极化子运动速度成正比。与此相反,门电压控制下极化子进动的周期长度与速度无关,其进动的周期时间与极化子运动速度成反比。据此,在自旋输运调控过程中,我们可以根据需要选择合适的控制方法。
In the past few decades,the potential applications of electronic spin in semiconductor devices have attracted a lot of interest.Different from the classical electronics,spintronics involves both the electronic and spin characters of an electron.Fundamental studies of spintronics include investigations of spin injection,transport and detection in electronic materials,as well as spin manipulation.The goal is to understand the interaction between the spin of electrons and crystalline environments and to make useful devices.
Among the fundamental studies of spintronics,magnetic semiconductors and half metals are the key materials.The basic structure of a spintronics device is a semiconductor heterojunction which is used to manipulate the spin injection,transport and detection. Therefore,the spin transport in magnetic semiconductors,half metals and a semiconductor heterojunction is one of the central questions.Recently,several model devices have been schemed out.For example,Datta and Das designed the first spin transistor.Zutic et al. provided a new type of magnetic field-effect transistor.However,the low efficiency of spin injection into semiconductor blocks the practical applications of such devices.The spin polarization of injected current is closely related to the ratio between resistances of the two layers.Therefore,more attentions should be paid to the influence of interfacial effects on the spin injection and spin evolution during the transport process.
Compared with conventional semiconductors,soft organic semiconductors are good candidates as the contact materials between magnetic layers.The spin relaxation length is much longer than that of conventional semiconductors due to the weak spin-orbital and hyperfine interactions.The electric,magnetic and optical properties are unique in both the organic molecules and conjugated polymers.Up to now,the electric properties of quasi-one dimensional conducting polymers have been well understood.However,there is a lack of full understanding about the spin proerty of organic polymer,since it is a new field called organic spintronics.
In the past few years,there have been extensive theoretical and experimental studies on organic spintronics.Dediu's group first observed the room temperature spin polarization injection into conjugated organic material sexithienyl(T_6) in La_(0.7)Sr_(0.3)MnO_3/T_6/La_(0.7)Sr_(0.3)MnO_3 planar hybrid junction.The negative magnetic resistance indicates the spin injection in polymer.Xiong et al.have also studied the spin injection and transport in La_(0.7)Sr_(0.3)MnO_3/Alq_3/Co organic spin valve.The measured magnetoresistance can be as high as 40%at low temperature.Subsequently,the molecular spin valves and nanowire organic spin valves have been fabricated experimentally.Due to the important influence of interfacial effect on spin injection,much attention has been paid to improve the capability of organic spin devices.Besides,in organic light emitting diodes(OLEDs),spin polarized materials can replace conventional electrodes,which shows that light emitting efficiency can be tuned by spin-dependent method.Besides the experimental investigations,the theoretical researches on spin injection and transport are mainly based on two methods:the macroscopic thermodynamic theory and the microscopic spin-charge coupling model.The former can well describe the spin diffusion of carriers in polymers whereas the latter is suitable for revealing the microscopic mechanism.
Although much effort has been devoted to the study of organic spintronics,many questions are still under debate,such as the character of spin carriers in polymers,the influence of interface coupling and conductance between polymers and electrodes on spin injection,spin diffusion of spin carriers in polymers,the impact of spin related interactions on spin transport,etc.It's worthy noting that,due to the strong electron-lattice interactions in conjugated polymers,the charge carriers are self-trapped states rather than ordinary electrons or holes.Considering the unique charge-spin relation of self-trapped states in polylmers,Xie et al.investigated the ground state of the ferromagnetic metal/polymer junction.However, such theoretical model is based on a static picture while the spin injection and transport in polymers is obviously a dynamic nonequilibrium process.Therefore,much is need to know about the spin dynamics.
In this paper,we investigated the dynamic process of spin injection and transport in polymers in the framework of the well-known Su-Schrieffer-Heeger(SSH) model and Brazovskii-Kirova(BK) model,which provide the basic quantum characteristic of the system. The detailed research contents include:
1.Theoretical research on spin injection in an organic device with a spin polarized self-assembled monolayer
To date,the spin injection has been achieved mainly by introducing the ferromagnetic electrodes in spin devices.Here we suggest another approach to achieve spin injection in an organic device by a spin polarized self-assembled monolayer(SPSAM).Self-assembled monolayers have been widely used to control the charge injection in organic electronic devices since they can change the coupling or barriers between the metal and the polymer for charge injection.However.the introduction of a SPSAM can break the spin degeneracy of the whole system.It was found that the coupling between metal and polymer is spin-dependent due to the involvement of SPSAM.In addition,the coupling between metal and polymer can be tuned by the inserted SPSAM.The spin injection is also tuned correspondingly.It should be mentioned that,the spin-independent manipulation can reduce injected spin polarization. Therefore,to achieve a high spin injection,the spin-independent manipulation should be depressed.
In addition,spin carriers transporting in the polymer layer are also discussed.A dynamic picture of spin carriers in polymer is given in the frame work of non-adiabatic approach. During the dynamic transport in an organic device,one electronic state may extend to the whole system,which includes both the electrode and the polymer layer.Contrasting with rigid semiconductors,the electronic state injected into the polymer layer is confined together to form a wave-packet due to strong electron-lattice interactions.A spin carrier with non-integral electronic charge and net spin has been obtained within the polymer side,which is defined as a spin quasi-bipolaron.Beyond the definition of a normal spin polaron or a spinless bipolaron, the spin quasi-bipolaron is a direct proof of spin injection.
2.Influence of spin angle between oppositely charged polarons on the yield of excitons
Recently in organic light emitting diodes(OLEDs),ferromagnetic materials can replace conventional electrodes,which is aimed to lift or tune light emitting efficiency by spin-dependent method.When the two ferromagnetic electrodes are parallel or antiparallel arranged,the yield of singlet exciton is 0 and 50%respectly.That is the yield of singlet excitons are greatly influenced by the angle between the polarized orientations of two ferromagnetic electrodes.Since the spin orientation of polarons injected into polymer is directly decided by the ferromagnetic electrode nearby,we studied the influence of spin angle between oppositely charged polarons on the yield of excitons.It was found that in the absence of electron-electron interactions,the yield of total excitons is independent onφ,while the yield of singlet and triplet exciton presents a linear variation with cosφ,whereφis the angle between the spin orientations of polarons.
The presence of electron-electron interactions shows a depression on the yield of singlet exciton.However,we found that the formation of singlet excitons from antiparallel spin oriented polaron is not influenced by the electron-electron interactions.
3.Spin precession of a polaron in organic polymers induced by a gate voltage
The spin manipulation during the transport process is also the basic issue for the application of organic spin devices.There are mainly two approaches to manipulate the spin of carriers.One is the spin-valve setup,where the output current is governed by the spin-polarization of ferromagnetic contact.Another is the so called spin field effect transistor. The spin-polarized current is injected from a ferromagnetic material and then collected by the second ferromagnetic material on the other side.During the transport process,the spin of a polaron appears periodic precession when it transports in polymer under a gate voltage.The localization of a polaron makes it easy to probe the spin precession dependence on the position of a polaron.It was found that the spin precession takes place only within the highest occupied state of the polaron.Such intralevel spin precession is different from the interlevel spin precession for normal semiconductor.For a localized spin carrier in polymer,the spin precession period length of a polaron is closely related to the strength of spin-orbit interaction. Therefore,by tuning the spin precession with respect to the magnetization direction of the ferromagnetic collector,the transmitted spin-polarized current can be modulated.
Besides appling the gate voltage,the spin precession can be also tuned by the magnetic field.When spin precession of the moving polaron is induced by a magnetic field,the periodic time of the spin precession is not dependent on the strength of driving electric fields, while the periodic length is related to the strength of driving electric field.When spin precession of the moving polaron is induced by a gate voltage,things are different.The periodic length of the spin precession is independent of the strength of driving electric fields, while the periodic time is in inverse proportion to the strength of driving electric field. Therefore we can choose appropriate spin control method according to actual need.
在自旋电子学领域,磁性半导体和半金属是自旋注入半导体异质结的关键材料,而自旋极化的电子在磁性半导体、半金属、半导体异质结中的输运是自旋电子学研究的核心问题。目前,一些原型器件已被设计出来,如Datta和Das设计了第一个自旋晶体管;Zutic等人提出了磁场效应晶体管的模型等。器件的实际应用需要解决如何有效地将自旋极化电流注入半导体的问题,这要求理论与实验研究中准确描述自旋注入、演变以及界面效应等。目前研究发现,注入电流的自旋极化与两层材料的电阻之比密切相关,而电阻不匹配正是传统材料难以实现高自旋注入效率的原因所在。
相对于普通固体材料,柔软的有机半导体(OSEs:Organic semiconductors)可以和磁性层形成良好的接触。由于有机聚合物弱的自旋—轨道耦合和超精细相互作用,载流子的自旋扩散长度比较长,因而有机聚合物是实现自旋极化输运的理想候选材料。从小分子到高分子,人们对有机材料的电磁光等特性的认识越来越深入。其中,对于具有一维特征的导电聚合物的结构和物性,特别是它的电特性,人们已经有了一个比较准确的理解。无论从量子化学还是物理建模出发,都得到了与实验基本一致的理论结果。而对于这种材料中的自旋相关特性在近几年刚刚开始起步,成为一个专门的方向——有机自旋电子学。
目前国内外许多课题组都开始进行有机自旋电子学的研究。Dediu等人首次研究了半金属庞磁电阻钙钛矿材料La_(0.7)Sr_(0.3)MnO_3(LSMO)向有机材料齐分子聚合物六噻吩T_6(sexithienyl)的自旋注入和输运,表明有机体内的输运电流是自旋极化的;Xiong等人设计了有机薄膜自旋阀结构,11K温度下测得该自旋阀磁电阻达40%;有机分子自旋阀,有机纳米线自旋阀等结构的实验研究结果相继报道;Majumdar和Zhan等人还研究了界面效应对自旋极化注入的影响。除此之外,人们也开始使用铁磁性电极材料取代通常有机发光二极管(OLED)中的普通电极,利用自旋极化来控制器件的发光效率。
有机自旋电子学的研究刚刚起步,在此领域还有一些关键问题尚不清楚,这些问题包括:聚合物中的载流子性质分析、聚合物与铁磁层的界面耦合、界面电导等因素对自旋注入的影响,自旋载流子在聚合物内的自旋扩散,各种自旋相互作用对自旋输运的影响等等。与无机材料相比,有机材料有它的特殊性,如结构上的低维特征与“软”性,其载流子不再是通常的电子或空穴,而是带电自陷态,如孤子、极化子、双极化子等等。因此要真正实现具有可操作性的有机自旋电子器件,需要对有机材料内自旋注入和输运的基本机理进行研究。
目前对有机材料中自旋注入与输运的理论研究有两大类:一是基于宏观热力学模型;二是基于自旋-电荷耦合的微观模型。前者能够描述自旋载流子在聚合物内的自旋扩散情况,而后者却在了解自旋极化的微观机理方面具有优势。Xie等人结合有机材料中元激发的具体电荷-自旋关系,研究了CMR/有机聚合物系统的基态性质,特别指出了极化子或双极化子在有机自旋电子学中的重要作用理论。然而,上述模型仅仅描述了一种静态的图像,实际上自旋注入有机体和输运载流子的传输为非平衡过程。本论文中我们针对有机材料内自旋注入和输运过程,在一维紧束缚SSH模型和BK模型的基础上,通过非绝热动力学的方法,研究了有机共轭聚合物中的自旋注入和输运的动态特性。在输运过程中,自旋受到自旋相关散射的影响,并非是一守恒量,也是它不同于电荷输运的地方。本论文具体的研究内容和结果如下:
1.利用自组装单层膜调节有机器件中的自旋注入
目前有机自旋器件的实验和理论研究工作中,人们多数采用铁磁电极实现自旋注入。我们提出了一种利用自旋极化的自组装单层膜修饰金属电极与聚合物界面,从而实现自旋极化注入的方法,并研究了有机层内的载流子的自旋极化特征。
自组装单层膜通常用来调节有机电子器件中的电荷注入,而使用自旋极化的材料作为自组装单层膜会造成整个系统内自旋简并破缺。计算结果表明,自旋极化的自组装单层膜改变了金属电极与聚合物层之间的耦合强度,并且二者之间的耦合是自旋相关的。利用自旋极化的自组装单层膜调节金属电极与聚合物层之间的耦合强度可以调节自旋极化的注入。另外,自旋注入过程中受到的自旋无关反转散射会使注入效率降低,因此在实验中需要注意该效应的影响。
自旋极化的注入会使有机材料内部的载流子发生变化,利用非绝热动力学方法,我们给出了聚合物中的自旋载流子的图像。在较强的电子-晶格耦合作用下,注入的电荷形成自陷态。而在动力学输运过程中,由于电子的波动性,它会在整个系统内扩展。当考虑聚合物内载流子的特征时,我们发现了一种具有非整数电荷和净自旋的载流子,它不同于普通的极化子和双极化子。我们将其称为自旋准极化子。自旋准极化子的存在从理论上证实了自旋极化注入的可能性。
2.极化子自旋取向对激子产率的影响
目前在有机发光二极管研究中,人们采用铁磁电极替代普通金属电极,用于提高和控制单态激子的产率。两个铁磁电极平行和反平行排列时,单态激子的产率分别为0和50%。当铁磁电极极化方向夹角变化时,单态激子的产生效率也会发生相应的变化。由铁磁电极注入的极化子的自旋方向直接决定于铁磁电极的极化方向,因此我们研究了极化子自旋取向的变化对碰撞后形成激子产率的影响。
研究发现,不存在电子-电子相互作用时,总的激子态产率并不随极化子自旋取向的变化而变化。单、三态激子的产生效率均与极化子自旋取向夹角的余弦值cosφ呈线性关系。单、三态激子产率的比值反比于cosφ。电子-电子相互作用会使总的激子态产率也随极化子自旋取向的改变而改变。尤其单态激子的产率会受电子-电子相互作用的影响而降低。然而对于自旋反平行的两个极化子碰撞形成的单态激子产率并不受电子-电子相互作用的影响。
3.有机聚合物中极化子的自旋进动
对于有机自旋器件,除了研究自旋极化的注入效率,对自旋输运的操控也是有机自旋器件研究的基本问题之一。自旋极化的载流子注入有机层之后,由于自旋—轨道耦合等自旋相关的相互作用或外加磁场的影响,载流子在内部传输的过程中可能发生自旋翻转。在有机自旋器件中,目前有两种主要的自旋控制方法,一种是在自旋阀中利用两边铁磁电极的自旋极化取向来控制自旋注入的极化方向,另一种就是利用门电压或外加磁场对输运过程中载流子的极化方向进行调控。我们研究了门电压作用下极化子的自旋进动现象,并将其与磁场作用下的自旋进动现象作了对比,为将来有机自旋操控器件的制备提供了理论基础。
门电压作用下聚合物内的极化子自旋具有自旋进动特征。研究发现,载流子的自旋进动具有周期性特点,且自旋进动的周期长度与自旋轨道耦合强度成反比。在刚性半导体中自旋进动发生在整个导带中,即存在不同能级间的电子跃迁,称为能级间进动。与此不同,通过对极化子瞬时本征能级上电荷占据的分析发现,在有机材料中的自旋进动现象只发生在极化子能级,即能级内进动。由于门电压可以控制极化子的进动末态,因此有可能将该效应应用到自旋相关的有机电子器件中。通过调节载流子的自旋进动,改变其从中间层注出时的自旋取向。这样,在注出层铁磁电极的自旋极化方向不变时,我们能通过控制载流子的自旋进动实现对电流大小的控制。
除门电压控制方式之外,聚合物中运动的极化子在外加磁场作用下也会产生自旋进动现象。通过调节驱动电场的强度,我们可以控制极化子运动的速度。结果发现,外加磁场下极化子进动的周期时间与极化子运动速度无关,其进动的周期长度与极化子运动速度成正比。与此相反,门电压控制下极化子进动的周期长度与速度无关,其进动的周期时间与极化子运动速度成反比。据此,在自旋输运调控过程中,我们可以根据需要选择合适的控制方法。
In the past few decades,the potential applications of electronic spin in semiconductor devices have attracted a lot of interest.Different from the classical electronics,spintronics involves both the electronic and spin characters of an electron.Fundamental studies of spintronics include investigations of spin injection,transport and detection in electronic materials,as well as spin manipulation.The goal is to understand the interaction between the spin of electrons and crystalline environments and to make useful devices.
Among the fundamental studies of spintronics,magnetic semiconductors and half metals are the key materials.The basic structure of a spintronics device is a semiconductor heterojunction which is used to manipulate the spin injection,transport and detection. Therefore,the spin transport in magnetic semiconductors,half metals and a semiconductor heterojunction is one of the central questions.Recently,several model devices have been schemed out.For example,Datta and Das designed the first spin transistor.Zutic et al. provided a new type of magnetic field-effect transistor.However,the low efficiency of spin injection into semiconductor blocks the practical applications of such devices.The spin polarization of injected current is closely related to the ratio between resistances of the two layers.Therefore,more attentions should be paid to the influence of interfacial effects on the spin injection and spin evolution during the transport process.
Compared with conventional semiconductors,soft organic semiconductors are good candidates as the contact materials between magnetic layers.The spin relaxation length is much longer than that of conventional semiconductors due to the weak spin-orbital and hyperfine interactions.The electric,magnetic and optical properties are unique in both the organic molecules and conjugated polymers.Up to now,the electric properties of quasi-one dimensional conducting polymers have been well understood.However,there is a lack of full understanding about the spin proerty of organic polymer,since it is a new field called organic spintronics.
In the past few years,there have been extensive theoretical and experimental studies on organic spintronics.Dediu's group first observed the room temperature spin polarization injection into conjugated organic material sexithienyl(T_6) in La_(0.7)Sr_(0.3)MnO_3/T_6/La_(0.7)Sr_(0.3)MnO_3 planar hybrid junction.The negative magnetic resistance indicates the spin injection in polymer.Xiong et al.have also studied the spin injection and transport in La_(0.7)Sr_(0.3)MnO_3/Alq_3/Co organic spin valve.The measured magnetoresistance can be as high as 40%at low temperature.Subsequently,the molecular spin valves and nanowire organic spin valves have been fabricated experimentally.Due to the important influence of interfacial effect on spin injection,much attention has been paid to improve the capability of organic spin devices.Besides,in organic light emitting diodes(OLEDs),spin polarized materials can replace conventional electrodes,which shows that light emitting efficiency can be tuned by spin-dependent method.Besides the experimental investigations,the theoretical researches on spin injection and transport are mainly based on two methods:the macroscopic thermodynamic theory and the microscopic spin-charge coupling model.The former can well describe the spin diffusion of carriers in polymers whereas the latter is suitable for revealing the microscopic mechanism.
Although much effort has been devoted to the study of organic spintronics,many questions are still under debate,such as the character of spin carriers in polymers,the influence of interface coupling and conductance between polymers and electrodes on spin injection,spin diffusion of spin carriers in polymers,the impact of spin related interactions on spin transport,etc.It's worthy noting that,due to the strong electron-lattice interactions in conjugated polymers,the charge carriers are self-trapped states rather than ordinary electrons or holes.Considering the unique charge-spin relation of self-trapped states in polylmers,Xie et al.investigated the ground state of the ferromagnetic metal/polymer junction.However, such theoretical model is based on a static picture while the spin injection and transport in polymers is obviously a dynamic nonequilibrium process.Therefore,much is need to know about the spin dynamics.
In this paper,we investigated the dynamic process of spin injection and transport in polymers in the framework of the well-known Su-Schrieffer-Heeger(SSH) model and Brazovskii-Kirova(BK) model,which provide the basic quantum characteristic of the system. The detailed research contents include:
1.Theoretical research on spin injection in an organic device with a spin polarized self-assembled monolayer
To date,the spin injection has been achieved mainly by introducing the ferromagnetic electrodes in spin devices.Here we suggest another approach to achieve spin injection in an organic device by a spin polarized self-assembled monolayer(SPSAM).Self-assembled monolayers have been widely used to control the charge injection in organic electronic devices since they can change the coupling or barriers between the metal and the polymer for charge injection.However.the introduction of a SPSAM can break the spin degeneracy of the whole system.It was found that the coupling between metal and polymer is spin-dependent due to the involvement of SPSAM.In addition,the coupling between metal and polymer can be tuned by the inserted SPSAM.The spin injection is also tuned correspondingly.It should be mentioned that,the spin-independent manipulation can reduce injected spin polarization. Therefore,to achieve a high spin injection,the spin-independent manipulation should be depressed.
In addition,spin carriers transporting in the polymer layer are also discussed.A dynamic picture of spin carriers in polymer is given in the frame work of non-adiabatic approach. During the dynamic transport in an organic device,one electronic state may extend to the whole system,which includes both the electrode and the polymer layer.Contrasting with rigid semiconductors,the electronic state injected into the polymer layer is confined together to form a wave-packet due to strong electron-lattice interactions.A spin carrier with non-integral electronic charge and net spin has been obtained within the polymer side,which is defined as a spin quasi-bipolaron.Beyond the definition of a normal spin polaron or a spinless bipolaron, the spin quasi-bipolaron is a direct proof of spin injection.
2.Influence of spin angle between oppositely charged polarons on the yield of excitons
Recently in organic light emitting diodes(OLEDs),ferromagnetic materials can replace conventional electrodes,which is aimed to lift or tune light emitting efficiency by spin-dependent method.When the two ferromagnetic electrodes are parallel or antiparallel arranged,the yield of singlet exciton is 0 and 50%respectly.That is the yield of singlet excitons are greatly influenced by the angle between the polarized orientations of two ferromagnetic electrodes.Since the spin orientation of polarons injected into polymer is directly decided by the ferromagnetic electrode nearby,we studied the influence of spin angle between oppositely charged polarons on the yield of excitons.It was found that in the absence of electron-electron interactions,the yield of total excitons is independent onφ,while the yield of singlet and triplet exciton presents a linear variation with cosφ,whereφis the angle between the spin orientations of polarons.
The presence of electron-electron interactions shows a depression on the yield of singlet exciton.However,we found that the formation of singlet excitons from antiparallel spin oriented polaron is not influenced by the electron-electron interactions.
3.Spin precession of a polaron in organic polymers induced by a gate voltage
The spin manipulation during the transport process is also the basic issue for the application of organic spin devices.There are mainly two approaches to manipulate the spin of carriers.One is the spin-valve setup,where the output current is governed by the spin-polarization of ferromagnetic contact.Another is the so called spin field effect transistor. The spin-polarized current is injected from a ferromagnetic material and then collected by the second ferromagnetic material on the other side.During the transport process,the spin of a polaron appears periodic precession when it transports in polymer under a gate voltage.The localization of a polaron makes it easy to probe the spin precession dependence on the position of a polaron.It was found that the spin precession takes place only within the highest occupied state of the polaron.Such intralevel spin precession is different from the interlevel spin precession for normal semiconductor.For a localized spin carrier in polymer,the spin precession period length of a polaron is closely related to the strength of spin-orbit interaction. Therefore,by tuning the spin precession with respect to the magnetization direction of the ferromagnetic collector,the transmitted spin-polarized current can be modulated.
Besides appling the gate voltage,the spin precession can be also tuned by the magnetic field.When spin precession of the moving polaron is induced by a magnetic field,the periodic time of the spin precession is not dependent on the strength of driving electric fields, while the periodic length is related to the strength of driving electric field.When spin precession of the moving polaron is induced by a gate voltage,things are different.The periodic length of the spin precession is independent of the strength of driving electric fields, while the periodic time is in inverse proportion to the strength of driving electric field. Therefore we can choose appropriate spin control method according to actual need.
引文
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