对称抛物势阱磁性隧道结中的自旋输运及磁电阻效应

研究了两端具有铁磁接触的对称抛物势阱磁性隧道结(F/SPW/F)中自旋相关的隧穿概率和隧穿磁电阻,讨论了量子尺寸效应和Rashba 自旋轨道耦合作用对自旋极化输运特性的影响.研究结果表明:隧穿概率和隧穿磁电阻随抛物势阱宽度的增加发生周期性的振荡.抛物势阱深度的增加减小了隧穿概率和隧穿磁电阻的振荡频率.Rashba 自旋轨道耦合强度的增加加大了隧穿概率和隧穿磁电阻的振荡频率.隧穿概率和隧穿磁电阻的振幅和峰谷比强烈依赖于两铁磁电极中磁化方向的夹角. 关键词： 磁性隧道结 Rashba 自旋轨道耦合 隧穿概率 隧穿磁电阻     

磁性隧道结电子输运特性的研究

在Slonczewsik自由电子理论模型下,研究了两铁磁性金属电极被一平面磁性势垒隔开的磁性隧道结零偏压下的隧穿电导、自旋极化率和隧穿磁阻比率,研究表明隧道结的磁结构对隧穿电导和隧穿磁阻的值有很大的影响,当两磁性电极分子场方向相同,且都与势垒层分子场反平行时,隧穿电导数值达到最大,两者平行时,其数值最小,同时还分析了分子场的相对取向等对磁性隧道结自旋极化电子输运性质的影响.研究结果对自旋电子器件的设计具有一定的指导意义.     

磁性纳米电机单电子晶体管的自旋输运特性

文章作者在研究磁性隧道结的自旋输运中引入量子点的机械振动自由度,将单电子隧穿和振动自由度耦合所导致的shuttle输运理论应用到自旋电子学中,研究结果表明,shuttle输运对自旋极化输运有很大的影响,其独特的输运性质可以用来设计自旋电子器件,文章在理论上提出具有巨磁效应的自旋阀、高性能的半导体自旋注入器以及电流的整流器.     

磁性隧道结电子输运特性的研究

在Slonczewsik自由电子理论模型下,研究了两铁磁性金属电极被一平面磁性势垒隔开的磁性隧道结零偏压下的隧穿电导、自旋极化率和隧穿磁阻比率,研究表明隧道结的磁结构对隧穿电导和隧穿磁阻的值有很大的影响,当两磁性电极分子场方向相同,且都与势垒层分子场反平行时,隧穿电导数值达到最大,两者平行时,其数值最小,同时还分析了分子场的相对取向等对磁性隧道结自旋极化电子输运性质的影响。研究结果对自旋电子器件的设计具有一定的指导意义。     

双势垒磁性隧道结的磁电阻效应及其在自旋晶体管中的应用

利用磁控溅射方法沉积双势垒磁性隧道结多层膜, 其中Al-O势垒层由等离子体氧化1 nm厚的 金属铝膜方式制备，然后采用深紫外光曝光和Ar离子刻蚀技术、微加工制备出长短轴分别为 6和3 μm大小的椭圆形双势垒磁性隧道结（DBMTJ），并在室温和低温下对其自旋电子输运 特性进行了研究. DBMTJ的隧穿磁电阻(TMR)比值在室温和42 K分别达到27％和423％， 结电阻分别为136 kΩ·μm²和175 kΩ·μm²，并在实验中观 察到平行状 态下存在低电阻态及共振隧穿效应，反平行态下呈现高电阻态以及TMR随外加偏压或直流电 流的增加而发生振荡现象. 由此，设计了一种基于这种双势垒磁性隧道结隧穿特性的自旋晶 体管. 关键词： 双势垒磁性隧道结 隧穿磁电阻 共振隧穿效应 自旋晶体管     

磁性隧道结自旋极化电子的隧穿特性

铁磁金属间通过中间层的自旋极化电子隧穿产生的磁性耦合,在自旋电子器件中有许多潜在的应用.考虑由一平面磁性势垒层隔开的两铁磁性金属电极构成的磁性隧道结,针对中间层形成的矩形势垒,在近自由电子模型的基础上,计算零偏压下的隧穿电导、自旋极化率和隧穿磁阻比率,分析势垒层特性、分子场强弱、分子场相对取向等对隧道结自旋极化电子隧穿特性的影响.计算结果对自旋电子器件的设计具有一定的指导意义.     

多层结构双自旋过滤隧道结中的电子输运特性

在转移矩阵方法及Mireles和Kirczenow的量子相干输运理论的基础上,研究了正常金属层/磁性半导体层/非磁绝缘层/磁性半导体层/正常金属层型双自旋过滤隧道结中Rashba自旋轨道耦合效应和自旋过滤效应对自旋相关输运的影响.讨论了隧穿磁电阻（TMR）、隧穿电导与各材料层厚度、Rashba自旋轨道耦合强度以及两磁性半导体中磁矩的相对夹角θ之间的关系.研究表明：含磁性半导体层的双自旋过滤隧道结由于磁性半导体层的自旋过滤效应和Rashba自旋轨道耦合作用可获得极大的TMR值.另外TMR和隧穿电导随着Rashba自旋轨道耦合强度的变化而振荡,振荡周期随Rashba自旋轨道耦合强度的增大逐渐减小. 关键词： 双自旋过滤隧道结 Rashba自旋轨道耦合 隧穿磁电阻 隧穿电导     

双势垒抛物势阱磁性隧道结隧穿磁阻及自旋输运性质的研究

采用相干量子输运理论和传递矩阵的方法,在抛物势阱磁性隧道结(F/PW/F)的铁磁和半导体势阱间插入另一种半导体作为势垒,构造具有双势垒的抛物势阱磁性隧道结作为研究对象,研究了抛物势阱宽度、自旋轨道耦合效应、角度效应及插入势垒厚度对隧穿磁阻及自旋输运性质的影响计算结果表明,通过适当调节Rashba自旋轨道藕合强度和插入势垒的厚度,可以实现隧穿磁阻(TMR)的调制,能获得较大的TMR值,这些特点有助于促进新型磁性隧道结的开发和应用.     

MgO基磁性隧道结温度-偏压相图的理论研究

MgO基磁性隧道结是自旋电子器件研究的热点问题,其温度特性和偏压特性在实际应用中极其重要.因此,亟需在理论上计算得到MgO基磁性隧道结的温度-偏压相图.本文构建了适用于单晶势垒层磁性隧道结的理论.该理论将单晶势垒层视作周期性光栅,利用光学衍射理论处理势垒层对隧穿电子的衍射,因此可以很好地计入隧穿电子波的相干性.根据此理论,同时计入温度和偏压的影响计算了MgO基磁性隧道结的温度-偏压相图.理论结果表明,通过调节MgO基磁性隧道结的铁磁电极半交换劈裂能D、化学势μ以及势垒层周期势v(Kh)可以优化其温度特性和偏压特性.该结果为MgO基磁性隧道结的应用提供了坚实的理论基础.     

处理具有任意形状势垒的磁性隧道结中电子输运的一个简单方法

在Slonczewski自由电子模型的基础上,提出了一个可用于处理具有任意形状势垒的磁性隧道结中磁电子输运的简单方法,并以三种常见构形的势垒,即梯形势垒,计入了镜像势的梯形势垒和抛物线势垒为例,讨论了势垒形状对隧穿磁电阻及其随偏压变化的影响. 关键词： 磁性隧道结 隧穿磁电阻 任意形状势垒 非零偏压     

Effect of ferroelectricity on electron transport in Pt/BaTiO3/Pt tunnel junctions

Based on first-principles calculations, we demonstrate the impact of the electric polarization on electron transport in ferroelectric tunnel junctions (FTJs). Using a Pt/BaTiO3/Pt FTJ as a model system, we show that the polarization of the BaTiO3 barrier leads to a substantial drop in the tunneling conductance due to changes in the electronic structure driven by ferroelectric displacements. We find a sizable change in the transmission probability across the Pt/BaTiO3 interface with polarization reversal, a signature of the electroresistance effect. These results reveal exciting prospects that FTJs offer as resistive switches in nanoscale electronic devices.     

单层有机器件的电子传输特性的数值模拟

在漂移扩散模型的基础上建立了单层有机器件的模型,包括了电荷注入、传输、空间电荷效应和陷阱的影响.电荷注入考虑了热电子发射电流和隧道电流.模拟得到的结果和文献中报道的实验测试数据一致.模拟研究了各个因素对器件J-V曲线的影响,电流和器件长度成反比,电流随着空穴注入势垒的减小而增加.电子注入势垒从1.7 eV减少到0.5 eV时,电流随着电子注入势垒的减小而减小,这主要是因为有机材料中电子迁移率太小,电子注入电流的增加可以忽略,而电子注入势垒的减小使内建势增加,在同样的电压下,场强 关键词： 有机器件 传输特性 数值模拟     

Imaging of spatial structures in superconducting tunnel junctions by electron-beam scanning

The voltage change caused in a current-biased superconducting tunnel junction by scanning the junction surface with an electron beam can serve to generate a two-dimensional image of spatial structures within the specimen. Depending upon the bias point during the scanning process, an image of local variations in the tunneling current density due to variations of the tunnel barrier resistance or in the energy gaps of the electrodes is obtained. PbAuIn/PbBi cross-line and SiO-window junctions have been used to demonstrate this imaging technique.     

Bias-induced reconstruction of hybrid interface states in magnetic molecular junctions

Based on first-principles calculations, the bias-induced evolutions of hybrid interface states in π-conjugated tricene and in insulating octane magnetic molecular junctions are investigated. Obvious bias-induced splitting and energy shift of the spin-resolved hybrid interface states are observed in the two junctions. The recombination of the shifted hybrid interface states from different interfaces makes the spin polarization around the Fermi energy strongly bias-dependent. The transport calculations demonstrate that in the π -conjugated tricene junction, the bias-dependent hybrid interface states work efficiently for large current, current spin polarization, and distinct tunneling magnetoresistance. But in the insulating octane junction, the spin-dependent transport via the hybrid interface states is inhibited, which is only slightly disturbed by the bias. This work reveals the phenomenon of bias-induced reconstruction of hybrid interface states in molecular spinterface devices, and the underlying role of conjugated molecular orbitals in the transport ability of hybrid interface states.     

Dynamic spin-polarized resonant tunneling in magnetic tunnel junctions

Precisely engineered tunnel junctions exhibit a long sought effect that occurs when the energy of the electron is comparable to the potential energy of the tunneling barrier. The resistance of metal-insulator-metal tunnel junctions oscillates with an applied voltage when electrons that tunnel directly into the barrier's conduction band interfere upon reflection at the classical turning points: the insulator-metal interface and the dynamic point where the incident electron energy equals the potential barrier inside the insulator. A model of tunneling between free electron bands using the exact solution of the Schr?dinger equation for a trapezoidal tunnel barrier qualitatively agrees with experiment.     

Are "pinholes" the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic pinholes in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlO(x)/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlO(x)/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement with experiment over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.     

Spin-polarized tunneling in a ferromagnetic graphene junction: Interplay between the exchange interaction and the orbital effect of the magnetic field

The influence of magnetic vector potential barrier (MVPB) on the spin-polarized transport of massless Dirac particles in ferromagnetic graphene is studied theoretically. The phenomenon of Klein tunneling of relativistic particles across a rectangular potential barrier prevents any of the massless fermions from being confined but they can be electrically confined by quantum dots with integrable dynamics (Bardarson et al., 2009) [36]. Utilization of only the in-plane exchange splitting in the ferromagnetic graphene cannot produce 100% spin polarization. This tunneling can be confined using the magnetic vector potential barrier, which leads to high degree of spin polarization. By combining the orbital effect and the Zeeman interaction in graphene junction, it is found that the junction mimics behavior of half-metallic tunneling junction, in which it acts as a metal to particles of one spin orientation but as an insulator or a semiconductor to those of the opposite orientation. The idea of the half-metallic tunneling junction can provide a source of ∼100% spin-polarized current, which is potentially very useful. Adjustment of the position of the Fermi level in ferromagnetic layer by placing a gate voltage on top of the ferromagnetic layer shows that reverse of the orientation of the completely spin-polarized current passing through the junction is controlled by adjusting the gate voltage. These interesting characteristics should lead to a practical gate voltage controlled spin filtering and spin-polarized switching devices as a perfect spin-polarized electron source for graphene-based spintronics.     

Effect of mechanical force on domain switching in BiFeO_3 ultrathin films

Ferroelectric polarization can be switched by an external applied electric field and may also be reversed by a mechanical force via flexoelectricity from the strain gradient.In this study,we report the mechanical writing of an epitaxial BiFeO3(BFO)thin film and the combined action of an applied mechanical force and electric field on domain switching,where the mechanical force and electric field are applied using the tip of atomic force microscopy.When the applied force exceeds the threshold value,the upward polarization of the BFO thin film can be reversed by pure mechanical force via flexoelectricity;when an electric field is simultaneously applied,the mechanical force can reduce the coercive electric field because both the piezoelectricity from the homogeneous strain and the flexoelectricity from strain gradient contribute to the internal electric field in the film.The mechanically switched domains exhibit a slightly lower surface potential when compared with that exhibited by the electrically switched domains due to no charge injection in the mechanical method.Furthermore,both the mechanically and electrically switched domains exhibit a tunneling electroresistance in the BFO ferroelectric tunnel junction.     

外电场下应变量子阱中电子与空穴的本征态

考虑自发与压电极化引起的内建电场,自由电子-空穴气屏蔽效应和外加电场,基于常微分数值计算,自洽求解电子与空穴的薛定谔方程和泊松方程以获得基态能级。以典型的GaN/A lxGa1-xN纤锌矿氮化物应变量子阱为例,通过数值求解,得到电子与空穴的本征基态能和相应本征波函数。计算结果表明:沿量子阱生长方向所施加的外加电场将抵消阱中内建电场的作用,阱结构的弯曲程度略显平缓,使电子(空穴)本征波函数逆(顺)着外电场方向移动,且均向阱中心移动,波峰峰值增加,隧穿几率减小;在固定外电场情况下,电子与空穴基态能级随阱宽的增加而减小,随掺杂组分的增加而增加,表明外加电场对内建电场有所削弱以及量子限制作用对电子(空穴)基态能有显著的影响。