Entanglement in One-Dimensional Random XY Spin Chain with Dzyaloshinskii--Moriya Interaction

The impurities of exchange couplings, external magnetic fields and Dzyaloshinskii-Moriya （DM） interaction considered as Gaussian distribution, and the entanglement in one-dimensional random XY spin systems is investigated by the method of solving the different spin-spin correlation functions and the average magnetization per spin. The entanglement dynamics at central locations of ferromagnetic and antiferromagnetic chains have been studied by varying the three impurities and the strength of DM interaction. （i） For the ferromagnetic spin chain, the weak DM interaction can improve the amount of entanglement to a large value, and the impurities have the opposite effect on the entanglement below and above critical DM interaction. （ii） For the antiferromagnetic spin chain, DM interaction can enhance the entanglement to a steady value. Our results imply that DM interaction strength, the impurity and exchange couplings （or magnetic field） play competing roles in enhancing quantum entanglement.     

金属表面Rashba自旋轨道耦合作用研究进展

自旋轨道耦合是电子自旋与轨道相互作用的桥梁, 它提供了利用外电场来调控电子的轨道运动、进而调控电子自旋状态的可能. 固体材料中有很多有趣的物理现象, 例如磁晶各向异性、自旋霍尔效应、拓扑绝缘体等, 都与自旋轨道耦合密切相关. 在表面/界面体系中, 由于结构反演不对称导致的自旋轨道耦合称为Rashba自旋轨道耦合, 它最早在半导体材料中获得研究, 并因其强度可由栅电压灵活调控而备受关注, 成为电控磁性的重要物理基础之一. 继半导体材料后, 金属表面成为具有Rashba自旋轨道耦合作用的又一主流体系. 本文以Au(111), Bi(111), Gd(0001)等为例综述了磁性与非磁性金属表面Rashba自旋轨道耦合的研究进展, 讨论了表面电势梯度、原子序数、表面态波函数的对称性, 以及表面态中轨道杂化等因素对金属表面Rashba自旋轨道耦合强度的影响. 在磁性金属表面, 同时存在Rashba自旋轨道耦合作用与磁交换作用, 通过Rashba自旋轨道耦合可能实现电场对磁性的调控. 最后, 阐述了外加电场和表面吸附等方法对金属表面Rashba自旋轨道耦合的调控. 基于密度泛函理论的第一性原理计算和角分辨光电子能谱测量是金属表面Rashba自旋轨道耦合的两大主要研究方法, 本文综述了这两方面的研究结果, 对金属表面Rashba自旋轨道耦合进行了深入全面的总结和分析.     

掺铌SrTiO_3中的逆自旋霍尔效应

采用磁控溅射法在未掺杂和掺杂的SrTiO_3基片上沉积了NiFe薄膜,通过翻转测试法分离出掺杂样品中的自旋整流电压和逆自旋霍尔电压.研究结果表明:在未掺杂的SrTiO_3基片中,翻转前后测试的电压曲线基本一致,为NiFe薄膜自旋整流效应产生的电压.对于掺Nb浓度x为0.028, 0.05, 0.1, 0.15, 0.2的SrTiO_3基片,分离出的逆自旋霍尔电压随掺杂浓度增加而减小,在掺杂浓度为0.15和0.2的样品中没有探测到明显的逆自旋霍尔电压.本文的结果表明,在SrTiO_3中掺入强自旋轨道耦合的杂质,通过掺杂浓度可以实现对SrTiO_3中逆自旋霍尔效应的调控,这类可调控的自旋相关研究为自旋电子器件的研究和开发提供了更多的可能性,具有很大的潜在应用价值.     

Vertex correction of the anisotropic magnetic impurities to the spin polarization of 2D electron gas

In terms of Kubo's formula and Green's function method, for the two-dimensional electron gas (2DEG) with Rashba spin-orbit coupling (SOC), we study the spin polarization due to the effect from magnetic impurities with anisotropic spin dependent delta type coupling to electrons when an external dc electric field in plane is applied. The vertex correction of impurities in ladder approximation is carried out in the limit of EF?1/τ, Δ. We find that the strength of spin polarization can be significantly modified by vertex correction and the spin polarization is relevant to the anisotropy coefficient γ, but the direction of net spin polarization cannot be changed.     

Some consequences of relativistic effects upon optical spectra

Large spin–orbit interaction produces large orbital magnetic moments in narrow energy bands. Since the orbital character of the wave functions is more important in orbital than in spin magnetism, the limitations of the local spin density approximation become evident. It is possible to keep the orbital dependence of the exchange interactions by using an orbital polarization scheme or by using Hartree–Fock theory with screened Slater integrals for exchange. This leads to an enhancement of the calculated orbital moment when the magnetism is strong. Recently calculated magnetic moments and calculated sum rules for X-ray magnetic circular dichroism in US are described. Received: 23 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

含stubs量子波导系统的电子自旋极化输运性质

理论上研究了含stubs的Rashba自旋轨道耦合（spin-orbit coupling, SOC）量子波导系统的自旋极化输运性质. 利用晶格格林函数方法,发现由于stubs和SOC产生的势阱使系统中出现束缚态,这些束缚态与传播态之间相互干涉导致电导中出现Fano共振结构,同时在对应的自旋极化率中也出现Fano共振或反共振结构. 此外,由于系统结构的突变使电子被反向散射和量子干涉效应,电导中出现一系列的共振峰. 但是,当系统加上外磁场后,所有这些效应都被抑制, 系统重新出现量子化电导, 同时自旋电导也出 关键词： 量子波导 自旋极化输运 自旋轨道耦合     

Kondo effect in the presence of magnetic impurities

We measure transport through gold grain quantum dots fabricated using electromigration, with magnetic impurities in the leads. A Kondo interaction is observed between dot and leads, but the presence of magnetic impurities results in a gate-dependent zero-bias conductance peak that is split due to a RKKY interaction between the spin of the dot and the static spins of the impurities. A magnetic field restores the single Kondo peak in the case of an antiferromagnetic RKKY interaction. This system provides a new platform to study Kondo and RKKY interactions in metals at the level of a single spin.     

Effect of non-uniform exchange field in ferromagnetic graphene

We have presented here the consequences of the non-uniform exchange field on the spin transport issues in spin chiral configuration of ferromagnetic graphene. Taking resort to the spin–orbit coupling (SOC) term and non-uniform exchange coupling term we are successful to express the expression of Hall conductivity in terms of the exchange field and SOC parameters through the Kubo formula approach. However, for a specific configuration of the exchange parameter we have evaluated the Berry curvature of the system. We also have paid attention to the study of SU(2) gauge theory of ferromagnetic graphene. The generation of anti damping spin–orbit torque in spin chiral magnetic graphene is also briefly discussed.     

具有Dzyaloshinskii-Moriya相互作用的一维随机量子XY模型中的纠缠特性

研究了一维随机量子XY自旋链中中心两量子位的纠缠特性,在该系统中引入了自旋间的交换耦合杂质、磁杂质和Dzyaloshinskii-Moriya相互作用,并且杂质满足高斯分布关系.通过数值计算,求出了自旋的关联函数和平均磁化强度,给出了Concurrence的解析表达式.结果表明：高斯分布和Dzyaloshinskii-Moriya相互作用对两量子位的纠缠有重要的影响,选择合适的交换耦合、外界磁场和Dzyaloshinskii-Moriya相互作用参数,可以控制和提高中心两量子位的纠缠. 关键词： 纠缠 XY模型')" href="#">随机量子XY模型 高斯分布 Dzyaloshinskii-Moriya相互作用     

The exchange interaction effect in the scanning tunneling spectroscopy of a two-orbital Anderson impurity on metallic surface

We investigate the scanning tunneling spectroscopy (STS) of a two-orbital Anderson impurity adsorbed on a metallic surface by using the numerical renormalization group (NRG) method. The density of state of magnetic impurity and the local conduction electron are calculated. We obtain the Fano resonance line shape in the STM conductance at zero temperature. For the impurity atom with antiferromagnetic inter-orbital exchange interaction and a spin singlet ground state, we show that a dip in the STM spectra around zero bias voltage regime and side peaks of spin excitation can be observed. The spin excitation energy is proportional to the exchange interaction strength. As the exchange interaction is ferromagnetic, the underscreened Kondo effect dominates the low energy properties of this system, and it gives rise to drastically different STM spectra as compared with the spin singlet case.     

Aharonov-Bohm effect as a probe of interaction between magnetic impurities

We study the effects of the RKKY interaction between magnetic impurities on the mesoscopic conductance fluctuations of a metal ring with dilute magnetic impurities. At sufficiently low temperatures and strong magnetic fields, the loss of electron coherence occurs mainly due to the scattering off rare pairs of strongly coupled magnetic impurities. We establish a relation between the dephasing rate and the distribution function of the exchange interaction within such pairs. In the case of the RKKY exchange interaction, this rate exhibits 1/B(2) behavior in strong magnetic fields. We demonstrate that the Aharonov-Bohm conductance oscillations may be used as a probe of the distribution function of the exchange interaction between magnetic impurities in metals.     

Effect of impurity clustering on ferromagnetism in dilute magnetic semiconductors

We establish a model to investigate the effect of clustering of impurities on the ferromagnetism in dilute magnetic semiconductors (DMS). The Curie temperature Tc is calculated by the mean-field theory on a lattice with randomly distributed clusters of magnetic impurities which are interacting with each other by carrier mediated RKKY exchange coupling together with the nearest-neighbor (NN) direct exchange interaction. We consider different types and sizes of the clusters and find that the clustering of impurities can either enhance or reduce Tc, depending on the type and strength of the NN exchange interaction. If the NN interaction is antiferromagnetic and strong compared with the RKKY interaction, the clustering will reduce Tc. On the other hand, if it is ferromagnetic interaction or weak antiferromagnetic one, the clustering can enhance Tc. The trend of enhancing Tc is magnified if the average size of clusters increases. The clustering also changes the distribution of polarizations of impurities. The obtained results provide natural explanations on the fact that the ferromagnetism of DMS samples depends on the preparing and annealing processes even though the density of the magnetic impurities is kept the same.     

Magnetic behavior and clustering effects in Mn-doped boron nitride sheets

Ab initio calculations are performed in the framework of density functional theory on Mn-doped boron nitride sheets, which are candidates for two-dimensional diluted magnetic semiconductors (DMSs). Each type of substitution reveals a qualitatively different magnetic behavior encompassing ferromagnetic, anti-ferromagnetic and spin glass ordering. The ability of formation of these defects is also discussed. We analyze the dependence of the exchange couplings on the distance between impurities and the typical range and distribution are extracted. Multiple-impurity configurations are considered and the results are mapped on an Ising-type Hamiltonian with higher order exchange interactions, revealing deviations from the standard two-spin models. The percolation of interacting magnetic moments is discussed and the critical concentration is determined for the underlying transition from a ferromagnetic to a super-paramagnetic state. We conclude our study by providing the optimal conditions for doping in order to obtain a ferromagnetic DMS.     

Controlling spin-dependent transport via inhomogeneous magnetic flux in double-dot Aharonov-Bohm interferometer

We study the spin-dependent electron transport through parallel coupled quantum dots (QDs) embedded in an Aharonov-Bohm (AB) interferometer connected asymmetrically to leads. Both the Rashba spin-orbit interaction (RSOI) inside one of the QDs, which acquires a spin-dependent phase factor in the tunnel-coupling strengths when the electrons flow through this arm of the AB ring, and an inhomogeneous magnetic flux penetrating the structure are taken into account. Due to the existence of the RSOI induced phase factor, magnetic flux and the interdot coupling, a spin-dependent Fano effect will arise. We pay special attention on the properties of the local density of states and the conductance when the electron phase factor is close to integer multiplies of a quantum of flux. It is shown that the roles and lifetimes of the bonding and antibonding states of the two spin components are very sensitive to the phase factor and can be well controlled accordingly. This manipulation of the spin degree of freedom relies on the existence of RSOI but can be fulfilled even when its strength is very weak. The proposed structure can be easily realized with present technology and might be of practical applications in spintronics devices and quantum computing.     

Current-induced polarization and the spin Hall effect at room temperature

Electrically induced electron spin polarization is imaged in n-type ZnSe epilayers using Kerr rotation spectroscopy. Despite no evidence for an electrically induced internal magnetic field, current-induced in-plane spin polarization is observed with characteristic spin lifetimes that decrease with doping density. The spin Hall effect is also observed, indicated by an electrically induced out-of-plane spin polarization with opposite sign for spins accumulating on opposite edges of the sample. The spin Hall conductivity is estimated as 3+/-1.5 Omega(-1) m(-1)/|e| at 20 K, which is consistent with the extrinsic mechanism. Both the current-induced spin polarization and the spin Hall effect are observed at temperatures from 10 to 295 K.     

Electronic structure and magnetism of Fe-doped SiC nanotubes

The electronic structure and magnetic properties of Fe-doped SiC nanotubes are investigated by using the first-principles method based on density functional theory(DFT) in the local spin density approximation(LSDA).The calculation results indicate that the SiC nanotube of Fe substitution for C exhibits antiferromagnetism while ferromagnetism features prominently when Fe substitutes Si.This is a kind of half-metal magnetic material.The formation energy calculation results show that the formation energy of ferromagnetic structure is 3.2 eV lower than that of antiferromagnetic structure.Fe atoms are more likely to replace Si atoms.Spin-orbit coupling induces electron spin polarization in the ground state.Also,the doping Fe atoms make relaxation towards the outside of the tube to some extent and larger geometric distortion occurs when Fe substitutes C,but the whole geometric structure of SiC nanotubes is not damaged due to the doping.It is revealed in the calculation of energy band structure and density of states that more dispersed distribution of energy levels is produced near the Fermi level.For Fe substitution for Si,obviously there are spin-split and intense p-d hybrid effects by Si 3p electron spins and Fe 3d electron spins localized at the exchanging interactions between magnetic transitional metal(TM) impurities.Spin electronic density results indicate that system magnetic moments are mainly generated by the unpaired 3d electrons of Fe atoms.All these results show that the transition metal doping SiC nanotube could be a potential route to fabricating the promising magnetic materials.     

Impact of Phosphorus superlattices on charge and spin dependent transport properties of zigzag silicene nanoribbons

To investigate charge and spin dependent conductance properties of Phosphorus doped zigzag silicene nanoribbons (ZSiNRs), we utilize recursive Green's function method and Landauer-Büttiker formalism. Our calculations are performed in the absence and presence of exchange magnetic fields with both parallel and antiparallel configurations. Considering a supperlattice of Phosphorus substituents in a periodic distribution at the edge of nanoribbon, the effect of increasing number of dopants and period of the distribution on transport properties are studied. It is found that transport properties of doped ZSiNRs vary with doping concentration according to being odd or even of number of dopants. For parallel configuration, doped ZSiNR with various concentrations works as a controllable spin filter with Fermi energy. Increasing doping concentration leads to increasing size of conductance gap and improvement of controlling quality of spin-filtering property while increasing period of Phosphorus atomic distribution has destructive effect on size of conductance gap and destroys spin-filtering property. Moreover, we show that although the same results are obtained for transport properties of doped ZSiNR with various concentrations of Phosphorus atoms in presence of antiparallel exchange magnetic fields, a completely controllable spin-filtering property cannot be achieved by Fermi energy changes.     

铁磁/非磁金属异质结中的拓扑霍尔效应

在铁磁/非磁金属异质结中,界面处的Dzyaloshinskii-Moriya相互作用会诱导诸如磁性斯格明子等手性磁畴壁结构的形成.当巡游电子通过手性磁畴壁结构时,会获得一个贝里相位,而相应的贝里曲率则等效于一个外磁场,它将诱导额外的霍尔效应,即拓扑霍尔效应.拓扑霍尔效应是当前磁性斯格明子和自旋电子学研究领域的热点之一.本文由实空间贝里相位出发,简要介绍了拓扑霍尔效应的物理机制;然后着重讨论了铁磁/非磁金属异质结中的拓扑霍尔效应,包括磁性多层膜中和MnGa/重金属双层膜中的拓扑霍尔效应.这两种结构都可以通过改变材料的厚度、种类、生长方式等调控界面Dzyaloshinskii-Moriya相互作用,从而有效地调控磁性斯格明子和拓扑霍尔效应.     

Spin transport in a chain of polygonal quantum rings with Dresselhaus spin-orbit coupling

Using a transfer matrix method, we investigate spin transport through a chain of polygonal rings with Dresselhaus spin-orbit coupling(DSOC). The spin conductance is dependent on the number of sides in the polygons. When DSOC is considered in a chain which also has Rashba spin-orbit coupling(RSOC) of the same magnitude, the total conductance is the same as that for the same chain with no SOC. However, when the two types of SOC have different values, there results a unique anisotropic conductance.     

CPA density of states and conductivity in a double-exchange system containing impurities

We study density of states and conductivity of the doped double-exchange system, treating interaction of charge carriers both with the localized spins and with the impurities in the coherent potential approximation. It is shown that under appropriate conditions there is a gap between the conduction band and the impurity band in paramagnetic phase, while the density of states is gapless in ferromagnetic phase. This can explain metal-insulator transition frequently observed in manganites and magnetic semiconductors. Activated conductivity in the insulator phase is numerically calculated. Received 13 June 2000 and Received in final form 5 January 2001