Impurity effect on surface states of Bi(111) ultrathin films

The surface impurity effect on the surface-state conductivity and weak antilocalization(WAL) effect has been investigated in epitaxial Bi(111) films by magnetotransport measurements at low temperatures. The surface-state conductivity is significantly reduced by the surface impurities of Cu, Fe, and Co. The magnetotransport data demonstrate that the observed WAL is robust against deposition of nonmagnetic impurities, but it is quenched by the deposition of magnetic impurities which break the time reversal symmetry. Our results help to shed light on the effect of surface impurities on the electron and spin transport properties of a 2D surface electron systems.     

超导材料的正常态下杂质锌引起的非均匀性

本文采用含有非磁性杂质的二维t-J模型来描述掺锌的高温氧化物超导材料,并利用非均匀双时格林函数理论对有限尺寸的材料进行了数值计算.在 Fermion-Spin 理论框架下,我们分别研究了非磁性杂质引起的电荷和自旋的不均匀性及对材料正常态特性的影响.我们发现:一方面非磁性杂质锌对其附近的自旋反铁磁关联有强烈的影响,能使周围的最近邻关联函数的绝对值比均匀情况增加很多;另一方面,空位子会受到其周围自旋的影响而出现相应的分布不均匀性,进而又影响自旋关联的不均匀性.并且,围绕杂质锌能够形成自旋激发和空位激发的局域态.     

Thermally induced spin polarization and thermal conductivities in a spin–orbit-coupled two-dimensional electron gas

The thermal conductivities and spin polarization induced by the temperature gradient are investigated in a Rashba spin–orbit-coupled two-dimensional electron gas. In this spin–orbit-coupled system in the presence of nonmagnetic or magnetic electron–impurity scattering, the Wiedemann–Franz law still holds. However, the spin polarization induced by the temperature gradient strongly depends on the property of impurities. The components of spin accumulation both perpendicular and parallel to the direction of the temperature gradient, and the thermally induced charge Hall conductivity may be nonzero for magnetic disorders.     

Magnetization of beryllium oxide in the presence of nonmagnetic impurities: Boron,carbon, and nitrogen

The electronic and magnetic states of a nonmagnetic insulator, namely, beryllium oxide, doped with nonmagnetic 2p elements (boron, carbon, and nitrogen) are studied using the density functional theory. The spin polarization of the 2p impurity states, as well as the transition of the doped BeO:(B,C,N) systems to the states of semiconducting or half-metallic magnets, is observed. The prospects for creating new magnetic materials by doping nonmagnetic insulators with nonmagnetic p impurities are discussed.     

Extrinsic spin Hall effect induced by iridium impurities in copper

We study the extrinsic spin Hall effect induced by Ir impurities in Cu by injecting a pure spin current into a CuIr wire from a lateral spin valve structure. While no spin Hall effect is observed without Ir impurity, the spin Hall resistivity of CuIr increases linearly with the impurity concentration. The spin Hall angle of CuIr, (2.1±0.6)% throughout the concentration range between 1% and 12%, is practically independent of temperature. These results represent a clear example of predominant skew scattering extrinsic contribution to the spin Hall effect in a nonmagnetic alloy.     

Photoemission spectroscopy of magnetic and nonmagnetic impurities on the surface of the Bi2Se3 topological insulator

Dirac-like surface states on surfaces of topological insulators have a chiral spin structure that suppresses backscattering and protects the coherence of these states in the presence of nonmagnetic scatterers. In contrast, magnetic scatterers should open the backscattering channel via the spin-flip processes and degrade the state's coherence. We present angle-resolved photoemission spectroscopy studies of the electronic structure and the scattering rates upon the adsorption of various magnetic and nonmagnetic impurities on the surface of Bi2Se3, a model topological insulator. We reveal a remarkable insensitivity of the topological surface state to both nonmagnetic and magnetic impurities in the low impurity concentration regime. Scattering channels open up with the emergence of hexagonal warping in the high-doping regime, irrespective of the impurity's magnetic moment.     

Self-consistent calculation of hyperfine fields and adiabatic potential of impurities in iron

Hyperfine fields of impurities of the atomic number Z=1–56 at the substitutional site and those of light impurities of Z=1–9 at the interstitial sites in ferromagnetic iron are calculated by the KKR method adapted to the system containing a single impurity atom. The potential of the impurity atom is determined self-consistently by use of the local spin density functional formalism. The results for nonmagnetic sp valence impurities agree with those of the previous nonself-consistent calculation by Katayama-Yoshida, Terakura and Kanamori except for a few cases, confirming their theory of the systematic variation of hyperfine fields. The calculation for magnetic impurities of transition elements is presented for the first time in this paper. The calculations mentioned so far assume that impurities are situated at the center of each site. For the purpose of discussing the stability of the impurity positions, the change of the adiabatic potential due to displacements from the center is calculated by carrying out similar self-consistent calculations for off-center impurity positions. It is concluded that positive muon and some light impurities including boron will be displaced from the center when trapped in a vacancy.     

Heat Generation by Electric Current in a Quantum Dot Molecular Coupled to Ferromagnetic Leads

We study the properties of the heat flow generated by electric current in a quantum dot(QD) molecular sandwiched between two ferromagnetic leads. The heat is exchanged between the QD and the phonon reservoir coupled to it. We find that when the leads' magnetic moments are in parallel configuration, the total heat generation is independent on the leads' spin-polarization regardless of the magnitude of the intradot Coulomb interaction. This behavior is similar to that of the electronic current. In the antiparallel configuration, however, the influences of the leads' ferromagnetism on the heat generation are quite different from those on the electric current. Under the conditions of weak intradot Coulomb interaction and small bias voltage, the heat generation is monotonously suppressed by increasing leads' spin-polarization.Whereas for sufficient large intradot Coulomb interaction and bias voltage, the heat generation shows non-monotonous behavior due to the electron-phonon interaction and the spin accumulation induced on the dot. Furthermore, the magnitude of the negative differential of the heat generation previously found in a QD connected to nonmagnetic leads can be weakened by the increase of the spin-polarization of the ferromagnetic leads.     

Heat Generation by Electric Current in a Quantum Dot Molecular Coupled to Ferromagnetic Leads

We study the properties of the heat flow generated by electric current in a quantum dot (QD) molecular sandwiched between two ferromagnetic leads. The heat is exchanged between the QD and the phonon reservoir coupled to it. We find that when the leads' magnetic moments are in parallel configuration, the total heat generation is independent on the leads' spin-polarization regardless of the magnitude of the intradot Coulomb interaction. This behavior is similar to that of the electronic current. In the antiparallel configuration, however, the influences of the leads' ferromagnetism on the heat generation are quite different from those on the electric current. Under the conditions of weak intradot Coulomb interaction and small bias voltage, the heat generation is monotonously suppressed by increasing leads' spin-polarization. Whereas for sufficient large intradot Coulomb interaction and bias voltage, the heat generation shows non-monotonous behavior due to the electron-phonon interaction and the spin accumulation induced on the dot. Furthermore, the magnitude of the negative differential of the heat generation previously found in a QD connected to nonmagnetic leads can be weakened by the increase of the spin-polarization of the ferromagnetic leads.     

Spin Hall effect induced by resonant scattering on impurities in metals

The spin Hall effect is a promising way for transforming charge currents into spin currents in spintronic devices. Large values of the spin Hall angle, the characteristic parameter of the yield of this transformation, have been recently found in noble metals doped with nonmagnetic impurities. We show that this can be explained by resonant scattering off impurity states split by the spin-orbit interaction. By using as an example copper doped with 5d impurities we describe the general conditions and provide a guide for experimentalists for obtaining the largest effects.     

Pure spin-current diode based on interacting quantum dot tunneling junction

A magnetic field-controlled spin-current diode is theoretically proposed, which consists of a junction with an interacting quantum dot sandwiched between a pair of nonmagnetic electrodes. By applying a spin bias V_S across the junction, a pure spin current can be obtained in a certain gate voltage regime,regardless of whether the Coulomb repulsion energy exists. More interestingly, if we applied an external magnetic field on the quantum dot, we observed a clear asymmetry in the spectrum of spin current I_S as a function of spin bias, while the charge current always decays to zero in the Coulomb blockade regime. Such asymmetry in the current profile suggests a spin diode-like behavior with respect to the spin bias, while the net charge through the device is almost zero. Different from the traditional charge current diode, this design can change the polarity direction and rectifying ability by adjusting the external magnetic field, which is very convenient. This device scheme can be compatible with current technologies and has potential applications in spintronics or quantum processing.     

Perspectives of electrically generated spin currents in ferromagnetic materials

Spin-orbit coupling enables charge currents to give rise to spin currents and vice versa, which has applications in non-volatile magnetic memories, miniature microwave oscillators, thermoelectric converters and Terahertz devices. In the past two decades, a considerable amount of research has focused on electrical spin current generation in different types of nonmagnetic materials. However, electrical spin current generation in ferromagnetic materials has only recently been actively investigated. Due to the additional symmetry breaking by the magnetization, ferromagnetic materials generate spin currents with different orientations of spin direction from those observed in nonmagnetic materials. Studies centered on ferromagnets where spin-orbit coupling plays an important role in transport open new possibilities to generate and detect spin currents. We summarize recent developments on this subject and discuss unanswered questions in this emerging field.     

自旋轨道耦合系统中的自旋流与自旋霍尔效应

作为自旋电子学的重要研究内容,如何在固态系统中产生、操控以及探测自旋流引起了研究人员的广泛兴趣。基于自旋轨道耦合的自旋霍尔效应为在非磁性半导体中产生自旋流提供了一种有效途径。然而,在具有自旋轨道耦合的系统中,自旋流并不守恒。如何理解这点并恰当地表述相应的连续性方程,成为自旋输运研究的基本问题之一。本文主要综述自旋轨道耦合系统中自旋流与自旋霍尔效应方面的研究进展。引入SU(2)规范势后,自旋流满足协变形式的连续性方程,该方程保证了SU(2)Kubo公式在不同规范固定下的自洽性。利用SU(2)场强张量,可以直接得到自旋密度和自旋流在SU(2)外场中受到的自旋力,该力在只有U(1)磁场时对应于Stern-Gerlach力。由于依赖杂质散射的外在自旋霍尔效应很难被利用,内在自旋霍尔效应的概念被提出:在非磁半导体中,U(1)电场会诱导出自旋流并导致系统边缘处的自旋积累。自旋霍尔效应已经在半导体和金属材料中被观察到。虽然在干净的二维电子气中自旋霍尔电导率是一普适常数e/8π,但杂质对它的影响却引起了人们的高度关注。通过引入退相干效应,自旋霍尔效应中杂质效应的一些令人困惑的理论结果,则得到清晰的解释。此外,本文还将介绍具有层间隧穿的双层二维电子气中的自旋输运现象。在能量简并点附近,自旋霍尔电导率和隧穿自旋电导率均会出现共振现象。当两层间的杂质势强度存在差异时,隧穿自旋电导率随门压的变化曲线呈现出非对称性,显示出自旋二极管效应。     

自旋轨道耦合系统中的自旋流与自旋霍尔效应

作为自旋电子学的重要研究内容,如何在固态系统中产生、操控以及探测自旋流引起了研究人员的广泛兴趣.基于自旋轨道耦合的自旋霍尔效应为在非磁性半导体中产生自旋流提供了一种有效途径.然而,在具有自旋轨道耦合的系统中,自旋流并不守恒.如何理解这点并恰当地表述相应的连续性方程,成为自旋输运研究的基本问题之一.本文主要综述自旋轨道耦合系统中自旋流与自旋霍尔效应方面的研究进展.引入SU(2)规范势后,自旋流满足协变形式的连续性方程,该方程保证了SU(2)Kubo公式在不同规范固定下的自洽性.利用SU(2)场强张量,可以直接得到自旋密度和自旋流在SU(2)外场中受到的白旋力,该力在只有U(1)磁场时对应于Stern-Gerlach力.由于依赖杂质散射的外在自旋霍尔效应很难被利用,内在自旋霍尔效应的概念被提出:在非磁半导体中,U(1)电场会诱导出自旋流并导致系统边缘处的自旋积累.自旋霍尔效应已经在半导体和金属材料中被观察到.虽然在干净的二维电子气中自旋霍尔电导率是一普适常数e/8π,但杂质对它的影响却引起了人们的高度关注.通过引入退相干效应,自旋霍尔效应中杂质效应的一些令人困惑的理论结果,则得到清晰的解释.此外,本文还将介绍具有层间隧穿的双层二维电子气中的自旋输运现象.在能量简并点附近,自旋霍尔电导率和隧穿白旋电导率均会出现共振现象.当两层间的杂质势强度存在差异时,隧穿自旋电导率随门压的变化曲线呈现出非对称性,显示出自旋二极管效应.     

Zinc impurities in d-wave superconductors

We study the two-dimensional Hubbard model with nonmagnetic Zn impurities modeled by binary diagonal disorder using quantum Monte Carlo within the dynamical cluster approximation. With increasing Zn content we find a strong suppression of d-wave superconductivity and an enhancement of antiferromagnetic spin correlations. T(c) vanishes linearly with Zn impurity concentration. The spin susceptibility changes from pseudogap to Curie-Weiss-like behavior indicating the existence of free magnetic moments in the Zn doped system. We interpret these results within the resonating-valence-bond picture.     

On the theory of impurities in a lattice of magnetic ions: Crystalline field effects

We present a formalism for treating the problem of impurities in a lattice of magnetic rare earth ions. Latter are subject to a crystalline field and special attention is paid to non-Kramers ions in a singlet ground state. Our calculations are restricted to the paramagnetic regime. We derive the conditions for magnetic localized modes to occur and discuss the appearance of local magnetic instabilities. It is shown that the impurity effects are especially large if the system is close to a magnetic phase transition. Furthermore we compute the influence of impurities on the magnetic transition temperature. For the case of vacancies or nonmagnetic impurities the dependence of the Curie temperature on impurity concentration is derived. It is demonstrated that small amounts of impurities can often completely suppress magnetic ordering.     

Spin-Hall transport in a two-dimensional electron system with magnetic impurities

The spin Hall transport properties in a two-dimensional electron system with both Rashba spin-orbit coupling (SOC) and magnetic impurities are investigated. Electrons are scattered by impurities through an exchange interaction that leads to spin flip-flop processes and so changes the spin Hall effect induced by the SOC. The spin Hall conductance is calculated in a 4-terminal system using the Landauer-Buttiker formula and Green function approach. In comparison with the simulation results on nonmagnetic impurities doping systems, our results reveal that the spin Hall conductance is still nonzero in a system with a large density of magnetic impurities and a finite intensity of the exchange interaction between the electrons and impurities, and its sign may be altered when the doping density and interaction strength are large enough.     

Density functional theory description of the mechanism of ferromagnetism in nitrogen-doped SnO2

Based on first-principles calculations, we have studied the occurrence of spin polarization in the magnetic metal oxide SnO₂ doped with nonmagnetic nitrogen (N) impurities. It was found that the local magnetic moments are localized mainly on the N dopant, causing a total moment of 0.95μB per cell. The long-range magnetic coupling of N-doped SnO₂ may be attributed to a p-p coupling interaction between the N impurity and host valence states.     

Quantum corrals, eigenmodes, and quantum mirages in s-wave superconductors

We study the electronic structure of magnetic and nonmagnetic quantum corrals embedded in two-dimensional s-wave superconductors. We demonstrate that a quantum mirage of an impurity bound state is projected from the occupied into the empty focus of a nonmagnetic quantum corral via the excitation of the corral's eigenmodes. We show that quantum corrals provide a new tool for manipulating the interaction between magnetic impurities by exciting oscillations in the corral's impurity potential. Finally, we discuss the form of eigenmodes in magnetic quantum corrals.     

Impurity-induced tuning of quantum-well States in spin-dependent resonant tunneling

We report exact model calculations of the spin-dependent tunneling in double magnetic tunnel junctions in the presence of impurities in the well. We show that the impurity can tune selectively the spin channels giving rise to a wide variety of interesting and novel transport phenomena. The tunneling magnetoresistance, the spin polarization, and the local current can be dramatically enhanced or suppressed by impurities. The underlying mechanism is the impurity-induced shift of the quantum well states (QWSs), which depends on the impurity potential, impurity position, and the symmetry of the QWS.