Magnetic properties of X<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>/Cu(001) overlayers,X = Fe,Mn, and Ir: ab-initio calculations

We study the effect of a magnetic impurity with spin-half on a single propagating electron in a one-dimensional model system via the tight-binding approach. Due to the spin-dependent interaction, the scattering channel for the flying qubit is split, and its transmission spectrum is obtained. It is found that, the spin orientation of the impurity plays the role as a spin state filter for a flying qubit.     

Spin-polarized transmission through a mesoscopic ring with a magnetic impurity

Based on one-dimensional quantum waveguide theory we study the symmetry of the spin-polarized transmission through an Aharonov–Bohm ring with a magnetic impurity, in which the spin-exchange interaction between an incident electron and the magnetic impurity leads to spin–flip scattering. It shows that for some special Fermi energies, both spin-up and spin-down transmission coefficients are symmetric under the flux reversal in the spin–flip scattering process and the spin-polarized conductance also is symmetric. In above case, AB oscillations of spin-down transmission and reflection are perfectly identical. The effect of the exchange interaction strength and Fermi wave vector on transmission behavior of spin-state electrons is examined.     

共轭高聚物链中极化子生成的掺杂效应

在紧束缚近似下,用绝热动力学演化的方法研究了共轭高聚物链中低浓度掺杂对极化子生成位置的影响。研究发现,掺入杂质与注入电子的先后次序不同,极化子生成的位置会有很大差别,先注入电子后掺入杂质的情况下,在链端自由区与过渡区,极化子生成位置受杂质影响较小;而先杂质后电子的次序下,极化子的生成位置明显受到杂质的控制。该研究表明共轭高聚物链中极化子的生成位置可通过调节掺杂与电荷注入的次序加以控制,从而可达到间接改变载流子迁移率的目的。     

The Fano antiresonance effect in the current-voltage characteristics of a nanostructure with a single magnetic impurity

A theoretical investigation is performed of quantum coherent electron transport through a nanostructure that contains an impurity ion with an uncompensated magnetic moment. It is shown that the transmission coefficient of spin-polarized electrons has the Fano antiresonance. This effect appears as a result of exchange interaction between the spin of transmitted electron and the spin of impurity ion. It is shown that Fano antiresonance leads to a qualitative modification of the current-voltage characteristic of the structure responsible for the large value of magnetoresistance.     

Absence of odd-even parity behavior for Kondo resonances in quantum dots

Zero-bias anomalies in the conductance through quantum dots have recently been identified as Kondo resonances and explained in terms of the Anderson impurity model. The effect requires a degeneracy and it has been proposed that this should occur for odd electron numbers on the dot. In this paper we present data, obtained on a split-gate quantum dot with a small number of electrons, which are in disagreement with this expectation. The mapping of the Anderson model on the quantum dot is discussed in terms of an interacting N electron system demonstrating why this expectation can fail.     

Smearing of the coulomb blockade by resonant tunneling

We study the Coulomb blockade in a grain coupled to a lead via a resonant impurity level. We show that the strong energy dependence of the transmission coefficient through the impurity level can have a dramatic effect on the quantization of the grain charge. In particular, if the resonance is sufficiently narrow, the Coulomb staircase shows very sharp steps even if the transmission through the impurity at the Fermi energy is perfect. This is in contrast to the naive expectation that perfect transmission should completely smear charging effects.     

Transport properties of surface acoustic wave based single electron transport device in the presence of an impurity potential

We report the transport properties of a surface acoustic wave based single electron transport device, which contains an unintentional quantum dot induced by background impurity potential fluctuations. It is found that the presence of the impurity potential can cause a deviation of the acoustoelectric current from its quantized value. Through the charging effect of the quantum dot induced by the impurity, we get an approximate relationship between the applied gate voltage and its corresponding electrostatic potential barrier height, together with the Coulomb charging energy needed to add a second electron into the dynamic quantum dot. Moreover, the amplitude of the surface acoustic wave is also estimated within a simple model.     

Heat flow through a plasma sheath in the presence of secondary electron emission from plasma‐wall interaction

The formation of a plasma sheath in front of a negative wall emitting secondary electron is studied by a one‐dimensional fluid model. The model takes into account the effect of the ion temperature. With the secondary electron emission (SEE ) coefficient obtained by integrating over the Maxwellian electron velocity distribution for various materials such as Be, C, Mo, and W, it is found that the wall potential depends strongly on the ion temperature and the wall material. Before the occurrence of the space‐charge‐limited (SCL ) emission, the wall potential decreases with increasing ion temperature. The variation of the sheath potential caused by SEE affects the sheath energy transmission and impurity sputtering yield. If SEE is below SCL emission , the energy transmission coefficient always varies with the wall materials as a result of the effect of SEE , and it increases as the ion temperature is increased. By comparison of with and without SEE , it is found that sputtering yields have pronounced differences for low ion temperatures but are almost the same for high ion temperatures.     

Efficient spin filtering in a disordered semiconductor superlattice in the presence of Dresselhaus spin-orbit coupling

The influence of the Dresselhaus spin-orbit coupling on spin polarization by tunneling through a disordered semiconductor superlattice was investigated. The Dresselhaus spin-orbit coupling causes the spin polarization of the electron due to transmission possibilities difference between spin up and spin down electrons. The electron tunneling through a zinc-blende semiconductor superlattice with InAs and GaAs layers and two variable distance In_xGa_(1−x)As impurity layers was studied. One hundred percent spin polarization was obtained by optimizing the distance between two impurity layers and impurity percent in disordered layers in the presence of Dresselhaus spin-orbit coupling. In addition, the electron transmission probability through the mentioned superlattice is too much near to one and an efficient spin filtering was recommended.     

Antiresonance induced spin-polarized current generation

According to the one-dimensional antiresonance effect (Wang X R, Wang Y and Sun Z Z 2003 Phys. Rev. B 65 193402), we propose a possible spin-polarized current generation device. Our proposed model consists of one chain and an impurity coupling to the chain. The energy level of the impurity can be occupied by an electron with a specific spin, and the electron with such a spin is blocked because of the antiresonance effect. Based on this phenomenon our model can generate the spin-polarized current flowing through the chain due to different polarization rates. On the other hand, the device can also be used to measure the generated spin accumulation. Our model is feasible with today's technology.     

Path-integral approach to the two-dimensional magneto-conductivity problem

We express Green's function and conductivity of an electron impurity system in the presence of a magnetic field or arbitrary strength in terms of Feynman path integrals and obtain an approximate formula for both the propagator and conductivity as the lowest order contribution of a systematic cumulant expansion. This approach takes the coherent scattering of an electron by impurity clusters at least approximately into account, in contrast to the self-consistent Born approximation (GBA) and similar approximations which are based on a partial summation of diagrams.In part II of this paper we apply this approach to a two-dimensional system, e.g. the surface channel of a field effect transistor, where the GBA leads to inconsistencies which arise from the discrete nature of the unperturbed energy spectrum in a quantizing magnetic field.     

Hydrogenic impurity ground state in quantum well: the envelope function revisited

We present a new variationnal method for calculating the ground state energy of an electron bound to an impurity located in a quantum well. This method relies on an envelope function which is determined exactly from a formal minimization procedure. The obtained energies are lower by as much as 10% than the ones found by the widely used free electron envelope function. Their large width limits are reached with exponentially small corrections as they should. We also find that, except for narrow wells, the shape of these exact envelope functions strongly depends on the impurity position, being consequently quite different from the usual free electron ones. In order to discuss the improvements brought by our new procedure in the most striking way, we have used a model semiconductor quantum well with infinite barrier height and simplified band structure. Extensions can be made to finite barrier and more realistic band structures, following the same technique. Received 11 December 2000     

Determination of the noise parameters in a one-dimensional open quantum system

We consider an electron magnetically interacting with a spin-1/2 impurity, embedded in an external environment whose noisy term acts only on the impurity's spin, and we find expressions for the electron transmission and reflection probabilities in terms of the phenomenological noise parameters. Moreover, we give a simple example of the necessity of complete positivity for physical consistency, showing that a positive but not completely positive dissipative map can lead to negative transmission probabilities.     

The bound polaron in an electric field in polar semiconductor heterostructures

We have calculated the ionization energy of a bound polaron confined in general step quantum wells (QWs) in the presence of an electric field, in which the coupling of an electron with confined bulk-like LO phonons, half-space LO phonons and interface phonons is considered. In particular, the interaction of the impurity with the various phonon modes is also included in QWs. Results have been obtained as a function of the barrier height, the well width, the electric field intensities and the position coordinates of the impurity in the QWs. Our numerical calculations clearly show that the interaction between the impurity and the phonon field plays an important role in screening the Coulomb interaction. It is shown that the cumulative effect of the electron–phonon coupling and the impurity–phonon coupling can contribute appreciably to the donor ionization energy. Only for a certain range of well widths can we neglect all the polaronic effects.     

Effect of a magnetic impurity on the optical properties of a spherical ZnSe quantum dot

We consider the electron and hole states in a semiconductor ZnSe spherical quantum dot, in the center of which a magnetic impurity atom of manganese is located. In calculations the quantum dot is approximated by a spherical rectangular well with a finite depth. Within the framework of perturbation theory, the effect of exchange spin interaction of an electron and a hole with a magnetic impurity on the band structure of the system is considered. The optical spectrum of the system for different polarizations of the incident light is studied also.     

Doping dependent tunneling conductance in SDW ordered copper oxide superconductors

It is observed that doping suppresses the long range anti-ferromagnetic order and induces superconducting phase for a suitable doping. In order to study this effect, we present a model study of the doping dependence of the tunneling conductance in high-T_c systems. The system is described by the Hamiltonian consisting of spin density wave (SDW) and s-wave type superconducting interaction in presence of varying impurity concentrations. The gap equations are calculated by using Green’s functions technique of Zubarev. The gap equations and the chemical potential are solved self-consistently. The imaginary part of the electron Green’s functions shows the quasi-particle density of states which represent the tunneling conductance observed by the scanning tunneling microscopy (STM). We investigate the effect of impurity on the gap equations as well as on the tunneling conductance. The results will be discussed based on the experimental observations.     

Local moment and ferromagnetic state formation in dilute,degenerate d-electron alloys: Ferromagnetic versus antiferromagnetic interactions

In this note the Kim [1] non-degenerate Anderson model (NDAM) of random dilute alloys treatment of local moment and ferromagnetic state formation is generalized to the ten-fold degenerate Anderson model (TDAM) of Siegel and Kemeny [2], Siegel [3,4] and Moriya [12]. We first determine how an impurity state is modified by neighboring impurities. For a simple binary alloy the local electron state at each impurity site depends upon the local distribution of other impurities. Second we derive a TDAM general relationship for the occurrence of a local moment on one impurity and the ferromagnetic ordering of the total impurity spins. Lastly we derive the impurity-impurity TDAM magnetic interaction; for the direct transfer interaction the impurity-impurity magnetic interaction can be ferromagnetic or antiferromagnetic depending upon the fractional occupation of impurity states. At each stage we compare our results with those of Kim's NDAM treatment.     

含有磁性杂质双AB环的自旋极化输运

我们研究了由电子通过含有磁性杂质双AB环结构的透射系数来表示的AB干涉振荡。在这里磁性杂质提供了自旋翻转机制。磁性杂质可以导致AB干涉的部分退相干。而且我们发现在磁通反向的情况下自旋向上和向下的透射系数关于磁通是不对称的，但反射系数和总的透射系数关于磁通却是对称的