Relation between magnetooptical effects and the topology of the Fermi surface of ferromagnetic metals

The anomalous magnetooptical effects in transition metals are related to the structure of the Fermi surface. The anisotropy parameters for the scattering of carriers by impurity centers is derived. The anisotropy parameter is calculated for the real and imaginary parts of the anomalous complex conductivity of iron for multiply connected, spin-down cylindrical surfaces around point H of the subband, for the single spin-down electron surface around point of the subband, and for the large spin-up hole surface around point H of the subband.Translated from Izvestiya VUZ. Fizika, No. 5, pp. 23–28, May, 1971.The author thanks professor E. I. Kondorskii.     

B与N掺杂对单层石墨纳米带自旋极化输运的影响

通过第一性原理计算研究了具有锯齿状边沿并且具有反铁磁构型的单层石墨纳米带的自旋极化输运.研究发现,在中心散射区同一位置掺入单个B和N原子,尽管对整个体系磁矩的影响完全相同,但对两个自旋分量电流的影响却完全相反.掺B时,自旋向上的电流显著大于自旋向下的电流;而掺N时,自旋向下的电流显著大于自旋向上的电流.这是由于不管掺B还是掺N都将打破自旋简并,使得导带和价带中自旋向上的能级比自旋向下的能级更高.掺B引入空穴,使完全占据的价带变为部分占据,从而自旋向上的能级正好处于费米能级,使得电子透射能力更强、电流更大,而自旋向下的能级则离费米能级较远使电子透射的能力较弱.掺N则引入电子,使得原来全空的导带变为部分占据,从而费米能级穿过导带中自旋向下的能级,使得自旋向下的电子比自旋向上的电子透射能力更强. 关键词： 自旋极化输运 单层石墨纳米带 第一性原理 非平衡格林函数     

Spin-dependent transmission of holes through a semiconductor quantum wire with multiple stub

The spin transport of holes through a quantum wire made of many identical T-shaped diluted magnetic semiconductor/semiconductor units is investigated theoretically. The spin-down and spin-up transmission coefficients have been studied as a function of stub parameters. The spin-up transmission coefficient as a function of the stub length is extremely negligible, in the case of multiple-stub quantum wire, while the spin-down transmission coefficient shows a nearly periodic behaviour with regions of large transmission separated by forbidden bands. The spin polarization switches periodically between one and zero as the stub length is changed and shows a square-wave pattern.     

The parametrization of the Fermi surfaces of the metallic elements

In the literature there is now much data available on the shapes of the Fermi surfaces of most of the metallic elements. It seems likely that the next few years will see the reduction of this data to parametric form; a start has already been made and the results published for a small number of metals. Parametrization schemes involving expansions in terms of appropriately symmetrized linear combinations of spherical harmonics or of plane waves are described. Tables of these symmetrized functions are already available in the literature. A plea is made for the standardization of procedure and of notation in future work on parametrizations of the Fermi surfaces of individual metals.     

Concentration dependent magnetism induced by hydrogen adsorption on graphene and single walled carbon nanotubes

We performed first principles calculations to study magnetic properties of hydrogenated graphene and single-walled carbon nanotubes (SWNTs) with different hydrogen concentrations. The hydrogen adsorptions on graphene and SWNTs generate localized states and accordingly flat bands near the Fermi level, opening substantial gaps. The magnetic properties of the compounds depend on hydrogen concentration. At high hydrogen concentration, the flat band splits into spin-up and spin-down branches located above and below the Fermi level, respectively, making the systems to have spontaneous magnetization. However, the spin-up and spin-down branches of the flat band are energetically degenerated at low hydrogen concentration and the systems are therefore nonmagnetic. This result is understandable from the point of view of direct interaction between unpaired π electrons of adjacent hydrogen-adsorption sites.     

Polaron, molecule and pairing in one-dimensional spin-1/2 Fermi gas with an attractive Delta-function interaction

Using solutions of the discrete Bethe ansatz equations, we study in detail the quantum impurity problem of a spin-down fermion immersed into a fully ploarized spin-up Fermi sea with weak attraction. We prove that this impurity fermion in the one-dimensional (1D) fermionic medium behaves like a polaron for weak attraction. However, as the attraction grows, the spin-down fermion binds with one spin-up fermion from the fully-polarized medium to form a tightly bound molecule. Thus it is seen that the system undergos a crossover from a mean field polaron-like nature into a mixture of excess fermions and a bosonic molecule as the attraction changes from weak attraction into strong attraction. This polaron-molecule crossover is universal in 1D many-body systems of interacting fermions. In a thermodynamic limit, we further study the relationship between the Fredholm equations for the 1D spin-1/2 Fermi gas with weakly repulsive and attractive delta-function interactions.     

NMR investigation of how free composite fermions are at nu=1 / 2

NMR measurements of the electron spin polarization (P) have been performed on a 2D electron system at and around half-filled lowest Landau level. Comparing the magnetic field and the temperature dependence of P to models of free and interacting composite fermions (CF), the imbalance of spin-up and spin-down CF Fermi seas is mapped as a function of Zeeman energy. Independent measurements of the CF effective mass, g factor, and Fermi energy are obtained from the thermal activation of P in tilted fields. The filling factor dependence of the P for 2 / 5相似文献   

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.     

Oblique propagation of longitudinal waves in magnetized spin-1/2 plasmas: Independent evolution of spin-up and spin-down electrons

We consider quantum plasmas of electrons and motionless ions. We describe separate evolution of spin-up and spin-down electrons. We present corresponding set of quantum hydrodynamic equations. We assume that plasmas are placed in an uniform external magnetic field. We account different occupation of spin-up and spin-down quantum states in equilibrium degenerate plasmas. This effect is included via equations of state for pressure of each species of electrons. We study oblique propagation of longitudinal waves. We show that instead of two well-known waves (the Langmuir wave and the Trivelpiece–Gould wave), plasmas reveal four wave solutions. New solutions exist due to both the separate consideration of spin-up and spin-down electrons and different occupation of spin-up and spin-down quantum states in equilibrium state of degenerate plasmas.     

Asymptotic correlation functions and FFLO signature for the one-dimensional attractive spin-1/2 Fermi gas

We investigate the long distance asymptotics of various correlation functions for the one-dimensional spin-1/2 Fermi gas with attractive interactions using the dressed charge formalism. In the spin polarized phase, these correlation functions exhibit spatial oscillations with a power-law decay whereby their critical exponents are found through conformal field theory. We show that spatial oscillations of the leading terms in the pair correlation function and the spin correlation function solely depend on ΔkF and 2ΔkF, respectively. Here ΔkF=π(n_↑−n_↓) denotes the mismatch between the Fermi surfaces of spin-up and spin-down fermions. Such spatial modulations are characteristics of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. Our key observation is that backscattering among the Fermi points of bound pairs and unpaired fermions results in a one-dimensional analog of the FFLO state and displays a microscopic origin of the FFLO nature. Furthermore, we show that the pair correlation function in momentum space has a peak at the point of mismatch between both Fermi surfaces k=ΔkF, which has recently been observed in numerous numerical studies.     

Non-universal Fermi polaron in quasi two-dimensional quantum gases

We consider an impurity problem in a quasi-two-dimensional Fermi gas, where a spin-down impurity is immersed in a Fermi sea of N spin-up atoms. Using a variational approach and an effective two-channel model, we obtain the energy for a wide range of interaction strength and for various different mass ratios between the impurity and the background fermion in the context of heteronuclear mixture. We demonstrate that in a quasi-two-dimensional Fermi gas there exists a transition of the ground state from polaron in the weakly interacting region to molecule in the strongly interacting region. The critical interaction strength of the polaron-molecule transition is non-universal and depends on the particle density of the background Fermi sea. We also investigate the excited repulsive polaron state, and find similar non-universal behavior.     

应变对两层半氢化氮化镓薄膜电磁学性质的调控机理研究

采用基于密度泛函理论的第一性原理模拟计算,研究了在应变作用下两层半氢化氮化镓纳米薄膜的电学和磁学性质.没有表面修饰的两层氮化镓纳米薄膜的原子结构为类石墨结构,并具有间接能隙.然而,当两层氮化镓纳米薄膜的一侧表面镓原子被氢化时,该纳米薄膜却依然保持纤锌矿结构,并且展示出铁磁性半导体特性.在应变作用下,两层半氢化氮化镓纳米薄膜的能隙可进行有效调控,并且它将会由半导体性质可转变为半金属性质或金属性质.这主要是由于应变对表面氮原子的键间交互影响和p-p轨道直接交互影响之间协调作用的结果.该研究成果为实现低维半导体纳米材料的多样化提供了有效的调控手段,为其应用于新型电子纳米器件和自旋电子器件提供重要的理论指导.     

Spin-polarized transport induced by photoirradiation in zigzag silicene nanosystem

We study the spin-polarized transport induced by photoirradiation in zigzag silicene nanosystem, based on tight-binding approach, Green's function method and Landauer–Büttiker formula. By applying strong circular polarized light, silicene nanosystem can be transformed into a quantum Hall insulator, where the spin-down subband is gapped while the spin-up subband persists gapless edge state. Therefore, the dc conductance is dominated by the spin-up electrons, and the spin polarization can reach almost 100% around the Fermi energy. The spatial-resolved local density of states confirm that the spin-up electrons transport at two edges of the nanosystem in opposite current directions. Furthermore, because of the topological origin of the edge state, the spin-polarized transport is very robust against the size change of the nanosystem.     

Spin-dependent shot noise in diluted magnetic semiconductor/semiconductor heterostructures with a nonmagnetic barrier

We investigate quantum size effect on the spin-dependent shot noise in the diluted magnetic semiconductor (DMS)/semiconductor heterostructure with a nonmagnetic semiconductor (NMS) barrier in the presence of external magnetic and electric fields. The results demonstrate that the NMS barrier plays a quite different role from the DMS layer in the electron transport process. It is found that spin-down shot noise shows relatively regular oscillations as the width of DMS layer increases, while the spin-up shot noise deceases monotonically. However, as the width of NMS layer increases, the spin-down shot noise displays irregular oscillations at first and then decreases while the spin-up shot noise decreases at a quite different rate. The results indicate that the shot noise can be used as a sensitive probe in detecting material type and its size.     

Superfluid transition in a chiron gas

Low temperature measurements of the magnetic susceptibility of LaCuO₄ doped with Sr (LSCO) suggest that superconducting transition is associated with the disappearance of the vortex liquid. This note draws attention to the possibility that these vortices can be identified with the soliton-like charge carriers introduced in a previous theory of spin–orbit localization in layered poor conductors. In two-dimensions, these solitons are vortex-like, while in three-dimensional systems they are monopole-like. In either case, there is a natural mechanism for the pairing of spin-up and spin-down solitons. At low temperatures, there is a also cross-over transition which is a function of carrier density between a state where the solitons can move independently and a condensate state where the spin-up and spin-down solitons in neighbouring layers are paired.     

All-electric-controlled spin current switching in single-molecule magnet-tunnel junctions

A single-molecule magnet (SMM) coupled to two normal metallic electrodes can both switch spin-up and spin-down electronic currents within two different windows of SMM gate voltage. Such spin current switching in the SMM tunnel junction arises from spin-selected single electron resonant tunneling via the lowest unoccupied molecular orbit of the SMM. Since it is not magnetically controlled but all-electrically controlled,the proposed spin current switching effect may have potential applications in future spintronics.     

Spin Polarization and Andreev Conductance through a Diluted Magnetic Semiconductor Quantum Wire with Spin--Orbit Interaction

Spin-dependent Andreev reflection and spin polarization through a diluted magnetic semiconductor quantum wire coupled to normal metallic and superconductor electrodes are investigated using scattering theory. When the spin-orbit coupling is considered, more Andreev conductance steps appear at the same Fermi energy. Magnetic semiconductor quantum wire separates the spin-up and spin-down electrons. The Fermi energy, at which different- spin-state electrons begin to separate, becomes lower due to the effect of the spin-orbit interaction. The spin filter effect can be measured more easily by investigating the Andreev conductance than by investigating the normal conductance.     

Spin-polarized tunneling through a thin-film

The effect of spin-disorder scattering on perpendicular transport in a magnetic monolayer is considered within the single-site Coherent Potential Approximation (CPA). The exchange interaction between a conduction electron and localized moment of the magnetic ion is treated with the use of the s–f model. Electron-spin polarization is evaluated in the tunnel current which comes from the different densities of spin-up, spin-down conduction electrons at the Fermi level in a ferromagnetic semiconductor (EuS). Calculated results are compared with some tunneling experiments.     

Spin-dependent resonant tunneling in a periodic non-magnetic heterostructure with spin-orbit effects

We theoretically investigate the electron transport in a periodic non-magnetic heterostructure with both Dresselhaus and Rashba spin-orbit effects. We show that the transport properties obviously depend on the number of periods and the large spin polarization can be achieved in such a structure. We also show that for m>1, the resonance splitting occurs in the transmission curves of both spin-up and spin-down electrons when the transmission curves are plotted as a function of the electron energy or the well width.