The Andreev tunneling behaviors in the FM/QD/SC system with intradot spin-flip scattering

The resonant behaviors of spin-dependent linear AR conductance, the spin-dependent AR current, the electron occupation number and spin accumulation in the QD are theoretically investigated in the FM/QD/SC system with intradot spin-flip scattering. The novel resonant behaviors of spin-dependent AR conductance versus Fermi energy are revealed, which are rather different from the AR conductance versus the dot's energy level case [Cao et al., Phys. Rev. B 70 (2004) 235341]. It is proved that the split of the resonant peak can be induced by the competition between the coupling strengths to the FM and SC leads, the intradot spin-flip scattering, and the gate voltage. The number, the widths, and the distance of the peaks could be controlled by tuning the relevant parameters. The resonance of AR current can take place only when the energy level of QD lines up with the right lead chemical potential and blows the left lead chemical potential. The magnitude of the resonant AR current depends on the number of resonant levels involved in the Andreev tunneling process. It is also proved that the spin-flip scattering can suppress the spin accumulation effectively, and induce the spin polarization of AR conductance and AR current simultaneously. The results make us understand better the fundamental in this system, and are useful for the design of spintronic devices.     

Electron transport in double-walled carbon nanotubes

The transport properties of finite length double-walled carbon nanotubes subject to the influences of a transverse electric field and a magnetic field with varying polar angles are investigated theoretically. The electrical conductance, thermal conductance and Peltier coefficient dependences on the external fields and symmetric configuration are studied in linear response regime. Prominent peak structures of the electrical conductance are predicted when varying the electric field strength. The features of the conductance peaks are found to be strongly dependent on the external fields and the intertube interactions. The heights of the electrical and thermal conductance peaks display the quantized behavior, while those of the Peltier coefficient do not. The conductance peaks are found to be broadened by the finite temperature.     

Influence of intradot Coulomb correlations on the Andreev reflection in a ferromagnetic metal/quantum dot/superconductor hybrid junction

Spin-dependent tunneling through a quantum dot coupled to one ferromagnetic and onesuperconducting electrodes is studied in the Andreev reflection (AR) regime. Electricalconductance is calculated within the nonequilibrium Green function technique. Features ofthe AR current involved by the intradot Coulomb correlations (or the dot’s chargingenergy U) and in the presence of the Zeeman splitting of the dotdiscrete level are analyzed in both linear and nonlinear transport regimes. A newinterference effect due to AR is predicted to appear in the case of a weak on-dotrepulsion. Strong Coulomb correlations studied in nonequilibrium situation revealedsignificant modifications of the AR differential conductance occurring only in case ofspin-polarized transmission. Origin of a variety of the multipeak structure of theconductance for the system with the interacting quantum dot, as well as the conditions forthe perfect U-dependent AR transmission are also discussed.     

铁磁绝缘层对正常金属/铁磁绝缘层/正常金属/p波超导结隧道谱的影响

在正常金属/铁磁绝缘层/正常金属/自旋三重态p波超导隧道结中,考虑到铁磁绝缘层的磁散射和粗糙界面散射,运用Bogoliubov-de Gennes(BdG)方程和Blonder-Tinkham-Klapwijk(BTK)理论模型,研究了铁磁绝缘层对隧道结微分电导的影响.研究表明:(1)对于px波,粗糙界面散射和磁散射都能使零偏压电导峰变低,能隙处凹陷升高;随着磁散射的增强,谱线的尖锐峰消失,宽峰逐渐变为凹陷;(2)对于py波,粗糙界面散射和磁散射都能使零能凹陷上移,能隙峰变低,随着粗糙界面散射的增强,两能隙峰间距减小;随着中间正常金属层厚度的增加,能隙内电导随外加偏压呈现振荡行为,能隙外电导仅与普通势垒有关;(3)对于px+ipy波,随着粗糙界面散射的增强,零偏压电导峰被压低,双凹陷处的值逐渐增大为小的能隙峰,而磁散射并不改变谱线中各凹陷处的电导值.     

Magnetotransport through ac driven ferromagnetic graphene nanoribbons

We present a theoretical study on the spin-dependent transport through the ferromagnetic graphene nanoribbons in the presence of a magnetic and an in-plane ac electric field, and find that when the ac field is applied, in the two-terminal ferromagnetic graphene device, for the parallel configurations of the electrodes' magnetizations, the width of the even-number conductance plateaus decrease, the new conductance plateaus appear at the odd-number positions, and the even-number conductance plateaus at the high energy are quenched under the sufficiently strong ac field. In contrast, for the antiparallel configuration of the electrodes' magnetizations, the odd-plateaus of the conductance shrink, and the new plateaus developed at the even-number positions. The magnetic resistance exhibits a successive rectangular-like oscillation structure close to the band edge, whereas experiences an alternative transition between the sharp peak and dip near the zero energy with increasing the ac field strength. In the six-terminal ferromagnetic graphene device, the variations of the longitudinal and Hall resistances' plateaus as well as the addition of the new quantized plateaus with the rise of the ac field strength are also revealed.     

Spin-dependent transport through quantum dot with inelastic interactions

Using the Keldysh nonequilibrium Green function method, we theoretically investigate the electron transport properties of a quantum dot coupled to two ferromagnetic electrodes, with inelastic electron-phonon interaction and spin flip scattering present in the quantum dot. It is found that the electron-phonon interaction reduces the current, induces new satellite polaronic peaks in the differential conductance spectrum, and at the same time leads to oscillatory tunneling magnetoresistance effect. Spin flip scattering suppresses the zero-bias conductance peak and splits it into two, with different behaviors for parallel and anti-parallel magnetic configuration of the two electrodes. Consequently, a negative tunneling magnetoresistance effect may occur in the resonant tunneling region, with increasing spin flip scattering rate.     

Electronic and magnetic properties of basic nanosystems of early 3d transition metals (Sc,Ti, V,Cr, and Mn)

This detailed and systematic theoretical study on the behavior of basic low dimensional (one- and two-dimensional) systems of early 3d transition metals should serve as a guideline to experimentalists as well as to theoreticians. We find that, lowering of dimensionality is favorable for emergence of magnetic ordering in all the systems studied, except Ti monolayers (MLs). For Ti MLs, both nonmagnetic and ferromagnetic states are degenerate within the numerical limits. For such a case, the interactions with substrate would play a decisive role in the magnetic ordering of the atoms in the ML. The total energy calculations show that the nonmagnetic and ferromagnetic states are almost degenerate for Cr and V MLs too; however, anti-ferromagnetic ordering is favored in these. The ferromagnetic ordering in Sc linear chains and anti-ferromagnetic ordering in MLs of Mn and Cr are found to be favored by a relatively larger margin showing good stability. Some low dimensional systems, showing electrons with only one kind of spin available at Fermi energy, may be suitable for spintronics related applications. The linear chains of Cr and Mn, and MLs of Sc are likely to form stable magnetic nanosystems as these exhibit almost saturated magnetic moment per atom around the equilibrium separation. The magnetic moment strengthens considerably as one goes from two- to one-dimension. Our results are supported qualitatively by available experimental results and offer a good insight into these nanosystems.     

Enhancement of charge and spin Seebeck effect in triple quantum dots coupling to ferromagnetic and superconducting electrodes

We theoretically study the thermoelectric transport properties through a triple quantum dots (QDs) device with the central QD coupled to a ferromagnetic lead, a superconducting one, and two side QDs with spin-dependent interdot tunneling coupling. The thermoelectric coefficients are calculated in the linear response regime by means of nonequilibrium Green's function method. The thermopower is determined by the single-electron tunneling processes at the edge of superconducting gap. Near the outside of the gap edge the thermopower is enhanced while thermal conductance is suppressed, as a result, the charge figure of merit can be greatly improved as the gap appropriately increases. In the same way, charge figure of merit also can be greatly improved near the outside of the gap edge by adjusting interdot tunneling coupling and asymmetry coupling of the side QDs to central QD. Moreover, the appropriate increase of the interdot tunneling splitting and spin polarization of ferromagnetic lead not only can improve charge thermopower and charge figure of merit, but also can enhance spin thermopower and spin figure of merit. Especially, the interdot tunneling splitting scheme provides a method of controlling charge (spin) figure merit by external magnetic field.     

Transport properties of a double quantum dot molecule in the presence of impurity effects

In this Letter, we studied the electronic transport through a parallel-coupled double quantum dot (DQD) molecule including impurity effects at zero temperature. The linear conductance can be calculated by using the Green's function method. An obvious Fano resonance arising from the impurity state in the quantum dot is observed for the symmetric dot-lead coupling structure in the absence of the magnetic flux through the quantum device. When the magnetic flux is presented, two groups of conductance peaks appear in the linear conductance spectra. Each group is decomposed into one Breit-Wigner and one Fano resonances. Tuning the system parameters, we can control effectively the shapes of these conductance peaks. The Aharonov-Bohm (AB) oscillation for the magnetic flux is also studied. The oscillation period of the linear conductance with π, 2π or 4π may be observed by tuning the interdot tunneling coupling or the dot-impurity coupling strengths.     

Quantum Phase Transition and Ferromagnetic Spin Correlation in Parallel Double Quantum Dots

We investigate electronic transport through a parallel double quantum dot （DQD） system with strong on-site Coulomb interaction, as well as the interdot tunnelling. By applying numerical renormalization group method, the ground state of the system and the transmission probability at zero temperature are obtained. For a system of quantum dots with degenerate energy levels and small interdot tunnel coupling, the spin correlations between the DQDs is ferromagnetic, and the ground state of the system is a spin-1 triplet state. The linear conductance will reach the unitary limit （2e^2/h） due to the Kondo effect at low temperature. As the interdot tunnel coupling increases, there is a quantum phase transition from ferromagnetic to anti-ferromagnetic spin correlation in DQDs and the linear conductance is strongly suppressed.     

Spin-Polarized Transport Through a Quantum Dot Coupled to Ferromagnetic Leads:Kondo Correlation Effect

We investigate the linear and nonlinear transport through a single level quantum dot connected to two ferromagnetic leads in Kondo regime, using the slave-boson mean-field approach for finite on-site Coulomb repulsion. We find that for antiparallel alignment of the spin orientations in the leads, a single zero-bias Kondo peak always appears in the voltage-dependent differential conductance with peak height going down to zero as the polarization grows to P=1. For parallel configuration, with increasing polarization from zero, the Kondo peak descends and greatly widens with the appearance of shoulders, and finally splits into two peaks on both sides of the bias voltage around P～0.7 until disappearing at even larger polarization strength. At any spin orientation angle θ, the linear conductance generally drops with growing polarization strength. For a given finite polarization, the minimum linear conductance always appears at θ=π.     

Enhanced granular magnetoresistance due to ferromagnetic layers

Giant magnetoresistance (GMR) of sequentially evaporated Fe-Ag structures has been investigated. Direct experimental evidence is given, showing that inserting ferromagnetic layers into a granular structure significantly enhances the magnetoresistance. The increase in the GMR effect is attributed to spin polarization effects. The large enhancement (up to more than a fourfold value) and the linear variation of the GMR in low magnetic fields are explained by scattering of the spin polarized conduction electrons on paramagnetic grains.     

Charge-carrier density collapse in La0.67Ca0.33MnO3and La0.67Sr0.33MnO3 epitaxial thin films

We measured the temperature dependence of the linear high field Hall resistivity of ( K) and ( K) thin films in the temperature range from 4 K up to 360 K in magnetic fields up to 20 T. At low temperatures we find a charge-carrier density of 1.3 and 1.4 holes per unit cell for the Ca- and Sr-doped compound, respectively. In this temperature range electron-magnon scattering contributes to the longitudinal resistivity. At the ferromagnetic transition temperature a dramatic drop in the number of charge-carriers n down to 0.6 holes per unit cell, accompanied by an increase in unit cell volume, is observed. Corrections of the Hall data due to a non saturated magnetic state will lead a more pronounced charge-carrier density collapse. Received 22 July 1999 and Received in final form 7 October 1999     

A valley-filtering switch based on strained graphene

We investigate valley-dependent transport through a graphene sheet modulated by both the substrate strain and the fringe field of two parallel ferromagnetic metal (FM) stripes. When the magnetizations of the two FM stripes are switched from the parallel to the antiparallel alignment, the total conductance, valley polarization and valley conductance excess change greatly over a wide range of Fermi energy, which results from the dependence of the valley-related transmission suppression on the polarity configuration of inhomogeneous magnetic fields. Thus the proposed structure exhibits the significant features of a valley-filtering switch and a magnetoresistance device.     

Thermodynamics of a quasi-one-dimensional spin system for molecule-based ferrimagnets

A physical picture of electron spin alignments in organic molecule-based ferrimagnets is given from numerical calculations of magnetic specific heat (C) and magnetic susceptibility (χ) as functions of temperature and static magnetic field (B) in terms of an Ising Hamiltonian for an alternating spin chain. The double-peak structure of specific heat appears for different parameter ratios and different magnetic field B, indicating that one peak originates from the ferromagnetic nature and the other from the antiferromagnetic nature. Meanwhile, we study successively the influence of intermolecular and intramolecular interaction on the magnetic susceptibility, showing that the ferromagnetic spin alignment in the alternating molecular chains of biradicals and monoradicals is equivalent to the ferromagnetic alignment of the effective S=1/2 spins. Our results are consistent with those of the Quantum Monte Carlo simulations and the exact diagonalization method and in qualitative agreement with the experimental ones.     

Spin polarized Kondo transport through a quantum dot connected to ferromagnetic leads

We study the spin dependent transport through a quantum dot connected to ferromagnetic leads. Using the non-equilibrium generalization of the non-crossing approximation for finite Coulomb repulsion U, we compute the spin polarized conductance, the local average occupancies and the local densities of states in the Kondo regime. We show that transport properties are strongly affected if we allow double occupancy by using a finite value for U. In the framework of our model, we have successfully reproduced the recent experimental finding of an electrically controlled magnetic moment on a carbon nanotube quantum dot coupled to ferromagnetic nickel leads [3]. Besides, in addition to the well known splitting of the Kondo peak in the density of states due to the presence of ferromagnetic leads, we find that the additional splitting due to non-zero bias voltage leads to an unexpected increase of the total conductance, which has also been observed by Hauptmann et al.     

Fingerprints of the magnetic polaron in nonequilibrium electron transport through a quantum wire coupled to a ferromagnetic spin chain

We study nonequilibrium quantum transport through a mesoscopic wire coupled via local exchange to a ferromagnetic spin chain. Using the Keldysh formalism in the self-consistent Born approximation, we identify fingerprints of the magnetic polaron state formed by hybridization of electronic and magnon states. Because of its low decoherence rate, we find coherent transport signals. Both elastic and inelastic peaks of the differential conductance are discussed as a function of external magnetic fields, the polarization of the leads, and the electronic level spacing of the wire.     

DC magnetization of ferromagnetic amorphous wires with local deformation

Experimental DC (VSM) and theoretical hysteresis loops of a ferromagnetic amorphous wire with a deformation in the middle or exposed to local magnetic fields are investigated. Hysteresis loops show two-stage Barkhausen jumps and staircase relaxation. With a local field at the same position, the loop drastically looses its shape and symmetry depending on the magnitude and the direction of the magnetic bias. A model to explain this behavior is proposed. The proposed model is based on the calculation of the magnetic moment distribution of the domain as a result of domain wall motion and nucleation in the inner core of a ferromagnetic wire and is in a good agreement with the experimental results.     

Mode selective control of flow turbulence

We study spin transport in normal/ferromagnetic/normal/ferromagnetic.../normal graphene superlattices, which can be realized by putting a series of magnetic insulator bars on top of a graphene sheet. Owing to magnetic proximity effect, local exchange splittings will be induced in the graphene sheet, effectively forming a magnetic graphene superlattice. The spin polarization of tunneling conductance and the magneto resistance (MR) exhibit oscillatory behavior with the gate voltage. The superlattice structure leads to an enhanced spin polarization and MR ratio, making the magnetic graphene superlattice become very promising in spintronics applications.     

Fano versus Kondo Resonances in a Closed Aharonov-Bohm Interferometer Coupled to Ferromagnetic Electrodes

Using the Keldysh nonequilibrium Green function and equation-of-motion technique, we investigate Fano versus Kondo resonances in a closed Aharonov-Bohm interferometer coupled to ferromagnetic leads and study their effects on the conductance of this system. The conductance with both parallel and antiparallel lead-polarization alignments is analysed for various values of the magnetic flux. Our results show that this system can provide an excellent spin filtering property, and a large tunnelling magnetoresistance can arise by adjusting the system parameters, which indicates that this system is a possible candidate for spin valve transistors and has important applications in spintronics.