Photo-induced magnetic-solitons and spin dynamics in diluted magnetic semiconductor quantum wells

The localization mechanism of transport property in the randomly distributed system of the hole-induced magnetic solitons with the alloy potential fluctuations in diluted magnetic semiconductors has been proposed, by using the effective Lagrangian of diffusion modes. The mechanism of the long relaxation of the spin dynamics below Curie temperature in diluted magnetic semiconductor wells and the bulk system has been discussed.     

Magneto-optical study of nonmagnetic quantum dots coupled to a magnetic semiconductor quantum well

We have investigated a series of double-layer structures consisting of a layer of self-assembled non-magnetic CdSe quantum dots (QDs) separated by a thin ZnSe barrier from a ZnCdMnSe diluted magnetic semiconductor (DMSs) quantum well (QW). In the series, the thickness of the ZnSe barrier ranged between 12 and 40 nm. We observe two clearly defined photoluminescence (PL) peaks in all samples, corresponding to the CdSe QDs and the ZnCdMnSe QW, respectively. The PL intensity of the QW peak is observed to decrease systematically relative to the QD peak as the thickness of the ZnSe barrier decreases, indicating a corresponding increase in carrier tunneling from the QW to the QDs. Furthermore, polarization-selective PL measurements reveal that the degree of polarization of the PL emitted by the CdSe QDs increases with decreasing thickness of the ZnSe barriers. The observed behavior is discussed in terms of anti-parallel spin interaction between carriers localized in the non-magnetic QDs and in the magnetic QWs.     

Nonequilibrium spin transport through a diluted magnetic semiconductor quantum dot system with noncollinear magnetization

The spin-dependent transport through a diluted magnetic semiconductor quantum dot (QD) which is coupled via magnetic tunnel junctions to two ferromagnetic leads is studied theoretically. A noncollinear system is considered, where the QD is magnetized at an arbitrary angle with respect to the leads’ magnetization. The tunneling current is calculated in the coherent regime via the Keldysh nonequilibrium Green’s function (NEGF) formalism, incorporating the electron–electron interaction in the QD. We provide the first analytical solution for the Green’s function of the noncollinear DMS quantum dot system, solved via the equation of motion method under Hartree–Fock approximation. The transport characteristics (charge and spin currents, and tunnel magnetoresistance (TMR)) are evaluated for different voltage regimes. The interplay between spin-dependent tunneling and single-charge effects results in three distinct voltage regimes in the spin and charge current characteristics. The voltage range in which the QD is singly occupied corresponds to the maximum spin current and greatest sensitivity of the spin current to the QD magnetization orientation. The QD device also shows transport features suitable for sensor applications, i.e., a large charge current coupled with a high TMR ratio.     

Spin transport and relaxation in graphene

We review our recent work on spin injection, transport and relaxation in graphene. The spin injection and transport in single layer graphene (SLG) were investigated using nonlocal magnetoresistance (MR) measurements. Spin injection was performed using either transparent contacts (Co/SLG) or tunneling contacts (Co/MgO/SLG). With tunneling contacts, the nonlocal MR was increased by a factor of ∼1000 and the spin injection/detection efficiency was greatly enhanced from ∼1% (transparent contacts) to ∼30%. Spin relaxation was investigated on graphene spin valves using nonlocal Hanle measurements. For transparent contacts, the spin lifetime was in the range of 50-100 ps. The effects of surface chemical doping showed that for spin lifetimes in the order of 100 ps, charged impurity scattering (Au) was not the dominant mechanism for spin relaxation. While using tunneling contacts to suppress the contact-induced spin relaxation, we observed the spin lifetimes as long as 771 ps at room temperature, 1.2 ns at 4 K in SLG, and 6.2 ns at 20 K in bilayer graphene (BLG). Furthermore, contrasting spin relaxation behaviors were observed in SLG and BLG. We found that Elliot-Yafet spin relaxation dominated in SLG at low temperatures whereas Dyakonov-Perel spin relaxation dominated in BLG at low temperatures. Gate tunable spin transport was studied using the SLG property of gate tunable conductivity and incorporating different types of contacts (transparent and tunneling contacts). Consistent with theoretical predictions, the nonlocal MR was proportional to the SLG conductivity for transparent contacts and varied inversely with the SLG conductivity for tunneling contacts. Finally, bipolar spin transport in SLG was studied and an electron-hole asymmetry was observed for SLG spin valves with transparent contacts, in which nonlocal MR was roughly independent of DC bias current for electrons, but varied significantly with DC bias current for holes. These results are very important for the use of graphene for spin-based logic and information storage applications.     

Spin relaxation and antiferromagnetic coupling in semiconductor quantum dots

We report carrier spin dynamics in highly uniform self-assembled InAs quantum dots and the observation of antiferromagnetic coupling between semiconductor quantum dots. The spin relaxation times in the ground state and the first excited state were measured to be 1.0 and 0.6 ns, respectively, without the disturbance of inhomogeneous broadening. The measured spin relaxation time decreases rapidly from 1.1 ns at 10 K to 200 ps at 130 K. This large change in the spin relaxation time is well-explained in terms of the mechanism of acoustic phonon emission. In coupled quantum dots, the formation of antiferromagnetic coupling is directly observed. Electron spins are found to flip at 80 ps after photoexcitation via the interdot exchange interaction. The antiferromagnetic coupling exists at temperatures lower than 50–80 K. A model calculation based on the Heitler–London approximation supports the finding that the antiferromagnetic coupling is observable at low temperature. These carrier spin features in quantum dots are suitable for the future quantum computation.     

Material dependence of spin transport modulated by magnetic barriers in semiconductor nano-wires

We present the properties of ballistic spin transport through magnetic barrier structures in semiconductor nano-wires. The Landauer's approach is adopted to calculation of the transmission probability and the conductance for various host material nano-wires which are different remarkably from each other in effective g-factors. A host material having small effective g-factor is quantized in the conductance and the spin-dependence is disappeared in it. Nevertheless this kind of behavior is broken for the host material having large effective g-factor and the spin-dependent splitting is shown.     

Electron conductance in curved quantum structures

A differential-geometry analysis is employed to investigate the transmission of electrons through a curved quantum-wire structure. Although the problem is a three-dimensional spatial problem, the Schrödinger equation can be separated into three general coordinates. Hence, the proposed method is computationally fast and provides direct (geometrical) parameter insight as regards the determination of the electron transmission coefficient. We present, as a case study, calculations of the electron conductivity of a helically shaped quantum-wire structure and discuss the influence of the quantum-wire centerline radius of curvature and pitch length for the conductivity versus the chemical potential.     

Spin dynamics in magnetic semiconductor nanostructures

The studies of the magnetic and electrical transport properties of ordered magnetic semiconductor nanostructures have been generalized. This new area lies at the intersection of nanotechnologies and fundamental problems of magnetism. The prospects for application of ferromagnetic semiconductors in spintronics have been discussed. A comparative analysis of the magnetic and electrical transport properties of nanowires, thin films, and bulk elemental semiconductors doped with transition metals has been performed. The influence of size effects on the spin dynamics, magnetization, and magnetoresistance of nanostructures has been considered.     

Spin valve effect and negative tunnel magnetoresistance in a quantum dot transistor

We study the spin polarized transport through a quantum dot transistor. It is shown that the interplay of large Coulomb interaction and optically induced spin accumulation gives rise to the spin valve effect over a range of bias. We also find negative tunnel magnetoresistance for system with ferromagnetic electrodes.     

Dynamics of spins in semiconductor quantum wells under drift

The dynamics of spins in semiconductor quantum wells under applied electric bias has been investigated by photoluminescence (PL) spectroscopy. The bias-dependent polarization of PL (P_PL) was measured at different temperatures. The P_PL was found to decay with an enhancement of increasing the strength of the negative bias, with an exception occurred for a low value of the negative bias. The P_PL was also found to depend on the temperature. The P_PL in the presence of a transverse magnetic field was also studied. The results showed that P_PL in the magnetic field oscillates under an applied bias, demonstrating that the dephasing of electron spin occurs during the drift transport in semiconductor quantum wells.     

Resonant tunneling in magnetic semiconductor tunnel junctions with arbitrary magnetic alignments

Combining an extended Julliere model with transfer matrix method, we study the spin-polarized resonant tunneling in GaMnAs/AlAs/GaAs/AlAs/GaMnAs double barrier ferromagnetic semiconductor (FS) tunnel junctions with the arbitrary angle θ between the magnetic directions of two FS's. It is shown that tunneling magnetoresistance (TMR) ratio linearly varies with sin²(θ/2). We also demonstrate that for the heavy and light holes, the properties of the spin-polarized resonant tunneling are obviously different. The present results are expected to be instructive for manufacturing the relevant semiconductor spintronic devices.     

The infrared spin-galvanic effect in semiconductor quantum wells

The spin-galvanic effect generated by homogeneous optical excitation with infrared circularly polarized radiation in quantum wells (QWs) is reviewed. The spin-galvanic current flow is driven by an asymmetric distribution of spin-polarized carriers in k-space of systems with lifted spin degeneracy due to k-linear terms in the Hamiltonian. Spin photocurrents provide methods to investigate the spin-splitting of the band structure and to make conclusion on the in-plane symmetry of QWs.     

Spin current pumping with two orthogonal electric fields in quantum wire

We study the problem of spin current pumping in a one-dimensional quantum wire when there exist two orthogonal Rashba spin-orbit couplings (SOCs) in different regions which evolve with time and can be induced by the perpendicular electric fields. On one hand, we demonstrate that the time-evolving Rashba SOC is equivalent to the spin-dependent electric field and the scheme may lead to the pure spin current associated with well suppressed charge current. On the other hand, we adopt the non-equilibrium Green's function method and numerically find that the parameter loop must satisfy certain condition for the successful pumping. We also study the effect of the Fermi energy and the inevitable disorder on the spin current. The implications of these results are discussed.     

Spin filtering magnetic modulation and spin-polarization switching in hybrid ferromagnet/semiconductor structures

Electron spin-polarization modulation with a ferromagnetic strip of in-plane magnetization is analyzed in a hybrid ferromagnet/semiconductor filter device.The dependencies of electron spin-polarization on the strip’s magnetization strength,width and position have been systematically investigated.A novel magnetic control spin-polarization switch is proposed by inserting a ferromagnetic metal(FM)strip eccentric in relation to off the center of the spin filter,which produces the first energy level spin-polarization reversal.It is believed to be of significant importance for the realization of semiconductor spintronics multiple-value logic devices.     

Spin transport in the anisotropic easy-plane two-dimensional Heisenberg antiferromagnet

Spin transport in the easy-plane two-dimensional anisotropic Heisenberg antiferromagnet with S=1 is studied. Regular part of spin conductivity is calculated, at zero temperature, using a self-consistent harmonic approximation and the Kubo formalism. Three magnon processes provide the dominant contribution to the spin conductivity. Furthermore, the transport is ballistic and characterized by finite Drude weight.     

Optimal control of fast and high-fidelity quantum state transfer in spin-1/2 chains

Spin chains are promising candidates for quantum communication and computation. Using quantum optimal control (OC) theory based on the Krotov method, we present a protocol to perform quantum state transfer with fast and high fidelity by only manipulating the boundary spins in a quantum spin-1/2 chain. The achieved speed is about one order of magnitude faster than that is possible in the Lyapunov control case for comparable fidelities. Additionally, it has a fundamental limit for OC beyond which optimization is not possible. The controls are exerted only on the couplings between the boundary spins and their neighbors, so that the scheme has good scalability. We also demonstrate that the resulting OC scheme is robust against disorder in the chain.     

Photoluminescence inner core excitation in semiconductor quantum structures

We introduce a photoluminescence inner core excitation (PLICE) for the studies of semiconductor quantum structures. This novel method, in which we use synchrotron radiation as tunable excitation source, is expected to facilitate us to obtain electronic and compositional information about buried quantum structures. Here we report experimental results on quantum dots (QDs) and quantum wires (QWRs), in order to demonstrate potential applicability of the method to the semiconductor nanostructure studies.     

Spin polarization of the neutral exciton in a single quantum dot

While efficient nuclear polarization has earlier been reported for the charged exciton in InAs/GaAs quantum dots at zero external magnetic field, we report here on a surprisingly high degree of circular polarization, up to ≈60%$\approx 60\%$, for the neutral exciton emission in individual InAs/GaAs dots. This high degree of polarization is explained in terms of the appearance of an effective nuclear magnetic field which stabilizes the electron spin. The nuclear polarization is manifested in experiments as a detectable Overhauser shift. In turn, the nuclei located inside the dot are exposed to an effective electron magnetic field, the Knight field. This nuclear polarization is understood as being due to the dynamical nuclear polarization by an electron localized in the QD. The high degree of polarization for the neutral exciton is also suggested to be due to separate in-time capture of electrons and holes into the QD.     

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.     

Quasibound states in semiconductor quantum well structures

We present a study on quasibound states in multiple quantum well structures using a finite element model (FEM). The FEM is implemented for solving the effective mass Schrödinger equation in arbitrary layered semiconductor nanostructures with an arbitrary applied potential. The model also includes nonparabolicity effects by using an energy dependent effective mass, where the resulting nonlinear eigenvalue problem was solved using an iterative approach. We focus on quasibound/continuum states above the barrier potential and show that such states can be determined using cyclic boundary conditions. This new method enables the determination of both bound and quasibound states simultaneously, making it more efficient than other methods where different boundary conditions have to be used in extracting the relevant states. Furthermore, the new method lifted the problem of quasibound state divergence commonly seen with many other methods of calculation. Hence enabling accurate determination of dipole matrix elements involving both bound and quasibound states. Such calculations are vital in the design of intersubband optoelectronic devices and reveal the interesting properties of quasibound states above the potential barriers.