Depolarization of the photoluminescence and spin relaxation in n-doped GaAs

Optically oriented electron spin lifetime in n-doped gallium arsenide was measured via depolarization of the photoluminescence (PL) in a transverse magnetic field (Hanle effect). In order to measure the PL polarization, a time-resolved pump-probe experiment, where a pump pulse generates spin-polarized electrons and a probe pulse monitors their polarization, was employed. The PL polarization in dependences of the pump-probe delay, external magnetic field as well as of the sample temperature was studied. The PL polarization was found to decay exponentially with the pump-probe delay, from which the spin lifetime of the electrons was measured. The measured value was found to depend on the strength of the magnetic field and sample temperature.     

本征GaAs中电子自旋极化的能量演化研究

采用时间分辨圆偏振光抽运-探测光谱,研究9.6 K温度下本征GaAs中电子自旋相干弛豫动力学,发现反映电子自旋相干的吸收量子拍的振幅随光子能量的增加呈非单调性变化.考虑自旋极化依赖的带填充效应和带隙重整化效应,发展了圆偏振光抽运-探测光谱的理论模型.该模型表明量子拍的振幅依赖于所探测能级的电子初始自旋极化度,自旋探测灵敏度以及带填充因子,三者的乘积导致了量子拍振幅的非单调变化,与实验结果一致.给出了能级分裂的二能级系统中电子自旋极化度定义.发现在高能级上可以获得100%的初始电子自旋极化度. 关键词： 圆偏振光抽运-探测光谱 吸收量子拍 电子自旋极化度 GaAs     

Ultrafast electron spin dynamics in bulk CdTe investigated by femtosecond pump-probe reflection spectroscopy

The transient spin polarization dynamics in bulk cadmium telluride (CdTe) at 70 K is investigated by the circularly polarized pump-probe reflection technique. A general expression is derived from the rate equations of a two-level system with small signal approximation to describe the light-helicity-dependent reflection spectrum. The initial degree of electron spin polarization in the excited state and the electron spin relaxation time in bulk CdTe at low temperature with different carrier density are analyzed according to this model. Our experimental results reveal that the D?yakonov–Perel? mechanism based on a fully microscopic kinetic spin Bloch equation (microscopic KSBE) approach dominates in the electron spin relaxation process in bulk CdTe crystal.     

Electronic-spin generation in multi-photon processes: Cubic anisotropy of pumping

We study the spin generation in multi-photon absorption processes in a bulk semiconductor, pumped it by a circularly polarized intense light by varying the angle of incidence. The generated spin polarization is calculated using the eight-band Kane model in the limit of large spin–orbit splitting and on the basis of the multi-photon photo-generation rate of the conduction electron spin density. It is found that the spin polarization strongly depends on the pumping photon energy outside the band edge. Cubic anisotropy in crystal pumping is also calculated. The results show that due to this anisotropy the spin generation differs by ∼8%, in consistence with earlier results obtained by others.     

本征GaAs中电子自旋极化对电子复合动力学的影响研究

采用时间分辨圆偏振光和线偏振光抽运-探测光谱,研究了9.6 K温度下本征GaAs中自旋极化电子与非极化电子的复合动力学及其随光子能量演化.发现自旋极化对电子复合动力学具有显著影响.仅在导带底附近测量时,两种方法测试到的复合寿命一致,而在高过超能量电子态测量时,两种方法测试到的复合寿命不一致.指出时间分辨法拉第光谱中,用于反演求解电子自旋相干寿命的电子复合寿命应该使用圆偏振光抽运-探测获得的复合寿命,而不是线偏振光抽运-探测获得的寿命.理论计算与实验结果吻合较好. 关键词： 圆偏振光抽运-探测光谱 自旋量子拍 自旋极化 GaAs     

Generation and detection of spin polarization in parallel coupled double quantum dots connected to four terminals

A four-terminal parallel double quantum dots (QDs) device is proposed to generate and detect the spin polarization in QDs. It is found that the spin accumulation in QDs and the spin-polarized currents in the upper and down leads can be generated when a bias voltage is applied between the left and right leads. It is more interesting that the spin polarization in the QDs can be detected using the upper and down leads. Moreover, the direction and magnitude of the spin polarization in the QDs, and in the upper and down leads can be tuned by the energy levels of QDs and the bias.     

一维铁磁/有机系统的基态与自旋分布研究

文中用一维紧束缚模型描述铁磁金属,用一维非简并的Su-Schrieffer -Heeger (SSH)模型描述共轭聚合物,研究了在一维铁磁/共轭聚合物系统和一维CMR材料/ 聚合物系统中的电子转移和自旋转移.发现在聚合物部分没有自旋的双极化子比有自旋的极化子具有较低的能量而容易产生.然而在铁磁CMR材料/聚合物系统中极化子的产生能低于聚合物中极化子的产生能,增加了有机物中自旋极化输运的可能性.     

Transmission of electrons through ferromagnetic material and applications to detection of electron spin polarization

The salient features of the total low energy inelastic electron scattering cross section in transition metals are described by a constant term σ₀ plus a term σ_d that is proportional to the number of unoccupied d-orbitals. This simple model predicts that low energy electrons transmitted through a ferromagnetic ultrathin film acquire a transport spin polarization a(χ). Using the ratio σ₀/σ_d as the only adjustable parameter, the model predicts the enhancement of the spin polarization of the low energy cascade electrons as well as a(χ) in reasonable agreement with the existing observations on Fe, Co and Ni. A detector for electron spin polarization P based on the spin dependent transmission of electrons through ferromagnetic material is proposed which should be superior to existing P-detectors by 1–2 orders of magnitude.     

Spin-polarization dependent carrier recombination dynamics and spin relaxation mechanism in asymmetrically doped (110) n-GaAs quantum wells

Carrier recombination and electron spin relaxation dynamics in asymmetric n-doped (110) GaAs/AlGaAs quantum wells are investigated with time-resolved pump-probe spectroscopy. The experiment results reveal that the measured carrier recombination time depends strongly on the polarization of pump pulse. With the same pump photon flux densities, the recombination time of spin-polarized carriers is always longer than that of the spin-balanced carriers except at low pump photon flux densities, this anomaly originates from the polarization-sensitive nonlinear absorption effect. Differing from the traditional views, in the low carrier density regime, the D'yakonov–Perel' (DP) mechanism can be more important than the Bir–Aronov–Pikus (BAP) mechanism, since the DP mechanism takes effect, the spin relaxation time in (110) GaAs QWs is shortened obviously via asymmetric doping.     

Energy behavior of spin electron cyclotron wave in a spin polarized plasma

In degenerate quantum plasma the energy behavior of electrostatic modes propagating perpendicular to the external magnetic field is studied by employing the separated spin evolution quantum hydrodynamic (SSE-QHD) model. This model reveals that spin electron cyclotron wave (SECW) appears additionally with the upper hybrid wave (UHW). In case of SECW, the curves for the energy flow speed at different levels of spin polarization effect flip over at a particular value of wave number. The spin polarization effect enhances the energy flow speed before this value of wave number and then suppresses it afterward. The energy flow speed is enhanced by spin polarization effect in the entire range of wave number for the propagation of UHW. The Bohm potential effect drastically increases the energy flow speed at high wave number domain in both the waves. This study may find its applications to understand the energy behavior inspin polarized solid state plasmas     

Heat transport and thermoelectric efficiency of two-level quantum dot attached to ferromagnetic electrodes

Energy transport, in the linear response regime, through a two-level quantum dot/molecule attached to ferromagnetic leads is studied in the Coulomb blockade region with use of the Green function formalism. Thermal conductance and figure of merit ZT are calculated and discussed for two configurations of magnetic moments and different polarization factor in the leads. A strong dependence of ZT on polarization is found. A substantial enhancement of efficiency can be observed in molecular junctions with one of energy levels weakly coupled to the leads. Moreover, in systems, in which spin accumulation in electrodes is important, a quite considerable spin efficiency can be expected.     

Spin polarization and spin separation realized in the double-helical molecules

We investigate the electron transport through one double-helical molecule with four terminals, by considering one terminal to be the source and others to be the drains. It is found that notable spin polarizations simultaneously occur during the processes of intra-chain electron tunneling and inter-chain electron reflection. More importantly, in these two processes, the spin polarizations always show similar strengths and opposite directions. Based on these results, we consider that the spin polarization and spin separation can be co-realized in this system.     

Diamagnetism of microcavity polaritons induced by spin-dependent polariton–polariton interactions

Diamagnetism of condensed microcavity polaritons in a vertically applied magnetic field is theoretically studied by using the density of free energy of polaritons. The magnetic dependence of polariton–polariton interactions and spin polarization degree of polaritons are derived, and are used to show the diamagnetic behavior of the polariton spin polarization, which is discussed for GaAs-based microcavities. We show that for strong magnetic field the spin polarization of the polaritons is paramagnetic as usual, while around positive exciton–photon detuning and special Rabi splitting, the spin polarization of the polaritons could be diamagnetic. In addition, weak magnetic field and high polariton density are beneficial to observe the polariton diamagnetism.     

Current-induced modification of spin wave mode interference

We have studied the effect of adiabatic spin-transfer torque on mode interference of spin waves. The mode interference generates amplitude-localized spots at special positions which do not move with time. When applying current, the wavevector of spin wave is modified, resulting in current-dependent displacement of amplitude-localized spots. This current-dependent change in the mode interference may allow to probe current-induced spin wave Doppler shift in space-domain. In favorable situations, it can be used to estimate the intrinsic properties of magnetic materials such as spin polarization.     

Electrically controllable spin transport in bilayer graphene with Rashba spin-orbit interaction

We study the spin transport in bilayer graphene nanoribbons (BGNs) in the presence of Rashba spin-orbit interaction (SOI) and external gate voltages. It is found that the spin polarization can be significantly enhanced by the interlayer asymmetry or longitudinal mirror asymmetry produced by external gate voltages. Rashba SOI alone in BGNs can only generate current with spin polarization along the in-plane y direction, but the polarization components can be found along the x, y and z directions when a gate voltage is applied. High spin polarization with flexible orientation is obtained in the proposed device. Our findings shed new light on the generation of highly spin-polarized current in BGNs without external magnetic fields, which could have useful applications in spintronics device design.     

Theory of spin polarization in mesoscopic spin–orbit coupling systems

We establish a general formalism of the bulk spin polarization (BSP) and the current-based spin polarization (CSP) for mesoscopic ferromagnetic and spin–orbit interaction (SOI) semiconducting systems. Based on this formalism, we reveal the basic properties of BSP and CSP and their relationships. The BSP describes the intrinsic spin polarized properties of devices. The CSP depends on both intrinsic parameters of device and the incident current. For the non-spin-polarized incident current with the in-phase spin-phase coherence, CSP equals to BSP. We give analytically the BSP and CSP of several typical nanodevice models, ferromagnetic nanowire, Rashba nanowire and rings. These results provide basic physical behaviors of BSP and CSP and their relationships.     

Dynamics of coherent and incoherent spin polarizations in ensembles of quantum dots

The temperature dependence of spin coherence in InGaAs quantum dots is obtained from quantum beats observed in polarization-resolved pump-probe experiments. Within the same sample we clearly distinguish between coherent spin dynamics leading to quantum beats and incoherent long-lived spin-memory effects. Analysis of the coherent data using a theoretical model reveals approximately 10 times greater stability of the spin coherence at high temperature compared to that found previously for exciton states in four-wave-mixing experiments by Borri et al. [Phys. Rev. Lett. 87, 157401 (2001)]]. The data on incoherent polarization reveal a new form of spin memory based on charged quantum dots.     

Polarization and readout of coupled single spins in diamond

We study the coupling of a single nitrogen-vacancy center in diamond to a nearby single nitrogen defect at room temperature. The magnetic dipolar coupling leads to a splitting in the electron spin resonance frequency of the nitrogen-vacancy center, allowing readout of the state of a single nitrogen electron spin. At magnetic fields where the spin splitting of the two centers is the same, we observe a strong polarization of the nitrogen electron spin. The amount of polarization can be controlled by the optical excitation power. We combine the polarization and the readout in time-resolved pump-probe measurements to determine the spin relaxation time of a single nitrogen electron spin. Finally, we discuss indications for hyperfine-induced polarization of the nitrogen nuclear spin.     

Spin injection in a coupled system of a diluted magnetic semiconductor Zn0.80Mn0.20Se and self-assembled quantum dots of CdSe

Spin injection processes from a Zn_0.80Mn_0.20Se diluted magnetic semiconductor (DMS) to adjacent self-assembled CdSe quantum dots (QDs) were investigated by cw and time-resolved magneto-optical spectroscopy in combination with tunable laser excitation. Direct experimental evidence for the spin injection was provided from the generation of the spin polarization in the QDs, which was opposite to their expected intrinsic polarization, by resonantly generating the spins in the DMS. The observed limited spin polarization generated by the spin injection, together with the sensitivity of the spin injection efficiency on structure design, indicates severe spin loss during the process and calls for further investigations to optimize spin injection efficiency in quantum structures.     

Interferometric control of spin-polarized electron populations at a metal surface observed by multiphoton photoemission

In an interferometric pump-probe experiment, we demonstrate the phase tuning of the spin polarization of photoelectrons emitted in a three-photon process from Cu(001). A phase shift of pi between delayed ultrafast circularly polarized light pulses can switch the spin polarization from +/-20% to -/+40%. In the delay regime of overlapping pulses, we show the dominating role of optical interference effects in determining the spin polarization. For longer delays, we detect the influence of the coherent material response, manifested in both the final state electron population as well as the final state spin polarization.