Spin diffusion of optically oriented electrons and photon entrainment in n-gallium arsenide

An experimental and theoretical study of spin transport in the n-GaAs semiconductor is reported. Transport of average electron spin from the photoexcited crystal surface is shown to be determined by the spin diffusion process. At the same time the transport of photoexcited carriers takes place primarily through photon entrainment, which transfers nonequilibrium carriers into the bulk of the semiconductor to distances considerably in excess of the electron spin diffusion length. A comparison of the experimental results with theory permits one to determine the average-spin diffusion length and electron-spin relaxation time. Fiz. Tverd. Tela (St. Petersburg) 39, 1975–1979 (November 1997)     

Coherent spin dynamics of electrons and excitons in nanostructures (a review)

The studies of spin phenomena in semiconductor low-dimensional systems have grown into the rapidly developing area of the condensed matter physics: spintronics. The most urgent problems in this area, both fundamental and applied, are the creation of charge carrier spin polarization and its detection, as well as electron spin control by nonmagnetic methods. Here, we present a review of recent achievements in the studies of spin dynamics of electrons, holes, and their complexes in the pump-probe method. The microscopic mechanisms of spin orientation of charge carriers and their complexes by short circularly polarized optical pulses and the formation processes of the spin signals of Faraday and Kerr rotation of the probe pulse polarization plane as well as induced ellipticity are discussed. A special attention is paid to the comparison of theoretical concepts with experimental data obtained on the n-type quantum well and quantum dot array samples.     

Surface-sensitive NMR in optically pumped semiconductors

We present a scheme of surface-sensitive nuclear magnetic resonance in optically pumped semiconductors, where an NMR signal from a part of the surface of a bulk compound semiconductor is detected apart from the bulk signal. It utilizes optically oriented nuclei with a long spin-lattice relaxation time as a polarization reservoir for the second (target) nuclei to be detected. It provides a basis for the nuclear spin polarizer [IEEE Trans. Appl. Supercond. 14:1635, 2004], which is a polarization reservoir at the surface of the optically pumped semiconductor that polarizes nuclear spins in a target material in contact through the nanostructured interfaces.     

Spin-dependent transport of holes in microstructures periodically modulated by diluted magnetic semiconductor sections

We study spin transport of holes through microstructures modulated periodically by diluted magnetic semiconductor (DMS) sections, stubless or stubbed. The stubs are symmetric or asymmetric and the magnetizations of consecutive DMS sections are parallel or antiparallel. The transmission coefficients of holes with spin up (T⁺) or down (T^-) are drastically different since the spins feel different potential profiles in the DMS sections. As a result, nearly square-wave patterns, or wide plateaus and oscillations, can be obtained for the transmission and the spin polarization as functions of the incident energy or of various parameters of the periodically repeated unit. Results for simple and composite units with and without deviations from perfect periodicity are reported. Some of the structures considered exhibit a strong spin-filtering behavior.     

GaAs自旋注入及巨霍尔效应的研究

在自旋电子学研究中,一般以超晶格结构、自旋阀、隧道结来实现,另一种自旋注入典型方法是稀磁半导体材料,如GaMnAs,本文通过颗粒膜实现自旋注入,利用磁控溅射法将Fe颗粒嵌入GaAs阵体上,制备了(GaAs)₁₉Fe₈₁颗粒膜样品,在室温条件下观测到15 μΩ ·cm最大饱和霍尔电阻率,该效应比纯铁的饱和霍尔电阻率大了一个数量级,成功地实现了自旋注入. 关键词： 自旋注入 颗粒膜 巨霍尔效应     

半导体核磁共振显微压力的质子全自旋量子门的实现

以实现质子全自旋量子门、观察半导体核子自旋态和量子计算为目的, 依据样品的自旋-晶格弛豫时间和自旋-自旋弛豫时间,采用脉冲调制序列控制磁共振的条件和翻转旋转框架,计算了共振显微压力. 结果表明,质子全自旋量子门具有高灵敏度和高Q操控性,通过扫描片段和激光干涉可以得到磁共振压力. 共振压力兼具MRI和AFM优点,是一种强有力的通过核自旋实现量子计算获得量子信息的有效方法. 关键词： 空间分辨率 共振显微 半导体光刻 电子束刻印     

Semiconductor with intrinsic spin: a hybrid structure of zigzag edge graphene nanoribbon/single‐walled carbon nanotube

A hybrid structure of n ‐ZGRN/(m,m)SWCNT, namely n ‐ZGNR‐(m,m)SWCNT, is predicted. It is found that the n ‐ZGNR‐(m,m)SWCNT is a ferromagnetic semiconductor with intrinsic spin in which the manipulation of spin‐polarized currents can be achieved just simply by applying a gate voltage. Moreover, compared to n ‐ZGNR, the n‐ ZGNR‐(m,m)SWCNT possesses enhanced local magnetic moment. Semiconductors with intrinsic spin represent a new direction in the exploration of materials for spintronics and good prospects for practical spintronic applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Demonstration of electrical spin injection into a semiconductor using a semimagnetic spin aligner

Spin injection into semiconductors has been a field of growing interest during recent years, because of the large possibilities in basic physics and for device applications that a controlled manipulation of the electrons spin would enable. However, it has proven very difficult to realize such a spin injector experimentally. Here we demonstrate electrical spin injection and detection in a GaAs/AlGaAs p-i-n diode using a semimagnetic II–VI semiconductor (Zn_{1 − x − y}Be_xMn_ySe) as a spin aligner. The degree of circular polarization of the electroluminescence from the diode is related to the spin polarization of the conduction electrons. Thus, it may be used as a detector for injected spin-polarized carriers. Our experimental results indicate a spin polarization of the injected electrons of up to 90% and are reproduced for several samples. The degree of optical polarization depends strongly on the Mn concentration and the thickness of the spin aligner. Electroluminescence from a reference sample without spin aligner as well as photoluminescence after unpolarized excitation in the spin aligner sample show only the intrinsic polarization in an external magnetic field due to the GaAs bandstructure. We can thus exclude side effects from Faraday effect or magnetic circular dichroism in the semimagnetic layer as the origin of the observed circularly polarized electroluminescence.     

Microwave electric field induced spin transitions of AlAs 2D electrons

We present the first direct electron spin resonance (ESR) on a 2D electron gas in a III–V semiconductor. ESR on a high mobility 2D electron gas in a single AlAs quantum well reveals an electronic g-factor of 1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimal linewidth of 7 Gauss. Both the signal amplitude and its dependence on the position and orientation of the sample in the cavity unambiguously demonstrate that the spin transitions in our experiment are caused by the microwave electric field. We present a model that ascribes the spin transitions to the effective magnetic field acting on the electron spins that arises from (Bychkov–Rashba) spin-orbit interaction and the modulation of the electron wavevector around k_F induced by the microwave electric field.     

Determining the sign of g factor via time-resolved Kerr rotation spectroscopy with a rotatable magnetic field

Time-resolved Kerr rotation spectroscopy is used to determine the sign of the g factor of carriers in a semiconductor material,with the help of a rotatable magnetic field in the plane of the sample.The spin precession signal of carriers at a fixed time delay is measured as a function of the orientation of the magnetic field with a fixed strength B.The signal has a sine-like form and its phase determines the sign of the g factor of carriers.As a natural extension of previous methods to measure the (time-resolved) photoluminescence or time-resolved Kerr rotation signal as a function of the magnetic field strength with a fixed orientation,such a method gives the correct sign of the g factor of electrons in GaAs.Furthermore,the sign of carriers in a (Ga,Mn)As magnetic semiconductor is also found to be negative.     

Spin waves in degenerate ferromagnetic semiconductors at low temperatures

Thes-d(f) exchange model of a ferromagnetic semiconductor is considered. With the use of the perturbation expansion in the formal parameter (2S)^–1,S being the magnitude of magnetic ion spin, the magnon Green function is calculated. The spectrum and damping of spin waves in the presence of conduction electrons is investigated. It is demonstrated that the change of sign of the temperature dependent correction to the spin-wave stiffness is possible provided that the Fermi energy exceeds some critical value. This may lead to the increase of the spin-wave stiffness with temperature unlike the case of the Heisenberg ferromagnet. The estimations of this effect for doped EuO and EuS are carried out.     

Spin polarization of semiconductor carriers by reflection off a ferromagnet

We present a theory of generation or alteration of the electron spin coherence and population in an n-doped semiconductor by reflection at the interface with a ferromagnet. The dependence of the spin reflection on the Schottky barrier height and the doping concentration in the semiconductor was computed for a generic model. The theory provides an explanation for the spontaneous electron spin coherence and nuclear polarization in the semiconductor interfaced with a ferromagnet and associated phenomena recently observed by time-resolved Faraday rotation experiments. The study also points to an alternative approach to spintronics different from spin injection.     

Optical orientation of particles with a random g-factor in semiconductor nanostructures

In real quasi-two-dimensional semiconductor nanostructures (quantum wells, quantum dots), the transverse g-factor of holes is a stochastic quantity. This fact should be taken into account in analyzing the optical orientation and Hanle effect of holes. The Hall effect for an ensemble of particles with a “random” g-factor has been treated theoretically. In the case where the spin relaxation time of a hole with a characteristic g-factor is shorter than the hole lifetime, there can occur a narrowing of the depolarization contour and an increase in its amplitude. In the opposite case of long spin relaxation times (trions in quantum dots), a formula has been derived, which generalizes the previously obtained result to the case of an arbitrary tilt angle of the magnetic field with respect to the plane of the layer (Hanle effect in the tilted form).     

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.     

Current-voltage characteristics of a spin half-metallic transistor

A new design of spin transistor based on half-metallic ferromagnets (referred to as a spin half-metallic transistor) is suggested, and its current-voltage characteristics are studied theoretically. Like a bipolar transistor, the new device can amplify current. At the same time, the properties of a spin half-metallic transistor depend considerably on the mutual orientation of the magnetizations of its three contacts. We also propose a device based on an F ^↑-F ^↓ junction. This device consists of two single-domain half-metallic parts with opposite magnetizations. There is a range of voltages where the current-voltage characteristics of an F ^↑-F ^↓ junction and a semiconductor diode are similar. The behavior of an F ^↑-F ^↓ junction under different conditions is studied.     

Effect of Rashba spin–orbit coupling on the spin polarized transport in diluted magnetic semiconductor barrier structures

We investigate theoretically the effects of Rashba spin–orbit coupling on the spin dependent transport through diluted magnetic semiconductor single and double barrier structures in the presence of a magnetic field. We find that the Rashba spin–orbit coupling gives rise to an enhancement of the negative tunnelling magnetoresistance of the diluted magnetic semiconductor single barrier structure and a pronounced beating pattern in the tunnelling magnetoresistance and spin polarization of the diluted magnetic semiconductor double barrier structure.     

Spin dynamics of semiconductor electrons in a hybrid ferromagnet/semiconductor structure

We analyze the recent experimental study by R.J. Epstein et al. [Phys. Rev. B 65, 121202 (2002)] on the spin dynamics of semiconductor electrons in a hybrid ferromagnet/semiconductor structure by using a simple model based on the Bloch equations. A comparison between the model calculations and the experimental observations shows that the spin relaxation rate is strongly anisotropic. We interpret this anisotropy as a manifestation of the exchange interaction between metallic and semiconductor electrons at the ferromagnet/semiconductor interface.     

自旋电子学简介及其研究进展

自旋电子学主要研究电子自旋在固体物理中的作用,是一门结合磁学与微电子学的交叉学科,其研究对象包括电子的自旋极化、自旋相关散射、自旋弛豫以及与此相关的性质及其应用等.基于电子自旋的自旋电子器件能够大大提高信息处理速度和存储密度,而且具有非易失性、低能耗等优点.简单介绍了自旋电子学的概念及其研究内容,综述了自旋电子学目前的研究及应用进展.     

极化子比率变化对自旋注入有机半导体性质的影响

从自旋扩散方程和欧姆定律出发研究了铁磁层到有机半导体的自旋注入,得到了系统的电流自旋极化率。有机半导体中的载流子为自旋极化子和不带自旋的双极化子,极化子比率在有机半导体内随输运距离变化。通过计算发现匹配的铁磁和有机半导体电导率有利于自旋注入;通过调节界面电阻自旋相关性,电流自旋极化率可获得很大程度提高;极化子比率衰减速率对有机半导体电流自旋极化率具有非常重要的影响。     

Magnetic susceptibility, heat capacity, and optical conductivity of LiPc and LiPcI

The magnetic susceptibility, using dc and electron spin resonance (ESR) methods, the specific heat, and the infrared properties of the one-dimensional molecular semiconductors lithium phthalocyanine (LiPc) and the iodinated compound LiPcI have been investigated for temperatures K. LiPc has a half-filled conduction band and is expected to be an organic metal. However, due to the strong Coulomb repulsion the system is a one-dimensional Mott-Hubbard insulator with a Hubbard gap of 0.75 eV as inferred from optical measurements. The localized electrons along the molecular stacks behave like a S = 1/2 antiferromagnetic spin chain. The spin susceptibility, as determined by ESR experiments, and the magnetic contribution to the heat capacity show a Bonner-Fisher type of behavior with an exchange constant K. LiPcI is an intrinsic narrow-gap semiconductor with an optical gap of 0.43 eV. In ESR experiments it is silent, indicating that all the unpaired electrons have been removed from the macrocycle via doping with iodine. Received: 16 June 1998 / Accepted: 14 July 1998