Anisotropy of the relaxation of the electron spin caused by competition of the Rashba and Dresselhaus mechanisms

Anisotropy of the relaxation of electron spin in AlGaAs quantum wells with an asymmetric profile in the growth-axis direction is studied by the technique of optical orientation. It is demonstrated that the difference between the Hanle curves for [110] and [1 $ \bar 1 Anisotropy of the relaxation of electron spin in AlGaAs quantum wells with an asymmetric profile in the growth-axis direction is studied by the technique of optical orientation. It is demonstrated that the difference between the Hanle curves for [110] and [10] orientations does originate from the spin-relaxation anisotropy rather than the electron g-factor anisotropy. Contributions to the Hanle curves both from free and localized electrons, characterized by different lifetimes, are observed. Original Russian Text ? A.M. Smirnov, V.A. Oseptsova, A.V. Platonov, A.S. Gurevich, V.P. Kochereshko, A.S. Shkolnik, V.P. Evtikhiev, V.V. Petrov, Yu.K. Dolgikh, Yu.P. Efimov, S.A. Eliseev, 2009, published in Fizika i Tekhnika Poluprovodnikov, 2009, Vol. 43, No. 7, pp. 933–937.     

Electrical Spin Injection into the 2D Electron Gas in AlN/GaN Heterostructures with Ultrathin AlN Tunnel Barrier

The spin injection into 2D electron gas (2DEG) in AlN/GaN heterostructures is studied by magneto-transport measurements. An ultrathin AlN layer at the hetero-interface acts as a barrier to form high-quality 2DEG in the triangular quantum well and a tunneling barrier for the spin injection to overcome the conductance mismatch issue. In this study, Hanle signals and inversed Hanle signals are observed, proving that the spin injection is achieved in the 2DEG in the AlN/GaN heterostructure rather than in the interfacial states. The spin-relaxation time in 2DEG at 8 K is found to be as long as 860 ps, which almost keeps constant with bias and decreases with increasing temperature. The spin-relaxation process is illustrated as Rashba spin-orbit coupling dominated D'yakonov Perel’ mechanisms above 8 K. These results show the promising potential of 2DEG in AlN/GaN heterostructures for spin field-effect transistor applications.     

Ba原子6s2 1S0 - 6s6p3P1跃迁Hanle效应实验研究

Ba原子是光频标的候选者之一,对其进行有效的激光冷却与囚禁需要相关能级的寿命和跃迁几率的信息。Ba原子激发态6s6p 3P1能级在激光冷却实验中很重要,通过Hanle效应实验测量了这一能级的寿命和自发辐射率,从理论和实验上研究了探测激光有限线宽和光强对Ba原子基态6s2 1S0与激发态6s6p 3P1之间跃迁（波长791 nm）的Hanle效应的荧光信号的影响。在考虑了激光线宽和光强因素后所得到的激发态6s6p 3P1的能级寿命和自发辐射率与其他方法给出的结果很好符合。     

The effect of silicon crystallographic orientation on the formation of silicon nanoclusters during anodic electrochemical etching

Possible ways for increasing the photoluminescence quantum yield of porous silicon layers have been investigated. The effect of the anodization parameters on the photoluminescence properties for porous silicon layers formed on silicon substrates with different crystallographic orientations was studied. The average diameters for silicon nanoclusters are calculated from the photoluminescence spectra of porous silicon. The influence of the substrate crystallographic orientation on the photoluminescence quantum yield of porous silicon is revealed. A model explaining the effect of the substrate orientation on the photoluminescence properties for the proous silicon layers formed by anode electrochemical etching is proposed.     

Molecular orientation and thermal stability of thin-film organic semiconductors

Molecular orientation in organic semiconductors plays a critical role in maximizing external quantum efficiencies of organic light-emitting diodes. It was generally believed that the molecular packing of organic semiconductors is either amorphous or liquid-crystal-like with a preferred molecular orientation distributed uniformly throughout the film. In this paper, however, we report that the orientation of organic molecules in physical-vapor deposited films varies drastically depending on thickness. The thermal stability of the molecular network, measured by its characteristic glass transition temperature, also varies as a function of the film thickness. Based on a two-layered film-structure model, we propose a simple function to quantify the molecular dipole orientation S parameter as a function of film thickness. This function describes well experimental data. In addition to contributing to external quantum efficiency, the molecular orientation parameter S is found to have a strong impact on disruptive change in material density after thermal anneal and glass transition.     

220 GHz Zenith Atmospheric Transparency at IAO, Hanle

We present 220 GHz (1.36 mm) measurements of zenith optical depth obtained to characterise the Indian Astronomical Observatory, Hanle (Ladakh, India) during the period from late December 1999 to early May 2000 and early October 2000 to September 2001. The data were sampled at an interval of 10 minutes. We describe the automated 220 GHz tipping radiometer used, its basic principle, operation, data acquisition method and data reduction scheme in detail. The 220 GHz opacity is found to be less than 0.06 for a significant fraction (40%) of the time during the winter months, indicating that Hanle is one of the good observing sites for submillimeter-wave astronomy. We make a preliminary correlation with the precipitable water vapour derived from surface relative humidity and air temperature measurements made during the same period with a weather station installed at the site. We also compare the Hanle site with other high-altitude sites like Mauna Kea and Atacama desert.     

Thermal orientation of electron spins

It is shown that the spin orientation of free electrons occurs in low-symmetry semiconductor structures if the electron gas is driven out of thermal equilibrium with the crystal lattice. The proposed mechanism of such a thermal orientation of electron spins is based on spin dependence of the electron-phonon interaction, which tends to restore equilibrium. The microscopic theory of the effect is developed here for asymmetric (110)-grown quantum wells where the electron gas heating leads to the spin orientation along the [] axis in the quantum well plane. The text was submitted by the authors in English.     

Method of Relaxation Rates Measurement in Proton-Containing Materials

A method of the relaxation time measurement in a flowing sample, based on the comparison of the amplitudes of nuclear magnetic resonance (NMR) signals observed from the reference and tested flowing samples was represented. The inductive and optical methods of NMR signal detection in the flowing sample were compared. The levels of NMR signal (10,000 ​Hz^−1/2) and the Hanle magnetometers signal-to-noise ratio (3500 ​Hz^−1/2) were calculated and the possibility of the NMR signal detection with the small-size Hanle magnetometer was demonstrated. Additionally, the absence of a magnet-analyzer and the ability to measure both longitudinal and transverse relaxation time with high accuracy was demonstrated.     

入射角对光量子阱束缚态影响的研究

为了探讨入射角对光量子阱传输特性的影响,用传输矩阵法计算了不同入射角对光量子阱结构的传输特性,得到了光量子阱能应用于多通道滤波和光开关的结论.结果表明,光子的束缚效应将导致频率的量子化,通过微小地改变入射角可改变束缚态的频率,且入射角的变化与束缚态频率的变化呈3次多项式关系.此结果为该结构实现多通道滤波和方位开关提供了理论依据,也为得到所需要的束缚态频率提供了理论基础.     

Vertical transport of hot electrons in GaAs/AlAs superlattices

Ballistic transport of hot photoexcited electrons injected from a superlattice into an enlarged quantum well is studied using the polarized hot photoluminescence technique. It is established that most photoexcited electrons thermalize before they are captured by the enlarged QW; however, a minor fraction of them reach the enlarged quantum well ballistically, keeping their momentum-distribution anisotropy or spin orientation arising due to the absorption of linearly or circularly polarized light in the superlattice.     

第Ⅱ类量子阱中的激子束缚能(英文)

We calculate variationally the binding energy of excitons in type-Ⅱ quantum well structures with the staggered or the misaligned configuration. Particularlly, we have investigated the effect of the longitudinal electric field on the exciton binding energy in In1-xGaxAs/GaSb1-yA.sy systems. It is shown that our results of the InAs/GaSb structure for zero field are much larger than those previously reported by other authers. For the non-zero field case, the variation of the binding energy obviously depends on the orientation of the external electric field. With increasing the absolute value of the field strength, the binding energy increases for F<0, while decreases for F>0. We also find that the electric field slightly affects the critical width for the semiconductor-to-semimetal transition in misaligned type-Ⅱ quantum wells. These results have very clear physical meaning.     

四终端量子环自旋输运的性质

提出了铁磁/半导体/铁磁结构的四终端量子环模型,研究表明:透射概率随半导体环增大做周期性等幅振荡,并与电子的自旋方向和铁磁电极的磁矩方向相关。上下电极的平均透射概率均比右侧电极的大,但四终端量子环的平均透射概率及其振荡频率均比两终端量子环的低。Rashba自旋轨道耦合具有促使透射概率产生零点的效应,AB磁通对透射概率具有影响。     

Homogeneous Linewidth and Saturation Parameter of Infrared Laser Transitions

We investigated the behavior of infrared (IR) absorption laser transitions of methanol isotopes in the presence of an uniform electric field, in order to determine Doppler-free spectroscopic data such as homogeneous linewidth and saturation parameter. For this we combine photoacoustic detection with the high resolution provided by the Nonlinear Hanle Effect (NLHE) on molecular transitions.     

Optical orientation of electrons in compensated semiconductors

The theory of the optical orientation of charge carriers in compensated III-V semiconductors and quantum wells for the case where electrons are excited to the conduction band from Mn-charged acceptor states is presented. It is shown that, in GaAs/AlGaAs quantum wells, the degree of the spin orientation of conduction-band electrons in this excitation scheme can be as high as 85%. This spin-orientation enhancement results from an increase in the heavy-hole contribution to the acceptor state in the vicinity of the defect center rather than from level splitting caused by quantum confinement. It is shown that the degree of circular polarization of the photoluminescence emitted upon the recombination of electrons thermalized at the bottom of the band with holes occupying the acceptor ground state in a quantum well can exceed 70%.     

Electric field effects on the efficiency of optically pumped submillimeter lasers

The FIR power dependence on an applied electric field is carefully investigated for optically pumped lasers. The enhancement effect observed for many laser lines, both "orthogonal" and "parallel," is analyzed as a function of different parameters such as gas pressure, laser threshold, cavity tuning, and transitions selection rules. Measurements of the IR absorption of the gases are also carried out by a Stark optoacoustic technique. Absorption increases are observed at low electric field values. The experimental results disagree with the previously proposed linear Hanle effect explanation. They are explained with the increase in the pump saturation intensity, also detected with the transferred Lamb-dip technique. The combined effects of the absorption increase and of the small-signal gain curve decrease are shown to qualitatively explain the observed FIR enhancement features.     

Spontaneous lifetime and quantum efficiency in light emittingdiodes affected by a close metal mirror

The spontaneous lifetime and quantum efficiencies of dipoles placed close to a metal mirror are calculated. The importance of these effects is demonstrated in the recent experimental work on InGaAs-GaAs quantum-well light-emitting diodes (QW LEDs) in which the modulations rates were varied from 0.8 GHz to 1.4 GHz through the placement of a QW next to a Ag mirror (D.G. Deppe et al., 1990). Classical energy transfer theory is used to treat the spontaneous radiation of a QW in a semiconductor system. The spontaneous radiative lifetime and internal quantum efficiency are strongly affected by the location of the QW, which confines the dipoles, by the orientation of the dipole in the well, by the metal reflector material, and by the metal mirror thickness. In general, the internal quantum efficiency decreases when the dipole is closer to the metal, because of the nonradiative energy transfer from the dipole to the absorptive metal. The overall effect of closely spaced metal mirrors on the measurable external quantum efficiency, which is important for fast light emitting diode design, is presented     

表面结构和生长方向对WO3纳米线电子结构影响的第一性原理研究

The effects of the surface and orientation of a WO₃ nanowire on the electronic structure are investigated by using first principles calculation based on density functional theory（DFT）.The surface of the WO3 nanowire was terminated by a bare or hydrogenated oxygen monolayer or bare WO₂ plane,and the[010]- and[001]-oriented nanowires with different sizes were introduced into the theoretical calculation to further study the dependence of electronic band structure on the wire size and orientation.The calculated results reveal that the surface structure, wire size and orientation have significant effects on the electronic band structure,bandgap,and density of states （DOS） of the WO₃ nanowire.The optimized WO₃ nanowire with different surface structures showed a markedly dissimilar band structure due to the different electronic states near the Fermi level,and the O-terminated[001] WO₃ nanowire with hydrogenation can exhibit a reasonable indirect bandgap of 2.340 eV due to the quantum confinement effect,which is 0.257 eV wider than bulk WO₃.Besides,the bandgap change is also related to the orientation-resulted surface reconstructed structure as well as wire size.     

分子束外延InAs量子点的RHEED实时原位分析

介绍了利用反射式高能电子衍射(RHEED)方法在自组装InAs量子点制备过程中进行结构分析的理论研究与实验工作的最新进展。从反射式高能电子衍射在InAs量子点临界转变状态测定、量子点表面取向、量子点应力分布测定、量子点形核长大动力学过程研究等方面的应用,可以看出RHEED在InAs量子点形成过程中对多种结构特征的原位分析具有突出优势。反射式高能电子衍射仪作为分子束外延系统中的标准配置,已成为一种对InAs量子点微观结构进行分析的简易而理想的分析测试工具。随着反射式高能电子衍射以及衍射理论的进一步发展,必将促进InAs量子点结构的精确表征水平的提高,进而实现更加理想结构的InAs量子点的制备及其应用。     

Quantum Theory of Contact Electrification for Fluids and Solids

A unified quantum‐mechanical model of contact electrification that provides a microscopic basis for the Volta–Helmholtz–Montgomery hypothesis is presented. The model can represent metals, semiconductors, or insulators, in either fluid or solid phase, and with an effective electron transfer parameter as the driving mechanism. Known experimental results such as the charging of similar materials, the charging of similar materials with different contact orientation, the surface charge mosaic, and the higher efficiency of charge transfer for a liquid–solid contact, compared to a solid–solid one, are reproduced. A quantum‐mechanical charge oscillation in the femtosecond to picosecond regime is predicted to take place. Coulomb interaction is found to have an impact on not just the charge transferred but also the period of charge oscillation.     

Microwave frequency doubling in ruby

Investigations of quantum mechanical frequency doubling in ruby are reported. Experimental results are given for the 4.04-GHz output from a ruby sample driven at 2.02 GHz when 4.04 GHz corresponds to paramagnetic resonance. The variation of output power with orientation differs appreciably from the well-known variation of the linear (maser) effect. Good agreement with theoretical results has been achieved.