Experimental evidence of resonant tunneling via localized DQW states in an asymmetric triple barrier structure

In this work we report on field-induced features appearing in the tunneling current traces of a biased asymmetric triple barrier resonant tunneling device in the presence of an in-plane magnetic field. A theoretical model that satisfactorily explains the origin of these features is discussed. The reported data evidences the localized nature of the quantum states in thin layer asymmetric double-quantum-well structures.     

Direct numerical simulations of second-order Stokes wave driven smooth-walled oscillatory channel: investigation of net current formation

In wall-bounded time-periodic flows, nonlinearity, associated with higher harmonic term(s) in velocity and/or acceleration outside the boundary layer, can significantly change the wall turbulence compared with that in the linear Stokes Boundary Layer. A significant feature of a nonlinear wall-bounded turbulent time-periodic flow is the formation of a net current which has not yet been mechanistically explained. This study investigates the effects of asymmetric velocity outside the boundary layer on wall turbulence and net current formation through Direct Numerical Simulations of a smooth-walled planar channel driven by the Second-order Stokes Wave. Simulation results suggest that net current characteristics depend on whether developed turbulence is present. When turbulence is developed, asymmetric viscous length scale is found to be the primary reason of the net current whereby a vertical offset between negative and positive Reynolds shear stress profiles, associated with forward and reverse flows, respectively, is created in a cycle. After averaging over a cycle, residual Reynolds shear stress, which drives the net current, is observed to be within the offset layer.     

Features of the electron-photon drag effect in a spiral ribbon in the external magnetic field

The features of the photon drag effect in a spiral two-dimensional ribbon, which are associated with an asymmetric electron energy spectrum in a longitudinal magnetic field, have been studied theoretically. The effect of the anisotropic transfer of the photon momentum to the electron system in a spectral dependence of the photon drag current density has been revealed.     

Competing mechanisms of plasma transport in inhomogeneous configurations with velocity shear: the solar-wind interaction with earth's magnetosphere

Two-dimensional simulations of the Kelvin-Helmholtz instability in an inhomogeneous compressible plasma with a density gradient show that, in a transverse magnetic field configuration, the vortex pairing process and the Rayleigh-Taylor secondary instability compete during the nonlinear evolution of the vortices. Two different regimes exist depending on the value of the density jump across the velocity shear layer. These regimes have different physical signatures that can be crucial for the interpretation of satellite data of the interaction of the solar wind with the magnetospheric plasma.     

Dynamical evolution of coherent structures in intermittenttwo-dimensional MHD turbulence

Recent satellite observations indicate that the Earth's magnetotail is generally in a state of intermittent turbulence. A model of sporadic localized merging of coherent structures has recently been proposed by Chang to describe the dynamics of the Earth's magnetotail. The authors report the results of MHD simulations regarding the development and merging of two dimensional coherent structures. With a magnetic shear, such coherent structures are generated in alignment with the imposed current sheet. The calculated fluctuation spectra suggest long-ranged correlations with power-law characteristics     

Saturation of relativistic Weibel instability and the formation of stationary current sheets in collisionless plasma

We have studied the features of formation and the possible stationary structures of a self-consistent magnetic field in a relativistic collisionless plasma, which are characteristic of a simple geometry of the Weibel instability that is well known in the nonrelativistic case. The universal condition is established, the growth rate is determined, and the criteria of saturation of the Weibel instability are analyzed for a broad class of anisotropic particle distribution functions (for definiteness, in application to an electron-positron plasma). A nonlinear equation of the Grad-Shafranov type describing the potential current structures is derived and its solutions are analytically studied. Special attention is paid to spatially harmonic, nonlinear current configurations with parameters determined by the properties of the initial homogeneous plasma subject to the Weibel instability. It is demonstrated that the magnetic field energy density in the obtained solutions (both harmonic and nonharmonic) can be comparable with the kinetic energy density of plasma particles.     

Investigation of silicon structures with periodically varying porosity by x-ray diffractometry methods

The features of formation of silicon structures with a periodically varying porosity are investigated by X-ray diffractometry. It is revealed that the magnitude of anode current density corresponding to the formation of a layer with higher porosity that is part of a multilayer porous structure also affects the porosity of a second, less porous layer. As time elapses, the single-crystalline matrix of which porous layers consist is amorphized and a structure consisting of amorphous layers with a periodically varying density arises.     

Controlling the motion of magnetic flux quanta

We study the transport of vortices in superconductors with triangular arrays of boomerang- or V-shaped asymmetric pinning wells, when applying an alternating electrical current. The asymmetry of the pinning landscape induces a very efficient "diode" effect, that allows the sculpting at will of the magnetic field profile inside the sample. We present the first quantitative study of magnetic "lensing" of fluxons inside superconductors. Our proposed vortex lens provides a near threefold increase of the vortex density at its "focus" regions. The main numerical features have been derived analytically.     

Spin torque transfer structure with new spin switching configurations

Spin torque transfer structures with new spin switching configurations are proposed, fabricated and investigated in this paper. The non-uniform current-induced magnetization switching is implemented based on both GMR and MTJ nano devices. The proposed new spin transfer structure has a hybrid free layer that consists of a layer with conductive channels (magnetic) and non-conductive matrix (non-magnetic) and traditional free layer(s). Two mechanisms, a higher local current density by nano-current-channels and a non-uniform magnetization switching (reversal domain nucleation and growth) by a magnetic nanocomposite structure, contribute in reducing the switching current density. The critical switching current density for the new spin transfer structure is reduced to one third of the typical value for the normal structure. It can be expected to have one order of magnitude or more reduction for the critical current density if the optimization of materials and fabrication processes could be done further. Meanwhile, the thermal stability of this new spin transfer structure is not degraded, which may solve the long-standing scaling problem for magnetic random access memory (MRAM). This spin transfer structure, with the proposed and demonstrated new spin switching configurations, not only provides a solid approach for the practical application of spin transfer devices but also forms a unique platform for researchers to explore the non-uniform current-induced switching process.     

Surface damages on single-crystal silicon during irradiation by a powerful ion beam

Damages on single-crystal silicon under the action of a powerful proton-carbon beam of nanosecond duration have been studied. The formation of various surface structures at a beam current density of less than 100 A/cm² has been observed. The morphological features of these structures have been described and the effect of natural dioxide silicon on their formation has been considered.     

Magnetic aftereffect in Co-P trilayer films

This paper reports on the results of experimental investigations of the magnetic properties of a trilayer system that consists of high-coercive polycrystalline and low-coercive amorphous magnetic Co-P layers and a nonmagnetic amorphous Ni-P interlayer. It has been established that the coercive force and the bias field of the hysteresis loop of the low-coercive layer undergo anomalous changes in the range of small thicknesses of the hard magnetic layer. In the same range of thicknesses, a magnetic aftereffect is found to manifest itself, which is unusual for this type of magnetic structures. The observed features are associated primarily with the kinetics of formation of a crystal structure of the high-coercive layer.     

Internal transport barrier with ion-cyclotron-resonance minority heating on tore supra

Recently, reversed magnetic shear operation was performed using only ion-cyclotron-resonance frequency minority heating (ICRH) during current ramp-up. A wide region of reversed magnetic shear has been obtained. For the first time, an electron internal transport barrier sustained by ICRH is observed, with a dramatical drop of density fluctuations. This barrier was maintained, on the current flat top, for about 2 s.     

中纬度地区电离层偶发E层对量子卫星通信性能的影响

电离层偶发E层是指在距离地面高度80—150 km之间,在风剪切作用下,电子密度急剧增加的不规则电离薄层,它会对量子卫星光信号的传输造成极大的影响.然而,有关电离层偶发E层与星地间量子通信信道参数关系的研究,迄今尚未展开.为了研究偶发E层对量子卫星通信性能的影响,首先分析了它的形成过程,得出自由电子密度随高度变化的关系;然后建立了自由电子密度、偶发E层的厚度对量子卫星链路衰减的模型;针对振幅阻尼信道,给出自由电子密度对信道容量、纠缠保真度、误码率和安全密钥产生率的定量关系.理论分析和仿真结果表明,当偶发E层的厚度为1 km、电子密度由3×10~5cm~(-1)增加到27×10~5cm~(-1)时,信道容量由0.8304衰减到0.1319,纠缠保真度由0.9386下降到0.3606,量子误码率由0.0093增加到0.0769,安全密钥产生率由9.968×10~(-5)减小到1.91×10~(-6).由此可见,电子密度的大小和偶发E层的厚度对量子卫星通信性能有显著的影响.因此,在进行量子卫星通信时,应根据对电离层参数的探测情况,自适应调整卫星系统的各项指标,以确保量子通信的可靠性.     

Towards hybrid silicon-organic molecular electronics: The stability of acetone on the Si(0 0 1) surface

We present the results of a combined study using scanning tunneling microscopy (STM) and density functional theory (DFT) of the interaction of acetone [(CH₃)₂CO] with the Si(0 0 1) surface. Three distinct adsorbate features were observed using atomic-resolution STM. One of the features appears as a bright protrusion located above a Si-Si dimer, while the other two are asymmetric about the dimer row and involve a second neighboring Si-Si dimer. One of the two asymmetric features has a protrusion located between the two dimers, while the other has a protrusion which is located at the site of a single dimer and exhibits a dimer sized depression on the adjacent dimer. DFT calculations have been performed for two structures; the four-membered ring structure and dissociation structure. Our calculations show that the bright single-dimer sized feature observed in the STM images could be attributed to either of these two calculated structures. However, neither of the two calculated structures can explain the appearance of the two-dimer wide asymmetric features observed in the experiment.     

Numerical evidence of undriven, fast reconnection in the solar-wind interaction with earth's magnetosphere: formation of electromagnetic coherent structures

We give evidence for the first time of the onset of undriven fast, collisionless magnetic reconnection during the evolution of an initially homogeneous magnetic field advected in a sheared velocity field. We consider the interaction of the solar wind with the magnetospheric plasma at low latitude and show that reconnection takes place in the layer between adjacent vortices generated by the Kelvin-Helmholtz instability. This process generates coherent magnetic structures with a size comparable to the ion inertial scale, much smaller than the system dimensions but much larger than the electron inertial scale. These magnetic structures are further advected in the plasma in a complex pattern but remain stable over a time interval much longer than their formation time. These results can be crucial for the interpretation of satellite data showing coherent magnetic structures in the Earth's magnetosheath or the magnetotail.     

The Influence of Stochastic Layers of Magnetic Field Lines on Transport Barrier Formation in a Stellarator System

The results of local measurements of RF discharge plasma parameters in the process of internal transport barriers (ITB) formation in the vicinity of rational magnetic surfaces in the Uragan-3M torsatron are presented. The following phenomena were observed in the process of ITB formation: widening of the radial density distribution, formation of plateaus on radial density and electron temperature distributions, formation of regions with high shear of poloidal plasma rotation velocity and radial electric field in the vicinity of stochastic layers of magnetic field lines, decrease of density fluctuations and their radial correlation length, decorrelation of density fluctuations, and increase of the bootstrap current.After the ITB formation, the transition to the improved plasma confinement regime takes place. The transition moves to the beginning of the discharge with the increase of heating power. The possible mechanism of ITB formation near rational surfaces is discussed.     

Anomalous electrical properties of Si/Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> heterostructures with an electron transport channel in Si layers

The electrical properties of elastically stressed FET-like Si/Si_{1 ? x} Ge _x layered heterostructures (x < 0.2) with a modulated dopant concentration are studied. It is presumed that the amplitude of a narrow (～6-nm-thick) potential well in the Si layer corresponds to the amplitude of a fluctuation potential giving rise to nanoscale inhomogeneities in the Si channel. The structures in question undergo a phase transition from the insulating to metallic behavior as the electron density in the Si channel is varied. In a magnetic field, the structures with metallic conductivity have negative magnetoresistance (NMR) for both the longitudinal and transverse directions of the applied magnetic field with respect to the transport current passing through the structure. Investigating the dependence of NMR on the magnetic field strength showed that the main contribution to NMR is due to weak localization. Peculiar properties are also found in electrical measurements of the diode characteristics of the structures in question. In particular, the capacitance-voltage characteristics of these structures show well-defined resonance features, possibly related to one-and quasi-zero-dimensional inclusions existing in addition to two-dimensional charge carriers in the inhomogeneous two-dimensional film.     

Negative tunneling magnetoresistance by canted magnetization in MgO/NiO tunnel barriers

The influence of the insertion of an ultrathin NiO layer between the MgO barrier and the ferromagnetic electrodes in magnetic tunnel junctions has been investigated from measurements of the tunneling magnetoresistance and via x-ray magnetic circular dichroism (XMCD). The magnetoresistance shows a high asymmetry with respect to bias voltage, giving rise to a negative value of up to -16% at 2.8 K. We attribute this effect to the formation of noncollinear spin structures at the interface of the NiO layer as inferred from XMCD measurements. The magnetic moments of the interface Ni atoms tilt from their easy axis due to exchange coupling with the neighboring ferromagnetic electrode, and the tilting angle decreases with increasing NiO thickness. The experimental observations are further supported by noncollinear spin density functional calculations.     

冲击射流激波振荡与抑制

欠膨胀冲击射流具有复杂的激波结构,并伴随产生高幅值的离散频率单音.通过高速摄像获取的纹影图像并结合噪声测量,对欠膨胀冲击射流激波振荡过程、剪切层不稳定波的模态和离散频率单音的产生进行了系列研究.给出了冲击距离为5倍喷嘴出口直径的复杂流动实验结果分析,射流剪切层不稳定波有对称和非对称两种模态,发现不同模态下的离散频率单音...     

利用低杂波改善约束的实验研究

介绍了在HT-6M托卡马克上,利用双低杂波的组合,成功地实现了准稳态的高约束模式运行. 能量约束时间提高两倍以上,粒子的约束在较高密度下依然改善3倍以上.通过应用波扩散及 电流径向扩散方程计算低杂波电流传播的方法,对一组典型的数据进行数值模拟.计算表明 ,在HT-6M低杂波实验中,由于纵场较低,密度较高,低杂波的能量沉积在离磁轴较远的位 置,使等离子体电流密度分布成为反剪切位形,内部输运垒地形成,大大提高了等离子体的 约束状况.实验数据也给出了反剪切的证据. 关键词： 托卡马克 低杂波 约束改善