The effects of current-path patterns on magnetotransport in spatially-confined structures by Monte Carlo simulation

Simulations are performed on clusters of finite size to study the effects of size and current-path structure on magnetotransport in spatially-confined samples. Magnetotransport networks are established and calculated based on fractal structures including Koch curves and percolation backbones extracted from regular lattices. The structure pattern of clusters is shown to play an important role in the magnetotransport behaviours by affecting the magnetore- sistance fluctuations due to spin disorder in the systems of small size, which suggests the possibility of controlling the magnetotransport by the design of current-path configurations.

Magnetotransport Properties of a Nodal Line Semimetal TiSi

We report the magnetoresistance（MR）, de Haas-van Alphen（dHvA） effect and Hall effect measurements on a single crystal of TiSi, which is predicted to be a nodal line semimetal. With application of a magnetic field, a metal-to-insulator-like transition in ρ（T） and a nonsaturating MR are observed at low temperatures. The dHvA oscillations reveal a small Fermi-surface pocket with a nontrivial Berry phase. The analysis of the nonlinear Hall resistivity shows that TiSi is a multiband system with low carrier densities and high mobilities. All these results unambiguously prove the existence of Dirac fermions in TiSi.

Magnetotransport properties of two-dimensional electron gas in AlGaN/AlN/GaN heterostructures

Magnetotransport measurements are carried out on the A1GaN/A1N/GaN in an SiC heterostructure, which demon- strates the existence of the high-quality two-dimensional electron gas （2DGE） at the A1N/GaN interface. While the carrier concentration reaches 1.32×10^13 cm^-2 and stays relatively unchanged with the decreasing temperature, the mobility of the 2DEG increases to 1.21 × 10^4 cm2/（V.s） at 2 K. The Shubnikov-de Haas （SdH） oscillations are observed in a magnetic field as low as 2.5 T at 2 K. By the measurements and the analyses of the temperature-dependent SdH oscillations, the effective mass of the 2DEC is determined. The ratio of the transport lifetime to the quantum scattering time is 9 in our sample, indicating that small-angle scattering is predominant.

Magnetotransport in a dual waveguide coupled by a finite barrier： Energy filter and directional coupler

We propose in this paper that a dual waveguide coupled by a finite barrier be able to serve as an energy filter under a perpendicular magnetic field. In the waveguide direction, the conductance exhibits a periodic square-wave pattern in which the miniband is controlled by the magnetic and potential modulation. The electrons with energies in the miniband can completely transfer along one waveguide while the other electrons undergo filtration. Compared with the coupled waveguide without magnetic modulation, the structure under magnetic field is found to be a good directional coupler. By adjusting the potential barrier and magnetic field, the electrons input from one port of waveguide can transfer to any other ports.

Evolution of electrical and magnetotransport properties with lattice strain in La0.7Sr0.3MnO3 film

In this paper,we investigate the effects of lattice strain on the electrical and magnetotransport properties of La0.7Sr0.3MnO3(LSMO)films by changing film thickness and substrate.For electrical properties,a resistivity upturn emerges in LSMO films,i.e.,LSMO/STO and LSMO/LSAT with small lattice strain at a low temperature,which originates from the weak localization effect.Increasing film thickness weakens the weak localization effect,resulting in the disappearance of resistivity upturn.While in LSMO films with a large lattice strain(i.e.,LSMO/LAO),an unexpected semiconductor behavior is observed due to the linear defects.For magnetotransport properties,an anomalous in-plane magnetoresistance peak(pMR)occurs at low temperatures in LSMO films with small lattice strain,which is caused by two-dimensional electron gas(2DEG).Increasing film thickness suppresses the 2DEG,which weakens the pMR.Besides,it is found that the film orientation has no influence on the formation of 2DEG.While in LSMO/LAO films,the 2DEG cannot form due to the existence of linear defects.This work can provide an efficient way to regulate the film transport properties.

Magnetic and electrical transport properties in GdAlSi and SmAlGe

We conduct a detailed examination of the magnetic and electrical transport properties in GdAlSi and SmAlGe crystals,which possess a LaPtSi-type structure(space group I4_(1)md).The magnetic susceptibility data unambiguously reveal magnetic ordering below a characteristic transition temperature(T_(N)).For GdAlSi,a hysteresis loop is observed in the magnetization and magnetoresistance curves within the ab plane when the magnetic field is applied below T_(N),which is around32 K.Notable specific heat anomalies are detected at 32 K for GdAlSi and 6 K for SmAlGe,confirming the occurrence of magnetic transitions.In addition,the extracted magnetic entropy at high temperatures is consistent with the theoretical value of Rln(2J+1) for J=7/2 in Gd^(3+) and J=5/2 in Sm^(3+),respectively.SmAlGe also exhibits Schottky-like specific heat contributions.Additionally,both GdAlSi and SmAlGe exhibit positive magnetoresistance and a normal Hall effect.

Anomalous magnetotransport in LaMn_(1-x)Te_xO_3

The LaMn1-xTexO3+δ (x=0.1, 0.2, 0.3) were synthesized using solid-state reaction method for the first time. X-ray photoemission spectrum (XPS) shows that in the samples the Te ions have a valence of Te4+, and Mn3+ ions are partly converted into Mn2+ and Mn4+ due to the excess oxygen and Te doping. The magnetotransport associated with Mn2+, Mn3+ and Mn4+ was investigated. The experimental results show that the samples are insulator at 0 T when the amount of Mn3+ is much larger or less than the sum of Mn2+ and Mn4+; by contrast, the samples display metal to insulator transition with increasing temperature when the amount of Mn3+ is close to the sum of Mn2+ and Mn4+. These anomalous magnetotransport behaviors were analyzed in the frame of the double-exchange (DE) mechanism.

Weak localization in disordered spin-1 chiral fermions

We theoretically investigate the quantum interference theory of magnetotransport of the three-component or spin-1 chiral fermions, which possess two linear Dirac bands and a flat band. For isotropic scalar impurities, the correction of conductivity from the coherent backscatter and non-coherent backscatter contributions cancel out in the intravalley scattering, leading to a weak localization correction to the Drude conductivity from the intervalley scattering. For the anisotropic impurities, the above cancelation is removed, we find the approximative quantum interference conductivity in the weak anisotropy case. The contributions from the chiral anomaly and classical Lorentz force are also discussed. Our work reveals some intriguing and detectable transport signatures of the novel spin-1 chiral fermions.

Recent Progress in the Fabrication, Properties, and Devices of Heterostructures Based on 2D Materials

With a large number of researches being conducted on two?dimen?sional(2D) materials, their unique properties in optics, electrics, mechanics, and magnetics have attracted increasing attention. Accordingly, the idea of combining distinct functional 2D materials into heterostructures naturally emerged that pro?vides unprecedented platforms for exploring new physics that are not accessible in a single 2D material or 3D heterostructures. Along with the rapid development of controllable, scalable, and programmed synthesis techniques of high?quality 2D heterostructures, various heterostructure devices with extraordinary performance have been designed and fabricated, including tunneling transistors, photodetectors, and spintronic devices. In this review, we present a summary of the latest progresses in fabrications, properties, and applications of di erent types of 2D heterostruc?tures, followed by the discussions on present challenges and perspectives of further investigations.

Quantitative Mobility Spectrum Analysis for Determination of Electron and Magneto Transport Properties of Te-Doped GaSb

Resistivity, magnetoresistivity and Hall effect measurements in n-type Te-doped GaSb grown by the liquid encapsuled Czochralski technique are carried out as functions of temperature （35-350 K） and magnetic field （0-1.35 T）. The power law model is used to explain the temperature-dependent resistivity. The magnetic-field-dependent data are analysed using the quantitative mobility spectrum analysis technique. The effect of individual band parameters （nL, nr, μL, μr, p and μp） on both the electron and magneto transports have been discussed. The EL - Er energy separation between the L and conduction band edges is also derived.

n⁺-p-p⁺Silicon Solar Cell Base Optimum Thickness Determination under Magnetic Field

Base optimum thickness is determined for a front illuminated bifacial silicon solar cell n+-p-p+ under magnetic field. From the magneto transport equation relative to excess minority carriers in the base, with specific boundary conditions, the photocurrent is obtained. From this result the expressions of the carrier’s recombination velocity at the back surface are deducted. These new expressions of recombination velocity are plotted according to the depth of the base, to deduce the optimum thickness, which will allow the production, of a high short-circuit photocurrent. Calibration relationships of optimum thickness versus magnetic field were presented according to study ranges. It is found that, applied magnetic field imposes a weak thickness material for solar cell manufacturing leading to high short-circuit current.

Siδ掺杂GaAs多量子阱的磁量子输运

使用ＭＢＥ技术低温生长了含有３个Ｓｉδ掺杂层的ＧａＡｓ外延片，形成三量子阶结构．在４．２～３００Ｋ温区０．１Ｔ的电磁铁上测量了样品载流子浓度和迁移率随温度的变化．在０．３～４．２Ｋ（３Ｈｅ温区）和０～７Ｔ强磁场下测量了样品的横向磁阻、纵向磁阻和霍尔电阻．观察到了横向磁阻的ＳｄＨ振荡和纵向磁阻的抗磁ＳｄＨ振荡．根据实验结果着重讨论了纵向磁阻振荡的起源以及霍尔振荡的含义，简单分析了δ掺杂二维电子气中磁致金属一绝缘体转变的实验条件．

AlxGa1-xN/GaN异质结二维电子气磁输运性质研究（英文）

在 1 5K低温和 0～ 9T的高磁场下研究了AlGaG/GaN异质结二维电子气的磁输运性质。实验结果在 4块样品中都观察到了Shubnikov daHass振荡的双周期行为。表明异质结的三角势阱中有两个子带被电子占据。通过电子子带占据时电子浓度分配的线形行为得到第二子带被占据的阈值浓度为 7 2× 1 0 12 cm-2 。通过对不同样品量子散射时间和输运迁移率的研究 ,说明在 1 5K下远程离化施主散射在量子散射时间中起主要作用。

Al_xGa_(1-x)N/GaN异质结二维电子气磁输运性质研究(英文)

在 1 5K低温和 0～ 9T的高磁场下研究了AlGaG/GaN异质结二维电子气的磁输运性质。实验结果在 4块样品中都观察到了Shubnikov daHass振荡的双周期行为。表明异质结的三角势阱中有两个子带被电子占据。通过电子子带占据时电子浓度分配的线形行为得到第二子带被占据的阈值浓度为 7 2× 1 0 12 cm-2 。通过对不同样品量子散射时间和输运迁移率的研究 ,说明在 1 5K下远程离化施主散射在量子散射时间中起主要作用。

Al_xGa_(1-x)N/GaN异质结构中二维电子气的自旋性质

Ⅲ族氮化物材料有很长的电子自旋弛豫时间以及很高的居里温度,成为近年来半导体自旋电子学研究的重要材料体系之一。介绍了目前两种最主要的研究AlxGa1-xN/GaN异质结构中二维电子气(2DEG)自旋性质的物理效应:磁电阻的舒伯尼科夫-德哈斯拍频振荡和弱反局域效应,回顾了AlxGa1-xN/GaN异质结构中2DEG自旋性质的研究进展。AlxGa1-xN/GaN异质结构材料中有很强的极化电场,诱导产生很高浓度的2DEG,能够产生相当大能量的自旋分裂,并且这种自旋分裂可以被栅压所调控,因此在自旋场效应晶体管方面有很好的应用前景。然而要实现GaN基自旋电子学器件的应用,GaN中自旋注入效率是目前所面临的问题。

La_(2/3)Ca_(1/3)MnO_3/LaMnO_3复合体系中颗粒间界对电磁输运行为的影响

将奈尔温度为145 K的反铁磁性绝缘体LaMnO_3作为第二相复合到La_(2/3)Ca_(1/3)MnO_3颗粒间界处,研究反铁磁性绝缘体对复合体系的电磁输运性质的影响.在(1-x)La_(2/3)Ca_(1/3)MnO_3/xLaMnO_3复合体系中,随着x增加,样品的金属-绝缘体转变温度Tp降低,峰值电阻增加.电输运行为表明:随着反铁磁性第二相LaMnO_3的引入,电子-声子散射以及电子-磁振子散射对输运行为的影响变大.在低磁场0.3 T下,相对于纯La_(2/3)Ca_(1/3)MnO_3,复合样品在金属-绝缘体转变温区附近的磁电阻大大增强;在高场3 T下,所有样品都存在着磁电阻平台现象,且复合样品的磁电阻值在低温区域都明显大于纯La_(2/3)Ca_(1/3)MnO_3的磁电阻值.

An Explicit Function Expression for dc Bias and Temperature Dependence of Magnetoresistances in Magnetic Tunnel Junctions

An explicit function expression for the bias voltage or/and temperature dependences of tunnel magnetoresistance ratio and resistances were obtained with a unique set of intrinsic parameters. Two of these intrinsic parameters are the Curie temperature TC and the density of state (DOS) for itinerant majority and minority electrons ξ(ρM/ρm), which are the eigen parameters of ferromagnetic electrodes. Others are the spin-dependent matrix-element ratio (i.e., |Td|2/|TJ|2 ) and the anisotropic-wavelength-cutoff energy ECγ of spin-wave spectrum in magnetic tunnel junction (MTJ), which are the structure parameters of an MTJ. These intrinsic parameters can be predetermined using the experimental measurement or, in principle, using the first-principle calculation method for an MTJ with the three key layers of FM/I/FM. Furthermore, a series of experimental data for an MTJ, for example, a spin-valve-type MTJ of Ta (5 nm)/Ni79Fe21(25 nm)/lr22Mn78(12 nm)/Co75Fe25(4 nm)/AI(0.8 nm)-oxide/Co75Fe25(4 nm)/Ni79Fe21(20 nm)/Ta (5 nm) in this work, can be self-consistently evaluated and explained using such concise explicit function formulations.

Hopping Conductivity in a Single-Walled Carbon Nanotube Network

We investigate the resistance and magnetoresistance （MR） of an entangled single-walled carbon nanotube （SWNT） network. The temperature dependence of conductance is fitted by formula G（T） = Go exp[-（To/T）^1/2] with To = 15.8 K at a wide temperature range from 4 K to 300K. The MR defined by [R（T, H） - R（T, 0）]/ R（T, 0） as a function of temperature and magnetic field perpendicular to the tube axis is negative at low temperatures. The MR amplitude increases as the temperature decreases at relative high temperature, but becomes decrease when temperature below 4 K. The results are explained in terms of the coherent hopping of carriers in the presence of a Coulomb gap at low temperature.

Synthesis and Characterization of Li-doped LaMnO_3 CMR Materials

A series of bulk polycrystalline La1-xLixMnO3 samples with x ranging from 0.1 to 0.5 was prepared by sol-gel method,X-ray diffraction patterns show that the crystal structures are single rhombohedral perorskite for the x≤0.3 sample and the impurity appears when x〉0.3.Under the same synthesized conditions,the higher Li content samples display a higher content of liquid phase content and larger mean grain sizes,which leads to the increases of the effect of the grain boundaries.The experimental results show that the change of the ferromagnetic transition temperature and the resistivity can attribute to the effect of the grain boundary and the connectivity of the inter grains as well as the ratio of Mn^3＋ to Mn^4＋.

Growth and transport properties of topological insulator Bi2Se3 thin film on a ferromagnetic insulating substrate

Exchange coupling between topological insulator and ferromagnetic insulator through proximity effect is strongly attractive for both fundamental physics and technological applications. Here we report a comprehensive investigation on the growth behaviors of prototype topological insulator Bi2Se3 thin film on a single-crystalline LaCoO3 thin film on SrTiO3 substrate, which is a strain-induced ferromagnetic insulator. Different from the growth on other substrates, the Bi2Se3 films with highest quality on LaCoO3 favor a relatively low substrate temperature during growth. As a result, an inverse dependence of carrier mobility with the substrate temperature is found. Moreover, the magnetoresistance and coherence length of weak antilocalization also have a similar inverse dependence with the substrate temperature, as revealed by the magnetotransport measurements. Our experiments elucidate the special behaviors in Bi2Se3/LaCoO3 heterostructures, which provide a good platform for exploring related novel quantum phenomena, and are inspiring for device applications.