Effect of the Morphology of an Ensemble of ZnO Microrods on the Optical and Luminescence Properties

JETP Letters - From measurements of the oscillatory magnetoresistance in weak perpendicular magnetic fields, we found that the quantum oscillations in two-dimensional electron systems in Si-MOS...     

Hopping magnetotransport via nonzero orbital momentum states and organic magnetoresistance

In hopping magnetoresistance of doped insulators, an applied magnetic field shrinks the electron (hole) s-wave function of a donor or an acceptor and this reduces the overlap between hopping sites resulting in the positive magnetoresistance quadratic in a weak magnetic field, B. We extend the theory of hopping magnetoresistance to states with nonzero orbital momenta. Different from s states, a weak magnetic field expands the electron (hole) wave functions with positive magnetic quantum numbers, m>0, and shrinks the states with negative m in a wide region outside the point defect. This together with a magnetic-field dependence of injection/ionization rates results in a negative weak-field magnetoresistance, which is linear in B when the orbital degeneracy is lifted. The theory provides a possible explanation of a large low-field magnetoresistance in disordered π-conjugated organic materials.     

Electron dephasing of a GaAs/AlGaAs quantum well with self-assembled InAs dots

We study the magnetotransport of a GaAs/AlGaAs quantum well with self-assembled InAs quantum dots. Negative magnetoresistance is observed at low field and analysed by weak localization theory. The temperature dependence of the extracted dephasing rate is linear, which shows that the inelastic electron-electron scattering processes with small energy transfer are the dominant contribution in breaking the electron phase coherence. The results are compared with those of a reference sample that contains no quantum dots.     

Electron scattering with spin flip in indium antimonide and indium arsenide

The longitudinal magnetoresistance has been investigated at temperatures in the range from 2.8 to 200 K in a magnetic field of up to 200 kOe with the aim of determining the temperature range and the magnetic field strength at which charge carrier scattering with spin flip occurs in n-type indium arsenide and n-type indium antimonide. It is established that quantum oscillations of the longitudinal magnetoresistance of indium arsenide exhibit weak zero maxima due to electron scattering with spin flip at temperatures in the range from 4 to 35 K in a magnetic field of 146 kOe. For the longitudinal magnetoresistance of indium antimonide, zero maxima caused by electron scattering with spin flip are revealed in the temperature range from 60 to 80 K in a magnetic field of 132 kOe.     

Quantum transport in lattices of coupled electronic billiards

A new system with dynamic chaos—2D lattice of single Sinai billiards coupled through quantum dots—is studied experimentally. Localization in such a system was found to be substantially suppressed, because the characteristic size of the billiard for g≤1 (g is conductance measured in e ₂/h units) is the localization length rather than the de Broglie wavelength of an electron, as in the usual 2D electron system. Lattice ballistic effects (commensurate peaks in the magnetoresistance) for g?1, as well as extremely large magnetoresistance caused by the interference in chaotic electron trajectories, were found. Thus, this system is shown to be characterized by simultaneous existence of effects that are inherent in order (commensurate peaks of magnetoresistance), disorder (percolation charge transport), and chaos (weak localization in chaotic electron trajectories).     

HgCdTe反型层的磁输运性质

利用成本低廉的液相外延技术, 成功制备了具有金属-绝缘体-半导体结构的HgCdTe场效应管器件. 在该器件中, 观察到清晰的Shubnikov-de Hass振荡和量子霍尔平台, 证明样品具有较高的质量. 测量零场附近的磁阻曲线, 在HgCdTe-基器件中观察到反弱局域效应, 表明样品中存在较强的自旋-轨道耦合作用. 利用Iordanskii-Lyanda-Pikus理论, 很好地拟合了反弱局域曲线. 由拟合得到的自旋分裂能随电子浓度的增大而增大, 最大达到9.06 meV. 根据自旋分裂能得到的自旋-轨道耦合系数同样随电子浓度的增大而增大, 与沟道较宽的量子阱中所得到的结果相反.     

自旋场效应晶体管中隧道磁阻的势垒相关反转效应

考虑自旋场效应晶体管中Rashba自旋轨道相互作用和自旋输运量子相干性,研究了势垒强度对自旋场效应晶体管的自旋相关量子输运的影响. 研究发现,势垒强度较低时,隧道结电导随Rashba自旋轨道相互作用强度的变化呈现明显的振荡现象,势垒强度较高时,电导表现出明显的势垒相关“电导开关”现象. 当势垒强度逐渐增强时,平行结构电导呈现出单调下降趋势,而反平行结构电导产生波动,这种波动导致该隧道磁阻也随势垒强度的变化表现出振荡现象,且在合适的准一维电子气厚度情况下隧道磁阻值可以产生正负反转,这个效应将会在基于自旋的电子器件信息的存储上获得应用. 关键词： 自旋场效应管 开关效应 量子相干 隧道磁阻     

EDESR and ODMR of Impurity Centers in Nanostructures Inserted in Silicon Microcavities

We present the first findings of the new electrically and optically detected magnetic resonance technique [ED electron spin resonance (EDESR) and ODMR], which reveal single point defects in the ultra-narrow silicon quantum wells (Si-QW) confined by the superconductor δ-barriers. This technique allows the ESR identification without application of an external cavity, as well as a high frequency source and recorder, and with measuring only the magnetoresistance (EDESR) and transmission (ODMR) spectra within the frameworks of the excitonic normal-mode coupling caused by the microcavities embedded in the Si-QW plane. The new resonant positive magnetoresistance data are interpreted here in terms of the interference transition in the diffusive transport of free holes, respectively, between the weak antilocalization regime in the region far from the ESR of a paramagnetic point defect located inside or near the conductive channel and the weak localization regime in the nearest region of the ESR of that defect.     

Semiclassical negative magnetoresistance of a 2D electron gas caused by scattering by short-range and long-range potentials

The magnetoresistance of a 2D electron gas confined in narrow GaAs quantum wells with AlAs/GaAs superlattice barriers is studied in the classical range of magnetic fields. It is shown that the negative magnetoresistance observed in this kind of structures with nonplanar heterointerfaces is semiclassical and qualitatively agrees with the model of negative magnetoresistance due to the scattering of charge carriers by two types of random potential, namely, the short-range and long-range ones.     

Magnetoresistance calculations for a quantum wire with periodically modulated width

We present numerical calculations of the magnetoresistance in a ballistic quantum wire with periodically modulated width. Surprisingly, the negative magnetoresistance peak is found to persist to a field strength which is two orders of magnitude larger than in single quantum dots. In the weakB-field regime, our results are in good agreement with recent experiments. We argue that the wide peak is due to multiple backscattering among the segments of the wire, leading to a novel weak localization effect.     

The effects of macroscopic inhomogeneities on the magnetotransport properties of the electron gas in two dimensions

In experiments on electron transport, the macroscopic inhomogeneities in the sample play a fundamental role. In this paper and a subsequent one, we introduce and develop a general formalism that captures the principal features of sample inhomogeneities (density gradients, contact misalignments) in the magnetoresistance data taken from low-mobility heterostructures. We present detailed assessments and experimental investigations of the different regimes of physical interest, notably the regime of semiclassical transport at weak magnetic fields, the plateau–plateau transitions as well as the plateau–insulator transition that generally occurs at much stronger values of the external field only.It is shown that the semiclassical regime at weak fields plays an integral role in the general understanding of the experiments on the quantum Hall regime. The results of this paper clearly indicate that the plateau–plateau transitions, unlike the plateau–insulator transition, are fundamentally affected by the presence of sample inhomogeneities. We propose a universal scaling result for the magnetoresistance parameters. This result facilitates, amongst many other things, a detailed understanding of the difficulties associated with the experimental methodology of H.P. Wei et al. in extracting the quantum critical behavior of the electron gas from the transport measurements conducted on the plateau–plateau transitions.     

Electron localization in a nondegenerate semiconductor with a random potential due to charged impurities

The saturation of the electron mobility, as determined according to the magnetoresistance, was observed in a semiconductor with a large-scale potential due to charged impurities. It was shown that the saturation is due to the existence of a quantum mobility threshold. A negative magnetoresistance of nondegenerate electrons, which is due to the suppression of quantum interferences corrections to the conductivity by the magnetic field, was found. The magnitude of these effects near the mobility threshold was explained by the absence of short, closed, electronic trajectories in the large-scale potential. A relation was established between the amplitude of the random potential and the saturated values of the mobility and the quantum corrections to the conductivity.     

Low temperature magnetoresistance of GaAs/AiGaAs corrugated-gate wires

We study quantum interference effects which are observed in the low temperature magnetoresistance of corrugated-gate wires fabricated in the GaAs/AlGaAs modulation-doped heterostructure system. Negative magnetoresistance, which appears over a large range of magnetic field, is observed. In addition, small oscillations in the magnetoresistance are observed. We believe these results are caused by boundary (or geometry-induced) scattering, within the gated wire, which evolves as the nature of the system changes from a quantum-dot array to a wire with weak modulation.     

Feature of the magnetotransport of a quasi-one-dimensional electrons on the superfluid helium

The magnetotransport in a nondegenerate quasi-one-dimensional (Q1D) electron system over superfluid helium has been investigated experimentally. The measurements are performed in the presence of a perpendicular magnetic field B up to 2.6 T in the temperature range T=0.48–2.05 K in the system of conducting channels of 100–400 nm width. It is shown that the value of longitudinal magnetoresistance ρ_xx increases with B. In the electron-gas scattering region (T>0.9 ), the behaviour of ρ_xx agrees with classical Drude law. In the quantum transport regime, the self-consistent Born approximation (SCBA) theory for a 2D electron system over liquid helium describes the experimental data qualitatively. The deviation due to the difference of the experimentally studied Q1D system of the electrons in a parabolic potential well differs from theoretically analysed one. The experimental data agree with the theoretical calculation for the Q1D electron system at the weak magnetic field and the low temperature.

The negative magnetoresistance of the conducting channels has been observed in both the gas- and the ripplon-scattering region. These effects have been explained by weak carrier localization on the gas atoms at high temperature and by display of the quantum magnetotransport features in a mesoscopic system at low temperature.     

Quantum oscillations in a novel organic quasi-two-dimensional (BEDO-TTF)5[RbHg(SCN)4]2 metal

Quantum magnetization and magnetoresistance oscillations are detected in the quasi-two-dimensional organic metal (BEDO-TTF)₅[RbHg(SCN)₄]₂ for the first time. The magnetization oscillation spectrum corresponds to a calculated Fermi surface provided that a magnetic breakdown is realized. The magnetoresistance oscillation spectrum contains additional frequencies, one of which can unambiguously be related to quantum interference. An analysis of the angular dependence of the magnetoresistance oscillation amplitude indicates that the many-body interactions in this metal are weak.     

Quantum interference effects in the magnetopiezoresistance of InAs/AlGaSb quasi-one-dimensional electron systems

We measured the low temperature magnetopiezoresistance of a quasi-one-dimensional electron system by fabricating an InAs/AlGaSb micromechanical cantilever. The magnetopiezoresistance curve showed aperiodic but reproducible oscillation, which was similar to the differential magnetoresistance curve obtained for the same device. A detailed comparison with model calculations strongly suggests that the quantum interference effects that cause the conductance fluctuations in the magnetoresistance are responsible for the peculiar behavior of the magnetopiezoresistance.     

Quantum transport in topological semimetals under magnetic fields

Topological semimetals are three-dimensional topological states of matter, in which the conduction and valence bands touch at a finite number of points, i.e., the Weyl nodes. Topological semimetals host paired monopoles and antimonopoles of Berry curvature at the Weyl nodes and topologically protected Fermi arcs at certain surfaces. We review our recent works on quantum transport in topological semimetals, according to the strength of the magnetic field. At weak magnetic fields, there are competitions between the positive magnetoresistivity induced by the weak anti-localization effect and negative magnetoresistivity related to the nontrivial Berry curvature. We propose a fitting formula for the magnetoconductivity of the weak anti-localization. We expect that the weak localization may be induced by inter-valley effects and interaction effect, and occur in double-Weyl semimetals. For the negative magnetoresistance induced by the nontrivial Berry curvature in topological semimetals, we show the dependence of the negative magnetoresistance on the carrier density. At strong magnetic fields, specifically, in the quantum limit, the magnetoconductivity depends on the type and range of the scattering potential of disorder. The high-field positive magnetoconductivity may not be a compelling signature of the chiral anomaly. For long-range Gaussian scattering potential and half filling, the magnetoconductivity can be linear in the quantum limit. A minimal conductivity is found at the Weyl nodes although the density of states vanishes there.     

Magnetoresistance of a two-dimensional electron gas in a parallel magnetic field

The conductivity of a two-dimensional electron gas in a parallel magnetic field is calculated. We take into account the magnetic-field-induced spin-splitting, which changes the density of states, the Fermi momentum, and the screening behavior of the electron gas. For impurity scattering, we predict a positive magnetoresistance for low electron density and a negative magnetoresistance for high electron density. The theory is in qualitative agreement with recent experimental results found for Si inversion layers and Si quantum wells.     

Anisotropic positive magnetoresistance of a nonplanar 2D electron gas in a parallel pagnetic field

Magnetotransport properties of a 2D electron gas in narrow GaAs quantum wells with AlAs/GaAs superlattice barriers were studied. It is shown that the anisotropic positive magnetoresistance observed in selectively doped semiconductor structures in a parallel magnetic field is caused by the spatial modulation of the 2D electron gas.