Vertex corrections to the anomalous Hall effect in spin-polarized two-dimensional electron gases with a Rashba spin-orbit interaction

We study the effect of disorder on the intrinsic anomalous Hall conductivity in a magnetic two-dimensional electron gas with a Rashba-type spin-orbit interaction. We find that anomalous Hall conductivity vanishes unless the lifetime is spin-dependent, similar to the spin Hall conductivity in the nonmagnetic system. In addition, we find that the spin Hall conductivity does not vanish in the presence of magnetic scatterers.     

Electronic transport and specific heat of 1T-V Se2

The low-temperature thermoelectric power and the specific heat of 1T-V Se₂ (vanadium diselenide) have been reported along with the electrical resistivity and Hall coefficient of the compound. The charge density wave (CDW) transition is observed near 110 K in all these properties. The thermoelectric power has been measured from 15 K to 300 K, spanning the incommensurate and commensurate CDW regions. We observed a weak anomaly at the CDW transition for the first time in the specific heat of V Se₂. The linear temperature dependence of the resistivity and thermoelectric power at higher temperatures suggests a normal metallic behavior and electron–phonon scattering above the CDW transition. The positive thermoelectric power and negative Hall coefficient along with strongly temperature-dependent behavior in the CDW phase suggest a mixed conduction related to the strongly hybridized s–p–d bands in this compound.     

Quantum Hall effect in intentionally disordered two‐dimensional electron systems

In this work intentionally disordered two‐dimensional electron systems in modulation doped GaAs/GaAlAs heterostructures are studied by magnetotransport experiments. The disorder is provided by a δ‐doped layer of negatively charged beryllium acceptors. In low magnetic fields a strong negative magnetoresistance is observed that can be ascribed to magnetic‐field‐induced delocalization. At increased magnetic fields the quantum Hall effect exhibits broad Hall plateaus whose centers are shifted to higher magnetic fields, i.e. lower filling factors. This shift can be explained by an asymmetric density of states. Consistently, the transition into the insulating state of quantum Hall droplets in high magnetic fields occurs at critical filling factors around ν_c=0.4, i.e. well below the value 1/2 that is expected for symmetric disorder potentials. The insulator transition is characterized by the divergence of both the longitudinal resistance as well as the Hall resistance. This is contrary to other experiments which observe a finite Hall resistance in the insulating regime and has not been observed previously. According to recent theoretical studies the divergence of the Hall resistance points to quantum coherent transport via tunneling between quantum Hall droplets. The magnetotransport experiments are supplemented by simulations of potential landscapes for random and correlated distributions of repulsive scatterers, which enable the determination of percolation thresholds, densities of states, and oscillator strengths for far‐infrared excitations. These simulations reveal that the strong shift of the Hall plateaus and the observed critical filling factor for the insulator transition in high magnetic fields require an asymmetric density of states that can only be generated by a strongly correlated beryllium distribution. Cyclotron resonance on the same samples also indicates the possibility of correlations between the beryllium acceptors.     

Probing the onset of strong localization and electron–electron interactions with the presence of a direct insulator–quantum Hall transition

We have performed low-temperature transport measurements on a disordered two-dimensional electron system (2DES). Features of the strong localization leading to the quantum Hall effect are observed after the 2DES undergoes a direct insulator–quantum Hall transition on increasing the perpendicular magnetic field. However, such a transition does not correspond to the onset of strong localization. The temperature dependences of the Hall resistivity and Hall conductivity reveal the importance of the electron–electron interaction effects for the observed transition in our study.     

Spin Hall effect and spin current generation in two-dimensional systems with random Rashba spin–orbit coupling

We consider a new effect induced by spin–orbit coupling in a two-dimensional electron gas confined in a semiconductor quantum well, i.e. the possibility of spin current generation by fluctuating random Rashba spin–orbit interaction, with the corresponding mean value of the interaction being equal to zero. Our main results suggest that – in contrast to the spatially uniform Rashba spin–orbit interaction – the spin Hall effect does not vanish for typical disorder strengths. We also point out some other possibilities of using such a random Rashba coupling for the generation of spin density and spin current in two-dimensional nonmagnetic structures.     

Disorder and the fractional quantum Hall effect

The fractional quantum Hall effect is studied in the 2D electron gas of four GaAs-Al_xGa_1?xAs heterostructures. Localization due to disorder, known to give rise to the wide integral quantum Hall plateaus, is demonstrated to inhibit the fractional effect, which is observed only in the higher mobility samples.     

电子温度各向异性对霍尔推力器BN绝缘壁面鞘层特性的影响

建立多价态多组分等离子体一维流体鞘层模型,引入电子温度各向异性系数并考虑出射电子速度分布,研究了电子温度各向异性对霍尔推力器中的BN绝缘壁面鞘层特性和近壁电子流的影响.分析结果表明,相比于纯一价氙等离子体鞘层参数,推力器中的多价态氙等离子体鞘层电势降略有降低,电子壁面损失增加,临界二次电子发射系数减小.推力器中的电子温度各向异性现象可以显著地加大出射电子能量系数,进而降低鞘层电势降,增强电子壁面相互作用.数值结果表明,空间电荷饱和机制下电子温度各向异性对鞘层空间电势分布影响显著. 关键词： 霍尔推力器 电子温度各向异性 空间电荷饱和鞘层     

Doped single‐walled carbon nanotubes and low‐mobility graphene: impact of disorder and dopants on electronic magnetotransport

The impact of disorder introduced by intentional and unintentional (environmental) dopants on the charge transport has been investigated in boron‐ and nitrogen‐doped single‐walled carbon nanotubes, and in low‐mobility monolayer graphene. For doped single‐walled nanotubes a theoretically not predicted plateau‐like region and onsets of oscillatory behaviour in the conductance for boron‐ and nitrogen‐doped nanotubes were observed, respectively. The oscillatory behaviour suggests interplay between the dopants and the helical movement of charges along the tube due to the magnetic field. For low‐mobility graphene the simultaneous appearance of the filling‐factor 2 and 3 plateau was observed in the Quantum‐Hall regime. This is attributed to an electron‐spin–isospin interaction mediated through the high disorder. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

非简并p型Hg1-xMnxTe单晶(x>0.17)的负磁电阻机理研究

研究磁性半导体中负磁电阻产生机理对正确理解载流子与磁性离子间的sp-d磁交换作用 是非常重要的.通过变温(10---300 K)磁输运和变温(5---300 K)磁化率实验研究了一系列不同Mn含量 非简并p型Hg_1-xMn_xTe单晶(x>0.17)的负磁电阻和顺磁增强效应. 实验结果表明其负磁电阻与温度的关系和磁化率与温度的关系基本一致, 两者都包含一个呈指数型变化的温度函数exp(-K/T).根据磁性半导体的杂质能级理论, 非简并p型Hg_1-xMn_xTe单晶在低磁场范围内出现负磁电阻效应的主要物理机理 为外磁场的磁化效应使得受主杂质或受主型束缚磁极化子的有效电离能减小.     

Topology of a <Superscript>3</Superscript>He-A Film on a Corrugated Graphene Substrate

A thin film of superfluid ³He on a corrugated graphene substrate represents topological matter with a smooth disorder. It is possible that the atomically smooth disorder produced by the corrugated graphene does not destroy the superfluidity even in a very thin film, where the system can be considered as quasi two-dimensional topological material. This will allow us to study the effect of disorder on different classes of the 2 + 1 topological materials: the chiral ³He-A with intrinsic quantum Hall effect and the time reversal invariant planar phase with intrinsic spin quantum Hall effect. In the limit of smooth disorder, the system can be considered as a Chern mosaic, i.e., a collection of domains with different values of Chern numbers. In this limit, the quantization of the Hall conductance is determined by the percolated domain, while the density of the fermionic states is determined by the edge modes on the boundaries of the finite domains. This system can be useful for the general consideration of disorder in the topological matter.     

Anomalous Hall Effect in Spin-Polarized Two-Dimensional Hole Gas with Cubic-Rashbsa Spin-Orbit Interaction

Based on the Kubo formalism, the anomalous Hall effect in a magnetic two-dimensional hole gas with cubic-Rashba spin-orbit coupling is studied in the presence of δ-function scattering potential. When the weak, short-ranged disorder scattering is considered in the Born approximation, we find that the self-energy becomes diagonal in the helicity basis and its value is independent of the wave number, and the vertex correction to the anomalous Hall conductivity due to impurity scattering vanishes when both subbandsare occupied. That is to say, the anomalous Hall effect is not vanishing or influenced by the vertex correction for two-dimensional heavy-hole system, which is in sharp contrast to the case of linear-Rashba spin-orbit coupling in the electron band when the short-range disorder scattering is considered and the extrinsic mechanism as well as the effect of external electric field on the SO interaction are ignored.     

Electron interactions and transport between coupled quantum Hall edge states

We examine the effects of electron-electron interactions on transport between edge states in a multilayer integer quantum Hall system. The edge states of such a system, coupled by interlayer tunneling, form a two-dimensional, chiral metal at the sample surface. We calculate the temperature-dependent conductivity and the amplitude of conductance fluctuations in this chiral metal, treating Coulomb interactions and disorder exactly in the weak-tunneling limit. We find that the conductivity increases with increasing temperature, as observed in recent experiments, and we show that the correlation length characterizing conductance fluctuations varies inversely with temperature.     

Electron-electron interaction in the magnetoresistance of graphene

We investigate the magnetotransport in large area graphene Hall bars epitaxially grown on silicon carbide. In the intermediate field regime between weak localization and Landau quantization, the observed temperature-dependent parabolic magnetoresistivity is a manifestation of the electron-electron interaction. We can consistently describe the data with a model for diffusive (magneto)transport that also includes magnetic-field-dependent effects originating from ballistic time scales. We find an excellent agreement between the experimentally observed temperature dependence of magnetoresistivity and the theory of electron-electron interaction in the diffusive regime. We can further assign a temperature-driven crossover to the reduction of the multiplet modes contributing to electron-electron interaction from 7 to 3 due to intervalley scattering. In addition, we find a temperature-independent ballistic contribution to the magnetoresistivity in classically strong magnetic fields.     

Integer quantum Hall effect in trilayer graphene

By using high-magnetic fields (up to 60 T), we observe compelling evidence of the integer quantum Hall effect in trilayer graphene. The magnetotransport fingerprints are similar to those of the graphene monolayer, except for the absence of a plateau at a filling factor of ν=2. At a very low filling factor, the Hall resistance vanishes due to the presence of mixed electron and hole carriers induced by disorder. The measured Hall resistivity plateaus are well reproduced theoretically, using a self-consistent Hartree calculations of the Landau levels and assuming an ABC stacking order of the three layers.     

Magneto-transport of InAs-quantum wells using InP0.69Sb0.31as a barrier material

InAs/InP_0.69Sb_0.31quantum-well structures grown by metal organic vapor-phase epitaxy are studied by temperature-dependent Hall measurements and by quantum Hall and Shubnikov de Haas effect measurements. At temperatures below 0.3 K a two-dimensional electron gas without a conductive by-pass was demonstrated. For a two-dimensional electron gas with a sheet electron concentration of 2.2 × 10¹²cm⁻²mobilities as high as 118 000 cm²(Vs)⁻¹were observed. In contrast to samples doped on both sides of the quantum well, a beating pattern in the longitudinal resistance was observed for samples which were doped on only one side. This effect is explained by spin–orbit coupling of the electrons in the quantum well which leads to a separation in two spin-splitted subbands. A spin-split energy in the range from 6.9 meV to 8.4 meV was extracted from the Shubnikov de Haas measurements.     

Antisymmetric magnetoresistance in magnetic multilayers with perpendicular anisotropy

While magnetoresistance (MR) has generally been found to be symmetric in applied field in nonmagnetic or magnetic metals, we have observed antisymmetric MR in Co/Pt multilayers. Simultaneous domain imaging and transport measurements show that the antisymmetric MR is due to the appearance of domain walls that run perpendicular to both the magnetization and the current, a geometry existing only in materials with perpendicular magnetic anisotropy. As a result, the extraordinary Hall effect gives rise to circulating currents in the vicinity of the domain walls that contributes to the MR. The antisymmetric MR and extraordinary Hall effect have been quantitatively accounted for by a theoretical model.     

High-field magnetoresistance and Hall effect in Bi2Sr2CuO x single crystals

We investigated the in-plane magnetoresistance and the Hall effect of high-quality Bi₂Sr₂CuO_x single crystals with T _c (midpoint) = 3.7–9.6 K in dc magnetic fields up to 23 T. For T < 10 K, the crystals show the classical positive magnetoresistance. Starting at T ≈ 14 K, an anomalous negative magnetoresistance appears at low magnetic fields; for T ≥ 40 K, the magnetoresistance is negative in the whole studied range of magnetic fields. Temperature and magnetic field dependences of the negative-magnetoresistance single crystals are qualitatively consistent with the electron interaction theory developed for simple semiconductors and disordered metals. As is observed in other cuprate superconductors, the Hall resistivity is negative in the mixed state and changes its sign with increasing field. The linear T-dependence of cotθ_H for the Hall angle in the normal state closely resembles that of the normal-state resistivity as expected for a Fermi liquid picture.     

Metal insulator transition in modulated quantum Hall systems

The quantum Hall effect is studied numerically in modulated two-dimensional electron systems in the presence of disorder. Based on the scaling property of the Hall conductivity as well as the localization length, the critical energies where the states are extended are identified. We find that the critical energies, which are distributed to each of the subbands, combine into one when the disorder becomes strong, in the way depending on the symmetry of the disorder and/or the periodic potential.     

Low field Hall effect in potassium

The anisotropic electron relaxation time in K caused by the freezing out of Umklapp processes on certain areas of the Fermi surface is predicted to give a temperature-dependent component to the low-field Hall effect at low temperatures. A previous attempt to demonstrate this component was defeated by the unsuitability of the existing data which contain an overwhelming high-field contribution in the temperature range of interest. To provide more suitable data, we have measured the Hall effect of polycrystalline K as the conditions tend towards the low-field limit. It is shown how the results can be qualitatively interpreted in terms of a competition between this predicted Umklapp component and the high-field tendencies inherent in the galvano-magnetic effect.     

Skyrmion Hall effect with spatially modulated Dzyaloshinskii–Moriya interaction

The skyrmion Hall effect is theoretically studied in the chiral ferromagnetic film with spatially modulated Dzyaloshinskii–Moriya interaction. Three cases including linear, sinusoidal, and periodic rectangular modulations have been considered, where the increase, decrease, and the periodic modification of the size and velocity of the skyrmion have been observed in the microscopic simulations. These phenomena are well explained by the Thiele equation, where an effective force on the skyrmion is induced by the inhomogeneous Dzyaloshinskii–Moriya interaction. The results here suggest that the skyrmion Hall effect can be manipulated by artificially tuning the Dzyaloshinskii–Moriya interaction in chiral ferromagnetic film with material engineering methods, which will be useful to design skyrmion-based spintronics devices.