First Principles Calculation of Universal Conductance Fluctuation in Monatomic Metal Chains

Combining the non-equilibrium Green＇s function method and density functional theory, we provide a first-principle scheme to calculate the universal conductance fluctuation （UCF） in quasi one-dimensional monatomic chains subject to a magnetic field. Our results show that for these monatomic chains, the amplitude of the UCF is much smaller than the previous theoretical prediction for mesoscopic conductors by Lee et al. [Phys. Rev. Lett. 55 （1985） 1622; Phys. Rev. B 36 （1987） 1039] The reason is that the ergodic hypothesis fails in these nanowires due to the confinement of geometry. We ascribe the phenomenon to the flux-dependent density of states fluctuation.     

An experimental scheme to verify the dynamics of the Aharonov--Bohm effect

There are two different viewpoints on the Aharonov--Bohm (A--B) effect. One asserts that the A--B effect is due to the existence of the vector potential A. The other asserts that the A--B effect is due to the interaction energy between the magnetic field produced by the moving charges and the magnetic field in the solenoid. The difference of these two viewpoints is analyzed in this paper. To judge which viewpoint is right, this paper suggests a new experimental method.     

拓扑绝缘体的普适电导涨落

拓扑绝缘体因其无能量耗散的拓扑表面输运而备受关注, 揭示拓扑表面态因其 的贝利相位而产生的拓扑输运现象, 将有助于拓扑绝缘体相关器件的应用开发. 本文回顾了普适电导涨落(UCF) 揭示拓扑绝缘体奇异输运性质的研究进展. 通过调控温度、角度、门电压、垂直磁场和平行磁场等外部参量, 实现了对拓扑绝缘体的UCF 效应的系统研究, 证实了拓扑绝缘体中二维UCF 的输运现象, 并通过尺寸标度规律获得了UCF 的拓扑起源的实验证据, 讨论了拓扑表面态的UCF 的统计对称规律. 从而实现了对拓扑绝缘体UCF 效应的较为完整的理解.     

Shubnikov-de Haas effect on conductance fluctuations in two-dimensional random magnetic fields

Conductance fluctuations in two-dimensional random magnetic fields are investigated numerically in the case where the mean and the fluctuation of the random magnetic fields are of the same order. The conductance is evaluated by means of the Landauer formula. It is found that for a system with edge states, the conductance fluctuation exhibits clearly a Shubnikov-de Haas type oscillation in the weak field regime.     

Oscillatory magnetoconductance of narrow quantum waveguides in the presence of inhomogeneous magnetic fields

We present quantum mechanical calculations of magnetoconductance of narrow quantum waveguides in the presence of inhomogeneous perpendicular magnetic field with the use of a model of two coupled tight-binding chains and the transfer-matrix method. The variation of the magnetoconductance with the magnetic flux φ threading one unit cell in the chains for different Fermi energies of the electrons is presented. The effect of magnetically defined ‘barriers’ on the conductance as a function of the Fermi energy is studied in detail for various samples with different magnetically structural configurations. The profile of the conductance depends on the magnitude and the relative direction of the magnetic field piercing the magnetic ‘barriers’. The behaviors of the conductance for the linear-variation and other modulation functions of the magnetic field in a finite region are shown. The abrupt change of the magnetic field in the interface between two adjacent regions causes striking oscillation structures imposed upon the conductance steps. When the magnetic field is varied smoothly (adiabatically) the oscillation structures in the conductance are substantially suppressed and smeared out and finally replaced by the rounded conductance step in the corner. The presence of a magnetically defined cavity in the waveguide leads to pronounced oscillations and the appearance of resonant dip-peak pair in the conductance.     

Curie温度附近稀磁半导体(Ga,Mn)As的电学噪声谱性质

建立了自发噪声谱测量系统来研究稀磁半导体（Ga,Mn）As的电学噪声性质.通过测量（Ga,Mn）As材料的自发噪声谱,发现（Ga,Mn）As的自发涨落会随温度升高而逐渐增大,同时,外加磁场会降低（Ga,Mn）As的自发涨落,这来源于外加磁场导致的（Ga,Mn）As磁畴部分有序化.此外,不同频率的噪声随温度的变化规律有很大差异：当频率低于30 kHz的时候,噪声谱和温度的变化关系和热噪声很相似,但数值上明显大于热噪声的值;当频率在30 kHz左右的时候,噪声大小和温度成线性关系;当频率大于30 kHz以后,在相变点附近噪声大小和温度的关系出现了明显的转折,高频高温噪声的大小和热噪声的理论值非常接近.这些结果有助于深入理解（Ga,Mn）As磁性起源的物理机制. 关键词： 自旋电子学 稀磁半导体 自发涨落谱     

Quantum transport in diffusive microstructures in high magnetic fields

We have studied magnetotransport in diffusive submicron wires (n⁺-GaAs) at high magnetic fields where ω_cτ>1. The system allowed us to follow a transition from orthodox mesoscopic behaviour to the regime of edge state transport.We found that for ω_cτ>1 the universal conductance fluctuations (UCF) can no longer be scaled in terms of only one parameter, the phase coherence length. This breakdown of universal scaling is seen as a large increase of the Lee-Stone correlation field while the magnitude of UCF is unchanged in strong disagreement with the theoretical prediction.A qualitatively new magnetoresistance oscillatory effect has been observed at intermediate temperatures (10K<T<50K) when the quantum transport along the edges coexists with classical (diffusive and dissipative) conduction in the bulk. The effect arises due to Landau quantization but entirely disappears at low temperatures. The latter distinguishes it from the other quantum transport phenomena and indicates the importance of dissipation in resistance measurement.     

Universal quantized spin-Hall conductance fluctuation in graphene

We report theoretical investigations of the quantized spin-Hall conductance fluctuation of graphene in the presence of disorder. Two graphene models that exhibit the quantized spin-Hall effect (QSHE) are analyzed. Model I is with unitary symmetry under an external magnetic field B not = 0 but with a zero spin-orbit interaction, t(SO)=0. Model II is with symplectic symmetry where B=0 but t(SO) not = 0. The two models give exactly the same universal QSHE conductance fluctuation value 0.285+/-0.005e/4pi regardless of symmetry. We also examined a third model that exhibits QSHE but with quadratic dispersion and obtained the same results. Finally, all three models of QSHE have a one-sided log-normal distribution for spin-Hall conductance. Our results strongly suggest that the quantized spin-Hall conductance fluctuation belongs to a new universality class.     

Magnetic-field-induced non-Gaussian fluctuations in macroscopic equilibrium systems

We calculate the magnetic-field-dependent nonlinear conductance and noise in a two-dimensional macroscopic inhomogeneous system. If the system does not possess a specific symmetry, the magnetic field induces a nonzero third cumulant of the current even at equilibrium. This cumulant is related to the first and second voltage derivatives of the spectral density and average current in the same way as for mesoscopic quantum-coherent systems, but these quantities may be much larger. The system provides a robust test of a nonequilibrium fluctuation relation.     

Negative and positive magnetoresistance in bilayer graphene: Effects of weak localization and charge inhomogeneity

We report measurements of magnetoresistance in bilayer graphene as a function of gate voltage (carrier density) and temperature. We examine multiple contributions to the magnetoresistance, including those of weak localization (WL), universal conductance fluctuations (UCF), and inhomogeneous charge transport. A clear WL signal is evident at all measured gate voltages (in the hole doped regime) and temperature ranges (from 0.25 to 4.3 K), and the phase coherence length extracted from the WL data does not saturate at low temperatures. The WL data is fit to demonstrate that the electron-electron Nyquist scattering is the major source of phase decoherence. A decrease in UCF amplitude with increase in gate voltage and temperature is shown to be consistent with a corresponding decrease in the phase coherence length. In addition, a weak positive magnetoresistance at higher magnetic fields is observed, and attributed to inhomogeneous charge transport.     

Coulomb Interaction in Quantum Dot with a Precessing Magnetic Field

We study electronic transport through a quantum dot （QD） with a precessing magnetic field. By using the Keldysh nonequilibrium Green function method, formulas of local density of states （LDOS） and conductance of QD are derived self-consistently. It shows that the LDOS and conductance have obvious changes with the Coulomb blockade interaction. The intensity and angle of the magnetic field or temperatures, which reflect the mesoscopic structure of the QD are derived. The superiority of this device is that the QD can be controlled easily by the magnetic field, so it is valuable to apply in generating, manipulating and probing spin state.     

Spatial fluctuations of the density of states in magnetic fields observed with scanning tunneling spectroscopy

Scanning tunneling spectroscopy images on n-InAs(110) exhibit a strong magnetic field dependent contrast on the 50 nm length scale, indicating fluctuations in the density of states of the sample. The contrast is correlated to previously observed Landau oscillations in dI/dV curves. Its origin is a spatial fluctuation of the Landau level energy of 3-4 meV caused by the inhomogeneous distribution of dopant atoms. Besides inducing large-scale fluctuations in the density of states, dopants preserve their ability to scatter electron waves. The resulting wave pattern is found to depend on the magnetic field. It is suggested that the dependence is guided by the condensation of the electronic states on Landau tubes.     

Quantum interference in carbon-nanotube electron resonators

A new mechanism is proposed to explain the slow conductance fluctuations in the conductance-gate voltage plot observed in the nanotube electron resonators. It is found that the slow conductance fluctuation is an intrinsic quantum interference phenomenon and exists in all metallic nanotube resonators except zigzag ones. Analytical expressions for both slow and rapid oscillation periods of the conductance fluctuations have been derived, which are well consistent with the existing experiments. It is predicted that the ratio of the slow oscillation period to the rapid one is independent of the gate-voltage efficiency, and determined only by the nanotube length used in experiments.     

Spatial distribution of magnetic field and persistent currents in Condon domain phase

The distributions of magnetic field above the surface of the sample due to existence of the diamagnetic domain structure are found. It is shown, that the constant magnetic induction splitting inside of a sample is caused by the magnetization current density, localised in the boundaries between adjacent domains, close to the sample surface. The properties of this current are studied. The influence of the domain wall thickness on the spatial distribution of magnetic field and magnetization current density is present. A possibility of detection of the changes in the magnetic field distribution in vacuum, close to the surface of the sample, by means of Hall probes, is discussed. The measurement of the spatial distribution of magnetic field can give lacking information about characteristic sizes of magnetic domain formation at the conditions of the strong de Haas van Alphen effect.     

Density of states of amorphous GdxSi1-x at the metal-insulator transition

We have determined the electronic density of states of amorphous Gd xSi (1-x), N(GdSi)(E), in the vicinity of the metal-insulator transition by measuring the tunneling conductance dI/dV across a Gd xSi (1-x)/oxide/Pb tunnel junction at low T (T approximately 100 mK). By applying a magnetic field we can tune through the metal-insulator transition and simultaneously measure the transport and N(E) on a single sample. We find a smooth transition from a metal with strong Coulomb interactions to a developing Coulomb gap in the insulating regime. In the metallic region N(GdSi)(0) scales approximately with sigma(2).     

Model for Influences of Magnetic Fields on Intracellular Calcium Oscillations

Based on the model of the two calcium stores developed by Goldbeter, the influence of external magnetic field on the calcium concentration has been discussed. We believe that the cell membrane is a major site of interaction for extremely-low-frequency (ELF) electromagnetic fields, and the permeability of ions can be changed with the changing electromagnetic fields. It is shown that magnetic field initiatesintracellular calcium oscillation with a threshold in flux density, and that the calcium oscillations do not occur if the density of magnetic field is below the threshold. The results of theoretical calculation are consistent with that of the experiment. The intracellular free calcium concentrations of different cells exposed to the same magnetic fields are different from each other. It is indicated that the different behaviors such as oscillation, rise and invariability of calcium concentration are associated with the sort of cells.     

Novel magneto-oscillatory behavior of confined electrons at a twin boundary in ZnSe and at an interface in GaAs/AlGaAs heterostructures

Electronic properties of confined electrons at naturally formed twin boundaries in ZeSe have been investigated by microwave absorption measurements. Cyclotron resonance signal of the confined electrons was observed. On the cyclotron resonance peak, novel oscillation of the absorption intensity was observed. Similar oscillatory behavior was also observed in GaAs/AlGaAs heterostructures near cyclotron resonance filed at which new Shubnikov–de Haas-like oscillation was observed by Zudov. The oscillatory structure has a period not in inverse magnetic field but magnetic field. The origin of the oscillation observed is caused by Rayleigh interference of two-dimensional electron gas.     

Experimental investigation of the breakdown of the Onsager-Casimir relations

We use magnetoconductance fluctuation measurements of phase-coherent semiconductor billiards to quantify the contributions to the nonlinear electric conductance that are asymmetric under reversal of magnetic field. We find that the average asymmetric contribution is linear in magnetic field (for magnetic flux much larger than 1 flux quantum) and that its magnitude depends on billiard geometry. In addition, we find an unexpected asymmetry in the power spectrum of the magnetoconductance with respect to reversal of magnetic field and bias voltage.     

Polarization of nuclear spins from the conductance of quantum wire

We devise an approach to measure the polarization of nuclear spins via conductance measurements. Specifically, we study the combined effect of external magnetic field, nuclear spin polarization, and Rashba spin-orbit interaction on the conductance of a quantum wire. Nonequilibrium nuclear spin polarization affects the electron energy spectrum making it time dependent. Changes in the extremal points of the spectrum result in time dependence of the conductance. The conductance oscillation pattern can be used to obtain information about the amplitude of the nuclear spin polarization and extract the characteristic time scales of the nuclear spin subsystem.     

Magnetic breakdown at high fields: semiclassical and quantum treatments

The effects of finite temperature and noninteracting spins on magnetic breakdown (MB) in a quasi-two-dimensional organic conductor have been determined by computing the field-dependent free energy using a realistic crystal structure with no adjustable parameters. The de Haas-van Alphen oscillation spectra, including the MB phenomena, are thereby obtained microscopically. Within the range of computed magnetic field, from 170 to 1400 T, we find remarkable agreement between the predictions of the semiclassical and quantum treatment. We also find that the Zeeman effect leads to splitting of a frequency corresponding to the fundamental orbit.