准一维多链无序体系跳跃电导特性

在单电子紧束缚近似下,建立了准一维多链无序体系直流、交流电子跳跃输运模型,通过计算探讨了无序模式、维度效应、温度及外场对其直流、交流电导率的影响.计算结果表明:准一维多链无序体系的直流、交流电导率随着格点能量无序度的增大而减小,非对角无序具有增强体系电子输运能力的作用.随着链数的增加,体系的直流、交流电导率增大,但格点能量无序度较小时,维度效应的影响不明显.在对角无序情况下准一维多链无序体系的交流电导率随温度的升高而增大,而在非对角无序模式下却随温度的升高而减小,但对于直流情况,体系的直流电导率随温度的升     

Magnetotransport through ac driven ferromagnetic graphene nanoribbons

We present a theoretical study on the spin-dependent transport through the ferromagnetic graphene nanoribbons in the presence of a magnetic and an in-plane ac electric field, and find that when the ac field is applied, in the two-terminal ferromagnetic graphene device, for the parallel configurations of the electrodes' magnetizations, the width of the even-number conductance plateaus decrease, the new conductance plateaus appear at the odd-number positions, and the even-number conductance plateaus at the high energy are quenched under the sufficiently strong ac field. In contrast, for the antiparallel configuration of the electrodes' magnetizations, the odd-plateaus of the conductance shrink, and the new plateaus developed at the even-number positions. The magnetic resistance exhibits a successive rectangular-like oscillation structure close to the band edge, whereas experiences an alternative transition between the sharp peak and dip near the zero energy with increasing the ac field strength. In the six-terminal ferromagnetic graphene device, the variations of the longitudinal and Hall resistances' plateaus as well as the addition of the new quantized plateaus with the rise of the ac field strength are also revealed.     

Theory of ac quantum conduction through an ideal constriction

A scattering theory is formulated for time-dependent (ac) transport through quantum constrictions or quantum point contacts. This is an extension of the standard scattering treatment for the time-independent (dc) case where quantized conductance steps and resonances occur. For an ideal constriction, the first-order transmission sidebands are derived when a time-dependent sinusoidal potential is applied. The frequency dependence of the conductance is discussed, and possible experiments are suggested.     

The invisible Majorana bound state at the helical edge

The presence of a Majorana bound state in condensed matter systems is often associated to a zero bias peak in conductance measurements. Here, we analyze a system were this paradigm is violated. A Majorana bound state is always present at the interface between a quantum spin Hall system that is magnetically gapped and a quantum spin Hall system gapped by proximity induced s-wave superconductivity. However, the linear conductance could be either zero or non-zero and quantized depending on the energy and length scales of the barriers. The transition between the two values is reminiscent of the topological phase transition in proximitized spin–orbit coupled quantum wires in the presence of an applied magnetic field. We interpret the behavior of the conductance in terms of scattering states at both zero and non-zero energy.     

Improving the ballistic AC conductivity through quantum resonance in branched nanowires

Frequency-dependent electrical conductivity is studied by means of the Kubo-Greenwood formula and a real-space renormalization plus convolution method. An analytical solution of the alternating current (AC) conductivity is found for periodic chains. In this article, we report enhancements to this ballistic AC conductivity when periodically or quasiperiodically placed Fano-Anderson impurities are introduced to an otherwise periodic chain, which is connected to two semi-infinite periodic leads at its ends. Moreover, the temperature effects on these resonant AC conducting states are remarkably different in periodic and in quasiperiodic systems. Finally, such enhancement is further analysed in branched nanowires with a small cross section.     

一维二元非对角关联无序体系跳跃电导特性

在单电子紧束缚无序模型基础上，建立了一维二元关联无序体系电子跳跃输运直流电导模型，并推导了其直流电导公式，通过计算其直流电导率，探讨了格点能量无序度、非对角关联及温度、外场对体系跳跃电导的影响.计算结果表明，一维二元无序体系的直流电导率随着格点能量无序度的增大而减小；当引入非对角关联时，体系出现退局域化现象，从而使体系的直流电导率增大；温度对体系的电子输运的影响表现为体系的直流电导率随温度的升高而增大；在外加电场的调制下，体系的直流电导率在强场区随电场强度增加而增长很快，呈现出非欧姆定律特性，但在弱场区外场的作用不明显. 关键词： 二元无序体系 跳跃电导 格点能量无序度 非对角关联     

Schottky-type grain boundaries in CCTO ceramics

In this work we studied electrical barriers existing at CaCu₃Ti₄O₁₂ (CCTO) ceramics using dc electrical measurements. CCTO pellets were produced by solid state reaction method and X-ray diffractograms showed which single phase polycrystalline samples were obtained. The samples were electrically characterized by dc and ac measurements as a function of temperature, and semiconductor theory was applied to analyze the barrier at grain boundaries. The ac results showed the sample’s permittivity is almost constant (10⁴) as function of temperature at low frequencies and it changes from 100 to 10⁴ as the temperature increases at high frequencies. Using dc measurements as a function of temperature, the behavior of barriers was studied in detail. Comparison between Schottky and Poole-Frenkel models was performed, and results prove that CCTO barriers are more influenced by temperature than by electric field (Schottky barriers). Besides, the behavior of barrier width as function of temperature was also studied and experimental results confirm the theoretical assumptions.     

Lee-Yang zeros of periodic and quasiperiodic anisotropic XY chains in a transverse field

The partition function zeros of the anisotropic XY chain in a complex transverse field are studied analytically and numerically. It is found that the partition function zeros of the periodic and quasiperiodic quantum Ising chain lie on the circle at zero temperature and the radius equal to the values of the critical field. For the periodic and quasiperiodic anisotropic XY chains, the closed curves are split to two or three closed curves as the anisotropic parameter gamma decreases at a given ratio of two kinds of exchange interactions. For the isotropic XX case, the partition function zeros lie on the straight segments which are parallel to the real axis and the segments move towards the real axis as the temperature goes to zero.     

Hysteresis phenomena in the course of polarization evolution in a sinusoidal electric field in lead magnesium niobate crystals

The processes of polarization evolution in single crystals of the PbMg_1/3Nb_2/3O₃ model ferroelectric relaxor in a sinusoidal electric field are investigated at temperatures near and above the temperature T _d 0 of destruction of the induced ferroelectric state upon heating in zero electric field. The polarization switching current loops are measured in the ac electric field applied along the 〈111〉 and 〈110〉 pseudocubic directions. The electroluminescence intensity loops are obtained under the combined action of ac and dc electric fields applied along the 〈100〉 direction. In a certain temperature range above T _d 0 and the freezing temperature T _f in lead magnesium niobate, there are electric current anomalies, that correspond to the dynamic formation and subsequent destruction of the ferroelectric macroregions throughout each half-cycle of the ac electric field. The measurements of electroluminescence hysteresis loops demonstrate that the observed depolarization delay (related to the ac electric field amplitude) increases with an increase in the dc electric field and decreases as the ac field amplitude increases. The nature of the observed phenomena is discussed.     

Probing the discrete motion of vortices with rf excitations

In this work we present experimental results on the rectification of vortices in a superconductor/ferromagnet system under a high frequency drive. The two-dimensional pinning landscape, induced by the stray fields of the ferromagnetic template, has no intrinsic asymmetry. Nevertheless, an asymmetric potential is artificially induced by an applied dc bias. The experimental results unambiguously show a biased, discrete motion of the vortices in the periodic potential at frequencies above 10 MHz. This synchronized motion is very sensitive to the external applied field. Increasing temperature leads to a reduction of the pinning potential, which in turn results in a lower ac power needed to drive the vortex lattice.     

Rectification and phase locking for particles on symmetric two-dimensional periodic substrates

We demonstrate a rectification phenomenon for overdamped particles interacting with a 2D symmetric periodic substrate when driven with a dc and a circular ac drive. As a function of longitudinal dc amplitude, the longitudinal velocity increases in a series of quantized steps distinct from Shapiro steps with transverse rectification occurring near these transitions. The rectification phenomenon is explained using symmetry arguments and a simple model.     

一维Fibonacci准周期Frustrated Ising模型的热力学特性

分别给出周期和自由边界条件下求解一维Fibonacci准周期Frustrated Ising模型的配分函数的方法。研究它的低温热力学性质,发现当温度趋于零时,其热力学函数在参数b/J=2/(2m+1)(m为正整数)处发生尖锐的变化。分析了零温时系统在不同参数范围内的基态构形,对计算的结果进行了解释。 关键词：     

Electron transport in double-walled carbon nanotubes

The transport properties of finite length double-walled carbon nanotubes subject to the influences of a transverse electric field and a magnetic field with varying polar angles are investigated theoretically. The electrical conductance, thermal conductance and Peltier coefficient dependences on the external fields and symmetric configuration are studied in linear response regime. Prominent peak structures of the electrical conductance are predicted when varying the electric field strength. The features of the conductance peaks are found to be strongly dependent on the external fields and the intertube interactions. The heights of the electrical and thermal conductance peaks display the quantized behavior, while those of the Peltier coefficient do not. The conductance peaks are found to be broadened by the finite temperature.     

D相AlCuCo准晶各向异性热电势的测量

采用激光交流加热的方法测量了D相AlCuCo准晶周期与准周期两个方向的热电势,温度测量区间为300—1200 K.通过测量发现沿周期方向的热电势符号为负,沿准周期方向的热电势符号为正;沿周期方向在1123 K（850 ℃）热电势有一个跳变;在高温区,热电势并不是单调变化的;实验所用方法能比较灵敏地测量材料相变时的温度点. 关键词： AlCuCo准晶 高温 热电势 交流测量法     

Non-Hermitian Kitaev chain with complex periodic and quasiperiodic potentials

We study the topological properties of the one-dimensional non-Hermitian Kitaev model with complex either periodic or quasiperiodic potentials. We obtain the energy spectrum and the phase diagrams of the system by using the transfer matrix method as well as the topological invariant. The phase transition points are given analytically. The Majorana zero modes in the topological nontrivial regimes are obtained. Focusing on the quasiperiodic potential, we obtain the phase transition from the topological superconducting phase to the Anderson localization, which is accompanied with the Anderson localization–delocalization transition in this non-Hermitian system. We also find that the topological regime can be reduced by increasing the non-Hermiticity.     

Widths of the Hall Conductance Plateaus

We study the charge transport of the noninteracting electron gas in a two-dimensional quantum Hall system with Anderson-type impurities at zero temperature. We prove that there exist localized states of the bulk order in the disordered-broadened Landau bands whose energies are smaller than a certain value determined by the strength of the uniform magnetic field. We also prove that, when the Fermi level lies in the localization regime, the Hall conductance is quantized to the desired integer and shows the plateau of the bulk order for varying the filling factor of the electrons rather than the Fermi level.     

基于垂直引线和调制电流的三线环形磁导引

提出了一种在单层原子芯片上实现闭合且导引中心无磁场零点的环形磁导引的新方案. 芯片表面刻蚀的导线结构由同心等距三环线构成, 三环线的电流引线垂直于芯片表面. 加载直流电流后, 这种构型即可在芯片表面附近产生闭合的环形磁导引. 交流调制三环线电流后, 环形磁导引的势能极小值附近不再存在磁场零点且其磁场起伏小. 这种方案可用于基于物质波干涉的原子芯片陀螺仪研究.     

Quantum antidot as an electrometer:Observation of fractional charge

In experiments on resonant tunneling through a quantum antidot in the quantum Hall (QH) regime, we observe periodic conductance peaks both versus magnetic field and a global gate voltage, i.e., electric field. Each conductance peak can be attributed to tunneling through a quantized antidot-bound state. The fact that the variation of the uniform electric field produces conductance peaks implies that the deficiency of the electrical charge on the antidot is quantized in units of charge of quasiparticles of surrounding QH condensate. The period in magnetic field gives the effective area of the antidot state through which tunneling occurs, the period in electric field (obtained from the global gate voltage) then constitutes a direct measurement of the charge of the tunneling particles. We obtain electron charge C in the integer QH regime, and quasiparticle charge C for the QH state.     

Magnetoelectric effect from the direct coupling of the Lorentz force from a brass ring with transverse piezoelectricity in a lead zirconate titanate (PZT) disk

A novel composite of brass ring and PZT disk shows a high dc magnetic field (H_dc) response when using the product effect of the Lorentz force effect from a metal ring in a dc magnetic field applied with ac electrical current, and the piezoelectric effect of piezoelectric material. The output voltage between the two faces of PZT shows a good linear response to the dc magnetic field (<1 kOe) under different ac electrical current inputs (<300 mA). The magnetoelectric coefficient is about ∼33.2 mV/T A. Simultaneously, the magnitude of its magnetoelectric coefficient can be manually controlled by an applied electrical current. This composite has the potential for applications in magnetoelectric transducers and sensors that work without coils even for static magnetic fields. PACS 85.80.Jm; 77.84.-S; 75.80.+q; 77.84.Dy; 77.65.-j     

Spin-glass behaviour of novel ternary uranium aluminide U3Co4+ x Al12− x (x = 0.55)

Experimental results on dc and ac susceptibility, magnetization and magnetic relaxation, specific heat, electrical resistivity and magnetoresistivity up to 8?T are reported for the novel ternary uranium aluminide U₃Co_4.55Al_11.45. The temperature dependence of the dc susceptibility shows a cusp at a characteristic temperature T _f?=?8–10?K that depends weakly on the applied magnetic field. The observed pronounced difference between the ZFC and FC magnetizations, as well as the decay in the remanent, both give evidence that a highly irreversible, frozen state is formed below T _f which is reminiscent of spin-glass behaviour. The real and imaginary parts of the ac susceptibility show that the corresponding T _f peaks are only slightly dependent on frequency. Electrical resistivity measured at zero and in fields up to 8?T indicates that the Kondo-like state becomes dominant at temperatures above T _f.