Spin-dependent negative differential conductance in transport through single-molecule magnets

Transport properties are theoretically studied through an anisotropy single-molecule magnet symmetrically connected to two identical ferromagnetic leads. It is found that even though in parallel configuration of leads’ magnetizations, the total current still greatly depends on the spin polarization of leads at certain particular bias region, and thus for large polarization a prominent negative differential conductance (NDC) emerges. This originates from the joint effect of single-direction transitions and spin polarization, which removes the symmetry between spin-up and spin-down transitions. The present mechanism of NDC is remarkably different from the previously reported mechanisms. To clarify the physics of the NDC, we further monitored the shot noise spectroscopy and found that the appearance of the NDC is accompanied by the rapid decrease of Fano factor.     

Double band-inversions of bilayer phosphorene under strain and their effects on optical absorption

Strain is a powerful tool to engineer the band structure of bilayer phosphorene.The band gap can be decreased by vertical tensile strain or in-plane compressive strain.At a critical strain,the gap is closed and the bilayer phosphorene is turn to be a semi-Dirac semimetal material.If the strain is stronger than the criterion,a band-inversion occurs and it re-happens when the strain is larger than another certain value.For the zigzag bilayer phosphorene ribbon,there are two edge band dispersions and each dispersion curve represents two degenerate edge bands.When the first band-inversion happens,one of the edge band dispersion disappears between the band-cross points while the other survives,and the latter will be eliminated between another pair of band-cross points of the second band-inversion.The optical absorption of bilayer phosphorene is highly polarized along armchair direction.When the strain is turn on,the optical absorption edge changes.The absorption rate for armchair polarized light is decreased by gap shrinking,while that for zigzag polarized light increases.The bandtouch and band-inversion respectively result in the sublinear and linear of absorption curve versus light frequency in low frequency limit.     

基于比例积分控制原理预测钻井全过程原始地层温度的新方法研究

应用比例积分控制原理将瞬态传热模型预测结果与出口 温度实测数据逐步进行反馈可准确预测原始静态地层温度. 为此, 本文基于井下各控制组件质量、动量及能量守恒原理, 建立了实际井身结构与钻具组合条件下循环和停止循环期间井筒-地层温度分布全瞬态传热模型, 应用全隐式有限差分法进行求解, 并引入比例积分控制原理对比分析实测温度与预测温度的误差范围进而精确、 快速获取原始地层温度. 结合一口深井基础数据计算表明, 套管下入深度改变了井筒-地层间热交换效率, 进而影响了近井壁地层温度分布状况; 同时, 钻井过程中循环和停止循环作业过程改变了井下各控制组件的初始条件与边界条件, 致使近井壁原始地层温度分布距离产生变化. 建立的数学模型和研究方法可为石油钻井、地热井开采及地球深部原始地层温度信息准确、 经济、快速获取提供理论基础. 关键词： 原始地层温度 循环与停止循环 瞬态传热模型 比例积分控制原理     

THz-field-induced electronic transmission step-structure for a quantum wire

We study theoretically the low-temperature electronic transport property of a straight quantum wire under the irradiation of a finite-range transversely polarized external terahertz (THz) electromagnetic (EM) field. Using the freeelectron model and the scattering matrix approach, we show an unusual behaviour of the electronic transmission of this system. A sharp step-structure appears in the electronic transmission probability as the EM field strength increases to a threshold value when a coherent EM field is applied. We demonstrate that this effect physically comes from the inelastic scattering of electrons with lateral photons through intersubband transitions.     

钻井液径向温度梯度与轴向导热对井筒温度分布影响

本文基于井筒与地层间能量平衡原理, 将井筒钻井液划分成不同径向单元网格, 建立了考虑径向温度梯度条件下钻井液层间温度模型; 同时引入钻井液轴向导热项, 建立了钻井液轴向导热温度模型, 将数学模型应用隐式有限差分法离散与求解. 计算结果表明: 钻井液径向温度梯度对井筒径向与轴向温度产生的误差分别为0.15 ℃和0.2 ℃左右; 而钻井液轴向导热对井筒温度分布几乎不产生影响. 因此, 通过建立的数学模型进行系统分析表明, 在建立井筒-地层耦合瞬态传热模型时可忽略两者对井筒温度分布的影响. 基于数学建模方法验证了以前学者模型假设条件的正确性, 为油气井与地热井井下温度分布规律深入研究奠定了可靠的理论基础. 关键词： 径向温度梯度 轴向导热 井筒温度 瞬态传热模型     

An easy and efficient way to treat Green's function for nano-devices with arbitrary shapes and multi-terminal configurations

The efficiency of the calculation of Green's function (GF) for nano-devices is very important because the calculation is often needed to be repeated countlessly. We present a set of efficient algorithms for the numerical calculation of GF for devices with arbitrary shapes and multi-terminal configurations. These algorithms can be used to calculate the specified blocks related to the transmission, the diagonals needed by the local density of states calculation, and the full matrix of GF, to meet different calculation levels. In addition, the algorithms for the non-equilibrium occupation and current flow are also given. All these algorithms are described using the basic theory of GF, based on a new partition strategy of the computational area. We apply these algorithms to the tight-binding graphene lattice to manifest their stability and efficiency. We also discuss the physics of the calculation results.     

自旋轨道耦合作用下石墨烯pn结的电子输运性质

本文研究了自旋轨道耦合作用下石墨烯纳米带pn结的电子输运性质. 当粒子的入射能量处于pn结两端势能之间时, 粒子将会以隧穿的形式通过石墨烯pn结, 同时伴随着电子空穴转换. 电导随费米能的变化曲线呈不等高阶梯状, 并在费米能位于pn结两端能量中点时取得最大值. 随着石墨烯pn结长度的增加, 电导以指数形式衰减. 自旋轨道耦合作用导致的能隙会使电导显著减小, 而边缘态的粒子则可以几乎毫无阻碍地通过pn结. 本文用一个简单的子带隧穿模型解释了上述特征. 最后还研究了在pn转换区中掺入替位杂质的情况. 在弱杂质下, 电导随费米能变化的曲线将不再对称; 当杂质较强时, 仅边界态的形成的电导台阶能够保持.     

太赫兹电磁场照射下量子线中的纳米电子力

研究两端连接电子库的量子线在太赫兹(THz)电磁场横向辐射下的电子力学问题. 在自由电子近似下求得了弹道区二能级电子波函数解析表达式,发现电子概率在两个能态间随线轴坐标作Rabi振荡, 其幅度与外场强度和频率有关. 利用热力学统计算符方法并利用波函数计算出了传导电子对量子线中张力贡献的大小. 其计算结果可表示成与外场无关和与外场有关的两部分之和,前者与已有的实验观测和理论计算结果一致, 而与外场参量有关的部分可用量子相干理论解释. 关键词： 量子线 THz电磁场 纳米电子力     

磁场对单壁碳纳米管电子结构和光学性质的影响

采用半经验的模型、用单激发组态相互作用方法计算并讨论了外加轴向磁场对单壁碳纳米管电子结构和光学性质的影响。由于电子电子间相互作用的影响,磁场导致碳纳米管吸收峰能级分裂与磁场不成正比。该结果与简单的能带理论所给出的结果在低磁场情况下有本质的区别,并与实验结果有更高的符合度。该研究进一步证明了电子电子间相互作用以及激子在决定碳纳米管电子结构和光学性质中的重要作用。