多量子点接触中声表面波驱动下的电子输运

本文研究了多量子点接触中声表面波驱动下的电子输运的声电电流特性.改变每个量子点接触的栅电压用以实现不同高度的静态势垒,从而调节声电电流.并用电子泵模型解释了不同势垒之间的耦合对于电流的影响,根据此模型得到的模拟结果与实验吻合.     

由Rashba效应和横向自旋轨道耦合诱发的量子点接触中的0.7反常结构研究

本文研究了在非对称限制势下由Rashba效应和横向自旋-轨道耦合诱发的量子点接触系统中的反常量子输运行为. 研究发现,在一定范围的Rashba相互作用强度下, 电导在0.8×2e2/h附近有一个较弱的坪台. 该坪台电导的值与非对称限制势的偏压有关. 在某个范围的偏压下, 它会随着偏压的增大而减小. 另外, 由于Rashba自旋-轨道耦合效应, 在非对称限制势作用下电子将会自旋极化. 因此, 在没有任何外加磁场的情况下, 采用纯电学手段即可做成量子点接触自旋偏振器.     

椭球量子点中施主杂质的三阶非线性光学极化率

在有效质量近似下,采用变分方法,研究椭球量子点中施主杂质的非线性光学性质.基于计算能量和波函数,以典型的半导体材料砷化镓为例,讨论椭球量子点的几何尺寸和各向异性度对施主杂质的三阶非线性极化率的影响.结果显示这些因素对椭球量子点中施主杂质的非线性光学性质有强烈的影响.     

拓扑绝缘体量子点中的磁交换相互作用

从理论上研究了拓扑绝缘体量子点中的磁交换相互作用.在拓扑绝缘体量子点中,边缘态电子数可以通过量子点的尺寸和外加电场进行调控.当量子点中掺入单个磁离子并且边缘态填充奇数电子时,电子与单个磁离子之间的交换相互作用达到最大值;而边缘态填充偶数电子时,电子与单个磁离子之间的交换相互作用消失.当量子点中掺入2个磁离子时,电子与Mn离子的sp-d相互作用会出现奇偶振荡行为,Mn离子间的相互作用取决于Mn离子间距和量子点壳层中的电子数,表现出典型的Ruderman-Kittel-Kasuya-Yosida型间接交换机制.工作澄清了拓扑绝缘体量子点壳层结构对其磁性的影响,有助于人们设计基于拓扑绝缘体量子点的自旋电子学或量子信息器件.     

Dimensional reduction at a quantum critical point

Competition between electronic ground states near a quantum critical point (QCP)--the location of a zero-temperature phase transition driven solely by quantum-mechanical fluctuations--is expected to lead to unconventional behaviour in low-dimensional systems. New electronic phases of matter have been predicted to occur in the vicinity of a QCP by two-dimensional theories, and explanations based on these ideas have been proposed for significant unsolved problems in condensed-matter physics, such as non-Fermi-liquid behaviour and high-temperature superconductivity. But the real materials to which these ideas have been applied are usually rendered three-dimensional by a finite electronic coupling between their component layers; a two-dimensional QCP has not been experimentally observed in any bulk three-dimensional system, and mechanisms for dimensional reduction have remained the subject of theoretical conjecture. Here we show evidence that the Bose-Einstein condensate of spin triplets in the three-dimensional Mott insulator BaCuSi2O6 (refs 12-16) provides an experimentally verifiable example of dimensional reduction at a QCP. The interplay of correlations on a geometrically frustrated lattice causes the individual two-dimensional layers of spin-(1/2) Cu2+ pairs (spin dimers) to become decoupled at the QCP, giving rise to a two-dimensional QCP characterized by linear power law scaling distinctly different from that of its three-dimensional counterpart. Thus the very notion of dimensionality can be said to acquire an 'emergent' nature: although the individual particles move on a three-dimensional lattice, their collective behaviour occurs in lower-dimensional space.     

GaN/AlxGa 1-xN量子点中类氢杂质的结合能

利用有效质量方法和变分原理,考虑内建电场(BEF)效应和量子点的三维约束效应,研究了纤锌矿结构的GaN/A lxGa1-xN柱形量子点中类氢杂质的结合能随量子点高度、A l含量和杂质位置的变化规律。结果表明:类氢杂质位于量子点中心时,杂质结合能随量子点高度的增加先增大后减小,存在最大值;随着A l含量的增加,杂质结合能增大。而杂质从量子点下界面沿Z轴上移至上界面时,杂质结合能先增大后减小,存在最大值。     

Hall-effect evolution across a heavy-fermion quantum critical point

A quantum critical point (QCP) develops in a material at absolute zero when a new form of order smoothly emerges in its ground state. QCPs are of great current interest because of their singular ability to influence the finite temperature properties of materials. Recently, heavy-fermion metals have played a key role in the study of antiferromagnetic QCPs. To accommodate the heavy electrons, the Fermi surface of the heavy-fermion paramagnet is larger than that of an antiferromagnet. An important unsolved question is whether the Fermi surface transformation at the QCP develops gradually, as expected if the magnetism is of spin-density-wave (SDW) type, or suddenly, as expected if the heavy electrons are abruptly localized by magnetism. Here we report measurements of the low-temperature Hall coefficient (R(H))--a measure of the Fermi surface volume--in the heavy-fermion metal YbRh2Si2 upon field-tuning it from an antiferromagnetic to a paramagnetic state. R(H) undergoes an increasingly rapid change near the QCP as the temperature is lowered, extrapolating to a sudden jump in the zero temperature limit. We interpret these results in terms of a collapse of the large Fermi surface and of the heavy-fermion state itself precisely at the QCP.     

量子点接触中的动态量子输运

实验测量量子点接触结构的动态量子输运.观察到导纳的实部和虚部都出现明显的台阶,为负值的虚部反应了器件的电感性响应的特性.但是零磁场条件下的导纳坪台有波动,这是背散射导致的结果.外加磁场可以有效地抑制背散射,改善坪台的平整程度,实验观察到当磁场为0.3 T左右时就可以得到比较理想的坪台.当磁场进一步增大时,可以观察到磁耗散效应,并且由此可以估算器件的零磁场子能带的间距.     

两电子量子环的磁场效应

使用矩阵对角化方法,研究了磁场中两电子量子环自旋单态和三重态的电学和光学性质,发现磁场强度对两电子量子环的能谱和光吸收有强烈的影响.计算结果显示两电子量子环的光吸收系数可以达到107/m,这个结果要比两电子量子点高一个数量级.通过引入磁场和改变量子环半径可以获得较大的吸收系数.     

The break-up of heavy electrons at a quantum critical point

The point at absolute zero where matter becomes unstable to new forms of order is called a quantum critical point (QCP). The quantum fluctuations between order and disorder that develop at this point induce profound transformations in the finite temperature electronic properties of the material. Magnetic fields are ideal for tuning a material as close as possible to a QCP, where the most intense effects of criticality can be studied. A previous study on the heavy-electron material YbRh2Si2 found that near a field-induced QCP electrons move ever more slowly and scatter off one another with ever increasing probability, as indicated by a divergence to infinity of the electron effective mass and scattering cross-section. But these studies could not shed light on whether these properties were an artefact of the applied field, or a more general feature of field-free QCPs. Here we report that, when germanium-doped YbRh2Si2 is tuned away from a chemically induced QCP by magnetic fields, there is a universal behaviour in the temperature dependence of the specific heat and resistivity: the characteristic kinetic energy of electrons is directly proportional to the strength of the applied field. We infer that all ballistic motion of electrons vanishes at a QCP, forming a new class of conductor in which individual electrons decay into collective current-carrying motions of the electron fluid.     

Magnetic and non-magnetic phases of a quantum spin liquid

A quantum spin-liquid phase is an intriguing possibility for a system of strongly interacting magnetic units in which the usual magnetically ordered ground state is avoided owing to strong quantum fluctuations. It was first predicted theoretically for a triangular-lattice model with antiferromagnetically coupled S = 1/2 spins. Recently, materials have become available showing persuasive experimental evidence for such a state. Although many studies show that the ideal triangular lattice of S = 1/2 Heisenberg spins actually orders magnetically into a three-sublattice, non-collinear 120° arrangement, quantum fluctuations significantly reduce the size of the ordered moment. This residual ordering can be completely suppressed when higher-order ring-exchange magnetic interactions are significant, as found in nearly metallic Mott insulators. The layered molecular system κ-(BEDT-TTF)(2)Cu(2)(CN)(3) is a Mott insulator with an almost isotropic, triangular magnetic lattice of spin-1/2 BEDT-TTF dimers that provides a prime example of a spin liquid formed in this way. Despite a high-temperature exchange coupling, J, of 250 K (ref. 6), no obvious signature of conventional magnetic ordering is seen down to 20 mK (refs 7, 8). Here we show, using muon spin rotation, that applying a small magnetic field to this system produces a quantum phase transition between the spin-liquid phase and an antiferromagnetic phase with a strongly suppressed moment. This can be described as Bose-Einstein condensation of spin excitations with an extremely small spin gap. At higher fields, a second transition is found that suggests a threshold for deconfinement of the spin excitations. Our studies reveal the low-temperature magnetic phase diagram and enable us to measure characteristic critical properties. We compare our results closely with current theoretical models, and this gives some further insight into the nature of the spin-liquid phase.     

应变GaN/AlxGa1-xN柱形量子点中杂质态的结合能

在有效质量近似下,利用变分原理研究了有限高应变GaN/AlxGa1-xN柱形量子点中杂质态的结合能,计算了结合能随量子点高度、半径和杂质位置的变化,讨论了应变对结合能的影响.数值计算表明,杂质态结合能随量子点半径的增大而减小,随量子点高度的增加将先增大到一极大值然后减小.当杂质位置在量子点中心时杂质态的结合能最大,且Al组分的增加使杂质态的结合能增大.研究还指出内建电场使得杂质态的结合能明显降低.     

地日系统中Lagrange点附近的编队队形重构

Lagrange点附近的编队研究对深空探测有着非常重要的意义。该文研究了利用双脉冲变轨实现三体问题中Halo轨道附近编队的队形重构问题。证明了在短时间重构时,三体问题中存在着与Hohmann变轨类似的结论:始末脉冲法是双变轨法中能量最优的重构方法。给出了实现重构所需能量的近似解析表达式和精确计算的数值方法。短时间重构的情况下,数值方法与解析方法得到的结果几乎相同。结果表明:在使用始末法实现重构时,重构所需要能量随重构时间的增加而减少,随重构半径的增加而增加。     

地日系统中Lagrange点附近的编队队形重构

L agrange点附近的编队研究对深空探测有着非常重要的意义。该文研究了利用双脉冲变轨实现三体问题中H a lo轨道附近编队的队形重构问题。证明了在短时间重构时,三体问题中存在着与Hohm ann变轨类似的结论:始末脉冲法是双变轨法中能量最优的重构方法。给出了实现重构所需能量的近似解析表达式和精确计算的数值方法。短时间重构的情况下,数值方法与解析方法得到的结果几乎相同。结果表明:在使用始末法实现重构时,重构所需要能量随重构时间的增加而减少,随重构半径的增加而增加。     

量子棒中强耦合磁极化子基态能量的磁场和温度依赖性

基于Huybrechts-Lee-Low-Pines变分法,研究量子棒中强耦合磁极化子基态能量的磁场和温度依赖性.结果表明:磁极化子基态能量的绝对值随温度参数的增加而增大,随电子-声子耦合强度的增加而增大,随磁场的回旋频率的增加而减小,随量子棒受限强度的增加而减小;磁极化子的基态能量随量子棒纵横比的变化规律呈""型曲线,并受到温度的显著影响.     

Structural,elemental, optical and magnetic study of Fe doped ZnO and impurity phase formation

We have prepared a series of(ZnO)1-x(Fe2O3)x≤0.10bulk samples with various concentrations of Fe dopant by ball milling and investigated their structural, compositional, optical and magnetic properties by means of X-ray diffraction(XRD), X-ray photoelectron spectroscopy(XPS),Raman spectrometer and vibrating sample magnetometer(VSM), respectively. Information about different impurity phases was obtained through Rietveld refinements of XRD data analysis. XPS results showed different valence states(Fe2+ tand Fe3+) supported by shaking satellite peaks in samples. With increasing Fe doping percentage, the crystal quality deteriorated and a shift of E2 low band(characteristic of ZnO) has been observed in Raman spectra. Energy band gap estimated from reflectance UV–vis spectroscopy showed shift for all bulk samples. The magnetic behavior was examined using a vibrating sample magnetometer(VSM), indicating ferromagnetic behavior at room temperature(300 K). The effective magnetic moment per Fe atom decreases with increase in doping percentage which indicates that ferromagnetic behavior arises from the substitution of Fe ions in the ZnO lattice.