Multiferroic tunnel junctions

Magnetic tunnel junctions with ferroelectric barriers, often referred to as multiferroic tunnel junctions, have been proposed recently to display new functionalities and new device concepts. One of the notable predictions is that the combination of two charge polarizing states and the parallel and antiparallel magnetic states could make it a four resistance state device. We have recently studied the ferroelectric tunneling using a scanning probe technique and multiferroic tunnel junctions using ferromagnetic La_0.7Ca_0.3MnO₃ and La_0.7Sr_0.3MnO₃ as the electrodes and ferroelectric (Ba, Sr)TiO₃ as the barrier in trilayer planner junctions. We show that very thin (Ba, Sr)TiO₃ films can sustain ferroelectricity up till room temperature. The multiferroic tunnel junctions show four resistance states as predicted and can operate at room temperatures.     

Properties of a dielectric plate using entangled two-photon states

We theoretically study the properties of a dielectric plate with a modified Hong-Ou-Mandel interferometer. The fourth-order correlation functions are calculated in two regimes, which are divided depending on the relative size between the thickness of the dielectric plate and the one-photon coherence length. When the thickness of the dielectric plate is less than the one-photon coherence length, a novel modulation behavior of the coincidence rate is observed, which has not been discussed before. If the thickness of the dielectric plate is larger than the one-photon coherence length, coalescence and anti-coalescence are observed. The obtained results highlight the effects of a linear optical element on fourth-order interference.     

Eshelby’s problem in an anisotropic multiferroic bimaterial plane

We solve analytically the Eshelby’s problem in an anisotropic multiferroic bimaterial plane. The solution is based on the extended Stroh formalism of complex variables, and is valid for the inclusion of arbitrary shapes, described by a Laurent polynomial, a polygon, or the one bounded by a Jordan curve. Furthermore, the results in the corresponding half plane and full plane can be reduced directly from the bimaterial-plane solution. As such, the solution unifies the complex variable method and the Green’s function method, extending further to the multiferroic bimaterial plane of general anisotropy. The essential eigenfunctions are also identified by which the induced fields can be simply determined. Numerical results are presented to investigate the features of these eigenfunctions as well as the strain, electric and magnetic fields (components of the extended Eshelby tensor). Particularly, we present the values of these fields at the center of the N-side regular polygonal inclusion and also the average values of these fields over the inclusion area. The effect of the half-plane traction-free surface condition as well as the effect of various couplings on the induced fields is discussed in detail. For the N-side regular polygonal inclusion, it is found that, when the inclusion is in the full plane, both the center and average values of the Eshelby tensor are independent of the side number N, except for N = 4. We further show that the piezoelectric and piezomagnetic coupling coefficients could significantly affect the Eshelby tensor. These features should be useful in controlling the Eshelby tensor for the design of better multiferroic composites. Typical contours of the field quantities in and around the inclusion bounded by both straight and curved line segments in a multiferroic bimaterial plane are also presented.     

多自由度Maxwell阻尼器减震结构(Ⅱ)——等效阻尼

对多自由度带支撑Maxwell阻尼器减震结构的等效阻尼比进行了系统研究。构建了耗能结构一般微分和积分混合地震响应方程组;基于与多自由度随机平均法分析完全相同的等效准则,建立了耗能结构各振型等效阻尼比的一般解析计算式。与一些典型问题的模态应变能法的计算精度进行对比分析,结果显示:本文方法的计算结果在特定条件下与精确解完全相同;在一个二自由度系统中相对位移响应方差的误差分别为0.99%和0.45%,优于应变能法的8%和5.4%,表明了本文方法的有效性。从而为建立此类耗能结构等效阻尼分析的完备解析解法、直接应用反应谱法进行实际工程设计提供了依据。     

氨基络合物制备氨硼烷及放氢性能研究

以NaBH₄为硼源、氨基络合物Ni(NH₃)₆Cl₂为氨源制备高储氢容量的氨硼烷(NH₃BH₃, Ammonia Borane, AB)及其放氢性能研究. 通过XRD, FTIR, ¹¹B NMR, ICP等手段分析表征了所制备产物的组成和纯度, 在此基础上探究了原料比例、反应温度、时间和溶剂等因素对产物的影响. 同时, 对不同原料比制得氨硼烷的热解放氢性能进行了研究. 实验结果表明: 当物质的量NaBH₄∶Ni(NH₃)₆Cl₂＝2∶1经过10 h的反应, 得到了纯度非常高的氨硼烷(纯度＞99%)|以NaBH₄∶Ni(NH₃)₆Cl₂＝3∶1得到的氨硼烷, 当以2 ℃/min进行升温时, 氢气释放主要集中在第一步, 并且没有硼烷和硼嗪等杂质气体的产生. 另外, 在产物中得到了金属Ni纳米颗粒, 经洗涤干燥后其粒径大小可控制在10 nm左右, 在催化氨硼烷等材料的水解放氢方面具有潜在的应用价值.     

第一性原理研究Zr的掺杂对Xe在UO_2中溶解能力的影响

Zr既是反应堆中核燃料组件的包壳材料,也是核燃料UO_2的一种裂变产物,不可避免地会掺杂到UO_2中,对其性质等产生一定的影响.本文通过第一性原理密度泛函理论计算,研究了Zr掺杂所引起的Xe在UO_2中溶解能力的变化.首先应用引入Hubbard U修正的广义梯度近似密度泛函计算了U,O间隙和空位缺陷的形成能,结果与文献值符合,验证了计算方法的可靠性.在此基础上对Zr掺杂后空位缺陷的形成能及Xe吸附到空位缺陷所需的结合能的变化情况进行了研究.结果表明,Zr的掺杂会增加空位缺陷的形成能,减小大部分Xe吸附的结合能,且空位缺陷形成能的变化量普遍更大,从而在整体上增加了Xe在UO_2中的溶解能.说明在UO_2中,Zr掺杂主要是通过增加缺陷的形成难度而减弱了Xe在其中的溶解能力.     

具有氧空位Bi_xWO_6(1.81≤x≤2.01)的第一性原理计算和光催化性能研究

为了探索影响Bi_2WO_6光催化性能的内在机制,采用密度泛函理论(DFT)计算和实验研究了非化学计量和氧空位对Bi_2WO_6晶体结构、电子结构和显微结构的影响.采用溶剂热法合成了具有氧空位的非化学计量Bi_xWO_6(x=1.81,1.87,1.89,1.92,2.01)光催化剂.DFT计算结果表明,氧空位的存在可显著减小Bi_2WO_6的带隙,有利于光生电子的生成.实验结果表明,当Bi元素的含量小于化学计量比时,Bi_2WO_6的晶体结构发生了微小变形,Bi元素含量对Bi_2WO_6的显微结构影响不大,但会影响产品氧空位的含量和光吸收性能,并有效抑制电子空穴的复合.Bi_(1.89)WO_6产品的光催化性能最佳,可见光照射180 min后,可降解98%的罗丹明B.采用适当方法,使产品具有氧空位和非化学计量是获得高光催化活性材料的一种有效方法.     

Mean-field approximations of fixation time distributions of evolutionary game dynamics on graphs

The mean fixation time is often not accurate for describing the timescales of fixation probabilities of evolutionary games taking place on complex networks. We simulate the game dynamics on top of complex network topologies and approximate the fixation time distributions using a mean-field approach. We assume that there are two absorbing states. Numerically, we show that the mean fixation time is sufficient in characterizing the evolutionary timescales when network structures are close to the well-mixing condition. In contrast, the mean fixation time shows large inaccuracies when networks become sparse. The approximation accuracy is determined by the network structure, and hence by the suitability of the mean-field approach. The numerical results show good agreement with the theoretical predictions.     

Simulating a low-temperature Maxwellian plasma using SH-HtscEBIT

This paper reports an experimental realization of a low-temperature quasi-Maxwell–Boltzmann plasma in an electron beam ion trap (EBIT) by employing a special energy sweep technique. A Maxwellian C IV plasma with electron temperatures in the range 10–40?eV is simulated in the Shanghai high-temperature superconducting EBIT (SH-HtscEBIT), which requires stable performance of the EBIT under the extreme condition of ultralow electron beam energy. The C IV spectra are obtained by an extreme ultraviolet spectrometer in the wavelength region 290–400?Å. The measured spectral line intensity ratios are compared with the results of calculations using a collisional-radiative model. The mean difference between the experimental and theoretical results in the simulated temperature region is about 14%.     

Tuning spin-filtering,rectifying, and negative differential resistance by hydrogenation on topological edge defects of zigzag silicene nanoribbons

By first-principles calculations, we propose three heterojunction nanodevices based on zigzag silicene nanoribbons with different edge-hydrogenated topological line defects. The devices all present excellent spin-filtering properties with 100% spin polarization as well as remarkable rectifying effect (with rectification ratio around 10²) and negative differential resistance behaviors. Our findings shed new light on the design of silicon-based nanodevices with intriguing spintronic applications.