Conformational change-modulated spin transport at single-molecule level in carbon systems

Controlling the spin transport at the single-molecule level, especially without the use of ferromagnetic contacts, becomes a focus of research in spintronics. Inspired by the progress on atomic-level molecular synthesis, through first-principles calculations, we investigate the spin-dependent electronic transport of graphene nanoflakes with side-bonded functional groups, contacted by atomic carbon chain electrodes. It is found that, by rotating the functional group, the spin polarization of the transmission at the Fermi level could be switched between completely polarized and unpolarized states. Moreover, the transition between spin-up and spin-down polarized states can also be achieved, operating as a dual-spin filter. Further analysis shows that, it is the spin-dependent shift of density of states, caused by the rotation, that triggers the shift of transmission peaks, and then results in the variation of spin polarization. Such a feature is found to be robust to the length of the nanoflake and the electrode material, showing great application potential. Those findings may throw light on the development of spintronic devices.     

基于反应力场的分子动力学方法研究氢在钯表面的分解

基于二阶矩近似反应力场方法构建的全维度势能面研究了氢分子及其同位素分子在钯表面的分解过程.在构建势能面的过程中数据库中只包含了氢分子与钯（111）表面相互作用的相关信息,该势能面在研究氢分子在钯（100）表面上的分解过程中表现出了非常好的可转移性.结果表明,氢分子及其同位素分子在钯（111）与钯（100）表面上的分解系数S₀均随着入射能量的增加呈现非单调变化,并且通过固定分子取向的方法发现同核分子（H₂、D₂和T₂）最有利分解取向角为90°,而异核分子（HD、HT和DT）受质心偏移的影响,其最有利分解取向角向大角度偏移.     

Ground state properties of a Bose－Einstein condensate confined in an anharmonic external potential

In light of the interference experiment of Bose－Einstein condensates, we present an anharmonic external potential model to study ground state properties of Bose－Einstein condensates. The ground state energy and the chemical potential have been analytically obtained, which are lower than those in harmonic trap. Additionally, it is found that the anharmonic strength of the external potential has an important effect on density and velocity distributions of the ground state for the Thomas－Fermi model.     

Phonon spectrum boron nitrideof single-wallednanotubes

Based on a force constant model, we investigated the phonon spectrum and then specific heat of single-walled boron nitride nanotubes. The results show that the frequencies of Raman and infrared active modes decrease with increasing diameter in the low frequency, which is consistent with the results calculated by density functional theory. The fitting formulae for diameter and chirality dependence of specific heat at 300K are given.     

Magnetotransport in a dual waveguide coupled by a finite barrier： Energy filter and directional coupler

We propose in this paper that a dual waveguide coupled by a finite barrier be able to serve as an energy filter under a perpendicular magnetic field. In the waveguide direction, the conductance exhibits a periodic square-wave pattern in which the miniband is controlled by the magnetic and potential modulation. The electrons with energies in the miniband can completely transfer along one waveguide while the other electrons undergo filtration. Compared with the coupled waveguide without magnetic modulation, the structure under magnetic field is found to be a good directional coupler. By adjusting the potential barrier and magnetic field, the electrons input from one port of waveguide can transfer to any other ports.     

Strain Induced Insulator-Metal Transition in Single Wall Carbon Nanotubes

In terms of a single-π orbital model, an analytical expression of the lowest-lying conduction-band and the highestlying valence-band is derived for single wall carbon nanotubes under both the uniaxial and torsional strains. We observe not only semiconductor-metal transitions in primary metallic tubes, but also insulator-metal transitions     

多Nb原子掺杂氮化碳与H离子协同作用促进氮高效还原

电化学合成氨因其可以低能耗产氨而备受关注.目前,迫切需要一种稳定、无污染、活性好和选择性高的催化剂来促进电化学合成氨.石墨相氮化碳由于制备简单且具有较好的物理化学性质,是一种有广阔应用前景的基底材料.研究表明,纯石墨相氮化碳不具有电化学合成氨的性能,因此需要对其进行改性.元素掺杂是一种常见的改性方式,其中过渡金属元素因其具有活跃的d电子而被广泛用于掺杂改性.目前实验上已经可以制备过渡金属单原子或者多原子掺杂的石墨相氮化碳基材料.尽管在实验以及理论上都表明通过电催化可以实现还原氮气合成氨且具有较好的催化合成氨性能,但是人们对该反应机理的理解还不够深入,并且多数研究没有考虑到溶液中最常见的H离子对活性的影响.因此,本文以Nb原子掺杂的石墨相氮化碳为基础,对多活性位点下H离子助力氮还原反应进行研究,以揭示其内在机理.本文研究所采用的密度泛函理论计算软件为VASP.通过构建石墨相氮化碳的单层结构,计算了中心空穴位置吸附1-5个Nb原子以及不同Nb原子吸附结构的稳定性,并在较稳定的模型上探索其氮气还原制备氨(NRR)的催化性能.结果表明,当金属原子数增加时,活性位点附近可供吸附的空间变大,导致反应过程中H原子更容易被金属原子捕获,而非直接与N结合形成NHx中间体,从而加快了反应进程.以Nb₃@g-C₃N₄结构为例,详细探讨了额外的H原子吸附对整个NRR反应的影响.结果表明,H的吸附可以把反应能垒较高的*NH₂→*NH₃步骤转变为低能垒的两步进行,即*NH₂+H→*NH₂+*H和*NH₂+*H+H→*NH₃+*H,从而降低反应需要的能量.另一个有意义的发现是,由于高的产氢能垒,额外的H原子在此过程中不会形成H₂,且在上述多步骤反应结束后,又与N₂结合形成*NNH中间体,从而持续进行生成氨反应.同样,在Zr₃@g-C₃N₄、Mo₃@g-C₃N₄结构中进行了验证.通过对*NH₂+*H中间体态密度等电子结构性质进行分析发现,吸附在金属原子上的H原子可以有效地削弱金属原子对*NH₂中间体的吸附强度,从而使其更容易进一步氢化.上述研究结果表明,在催化过程中H原子可以在催化剂的表面起到助催化作用.本文通过密度泛函理论计算,对石墨相氮化碳负载的多金属Nb原子的NRR催化过程进行了系统的研究,阐明了H离子影响NRR过程的机理,并且发现Nb₃@g-C₃N₄体系是一个催化性能良好且具有高选择性、低反应过电位的催化剂材料.同时,上述机理在Zr和Mo的体系也进一步得以验证.     

纳米结构链的hopping电导实空间重正化群方法

基于Miller-Abrahams理论,发展了一种重正化群方法,研究了一维纳米结构体系的hopping电导.研究表明在纳米结构体系中晶粒种类、晶粒尺寸对hopping电导有显著的调制作用,界面结构及晶格畸变等对hopping电导也有不同程度的影响.     

考虑次近邻相互作用下一维单原子链中的孤立波

利用多重尺度法结合准不连续性近似,研究了涉及次近邻相互作用下的一维单原子链中的波动问题,得到了其新的色散关系.结果表明,在同时考虑次近邻相互作用和非谐相互作用的情况下,一维单原子链中不仅存在包络孤立波、扭状和反扭状孤立波,而且存在另一种孤立波形式的元激发——呼吸子. 关键词： 一维单原子链 非谐相互作用 孤立波     

阻尼作用下一维非线性单原子链中的孤子

利用多重尺度法,研究了弱阻尼作用下一维非线性单原子链中的波动行为,发现在O(ε)级阻尼作用下其格波幅度减小,群速变慢.在晶格动力学中导出了参数耗散型非线性Schr?dinger方程,对方程进行解析求解.结果表明:在O(ε)级阻尼作用下一维非线性单原子链中的格孤波为传播的包络孤子;在O(ε²)级阻尼作用下,格孤波为钟状或扭结的孤子.