三角形石墨烯纳米片电子和磁学性质的尺寸效应

基于密度泛函理论, 本文研究了氢钝化锯齿形边缘三角形石墨烯纳米片的电子结构和磁学性质, 这种石墨烯纳米结构的基态表现出强烈的磁性边缘态和量子尺寸效应。 我们应用多种交换关联泛函, 对体系的自旋密度和电子结构进行了第一性原理计算和理论分析, 结果表明三角形石墨烯纳米片的总磁矩和自旋随尺寸线性变化,平均磁矩随着尺寸变大而增加, 并逐渐趋于一个定值。 与此同时, 体系的能隙随着尺寸增加而减小, 其中自旋不变能隙的调控对光学响应和光子激发有着重要意义。 计算得到的单电子能谱表明, 费米能级的简并度与体系尺寸成正比。 应用多种交换关联泛函的计算结果表明, 三角形石墨烯纳米片具有可调控的自旋和能隙, 为其在纳米级光电器件和磁性半导体的应用方面提供了理论依据.     

半氢化石墨烯与单层氮化硼复合体系的电子结构和磁性的调控

采用密度泛函理论第一性原理的PBE-D_2方法,对半氢化石墨烯与单层氮化硼(H-Gra@BN)复合体系的结构稳定性、电子性质和磁性进行了系统的研究.计算了六种可能的堆叠方式,结果表明:H-Gra@BN体系的AB-B构型是最稳定的,为铁磁性半导体,上、下自旋的带隙分别为3.097和1.798 e V;每个物理学原胞具有约1μB的磁矩,该磁矩主要来源于由未氢化的C_2原子的贡献;在z轴方向压应力的作用下,最稳的H-Gra@BN体系的电子性质由磁性半导体转变为半金属,再转变为非磁性金属;预测了一种能方便地通过应力调控电子性质和磁性质的新型材料,有望应用在纳米器件以及智能建筑材料等领域.     

单层正三角锯齿型石墨烯量子点的电子结构和磁性

采用基于密度泛函理论的广义梯度近似(GGA),对不同尺寸(N=2—11)的单层正三角锯齿型石墨烯量子点(Z_N -GNDs)的结构进行优化,得到与实验数据较好符合的晶格常数,进一步计算得到不同尺寸下体系的自旋多重度、磁矩、电子态密度以及自旋电子密度.结果表明:所有体系都呈现金属性,在尺寸较小的体系中量子尺寸效应对电子结构的影响比较明显;与单层石墨烯片一样,sp²杂化作用和非键态电子在量子点中仍起到非常重要的作用;费米能级上有自旋向上的电子分布,体系的 关键词： 石墨烯 量子点 电子结构 磁性     

石墨烯-六方氮化硼面内异质结构的扫描隧道显微学研究

石墨烯-六方氮化硼面内异质结构因可调控石墨烯的能带结构而受到广泛关注. 本文介绍了在超高真空体系内, 利用两步生长法在两类对石墨烯分别有强和弱电子掺杂的基底, 即Rh(111)和Ir(111)上制备石墨烯-六方氮化硼单原子层异质结构. 通过扫描隧道显微镜及扫描隧道谱对这两种材料的形貌和电子结构进行研究发现: 石墨烯和六方氮化硼倾向于拼接生长形成单层的异质结构, 而非形成各自分立的畴区; 在拼接边界处, 石墨烯和六方氮化硼原子结构连续无缺陷; 拼接边界多为锯齿形型, 该实验结果与密度泛函理论计算结果相符合; 拼接界面处的石墨烯和六方氮化硼分别具有各自本征的电子结构, 六方氮化硼对石墨烯未产生电子掺杂效应.     

氮化硼纳米片的电子结构和自旋调控

单层氮化硼纳米材料具有与石墨烯相似的原子排列方式, 但是由于硼原子和氮原子之间的电荷转移, 两种材料的电子特性具有较大的差异. 本文采用Hubbard模型和量子力学第一性原理计算相结合的方法研究了具有氢原子饱和的锯齿型边界的三角形氮化硼纳米片(Nanoflake) 的电子结构, 发现：与相应的石墨烯纳米片不同, 出现在氮化硼纳米片费米能级附近的零能态(zero-energy-states)要么被电子完全占据, 要么是全空的, 表现出自旋简并的特点; 通过对氮化硼纳米片进行电子(或空穴)掺杂可以有效地调控"零能态"上的电子占据, 进而对氮化硼纳米片的自旋进行调控. 这将为氮化硼纳米材料在自旋电子学等领域的应用提供重要的理论依据. 关键词： 氮化硼纳米片 电子结构调控 Hubbard 模型 量子力学第一性原理     

5d过渡金属原子吸附氮化硼纳米管的第一性原理计算

采用基于密度泛函理论的第一性原理计算方法, 研究了氮化硼纳米管六元环中心吸附5d过渡金属原子后体系的几何结构, 电子结构和磁性性质. 研究发现, 吸附原子向一个氮原子或硼原子偏移; 吸附体系在费米能级附近出现明显的杂质能级; 各个体系的总磁矩随原子序数出现规律性变化, 局域磁矩主要分布在吸附原子上.     

六方氮化硼单层有序缺陷和无序缺陷体系几何结构、电子结构和磁性性质的第一性原理计算

采用基于密度泛函理论(DFT)的第一性原理计算方法,系统研究了12×12的六方氮化硼单层(h-BNS)有序缺陷和无序缺陷对体系几何结构、电子结构和磁性性质的影响,并与理想的h-BNS、一个B原子缺陷体系(V_B)、一个N原子缺陷体系(V_N)进行对比.研究发现:缺陷周围原子位置发生明显改变;硼原子缺陷体系的费米能级向下移动而氮原子缺陷体系的费米能级向上移动,并且硼原子缺陷体系费米能级的相对移动比氮原子缺陷体系费米能级的相对移动大;h-BNS本身没有磁矩,但缺陷体系都有磁矩,其中V_B和V_N体系的总磁矩为1μB,其余的有序和无序缺陷体系的总磁矩也都不为零且B原子缺陷体系的总磁矩明显大于N原子缺陷体系的总磁矩.     

5d过渡金属原子吸附氮化硼纳米管的第一性原理计算

采用基于密度泛函理论的第一性原理计算方法,研究了氮化硼纳米管六元环中心吸附5d过渡金属原子后体系的几何结构,电子结构和磁性性质.研究发现,吸附原子向一个氮原子或硼原子偏移;吸附体系在费米能级附近出现明显的杂质能级;各个体系的总磁矩随原子序数出现规律性变化,局域磁矩主要分布在吸附原子上.     

六方氮化硼单层有序缺陷和无序缺陷体系几何结构、电子结构和磁性性质的第一性原理计算

采用基于密度泛函理论(DFT)的第一性原理计算方法,系统研究了12×12的六方氮化硼单层(h-BNS)有序缺陷和无序缺陷对体系几何结构、电子结构和磁性性质的影响,并与理想的h-BNS、一个B原子缺陷体系(VB)、一个N原子缺陷体系(VN)进行比较. 研究发现：缺陷周围原子位置发生明显改变;硼原子缺陷体系的费米能级向下移动而氮原子缺陷体系的费米能级向上移动,并且硼原子缺陷体系费米能级的相对移动比氮原子缺陷体系费米能级的相对移动大;h-BNS本身没有磁矩,但缺陷体系都有磁矩,其中VB 和VN体系的总磁矩为1μB,其余的有序和无序缺陷体系的总磁矩也都不为零且B原子缺陷体系的总磁矩明显大于N原子缺陷体系的总磁矩。     

六方氮化硼单层有序缺陷和无序缺陷体系几何结构、电子结构和磁性性质的第一性原理计算

采用基于密度泛函理论(DFT)的第一性原理计算方法,系统研究了12×12的六方氮化硼单层(h-BNS)有序缺陷和无序缺陷对体系几何结构、电子结构和磁性性质的影响,并与理想的h-BNS、一个B原子缺陷体系(VB)、一个N原子缺陷体系(VN)进行比较. 研究发现：缺陷周围原子位置发生明显改变;硼原子缺陷体系的费米能级向下移动而氮原子缺陷体系的费米能级向上移动,并且硼原子缺陷体系费米能级的相对移动比氮原子缺陷体系费米能级的相对移动大;h-BNS本身没有磁矩,但缺陷体系都有磁矩,其中VB 和VN体系的总磁矩为1μB,其余的有序和无序缺陷体系的总磁矩也都不为零且B原子缺陷体系的总磁矩明显大于N原子缺陷体系的总磁矩。     

Gauge invariant unified field structures,quantizable dynamical systems,charge, and spin structures

Gauge invariant unified field structures on a manifold B are introduced. Necessary and sufficient conditions for their existence are studied. The connection with charge is studied; it is shown that such gauge invariant structures, e.g. quantizable dynamical systems, over simply connected manifolds B are completely classified by charge. Complex analytic gauge invariant unified field structures are studied. These structures over a complex analytic manifold B whose square is the canonical line bundle are in bijective correspondence with the spin structures on B. Finally, a class of homogeneous quantizable dynamical systems are shown not to carry spin structure.     

Remarks on unified field structures,spin structures and canonical quantization

Unified field structures are defined and reviewed. Under certain conditions these are shown to be dynamical systems. And quantizable dynamical systems are shown to be unified field structures with invariant Riemannian metric. Spin structure is reviewed and manifoldsM ^8k+4 with spin structure are shown to be symplectic.This research was supported in part by NSF GP-13375.     

On dips, structures and eikonalization

We have investigated several models of Pomeron and Odderon contributions to high energy elastic pp and scattering. The questions we address concern their role in this field, the behavior of the scattering amplitude (or of the total cross-section) at high energy, and how to fit all high energy elastic data. The data are quite well reproduced by our approach at all momenta and for sufficiently high energies. The relative virtues of Born amplitudes and of different kinds of eikonalizations are considered. An important point in this respect is that secondary structures are predicted in the differential cross-sections at increasing energies and these phenomena appear to be quite directly related to the procedure of eikonalizing the various Born amplitudes. We conclude that these secondary structures arise naturally within the eikonalized procedure (although their precise localization turns out to be model dependent). The fitting procedure naturally predicts the appearance of a zero at small in the real part of the even amplitude as anticipated by general theorems. We would like to stress, once again, how important it would be to have at LHC both pp and options for many questions connected to the general properties of high energy hadronic physics and for a check of our predictions. Received: 20 January 2000 / Revised version: 28 February 2000 / Published online: 6 July 2000     

Periodicity, planarity, residual dipolar coupling, and structures

The periodic behavior of residual dipolar couplings (RDCs) arising from nucleic acid and protein secondary structures is shown to be more complex and information-rich than previously believed. We have developed a theoretical framework which allows the bond vector orientation of nucleic acids and the peptide plane orientations of protein secondary structures to be extracted from their Dipolar waves. In this article, we focus on utilizing "Dipolar waves" of peptides to extract structure information, and describe in more detail the fundamental principles of the relationship between the periodicities in structure and RDCs, the practical procedure to extract peptide plane orientation information from RDC data, and assessment of errors using Monte-Carlo simulations. We demonstrate the utility of our method for two model alpha-helices, one kinked and one curved, and as well as an irregular beta-strand.     

Tetraplectic structures, tri-momentum maps, and quaternionic flag manifolds

The purpose of this note is to define tri-momentum maps for certain manifolds with an Sp(1)ⁿ-action. We exhibit many interesting examples of such spaces using quaternions. We show how these maps can be used to reduce such manifolds to ones with fewer symmetries. The images of such maps for quaternionic flag manifolds, which are defined using the Dieudonné determinant, resemble the polytopes from the complex case.     

One-dimensional strings, random fluctuations, and complex chaotic structures

The effect of random fluctuations on the set of equations resulting from coupling 200 points with one-dimensional maps is studied. This dynamical system exhibits a spatial exponential growth in fluctuations, resulting in a complex chaotic structure. The system suggests a closer look at stability in complex systems.     

Branching, capping, and severing in dynamic actin structures

Branched actin networks at the leading edge of a crawling cell evolve via protein-regulated processes such as polymerization, depolymerization, capping, branching, and severing. A formulation of these processes is presented and analyzed to study steady-state network morphology. In bulk, we identify several scaling regimes in severing and branching protein concentrations and find that the coupling between severing and branching is optimally exploited for conditions in vivo. Near the leading edge, we find qualitative agreement with the in vivo morphology.     

Energetics,electronic structures and geometries of didehydroazines

When calibrated against the available experimental data for didehydrobenzenes, RB3LYP/cc-pVTZ, QCISD/cc-pVTZ, CCSD(T)/cc-pVTZ, and G3 electronic structure calculations provide reliable predictions of standard enthalpies and singlet—triplet splittings in all possible isomers of didehydroazines that (with a possible exception of 2,6-didehydropyridine) possess singlet ground states. Singlet didehydroazines with larger numbers of nitrogen atoms turn out to be more prone to ring opening, as indicated by the fact that out of the 6, 11, 6 and 3 possible didehydropyridines, didehydrodiazines, didehydrotriazines, and didehydrotetrazines, respectively, 5, 7, 2 and none are actually found. Immediate proximity of the nitrogen atom to the formally triple carbon—carbon bond confers decreased thermodynamic stabilities and smaller singlet—triplet splittings on the species of the 1,2-didehydro type. Some of the aza analogues of singlet 1,3-didehydrobenzene are as stable as their 1,2-didehydro counterparts. The only existing aza analogue of singlet 1,4-didehydrobenzene is 2,5-didehydropyrazine, which is particularly stable and possesses a large singlet—triplet splitting, making it a feasible synthetic target. The present calculations indicate that the experimental standard enthalpies of formation of pyrimidine and pyrazine are in error.