Gravitational force between two electrons in superconductors

The attractive gravitational force between two electrons in superconductors is deduced from the Eddington–Dirac large number relation, together with Beck and Mackey electromagnetic model of vacuum energy in superconductors. This force is estimated to be weaker than the gravitational attraction between two electrons in the vacuum.     

Structure and melting of dipole clusters

Two-dimensional clusters of particles, repelling due to dipole-dipole interactions and confined by an external parabolic potential, are considered. The model describes different physical systems, particularly electrons in semiconductor structures, or electrons above a drop of He near a metal electrode, a drop of colloid liquid etc. Two kinds of ordering are in competition in the clusters: a triangular lattice and a shell structure. The ground-state configurations corresponding to the local and global minima of the potential energy for clusters with N = 1 – 40 “particles” are calculated. The structure, the potential energy and the radial and angular r.m.s. displacements as functions of temperature are also calculated. Analysing these quantities the melting of clusters is studied. One- or two-stage melting occurs depending on the number of particles in the cluster. In the case of clusters consisting of two shells melting has two stages: at lower temperature reorientation of neighbouring shells (“orientational melting”) arises; at much higher temperatures the radial shell order disappears. In clusters consisting of more than two shells total melting occurs as a first-order one-stage transition (analogously to a dipole crystal). This is connected with the barrier of rotation being less than the barrier of interchange of particles between shells for small microclusters while the barriers are of equal order for clusters with a greater number of particles.     

Aln(n=2～7)团簇的结构和能级分布

采用密度泛函B3LYP的方法研究了小原子团簇Al2～7的几何结构和能级分布,分析了随团簇原子数的增加,团簇的几何结构和费米能级的变化情况.研究结果表明:Al2～7的团簇的几何结构在5个原子以前为平面结构,而从六个原子开始为空间立体的稳定结构.电子壳层结构表明,在铝团簇中没有出现非常明显的象碱金属那样的稳定幻数结构.在Al2～Al7团簇中,能级结构呈现明显的分立特征,费米能级随着原子个数的增加而减小,到Al7时又有所增加,且团簇的能量间隙最小.     

不同构型(In,Al)GaN合金发光机理的第一性原理研究

基于第一性原理的密度泛函理论,研究了纤锌矿（In,Al）GaN合金的4种构型（均匀、短链、小团簇、团簇-链共存模型）的电子结构和发光微观机理。结果表明,在InGaN合金中,短In-N-链和小In-N团簇都局域电子在价带顶（VBM）态。当小团簇与短链共存时,前者局域电子的能力明显强于后者,是辐射复合发光中心。然而,在AlGaN合金中,电子在VBM态的局域受短Al-N链和小Al-N团簇的影响并不显著。合金微观结构的不同会引起电子局域的改变,从而影响材料的发光性能,并对带隙和弯曲系数有重要影响。     

Hole core in superconductors and the origin of the Spin Meissner effect

It is proposed that superconductors possess a hidden ‘hole core’ buried deep in the Fermi sea. The proposed hole core is a small region of the Brillouin zone (usually at the center of the zone), where the lowest energy states in the normal state reside. We propose that in the superconducting state these energy states become singly occupied with electrons of a definite spin helicity. In other words, that holes of a definite spin helicity condense from the top to the bottom of the band in the transition to superconductivity, and electrons of that spin helicity ‘float’ on top of the hole core, thus becoming highly mobile. The hole core has radius q₀ = 1/2λ_L, with λ_L the London penetration depth, and the electrons expelled from the hole core give an excess negative charge density within a London penetration depth of the real space surface of the superconductor. The hole core explains the development of a spin current in the transition to superconductivity (Spin Meissner effect) and the associated negative charge expulsion from the interior of metals in the transition to superconductivity, effects we have proposed in earlier work to exist in all superconductors and to be at the root of the Meissner effect.     

Rhn(n=2～20)团簇结构和磁特性

使用紧束缚近似多体势和遗传算法,计算了Rhn(n=2 ~20)团簇的基态结构.据优化结构计算了平均键长(-R) ,平均配位数(CN) ,单原子平均束缚能Eb,使用自洽的4d,5s ,5p价电子紧束缚模型,计算了单原子平均磁矩-μn和Rhn(n=15 ,17 ,18 ,19)团簇的总的平均电子态密度.结果初步揭示了小铑团簇结构、电子和磁特性之间的关系.     

Solar cells as pseudo-Josephson junctions

We have identified pseudo-Josephson effects within currents in solar cells. Compared to the original Josephson junction of superconductors, we were able to obtain theoretical results in one-to-one correspondence with experimental observation of solar cell currents. We applied a pseudo-Josephson junction to the current along the axis of energy, that is, the momentum space. The theoretical form of DC-type currents before integration over frequency was also found to be of the tan-function. This pseudo-Josephson junction is, however, not in real space but in momentum space. Currents of solar cells were not formulated in terms of the unique semiconductor differences between electrons and holes but with our pseudo-Josephson junction in terms of conduction band electrons, energy gap, and valence band electrons in the solar cell materials. The mechanism of solar cell function is thus ascribed to a pseudo-Josephson effect in our scheme.     

Short-range order,large-scale potential fluctuations,and photoluminescence in amorphous SiN<Subscript>x</Subscript>

The short-range order and electron structure of amorphous silicon nitride SiN_x (x<4/3) have been studied by a combination of methods including high-resolution X-ray photoelectron spectroscopy. Neither random bonding nor random mixture models can adequately describe the structure of this compound. An intermediate model is proposed, which assumes giant potential fluctuations for electrons and holes, caused by inhomogeneities in the local chemical composition. The characteristic scale of these fluctuations for both electrons and holes is about 1.5 eV. The photoluminescence in SiN_x is interpreted in terms of the optical transitions between quantum states of amorphous silicon clusters.     

稳态Cu-Zr二十面体团簇电子结构的密度泛函研究

采用第一原理对以Cu为心的低能稳态Cu_nZ_(r13-n)(n=6,7,8,9)二十面体团簇的电子结构进行计算,结果表明:同一化学组分下,以Cu为心的Cu-Zr二十面体团簇中出现的同类原子聚集现象可以增强团簇的稳定性,降低费米能级(EF)上的电子数N(EF),这为低能稳态团簇拥有较小的N(EF)提供了深层次的理论解释.进一步的差分电子密度与Mulliken布居分析得知,Cu-Zr二十面体中共价键与离子键共存,成键态与反键态共存,且团簇在形成时壳层Zr与中心Cu原子是电子的提供者,壳层Cu是电子的获得者.该电荷转移方向是金属玻璃中以Cu为心的Cu-Zr二十面体团簇普遍遵循的规律,不随团簇的化学序参数及化学组分的变化而变化.计算的红外振动谱为实验上准确表征不同二十面体原子团提供了一种新的思路.     

从聚酰亚胺单分子链电荷陷阱特性的改变探讨体材料的沿面放电现象

沿面放电是破坏绝缘系统性能的原因之一.聚酰亚胺常用于高频电力设备的气-固绝缘中,为此利用密度泛函理论,从原子分子层面探讨了在外电场下聚酰亚胺及其受极性基团OH~–影响后的单分子链结构、能级与态密度、静电势、激发态等微观参数对陷阱形成以及沿面放电的影响.结果表明,外电场下,聚酰亚胺分子结构卷曲,偶极矩增加,易于积聚电荷形成空间电荷中心,尤属引入极性基团OH~–后变化较明显;聚酰亚胺分子中,苯环区域形成空穴陷阱,酰亚胺环区域形成电子陷阱,且电子陷阱能级的数量较多,其中空间电荷陷阱深度随外电场的增加逐渐变深;聚酰亚胺分子在引入极性基团OH~–后激发能降低,使得分子内部的电子变得容易被激发;电子与空穴的空间分离度随电场增加而降低,利于空穴与电子的复合而发出光子.     

Electron—hole superconductivity. Influence of structure defects

The ground state and the electromagnetic properties of the superconductor with the pairing of the spatially separated electrons and holes (EHSC) are investigated taking into account the defects, impurities and the films roughness. The bound states into the energy gap are found. The critical value of the free path of the quasiparticles at the films (destroying the pairing) is obtained (the result is also applied to the superconductors with pairing of the electrons from different layers). The electromagnetic response in EHSC, taking into consideration the scattering by the defects, is calculated. It corresponds to the ideal conductivity.     

Cluster-model study on the K-shell excited states of crystalline lithium under intense laser irradiation

Molecular-orbital calculations are performed to elucidate electronic structures and optical properties of lithium clusters in which several K-shell electrons are simultaneously excited to the valence levels. It is shown that relaxation of valence electrons around localized core holes influences the photoabsorption near-edge spectra significantly. The spectra in the excited state are modified from those in the ground state due to the presence of initial core holes. Potential energy surfaces are calculated for core-ionized Li₉ ^z+ clusters, which exhibit bound states for z≤3. The present cluster calculations would serve as prototypical models of laser-excited hollow atom solids with applications to X-ray optics.     

Charge-induced anisotropic distortions of semiconducting and metallic carbon nanotubes

To accommodate extra electrons or holes injected into a single-wall carbon nanotube, carbon-carbon bonds adjust their lengths. Resulting changes in carbon-nanotube length as a function of charge injection provide the basis for electromechanical actuators. We show that a key mechanism at low injection levels, modulation of electron kinetic energy, provides nanotube deformations that are both anisotropic and strongly dependent on nanotube structure. Nanotubes can exhibit both expansion and contraction, as well as nonmonotonic size changes. The magnitude of the actuation response of semiconducting carbon nanotubes may be substantially larger than that of graphite.     

Resonant ionic bonding and high TC superconductivity

The implications of an electronic coupling model in high T_C superconductors of YBCO are discussed in which the pairing mechanism is based upon the ionic bonding. Specifically,the bonding reaction Cu + O [rlhar2] Cu^-- resonates at a frequency of I/2π ; in the CuO planes, where I = 0.4 eV is the ionic bonding energy. We formulate an Hamiltonian which allows for the reduction of the total energy by I each time two holes or two electrons form a pair. The total energy consists of kinetic energy calculable from band theory, and the ionic bonding energy proportional to I. We calculate (i) the effective Ginzburg-Landau free energy for all T < T_c, (ii) the energy gap, (iii) the anisotropic London penetration depths, and (iv) the anisotropic coherence lengths. General agreement is found between our calculations and experimental data.     

Properties of holons in the quantum dimer model

I introduce a doped two-dimensional quantum dimer model describing a doped Mott insulator and retaining the original Fermi statistics of the electrons. This model shows a rich phase diagram including a d-wave hole-pair unconventional superconductor at small enough doping and a bosonic superfluid at large doping. The hole kinetic energy is shown to favor binding of topological defects to the bare fermionic holons turning them into bosons, in agreement with arguments based on resonating valence bond wave function. Results are discussed in the context of cuprate superconductors.     

Primordial black hole formation by stabilized embedded strings in the early universe

Primordial black hole formation by cosmic string collapses is reconsidered in the case where the winding number of the string is larger than unity. The line energy density of a multiple winding string becomes greater than that of a single winding string so that the probability of black hole formation by string collapse during loop oscillation would be strongly enhanced. Moreover, this probability could be affected by changes in gravity theory due to large extra dimensions based on the brane universe model. In addition, a wider class of strings which are stable compared to conventional cosmic strings can contribute to such a scenario. Although the production of the multiple winding defect is suppressed and its number density should be small, the enhancement of black hole formation by the increased energy density may provide a large number of evaporating black holes in the present universe which gives more stringent constraints on the string model compared to the ordinary string scenario.     

Ehrenfest theorem for inhomogeneous superconductors and supercurrent force on Andreev reflected quasiparticles

Quasiclassical equations of motion are derived for quasiparticle wave packets in inhomogeneous superconductors with electromagnetic fields. Besides the Lorentz forces on electrons and holes there are two off-diagonal pair potential forces. The first one is due to gradients of the modulus of the pair potential and responsible for electron-hole scattering from inhomogeneities of the pair potential. The second one results from interactions with the ground state condensate and is proportional to the gauge invariant Cooper pair velocity of surface supercurrents in the interfaces between normal and superconducting regions. The most favorable experimental conditions for the observation of this new force may be found inN-S junctions composed of clean normal metals and type II superconductors with high critical fields, large London penetration depths, and large energy gaps.     

铁基超导体中的化学掺杂研究

目前已经发现的绝大部分铁基超导体都是通过化学掺杂而得到的。铁基超导体的母体一般在200K以下经历自旋密度波（SDW）转变：即其基态是一类巡游电子反铁磁不良导体。通过适当的元素替代可以在FeAs层产生额外的电子、空穴、巡游性或化学压力,从而有效地抑制SDW序,实现超导电性。本文侧重作者所在小组的相关研究结果,将铁基超导体中的元素替代研究分为FeAs层外和FeAs层内掺杂两大类,依次介绍和评述两年来国际上对4种主要铁基化合物中的化学掺杂研究进展。     

Proposed design of a Josephson diode

We propose a new type of Josephson junction formed by two superconductors close to the superconductor-Mott-insulator transition, one of which is doped with holes and the other is doped with electrons. A self-organized Mott-insulating depletion region is formed at the interface between two superconductors, giving rise to an asymmetric response of current to the external voltage. The collective excitations of the depletion region result in a novel phase dynamics that can be measured experimentally in the noise spectrum of the Josephson current.     

Ni-X-In(X=Mn,Fe和Co)合金的缺陷稳定性和磁性能的第一性原理研究

Ni-Mn-In是一种新型的磁控形状记忆合金, 它通过磁场诱导逆马氏体相变实现形状记忆效应. 实验中常围绕化学计量比Ni₂MnIn合金进行成分调整, 以获得适宜的马氏体相变温度与居里温度, 在这个过程中必然会产生多种点缺陷. 本文使用量子力学计算软件包VASP, 在密度泛函理论的框架下通过第一原理计算, 系统地研究了非化学计量比Ni-X-In(X=Mn, Fe 和Co)合金的缺陷形成能和磁性能. 反位缺陷中, In和Ni在X亚晶格的反位缺陷(In_X和Ni_X)的形成能最低, Ni和X反位于Y的亚晶格(Ni_Y和X_Y)得到较高的形成能. 因此, In原子可以稳定立方母相的结构, 而X原子对母相结构稳定性的影响则相反; 空位缺陷中最高的形成能出现在In空位缺陷, 再次肯定了In原子对稳定母相结构的作用. 此外, 详细研究了点缺陷周围原子的磁性能以及电荷分布. 本文的计算结果在指导实验中的成分设计和开发新型磁控形状记忆合金方面具有重要意义.