Low-energy electronic properties of pairs of identical carbon nanotubes

The tight-binding model is employed to study the low-energy electronic properties of aligned pairs of identical single-wall carbon nanotubes with the intertube interactions. The rotational symmetry about the tube axes is totally broken, and the intertube interactions hybridize the atomic states on each tube to create new sub-bands. Sub-band spacing, sub-band curvature, band-edge states, and energy gaps are sensitive to stacking types and are also dependent on the radius and the chirality of the tubes. The systems could be metal, semimetal, or semiconductor depending on their stacking types. In particular, an armchair pair keeps the band structures linear like a single tube if the pair has a glide symmetry with respect to the plane between its constituent tubes. Breaking this symmetry makes the pair semimetallic or semiconducting. However, there are no such properties for chiral and zigzag pairs. The variations in electronic structures of these pairs are more complicated and more sensitive to the tube radii. Instead of being like a rope or a large bundle, the stacking-type dependent behavior is more similar to commensurate double-wall carbon nanotubes.     

Strain-induced interference effects on the resonance Raman cross section of carbon nanotubes

In this Letter, we report the effects of strain on the electronic properties of single-wall carbon nanotubes. When we normalize the electronic transition energies to the corresponding values obtained for unstrained tubes, we obtain that, regardless of the tube diameter, all the data collapse onto universal curves following an n - m = constant family pattern. In the case of metallic tubes, quantum interference effects on the Raman cross section are predicted for strained tubes when the energies of the lower and the upper components have nearly the same values. Experimental evidence for the strain-induced Raman cross section changes is observed in single nanotube spectroscopy.     

Stone-Wales defects can cause a metal-semiconductor transition in carbon nanotubes depending on their orientation

It has been shown that the two different orientations of Stone-Wales (SW) defects, i.e. longitudinal and circumferential SW defects, on carbon nanotubes (CNTs) result in two different electronic structures. Based on density functional theory we have shown that the longitudinal SW defects do not open a bandgap near the Fermi energy, while a relatively small bandgap emerges in tubes with circumferential defects. We argue that the bandgap opening in the presence of circumferential SW defects is a consequence of long-range symmetry breaking which can spread all the way along the tube. Specifically, the distribution of contracted and stretched bond lengths due to the presence of defects, and hopping energies for low-energy electrons, i.e. the 2p(z) electrons, show two different patterns for the two types of defects. Interplay between the geometric features and the electronic properties of the tubes have also been studied for different defect concentrations. Considering π-orbital charge density, it has also been shown that the deviations of bond lengths from their relaxed length result in different doping for two defect orientations around the defects-electron-rich for a circumferential defect and hole-rich for a longitudinal one. We have also shown that, in the tubes having both types of defects, circumferential defects would dominate and impose their electronic properties.     

Tuning Fermi-surface properties through quantum confinement in metallic metalattices: new metals from old atoms

We describe a new class of nanoscale structured metals wherein the effects of quantum confinement are combined with dispersive metallic electronic states to induce modifications to the fundamental low-energy microscopic properties of a three-dimensional metal: the density of states, the distribution of Fermi velocities, and the collective electronic response.     

Electronic bisection of a single-wall carbon nanotube by controlled chemisorption

Conversion of two diametrically opposed atomic rows on a carbon nanotube to sp(3) hybridization produces two identical weakly coupled one-dimensional electronic systems within a single robust covalently bonded package: a biribbon. Arm-chair tubes, when so divided, acquire a pair of narrow spin-polarized bands at the Fermi energy; interaction across the sp(3) dividers produces a tunable band splitting in the THz range. For semiconducting tubes, the eigenvalues of the low-energy electronic states are surprisingly unaffected by the bifurcation; however, the tubes' response functions to external electric fields are dramatically altered. These modified tubes could be produced by uniaxial compression transverse to the tube axis followed by site-selective chemisorption.     

碳掺杂ZnO的电子结构和光学性质

采用基于密度泛函理论框架下的第一性原理计算研究碳掺杂ZnO的电子结构和光学性质.计算结果表明：C原子替代O原子和C原子替代Zn原子两种掺杂体系的电子结构存在明显差异,这主要是由于C原子的电子分布及对周围原子的影响不同;碳掺杂ZnO光学性质的变化集中在低能量区,而高能量区的光学性质没有明显变化.结合电子结构定性解释了光学性质的变化. 关键词： ZnO 碳掺杂 电子结构 光学性质     

Dynamical spin and charge response functions in the doped two-dimensional Hubbard model

Dynamical properties of the spin and charge response functions in the doped two-dimensional Hubbard model are calculated by taking into account the drastic separation of the single-particle spectral function into the low-energy coherent and high-energy incoherent parts due to the strong Coulomb interaction. We show that this evolution of the electronic states is the origin of the broad and structureless feature in the charge response function. In the weak coupling regime the low-energy enhancement of the spin excitation is produced which can be explained within the random phase approximation. However, for the larger interaction close to the antiferromagnetic Stoner condition, the low-energy intensity of the spin excitation is suppressed. Received: 25 September 1997 / Revised: 19 December 1997 / Accepted: 9 January 1998     

W掺杂ZnO电子结构与光学性质第一性原理计算

采用了基于密度泛函理论(DFT)的第一性原理平面波超软赝势方法,计算本征ZnO和不同W掺杂浓度下W:ZnO体系的电子结构和光学性质.计算结果表明:W掺杂可以提高ZnO的载流子浓度,从而改善ZnO的导电性.掺杂后,吸收光谱发生红移现象,且光学性质变化集中在低能量区,而高能量区的光学性质没有太大变化,计算结果与相关实验结果相符合.最后,结合电子结构定性分析了光学性质的变化.     

Ultraslow phonon-assisted collapse of tubular image states

We predict ultraslow collapse of “tubular image states” (TIS) on material surfaces. TIS are bound Rydberg-like electronic states formed at large distances (∼30 nm) from the surfaces of suspended circularly-symmetric nanowires, such as metallic C nanotubes. The states are formed in potential wells, resulting from a combination of the TIS-electron attraction to image charges in the nanotube and its centrifugal repulsion, caused by spinning around the tube. We demonstrate that TIS can collapse on the tube surface by passing their angular momentum l to circularly polarized flexural phonons excited in the tube. Our analysis shows that for highly detached TIS with l ? 6 the relaxation lifetimes are of the order of 10 ns-1 μs, while for l < 6 these lifetimes are reduced by several orders of magnitude.     

Global structure optimization study on Au 2-20

The geometries and electronic properties of the most stable small Au_n clusters with n=2 to 20 are presented. An intensive search for low-energy minima of Au_n clusters was carried through using a density-functional tight-binding method combined with genetic algorithms for an unbiased global structure optimization. The structural and energetic properties of the small gold clusters are compared with those of planar Au_n clusters with n=5 to 15.     

Density functional studies on lanthanide (III) texaphyrins (Ln-Tex2+, Ln = La,Gd, Lu): structure,stability and electronic excitation spectrum

Density functional calculations have been performed to study the molecular structure and chemical properties of selected lanthanide(III) texaphyrins (Ln-Tex²⁺, Ln = La, Gd, Lu). The lanthanide element is found to reside above the mean N₅ texaphyrin plane, and the larger the cation, the greater the observed out-of-plane displacement. It is concluded that the lanthanide cation is tightly bound to the macrocyclic skeleton, yielding a stable structure. However, the chemical properties of Ln-Tex²⁺ are found to be only slightly affected by the substitution of the lanthanide element. A low-energy LUMO is found for the Ln-Tex²⁺ (Ln = La, Gd, Lu), which are therefore easily reduced in an electron-rich environment. Two characteristic bands are obtained in the calculated electronic excitation spectrum (a high-energy band at 454–462 nm and a low-energy band at 681–686 nm). The intensity of the high-energy band is much larger than that of the low-energy one, yielding a rather unique spectral feature.     

Elastic stability and electronic structure of low energy tetragonal and monoclinic PdN2 and PtN2

This paper studies the elastic and electronic structure properties of two new low-energy structures of PdN2 and PtN2 by first-principles calculations.It finds that tetragonal and monoclinic structures are more stable than a pyrite one.The always positive eigenvalues of the elastic constant matrix confirm that both the tetragonal and monoclinic structures are elastically stable.The origin of the low bulk modulus of the two structures is discussed.The results of the calculated density of states show that both of the two low-energy structures are metallic.     

闪锌矿结构AlSb空位缺陷性质的第一性原理研究

采用基于密度泛函理论(DFT)第一性原理的平面波超软赝势方法，计算含有锑空位和铝空位的电子结构，发现空位引起周围原子弛豫，晶体结构发生畸变。在此基础上研究了空位缺陷对闪锌矿型AlSb体系电子结构的影响，结果表明，铝空位缺陷使锑化铝费米能级降低，带隙变窄；而锑空位缺陷则使锑化铝费米能级升高，带隙变窄，同时，锑化铝的半导体类型由p型变为n型。对光学性质的研究发现，由于空位的引入使其邻近原子电子结构发生变化，使得空位缺陷系统光学性质的变化主要集中在低能量区域。     

闪锌矿结构AlSb空位缺陷性质的第一性原理研究

采用基于密度泛函理论(DFT)第一性原理的平面波超软赝势方法,计算含有锑空位和铝空位的AlSb电子结构,发现空位引起周围原子弛豫,晶体结构发生畸变.在此基础上研究了空位缺陷对闪锌矿型AlSb体系电子结构的影响,结果表明,铝空位缺陷使锑化铝费米能级降低,带隙变窄;而锑空位缺陷则使锑化铝费米能级升高,带隙变窄,同时,锑化铝的半导体类型由p型变为n型.对光学性质的研究发现,由于空位的引入使其邻近原子电子结构发生变化,使得空位缺陷系统光学性质的变化主要集中在低能量区域.     

Structural and electronic properties of sulphur-doped boron nitride nanotubes

First-principles calculations based on density functional theory were performed to study the structural and electronic properties of sulphur substitution-doped boron nitride (BN) nanotubes, using the theory as implemented in SIESTA code, which uses non-conserving pseudo-potentials in fully non-local form and atomic orbitals as the basis set. The generalized gradient approximation (GGA) was used for the exchange–correlation (XC) potential. The tube selected was a (10, 0) BN nanotube that fell in the range of energy gap independent of the tube diameter. The electronic and structural properties for sulphur substitution in the boron and the nitrogen sites were studied. The structural arrangement in equilibrium conditions for S shows an outward radial deformation around the sulphur atom in the tube. The bandgap of the pristine BN nanotubes was found to be significantly modified on doping.     

Equilibration of a one-dimensional quantum liquid

We review some of the recent results on equilibration of one-dimensional quantum liquids. The low-energy properties of these systems are described by the Luttinger liquid theory, in which the excitations are bosonic quasiparticles. At low temperatures, the relaxation of the gas of excitations toward full equilibrium is exponentially slow. In electronic Luttinger liquids, these relaxation processes involve backscattering of electrons and give rise to interesting corrections to the transport properties of one-dimensional conductors. We focus on the phenomenological theory of the equilibration of a quantum liquid and obtain an expression for the relaxation rate in terms of the excitation spectrum.     

Effet de l'oxydation en surface et en volume sur les spectres de pertes caractéristiques d'énergie d'électrons dans les couches minces de titane

Transmission and reflection electron energy loss spectra, together with electronic diffraction patterns and Auger spectra have been registered on evaporated titanium thin films. The optical, crystallographic and chemical properties for the bulk and the surface of the films are compared. The 25 eV energy loss observed in the spectra of the low-energy electrons seems to give evidence for the formation of a superficial oxide layer. This assumption is confirmed by the transmission energy loss spectrum of pure rutile which is also presented.     

Ca掺杂的氧化铍纳米管吸附H2O和NH3的选择传感特

通过密度泛函计算, 借助NH₃和H₂O分子对未掺杂以及钙掺杂的BeO碳纳米管的结构和电传导性进行了研究. 结果发现,NH₃和H₂O分子可以吸附在纳米管侧壁的Be原子上,吸附能分别为约36.1和39.0 kcal/mol. 态密度分析显示BeO纳米管的电传导性在吸附后稍有变化. 对于NH₃和H₂O分子,纳米管表面的钙原子替换Be原子可使吸附能分别增加约7.4和14.7 kcal/mol. 与未掺杂纳米管不同的是,钙掺杂BeONT吸附NH₃和H₂O分子的电传导性更加敏感,且H₂O分子比NH₃分子更敏感.     

Non-Linear Excitation and Decay of Low-Energy Nuclear Isomers Produced under Femtosecond Laser–Plasma Interaction

The properties of internal electronic conversion of nucleus excited in hot dense laser plasma are considered. The important role of dynamics of plasma charge state in residual gas is shown. We are also considering the role of multi-quanta photoexcitation of low-energy nuclear levels in this plasma. This revised version was published online in August 2006 with corrections to the Cover Date.     

First principle study of the influence of vacancy defects on optical properties of GaN

We employ plane-wave with ultrasoft pseudopotential method to calculate and compare the total density of states and partial density of states of bulk-phase GaN,Ga0.9375 N,and GaN0.9375 systems based on the first-principle density-functional theory(DFT).For Ga and N vacancies,the electronic structures of their neighbor and next-neighbor atoms change partially.The Ga0.9375 N system has n-type semiconductor conductive properties,whereas the GaN0.9375 system has p-type semiconductor conductive properties.By studying the optical properties,the influence of Ga and N vacancy defects on the optical properties of GaN has been shown as mainly in the low-energy area and very weak in high-energy area.The dielectric peak influenced by vacancy defects expands to the visible light area,which greatly increases the electronic transition in visible light area.