Quantum interference in carbon nanotube intramolecular junction

The quantum conductance oscillations (QCOs) of the intramolecular junction (IMJ) composed of two single-wall carbon nanotubes (SWNTs) have been studied by using a π-orbital only tight-binding (TB) model and a Green’s function technique. It is found that in the IMJs in addition to the rapid QCO frequencies corresponding to the constituent tubes there exist also their sum frequencies. The slow QCO frequencies of the IMJ will be different from those of its corresponding two perfect tubes if they have different chiral angles.     

Raman spectral imaging of a carbon nanotube intramolecular junction

Confocal Raman spectral imaging results are presented for a metal-to-semiconductor intramolecular junction (IMJ) on an isolated carbon nanotube. Spectra observed at the junction are consistent with the symmetry lowering expected from the occurrence of pentagon-heptagon defects to produce the chirality shift. The IMJ transition zone is sharp and preserves the nanotube diameter. These results have significant implications for the future use of IMJs as electronic devices, including how prevalent these structures are and how their growth may be rationally targeted. Raman imaging has been demonstrated to be a powerful tool for IMJ studies and provides a more accessible method for further studies of IMJ structure and growth.     

Coherent transport through intramolecular junction of single-wall carbon nanotubes

We have constructed four types single-wall carbon nanotube intramolecular junctions (IMJs) of (5,5)/(8,0), (5,5)/(10,0), (5,5)/(9,0)A, and (5,5)/(9,0)B along a common axis, and calculated their electronic and transport properties using a tight binding-based Green's function approach that is particular suitable for realistic calculation of electronic transport property in extended system. Our results show that quasi-localized states can appear in the metal/semiconductor heterojunctions ((5,5)/(8,0) and (5,5)/(10,0)junctions), which is desirable for the design of a quantum device; and the conductance of M-M IMJs is very sensitive to the connectivity of the matching tubes, certain configurations of connection completely stop the flow of electron, while others permit the transmission of the current through the interface. These results may have implications for the device assembly and manipulation process of all carbon nanotubes-based microelectronic elements. Received 14 January 2003 / Received in final form 25 February 2003 Published online 4 June 2003 RID="a" ID="a"e-mail: lfyzz@yahoo.com.cn     

Wave propagation in carbon nanotube intramolecular junctions: finite element calculations

The dispersion of longitudinal and transverse waves in (n,0)–(2n,0) intramolecular junctions (IMJs) are investigated using an atomistic finite element method (FEM). The transient responses of IMJs with different connection types subjected to harmonic incident wave were modelled using three-dimensional elastic beams of carbon bonds and point masses. The linkage between the force-field constants of molecular mechanics and input parameters of beam and mass elements was established through the molecular structural mechanics approach. The wave dispersion simulated by FEM shows good agreement with that of the non-local elastic model in a wide frequency range up to the terahertz region. It is shown that both the microstructure of conical part (connection part) and the coupling of longitudinal vibration and transverse vibration brought by the conicity play important roles in the dispersion of longitudinal and transverse wave in a single-walled IMJ. The amplitude decay of longitudinal wave depended on the distance propagating; the wavelength and the structure in connection part are examined. The results show that the dispersion of the decay of the wave amplitude in IMJ with less pentagon–heptagon defects has a better agreement with analytical results of macroscopic conical shell.     

Optical properties of 4 Å single-walled carbon nanotubes inside the zeolite channels studied from first principles calculations

The structural, electronic, and optical properties of 4 ? single-walled carbon nanotubes (SWNTs) contained inside the zeolite channels have been studied based upon the density-functional theory in the local-density approximation (LDA). Our calculated results indicate that the relaxed geometrical structures for the smallest SWNTs in the zeolite channels are much different from those of the ideal isolated SWNTs, producing a great effect on their physical properties. It is found that all three kinds of 4 ? SWNTs can possibly exist inside the Zeolite channels. Especially, as an example, we have also studied the coupling effect between the ALPO₄-5 zeolite and the tube (5,0) inside it, and found that the zeolite has real effects on the electronic structure and optical properties of the inside (5,0) tube. Received 26 January 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: yxptl@hotmail.com     

Theoretical study of Si impurities in BN nanotubes

Density functional theory is employed to investigate the electronic and structural properties of substitutional Si impurities in a (10,0) BN nanotube. For the Si case, the band structure shows a level centered on the Si atom crossing the Fermi energy and no net spin is found. The Si introduces three localized exchange splitted Si levels in the gap. The formation energies show that the Si is likely to be present at N-rich conditions.Received: 9 December 2003, Published online: 28 May 2004PACS: 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 85.35.Kt Nanotube devices     

Effect of carbon network defects on the electronic structure of semiconductor single-wall carbon nanotubes

For a single-wall (14, 0) carbon nanotube, the total density of electronic states of the ideal structure and of some possible defect structures is calculated in the framework of the band theory approach using Gaussian-type orbitals and the approximation of the generalized density gradient. It is shown that allowance for defects of the atomic structure of a nanotube makes it possible to adequately describe the existing experimental data on nanotube electronic structure. In the framework of the same approach, the total density of electronic states is calculated for an intermolecular contact of (5, 5) and (10, 0) single-wall carbon nanotubes formed due to the creation of a 5–7 defect. It is shown that the electronic states related to the contact region and the 5–7 defect lie in vicinity of the Fermi level.     

First-principles study of the ferromagnetic and half-metallic properties of the fumarate-bridged polymer

Density-functional theory (DFT) with generalized gradient approximation (GGA) is applied to study the electronic structure and the magnetic properties of ferromagnet [Cu( -C₄H₂O₄)(NH₃)₂] _n (H₂O) _m . The density of states, the electronic band structure and the spin magnetic moment are calculated. The calculations reveal that the compound has a stable half-metal-ferromagnetic ground state, and that there exists a dominant ferromagnetic interaction arising from the alkoxo-bridged dimeric part of the compound. The spin magnetic moment 1.0 per molecule mainly comes from the Cu ion with little contribution from O, N, C anion.Received: 5 October 2003, Published online: 12 July 2004PACS: 75.50.Xx Molecular magnetsK.L. Yao: wl-zl41@163.com     

Spectroscopic studies of NaCs for the ground state asymptote of Na + Cs pairs

The ground state X of NaCs was studied by laser induced fluorescence Fourier-transform spectroscopy. An accurate potential energy curve was derived from more than 5000 transitions. This potential reproduces the experimental observations within their uncertainties of cm^-1 and covers about 99.97% of the potential well depth. Few vibrational levels of the shallow state a below the atomic ground state asymptote were observed. The identification is mainly done by the observed and quantitatively interpreted molecular hyperfine structure applying atomic parameters of the ground states of Na and Cs. An estimated potential curve for a is reported which can be used together with that of X for coupled channel calculations of cold collisions between Na and Cs. An example is given. Electronic supplementary material to this article is available at and is accessible for authorized users. Received: 10 September 2004, Published online: 23 November 2004 PACS: 31.50.Bc Potential energy surfaces for ground electronic states - 33.20.Kf Visible spectra - 33.20.Vq Vibration-rotation analysis - 33.50.Dq Fluorescence and phosphorescence spectra Supplementary tables (Tabs. I-III) are only available in electronic form at http: //www.eurphysj.org     

LDA and GGA investigations of some ground state properties of aluminium with the all electron MAPW method

Both in local-density approximation (LDA) and in generalised-gradient approximation (GGA) the electronic structure of Aluminium is evaluated by use of the modified augmented plane wave (MAPW) self-consistent scheme. The LDA based on the exchange correlation functional by Vosko, Wilk and Nusair gives the equilibrium lattice constant in good accord with its experimental value. The hole sheet of the Fermi surface, h₂, is well described by weakly distorted spheres with origin at and in the reciprocal lattice. Near and above the equilibrium lattice constant the electronic sheet, e₃, is found to be quite similar to the model originally proposed by Ashcroft. However, even moderate compressions induce a drastic variation.Received: 17 November 2003, Published online: 2 April 2004PACS: 71.18. + y Fermi surface: calculations and measurements; effective mass, g factor - 71.20.-b Electron density of states and band structure of crystalline solids     

Formation of cold bialkali dimers on helium nanodroplets

Cold alkali diatomic molecules (LiCs, NaCs) in the lowest vibrational state of the electronic triplet ground state are formed on superfluid helium nanodroplets. Using photoionization detection the excitation spectra of the transitions are recorded. The splitting of the vibrational structure in the LiCs spectrum, not observed in the NaCs spectrum, is interpreted in terms of molecular fine structure. The spectra are well reproduced by a model based on quantum chemistry potential curves including spin-orbit coupling, in combination with an asymmetric line shape function to account for cluster-induced broadening. Our refined potential curves provide important input data for the photoassociation of ultracold dipolar alkali molecules from atomic quantum gases.Received: 1 July 2004, Published online: 26 October 2004PACS: 36.40.Mr Spectroscopy and geometrical structure of clusters - 34.50.Gb Electronic excitation and ionization of molecules; intermediate molecular states (including lifetimes, state mixing, etc.) - 33.20.-t Molecular spectra     

Nematic liquid crystal around a spherical particle: Investigation of the defect structure and its stability using adaptive mesh refinement

We investigate the orientation profile and the structure of topological defects of a nematic liquid crystal around a spherical particle using an adaptive mesh refinement scheme developed by us previously. The previous work [J. Fukuda et al., Phys. Rev. E 65, 041709 (2002)] was devoted to the investigation of the fine structure of a hyperbolic hedgehog defect that the particle accompanies and in this paper we present the equilibrium profile of the Saturn ring configuration. The radius of the Saturn ring r_d in units of the particle radius R₀ increases weakly with the increase of , the ratio of the nematic coherence length to R₀. Next we discuss the energetic stability of a hedgehog and a Saturn ring. The use of adaptive mesh refinement scheme together with a tensor orientational order parameter allows us to calculate the elastic energy of a nematic liquid crystal without any assumption of the structure and the energy of the defect core as in the previous similar studies. The reduced free energy of a nematic liquid crystal, , with L₁ being the elastic constant, is almost independent of in the hedgehog configuration, while it shows a logarithmic dependence in the Saturn ring configuration. This result clearly indicates that the energetic stability of a hedgehog to a Saturn ring for a large particle is definitely attributed to the large defect energy of the Saturn ring with a large radius.Received: 10 December 2003, Published online: 2 March 2004PACS: 61.30.Cz Molecular and microscopic models and theories of liquid crystal structure - 61.30.Jf Defects in liquid crystals - 82.20.Wt Computational modeling; simulation     

Structural,electronic, and optical properties of beryllium monochalcogenides

The results of first-principles theoretical study of the structural, electronic and optical properties of beryllium monochalcogenides BeTe, BeSe and BeS, performed using the full potential linearized augmented plane wave (FP-LAPW) method are presented. The calculated structural parameters and band gaps compare very well with previous theoretical results. The trends of the band gap pressure coefficients and volume deformation potentials for these II-VI compounds are investigated. The linear pressure coefficients for the X and band gaps increase with decrease in anion atomic weight. The dependence of the direct and indirect band gaps on the relative change of lattice constant are found to follow almost the same type of trends in each of these compounds. The volume deformation potential ( ) for the direct ( ) and indirect ( ) gaps are positive, but negative for the indirect ( ) gap. Furthermore, , for transitions decreases with increase in anion atomic number whereas , increases. The optical properties have also been calculated. From the reflectivity spectra, the compounds will be useful for optical applications. The variation of the band gaps with respect to the application of pressure and the origin of some of the peaks in the optical spectra are discussed in terms the calculated electronic structure.Received: 26 September 2003, Published online: 18 June 2004PACS: 71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.) - 71.15.Mb Density functional theory, local density approximation, gradient and other corrections - 71.20.Nr Semiconductor compounds     

磁场中碳纳米管电子结构的紧束缚法研究

利用石墨平面碳原子轨道作sp2杂化时π电子的紧束缚模型,对磁场中直状单层碳纳米管(SWNTs)的电子结构进行理论推导和分析。磁场对碳纳米管的波矢产生影响,从而使碳纳米管的电子结构及能隙均以磁通量子Φ0(=h/e)为周期随磁通量Φ周期性变化。     

Mechanical property of carbon nanotubes with intramolecular junctions: Molecular dynamics simulations

Intramolecular junctions (IMJs) of carbon nanotubes hold a promise of potential applications in nano-electromechanical systems. However, their structure-property relation is still unclear. Using the revised second-generation Tersoff-Brenner potential, molecular dynamics simulations were performed to study the mechanical properties of single-walled to four-walled carbon nanotubes with IMJs under uniaxial tension. The dependence of deformation and failure behaviors of IMJs on the geometric parameters was examined. It was found that the rupture strength of a junction is close to that of its thinner carbon nanotube segment, and the rupture strain and Young's modulus show a significant dependence on its geometry. The simulations also revealed that the damage and rupture of multi-walled carbon nanotube junctions take place first in the innermost layer and then propagate consecutively to the outer layers. This study is helpful for optimal design and safety evaluation of IMJ-based nanoelectronics.     

ELECTRONIC PROPERTIES OF CARBON NANOTUBES WITH A PENTAGON-HEPTAGON PAIR DEFECT

The features of energetics and electronic properties of carbon nanotubes, containing a pentagon-heptagon pair (5/7) topological defect in the hexagonal network of the zigzag configuration, are investigated using the extended Su-Schriffer-Heeger model based on the tight binding approximation in real space. Our calculations show that this pentagon-heptagon pair defect in the nanotube structures is not only responsible for a change in nanotube diameter, but also governs the electronic behaviour around Fermi level. Furthermore, we have calculated the densities of states of the (9,0)-(8,0) and (8,0)-(7,0) systems. For the (9,0)-(8,0) system, a narrow gap exists in the vicinity of the Fermi energy. In contrast, for the (8,0)-(7,0) system, a little peak of the density of states occurs at the Fermi energy. These can be attributed to the addition of a pair of pentagon-heptagon defects in the interface between two isolated carbon nanotubes.     

Higher baryon resonances in carbon-carbon collisions at 4.2 GeV/c per nucleon

The production of the N(1440), the so-called Roper, and N(1520) resonances in high-energy collisions of carbon nuclei with the carbon nucleus, using a 2 m propane bubble chamber, was investigated. Attention was paid to the two-pion decay mode of the higher baryon resonances. From the invariant masses of three-particle states the mass and width of the resonances were obtained. The ratio of the resonances decay to and states was estimated.Received: 1 July 2003, Revised: 29 September 2003, Published online: 20 April 2004PACS: 25.70.Ef Resonances - 14.20.Gk Baryon resonances with S = 0     

Mass-selected Ag<Subscript>3</Subscript> clusters soft-landed onto MgO/Mo(100): femtosecond photoemission and first-principles simulations

The electronic structure of supported mass-selected Ag₃ clusters is analyzed by joint femtosecond photoemission spectroscopy and ab initio theoretical investigations. A wide band gap insulating magnesia ultra-thin film on Mo(100) has been chosen as substrate in order to minimize the electronic interaction between metal clusters and support. After magnesia ultra-thin film preparation no photoemission from the molybdenum substrate is observed anymore, instead very weak two photon photoemission is detected possibly originating from surface or subsurface oxide defect states. Soft-landing deposition of 2 of atomic monolayer equivalents of Ag₃ clusters results in the disappearance also of the MgO two photon photoemission signal, while a strong single photon photoemission signal is detected from states located directly below the Fermi level. The theoretical study of structural, electronic and optical properties of Ag₃ at two model sites of MgO (100), the stoichiometric MgO(100) and an F_S-center defect, based on the DFT method and the embedded cluster model provides insight into the interactions between the cluster and the support which are responsible for the characteristic spectroscopic features.