Linear Atomic Chains Research Articles

Structural, electronic and magnetic properties of chiral nanotubes filled with a linear chain of iron

Density-functional calculations of the electronic structure of (n,n/2) chiral carbon nanotubes filled with a linear chain of Fe atoms were conducted for the first time. It was found that upon encapsulating a chain of Fe atoms, the initially semiconducting nanotubes Fe5@(4,2), Fe5@(6,3) and Fe5@(8,4) became metallic. In the case of the Fe5@(8,4) structure, only the iron chain was conductive. All the considered nanotubes maintained large energy of the magnetic anisotropy, which is characteristic for a free linear chain of iron atoms. The magnetic moment on an iron atom was found to vary nonmonotonically with increasing the chiral index n; for the Fe5@(6,3) nanotube, the magnetic moment reached that of a free linear chain of iron atoms.

Bond length and electric current oscillation of long linear carbon chains: Density functional theory, MpB model, and quantum spin transport studies

Carbon linear atomic chains attached to graphene have experimentally been produced. Motivated by these results, we study the nature of the carbon bonds in these nanowires and how it affects their electrical properties. In the present study we investigate chains with different numbers of atoms and we observe that nanowires with odd number of atoms present a distinct behavior than the ones with even numbers. Using graphene nanoribbons as leads, we identify differences in the quantum transport of the chains with the consequence that even and odd numbered chains have low and high electrical conduction, respectively. We also noted a dependence of current with the wire size. We study this unexpected behavior using a combination of first principles calculations and simple models based on chemical bond theory. From our studies, the electrons of carbon nanowires present a quasi-free electron behavior and this explains qualitatively the high electrical conduction and the bond lengths with unexpected values for the case of odd nanowires. Our study also allows the understanding of the electric conduction dependence with the number of atoms and their parity in the chain. In the case of odd number chains a proposed π-bond (MpB) model describes unsaturated carbons that introduce a mobile π-bond that changes dramatically the structure and transport properties of these wires. Our results indicate that the nature of bonds plays the main role in the oscillation of quantum electrical conduction for chains with even and odd number of atoms and also that nanowires bonded to graphene nanoribbons behave as a quasi-free electron system, suggesting that this behavior is general and it could also remain if the chains are bonded to other materials.

Calculation of space localized properties in correlated quantum Monte Carlo methods with reweighting: the nonlocality of statistical uncertainties.

We study the efficiency of quantum Monte Carlo (QMC) methods in computing space localized ground state properties (properties which do not depend on distant degrees of freedom) as a function of the system size N. We prove that for the commonly used correlated sampling with reweighting method, the statistical fluctuations σ2(N) do not obey the locality property. σ2(N) grow at least linearly with N and with a slope that is related to the fluctuations of the reweighting factors. We provide numerical illustrations of these tendencies in the form of QMC calculations on linear chains of hydrogen atoms.

Unveiling the origin of oxygen atomic impurities in Au nanowires

The appearance of unusually large Au-Au bond distances in linear atomic chains (LACs) of Au nanowires is commonly attributed to the presence of atomic impurities. However, the origin of those contaminants is unknown. We present a study based on density functional theory calculations using quasistatic ($T=0$) and finite-temperature ab initio molecular-dynamics simulations of a possible route for the formation of atomic impurities in Au nanowires. This process starts with the adsorption of an O${}_{2}$ molecule followed by a CO molecule on Au LACs, leading to the formation of an intermediate O${}_{2}$CO complex. Upon thermal activation at finite temperatures, the complex is able to proceed to oxidation forming a CO${}_{2}$ molecule and leaving an atomic O impurity in the Au LAC.

Effect of alloying of magnetic and non-magnetic low reactivity atoms into atomic chain

We have theoretically studied the possible formation of evenly mixed Pt–X, Au–X, Pd–X bimetallic atomic chains (ACs) with X = Co, Fe, and Ni. The results show that Pt–Fe ACs are the most energetically favorable. First principles calculations revealed that the energy of alloy formation is dependent on the d-band filling of the magnetic component (Fe, Co, and Ni). Thus, we found that formation of stable evenly mixed ACs with Ni atoms as the magnetic component is not possible. Moreover, we found that the alloying energy is dependent on the geometry of the AC. We found that the energy of alloy formation remains unchanged in linear ACs and drastically decreases by ∼1 eV under chain contraction, accompanied by a transition of the AC from a linear to a zigzag configuration. Our electronic structure calculations revealed the emergence of a ferromagnetic transition under stretching of the AC in all the bimetallic chains with Fe atoms as the magnetic component (Pt–Fe, Pd–Fe, and Au–Fe) from the ferromagnetic to the antiferromagnetic state.

High-pressure close-packed structure of boron

Based on ab initio evolutionary algorithms, a high-pressure close-packed phase of boron with hexagonal P63/mcm symmetry is predicted, named as B10, which is stable over α-Ga phase above 375 GPa to at least 500 GPa. High pressure makes the typical B12 icosahedron collapse to form an incompressible linear atomic chain arrangement together with an isosceles triangle arrangement. The electron localization function calculations confirm that the B10 has strong covalency in this special atomic arranged structure. The vibration of the three atom's isosceles triangle in the framework of linear atomic chains induces an unusual superconductivity in B10. Electron–phonon calculations indicate that electron–phonon coupling parameter λ is 0.82 and the superconducting critical temperature is 44 K at 400 GPa.

Mechanical Properties of Graphene Nanowiggles

ABSTRACTIn this work we have investigated the mechanical properties and fracture patterns of some graphene nanowiggles (GNWs). Graphene nanoribbons are finite graphene segments with a large aspect ratio, while GNWs are nonaligned periodic repetitions of graphene nanoribbons. We have carried out fully atomistic molecular dynamics simulations using a reactive force field (ReaxFF), as implemented in the LAMPPS (Large-scale Atomic/Molecular Massively Parallel Simulator) code. Our results showed that the GNW fracture patterns are strongly dependent on the nanoribbon topology and present an interesting behavior, since some narrow sheets have larger ultimate failure strain values. This can be explained by the fact that narrow nanoribbons have more angular freedom when compared to wider ones, which can create a more efficient way to accumulate and to dissipate strain/stress. We have also observed the formation of linear atomic chains (LACs) and some structural defect reconstructions during the material rupture. The reported graphene failure patterns, where zigzag/armchair edge terminated graphene structures are fractured along armchair/zigzag lines, were not observed in the GNW analyzed cases.

Quantum conductance oscillation in linear monatomic silicon chains

The conductance of linear silicon atomic chains with n=1–8 atoms sandwiched between Au electrodes is investigated by using the density functional theory combined with non-equilibrium Green's function. The results show that the conductance oscillates with a period of two atoms as the number of atoms in the chain is varied. We optimize the geometric structure of nanoscale junctions in different distances, and obtain that the average bond-length of silicon atoms in each chain at equilibrium positions is 2.15±0.03Å. The oscillation of average Si–Si bond-length can explain the conductance oscillation from the geometric structure of atomic chains. We calculate the transmission spectrum of the chains in the equilibrium positions, and explain the conductance oscillation from the electronic structure. The transport channel is mainly contributed by px and py orbital electrons of silicon atoms. The even–odd oscillation is robust under external voltage up to 1.2V.

Gold Nanowires: A Time-Dependent Density Functional Assessment of Plasmonic Behavior

The surface plasmon resonance has been theoretically investigated in gold nanowires by means of time-dependent density functional theory. Linear chains of Au atoms and nanowires with the structure of the fcc bulk gold grown along the ⟨110⟩ and ⟨111⟩ directions have been considered. The effects of changing the length and the section on the plasmon have been studied. Strong photoabsorption is found when the length is above 2 nm: in that case the absorption profile is characterized by a sharp peak, and its analysis reveals that many configurations contribute to the transition, confirming its collective nature as an s ← s intraband transition. As expected, the effect of increasing the length is reflected in a red shift of the plasmon.

Enhanced many‐body effects in one‐dimensional linear atomic chains

AbstractWe present the quasiparticle energy and optical absorption spectrum results of 1D linear H, BN, C, and Au chains, which are the lower size limit of a periodic material, by using ab initio many‐body approaches. Unprecedentedly large quasiparticle corrections and excitonic effects are revealed in these extreme systems compared with quasi‐1D, 2D, and 3D periodic systems. The binding energy of the bound exciton is up to 3.55 eV in the semiconducting BN chain and 0.5 eV in the metallic C chain, the latter of which is the largest in a metallic system.

Plasmon resonances and plasmon‐induced charge transport in linear atomic chains

In linear hydrogen atomic chains, plasmon resonances and plasmon‐induced charge transport are studied by time‐dependent density functional theory. For the large linear chain, it is a general phenomenon that, in the longitudinal excitation, there are high‐energy resonances and a large low‐energy resonance. The energy of the large low‐energy resonance conforms to the results calculated by the classical Drude model. In order to explain the formation mechanism of the high‐energy resonances, we present a simple harmonic oscillator model. This model may reasonably account for the relationship between low‐energy and high‐energy resonances, and has a certain degree of universality. As the interatomic distance decreases, the current shows a gradual transition from insulator to metal. The current enhancement mainly depends on the local field enhancement associated with plasmon excitation, and the enhanced electron delocalization effect as a result of the decrease of the interatomic distance. © 2013 Wiley Periodicals, Inc.

Anomalous phonon behavior of carbon nanotubes: First-order influence of external load

External loads typically have an indirect influence on phonon curves, i.e., they influence the phonon curves by changing the state about which linearization is performed. In this paper, we show that in nanotubes, the axial load has a direct first-order influence on the long-wavelength behavior of the transverse acoustic (TA) mode. In particular, when the tube is force-free, the TA mode frequencies vary quadratically with wave number and have curvature (second derivative) proportional to the square-root of the nanotube's bending stiffness. When the tube has non-zero external force, the TA mode frequencies vary linearly with wave number and have slope proportional to the square-root of the axial force. Therefore, the TA phonon curves—and associated transport properties—are not material properties but rather can be directly tuned by external loads. In addition, we show that the out-of-plane shear deformation does not contribute to this mode and the unusual properties of the TA mode are exclusively due to bending. Our calculations consist of 3 parts: First, we use a linear chain of atoms as an illustrative example that can be solved in close-form; second, we use our recently developed symmetry-adapted phonon analysis method to present direct numerical evidence; and finally, we present a simple mechanical model that captures the essential physics of the geometric nonlinearity in slender nanotubes that couples the axial load directly to the phonon curves. We also compute the density of states and show the significant effect of the external load.

Plasmons in Linear Nickel-Doped Au Atomic Chains

By using the time-dependent density functional theory (TDDFT) in the time domain, we study the photoabsorption spectra of the nickel-doped Au atomic chains with up to 22 atoms, which is now possible to fabricate experimentally. It is found that the plasmon excitations can be tuned by doping Ni atoms. When the two Ni atoms substitute symmetrically the Au atoms in the chain, the dipole strength of the different transverse plasmon modes will be selectively suppressed depending on the positions of the Ni atoms, while the longitudinal plasmon mode is scarcely affected in both energy and strength. This behavior of the doping Ni atom as a local plasmonic tuner may find its applications in nanoplasmonics.

Sign-changeable spin-filter efficiency in linear carbon atomic chain

It is well known that there is spin-filter efficiency (SFE) of a linear carbon atomic chain. In this article, we examine the quantum transport calculations of a linear carbon atomic chain connected to two half-planar graphene electrodes by using the first-principle method and reveal for the first time that sign of the SFE of such carbon atomic chain is changeable with the bias. This makes the carbon atomic chains attractive to potential application of spintronics.

Time-Dependent Density Functional Theory Studies of Plasmons in Parallel Double Sodium Atomic Chains

The photoabsorption spectra of linear double Na atomic chains with different lengths and inter-chain spacings are investigated using the time-dependent density functional theory (TDDFT) in the frequency domain. The formation and development of the collective resonances in the spectra are studied in terms of the spacing and length. With the decreasing spacing, the enhanced coupling between the chains lead to the emergence of extra peaks. For the well coupled double chains, the excitations show a single longitudinal mode but twin bimodal structure for the transverse modes. Our calculation reveals the formation and evolution of plasmon excitations in atomic nanostructures.

Optical properties of collective excitations for an atomic chain with vacancies

Resonant dipole-dipole interaction modifies the energy and decay rate of electronic excitations for a collection of ultracold atoms. Collective excitations of a finite linear atomic chain can be divided into dark and bright modes. Typically the superradiant mode with maximum effective collective dipole dominates resonant light scattering. The generic case of two chain segments of different length and position exhibits an interaction blockade and spatially structured light emission. Ultimately, an extended system of several interfering segments models a long chain with randomly distributed defects of vacant sites. The corresponding emission pattern provides a sensitive tool to study structural and dynamical properties of the system.

Plasmon resonances and electron transport in linear sodium atomic chains

The plasmon resonances and electron transport in linear sodium atomic chains are investigated using time-dependent density functional theory. The dipole response and dynamic conductance are obtained as a function of inter-atomic distance d and the chain length, under an impulse excitation polarized along the chains axis. We found that the current enhancement associated with the plasmon-induced field enhancement is mainly due to plasmon excitation of the hybridized bonding dipolar plasmons (BDP) mode. And the high energy plasmon modes are relatively weaker and are more delocalized in space than the BDP mode. The intensity of the dipole response and the current show an opposite tendency as the d is decreased while a same tendency is found as the number of atom increased. This can be understood from the competition between global quantization and local atomic confinement.

Time-dependent quantum transport: An efficient method based on Liouville-von-Neumann equation for single-electron density matrix

Basing on our hierarchical equations of motion for time-dependent quantum transport [X. Zheng, G. H. Chen, Y. Mo, S. K. Koo, H. Tian, C. Y. Yam, and Y. J. Yan, J. Chem. Phys. 133, 114101 (2010)], we develop an efficient and accurate numerical algorithm to solve the Liouville-von-Neumann equation. We solve the real-time evolution of the reduced single-electron density matrix at the tight-binding level. Calculations are carried out to simulate the transient current through a linear chain of atoms, with each represented by a single orbital. The self-energy matrix is expanded in terms of multiple Lorentzian functions, and the Fermi distribution function is evaluated via the Padè spectrum decomposition. This Lorentzian-Padè decomposition scheme is employed to simulate the transient current. With sufficient Lorentzian functions used to fit the self-energy matrices, we show that the lead spectral function and the dynamics response can be treated accurately. Compared to the conventional master equation approaches, our method is much more efficient as the computational time scales cubically with the system size and linearly with the simulation time. As a result, the simulations of the transient currents through systems containing up to one hundred of atoms have been carried out. As density functional theory is also an effective one-particle theory, the Lorentzian-Padè decomposition scheme developed here can be generalized for first-principles simulation of realistic systems.

TDDFT study on quantization behaviors of nonadiabatic couplings in polyatomic systems

AbstractWe present extensive calculations on the quantization behaviors of first‐order nonadiabatic couplings (NAC) in polyatomic systems, using the method based on time‐dependent density functional theory (TDDFT). Along circular contours around Jahn–Teller conical intersections at various symmetries, such as D4h and D6h, the angular NAC are found to show apparent dependence on the contour angle, no matter how close the conical intersections are approached. This is in contrast to that the angular NAC in triatomic Jahn–Teller systems (D3h) converge to the quantized value of 1/2. The oscillatory feature of our calculated curves qualitatively resembles the analytic result of angular NAC around elliptic conical intersections in the small‐displacement limit. Furthermore, by performing the integral along the whole contour, the quantization rule on the integral of the angular NAC holds for all cases, which gives the geometric phase with a value of π. On the other hand, the angular NAC in the immediate vicinity of Renner–Teller intersections are shown to be always 1. This observation can be understood from the fact that the molecular symmetry property of the Renner–Teller intersections is always determined by the linear atomic chain. Further analysis of our results on Renner–Teller systems shows that although the quantization behavior of angular NAC can determine all NAC vectors on each atom for the triatomic systems, it cannot for the systems with more than three atoms. The NAC vectors of the latter systems are found to show both common and distinct features with respect to the atomic number and species by our TDDFT calculations. © 2012 Wiley Periodicals, Inc.

Isoelectronic Doping of Graphdiyne with Boron and Nitrogen: Stable Configurations and Band Gap Modification

Graphdiyne, consisting of sp- and sp(2)-hybridized carbon atoms, is a new member of carbon allotropes which has a natural band gap ~1.0 eV. Here, we report our first-principles calculations on the stable configurations and electronic structures of graphdiyne doped with boron-nitrogen (BN) units. We show that BN unit prefers to replace the sp-hybridized carbon atoms in the chain at a low doping rate, forming linear BN atomic chains between carbon hexagons. At a high doping rate, BN units replace first the carbon atoms in the hexagons and then those in the chains. A comparison study indicates that these substitution reactions may be easier to occur than those on graphene which composes purely of sp(2)-hybridized carbon atoms. With the increase of BN component, the band gap increases first gradually and then abruptly, corresponding to the transition between the two substitution motifs. The direct-band gap feature is intact in these BN-doped graphdiyne regardless the doping rate. A simple tight-binding model is proposed to interpret the origin of the band gap opening behaviors. Such wide-range band gap modification in graphdiyne may find applications in nanoscaled electronic devices and solar cells.