Effect Of Topological Defects Research Articles

Thermal Properties and Mass Spectra of Heavy Mesons in the Presence of a Point-Like Defect

In this research, the radial Schr¨odinger equation is solved analytically using the Nikiforov-Uvarov method with the Cornell potential. The energy spectrum and the corresponding wave function are obtained in close form. The effect of Topological Defect on the thermal properties and mass spectra of heavy mesons such as charmonium and bottomonium are studied with the obtained energy spectrum. It is found that the presence of the Topological Defect increases the mass spectra and moves the values close to the experimental data. Our results agreed with the experimental data and are seen to be improved when compared with other works.

Effects of Topological Defects and AB Fields on the Thermal Properties, Persistent Currents and Energy Spectra with an Exponential-Type Pseudoharmonic Potential

Effects of Topological Defects and AB Fields on the Thermal Properties, Persistent Currents and Energy Spectra with an Exponential-Type Pseudoharmonic Potential

THE EFFECT OF TOPOLOGICAL DEFECT ON THE MASS SPECTRA OF HEAVY AND HEAVY-LIGHT QUARKONIA

In this present study, the effect of Topological Defect on the mass spectra of heavy and heavy-light mesons such as charmonium, bottomonium, and charm-strange ,bottom-charm respectively are studied with the Hulthen plus Yukawa potential. The Schrödinger equation is solved analytically using the Nikiforov-Uvarov method. The approximate solutions of the energy spectrum and un-normalized wave function were obtained. We applied the present results to predict the mass spectra of heavy and heavy-light mesons in the presence and absence of a topological defect for different quantum states. We noticed that when the topological defect increases the mass spectra are shifted and move closer to the experimental data. However, when compared to the work of other researchers, the results established an improvement.

Non-locality effect of Majorana fermions via local entanglement entropy

In this paper, to characterize the effect of topological defects on the entanglement of topological states, we introduce the concept of local entanglement entropy. By using the spinless [Formula: see text] superconductor with quantized vortices, we numerically calculated the local entanglement entropy. In the topological superconducting (weak-pairing) phase, we found that the local entanglement entropy turns to 0.5 for a Majorana zero mode. For the system with two/four Majorana zero modes, by calculating local entanglement entropy, we found the non-locality effect of Majorana zero modes. In the future, we will use local entanglement entropy to characterize the defect effect on other many-body systems with long entanglement, for example, topological order and quantum spin liquid.

Effect of Topological Defects on the Elasticity of Near-Ideal Polymer Networks

AbstractIn recent years, new types of polymer gels have emerged, which have a well-controlled network structure and few topological defects. These so-called near-ideal polymer networks constitute a good model system to revisit the long-standing problem of structure–property relationships in polymer networks, as well as a promising platform for the development of polymer gels with outstanding mechanical properties. In this study, we investigate the relative contributions of network defects (dangling chains and second-order loops) on the stress–stretch response of near-ideal polymer networks using a computational discrete network model. We identify the average chain prestretch as a key parameter to capture the effect of network topology on the elastic modulus and maximum extensibility. Proper account of the chain prestretch further leads to scaling relations for the elastic properties in terms of topology parameters that differ from classical estimates of rubber elasticity theory. Stress–stretch curves calculated using the discrete network model are also compared to semi-analytical estimates.

Effect of Stone-Thrower-Walls Defect on Mechanical Properties of Bi-layer Graphene - A Molecular Dynamics Study

This paper mainly focusing on the effects of topological defects like Stone-Thrower-Wales (STW) defect on the mechanical properties of bilayer graphene. The effect of STW defects on crack propagation mechanism were analyzed using Molecular Dynamics (MD) simulation. The results obtained are plotted to visualize the failure characteristics and the maximum stress that the material can withstand before undergoing fracture. These data are then validated with literature and experimentally obtained data.

Study of Stone-wales Defect on Elastic Properties of Single-layer Graphene Sheets by an Atomistic based Finite Element Model

In this paper, an atomistic based finite element model is developed to investigate the influence of topological defects on mechanical properties of graphene. The general in-plane stiffness matrix of the hexagonal network structure of graphene is found. Effective elastic modulus of a carbon ring is determined from the equivalence of molecular potential energy related to stretch and angular deformation. A hexagonal carbon ring as a unit cell of graphene sheets is modeled by four-node elements and by applying three-node triangular elements, Stone-Wales (SW) defect as an important topological defect which leads to the formation of two heptagons and pentagons is modeled. In this method, both pristine structure of graphene and graphene with SW defect are considered and to get more real structure, an atomistic model of a small part of graphite sheet around the defect site, is modeled in Gaussian software and new arrangement around SW defect are obtained by minimizing its energy. Young’s modulus, shear modulus and Poisson’s ratio of the pristine single-layered graphene sheet (SLGS) and the effect of topological defects on the elastic properties of SLGS is examined. The numerical results from this new model show good agreement with data available in the literature.

Effect of topological defects on mechanical properties of graphene sheets: a molecular dynamics study

The mechanical properties of graphene may be strongly influenced by topological defects such as vacancies, Stone-Thrower-Wales (STW) defects. In this research article, the effects of Stone Thrower Wales (STW) defect on the mechanical properties such as intrinsic strength and fracture toughness of graphene sheet have been investigated by using molecular dynamics based simulations. The intrinsic properties of graphene sheet are strongly affected by the orientation of STW defects. For STW2 defected graphene sheet, the strength of the armchair direction is much higher than that of zigzag direction. While the scenario for STW1 defected graphene is opposite, the strength of the zigzag direction is much higher than that of armchair direction. Furthermore, we investigated that STW defects can be used for tailoring the fracture toughness of graphene sheet. Significant improvement in the fracture toughness of graphene in armchair direction was predicted, while STW defects were positioned in front of the crack-tip. Interaction of stress fields generated from crack tip as well as from STW defects helps in tailoring the fracture behavior of graphene.

Effect of topological defects on “nuclear pasta” observables

[Background] The "pasta" phase of nuclear matter may play an important role in the structure and evolution of neutron stars. Recent works suggest nuclear pasta has a high resistivity which could be explained by the presence of long lived topological defects. The defects act as impurities that decrease thermal and electrical conductivity of the pasta. [Purpose] To quantify how topological defects affect transport properties of nuclear pasta and estimate this effect using an impurity parameter $Q_{\text{imp}}$. [Methods] Contrast molecular dynamics simulations of up to 409\,600 nucleons arranged in parallel nuclear pasta slabs (perfect pasta) with simulations of pasta slabs connected by topological defects (impure pasta). From these simulations compare the viscosity and heat conductivity of perfect and impure pasta to obtain an effective impurity parameter $Q_{\text{imp}}$ due to the presence of defects. [Results] Both the viscosity and thermal conductivity calculated for both perfect and impure pasta are anisotropic, peaking along directions perpendicular to the slabs and reaching a minimum close to zero parallel to them. In our 409\,600 nucleon simulation topological defects connecting slabs of pasta reduce both the thermal conductivity and viscosity on average by about 37\%. We estimate an effective impurity parameter due to the defects of order $Q_{\text{imp}}\sim30$. [Conclusions] Topological defects in the pasta phase of nuclear matter have an effect similar to impurities in a crystal lattice. The irregularities introduced by the defects reduce the thermal and electrical conductivities and the viscosity of the system. This effect can be parameterized by a large impurity parameter $Q_{\text{imp}}\sim30$.

Effects of Topological Defects and Diatom Vacancies on Characteristic Vibration Modes and Raman Intensities of Zigzag Single-Walled Carbon Nanotubes

Defects are ubiquitous in carbon nanotubes (CNTs), despite their large formation energies, and have astounding effects on their physicochemical properties. In this study, we employ density-functional theory (DFT) calculations to study systematically the atomic structure, stability, and characteristic vibrations of pristine and defected zigzag CNTs, where the defects are of the form of Stone-Wales (SW) and diatom vacancies (DV). The DFT optimized structures and the phonon modes are subsequently used in conjunction with a semiempirical bond-polarization model to study the nonresonant Raman spectra. For each defect type, we find two CNT structures with defects parallel or oblique to the tube axis. For the SW defects, the two structures have similar formation energies, whereas for the DV defect, only defects parallel to the tube axis are likely to exist. The results show that the defects induce a blue shift in the radial breathing mode (RBM) of metallic CNTs, whereas this mode is not shifted for semiconducting CNTs. However, the RBM shift or its Raman profile is not sensitive to the defect type. The G-band showed more sensitivity to the defects in the form of a red/blue shift in the frequency, or a partial/complete defragmentation of the G bands.

Effect of topological defects and Coulomb charge on the low energy quantum dynamics of gapped graphene

We study the combined effect of a conical topological defect and a Coulomb charge impurity on the dynamics of Dirac fermions in gapped graphene. Beyond a certain strength of the Coulomb charge, quantum instability sets in, which demarcates the boundary between sub- and supercritical values of the charge. In the subcritical regime, for certain values of the system parameters, the allowed boundary conditions in a gapped graphene cone can be classified in terms of a single real parameter. We show that the observables such as local density of states, scattering phase shifts and the bound state spectra are sensitive to the value of this real parameter, which is interesting from an empirical point of view. For a supercritical Coulomb charge, we analyze the system with a regularized potential as well as with a zigzag boundary condition and find the effect of the sample topology on the observable features of the system.

Defect science and engineering of liquid crystals under geometrical frustration

Spontaneous symmetry breaking while preserving flow ability is a remarkable feature of nematic liquid crystals. When a nematic liquid crystal coexists with a solid, the surface field of the solid tends to anchor the director direction on the surface: anchoring effects. If geometrical frustration between nematic ordering and anchoring is strong enough, stable topological defects are formed. Defects in an ordered state are usually regarded as undesirable features. However, recent studies reveal that defects stabilized by a topological constraint from the solid surfaces are actually quite useful and open up novel possibilities for defect engineering of liquid crystals: self-organization of soft matter by defects, memory effects of topological origin, and control of flow of nematic liquid crystals and colloid motion by defects. For example, defect reconfiguration accompanying the change in the topology costs a very high energy far beyond the thermal energy, which overwhelms a typical energy scale in soft matter. This provides extreme stability for structures assembled by defects and information memorized in defect topology. Furthermore, effects of topological defects can easily be removed perfectly by a nematic-to-isotropic transition, which provides switchable functions. Defects also affect the motion of colloids immersed in a liquid crystal and flow behaviour of a liquid crystal. Here we review recent developments in science and engineering of topological defects in nematic liquid crystals, mainly based on our numerical simulation studies.

Diffusion of order reconstruction induced by 1/2 wedge disclination in a thin hybrid nematic liquid-crystal cell

Based on the Landau-de Gennes theory, the diffusion of order reconstruction induced by 1/2 wedge disclination in a thin hybrid cell is investigated by the relaxation iterative method. The core structure, the biaxial structure, and the diffusion of order reconstruction as the cell thickness decreases, are explored. The defect structure and the range of order reconstruction do not change when the cell thickness is larger than 15. As the thickness decreases from 15, the defect range broadens along the substrate direction, and the biaxial region as well as the range of order reconstruction also enlarges. When the thickness further decreases to below the critical value of 10, the biaxial region and the order reconstruction range merge into an entire cell, where the planar orientation is abruptly converted into the perpendicular one across the biaxial wall. The results obtained in this paper are important for further studying the regulating effect of topological defect on submicron colloidal particles in nematics.

Effect of topological defects on graphene geometry and stability.

The effect of two basic topological defects, mitosis and the Stone-Wales defect, is studied in the graphene structure. The topological rules of the curvatures due to the occurrence of the defects in different arrangements are determined. Despite the fact that the causes and the probability of these topological defects are not known today, this theoretical work studies the distortions caused by the defects geometry and stability of the graphene structure.

Effect of Topological Defects on Buckling Behavior of Single-walled Carbon Nanotube

Molecular dynamic simulation method has been employed to consider the critical buckling force, pressure, and strain of pristine and defected single-walled carbon nanotube (SWCNT) under axial compression. Effects of length, radius, chirality, Stone–Wales (SW) defect, and single vacancy (SV) defect on buckling behavior of SWCNTs have been studied. Obtained results indicate that axial stability of SWCNT reduces significantly due to topological defects. Critical buckling strain is more susceptible to defects than critical buckling force. Both SW and SV defects decrease the buckling mode of SWCNT. Comparative approach of this study leads to more reliable design of nanostructures.

Effects of topological defects and local curvature on the electronic properties of planar graphene

A formalism is proposed to study the electronic and transport properties of graphene sheets with corrugations as the one recently synthesized. The formalism is based on coupling the Dirac equation that models the low energy electronic excitations of clean flat graphene samples to a curved space. A cosmic string analogy allows to treat an arbitrary number of topological defects located at arbitrary positions on the graphene plane. The usual defects that will always be present in any graphene sample as pentagon–heptagon pairs and Stone–Wales defects are studied as an example. The local density of states around the defects acquires characteristic modulations that could be observed in scanning tunnel and transmission electron microscopy.

Effects of topological defects in the semiconductor carbon nanotube intramolecular junctions

Several intramolecular junctions (IMJs) connecting two semiconductor single-wall carbon nanotubes (SWNTs) have been realized by using the layer-divided technique and introducing the pentagon-heptagon topological defects. The atomic structure of each IMJ is optimized with a combination of density-functional theory (DFT) and the universal force field (UFF) method, based upon which a \(\pi \)-orbital tight-binding calculation is performed on its electronic properties. Obtained results indicate that different topological defects and their distributions on the interfaces of the IMJs have decisive effects on the electronic properties of the IMJs. The specific geometrical defects control the localized defect states chiefly, while the diameters of the SWNTs on both sides are also related to them. The influence on the experimental observation brought by the choice of the scanning line is also presented by comparing the scanning results performed on the defect side with those on the defect-free side. A new IMJ structure has been found, and it probably reflects the real atomic structures of the semiconductor-semiconductor (S-S) IMJ [Phys. Rev. Lett. 90, 216107 (2003)].

The effect of topological defects and oxygen adsorption on the electronic transport properties of single-walled carbon-nanotubes

Ab initio density functional theory (DFT) calculations of the interactions between isolated infinitely-long semiconducting zig-zag (10, 0) or isolated infinitely-long metallic arm-chair (5, 5) single-walled carbon-nanotubes (SWCNTs) and single oxygen-molecules are carried out in order to determine the character of molecular-oxygen adsorption and its effect on electronic transport properties of these SWCNTs. A Green’s function method combined with a nearest-neighbor tight-binding Hamiltonian in a non-orthogonal basis is used to compute the electrical conductance of SWCNTs and its dependence on the presence of topological defects in SWCNTs and of molecular-oxygen adsorbates. The computational results obtained show that in both semiconducting and metallic SWCNTs, oxygen-molecules are physisorbed to the defect-free nanotube walls, but when such walls contain topological defects, oxygen-molecules become strongly chemisorbed. In semiconducting (10, 0) SWCNTs, physisorbed O 2-molecules are found to significantly increase electrical conductance while the effect of 7-5-5-7 defects is practically annulled by chemisorbed O 2-molecules. In metallic (5, 5) SWCNTs, both O 2 adsorbates and 7-5-5-7 defects are found to have a relatively small effect on electrical conductance of these nanotubes.

Effect of topological defects in graphite

The simplest topological defect in a graphite sheet can be introduced by switching one bond with the movement of two carbon atoms, which transforms four hexagons to two pentagons and two heptagons. Putting the topological defects periodically in a graphite sheet, we study how the electronic state changes based on the tight binding model. Depending on the combination of geometrical parameters which specify the arrangement of the defects, we find that the electronic states can be classified into the zero-gap semiconducting, semimetallic and metallic cases. The localized charge density states at the defect sites emerge just above the Fermi level. Curious states having a flat band at the Fermi level appear in some particular geometrical arrangements of the defects. In addition, we propose to design a series of graphitic polymers with a flat lowest-unoccupied band.

Effect of topological defects and site impurities in conjugated polymer chains

The effect of branch points and site impurities on a conjugated trans-polyacetylene chain is studied using minimum energy calculations performed directly on the Su-Schrieffer-Heeger Hamiltonian. New mid-gap states found in the case of branched molecules are described. The evolution of nonlinear excitations as a site potential is turned on is investigated.