Tight-binding Results Research Articles

Multiple Valley Modulations in Noncollinear Antiferromagnets.

Two-dimensional valleys and magnetism are rising areas with intriguing properties and practical uses in advanced information technology. By coupling valleys to collinear magnetism, valley degeneracy is lifted in a large number of magnetic valley materials to exploit the valley degree of freedom. Beyond collinear magnetism, new coupling modes between valley and magnetism are few but highly desirable. By tight-binding calculations of a breathing Kagome lattice, we demonstrate a tunable valley structure and valley-contrasting physical properties in noncollinear antiferromagnets. Distinct from collinear magnetism, noncollinear antiferromagnetic order enables valley splittings even without spin-orbit coupling. Both the canting and azimuthal angles of magnetic moments can be used as experimentally accessible knobs to tune valley splittings. Our first-principles calculations of the Fe3C6O6-silicene-Fe3C6O6 heterostructure also exhibit tunable valley splittings in noncollinear antiferromagnetism, agreeing with our tight-binding results. Our work paves avenues for designing novel magnetic valley materials and energy-efficient valleytronic devices based on noncollinear magnetism.

Relation of alkyl chain length and corrosion inhibition efficiency of N-Acylated Chitosans over mild steel in acidic medium

The present investigation is focussed on unveiling the potential of ecologically safe Chitosan derived N-Acylated products (N-ACs) as anti-corrosive agents for mild steel in 0.5 M H2SO4. Electrochemical Impedance Spectroscopy (EIS) including potentiodynamic polarization (PDP) and potentiostatic electrochemical impedance were deployed to evaluate their anti-corrosive performance and the results of the same reported N-acylated chitosan derivatives project remarkable inhibition efficiency with the most effective performance of 96.80% at 250 mgL-1. The pronounced inhibition efficiency reported was conclusively accredited to the availability of heteroatoms in modified chitosan which further aid in the formation of a protective barrier over mild steel counterparts. The final acylated products were reported to be mixed type as demarcated by PDP results. Further, the prevention ability of modified chitosan was obtained via adsorption and the Langmuir adsorption model was best seen as suitable in this regard. Surface studies and theoretical modeling including Global Reactivity results, molecular Dynamic Modelling (MD), and Density Functional Bond Tight Binding Results (DFBT) were incorporated to gain insights into the molecular level of interactions between metal and N-ACs.

Moiré-modulated band gap and van Hove singularities in twisted bilayer germanene

Twisting bilayers of two-dimensional topological insulators has the potential to create unique quantum states of matter. Here, we successfully synthesized a twisted bilayer of germanene on Ge2Pt(101) with a 21.8° twist angle, corresponding to a commensurate (√7×√7) structure. Using scanning tunneling microscopy and spectroscopy, we unraveled the structural and electronic properties of this configuration, revealing a moiré-modulated band gap and a well-defined edge state. This band gap opens at AB/BA stacked sites and closes at AA stacked sites, a phenomenon attributed to the electric field induced by the scanning tunneling microscopy tip. Our study further revealed two van Hove singularities at −0.8 eV and +1.04 eV, resulting in a Fermi velocity of (8 ± 1) × 105 m s−1. Our tight-binding results uncover a unique quantum state, where the topological properties could be regulated through an electric field, potentially triggering two topological phase transitions.

Atomistic Hartree theory of twisted double bilayer graphene near the magic angle

Twisted double bilayer graphene (tDBLG) is a moiré material that has recently generated significant interest because of the observation of correlated phases near the magic angle. We carry out atomistic Hartree theory calculations to study the role of electron–electron interactions in the normal state of tDBLG. In contrast to twisted bilayer graphene, we find that such interactions do not result in significant doping-dependent deformations of the electronic band structure of tDBLG. However, interactions play an important role for the electronic structure in the presence of a perpendicular electric field as they screen the external field. Finally, we analyze the contribution of the Hartree potential to the crystal field, i.e. the on-site energy difference between the inner and outer layers. We find that the on-site energy obtained from Hartree theory has the same sign, but a smaller magnitude compared to previous studies in which the on-site energy was determined by fitting tight-binding results to ab initio density-functional theory (DFT) band structures. To understand this quantitative difference, we analyze the ab initio Kohn–Sham potential obtained from DFT and find that a subtle interplay of electron–electron and electron–ion interactions determines the magnitude of the on-site potential.

Tunable Electronic, Optical, and Thermal Properties of two- dimensional Germanene via an external electric field

In this paper, we present a tight-binding model based on DFT calculations for investigation the electronic and optical properties of monolayer Germanene. The thermal properties are investigated using Green function method. The required tight binding parameters including the onsite energies and third nearest neighbors hopping and overlap integrals are obtained based on our DFT calculations. Germanene is a semiconductor with zero band gap and linear band dispersion around the K point. The band gap opening occurs in the presence of bias voltage. The band gap is increased linearly with increase of the bias voltage strength. The tight binding results for position of the two first peaks in the optical Infrared region is same with the DFT results. By applying and increasing bias voltage, the dielectric function shows the blue shift by reduction the peak intensity in the energy range E < 1 eV. The thermal conductivity and heat capacity increase with increasing the temperature due to the increasing of thermal energy of charge carriers and excitation them to the conduction bands. The thermal properties of Germanene in the absence of bias U = 0 is larger than that U ≠ 0 and they decrease by further bias strength increasing, due to the increasing band gap with bias.

Bco-C24: A new 3D Dirac nodal line semi-metallic carbon honeycomb for high performance metal-ion battery anodes

Three-dimensional porous carbon materials play a significant role in gas separation and energy storage. However, their low performances in energy storage hamper their applications. Herein, we propose a new possible category of carbon honeycomb (CHC), i.e. CHCs with asymmetrical channels (denoted as asymmetrical CHCs), to increase their performance. As an example, by cross-linking graphene sheet with sp3–hybridized carbon chains, we prepare a new 3D porous asymmetrical CHC, i.e. bco-C24. It is dynamically, thermally, and mechanically stable. Our first-principles calculations and tight binding results demonstrate that it possesses a significant electronic band structure of Dirac nodal lines, which are ascribed to the px and py orbitals of α carbon atoms in the graphene ribbons that have both α and β carbon atoms. It provides high carrier mobility between 7.98 × 105 and 9.99 × 105 m/s. It has high theoretical capacities (743.8/495.9/991.8/743.8 mA h/g), low diffusion barriers (0.077/0.032/0.117/0.169 eV), low average voltages, and negligible volume changes for Na/K/Ca/Li-ion batteries. This study not only constructs a new concept of asymmetrical CHCs and provides a strategy of cross-linking graphene sheets to create its sample structures, but also suggests new carriers for ion storage in the superior next generation metal-ion battery anodes.

Importance of second-order deformation potentials in modeling of InAs/GaAs nanostructures

Accurate modeling of electronic properties of nanostructures is a challenging theoretical problem. Methods making use of continuous media approximation, such as k.p, sometimes struggle to reproduce results obtained with more accurate atomistic approaches. On the contrary, atomistic schemes generally come with a substantially larger cost of computation. Here, we bridge between these two approaches by taking 8-band k.p method augmented with non-linear strain terms fit to reproduce sp3d5s* tight-binding results. We illustrate this method on the example of electron and hole states confined in quantum wells and quantum dots of photonics applications relevant InAs/GaAs material system, and demonstrate a good agreement of a non-linear k.p scheme with empirical tight-binding method. We discuss limits of our procedure as well as provide non-linear 8-band k.p parameter sets for InAs and GaAs. Finally, we propose a parameterization for effective term used to improve the accuracy of the standard effective mass method.

Tight-binding model for electronic structure of hexagonal boron phosphide monolayer and bilayer

Graphene-like hexagonal boron phosphide with its moderate band gap and high carrier mobility is considered to be a high potential material for electronics and optoelectronics. In this work, the tight-binding Hamiltonian of hexagonal boron phosphide monolayer and bilayer with two stacking orders are derived in detail. Including up to fifth-nearest-neighbor in plane and next-nearest-neighbor interlayer hoppings, the tight-binding approximated band structure can well reproduce the first-principle calculations based on the screened Heyd–Scuseria–Ernzerhof hybrid functional level over the entire Brillouin zone. The band gap deviations for monolayer and bilayer between our tight-binding and first-principle results are only 2 meV. The low-energy effective Hamiltonian matrix and band structure are obtained by expanding the full band structure close to the K point. The results show that the iso-energetic lines of maximum valence band in the vicinity of K point undergo a pseudo-Lifshitz transition from h-BP monolayer to AB_B-P or AB_B-B bilayer. The mechanism of pseudo-Lifshitz transition can be attributed to two interlayer hoppings rather than one.

Optimizing the tight-binding parametrization of the quasi-one-dimensional superconductor K2Cr3As3

We study the tight-binding dispersion of the recently discovered superconductor K2Cr3As3, obtained from Wannier projection of Density Functional Theory (DFT) results. In order to establish quantitatively the actual degree of quasi-one-dimensionality of this compound, we analyze the electronic band structure for two reduced sets of hopping parameters: one restricted to the Cr-As tubes and another one retaining a minimal number of in-plane hoppings. The corresponding total and local density of states of the compound are also computed with the aim of assessing the tight-binding results with respect to the DFT ones. We find a quite good agreement with the DFT results for the more extended set of hopping parameters, especially for what concerns the orbitals that dominate at the Fermi level. Therefore, we conclude that one cannot avoid taking into account in-plane hoppings up to the next-nearest-neighbors cells even only to describe correctly the Fermi surface cuts and the populations along the kz direction. Such a choice of a minimal number of hopping parameters directly reflects in the possibility of correctly describing correlations and magnetic interactions.

Fragile aspects of topological transition in lossy and parity-time symmetric quantum walks

Quantum walks often provide telling insights about the structure of the system on which they are performed. In {mathscr{P}}{mathscr{T}}-symmetric and lossy dimer lattices, the topological properties of the band structure manifest themselves in the quantization of the mean displacement of such a walker. We investigate the fragile aspects of a topological transition in these two dimer models. We find that the transition is sensitive to the initial state of the walker on the Bloch sphere, and the resultant mean displacement has a robust topological component and a quasiclassical component. In {mathscr{P}}{mathscr{T}} symmetric dimer lattices, we also show that the transition is smeared by nonlinear effects that become important in the {mathscr{P}}{mathscr{T}}-symmetry broken region. By carrying out consistency checks via analytical calculations, tight-binding results, and beam-propagation-method simulations, we show that our predictions are easily testable in today’s experimental systems.

Passive parity-time-symmetry-breaking transitions without exceptional points in dissipative photonic systems [Invited

Over the past decade, parity-time ($\mathcal{PT}$)-symmetric Hamiltonians have been experimentally realized in classical, optical settings with balanced gain and loss, or in quantum systems with localized loss. In both realizations, the $\mathcal{PT}$-symmetry breaking transition occurs at the exceptional point of the non-Hermitian Hamiltonian, where its eigenvalues and the corresponding eigenvectors both coincide. Here, we show that in lossy systems, the $\mathcal{PT}$ transition is a phenomenon that broadly occurs without an attendant exceptional point, and is driven by the potential asymmetry between the neutral and the lossy regions. With experimentally realizable quantum models in mind, we investigate dimer and trimer waveguide configurations with one lossy waveguide. We validate the tight-binding model results by using the beam propagation method analysis. Our results pave a robust way toward studying the interplay between passive $\mathcal{PT}$ transitions and quantum effects in dissipative photonic configurations.

Kondo-Ising and tight-binding models for TmB4

In $TmB_4$, localized electrons with a large magnetic moment interact with metallic electrons in boron-derived bands. We examine the nature of $TmB_4$ using full-relativistic ab-initio density functional theory calculations, approximate tight-binding Hamiltonian results, and the development of an effective Kondo-Ising model for this system. Features of the Fermi surface relating to the anisotropic conduction of charge are discussed. The observed magnetic moment $\sim 6 \, \mu_B$ is argued to require a subtle crystal field effect in metallic systems, involving a flipped sign of the effective charges surrounding a Tm ion. The role of on-site quantum dynamics in the resulting Kondo-Ising type "impurity" model are highlighted. From this model, elimination of the conduction electrons will lead to spin-spin (RKKY-type) interaction of Ising character required to understand the observed fractional magnetization plateaus in $TmB_4$.

Spin textures on general surfaces of the correlated topological insulatorSmB6

Employing the $\mathbf{k}\cdot\mathbf{p}$ expansion for a family of tight-binding models for SmB$_6$, we analytically compute topological surface states on a generic $(lmn)$ surface. We show how the Dirac-cone spin structure depends on model ingredients and on the angle $\theta$ between the surface normal and the main crystal axes. We apply the general theory to $(001)$, $(110)$, $(111)$, and $(210)$ surfaces, for which we provide concrete predictions for the spin pattern of surface states which we also compare with tight-binding results. As shown in previous work, the spin pattern on a $(001)$ surface can be related to the value of mirror Chern numbers, and we explore the possibility of topological phase transitions between states with different mirror Chern numbers and the associated change of the spin structure of surface states. Such transitions may be accessed by varying either the hybridization term in the Hamiltonian or the crystal-field splitting of the low-energy $f$ multiplets, and we compute corresponding phase diagrams.

Membrane invagination induced by Shiga toxin B-subunit: from molecular structure to tube formation.

The bacterial Shiga toxin is composed of an enzymatically active A-subunit, and a receptor-binding homopentameric B-subunit (STxB) that mediates intracellular toxin trafficking. Upon STxB-mediated binding to the glycolipid globotriaosylceramide (Gb3) at the plasma membrane of target cells, Shiga toxin is internalized by clathrin-dependent and independent endocytosis. The formation of tubular membrane invaginations is an essential step in the clathrin-independent STxB uptake process. However, the mechanism by which STxB induces these invaginations has remained unclear. Using a combination of all-atom molecular dynamics and Monte Carlo simulations we show that the molecular architecture of STxB enables the following sequence of events: the Gb3 binding sites on STxB are arranged such that tight avidity-based binding results in a small increment of local curvature. Membrane-mediated clustering of several toxin molecules then creates a tubular membrane invagination that drives toxin entry into the cell. This mechanism requires: (1) a precise molecular architecture of the STxB binding sites; (2) a fluid bilayer in order for the tubular invagination to form. Although, STxB binding to the membrane requires specific interactions with Gb3 lipids, our study points to a generic molecular design principle for clathrin-independent endocytosis of nanoparticles.

Analytical Calculation of Energy levels of mono- and bilayer Graphene Quantum Dots Used as Light Absorber in Solar Cells

In this paper by solving Dirac equation, we present an analytical solution to calculate energy levels and wave functions of mono- and bilayer graphene quantum dots. By supposing circular quantum dots, we solve Dirac equation and obtain energy levels and band gap with relations in a new closed and practical form. The energy levels are correlated with a radial quantum number and radius of quantum dots. In addition to monolayer quantum dots, AA- and AB-stacked bilayer quantum dots are investigated and their energy levels and band gap are calculated as well. Also, we analyze the influence of the quantum dots size on their energy spectrum. It can be observed that the band gap decreases as quantum dots’ radius increases. On the other hand, increase in the band gap is more in AB-stacked bilayer quantum dots. Using the obtained relations, the band gap is obtained in each state. Comparing the energy spectra obtained from the tight-binding approximation with those of our obtained relations shows that the behavior of the energies as function of the dot size is qualitatively similar, but in some cases, quantitative differences can be seen. As quantum dots radius increases, the analytical results approach to the tight-binding method results.

Fine structure and real space analysis of neutral acceptor states in GaAs

The properties of neutral acceptor states in GaAs are re-examined in the frame of extended-basis tight-binding model. Spherical harmonics decomposition of microscopic local density of states (LDOS) allows for the direct analysis of the tight-binding results in terms of k · p approximation. Lifting of degeneracy by strain and electric field and their effect on LDOS are examined. The fine structure of magnetic impurity caused by exchange interaction of hole with impurity d-shell and its dependence on strain is studied. It is shown that exchange interaction makes the ground state more isotropic.

Tunable valley polarization by a gate voltage when an electron tunnels through multiple line defects in graphene

By means of an appropriate wave function connection condition, we study the electronic structure of a line defect superlattice of graphene with the Dirac equation method. We obtain the analytical dispersion relation, which can simulate well the tight-binding numerical result about the band structure of the superlattice. Then, we generalize this theoretical method to study the electronic transmission through a potential barrier where multiple line defects are periodically patterned. We find that there exists a critical incident angle which restricts the electronic transmission through multiple line defects within a specific incident angle range. The critical angle depends sensitively on the potential barrier height, which can be modulated by a gate voltage. As a result, non-trivial transmissions of K and K′ valley electrons are restricted, respectively, in two distinct ranges of the incident angle. Our theoretical result demonstrates that a gate voltage can act as a feasible measure to tune the valley polarization when electrons tunnel through multiple line defects.

Electronic and transport properties of T-graphene nanoribbon: Symmetry-dependent multiple Dirac points, negative differential resistance and linear current-bias characteristics

Based on the tight-binding method and density functional theory, band structures and transport properties of T-graphene nanoribbons are investigated. By constructing and solving the tight-binding Hamiltonian, we derived the analytic expressions of the linear dispersion relation and Fermi velocity of Dirac-like fermions for armchair T-graphene nanoribbons. Multiple Dirac points, which are triggered by the mirror symmetry of armchair T-graphene nanoribbons, are observed. The number and positions of multiple Dirac points can be well explained by our analytic expressions. Tight-binding results are confirmed by the results from density functional calculations. Moreover, armchair T-graphene nanoribbons exhibit negative differential resistance, whereas zigzag T-graphene nanoribbons have linear current-bias voltage characteristics near the Fermi level.

Molecular dynamics simulations of the amino acid-ZnO (10-10) interface: a comparison between density functional theory and density functional tight binding results.

We investigate the adsorption behavior of four different amino acids (glutamine, glutamate, serine, cysteine) on the zinc oxide (101̄0) surface, comparing the geometry and energy associated with a number of different adsorption configurations. In doing this, we highlight the benefits and limits of using density-functional tight-binding (DFTB) with respect to standard density functional theory (DFT). The DFTB method is found to reliably reproduce the DFT adsorption geometries. Analysis of the adsorption configurations emphasizes the fundamental role of the first hydration layer in mediating the interactions between the amino acids and the surface. Direct surface-molecule bonds are found to form predominantly via the carboxylate groups of the studied amino acids. No surface-mediated chemical reactions are observed, with the notable exception of a proton transfer from the thiol group of cysteine to a hydroxyl group of the surface hydration layer. The adsorption energies are found to be dominated both by the formation of direct or indirect surface-molecule hydrogen bonds, but also by the rearrangement of the hydrogen-bond network in surface proximity in a non-intuitive way. Energetic comparisons between DFTB and DFT are made difficult on one side by the long time necessary to achieve convergence of potential energy values in MD simulations and on the other side by the necessity of including higher-order corrections to DFTB to obtain a good description of the hydrogen bond energetics. Overall, our results suggest that DFTB is a good reference method to set the correct chemical states and the initial geometries of hybrid biomolecule/ZnO systems to be simulated with non-reactive force fields.

Modified Li chains as atomic switches

We present electronic structure and transport calculations for hydrogen and lithium chains, using density functional theory and scattering theory on the Green's function level, to systematically study impurity effects on the transmission coefficient. To this end we address various impurity configurations. Tight-binding results allow us to interpret our the findings. We analyze under which circumstances impurities lead to level splitting and/or can be used to switch between metallic and insulating states. We also address the effects of strongly electronegative impurities.