Tight-binding Band Structure Research Articles

Non-magnetic layers with a single symmetry-protected Dirac cone: Which additional dispersions must appear?

In two-dimensional (2D), non-magnetic materials, a single Dirac cone at high-symmetry point (HSP) of the Brillouin zone (BZ), akin to the one in graphenes’ band structure, cannot appear as the only quasiparticle at the Fermi level. Here we found two layer groups with time-reversal symmetry, among all possible both without and with spin-orbit coupling, that host one Dirac cone at HSP and we show which additional dispersions appear: a pair of Dirac lines on opposite BZ edges and a pair of Dirac cones that can be moved but not removed by symmetry preserving perturbations, on the other two BZ edges. We illustrate our theory by a tight-binding band structure and discuss real 2D materials that belong to one of the two symmetry groups. Finally, we single out smaller or bigger discrepancies among the published papers on the same or related topic.

High Curie temperature Chern insulator and spin-gapless semiconducting ferromagnetic h-CrC monolayer: A first-principles study

High Curie temperature Chern insulator and spin-gapless semiconducting ferromagnetic h-CrC monolayer: A first-principles study

Propagating charge carrier plasmons in Sr2RuO4

We report on studies of charge carrier plasmon excitations in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ by transmission electron energy-loss spectroscopy. In particular, we present results on the plasmon dispersion and its width as a function of momentum transfer. The dispersion can be qualitatively explained in the framework of random phase approximation calculations, using an unrenormalized tight-binding band structure. The constant long-wavelength width of the plasmon indicates that it is caused by a decay into interband transitions and not by quantum critical fluctuations. The results from these studies on a prototypical correlated metal system show that the long-wavelength plasmon excitations near 1 eV are caused by resilient quasiparticles and are not influenced by correlation effects.

Modeling of a vertical tunneling transistor based on Gr-hBN- χ 3 borophene heterostructure

We present a computational study on the electrical behavior of the field-effect transistor based on vertical graphene-hBN-χ3 borophene heterostructure and vertical graphene nanoribbon-hBN-χ3 borophene nanoribbon heterostructure. We use nonequilibrium the Green function formalism along with an atomistic tight-binding (TB) model. The TB parameters are calculated by fitting tight-binding band structure and first-principle results. Also, electrical characteristics of the device, such as ION/IOFF ratio, subthreshold swing, and intrinsic gate-delay time, are investigated. We show that the increase of the hBN layer number decreases subthreshold swing and degrades the intrinsic gate-delay time. The device allows current modulation 177 at room temperature for a 1.2 V gate-source bias voltage.

Impact of passivation on the Dirac cones of 2D topological insulators

Topological insulators have unique properties that make them promising materials for future implementation in next-generation electronic devices. However, topological insulators like stanene nanoribbons need to be passivated before they can be used in devices. We calculate the electronic band structure of stanene nanoribbons (SNRs) that are passivated by hydrogen (H), fluorine (F), chlorine (Cl), bromine (Br), iodine (I), or sodium (Na). We show that the difference between the electronegativity of the passivation material and the tin atoms defines the position of the Dirac cone of the topological insulator edge states. We develop a four-parameter tight-binding model based on the Kane–Mele model [Kane and Mele, Phys. Rev. Lett. 95, 226801 (2005); Kane and Mele, Phys. Rev. Lett. 95, 146802 (2005)]. The hopping parameters of the TB model are obtained by fitting the tight-binding model to the density functional theory (DFT) calculations. Finally, we demonstrate that the DFT band structures and the tight-binding model band structures are in good agreement with each other at low energies around the Dirac point, thereby capturing the complete physics of the passivated edge bands.

Tight-binding band structure of β- and α-phase Ga2O3 and Al2O3

Rapid design and development of the emergent ultrawide-bandgap semiconductors Ga2O3 and Al2O3 require a compact model of their electronic structures, accurate over the broad energy range accessed in future high-field, high-frequency, and high-temperature electronics and visible and ultraviolet photonics. A minimal tight-binding model is developed to reproduce the first-principles electronic structures of the β- and α-phases of Ga2O3 and Al2O3 throughout their reciprocal spaces. Application of this model to α-Ga2O3/α-Al2O3 superlattices reveals that intersubband transitions can be engineered to the 1.55μm telecommunications wavelength, opening new directions in oxide photonics. Furthermore, by accurately reproducing the bandgap, orbital character, effective mass, and high-energy features of the conduction band, this compact model will assist in the investigation and design of the electrical and optical properties of bulk materials, devices, and quantum confined heterostructures.

An atomistic approach for the structural and electronic properties of twisted bilayer graphene-boron nitride heterostructures

Twisted bilayer graphene (TBG) has taken the spotlight in the condensed matter community since the discovery of correlated phases. In this work, we study heterostructures of TBG and hexagonal boron nitride (hBN) using an atomistic tight-binding model together with semi-classical molecular dynamics to consider relaxation effects. The hBN substrate has significant effects on the band structure of TBG even in the case where TBG and hBN are not aligned. Specifically, the substrate induces a large mass gap and strong pseudo-magnetic fields that break the layer degeneracy. Interestingly, such degeneracy can be recovered with a second hBN layer. Finally, we develop a continuum model that describes the tight-binding band structure. Our results show that a real-space tight-binding model in combination with semi-classical molecular dynamics is a powerful tool to study the electronic properties of moiré heterostructures, and to explain experimental results in which the effect of the substrate plays an important role.

Impact of stacking sequence on the tight-binding electronic band structures of (BeX)m/(ZnX)m, X = S, Se and Te superlattices

A semiempirical sp 3 s* tight-binding method is used to study the electronic band structures of bulk Be-, Zn- chalcogenides (BeX, ZnX; X= S, Se, Te) and their representative (BeX) m /(ZnX) m (0 0 1) superlattices (SLs). For binary BeX and ZnX materials, our simulations of the energy band dispersions, E j k → have offered correct band gaps in good agreement with the first-principles calculations. Based on the quantum confinement effects, the impact of stacking sequences m on the band structures of (BeX) m /(ZnX) m SLs are carefully examined (see: indirect band gap (BeS) 2 /(ZnS) 2 and direct band gap (BeTe) 5 /(ZnTe) 5 SLs). Our systematic study by considering m ≤ 10 has strongly indicated that controlling m in (BeX) m /(ZnX) m SLs is probably an effective way to achieve direct bandgaps for assessing their potential use in technologically important optoelectronic devices. • A semiempirical sp 3 s* tight-binding method is used to study the electronic band structures of bulk Be-, Zn- chalcogenides (BeX, ZnX; X = S, Se, Te) and their representative (BeX) m /(ZnX) m (0 0 1) superlattices (SLs). • The impact of stacking sequences m on the band structures of (BeX) m /(ZnX) m SLs are carefully examined by considering m ≤ 10. • The systematic study has strongly indicated that controlling m in (BeX) m /(ZnX) m n SLs is probably an effective way to achieve direct bandgaps. By using a semi-empirical tight-binding sp 3 s* method, the results of comprehensive electronic band structures are reported for the zinc-blende beryllium- and zinc-chalcogenides (BeX and ZnX; X = S, Se, Te) as well as their representative (BeX) m /(ZnX) m (0 0 1) superlattices (SLs). For the bulk BeX and ZnX materials, the simulations of energy band dispersions E j k → have offered the correct band gaps in very good agreement with the first-principles calculations. The band-mixing effect through the interfaces of two constituent compounds (BeX indirect- and ZnX direct-band gap) has played an important role for determining the overall band lineup in the (BeX) m /(ZnX) m SLs over the entire Brillouin zone. Based on the quantum confinement effects, the impact of stacking sequence m ≤ 10 is carefully examined for assessing the band structures of SLs. The results have clearly revealed that the nature of energy bandgaps is quite sensitive to the choice of well (BeX) and barrier (ZnX) layer thickness. Obviously, this intuition has implied that controlling m to achieve direct bandgaps in novel (BeX) m /(ZnX) m SLs is probably an effective way of assessing their potential use in technologically important optoelectronic devices.

Scattering problems via real-time wave packet scattering

In this paper, we use a straightforward numerical method to solve scattering models in one-dimensional lattices based on a tight-binding band structure. We do this by using the wave packet approach to scattering, which presents a more intuitive physical picture than the traditional plane wave approach. Moreover, a general matrix diagonalization method that is easily accessible to undergraduate students taking a first course in quantum mechanics is used. Beginning with a brief review of wave packet transport in the continuum limit, comparisons are made with its counterpart in a lattice. The numerical results obtained through the diagonalization method are then benchmarked against analytic results. The case of a resonant dimer is investigated in the lattice, and several resonant values of the mean wave packet momentum are identified. The transmission coefficients obtained for a plane wave incident on a step potential and rectangular barrier are compared by investigating an equivalent scenario in a lattice. Finally, we present several short simulations of the scattering process, which emphasize how a simple methodology can be used to visualize some remarkable phenomena.

Band engineering of non-hexagonal 2D tetragonal-silicene sheet and nanoribbons: A theoretical approach

Abstract We report combined first-principle and tight-binding (TB) calculation to investigate the mechanical and electronic properties of non-hexagonal two-dimensional (2D) tetragonal-silicene (TS) sheet and nanoribbons (NRs). The results obtained by the TB method are consistent with the density functional theory (DFT) results. The elastic constants of the TS sheet are comparable to silicene. The existence of two Dirac cones is observed at different k → points in the irreducible Brillouin Zone. A spin-orbit bandgap larger than silicene is observed in TS. The criteria for the stability and robustness of the Dirac Cones on the TB parameters are extensively investigated. Both the Dirac points are robust and stable under a wide range of hopping parameters. The degeneracy of the Dirac cones can be removed by introducing asymmetry in the on-site energy. Comparing the DFT and TB band structures, the values of the TB hopping parameters are estimated. The electronic properties of the TSNRs show a strong dependence on the width and the edge states. The symmetric armchair TSNRs show a width dependent multiple Dirac cones in the Brillouin zone. The number of Dirac cones of the NR is found to be dependent on the TB hopping parameters as well. On the other hand, asymmetric armchair TSNRs are semiconducting. The tuning of the bandgap for asymmetric armchair TSNRs by modulating the TB hopping parameters is also explored. Zigzag TSNR behaves as a degenerate semiconductor with the presence of Dirac cones just below and above E F . We subsequently expect that these theoretical findings will give a better comprehension of Dirac materials and their NRs with its potential application in nano-electronics.

Electronic structure calculations of twisted multi-layer graphene superlattices

Quantum confinement endows two-dimensional (2D) layered materials with exceptional physics and novel properties compared to their bulk counterparts. Although certain two- and few-layer configurations of graphene have been realized and studied, a systematic investigation of the properties of arbitrarily layered graphene assemblies is still lacking. We introduce theoretical concepts and methods for the processing of materials information, and as a case study, apply them to investigate the electronic structure of multi-layer graphene-based assemblies in a high-throughput fashion. We provide a critical discussion of patterns and trends in tight binding band structures and we identify specific layered assemblies using low-dispersion electronic bands as indicators of potentially interesting physics like strongly correlated behavior. A combination of data-driven models for visualization and prediction is used to intelligently explore the materials space. This work more generally aims to increase confidence in the combined use of physics-based and data-driven modeling for the systematic refinement of knowledge about 2D layered materials, with implications for the development of novel quantum devices.

Electron correlation induced orbital selective Lifshitz transition in new hybrid 12442 iron based superconductors

Abstract A systematic detailed electronic structure study of the newly discovered ACa2Fe4As4F2 i.e., 12442 iron based compounds (where A = K, Rb, Cs) within density functional theory (DFT) based first principles calculations is presented. The orbital projected band structures reveal mixed multi-orbital multi-band nature of the 12442 compounds having contributions mostly from Fe-3d orbitals. Unlike other hybrid iron based superconductors, contribution of As-4 p z orbital is negligible in these compounds. Substitution of alkali atoms of gradually higher atomic radius exert chemical pressure inside the compound and induce orbital selective evolution of band structure as well as density of states. Orbital projected Fermi surfaces divulge orbital selective modification in intra orbital, inter-band nesting condition of the d x 2 - y 2 / z 2 and d xz / yz orbital derived FSs. Correlation induced orbital selective Lifshitz transition is revealed through GGA+U calculations. The tight binding band structure fitted with low energy DFT bands using maximally localized Wannier functions divulge a unique feature of the nearest neighbor hopping parameter. In contrast to many iron based compounds, intra-orbital hopping in d z 2 is larger than intra-orbital hopping in d x 2 - y 2 . Orbital selective decrease in intra-orbital hopping of d xy and d yz due to change in chemical pressure induced by substitution of alkali atoms Rb or Cs is revealed in these compounds.

Combining Chirality and Hydrogen Bonding in Methylated Ethylenedithio-Tetrathiafulvalene Primary Diamide Precursors and Radical Cation Salts

Methyl-and dimethyl-ethylenedithio-tetrathiafulvalene ortho-diamide donors Me-EDT-TTF(CONH2)2 (1a) and DM-EDT-TTF(CONH2)2 (1b) have been prepared by the direct reaction of the corresponding diester precursors with aqueous ammonia solutions. The neutral (rac)-1a, (S)-1a and (S,S)-1b donors have been characterized by single crystal X-ray diffraction. In the three compounds, which crystallized in the non-centrosymmetric monoclinic space-group P21, the amide groups are disordered, yet they form the classical intramolecular hydrogen bond for such ortho-diamide motif. Electrocrystallization experiments afforded the mixed valence radical cation salts [(S,S)-1b]2XO4 and [(R,R)-1b]2XO4 (X = Cl, Re) containing four independent donors in the asymmetric unit, with the positive charge localized essentially on two donors while the two others are neutral. The topology of the organic layer is of '-type. Single crystal resistivity measurements show semiconducting behavior for [(S,S)-1b]2ClO4 and [(R,R)-1b]2ReO4, with a room temperature conductivity of 5 10-5 S cm-1 and activation energies Ea ≈ 3000 K. Tight-binding band structure calculations of extended Huckel type in combination with DFT calculations are in agreement with the semiconducting behavior and suggest a localized Mott type semiconductor rather than a band gap semiconductor.

Wannier pairs in superconducting twisted bilayer graphene and related systems

Unconventional superconductivity often arises from Cooper pairing between neighboring atomic sites, stipulating a characteristic pairing symmetry in the reciprocal space. The twisted bilayer graphene (TBG) presents a new setting where superconductivity emerges on the flat bands whose Wannier wavefunctions spread over many graphene unit cells, forming the so-called Moir\'e pattern. To unravel how Wannier states form Cooper pairs, we study the interplay between electronic, structural, and pairing instabilities in TBG. For comparisons, we also study graphene on boron-nitride (GBN) possessing a different Moir\'e pattern, and single-layer graphene (SLG) without a Moir\'e pattern. For all cases, we compute the pairing eigenvalues and eigenfunctions by solving a linearized superconducting gap equation, where the spin-fluctuation mediated pairing potential is evaluated from materials specific tight-binding band structures. We find an extended $s$-wave as the leading pairing symmetry in TBG, in which the nearest-neighbor Wannier sites form Cooper pairs with same phase. In contrast, GBN assumes a $p+ip$-wave pairing between nearest-neighbor Wannier states with odd-parity phase, while SLG has the $d+id$-wave symmetry for inter-sublattice pairing with even-parity phase. Moreover, while $p+ip$, and $d+id$ pairings are chiral, and nodeless, but the extended $s$-wave channel possesses accidental {\it nodes}. The nodal pairing symmetry makes it easily distinguishable via power-law dependencies in thermodynamical entities, in addition to their direct visualization via spectroscopies.

Variability Predictions for the Next Technology Generations of n-type SixGe1-x Nanowire MOSFETs.

Using a state-of-the-art quantum transport simulator based on the effective mass approximation, we have thoroughly studied the impact of variability on channel gate-all-around nanowire metal-oxide-semiconductor field-effect transistors (NWFETs) associated with random discrete dopants, line edge roughness, and metal gate granularity. Performance predictions of NWFETs with different cross-sectional shapes such as square, circle, and ellipse are also investigated. For each NWFETs, the effective masses have carefully been extracted from tight-binding band structures. In total, we have generated 7200 transistor samples and performed approximately 10,000 quantum transport simulations. Our statistical analysis reveals that metal gate granularity is dominant among the variability sources considered in this work. Assuming the parameters of the variability sources are the same, we have found that there is no significant difference of variability between SiGe and Si channel NWFETs.

Tight-binding electronic band structure and surface states of Cu-chalcopyrite semiconductors

We report theoretical calculations of the electronic band structure and surface states of Cu-chalcopyrite semiconductors. The systems under consideration are CuGaS2, CuAlSe2, and CuGaS2 doped with Cr. The calculations are carried out using semi-empirical Tight-Binding formalism. By reproducing the band gap of these systems obtained by ab-initio calculations we report the Tight-Binding parameters required for the band structure calculations. We present bulk band structure calculations of the above mentioned semiconductors, and we analyze the (001) and (110) surface states of the corresponding semi-infinite semiconductors.

Tight-binding dispersion of the prismatic pentagonal lattice

Tight-binding Hamiltonian on the prismatic pentagonal lattice is exactly solved to obtain the analytic expressions of dispersion relations and eigenvectors. This lattice is made of prismatic pentagon which is different from Cairo pentagon. Six different dispersion relations and total density of states are obtained. Dispersion relations are symmetric about the zero energy at a particular point in the parameter space. Although a large gap is found for the Cairo pentagonal lattice, no gap as well as no Dirac cone is found to appear in the tight-binding band structure for this prismatic pentagonal lattice. Instead, a pair of van Hove singularities has been identified at two different energy values in the band structure.

Excitonic optical response of carbon chains confined in single-walled carbon nanotubes

It has been recently shown that long linear carbon chains (carbyne) can be formed inside multiwalled carbon nanotubes (CNTs). Encapsulation of carbyne inside the CNT affects the electronic structure of the chain by the long-range Coulomb interaction. This introduces an indirect band gap in the combined CNT-chain system and results in a change in the optical band gap. We study the excitonic optical response of the combined system using the Bethe-Salpeter and Wannier equations based on density functional theory and tight-binding band structures. The optical properties of isolated CNTs and chains are strongly affected by excitonic effects and the CNT-chain system follows a similar trend. The interaction between the CNT and chain results in new bright excitons as well as charge transfer excitons, where electrons are localized on the CNT and holes on the chain, yielding new dark excitons in the combined system.

Electronic transport properties and first-principles study of γ-graphyne, and γ-BN graphyne monolayers

Abstract The electron transport properties of an armchair γ-graphyne (γ-yne) nanoribbon and γ-BN graphyne (γ-BN-yne) nanoribbon embedded between two armchair γ-yne nanoribbons are systematically investigated by a tight-binding (TB) model. The parameters of tight biding hopping and on-site energies are obtained by comparing the tight binding band structure of γ-yne and γ-BN-yne monolayers with the first-principles calculations. We numerically compute the electronic transport properties in terms of transmission and current-voltage characteristic. It is shown that by increasing the length of γ-BN-yne nanoribbon transmission through the system decreases.

Transport in vertically stacked hetero-structures from 2D materials

In this work, the transport of tunnel field-effect transistor (TFET) based on vertically stacked hereto-structures from 2D transition metal dichalcogenide (TMD) materials is investigated by atomistic quantum transport simulations. WTe2-MoS2 combination was chosen due to the formation of a broken gap hetero-junction which is desirable for TFETs. There are two assumptions behind the MoS2-WTe2 hetero-junction tight binding (TB) model: 1) lattice registry. 2) The S − Te parameters being the average of the S − S and Te − Te parameters of bilayer MoS2 and WTe2. The computed TB bandstructure of the hetero-junction agrees well with the bandstructure obtained from density functional theory (DFT) in the energy range of interest for transport. NEGF (Non-Equilibrium Green’s Function) equations within the tight binding description is then utilized for device transfer characteristic calculation. Results show 1) energy filtering is the switching mechanism; 2) the length of the extension region is critical for device to turn off; 3) MoS2-WTe2 interlayer TFET can achieve a large on-current of 1000µA/µm with VDD = 0.3V, which suggests interlayer TFET can solve the low ON current problem of TFETs and can be a promising candidate for low power applications.