Tight-binding Hamiltonian Model Research Articles

Adjustment of optical absorption in phosphorene through electron-phonon coupling and an electric field.

This study investigates the optical absorption of monolayer phosphorene, focusing on its response to the electron-phonon coupling (EPC) and an electric field. Using a tight-binding Hamiltonian model based on the Barišic-Labbe-Friedel-Su-Schrieffer-Heeger model and the Kubo formula, we calculate the electronic band structure and optical absorption characteristics. The anisotropic dispersion of carriers along armchair and zigzag directions leads to distinct optical responses. Positive and negative EPC effects increase and decrease hopping parameters, respectively, enlarging and reducing/closing the band gap. Moreover, both EPCs cause an admixture of blue and red shift spectrum along the armchair direction, while a red (blue) shift spectrum is observed for positive (negative) EPC along the zigzag direction. Incorporating electric field effects in the EPC increases band gaps for both positive and negative EPC activities, resulting in shifted optical peaks along both directions.

Atomistic designing of 2D quantum materials heterostructures CdF/CrI3 for Berry curvature driven tunable intrinsic anomalous Hall state

The coupling between topology and magnetism can explore rich physics with fundamental interest. Passing through the phase of Bismuth-based topological insulators magnetized by the 3d/4f transition metal doping, currently the fabrication of quantum heterostructures by suitable new-generation 2D materials, has emerged as a prospective alternative. Following the current trends, the present investigation deals with the atomistic designing and investigation of the quantum heterostructures of the newly predicted massive Dirac semimetal CdF and well-known layered ferromagnetic insulator CrI3 using the first-principles density functional theory calculations supplemented by the low energy tight-binding model Hamiltonian. The designed strategy ensures the lattice mismatch should be within the permissible range. We have addressed the physical characteristics of heterostructures in terms of the non-trivial topological band inversion between Cd-5s and I-2p orbitals. Proximity effect induces magnetic interactions, breaks the time-reversal symmetry at the interface, and leads to Berry curvature-driven tunable intrinsic anomalous Hall conductance (AHC) at the Fermi energy. Our analysis reveals the electrons with high Fermi velocity (106 m s−1) in the heterostructures and the band topology at the Fermi level can be tuned effectively using very small external gate voltage or homogeneous electric field. Our investigation can open up new avenues for designing new topological phases in the heterostructure community and possible tailoring routes of the intrinsic AHC in moderate temperature.

Thermodynamics Properties of (6, 6, 12)-Graphyne Structure Due to Biaxial Strain and Magnetic Field Effects

In this paper, we apply a tight binding Hamiltonian model in the presence of magnetic field for investigation of the electronic and transport properties of (6, 6, 12)-graphyne layer. We have also considered the effects of in-plane biaxial strain on the electronic behavior of (6, 6, 12)-graphyne layer. Moreover the impact of strains on magnetic susceptibility and specific heat of the structure has been studied. Specially, the temperature dependence of static thermal conductivity of (6, 6, 12)-graphyne layer has been studied due to magnetic field and strain effects. We have exploited the linear response theory and Green’s function approach to obtain the temperature behavior of thermal conductivity, electrical conductivity and Seebeck coefficient. Our numerical results indicate, thermal conductivity increases upon increasing the temperature in the low amounts region. This fact comes from the increasing of thermal energy of charge carriers and excitation of them to the conduction bands. The temperature dependence of Seebeck coefficient shows that the thermopower of undoped (6, 6, 12)-graphyne layer gets positive sign on the whole range of temperatures in the absence of strain effects. The effects of both electron doping and magnetic field factors on temperature behavior of electrical conductivity of (6, 6, 12)-graphyne have been investigated in details. Moreover the effects of biaxial strain on thermal conductivity of single layer (6, 6, 12)-graphyne have been addressed.

Topological properties of half Heusler compounds: Tuned by the spin-orbit coupling and p-d hybridization

The topological properties of Half Heusler compounds XYZ (X=Na, K; Y=Cu, Ag, Au; Z=S, Se, Te) are studied by the density functional theory (DFT) and tight-binding model Hamiltonian. The Y-s, Y-d and Z-p states form the bands around the Fermi level and determine their topological properties. When the energy of Y-s state is much larger than the other states, the compounds are normal insulators, just like NaCuS and NaCuS. On the contrary, when the Y-s state energy is only a little higher than the other states, the compounds are topological insulators (TI) or Dirac semi-metal (DSM) depended on the effective spin orbital coupling (SOC), which can be tuned by the relation between positive SOC Z-p and Y-d states. The compounds with negative effective SOC are TI such as NaAgS, NaAuS, while the compounds possessing positive SOC are DSM, just like NaAgTe and NaAuTe. Our simple tight-binding model can be used to design topological materials before the time consuming DFT calculations.

MAGNETIC FIELD-DEPENDENT ELECTRIC CURRENT ON A PERIODIC POLY(DA)-POLY(DT) DNA MOLECULE STRUCTURE

An electric current flowing on the Poly(dA)-poly(dT) DNA molecule structure has been calculated. The current was calculated from transmission probabilities by employing Landauer-Buttiker formalism. Green’s function technique has been used for obtaining the transmission probabilities. The DNA molecule structure was modeled within the tight-binding Hamiltonian model approach. The model takes into account electron hopping parameters which are dependent on the magnetic field as well as the electric field. The presence of a magnetic field causes the Peierls phase factor on the electron hopping parameters. The calculation results at low electric voltages show that after decreasing, the electric current oscillates with the magnetic field. However, at higher electric voltages, the electric current is increasing and oscillates with the magnetic field.

Tunnel magnetoresistance of trilayer graphene-based spin valve

Using the non-equilibrium Green’s function method and within the framework of the tight-binding Hamiltonian model in Landauer–Büttiker formalism, the spin-dependent transport and tunnel magnetoresistance ( T M R ) of AAA- and ABC-stacked trilayer zigzag graphene nanoribbon (TLG) connected to two ferromagnetic ( F M ) semi-infinite single-layer zigzag graphene nanoribbons (FM/TLG/FM system) have been theoretically investigated. Results show that the T M R for both AAA- and ABC-stacked cases can be increased about 100% because of the band-selective rule, and the magnitude of this 100% T M R region, enhances with the increase of the magnetization strength in FM graphene nanoribbon electrodes . Also, the TLG dimension has a prominent effect on the T M R ratio of the system. It is shown that the large values of the TMR ratio can be achieved very nicely by the variation of the width and length of the FM/TLG/FM system. The present theoretical results may be useful for designing few-layer graphene-based spin valves . • The TMR of trilayer graphene-based spin valve have been investigated. • The TMR for both AAA- and ABC-stacked cases can be increased about 100%. • The magnitude of 100% TMR region enhances with the magnetization strength. • The effects of width and length on the TMR ratio have been investigated.

Magnetic Weyl and quadratic nodal lines in inverse-Heusler-based fully compensated ferrimagnetic half-metals

Heusler alloys, a class of easily prepared, highly ordered intermetallic compounds, were first reported in 1903. Since then, Heusler alloys have presented various physical phenomena in modern condensed-matter physics. Among Heusler alloys, Heusler-based fully compensated ferrimagnetic half-metals (FCF-HMs) are, particularly, relevant because they host fully spin polarization and have no net magnetic moment, making them have no stray field and less affected by external magnetic fields. Based on first-principles calculations and a tight-binding Hamiltonian model, we provide new insight into inverse-Heusler-based (IHB) FCF-HMs and reveal that they exhibit spin-polarized Weyl and quadratic nodal lines as well as spin-polarized drumheadlike surface states. This paper presents the electron-filling-based design rule and material candidates for IHB FCF-HMs and suggests that IHB FCF-HMs are promising candidates for follow-up investigations in the field of topological spintronics. Subsequent experimental confirmation of the topological states in IHB FCF-HMs is imminent.

Tuning electronic phase in noncentrosymmetric quantum spin Hall insulators through physical stimuli

In this work, the perturbation-induced phase transitions in noncentrosymmetric quantum spin Hall insulators (QSHIs) are analytically addressed. In particular, the dilute charged impurity, the electric field, and the Zeeman splitting field are considered within the tight-binding Hamiltonian model, Green’s function approach, and the Born approximation. Following the C 3v symmetry breaking in the PbBiI compound as a representative QSHI, the band gap becomes larger via the electric field, while the system transits to the semimetallic phase via the dilute charged impurities and Zeeman field, modifying the degenerate states in the electronic density of states. While the coexistence of electric field and impurities demonstrate that the system backs to its initial semiconducting phase, the combined Zeeman field and impurities do not alter the robust semimetallic phase.

The Effect of Magnetic Field on Optical Conductivity of Hexagonal Boron-Nitride Sheet

The main purpose of this work is to investigate the effect of external magnetic field on the optical absorption of single-layer hexagonal boron nitride. The optical absorption of the system can be found by calculating the imaginary part of the dielectric function. The optical absorption is proportional to from the real part of the dynamical electrical conductivity function. Based on linear response theory, dynamical conductivity function can be calculated in terms of the current-current correlation function. In this study, the tight-binding model Hamiltonian has been applied to study the electronic band spectrum of the system. Also, the energy spectrum of electrons is calculated in the presence of a magnetic field along perpendicular to the plane. The Green’s function approach has been explored to obtain current-current correlation function and optical conductivity. The results show applying magnetic field has significant effects on dependence of optical absorption of boron nitride system on photon frequency and temperature.

Effect of the inversion symmetry breaking on the orbital Hall effect: A model study

The orbital Hall effect (OHE) is the transverse flow of orbital moment in a solid in response to an applied electric field, analogous to the flow of spin moment in the spin Hall effect (SHE). Although the effect has not been directly observed, there is ample indirect evidence for its existence in a number of experiments. Here, we show that the OHE is enhanced in solids with broken inversion symmetry, which may be more suitable to observe the effect. The mechanism of the OHE is fundamentally different in solids with inversion symmetry, where the orbital moment is quenched in the Brillouin zone (BZ), from that in a solid with broken inversion symmetry, where an intrinsic orbital moment is already present, the motion of which under the applied electric field could lead to a robust OHE. Using a tight-binding model Hamiltonian of a simple cubic lattice with two atoms in the unit cell, we study the effect of the inversion symmetry breaking on the OHE. We show that with the increase in the strength of the broken symmetry, the magnitude of the intrinsic orbital moment in the Brillouin zone increases. This, in turn, enhances the orbital Hall conductivity, in particular, the part that is directly proportional to the orbital moment in the BZ, which we call the ``noncentrosymmetric contribution.'' If the spin-orbit coupling is present, which couples the orbital and spin moments, the OHE leads to the SHE, which also becomes enhanced by the broken inversion symmetry. Our work has important implications for experimenters, suggesting that noncentrosymmetric solids may be more suitable for direct observation of the OHE.

Plasmonic excitations and charge structure factor in armchair boron nitride nanotubes

In this paper we investigated theoretically both dynamical and static charge susceptibilities of armchair Boron-Nitride nanotube(BNNT). Using the imaginary part of the dynamical charge susceptibility function, one can find the plasmonic modes of the system. For this purpose we implement Green's function approach in the context of the tight-binding model hamiltonian. According to linear response theory the charge response function will be obtained via calculating the correlation function of charge density-density operators. Also we present the static charge structure factor which can be a criterion for charge ordering of the system. Especially the effect of gap parameter and chemical potential on the plasmon modes and static charge structure factor of armchair BNNT are investigated. Our results show that the chemical potential changes the number of plasmon modes and their positions. Furthermore chemical potential and gap parameter affect on the static charge susceptibility and consequently charge ordering of the BNNT is changed. • The investigation of charge structure factors of armchair nanotubes. • The study of chemical potential and gap parameter on dynamical susceptibility of nanotube. • The investigation of static structure factor behavior as a function of temperature.

I-V characteristic of Poly(dA)-poly(dT) DNA molecule: the role of internal electric field

The effect of an internal electric field built up along the Poly(dA)-poly(dT) DNA molecule on the electron transport in the molecule has been studied. The electric field dependent electron hopping amplitude has been used in the tight-binding Hamiltonian model of a Poly(dA)-poly(dT) DNA molecule. Green’s function technique is used in calculating electron transmission probability, which is then used for computing current as a function of voltage by employing the Landauer-Büttiker formalism. It is observed that the internal electric field can cause the current decreases with voltages. The current reaches its maximum at a lower voltage as the internal electric field increases.

Electric field tuning of the properties of monolayer hexagonal boron phosphide

External factors are known to significantly modify the properties of low-dimensional materials. Here, we show that a perpendicular electric field can tune the fundamental electrical, thermal, and magnetical features of monolayer boron phosphide (MBP). We use the traditional Boltzmann approach, two-band tight-binding Hamiltonian model, and the Green’s function technique and particularly focus on the bandgap, electronic density of states, electronic heat capacity (EHC), and paramagnetic susceptibility (PMS) of MBP. We clarify the unique role of bias polarity on these quantities. The system undergoes a pseudo-Lifshitz phase transition when the electric field is equal to the pristine bandgap of MBP and accordingly, the massive Dirac fermions become massless. Additionally, the electron–hole symmetry breaking stemming from the variation of the Van Hove singularities with the electric field is discussed. We further show that the modulated electronic phase of the system leads to the tuning of EHC and PMS depending on the electric field direction. We believe that the present results are useful for various logic applications based on MBP.

Spin susceptibilities of doped bilayer graphene in the presence of magnetic ordering

We study the dynamical spin susceptibilities of biased bilayer graphene due to spin polarization effects in the context of tight binding model Hamiltonian. Green's function approach has been implemented to find the behavior of dynamical spin susceptibility of bilayer graphene within linear response theory. We have found the frequency dependence of imaginary part of spin susceptibilities for different values of bias voltage. Also the dependence of both transverse and longitudinal spin susceptibilities on spin polarization has been investigated in details. The frequency position of sharp peaks in the frequency dependence of the susceptibilities introduce the collective magnetic excitation modes of itinerant electrons on bilayer system.

Optical properties of biased bilayer graphene due to gap parameter effects

We address the optical conductivity of undoped bilayer graphene in the presence of a finite bias voltage at finite temperature. The effects of gap parameter and stacking type on optical conductivity are discussed in the context of tight binding model Hamiltonian. Green’s function approach has been implemented to find the behavior of optical conductivity of bilayer graphene within linear response theory. We have found the frequency dependence of optical conductivity for different values of gap parameter and bias voltage. Also the dependence of optical conductivity on the temperature has been investigated in details. A peak appears in the plot of optical conductivity versus frequency for different values of temperatures and bias voltage. Furthermore we find the frequency position of broad peak in optical conductivity goes to higher values with increase of gap parameter for both bernal and simple stacked bilayer graphenes.

Mechanically modulated spin-orbit couplings in oligopeptides

Recently experiments have shown very significant spin activity in biological molecules such as DNA, proteins, oligopeptides and aminoacids. Such molecules have in common their chiral structure, time reversal symmetry and the absence of magnetic exchange interactions. The spin activity is then assumed to be due to either the pure Spin-orbit (SO) interaction or SO coupled to the presence of strong local sources of electric fields. Here we derive an analytical tight-binding Hamiltonian model for Oligopeptides that contemplates both intrinsic SO and Rashba interaction induced by hydrogen bond. We use a lowest order perturbation theory band folding scheme and derive the reciprocal space intrinsic and Rashba type Hamiltonian terms to evaluate the spin activity of the oligopeptide and its dependence of molecule uniaxial deformations. SO strengths in the tens of meV are found and explicit spin active deformation potentials. We find a rich interplay between responses to deformations both to enhance and diminish SO strength that allow for experimental testing of the orbital model. Qualitative consistency with recent experiments shows the role of hydrogen bonding in spin activity.

Electronic properties of different configurations of double-strand DNA-Like nanowires

The band structure, density of states (DOS), and electrical conductivity (EC) of different configurations of a double-strand poly-GC-poly-AT DNA-like nanowire are investigated within the tight-binding Hamiltonian model and the Green's function formalism. Three different configurations of the double-strand DNA-like nanowire were considered: infinite, finite, and cyclic. The change of behavior in the band structure due to the introduction of dimerization effects of the longitudinal hopping terms was studied. Without dimerization, the DNA-like nanowire behaves as a conductor and with dimerization, a semiconductor behavior was observed. This behavior change was also verified through the calculation of the DOS via the Green's function formalism. The effects of the finite size of the DNA-like nanowire and also the effect of length increase on the EC of the system were studied using Kubo formula. There exists a direct relationship between EC and length of DNA-like nanowire. As length increases, so does the EC of the DNA-like nanowire. This information can be useful in both theoretical research and experimental development of DNA-based molecular electronics.

Topological band structure transitions and goniopolar transport in honeycomb antimonene as a function of buckling

The electronic band topology of monolayer $\beta$-Sb (antimonene) is studied from the flat honeycomb to the equilibrium buckled structure using first-principles calculations and analyzed using a tight-binding model and low energy Hamiltonians. In flat monolayer Sb, the Fermi level occurs near the intersection of two warped Dirac cones, one associated with the $p_z$-orbitals, and one with the $\{p_x,p_y\}$-orbitals. The differently oriented threefold warping of these two cones leads to an unusually shaped nodal line, which leads to anisotropic in-plane transport properties and goniopolarity. A slight buckling opens a gap along the nodal line except at six remaining Dirac points, protected by symmetry. Under increasing buckling, pairs of Dirac points of opposite winding number annihilate at a critical buckling angle. At a second critical angle, the remaining Dirac points disappear when the band structure becomes a trivial semiconductor. Spin-orbit coupling and edge states are discussed.

Thermoelectric properties through a wire composed of isoprene molecules

In this work, we study the electrical and thermoelectric properties through carbon bonds acting as nanowires derived from linear unsaturated organic molecules with a π conjugated system composed of isoprene molecules (NWIM) linked to leads. The study of electrical properties is conducted through the length of the NWIM and molecular couplings, and that of thermoelectric properties is conducted through a hemiterpenoid with a single isoprenic unit as the chemical scaffold. We approach the system by modeling it based on a tight-binding Hamiltonian model and solving it by using analytical means such as the renormalization process and Green’s functions. We obtain the transmission probability by utilizing the Fisher–Lee relationship. In the linear response approximation, by analyzing the electronic conductance (G), the thermal conductance (κ), the Seebeck coefficient (S), and the figure of merit (ZT), the molecular system clearly shows a behavior similar to that of a semiconductor material, obtaining a better thermoelectric performance with an asymmetric transmission probability at the edges of the band. Remarkably, by careful selection of the Fermi energy, the system plays an important role in the effectiveness of the ZT. These results offer a novel approach to molecular-based device designs, where the change in conductance due to the length effect in the NWIM can produce changes in the insulator–conductor states.

Multi-band Tight-Binding Model of MoS2 Monolayer

In this theoretical study, the band structure of MoS2 monolayer was initially numerically calculated using an 11-band tight-binding Hamiltonian model. Then, the contributions of d and p orbitals on the density of states (DOS), electronic heat capacity (EHC), and Pauli magnetic susceptibility (PMS) of the system were investigated based on the mentioned model and the Green’s function method. Also, the temperature dependence of the EHC and PMS is compared for the d and the p orbitals. An obvious gap was observed in the band structure and DOS of the MoS2 monolayer system, which is in good agreement with previous works. Moreover, due to the orbital overlap, Van Hove singularities appear. Then, as a result of this, a crossover occurs in the curves of PMS, which are divided into a low- and high-temperature region. Further, a Schottky anomaly is observed in the EHC curves. Overall, in this paper, we have introduced a method to investigate the electronic properties of $${{ {MoS}}}_{2}$$ monolayer that can be applied to other monolayer dichalcogenides.