Magneto-electronic properties of the AA- and ABC-stacked graphites

In this study, we apply the tight-binding method to magneto-electronic properties of the AA- and ABC-stacked graphites, which are strongly dependent on the interlayer interactions, the magnetic field, and the stacking sequences. First of all, the interlayer interactions induce the significant changes in the energy dispersions, the band symmetry about the Fermi level, the overlap between valence and conduction bands, the band width, and the band-edge states or the symmetry points. Then, the magnetic field induces the Peierls phase in the Bloch functions and thus strongly affects the energy dispersions of the Landau Levels, the subband spacings, the energy width, and the special structures in density of states (DOS). Finally, the stacking sequences dominates over the low-energy band overlap and the anisotropy of energy bands. The effects mentioned above are exactly reflected in the density of state. Here, DOS exhibits the 3D, 2D, and 1D characteristics.     

The giant Stark effect in armchair-edge phosphorene nanoribbons under a transverse electric field

We study the variation of electronic properties for armchair-edge phosphorene nanoribbons (APNRs) modulated by a transverse electric field. Within the tight-binding model Hamiltonian, and by solving the differential Schrödinger equation, we find that a band gap closure appears at the critical field due to the giant Stark effect for an APNR. The gap closure has no field polarity, and the gap varies quadratically for small fields but becomes linear for larger ones. We attribute the giant Stark effect to the broken edge degeneracy, i.e., the charge redistributions of the conduction band minimum and valence band maximum states localized at opposite edges induced by the field. By combined with the Green's function approach, it is shown that in the presence of the critical field a gap of density of states (DOS) disappears and a high value DOS turns up at the energy position of the band gap closure. Finally, as the field increases, we find the band gap decreases more rapidly and the gap closure occurs at smaller fields for wider ribbons. Both the band gap and DOS variations with the field show an insulator-metal transition induced by a transverse electric field for the APNR. Our results show that wider APNRs are more appreciable to design field-effect transistors.     

Electronic properties of bearded graphene nanoribbons

We investigate the electronic properties of graphene nanoribbons with attachment of bearded bonds as a model of edge modification. The main effect of the addition of the beards is the appearance of additional energy subbands. The originally gapless armchair graphene nanoribbons become semiconducting. On the other hand, the originally semiconducting armchair graphene nanoribbons may or may not change to gapless systems depending on the width. With the inclusion of a transverse electric field, the band structures of bearded graphene nanoribbons are further altered. An electric field creates additional band-edge states, and changes the subband curvatures and spacings. Furthermore, the energy band symmetry about the chemical potential is lifted by the field. With varying width, the bandgap demonstrates a declining zigzag behavior, and touches the zero value regularly. Modifications in the electronic structure are reflected in the density of states. The numbers and energies of the density of state divergent peaks are found to be strongly dependent on the geometry and the electric field strength. The beard also causes electron transfer among different atoms, and alters the probability distributions. In addition, the electron transfers are modified by the electric field. Finally, the field introduces more zero values in the probability distributions, and removes their left–right symmetry.     

The effect of a transverse electric field on the electronic properties of an armchair carbon nanoscroll

In this work, we use the tight-binding model to study the low-energy electronic properties of carbon nanoscrolls subject to the influences of a transverse electric field. A carbon nanoscroll can be considered as an open-ended spirally wrapped graphene nanoribbon. The inter-wall interactions will alter the subband curvature, create additional band-edge states, modify the subband spacing or energy gap, and separate the partial flat bands. Furthermore, the energy band symmetry about the Fermi level is lifted by such interactions. The truncated Archimedean spiral ρ?=?r _a θ?+r is used to describe the spiral structures of carbon nanoscrolls. The energy gap is found to oscillate significantly with r, and exhibits complete energy gap modulations. With the inclusion of a transverse electric field, the band structures are further altered. Inter-wall hoppings will cause electron transfers between different atoms leading to distortions of the electron wavefunctions. The main features of the energy dispersions are directly reflected in the density of states. The numbers, heights, and energies of the density of states peaks are dependent on the electric field strength.     

Electronic properties of TiC(100) and polar TiC(111) surfaces

The energy bands of films of TiC have been calculated using the linear-combination-of-atomic-orbitals method with parameters obtained by a fit to the bulk band structure. The Madelung potentials and charge redistribution have been determined self-consistently. For the neutral TiC(100) surface, the density of states (DOS) is similar to that of the bulk. For the non-neutral Ti-covered TiC(111) surface, Ti 3d-derived surface states appear around the Fermi energy E_F. The long-range electric field produced by the polar surfaces is screened by the charge redistribution, and the polar surfaces are stabilized. Characteristic features of TiC(111) compared to other surfaces of TiC are attributed to the high surface DOS at E_F.     

Electro-optical properties of zigzag and armchair boron nitride nanotubes under a transverse electric field: Tight binding calculations

The electro-optical properties of zigzag and armchair BNNTs in a uniform transverse electric field are investigated within tight binding approximation. It is found that the electric field modifies the band structure and splits band degeneracy where these effects reflect in the DOS and JDOS spectra. A decrease in the band gap, as a function of the electric field, is observed. This gap reduction increases with the diameter and it is independent of chirality. An analytic function to estimate the electric field needed for band gap closing is proposed which is in good agreement with DFT results. In additional, we show that the larger diameter tubes are more sensitive than small ones. Number and position of peaks in DOS and JDOS spectra for armchair and zigzag tubes with similar radius are dependent on electric field strength.     

Electronic structure and optical properties of ordered compounds potassium tantalate and potassium niobate and their disordered alloys

The electronic energy band structure, site and angular momentum decomposed density of states (DOS) of cubic perovskite oxides KNbO₃ and KTaO₃ have been obtained from a first principles density functional based full potential linearized augmented plane wave (FLAPW) method within a generalized gradient approximation (GGA). The total DOS in valence region is compared with the experimental photo-emission spectra (PES). The calculated DOS is in good agreement with the experimental energy spectra and the features in the spectra are interpreted by comparison with the projected density of states (PDOS). The valence band PES is mainly composed of Nb-4d/Ta-5d and O 2p states in KNbO₃ and KTaO₃, respectively. Using the PDOS and the band structure we have analyzed the inter-band contribution to the optical properties of these materials. The real and imaginary parts of the dielectric function have been calculated and compared with experimental data. They are found to be in a reasonable agreement. The role of band structure on the optical properties have been discussed.     

Electric-field-tunable electronic properties of graphene quantum dots

The tight-binding method is employed to investigate the electronic properties of a square graphene quantum dot subject to an in-plane electric field (F). The electronic properties are strongly modified by tuning the field strength or altering the field direction. F will change state energies, alter energy gaps, and induce energy gap modulations. State energies show oscillatory behavior with the change of the field strength. The oscillating amplitude and period are further modulated by the change of the field direction. The field-orientation-dependent electronic properties originate in the geometrical anisotropy of the square graphene quantum dot. Moreover, the density of states (DOS), exhibiting many discrete peaks, directly reveals the characteristic of the electric-field-tunable electronic properties. The number and frequencies of DOS peaks are significantly dependent on the field strength and direction.     

碳掺杂硼氮纳米管电子场发射的第一性原理研究

对闭口硼氮纳米管（BNNT）顶层掺碳体系,运用第一性原理研究了电子场发射性能.结果表明,掺碳的BNNT体系电子结构变化显著;外电场愈强,体系态密度向低能端移动幅度愈大,且最高占据分子轨道（HOMO）/最低未占据分子轨道（LUMO）能隙愈小.体系态密度和局域态密度,HOMO和LUMO及其能隙分析一致表明,各种碳掺杂体系中C_eqBNNT的场发射性能最佳. 关键词： 硼氮纳米管 碳掺杂 第一性原理     

Electron transport in nanotube-ribbon hybrids

The electronic and transport properties of nanotube-ribbon hybrids subject to the influences of a transverse electric field are investigated theoretically. The energy dispersion relations are found to exhibit rich dependence on the nanotube-ribbon interactions, the field strength, and the geometry of the hybrids. The nanotube-ribbon coupling will modify the subband curvature, create additional band-edge states, and change the subband spacing or energy gap. The bandstructures are asymmetric and symmetric about the Fermi energy when the interactions are turned on and off, respectively. The inclusion of a transverse electric field will further alter the bandstructures and lift the degeneracy of the partial flat bands in hybrid (IV). The chemical-potential-dependent electrical and thermal conductance exhibit a stepwise increase behavior. Variations in the electronic structures with field strength will be reflected in the electrical and thermal conductance. Prominent peaks, as well as single-shoulder and multi-shoulder structures in the electrical and thermal conductance are predicted when varying the electric field strength and the nanotube location. The features of the conductance are found to be strongly dependent on the field strength, the geometry and the temperature.     

Hydrogen chemisorption on silicon (100) 2 × 1

Changes in the valence band density of states (DOS) of a (100) silicon surface that accompany he chemisorption of atomic hydrogen onto that surface are deduced from a study of the changes in the L_2,3VV Auger lineshape. Complementary changes in the conduction band DOS are inferred from changes in L_2,3VV-core-level characteristic loss spectra (CLS). The chemisorbed hydrogen layer is identified as the dihydride phase from low energy electron diffraction measurements. Upon hydrogen adsorption the DOS at the top of the valence band decreases and new energy levels associated with the Si-H bonds appear lower in the band. Assuming that the Auger signal from the hydrogen covered sample consists of a superposition of a signal from silicon atoms bonded to hydrogen in the dihydride layer and an elemental-Si signal from the substrate, a N(E) difference spectrum with features due only to the dihydride is obtained by subtracting the background corrected, loss deconvoluted L_2,3VV signal for a clean (100)Si surface rom the corresponding signal for the hydrogen covered surface. Comparisons of the energy position of the major peak in this difference spectrum with that of the main peak in a gas phase silane Si-L_2,3VV spectrum, and of the corresponding Auger energy calculated empirically, indicate a hole—hole interaction energy of ~8 eV for the two-hole final state in the gaseous system and zero for the dihydride surface system. Hydrogen induced changes in the conduction band DOS are less apparent than those of the valence band DOS with only the possibility of a decrease in the DOS at the bottom of the conduction band being inferred from the CLS measurements. Electron stimulated desorption of hydrogen from the dihydride layer is adduced from changes in the Auger lineshape under electron beam irradiation of the surface. Hydrogen induced changes in the near-elastic electron energy loss spectra (ELS) are also reported and compared with previously published ELS results.     

Valence band structure and density of states effective mass model of biaxial tensile strained silicon based on k·p theory

After constructing a stress and strain model, the valence bands of in-plane biaxial tensile strained Si is calculated by k·p method. In the paper we calculate the accurate anisotropy valance bands and the splitting energy between light and heavy hole bands. The results show that the valance bands are highly distorted, and the anisotropy is more obvious. To obtain the density of states (DOS) effective mass, which is a very important parameter for device modeling, a DOS effective mass model of biaxial tensile strained Si is constructed based on the valance band calculation. This model can be directly used in the device model of metal-oxide semiconductor field effect transistor (MOSFET). It also a provides valuable reference for biaxial tensile strained silicon MOSFET design.     

碳纳米锥电子场发射的第一性原理研究

运用第一性原理研究了不同锥角和结构的碳纳米锥 （CNC） 电子场发射性能.结果表明：随外电场 （E_add） 增强,CNC电子结构变化显著,费米能级 （E_f） 处态密度 （DOS） 明显增大;赝能隙减小;体系电荷移向尖端.DOS,HOMO/LUMO及Mulliken电荷分析表明：CNC的电子场发射性能除依赖于尖端结构外,很大程度上还取决于锥角大小,特别顶层6个原子的CNC₃和CNC₄场发射性能 关键词： 碳纳米锥 电子场发射 第一性原理     

N-M(Cd,Mg)共掺闭口氧化锌纳米管场发射第一性原理研究

采用基于密度泛函理论的第一性原理计算方法, 分别研究了N掺杂和N-M(Cd, Mg)共掺(9, 0)型闭口氧化锌纳米管(ZnONT)的几何结构和场发射性能.结果表明: N原子能够提高体系帽端结构的稳定性; 随外加电场增强, 体系的态密度向低能方向移动, 最高占据分子轨道(HOMO)-最低未占据分子轨道(LUMO)能隙及有效功函数变小, 电荷向帽端聚集程度愈高. 体系态密度/局域态密度, HOMO/LUMO, 能隙及Mulliken电荷分析一致表明, N-Cd共掺可提高ZnONT的场发射性能, N-Mg共掺反而抑制其电子发射. 关键词： 第一性原理 ZnO纳米管 场发射 共掺杂     

氮掺杂及水分子吸附碳纳米管电子场发射第一性原理研究

对闭口碳纳米管(CNT)顶端分层掺氮及吸附不同数目水分子体系,运用第一性原理研究了有电场存在时的电子场发射性能.结果表明：掺氮并吸附水分子的CNT结构稳定;外电场愈强、水分子数愈多,体系态密度(DOS)向低能端移动幅度愈大且最高分子占据轨道(HOMO)/最低分子空轨道(LUMO)能隙愈小.吸附能,DOS/LDOS,HOMO/LUMO及其能隙分析一致表明,第三层氮掺杂CNT吸附不同数目水分子体系的场发射性能最佳. 关键词： 氮掺杂 水吸附 密度泛函理论 电子场发射     

Density functional approach to study 5f electron behavior and electric field gradient in NpRh3

We have presented results on the electronic structure and the electric field gradient (EFG) of NpRh₃ within a framework of density functional theory within the local density approximation (LDA) and generalized gradient approximation (GGA) with and without spin-orbit coupling (SOC). The electronic density of states (DOS) shows that Np-f to Rh-d hybridization leads to a narrow band 5f-electron. Using the GGA and spinpolarized calculations, the calculated EFG shows that the dominant contribution to EFG comes from electrons with strong p-character.     

Controllable Absorption and Dispersion Properties of an RF-drivenFive-Level Atom in a Double-Band Photonic-Band-Gap Material

The probe absorption-dispersion spectra of a radio-frequency (RF)-driven five-level atom embedded in a photonic crystal are investigated by considering the isotropic double-band photonic-band-gap (PBG) reservoir. In the model used, the two transitions are, respectively, coupled by the upper and lower bandsin such a PBG material, thus leading to some curious phenomena. Numerical simulations are performed for the optical spectra. It is found that when one transition frequency is inside the band gap and the other is outside the gap, there emerge three peaks in the absorption spectra. However, for the case that two transition frequencies lie inside or outside the band gap, the spectra display four absorption profiles. Especially, there appear two sharp peaksin the spectra when both transition frequencies exist inside the band gap. The influences of the intensity and frequency of the RF-driven field on the absorptive and dispersive response are analyzed under different band-edge positions. It is found that a transparency window appears in the absorption spectra and is accompanied by a very steep variation of the dispersion profile by adjusting system parameters. These results show that the absorption-dispersion properties of the system depend strongly on the RF-induced quantum interference and the density of states (DOS) of the PBG reservoir.     

Effects of an electric field on the electronic and optical properties of zigzag boron nitride nanotubes

We have investigated the electro-optical properties of zigzag BNNTs, under an external electric field, using the tight binding approximation. It is found that an electric field modifies the band structure and splits the band degeneracy. Also the large electric strength leads to coupling the neighbor subbands which these effects reflect in the DOS and JDOS spectrum. It has been shown that, unlike CNTs, the band gap of BNNTs can be reduced linearly by applying a transverse external electric field. Also we show that the larger diameter tubes are more sensitive than small ones. The semiconducting metallic transition can be achieved through increasing the applied fields. The number and position of peaks in the JDOS spectrum are dependent on electric field strength. It is found that at a high electric field, the two lowest subbands are oscillatory with multiple nodes at the Fermi level.     

Destruction of Landau levels in asymmetric bilayer nanographene ribbons

Magneto-electronic properties of asymmetric bilayer nanographene ribbons are enriched by geometric structures, interlayer atomic interactions, magnetic quantization and finite-size confinement. There are drastic changes on the band symmetry, the degeneracy of the partial flat bands, the number of band-edge states, the energy dispersion, the carrier density, and the spatial symmetry of the wave function. Quasi-Landau levels might be converted into oscillating bands where extra band-edge states are created. When the upper ribbon is located at the ribbon centre, the Landau wave functions are completely destroyed. Meanwhile, a charge transfer between different layers or different sublattices in the same layer occurs. Furthermore, the density of states, reflecting the band structure, is also severely altered in terms of the number, structure, energy, and height of the prominent peaks.     

Screening influence on the Stark effect of impurity states in strained wurtzite GaN/AlxGa1-xN heterojunctions under pressure

The screening effect of the random-phase-approximation on the states of shallow donor impurities in free strained wurtzite GaN/Al x Ga 1 x N heterojunctions under hydrostatic pressure and an external electric field is investigated by using a variational method and a simplified coherent potential approximation.The variations of Stark energy shift with electric field,impurity position,Al component and areal electron density are discussed.Our results show that the screening dramatically reduces both the blue and red shifts as well as the binding energies of impurity states.For a given impurity position,the change in binding energy is more sensitive to the increase in hydrostatic pressure in the presence of the screening effect than that in the absence of the screening effect.The weakening of the blue and red shifts,induced by the screening effect,strengthens gradually with the increase of electric field.Furthermore,the screening effect weakens the mixture crystal effect,thereby influencing the Stark effect.The screening effect strengthens the influence of energy band bending on binding energy due to the areal electron density.