Ab Initio Band Calculations Research Articles

Fine details of sixfold Dirac fermions in pyrite-structured PdSb2

We report detailed angle-resolved photoemission measurements on the electronic structure of an unconventional multifold Dirac fermionic semimetal ${\mathrm{PdSb}}_{2}$ with a pyrite structure. By exploiting the photon energy and polarization dependence of the matrix element in photoemission intensity and by comparing photoemission data with ab initio band calculations, we experimentally identify the exact electronic structure, including the orbital characters of the electron pockets at the $R$ point in the Brillouin zone of ${\mathrm{PdSb}}_{2}$. Each electron pocket and hole-pocket-like structure consists of three doubly degenerate parabolic bands, respectively, which cross one another at the $R$ point, forming a sixfold Dirac fermion. The overall electronic structure is consistent with the band calculations, but the gap size between two sextuple points is very sensitive to the Wyckoff position of Sb atoms, which is a result of the competition between the band energies and crystal symmetries.

How water makes graphene metallic

A simple explanation is given on the tendency of graphene to become metallic when the amount of doped water is increased, an effect which was previously obtained from ab initio band calculations. It is clarified how the effect is mainly determined by oriented water electric dipoles, which create a step like potential at the separation distance between graphene and water planes. By using perturbation theory and an effective potential coupled with a image-charge tail potential, we showed that under increasing the water doping, the lowest energy free band in graphene starts lowering their energies by approaching to the Fermi level. Moreover, we demonstrated that this crossing induces a huge increase of states at the Fermi level, an effect akin to the magic-angle flat-band appearance in bilayer graphene.

Anisotropy of the Complex Permittivity of the Kagome-Staircase Compounds Co3V2O8 and Ni3V2O8: Experiment and Ab Initio Calculations

The anisotropy of the components of the complex permittivity of vanadate Co3V2O8 and Co3V2O8 single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c. The optical properties are also investigated along axis b. The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c. The maximum of the main interband absorption band of Co3V2O8 is shifted toward low energies by 0.25–0.3 eV as compared to Co3V2O8. The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.

Ground-state molecular and electronic structures of group-IV nanoribbons, nanorings and nanotubes.

Based on ab initio molecular orbital (MO) theory and first-principles band calculations, we systematically study the ground-state molecular and electronic structures of group-IV nanoribbons (NRBs), nanorings (NRGs) and nanotubes (NTBs) by substituting the honeycomb skeletal atoms with C, Si or Ge atoms. We then explore the energetics in the ground-state singlet-triplet (ST) crossover, particularly focusing on the configuration hybridization by electron correlation.

Are the triple surface plasmon resonances in Zn nanoparticles true?

It has been experimentally and numerically confirmed that zinc (Zn) nanoparticles (NPs) dispersed in silica exhibit two optical extinction peaks around ∼250 nm (1st peak) and ∼1050 nm (2nd peak), both of which were ascribed to surface plasmon resonances (SPRs) in the broad sense, i.e., the dual SPRs. Recently, Kuiri and Majhi (KM) observed the 3rd peak around ∼900 nm by calculations, and proposed the triple SPRs for Zn NPs without any experimental confirmation. This paper claims that the 3rd peak has never been observed in any experiments nor in any calculations except given by KM. They justified the triple resonances from an approximated SPR criterion, ε1Zn(ω) + 2ε1SiO2(ω) = 0, which is not valid for non-idealized metals like Zn, because the imaginary part of the dielectric function ε2Zn(ω) is not negligible. Instead, a rigorous SPR criterion predicts the dual resonances only. From comparisons with ab initio band calculations, the 1st and 2nd extinction peak are ascribed to resonantly enhanced inter-band transitions (so-called electronic resonance) and intra-band transitions (SPR in the narrow sense), respectively. Since either of the peaks arises from the resonant enhancement due to the dielectric function, both the peaks are regarded as SPRs in the broad sense, i.e. the dual SPRs.

Computational study of TiO2 Brookite (100), (010) and (210) surface doped with Ruthenium for application in Dye Sensitised Solar Cells

Titanium dioxide (TiO2) polymorphs are widely used in many energy-related applications due to their peculiar electronic and physicochemical properties. The electronic structures of brookite TiO2 surfaces doped with transition metal ruthenium have been investigated by ab initio band calculations based on the density functional theory with the planewave ultrasoft pseudopotential method. The generalized gradient approximation (GGA) was used in the scheme of Perdew-Burke-Ernzerhof (PBE) to describe the exchange-correlation functional. All calculations were carried out with CASTEP (Cambridge Sequential Total Energy Package) code in Materials Studio of Accelrys Inc. The surface structures of Ru doped TiO2 were constructed by cleaving the 1 × 1 × 1 optimized bulk structure of brookite TiO2. The results indicate that Ru doping can narrow the band gap of TiO2, leading to the improvement in the photoreactivity of TiO2, and simultaneously maintain strong redox potential. The theoretical calculations could provide meaningful guide to develop more active photocatalysts with visible light response.

Experimental Realization of Type-II Dirac Fermions in a PdTe_{2} Superconductor.

A Dirac fermion in a topological Dirac semimetal is a quadruple-degenerate quasiparticle state with a relativistic linear dispersion. Breaking either time-reversal or inversion symmetry turns this system into a Weyl semimetal that hosts double-degenerate Weyl fermion states with opposite chiralities. These two kinds of quasiparticles, although described by a relativistic Dirac equation, do not necessarily obey Lorentz invariance, allowing the existence of so-called type-II fermions. The recent theoretical discovery of type-II Weyl fermions evokes the prediction of type-II Dirac fermions in PtSe_{2}-type transition metal dichalcogenides, expecting experimental confirmation. Here, we report an experimental realization of type-II Dirac fermions in PdTe_{2} by angle-resolved photoemission spectroscopy combined with abinitio band calculations. Our experimental finding shows the first example that has both superconductivity and type-II Dirac fermions, which turns the topological material research into a new phase.

Schottky Barriers in Bilayer Phosphorene Transistors.

It is unreliable to evaluate the Schottky barrier height (SBH) in monolayer (ML) 2D material field effect transistors (FETs) with strongly interacted electrode from the work function approximation (WFA) because of existence of the Fermi-level pinning. Here, we report the first systematical study of bilayer (BL) phosphorene FETs in contact with a series of metals with a wide work function range (Al, Ag, Cu, Au, Cr, Ti, Ni, and Pd) by using both ab initio electronic band calculations and quantum transport simulation (QTS). Different from only one type of Schottky barrier (SB) identified in the ML phosphorene FETs, two types of SBs are identified in BL phosphorene FETs: the vertical SB between the metallized and the intact phosphorene layer, whose height is determined from the energy band analysis (EBA); the lateral SB between the metallized and the channel BL phosphorene, whose height is determined from the QTS. The vertical SBHs show a better consistency with the lateral SBHs of the ML phosphorene FETs from the QTS compared than that of the popular WFA. Therefore, we develop a better and more general method than the WFA to estimate the lateral SBHs of ML semiconductor transistors with strongly interacted electrodes based on the EBA for its BL counterpart. In terms of the QTS, n-type lateral Schottky contacts are formed between BL phosphorene and Cr, Al, and Cu electrodes with electron SBH of 0.27, 0.31, and 0.32 eV, respectively, while p-type lateral Schottky contacts are formed between BL phosphorene and Pd, Ti, Ni, Ag, and Au electrodes with hole SBH of 0.11, 0.18, 0.19, 0.20, and 0.21 eV, respectively. The theoretical polarity and SBHs are in good agreement with available experiments. Our study provides an insight into the BL phosphorene-metal interfaces that are crucial for designing the BL phosphorene device.

Electronic structure and magnetic properties of doped Al1–x Ti x N (x = 0.03, 0.25) compositions based on cubic aluminum nitride from ab initio simulation data

The phase stability, electronic structure, and magnetic properties of Al1–x Ti x N compositions based on the metastable aluminum nitride modification with the rock-salt structure at low (x = 0.03) and high (x = 0.25) concentrations of titanium in the system have been investigated using the results of ab initio band calculations. It has been shown that, at low values of x, the partial substitution is characterized by a positive enthalpy, which, however, changes sign with an increase in the titanium concentration. According to the results of the band structure calculations, the doped compositions have electronic conductivity. For x = 0.03, titanium impurity atoms have local magnetic moments (∼0.6 μB), and the electronic spectrum is characterized by a 100% spin polarization of near-Fermi states. Some of the specific features of the chemical bonding in Al1–x Ti x N cubic phases have been considered.

Early stage oxynitridation process of Si(001) surface by NO gas: Reactive molecular dynamics simulation study

Initial stage of oxynitridation process of Si substrate is of crucial importance in fabricating the ultrathin gate dielectric layer of high quality in advanced MOSFET devices. The oxynitridation reaction on a relaxed Si(001) surface is investigated via reactive molecular dynamics (MD) simulation. A total of 1120 events of a single nitric oxide (NO) molecule reaction at temperatures ranging from 300 to 1000 K are statistically analyzed. The observed reaction kinetics are consistent with the previous experimental or calculation results, which show the viability of the reactive MD technique to study the NO dissociation reaction on Si. We suggest the reaction pathway for NO dissociation that is characterized by the inter-dimer bridge of a NO molecule as the intermediate state prior to NO dissociation. Although the energy of the inter-dimer bridge is higher than that of the intra-dimer one, our suggestion is supported by the ab initio nudged elastic band calculations showing that the energy barrier for the inter-dimer bridge formation is much lower. The growth mechanism of an ultrathin Si oxynitride layer is also investigated via consecutive NO reactions simulation. The simulation reveals the mechanism of self-limiting reaction at low temperature and the time evolution of the depth profile of N and O atoms depending on the process temperature, which would guide to optimize the oxynitridation process condition.

Charge-spin-orbital fluctuations in mixed valence spinels: Comparative study ofAlV2O4andLiV2O4

Mixed valence spinels provide a fertile playground for the interplay between charge, spin, and orbital degrees of freedom in strongly correlated electrons on a geometrically frustrated lattice. Among them, AlV$_2$O$_4$ and LiV$_2$O$_4$ exhibit contrasting and puzzling behavior: self-organization of seven-site clusters and heavy fermion behavior. We theoretically perform a comparative study of charge-spin-orbital fluctuations in these two compounds, on the basis of the multiband Hubbard models constructed by using the maximally-localized Wannier functions obtained from the ab initio band calculations. Performing the eigenmode analysis of the generalized susceptibility, we find that, in AlV$_2$O$_4$, the relevant fluctuation appears in the charge sector in $\sigma$-bonding type orbitals. In contrast, in LiV$_2$O$_4$, optical-type spin fluctuations in the $a_{\rm 1g}$ orbital are enhanced at an incommensurate wave number at low temperature. Implications from the comparative study are discussed for the contrasting behavior, including the metal-insulator transition under pressure in LiV$_2$O$_4$.

Quantum-mechanical calculations for the electron structure of phosphorus-containing sulfides Sn2P2S6 and Tl3PS4

The electron structure of phosphorus-containing sulfides Sn2P2S6 and Tl3PS4 is studied experimentally (by means of X-ray spectroscopy) and theoretically (via ab initio band calculations). Partial electronic state densities calculated using the WIEN2k software package correspond to their experimental analogs (the X-ray K- and {vnL}2,3-spectra of P and S). As a result of the greater electronegativity of sulfur, the electron densities of the p- and {pms}-states of P are shifted by approximately 3.5 eV toward lower energies, relative to the similar electronic states of S.

First-principles study on transition metal-doped anatase TiO2

The electronic structures, formation energies, and band edge positions of anatase TiO2 doped with transition metals have been analyzed by ab initio band calculations based on the density functional theory with the planewave ultrasoft pseudopotential method. The model structures of transition metal-doped TiO2 were constructed by using the 24-atom 2 × 1 × 1 supercell of anatase TiO2 with one Ti atom replaced by a transition metal atom. The results indicate that most transition metal doping can narrow the band gap of TiO2, lead to the improvement in the photoreactivity of TiO2, and simultaneously maintain strong redox potential. Under O-rich growth condition, the preparation of Co-, Cr-, and Ni-doped TiO2 becomes relatively easy in the experiment due to their negative impurity formation energies, which suggests that these doping systems are easy to obtain and with good stability. The theoretical calculations could provide meaningful guides to develop more active photocatalysts with visible light response.

Calculation of the X-Ray emission K and L 2,3 bands of metallic magnesium and aluminum with allowance for multielectron effects

A procedure is proposed to calculate the shape of the characteristic Xray emission bands of metals with allowance for multielectron effects. The effects of the dynamic screening of a core vacancy by conduc� tion electrons and the Auger effect in the valence band are taken into account. The dynamic screening of a core vacancy, which is known to be called the MND (Mahan-Nozeieres-De Dominics) effect, is taken into account by an ab initio band calculation of crystals using the PAW (projected augmented waves) method. The Auger effect is taken into account by a semiempirical method using the approximation of a quadratic depen� dence of the level width in the valence band on the difference between the level energy and the Fermi energy. The proposed calculation procedure is used to describe the Xray emission K and L2,3 bands of metallic mag� nesium and aluminum crystals. The calculated spectra agree well with the experimental bands both near the Fermi level and in the lowenergy part of the spectra in all cases.

The length controllable synthesis and near-infrared photoluminescence of one-dimensional ternary Cu4Bi4S9 semiconductor nanobelts

The ternary Cu–Bi–S based semiconductor with abundant and environmental-friendly elements may provide alternative choice for near-infrared (NIR) light-emitting sources. High-quality copper bismuth sulfide Cu4Bi4S9 (CBS) nanobelts of controllable lengths up to 1cm were synthesized through a modified solvothermal route by using solvents of varied boiling points (BP) and changing reaction temperatures (RT) to control the competition of efficient collision of Cu4Bi4S9 seeds, intermediate species and precursors and growth of Cu4Bi4S9 nanobelts. Their NIR photoluminescence (PL) of Cu4Bi4S9 nanobelts was firstly observed in the range of 1400–2200nm, which showed the transition characteristics of mixed direct and indirect-bands nearby. The absorption and PL spectral data could be well accounted for by the ab initio band calculations of Cu4Bi4S9.

Semiconductor-metal and semiconductor-magnetic half-metal phase transitions in layered SrAgSeF phases doped with oxygen and nitrogen

Results of ab initio band calculations for a layered nonmagnetic SrAgSeF semiconductors consisting of [SrF]/[AgSe] alternating blocks show that the partial O → F substitution leads to a semiconductormetal phase transition due to “metallization” of the [AgSe] bocks. The oxygen-doped SrAgSeF1 − xOx phase possesses a metal/semiconductor periodic structure formed by alternating [AgSe] and [SrF1 − xOx] blocks, respectively. On the contrary, the partial N → F substitution induces a semiconductor-magnetic half-metal phase transition. The resulting SrAgSeF1 − xNx system may be of interest as a new material for spintronics.

Structure of Morpholinium Tribromoplumbate C4H8ONH2PbBr3 Studied Using Single-Crystal Neutron Diffraction

The structure of inorganic–organic perovskite morpholinium tribromoplumbate C 4 H 8 ONH 2 PbBr 3 has been studied using single-crystal neutron diffraction with the time-of-flight (TOF) Laue technique. Nuclear positions and anisotropic atomic displacement parameters (ADPs) of all the atoms are determined with high accuracy without any restraints. The accuracy of bond lengths and angles in the organic part was almost one digit higher than that of X-ray data, showing the advantage of the neutron Laue technique for the structural study of the present system. It is indicated that the bond lengths and ADPs in the organic part are affected by protonation and bondings between organic and inorganic parts while the conformation of the morpholine molecule in the crystal is retained. The blue-shift in the band gap owing to the distortion in the inorganic octahedra is found in the optical diffuse reflection spectrum and in the electron density of state calculated by ab initio band calculation.

Electronic Structure of Hole-Doped Transition Metal Cyanides

Electronic structures of hole-doped transition metal cyanides, Na0:84� xCo[Fe(CN)6]0:71 .3.8H2O (NCF71), Na0:72� xNi[Fe(CN)6]0:68 .5.1H2O (NNF68) and Na1:60� xCo[Fe(CN)6]0:90 .2.9H2O (NCF90), were investigated by means of the x-ray absorption spectroscopy and the valence differential spectroscopy. The x-ray absorption spectroscopy revealed that the holes are introduced on the Fe, Fe, and Co sites for the NCF71, NNF68 and NCF90 films, respectively. Owning to the valence differential spectroscopy, we unambiguously assigned the spectral components to the respective optical transitions. We further found that an ab initio band calculation based on the local density approximation with the onsite Columbic repulsion (LDA+U) semi-quantitatively explains the optical transitions.

Ab-initio calculations for defect energies in Co 2MnSi and Co 2CrAl

Astract The ab-initio band calculations predict that the full-Heusler ferromagnetic alloys at L21 structure, such as Co2MnSi and Co2CrAl, are half-metallic. However, the measured spin polarizations have been usually less than ∼ 60 % . The decrease may be attributed to defects of swaps and antisites. We give ab-initio calculations for the formation energies of isolated swaps in Co2MnSi and Co2CrAl and isolated antisites in Co2MnSi, which may be the important factor to determine the defect concentrations. The present calculations predict that two kinds of antisites (Si in Mn-site and Mn in Co-site) in Co2MnSi and one kind of swap (Cr–Al) in Co2CrAl, are very likely to be formed because of the small defect energies. Using the calculated results, we clarify the fundamental features of Co2MnSi and Co2CrAl with defects and discuss how to understand the discrepancies between the band calculation and experimental results.

“Pudding mold”-type band as an origin of large thermopower in [formula omitted]-type organic conductors

We study the origin of the large thermopower in quasi-two-dimensional τ -type organic conductor, τ − ( EDO − S , S − DMEDT − TTF ) 2 ( AuBr 2 ) 1 + y ( y ≤ 0.875 ), from the view point of a “pudding mold”-type band structure. We calculate the electronic band structure using an ab initio band calculation package, and obtain a tight binding model fit to the ab initio band structure. Using the model and Boltzmann's equation approach, we calculate the temperature dependence of the Seebeck coefficient. We conclude that the peculiar band structure is the origin of the large Seebeck coefficient and the appearance of the maximum value at a certain temperature.