First-principles study on structural and electronic properties of AlNSix heterosheet

Under generalized gradient approximation (GGA), the structural and electronic properties of AlN and Si sheets, hydrogen terminated AlN and Si nanoribbons with hexagonal morphology and 2, 4, 6 zigzag chains across the ribbon width and the hexagonally bonded heterosheets AlNSi_x (x=2, 4, and 6) consisting of hexagonal networks of AlN (h-AlN) strips and silicene sheets with zigzag shaped borders have been investigated using the first-principles projector-augmented wave (PAW) formalism within the density function theory (DFT) framework. The AlN sheet is an indirect semiconductor with a band gap of 2.56 eV, while the Si sheet has a metallic character since the lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) meet at one k point from Γ to Z. In the semiconductor 6-ZAlNNR, for example, the states of LUCB and HOVB at zone boundary Z are edge states whose charges are localized at edge Al and N atoms, respectively. In metallic 6-ZSiNR, a flat edge state is formed at the Fermi level E_F near the zone boundary Z because its charges are localized at edge Si atoms. The hybridizations between the edge states of h-AlN strips and silicene sheets result in the appearance of border states in the zigzag borders of heterosheets AlNSi_x whose charges are localized at two atoms of the borders with either bonding or antibonding π character.     

Magnetic structure of graphite ribbon

Anoble mechanism of spin polarization is proposed for finite graphite sheet with edge. For graphite ribbon with zigzag edge, there appear peculiar ‘edge states’. These localized states comprise nearly flat band at the Fermi level, which easily causes magnetic instability. Magnetic structure is suggested from Hartree-Fock analysis of the Hubbard model, where huge magnetic moments are induced at around both of edges by weak HubbardU and are coupled antiferromagnetically with each other.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Symmetrical metallic and magnetic edge states of nanoribbon from semiconductive monolayer PtS2

Transition metal dichalcogenides (TMD) MoS₂ or graphene could be designed to metallic nanoribbons, which always have only one edge show metallic properties due to symmetric protection. In present work, a nanoribbon with two parallel metallic and magnetic edges was designed from a noble TMD PtS₂ by employing first-principles calculations based on density functional theory (DFT). Edge energy, bonding charge density, band structure, density of states (DOS) and simulated scanning tunneling microscopy (STM) of four possible edge states of monolayer semiconductive PtS₂ were systematically studied. Detailed calculations show that only Pt-terminated edge state among four edge states was relatively stable, metallic and magnetic. Those metallic and magnetic properties mainly contributed from 5d orbits of Pt atoms located at edges. What's more, two of those central symmetric edges coexist in one zigzag nanoribbon, which providing two atomic metallic wires thus may have promising application for the realization of quantum effects, such as Aharanov–Bohm effect and atomic power transmission lines in single nanoribbon.     

Optical properties of layered superconductor LixZrNCl

We investigated band-insulator-to-superconductor transition in Li_xZrNCl driven by carrier doping by means of magnetization, resistivity, and optical reflectivity measurements. The magnetization and the resistivity measurements showed that the transition occurs at around x=0.05. The pristine β-ZrNCl exhibited reflectivity and optical conductivity spectra typical of an insulator, whereas in the spectrum of Li_0.37ZrNCl, Drude-like high reflectance band and associated plasma edge are apparently observed. This is the direct spectroscopic evidence of insulator-to-metal transition of Li_xZrNCl.     

Urbach rule and disordering processes in Cu6P(S1−xSex)5Br1−yIy superionic conductors

Compositional behavior of Urbach absorption edge is studied as well as the effect of compositional disordering on the parameters of exciton-phonon interaction, phase transition temperatures and electric conductivity in Cu₆P(S_1−xSe_x)₅Br_1−yI_y superionic solid solutions. The effect of different types of disordering on the optical absorption processes and specific features of compositional changes in the absorption edge spectra under S→Se and Br→I anion substitution in the mixed crystals are investigated. (x, T) phase diagrams for Cu₆P(S_1−xSe_x)₅X (X=I, Br) solid solutions are studied.     

Ab initio calculation of Ni50−xFexTi50

The structure, lattice dynamics and electronic band structure of Ni_43.75Fe_6.25Ti₅₀ were obtained using ab initio calculations. The phonon dispersion relations and phonon density of states were calculated using the direct method. The stability of Ni_50−xFe_xTi₅₀ structure for x=0.0, 6.25, 12.5, 25 has been investigated and shown that the orthorhombic structure is the most stable phase for x=25.     

Compositional behaviour of Urbach absorption edge and exciton-phonon interaction parameters in Cu6PS5I1−xBrx superionic mixed crystals

Temperature behaviour of optical absorption edge in Cu₆PS₅I_1−xBr_x mixed crystals is studied in the interval 77-325 K. It is shown that the absorption edge has Urbach shape in the 215-325 K temperature interval. The influence of temperature and compositional disorder on the Urbach absorption edge parameters is presented. The mechanism of the Urbach bundle formation and the effect of I→Br anionic substitution on the exciton-phonon interaction parameters is elucidated.     

Concentration variations in the optical pseudogap and refractive index of crystals of Cu6PS5I1–xClx solid solutions

The effect of anionic substitution of halogen atoms on the spectral position and shape of the optical absorption edge and the dispersion of the refractive index of crystals of Cu₆PS₅I_1–xCl_x solid solutions are studied. The concentration dependences of the width of the optical pseudogap, refractive index, and lattice parameters are shown to be interrelated in these solid solution crystals.     

Oxygen vacancy doping effect on the electrical and magnetic behavior of Ba5−xLaxNb4−xTixO15

We report electric and magnetic properties of oxygen deficient Ba_5−xLa_xNb_4−xTi_xO_15−δ phases, which have been prepared by solid-state reaction method followed by a controlled reduction process under hydrogen atmosphere. The extra electrons added by the formation of the oxygen vacancies (δ) introduce localized spins and the magnetic susceptibility can be described by a temperature-independent contribution and a Curie-Weiss term associated to the Ti³⁺ ion formation. Besides, the experimental resistivity (ρ) data of these four reduced compounds are well described in a wide temperature range with the equation , which suggests the presence of small polarons in the system. Although, all samples present electrical insulating behavior, the electrical resistivity decreases four orders of magnitude for intermediate x values. We interpreted this fact as a consequence of the mix between the localized bands of the Nb and Ti ions, which favors the promotion of carriers due to reduction of the band gap.     

Structural and electronic properties of a single Si chain doped zigzag AlN nanoribbon

The first-principles projector-augmented wave (PAW) potentials within the density function theory (DFT) framework have been used to determine the geometry structures and electronic properties of the zigzag edge AlN nanoribbons (ZAlNNRs) doped with a single Si chain under generalized gradient approximation (GGA). The average Al–Si, Si–Si, Al–N, Si–N, Al–H and N–H bond lengths are 2.39, 2.16, 1.83, 1.74, 1.59 and 1.03 Å, respectively. Pure 7-ZAlNNR is an indirect semiconductor with a large band gap of 2.235 eV, while a semiconductor to metal transformation is taken place after a single Si chain substituting for a single Al–N chain at various positions. In pure 7-ZAlNNR, the HVB and LCB are mainly attributed to the edge N and Al atoms, respectively, while in a single Si chain substituting doped 7-ZAlNNR, the HVB and LCB are mainly attributed to the Si atoms. The Al–N, Al–H and Al–Si bonds are ionic bond, the Si–Si and Si–H bonds are covalent bond, the N–H and N–Si bonds are covalent bond modified ionic bond.     

Structural, electronic and optical calculations of CaxZn1−xO alloys: A first principles study

First principles density functional calculations, using full potential linearized augmented plane wave (FP-LAPW) method, have been performed in order to investigate the structural, electronic and optical properties of Ca_xZn_1−xO alloy in B1 (NaCl) phase. Dependence of structural parameters as well as the band gap values on the composition x have been analyzed in the range 0?x?1. Calculated electronic structure and the density of states of these alloys are discussed in terms of the contribution of Zn d, O p and Ca p and d states. Furthermore, optical properties such as complex dielectric constants ε(ω), refractive index including extinction coefficient k(ω), normal-incidence reflectivity R(ω), absorption coefficient α(ω) and optical conductivity σ(ω) are calculated and discussed in the incident photon energy range 0-45 eV.     

Electronic structures of mixed ionic-electronic conductors SrCoOx

Electronic structures of SrCoO_x with x=3, 2.875 and 2.75 were calculated by DFT technique in SLDA approximation. Two kinds of oxygen vacancy ordering with energies of 0.22 and 0.01 eV lower in comparison with random vacancy distribution were revealed. The transition between these ordered vacancy systems with the activation energy 0.34 eV can be a step in the ionic conductivity mechanism. The calculated ion charges, magnetic moments and electron density of states were used to analyze chemical bonding in the crystals. All calculated compounds have metal electronic conductivity.     

Temperature dependence of electrical resistivity for Ca-doped perovskite-type Y1−xCaxCoO3 prepared by sol-gel process

The temperature dependences of DC electrical resistivity for perovskite-type oxides Y_1−xCa_xCoO₃ (0?x?0.1), prepared by sol-gel process, were investigated in the temperature range from 20 K up to 305 K. The results indicated that with increase of doping content of Ca the resistivity of Y_1−xCa_xCoO₃ decreased remarkably, which was found to be caused mainly by increase of carrier (hole) concentration. In the whole temperature range investigated the temperature dependence of resistivity ρ(T) for the un-doped (x=0) sample decreased exponentially with decreasing temperature (i.e. ln ρ∝1/T), with a conduction activation energy ; the resisitivity of lightly doped oxide (x=0.01) possessed a similar temperature behavior but has a reduced E_a (0.155 eV). Moreover, experiments showed that the relationship ln ρ∝1/T existed only in high-temperature regime for the heavily doped samples (T?82 and ∼89 K for x=0.05 and 0.1, respectively); at low temperatures Mott's ln ρ∝T^−1/4 law was observed, indicating that heavy doping produced strong random potential, which led to formation of considerable localized states. By fitting of the experimental data to Mott's T^−1/4 law, we estimated the density of localized states N(E_F) at the Fermi level, which was found to increase with increasing doping content.     

Topologically induced surface electron state on Si(1 1 1) surfaces

First-principle electronic structure calculation reveals the appearance of a new class of surface state on hydrogenated and clean Si(1 1 1) surfaces. The states are found to exhibit different characteristics to conventional surface electron states in terms of the peculiar distribution of the wavefunction depending on the wavenumber. In addition, the state results in flat dispersion bands in a part of the surface Brillouin zone having energy of about 8 eV below the top of the valence band. An analytic expression based on the tight-binding approximation corroborates the surface electron state results from the delicate balance of the electron transfer among the atoms situated near the surface. The obtained results give a possible extension and generalization of the edge state in graphite ribbons with zigzag edges.     

Inelastic X-ray scattering investigations of lattice dynamics in SmFeAsO1−xFysuperconductors

We report measurements of the phonon density of states as probed with inelastic X-ray scattering in SmFeAsO_1−xF_y powders. An unexpected strong renormalization of phonon branches around 23 meV is observed as fluorine is substituted for oxygen. Phonon dispersion measurements on SmFeAsO_1−xF_y single crystals allow us to identify the 21 meV A_1g in-phase (Sm,As) and the 26 meV B_1g (Fe,O) modes to be responsible for this renormalization, and may reveal unusual electron-phonon coupling through the spin channel in iron-based superconductors.     

Neutron powder diffraction study on the structures of LaNi5−xAlxDy compounds

The structures of LaNi_5−xAl_xD_y (x=0.75,0.25,y=1.01, 1.10, 1.91 and 3.1) were systematically investigated by neutron and X-ray diffraction. D atoms are found to enter the 6m site of the α-phase but not the reported 12n site, while the 6m and 12n sites of the β-phase. In the case of LaNi_4.75Al_0.25D_y with lower Al content and symmetry, D atoms do not enter the α-phase but occupy the 4h site besides the 6m and 12n sites of the β-phase. The relationship between structures and properties is also discussed.     

Recombination centers in phosphorous doped hydrogenated amorphous silicon

Optical absorption below the mobility gap of a-SiH_x:P films is derived from photoconductivity measurements and interpreted in terms of optical transitions from occupied localized states in the exponential valence band tail and dangling bond states 0.8 eV above the valence band edge to unoccupied free electron conduction band states. Collection efficiency measurements of Schottky barrier structures indicate that P doping introduces centers with large capture cross-section for holes.