First principles study of structural, elastic, electronic and optical properties of the cubic perovskite BaHfO3

First principles study of structural, elastic, electronic and optical properties of the cubic perovskite-type BaHfO₃ has been reported using the pseudo-potential plane wave method within the local density approximation. The calculated equilibrium lattice is in a reasonable agreement with the available experimental data. The elastic constants and their pressure dependence are calculated using the static finite strain technique. A linear pressure dependence of the elastic stiffnesses is found. Band structures show that BaHfO₃ is a direct band gap between the occupied O 2p and unoccupied Hf d states. The variation of the gap versus pressure is well fitted to a quadratic function. Furthermore, in order to understand the optical properties of BaHfO₃, the dielectric function, absorption coefficient, optical reflectivity, refractive index, extinction coefficient, and electron energy loss are calculated for radiation up to 30 eV. We have found that O 2p states and Hf 5d states play a major role in the optical transitions as initial and final states, respectively. This is the first quantitative theoretical prediction of the elastic, electronic and optical properties of BaHfO₃ compound, and it still awaits experimental confirmation.     

Unusual electronic properties of InN

Electronic structure of zinc-blende and wurtzite InN using linear combination of atomic orbitals and the latest approach of generalised gradient approximation within full potential linearised augmented plane wave schemes is reported. An unusual small band gap and real space analysis of our first ever experimental Compton profile are discussed.     

First-principles study of the structural and thermodynamic properties of AsNMg3 antiperovskite

The structural and elastic properties of the antiperovskite semiconductor AsNMg₃ are investigated using the full-potential linearized augmented plane wave plus local orbital (FP-LAPW+lo) method within the generalized gradient in the frame of the density functional theory. The ground state properties such as lattice constant, bulk modulus, pressure derivative of the bulk modulus and elastic constants are in good agreement with numerous experimental and theoretical data. Through the quasi-harmonic Debye model, in which the phononic effects are considered, we have obtained successfully the thermodynamic properties such as the thermal expansion coefficient, Debye temperature and specific heats in the whole pressure range from 0 to 30 GPa and temperature range from 0 to 1200 K.     

Electronic topological transition and phase transition in rhodium and iridium based inter metallic compounds

Of the Rh₃Y, Rh₃La, Ir₃Y and Ir₃La inter-metallic compounds, the compound Rh₃Y exists in hexagonal structure, Ir₃Y and Ir₃La exist in rhombohedral structure, whereas the compound Rh₃La exists in both hexagonal and rhombohedral structures. Based on our tight binding-linear muffin tin orbital (TB-LMTO) study of other rhodium and iridium-based Rh₃X and Ir₃X (where X=Ti, Zr, Hf, V, Nb, Ta and Sc) inter-metallic compounds of AuCu₃ type cubic structure, an attempt is made to examine whether the compounds Rh₃Y, Rh₃La, Ir₃Y and Ir₃La will undergo a structural phase transition to cubic structure from their experimentally reported structures. From our study, it is observed that the compounds Rh₃Y and Rh₃La undergo a structural phase transition to cubic phase at 4.5 and 10.1 GPa, respectively, from their experimentally reported hexagonal and rhombohedral phases. Further it is predicted that both the compounds Ir₃Y and Ir₃La can exist in the cubic phase itself at ambient condition, in contrary to the experimental observation. From the band structure outputs that have been plotted for the compounds under compression, it is observed that the compounds Rh₃La, Ir₃Y and Ir₃La undergo the Lifshitz type of transition which may change the Fermi surface topology and hence the physical properties of these compounds.     

Correlation of excitation-wavelength dependent photoluminescence with the fractal microstructures of porous silicon

The correlation of the excitation-wavelength dependent photoluminescence with the fractal microstructures of porous silicon has been investigated. As the excitation wavelength increases from 340 to 650 nm, the photoluminescence of porous silicon redshifts from 500 to 780 nm. The excitation-wavelength dependent photoluminescence suggests the existence of a size distribution for the large number of silicon nanocrystallites in porous silicon. Using scanning electron microscopy and computer simulation, we have investigated the fractal features of the microstructures of porous silicon. Our results have demonstrated that the fractal features in the microstructures of porous silicon indicate the existence of a size distribution for the silicon nanocrystallites in porous silicon. The recorded excitation-wavelength dependent photoluminescence of porous silicon can be interpreted in terms of the bond-order-length-strength correlation theory.     

Structural, Electronic Properties and Chemical Bonding of Borate Li4CaB2O6 under High Pressure： an Ab Initio Investigation

We calculate structural, electronic properties and chemical bonding of borate Li4CaB2O6 under high pressure by means of the local density-functional pseudopotential approach. The equilibrium lattice constants, density of states, Mulliken population, bond lengths, bond angles as well as the pressure dependence of the band gap are presented. Analysis of the simulated high pressure band structure suggests that borate Li4CaB2O6 can be used as the semi-conductor optical material. Based on the Mulliken population analysis, it is found that the electron transfer of the Li atom is very different from that of other atoms in the studied range of high pressures. The charge populations of the Li atom decrease with the pressure up to 60 GPa, then increase with the pressure.     

A note on RF photocathode guns triggered by synchrotron light

In this note we explore the conditions under which synchrotron radiation can be used to trigger a photocathode gun. We show that such a solution offers in principle the possibility of operating with a pulsed electron beam filling all the buckets of the accelerating radio frequency. Some comments on the e-beam characteristics are also presented.     

Visible photoluminescence from Ge+-implanted SiO2 films thermally grown on crystalline Si

+ -implanted SiO₂ films is studied as a function of different fabricating conditions (implantation dose, annealing temperature and time). The SiO₂ films containing Ge nanocrystals exhibit two photoluminescence (PL) bands peaked at 600 nm and 780 nm. There are two excitation bands in the PL excitation (PLE) spectra. With variation in Ge nanocrystal size, the PL and PLE peak energies show no appreciable shift. The PL and PLE spectral analyses suggest that during the PL process, electron–hole pairs are generated by the E(l) and E(2) direct transitions inside Ge nanocrystals, which then radiatively recombine via luminescent centers in the matrix or at the interface between the nanocrystal/matrix. Received: 27 January 1998/Accepted: 18 March 1998     

The Assodated Matrices of Ap, n, 4p-5 Polyhedra and their Admissible Onerations

In this paper we describe the decomposition problem of a special kind of Ap. n. 4p-5 polyhedra by using the associated matrices and their admissible operations.     

一类Schrdinger方程的唯一延拓性

本文研究一类具有Ｌｉｐ连续系数且带奇异位势的Ｓｃｈｒｄｉｎｇｅｒ方程Ｌｕ＝－ｄｉｖ(Ａ(ｘ)ｕ＋Ｖ(ｘ)ｕ(ｘ)＝０,得到了此类方程弱解的唯一延拓性以及弱解的绝对值属于某Ａｐ权．