P.S. Yadav

Ab initio calculation of electronic properties of Ga1 xAlxN alloys

The electronic properties of the wide-band-gap semiconducting ordered alloys (for x = 0.0, 0.25, 0.50, 0.75 and 1.0) and the random alloys have been investigated by using a full-potential self-consistent linear muffin tin orbital (LMTO) method. The calculated direct band gap for random distribution of cation nearest-neighbour tetrahedral clusters in the alloys for any arbitrary concentration x is seen to show a quite linear variation in agreement with the experiment. On the other hand, the indirect band gap remains invariant. We observe a direct to indirect band gap crossover at x = 0.59. The band gap bowing is seen to be very small.

Ab initio study of the physical properties of binary SimCn (m+n≤5) nanoclusters

An ab initio study of the stability, structural, electronic, vibrational and optical properties of the most stable silicon–carbon binary nanoclusters SimCn (m+n≤5) has been made. A B3LYP-DFT/6-311G(3df) method has been employed to optimize fully the geometries of the nanoclusters. The binding energies (BEs), highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps, bond lengths, ionization potentials (IPs), adiabatic electron affinities (EAs), vibrational frequencies, infrared intensities, relative infrared intensities and Raman scattering activities have been computed. In the more stable structures, the carbon atoms are in the majority whereas in the less stable structure the reverse is true. For the clusters containing all the carbon atoms except one silicon atom, the BE increases monotonically with the number of carbon atoms. The ground states of the clusters containing even numbers of the carbon atoms are, in general, lower than those containing odd numbers of carbon atoms. On the other hand, the lowest unoccupied states of the clusters containing even numbers of carbon atoms lie higher than those containing odd numbers of carbon atoms. All the predicted physical quantities are in good agreement with the experimental data wherever available. The growth of these most stable structures should be possible in the experiments.

Ab initio study of anomalous band-gap bowing in alloys

A full potential self-consistent linear muffin-tin orbital method in the local density approximation (LDA) has been employed to investigate the electronic structure of the 14 ordered alloys with x = 0.0, 0.037, 0.074, 0.111, 0.125, 0.25, 0.375, 0.407, 0.50, 0.625, 0.75, 0.875, 0.963 and 1.0. The lattice parameter is seen to have a non-linear variation with the concentration of the constituent atoms. In LDA, a near closure of the fundamental energy gap appears in the concentration range 12.5-62.5% of N atoms. A strong hybridization of N s states with the Ga (s, p) and As (s, p) states is seen at the bottom of the conduction band and these states descend into the fundamental gap, filling it either partially or completely. The band gap for the random alloys also shows an anomalous bowing.

Phonons and specific heat in high-Tc Tl1(2)Ba2Can−1CunO2n+3(4) superconductors

Abstract A simple Born model involving only a few adjustable parameters explains very well for the first time the experimental data available for the Tl-based Tl 1(2) Ba 2 Ca n −1 Cu n O 2n+3(4) , n =1 to 3 high- T c oxide superconductors. In particular, for the Tl 2 Ba 2 CaCu 2 O 8 system the calculated phonon density of states is in very good agreement with the neutron data. There are small discrepancies in the low frequency region where the calculated phonons appear, comparatively towards the low frequency side. The phonon bands show highly two-dimensional behaviour. Planar dispersions are significant only for the CuO 2 layers. The lattice specific heat in the low temperature region shows a non-Debye behaviour i.e., a variation different from a T 3 one. This puts into question the reliability of the magnitude and the temperature dependence of the non-lattice contributions determined in an analysis of the observed total specific heat after assuming a T 3 -dependent lattice contribution.

Structural and Electronic Properties of ZnO NANOCLUSTERs: A B3LYP DFT Study

An ab initio B3LYP-DFT/6-311G(3df) study has been performed for the stability, structural and electronic properties of forty Znm On (m + n = p = 2 to 4) nanoclusters. We also consider the zero point energy correction. The nanoclusters containing large number of strongly electronegative O atoms for p = 3 and 4 are found to be most stable as compared to the other nanoclusters of the same configuration. The most stable clusters have linear or planer structures and not the three dimensional ones. The observed trend of decrease of the HOMO-LUMO gap with the size of the nanocluster is in conformity with the quantum confined behavior.

Anomalous band gap bowing in AlAs1−x Nx alloys

Abstract We have employed a first principle self consistent linear muffin tin orbital method in the local density approximation to calculate the electronic structure of the nine AlAs1.xNx alloys with x=0.0, 0.125, 0.25, 0.375, 0.50, 0.625, 0.75, 0.875 and 1.0. The lattice parameter shows a non linear behaviour with the concentration of N-atoms. A quite smaller gap appears in the neighbourhood of x=0.125 concentration of N-atoms.. The state lying at the CBM, which is composed of N-s, Al(s,p), As-s descends into the fundamental gap filling it partially. For the random alloys the band gap also shows anomalous bowing.

Vibrational excitations in a-Si1−xCx:H alloys

Abstract We report here the results of a first calculation of the vibrational excitations of hydrogenated a-Si 1−x C x :H alloys using a cluster Bethe lattice method (CBLM). The study has been made for two types of distributions of constituent atoms of the alloy: a random sequence and a chemically ordered one. The infrared data of a-Si 1−x C x :H alloys can be well understood if one assumes the occurrence of a random distribution of Si and C atoms.

Vibrational excitations in a-Si3N4:H(D) alloys

The authors report here the first results of a theoretical study of the vibrational excitations in the amorphous Si3N4 and a-Si3N4:H(D) alloys using a cluster Bethe lattice method. The a-Si3N4 network has a nearest-neighbour tetrahedral coordination of N atoms around the Si atom and a threefold planar coordination of Si atoms around the N atom. The computed phonon density of states for a-Si3N4 and a-Si3N4:H alloys is in very good agreement with the available infrared and Raman data. The study shows the occurrence of the mono- and dihydrides both at the Si and N atoms. Detailed infrared and Raman measurements need to be performed on the a-Si3N4:H alloys for the different concentrations of H atoms, especially in the low-frequency region, to elucidate the microscopic structure of the alloys.

Vibrational excitations in a-A1−xBxC alloys

Abstract The vibrational excitations of amorphous A1-xBxC systems for different concentrations of the constituent atoms have been investigated in a cluster-Bethelattice formulation. The two-mode behaviour of the alloys is very clearly evident in the calculated local phonon density of states. The available experimental data in the optical phonon region are in excellent agreement with the computed phonon density for a number of alloys assuming a random distribution of varying atoms. The concept of a one-mode behaviour does not seem appropriate for any of the alloys. Sometimes, a two-mode behaviour is masked by the fact that the resonance-localized mode appearing near one end-component of the alloy is either very weak or highly degenerate with the bulk phonons lying in the region.

Phonons in high-Tc Tl2Ba2Can−1CunO2n + 4 superconductors

Abstract A simple Born model involving a few adjustable parameters has been seen to explain very well the experimental data available for the Tl-based Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 , n =1,2,3 high- T c oxide superconductors. In particular, for Tl 2 Ba 2 CaCu 2 O 8 system, the calculated phonon density of states is in agreement with the neutron data. There are small discrepancies in the low-frequency region where the calculated phonons appear, comparatively towards the low-frequency side. The phonon bands show highly two-dimensional behaviour. Planar dispersions are significant only for CuO 2 layers.