Savitri Agrawal

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 structural, electronic and optical properties of ultrathin bismuth nanowires

The energetics, structural, electronic and optical absorption properties of the bismuth nanowires Bin with n = 1, 6 have been investigated using density functional theory (DFT) in the local density approximation (LDA) including the spin–orbit coupling (SOI). The inclusion of the SOI appreciably affects all the physical properties of the wires. The stable structures form four groups: the planar structures, the caged configurations, the pyramidal structures and the helical configurations. This finding may be a guide for the construction of atomic configurations of the nanowires possessing a larger number of atoms per unit cell. The most stable wire configurations are the 5-Bi pentagonal, and the 6-Bi hexagonal and 6-Bi triple zigzag wires, which should be seen in the experiments. All the wires are metallic. The behaviour of the electron states of the second category structures is quite near to that of a linear chain where the parabolic bands cross the EF, and the number of the channels available for the electric conduction is large. Thus, one should grow the wire structures falling into the second category for achieving high conduction. For the 5-Bi pentagonal and 6-Bi hexagonal cross-sectional wires, the number of channels available for the electric conduction are ten and twelve, respectively. The SOI drastically affects the calculated optical absorption, especially in the low energy region. The absorption peaks are different in terms of the number and the energy locations for the different wires, and may be used for the characterization of the structure of a wire. Our analysis of the calculated electronic structure and the optical data of all the studied structures supports the occurrence of the 4-Bi double and/or 6-Bi triple zigzag chains in the samples of Romanov.

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.

Ab initio study of small diameter (6, 6) armchair carbon nanoropes: orientational dependent properties

A comprehensive ab initio investigation of the effects of the relative orientation (RO) between the adjacent tubes in a rope on the stability, structural, electronic, optical and Raman-active properties has been performed for the ropes of small diameter carbon (6, 6) nanotubes. A number of new features not discussed earlier are observed in the present study. The symmetric rope with an RO of 0° is metallic in all directions, whereas the asymmetric ropes with a non-zero value of RO are semiconductors along the tube axis but semi-metallic normal to rope axis. The band gap increases with RO up to an angle of 15° and thereafter reveals oscillatory behaviour. No dips appear in the symmetric rope but they do exist in the asymmetric rope. Strong optical absorption appears along the axis in the energy range 2.4–4.2 eV in the isolated tube. On the other hand, for the ropes, the strong absorption extends up to the energy region 1.8–4.5 eV. Strong peaks also occur at 0.05 and 0.15 eV for the ropes with RO = 0° and 15°, respectively. The even-parity Raman-active radial breathing mode (RBM) frequencies calculated here for the isolated (n,n), n = 3–6 tubes are seen to deviate from the usual law (where d is the tube diameter). For small diameter tubes, this shows an approximate variation, ω = 1/d1/2. The RBM frequencies for the ropes are either greater or smaller compared to the isolated tube, depending on the value of RO. A cubic anharmonicity of about 14% is seen in the potential for the radial mode vibrations. The RBM frequencies calculated here for some ropes, which are lower compared to that of the isolated tube, concur with the available Raman data.

First-principles calculation of electron surface states of the zinc-blende GaN(110) surface

Abstract The surface electronic structure for an unrelaxed as well as relaxed zinc-blende GaN(110) surface has been investigated within the local density approximation of density functional theory, employing a first-principles full-potential self-consistent linear muffin-tin orbital (LMTO) method and a supercell approach. Intrinsic surface states appear in the fundamental energy gap for an unrelaxed surface. These intrinsic gap surface states shift towards the bulk valence and the conduction band region on considering the relaxation of the surface atoms. Orbitals characters of surface states at Γ, X, M, X″ symmetry points have been identified. Several localized and resonance states are predicted for the first time in different energy regions.

Effect of spin?orbit interaction on the electronic and optical properties of ultrathin bismuth nanowires?a density functional approach

A first-principles study of the effects of spin–orbit coupling (SOI) on the structural, electronic and optical properties of 16 bismuth nanowires, Bin with n = 7–18, has been performed. The density functional theory (DFT) in the local density approximation (LDA) has been used. The inclusion of the SOI significantly alters the electronic and optical properties of the wires. The stable structures for the Bin wires with n = 7–18 form two groups: non-helical and helical configurations. In addition to the most stable non-helical 5-Bi pentagonal, 6-Bi hexagonal and 6-Bi triple-zigzag wire configurations found in a previous report, the present study adds to this list three more non-helical structures, namely the non-helical 7-Bi hexagonal, 8-Bi heptagonal and 11-Bi pentagonal cross-sectional wire configurations. The present result is in sharp contrast to the conclusions of our previous studies of Pb- and Tl-nanowires, where it was observed that, in general, a structure possessing a high coordination number value has large binding energy and, therefore, the helical wire structures are the most stable ones. All of the wires are metallic in the LDA. The number of channels in the nanowires is large which will lead to high quantum ballistic conduction. The consideration of the many body effects such as the GW approximation (GWA) may destroy the metallicity predicted here in the Bin wire configurations for n≤7. However, for the wire configurations having n≥8, we find that even in the GWA, one may not observe the opening of the energy gaps because of the violation of the electron counting principle. The optical absorption calculated with SOI is much stronger compared to the one obtained after neglecting the SOI. For the wires containing a large number of atoms in the unit cell, the optical absorption is multi-peaked, strong and extended over the whole energy region from infrared to the ultraviolet electromagnetic radiation including the visible region. These nanowires may thus be used as a source of white radiation.

Electronic structure and the van Hove singularity scenario in high-Tc HgBa2CuO4+δ superconductor pressure effects

Abstract The electronic structure and the hole concentrations in the recently discovered high- T c superconductor HgBa 2 CuO 4+ δ ( δ =0, 1) have been investigated by employing a first-principles full potential self-consistent linear muffin-tin orbital (LMTO) method with the local density functional theory. The scalar relativistic effects have been considered. The hole concentrations of the Cu d x 2 - y 2 and O p x,y orbitals are seen to be larger for the HgBaCuO 5 system than those of the HgBaCuO 4 solid. Howe ever, the van Hove singularity (vHS) induced Cu d and O p peak which is seen to lie above the Fermi level in the δ =1 system shifts below the Fermi level in the δ =0 system. Thus, the occurence of the superconducting behaviour observed in the HgBa 2 CuO 4+δ with δ =0.1 appears to originate by the hole filling of the δ =0 system by oxygenation to achieve the pinning of the vHS peak at the Fermi level. The Fermi surface nesting area in the δ =0 compound is seen to be larger than that seen in the δ =1 compound. The calculation reveals that the vHS peak approaches towards the Fermi level and the hole concentrations increase with the pressure on the crystal predicting a possibility of enhancement of T c at high pressures, a result which is in agreement with the experiments. The vHS is pinned at the Fermi level for a critical volume V / V 0 =0.65, V 0 being the normal volume. This volume may be achieved for a pressure of 24 GPa, a value in very good agreement with the experimental data.

An Ab-initio Study of Metallic and Semiconducting [001] SiC Nanowires

A detailed comprehensive ab-initio study of the structural, electronic and optical properties of the unpassivated and H-passivated SiC nanowires (NWs) grown along [001] direction having diameters lying in the range, 3.35 to 15.42 A has been made by employing the first-principles pseudopotential method within density functional theory (DFT) in the generalized gradient approximation (GGA). We investigate two types of the NWs having hexagonal and triangular cross-sections. The binding energy (BE) increases with the diameter of the NW because of decrease in the relative number of the unsaturated surface bonds. The band gap decreases with the diameter of the NW because of the quantum confinement. After atomic relaxation, appreciable distortion occurs in the NWs where the chains of Si- and C-atoms are curved in different directions. These distortions are reduced with the diameters of the NWs. The different NWs reveal different electronic properties, e.g., one NW is seen to be metallic whereas the other one is s...

An ab initio study of optical and Raman spectra of heavily Li-doped 4 Å carbon nanotubes

An extended and systematic ab initio investigation of the energetics, structural, electronic, optical and Raman-active properties of heavily Li-doped ultrathin 4 A diameter carbon nanotubes of different chiralities has been performed. A number of new features not discussed earlier are observed in the present study. Our values for the binding energies of the Li-doped tubes are higher than the ones reported earlier. We find the Li-doped (3, 3) tubes stable in contrast to an earlier prediction. For each type of the tube, the saturation intake of Li atoms inside the tube is 8–10% only, whereas on the surfaces of the (5, 0) and (4, 2) tubes, this intake is as high as 100%, and it forms a circular tube of Li+ ions around the tube. Li atoms residing on the axis inside the tube do not modify the symmetry of the host tube and thus the electronic structure (except the upward shifting of the Fermi surface, EF) in contrast to surface Li atoms which (apart from the (4, 2) tube, which possesses no symmetry) destroy the symmetry and drastically alter the electronic structure. In general, Li doping inside or outside the tube increases the electron density of states (DOS) several times except in the (5, 0) tube doped with a small concentration of Li atoms, where it decreases. For one inner Li atom in each tube, the peak structure in the optical absorption remains essentially the same as that of the pristine tube. In general, the optical absorption is different for the various nanotube–Li configurations studied. For each type of tube, in general, the RBM frequency reduces with the dilation of the diameter of the optimized tube. The high values of DOS obtained for certain Li concentrations may lead to high values of electrical conductivity and the superconducting transition temperatures in Li-doped tubes as has already been reported in pristine tubes and ropes.