D. G. Kanhere

Electronic structure of spinel oxides: zinc aluminate and zinc gallate

The electronic structure of zinc aluminate (ZnAl2O4) and that of zinc gallate (ZnGa2O4) were studied by the self-consistent tight-binding linearized muffin-tin orbital method with the atomic sphere approximation. The calculated results predict these zinc-based spinel oxides to be direct-gap materials. The direct gap at is found to be 4.11 eV for ZnAl2O4 and 2.79 eV for ZnGa2O4. With reference to the calculated band gap of 5.36 eV for MgAl2O4, the systematic decrease in the gap is attributed to the presence of 3d orbitals of Zn and Ga and the associated p-d hybridization in the upper valence band of zinc aluminate and gallate. Comparison of the contour maps of the electron localization function of ZnAl2O4 and ZnGa2O4 with that of MgAl2O4 clearly shows the bonding to be less ionic in the zinc-based spinel oxides. Finally, the calculations yield a smaller electron effective mass for zinc gallate as compared to that for zinc aluminate, suggesting a higher mobility of electrons in gallate.

Why Do Gallium Clusters Have a Higher Melting Point than the Bulk

Density functional molecular dynamical simulations have been performed on Ga17 and Ga13 clusters to understand the recently observed higher-than-bulk melting temperatures in small gallium clusters [Phys. Rev. Lett. 91, 215508 (2003)]]. The specific-heat curve, calculated with the multiple-histogram technique, shows the melting temperature to be well above the bulk melting point of 303 K, viz., around 650 and 1400 K for Ga17 and Ga13, respectively. The higher-than-bulk melting temperatures are attributed mainly to the covalent bonding in these clusters, in contrast with the covalent-metallic bonding in the bulk.

Structure, electronic properties, and magnetic transition in manganese clusters

We systematically investigate the structural, electronic, and magnetic properties of Mnn clusters n =2–2 0 within the ab initio pseudopotential plane wave method using generalized gradient approximation for the exchange-correlation energy. A new kind of icosahedral structural growth has been predicted in the intermediate size range. Calculated magnetic moments show an excellent agreement with the Stern-Gerlach experiment. A transition from ferromagnetic to ferrimagnetic Mn-Mn coupling takes place at n = 5 and the ferrimagnetic states continue to be the ground states for the entire size range. Possible presence of multiple isomers in the experimental beam has been argued. No signature of nonmetal to metal transition is observed in this size range and the coordination dependence of d-electron localization is discussed.

A systematic study of electronic structure from graphene to graphane

While graphene is a semi-metal, a recently synthesized hydrogenated graphene called graphane is an insulator. We have probed the transformation of graphene upon hydrogenation to graphane within the framework of density functional theory. By analysing the electronic structure for 18 different hydrogen concentrations, we bring out some novel features of this transition. Our results show that the hydrogenation favours clustered configurations leading to the formation of compact islands. The analysis of the charge density and electron localization function (ELF) indicates that, as hydrogen coverage increases, the semi-metal turns into a metal, showing a delocalized charge density, then transforms into an insulator. The metallic phase is spatially inhomogeneous in the sense it contains islands of insulating regions formed by hydrogenated carbon atoms and metallic channels formed by contiguous bare carbon atoms. It turns out that it is possible to pattern the graphene sheet to tune the electronic structure. For example, removal of hydrogen atoms along the diagonal of the unit cell, yielding an armchair pattern at the edge, gives rise to a bandgap of 1.4 eV. We also show that a weak ferromagnetic state exists even for a large hydrogen coverage whenever there is a sublattice imbalance in the presence of an odd number of hydrogen atoms.

Cohesive, electronic and magnetic properties of the transition metal aluminides FeAl CoAl and NiAl

Electronic structure calculations using the tight-binding linear muffin tin orbital (TB-LMTO) method have been performed for three transition metal aluminides, viz. FeAl, CoAl and NiAl. The band structures and density of states (DOS), valence electron charge density contours and Fermi surfaces have been obtained and compared with the available experimental results as well as with existing theoretical calculations. The lattice constants, cohesive energies and heat of formation at equilibrium lattice constants and bulk moduli agree with the experimental values. The calculations show varying degrees of charge transfer from Al site to the transition metal (TM) sites as one goes from FeAl to CoAl to NiAl. The magnetism of pure elements Fe, Co, Ni is mostly quenched in the stoichiometric phases, with only FeAl retaining a magnetic moment of about 0.7 mu B/atom within the framework of the LMTO.

Density functional investigation of the interaction of acetone with small gold clusters

The structural evolution of Au(n) (n=2, 3, 5, 7, 9, and 13) clusters and the adsorption of organic molecules such as acetone, acetaldehyde, and diethyl ketone on these clusters are studied using a density functional method. The detailed study of the adsorption of acetone on the Au(n) clusters reveals two main points. (1) The acetone molecule interacts with one gold atom of the gold clusters via the carbonyl oxygen. (2) This interaction is mediated through back donation mainly from the spd-hybridized orbitals of the interacting gold atom to the oxygen atom of the acetone molecule. In addition, a hydrogen bond is observed between a hydrogen atom of the methyl group and another gold atom (not involved in the bonding with carbonyl oxygen). Interestingly, the authors notice that the geometries of Au(9) and Au(13) undergo a significant flattening due to the adsorption of an acetone molecule. They have also investigated the role of the alkyl chain attached to the carbonyl group in the adsorption process by analyzing the interaction of Au(13) with acetaldehyde and diethyl ketone.

Aromaticity and antiaromaticity of LixAl4 clusters: Ring current patterns versus electron counting

Maps of magnetic-field induced current density are computed for a series of lithium–aluminium clusters based on the planar Al4 cycle: formal 2π systems LiAl4− (C4v), Li2Al4 (D4h, Cs), and formal 4π systems Li3Al4− (Cs), and Li4Al4 (C2h). All four species sustain a diatropic σ ring current in the Al4 cycle. In the 2π systems, although the 4n + 2 π electron count suggests aromaticity and hence diatropicity, the π orbital is magnetically inactive, as in the Al42− dianion. However, in the 4π formally antiaromatic systems, the π-like HOMO supports an additional paratropic current. Considerations of orbital symmetry and energy, but not electron counting alone, rationalise both computed currents. All calculations were carried out at the coupled Hartree–Fock level in a 6-31G** basis using the CTOCD-DZ (continuous transformation of origin of current density – diamagnetic zero), or ipsocentric, formulation of magnetic response, where current density at any point is obtained with that point itself chosen as the origin of vector potential.

Magic Melters Have Geometrical Origin

Recent experimental reports bring out extreme size sensitivity in the heat capacities of gallium and aluminum clusters. In the present work we report results of our extensive ab initio molecular dynamical simulations on Ga30 and Ga31, the pair which has shown rather dramatic size sensitivity. We trace the origin of this size sensitive heat capacities to the relative order in their respective ground state geometries. Such an effect of nature of the ground state on the characteristics of heat capacity is also seen in case of small gallium and sodium clusters, indicating that the observed size sensitivity is a generic feature of small clusters.

Ferromagnetism in Carbon-Doped Zinc Oxide Systems

We report spin-polarized density functional calculations of ferromagnetic properties for a series of ZnO clusters and ZnO solid containing one or two substitutional carbon impurities. We analyze the eigenvalue spectra, spin densities, molecular orbitals, and induced magnetic moments for ZnC, Zn(2)C, Zn(2)OC, carbon-substituted Zn(n)O(n) (n = 3-10, 12) clusters and the bulk ZnO. The results show that the doping induces magnetic moment of approximately 2 mu(B) in all the cases. All systems with two carbon impurities show ferromagnetic interaction, except when carbon atoms share the same zinc atom as the nearest neighbor. This ferromagnetic interaction is predominantly mediated via pi-bonds in the ring structures and through pi- and sigma-bonds in the three-dimensional structure. The calculations also show that the interaction is significantly enhanced in the solid, bringing out the role of dimensionality of the Zn-O network connecting two carbon atoms.

Thermodynamics of tin clusters

We report the results of detailed thermodynamic investigations of the