Pushpa Raghani

Spin and Exchange Coupling for Ti Embedded in a Surface Dipolar Network

Pushpa Raghani et al. — Thu, 15 Sep 2011 08:28:25 PDT

We have studied the spin and exchange coupling of Ti atoms deposited on a Cu2N/Cu(100) surface using density functional theory with generalized gradient approximation +U. In agreement with experiments, we find that Ti has the highest binding on top of Cu atoms. We also find that the spin of individual Ti atoms deposited on the Cu2N/Cu(100) surface increases as Ti coverage on the surface is decreased. For U=0, the spin of a Ti atom starts at S=0 at high coverages and increases to S=1/2 as the coverage is decreased, which agrees very well with results obtained from STM experiments. At higher values of U, the spin of Ti is found to be close to 1 regardless of coverage. We also calculate the exchange coupling for Ti dimers on the Cu2N/Cu(100) surface and we find that the exchange coupling across a 'void' of 3.6Å is antiferromagnetic, whereas indirect (superexchange) coupling through a N atom is ferromagnetic. For a square lattice of Ti on Cu2N/Cu(100), we nd a novel spin striped phase.

SixC1−xO2 Alloys: A Possible Route to Stabilize Carbon-Based Silica-Like Solids?

Pushpa Raghani — Sun, 30 Aug 2009 21:29:59 PDT

Novel extended tetrahedral forms of CO2 have been synthesized recently under high-pressure conditions. We perform ab initio density functional theory calculations to investigate whether doping with Si can extend the stability range of such tetrahedral forms of CO2 to ambient pressure. Calculations are performed with a simple cubic cell containing eight formula units in a β-cristobalite-like structure. Though we find that all the SixC1−xO2 structures considered by us are thermodynamically unstable with respect to decomposition into the end members at ambient pressures, the energy differences are small, suggesting that it might be possible for such phases to exist in metastable forms. At higher pressures, the heat of formation is found to be negative. The bonding between C and O atoms is more covalent than that between Si and O atoms. We also find indications that some C atoms may prefer three-fold coordination at low pressure.

Bond Stiffening in Small Nanoclusters and Its Consequences for Mechanical and Thermal Properties

Pushpa Raghani et al. — Sun, 30 Aug 2009 21:25:14 PDT

We have used density functional perturbation theory to investigate the stiffness of interatomic bonds in small clusters of Si, Sn, and Pb. As the number of atoms in a cluster is decreased, there is a marked shortening and stiffening of bonds. The competing factors of fewer but stiffer bonds in clusters result in softer elastic moduli but higher average frequencies as size is decreased, with clear signatures of universal scaling relationships. The stiffness of bonds is found to scale as the inverse tenth power of length. A significant role in understanding trends is played by the coordination number of the bulk structure: The higher this is, the lesser is the relative softening of elastic constants and the greater the relative damping of vibrational amplitudes for clusters compared to the bulk. Our results could provide a framework for understanding recent reports that some clusters remain solid above the bulk melting temperature. Our results suggest that Sn and Pb clusters (but not Si clusters) are more thermally stable than the bulk.

Interplay Between Bonding and Magnetism in the Binding of NO to Rh Clusters

Prasenjit Ghosh et al. — Sun, 30 Aug 2009 21:18:20 PDT

We have studied the binding of NO to small Rh clusters, containing one to five atoms, using density functional theory in both spin-polarized and non-spin-polarized forms. We find that NO bonds more strongly to Rh clusters than it does to Rh(100) or Rh(111), suggesting that Rh clusters may be good catalysts for NO reduction. However, binding to NO also quenches the magnetism of the clusters. This (local) effect results in reducing the magnitude of the NO binding energy, and also washes out the clear size-dependent trend observed in the nonmagnetic case. Our results illustrate the competition present between the tendencies to bond and to magnetize, in small clusters.