Nithaya Chetty

First principles LDA+U and GGA+U study of protactinium and protactinium oxides : dependence on the effective U parameter

The electronic structure and properties of protactinium and its oxides (PaO and PaO2) have been studied within the framework of the local density approximation (LDA), the Perdew-Burke-Ernzerhof generalized gradient approximation [GGA(PBE)], LDA + U and GGA(PBE) + U implementations of density functional theory. The dependence of selected observables of these materials on the effective U parameter has been investigated in detail. The examined properties include lattice constants, bulk moduli, the effect of charge density distributions, the hybridization of the 5f orbital and the energy of formation for PaO and PaO2. The LDA gives better agreement with experiment for the bulk modulus than the GGA for Pa but the GGA gives better structural properties. We found that PaO is metallic and PaO2 is a Mott-Hubbard insulator. This is consistent with observations for the other actinide oxides. We discover that GGA and LDA incorrectly give metallic behavior for PaO2. The GGA(PBE) + U calculated indirect band gap of 3.48 eV reported for PaO2 is a prediction and should stimulate further studies of this material.

EFFECTIVE-MEDIUM TIGHT-BINDING MODEL FOR SILICON

A method for calculating the total energy of Si systems, which is based on the effective-medium-theory concept of a reference system, is presented. Instead of calculating the energy of an atom in the system of interest, a reference system is introduced where the local surroundings are similar. The energy of the reference system can be calculated self-consistently once and for all while the energy difference to the reference system can be obtained approximately. We propose to calculate it using the tight-binding linear-muffin-tin-orbital scheme with the atomic-sphere approximation (ASA) for the potential, and by using the ASA with charge-conserving spheres we are able to treat open systems without introducing empty spheres. All steps in the calculational method are [ital ab] [ital initio] in the sense that all quantities entering are calculated from first principles without any fitting to experiment. A complete and detailed description of the method is given together with test calculations of the energies of phonons, elastic constants, different structures, surfaces, and surface reconstructions. We compare the results to calculations using an empirical tight-binding scheme.

Theoretical studies of iridium under pressure

Recent experiments on Ir under pressure (Cerenius and Dubrovinsky 2000 J. Alloys Compounds 306 26) show a transition to a superlattice structure comprising 14 atomic layers. This observation has implications for high-pressure applications since Ir, with its high bulk modulus and high thermal stability, is ideally suited for use as a gasket for high-temperature, high-pressure diamond anvil cell experiments. We perform first-principles total energy calculations to study the crystal phases and defect structures of Ir under pressure. We have extended the bond-orientation model (Chetty and Weinert 1997 Phys. Rev. B 56 10844) to compute all of the ~ 2N defect structures as a function of atomic volume. We find Ir in the FCC structure to be extremely stable for pressures up to about 60 GPa. We also calculate the stacking fault energies of Ir.

Optimized and transferable densities from first-principles local density calculations

From ab initio pseudopotential calculations of the solid the authors extract atomic-like electron densities which, when overlapped in the crystal, reproduce the self-consistent density except for those components for which the structure factor is zero. They demonstrate the universality of the optimized densities for different crystal structures at varying volumes. These densities are the optimized choice for the Harris functional, and they compare their result with the Finnis contraction of the free atom density and with the effective medium theory ansatz of embedding an atom in a homogeneous electron gas.

Mechanical properties of hydrogenated bilayer graphene

Using first principle methods, we study the mechanical properties of monolayer and bilayer graphene with 50% and 100% coverage of hydrogen. We employ the vdW-DF, vdW-DF-C09x, and vdW-DF2-C09x van der Waals functionals for the exchange correlation interactions that give significantly improved interlayer spacings and energies. We also use the PBE form for the generalized gradient corrected exchange correlation functional for comparison. We present a consistent theoretical framework for the in-plane layer modulus and the out-of-plane interlayer modulus and we calculate, for the first time, these properties for these systems. This gives a measure of the change of the strength properties when monolayer and bilayer graphene are hydrogenated. Moreover, comparing the relative performance of these functionals in describing hydrogenated bilayered graphenes, we also benchmark these functionals in how they calculate the properties of graphite.

Defect charge states in Si doped hexagonal boron-nitride monolayer

We perform ab initio density functional theory calculations to investigate the energetics, electronic and magnetic properties of isolated stoichiometric and non-stoichiometric substitutional Si complexes in a hexagonal boron-nitride monolayer. The Si impurity atoms substituting the boron atom sites SiB giving non-stoichiometric complexes are found to be the most energetically favourable, and are half-metallic and order ferromagnetically in the neutral charge state. We find that the magnetic moments and magnetization energies increase monotonically when Si defects form a cluster. Partial density of states and standard Mulliken population analysis indicate that the half-metallic character and magnetic moments mainly arise from the Si 3p impurity states. The stoichiometric Si complexes are energetically unfavorable and non-magnetic. When charging the energetically favourable non-stoichiometric Si complexes, we find that the formation energies strongly depend on the impurity charge states and Fermi level position. We also find that the magnetic moments and orderings are tunable by charge state modulation q  =  -2, -1, 0, +1, +2. The induced half-metallic character is lost (retained) when charging isolated (clustered) Si defect(s). This underlines the potential of a Si doped hexagonal boron-nitride monolayer for novel spin-based applications.

First principles molecular dynamics study of nitrogen vacancy complexes in boronitrene

We present the results of first principles molecular dynamics simulations of nitrogen vacancy complexes in monolayer hexagonal boron nitride. The threshold for local structure reconstruction is found to be sensitive to the presence of a substitutional carbon impurity. We show that activated nitrogen dynamics triggers the annihilation of defects in the layer through formation of Stone-Wales-type structures. The lowest energy state of nitrogen vacancy complexes is negatively charged and spin polarized. Using the divacancy complex, we show that their formation induces spontaneous magnetic moments, which is tunable by electron or hole injection. The Fermi level s-resonant defect state is identified as a unique signature of the ground state of the divacancy complex. Due to their ability to enhance structural cohesion, only the divacancy and the nitrogen vacancy carbon-antisite complexes are able to suppress the Fermi level resonant defect state to open a gap between the conduction and valence bands.

Enhancing the understanding of entropy through computation

We devise an algorithm to enumerate the microstates of a system comprising N independent, distinguishable particles. The algorithm is applicable to a wide class of systems such as harmonic oscillators, free particles, spins, and other models for which there are no analytical solutions, for example, a system with single particle energy spectrum given by ɛ(p,q) = ɛ0(p2 + q4), where p and q are non-negative integers. Our algorithm enables us to determine the approach to the limit N → ∞ within the microcanonical ensemble, and makes manifest the equivalence with the canonical ensemble. Various thermodynamic quantities as a function of N can be computed using our methods.

Suppressed bond-site percolation

Abstract We have developed a new model for correlated bond-site percolation, namely the suppressed bond-site percolation model. In this scheme, the sites are occupied with probability p s as is usual, but the bonds are now opened with a probability that depends on the occupancy of sites in the local vicinity cluster. The vicinity cluster is defined as the cluster of sites that involves the bond endpoint sites as well as the nearest-neighbour sites of these endpoint sites. We believe that the suppressed bond-site model is a useful description of polymerisation. We have studied this new model numerically, using Monte Carlo methods. We have discovered and investigated new features in the percolation threshold.

Construction of transferable spherically averaged electron potentials

A new scheme for constructing approximate effective electron potentials within density-functional theory is proposed. The scheme consists of calculating the effective potential for a series of reference systems, and then using these potentials to construct the potential of a general system. To make contact with the reference system the neutral-sphere radius of each atom is used. The scheme can simplify calculations with partial wave methods in the atomic-sphere or muffin-tin approximation, since potential parameters can be precalculated and then for a general system obtained through simple interpolation formulas. We have applied the scheme to construct electron potentials of phonons, surfaces, and different crystal structures of silicon and aluminium atoms, and found excellent agreement with the self-consistent effective potential. By using an approximate total electron density obtained from a superposition of atom-based densities, the energy zero of the corresponding effective potential can be found and the energy shifts in the mean potential between inequivalent atoms can therefore be directly estimated. This approach is shown to work well for surfaces and phonons of silicon.