Scientific Publications 2009
2009. "Ab initio investigation of phase stability of Y2Ti2O7 and Y2Zr2O7 under high pressure." Physical Review. B, Condensed Matter and Materials Physics 80(21):Art. No. 212102. doi:10.1103/PhysRevB.80.212102 Abstract The phase stabilities of Y2Ti2O7 and Y2Zr2O7 under high pressure were investigated by ab initio methods. Pyrochlore-structured Y2Ti2O7 and defect-fluorite Y2Zr2O7 exhibit different responses to high pressure. Both the defect-fluorite and defect-cotunnite structures are energetically more stable at high pressure in Y2Ti2O7, but comparison with experimental results suggest that only the transformation to the defect-fluorite structure is kinetically favored. For Y2Zr2O7, the defect-fluorite phase should undergo a structural transformation to the defect-cotunnite state under high pressure.
2009. "Threshold displacement energy in GaN; Ab initio molecular dynamics study." Journal of Applied Physics 105(12):123527, 1-5. doi:10.1063/1.3153277 Abstract Large-scale ab initio molecular dynamics method has been used to determine the threshold displacement energies, Ed, along five specific directions and to determine the defect configurations created during low energy events. The Ed shows a significant dependence on direction. The minimum Ed is determined to be 39 eV along the <-1010> direction for a gallium atom and 17.0 eV along the <-1010> direction for a nitrogen atom, which are in reasonable agreement with the experimental measurements. The average Ed values determined are 73.2 and 32.4 eV for gallium and nitrogen atoms, respectively. The N defects created at low energy events along different crystallographic directions have a similar configuration (a N-N dumbbell configuration), but various configurations for Ga defects are formed in GaN.
2009. "Photoelectron Angular Distribution and Molecular Structure in Multiply Charged Anions." Journal of Physical Chemistry A 113(6):945-948. Abstract Photoelectrons emitted from multiply charged anions (MCAs) carry information of the intramolecular Coulomb repulsion (ICR), which is dependent on molecular structures. Using photoelectron imaging, we observed the effects of ICR on photoelectron angular distributions (PAD) of the three isomers of benzene dicarboxylate dianions C6H4(CO2)22– (o-, m- and p-BDC2–). Photoelectrons were observed to peak along the laser polarization due to the ICR, but the anisotropy was the largest for p-BDC2–, followed by the m- and o-isomer. The observed anisotropy is related to the direction of the ICR or the detailed molecular structures, suggesting that photoelectron imaging may allow structural information to be obtained for complex multiply charged anions.
2009. "Photoelectron imaging of multiply charged anions:Effects of intramolecular Coulomb repulsion and photoelectron kinetic energies on photoelectron angular distributions." Journal of Chemical Physics 130(7):Article no. 074301. Abstract Multiply charged anions possess strong intramolecular Coulomb repulsion *ICR*, which has been shown to dictate photoelectron angular distributions PADs using photoelectron imaging. Here we report the effects of photoelectron kinetic energies on the PADs of multiply charged anions. Photoelectron images on a series of dicarboxylate dianions, −O2CCH2*nCO2 − Dn 2− , n=3–11 have been measured at two photon energies, 532 and 266 nm. The first photoemission band of Dn 2−, which is a perpendicular transition in the absence of the ICR, comes from electron detachment of an O lone pair orbital on the –CO2 − end groups. Recent photoelectron imaging studies at 355 nm show that the PADs of Dn 2− peak in the directions parallel to the laser polarization for small n due to the ICR, which directs the outgoing electrons along the molecular axis. The current data show much stronger parallel peaking at 532 nm, but much weaker parallel peaking in the 266 nm data, relative to the 355 nm data. These observations indicate that the ICR has greater influence on the trajectories of slow photoelectrons and much reduced effects on faster photoelectrons. This study demonstrates that the PADs of multiply charged anions depend on the interplay between ICR and the outgoing photoelectron kinetic energies.
2009. "Diffuse-interface model for smoothed particle hydrodynamics." Physical Review. E, Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics 79(3):Art. No. 036702. Abstract Diffuse-interface theory provides a foundation for the modeling and simulation of microstructure evolution in a very wide range of materials, and for the tracking/capturing of dynamic interfaces between different materials on larger scales. Smoothed particle hydrodynamics (SPH) is also widely used to simulate fluids and solids that are subjected to large deformations and have complex dynamic boundaries and/or interfaces, but no explicit interface tracking/capturing is required, even when topological changes such as fragmentation and coalescence occur, because of its Lagrangian particle nature. Here we developed an SPH model for single-two component singletwo- phase fluids that is based on diffuse-interface theory. In the model, the interface has a finite thickness and a surface tension that depend on the coefficient, k, of the gradient contribution to the Helmholtz free energy functional and the density dependent homogeneous free energy. In this model, there is no need to locate the surface (or interface) or to compute the curvature at and near the interface. One- and two-dimensional SPH simulations were used to validate the model.
2009. "Spectral/HP Element Method With Hierarchical Reconstruction for Solving Hyperbolic Conservation Laws." Acta Mathematica Scientia 29B(6):1737-1748. Abstract Hierarchical reconstruction (HR) has been successfully applied to prevent oscillations in solutions computed by finite volume, discontinuous Galerkin, spectral volume schemes when solving hyperbolic conservation laws. In this paper, we demonstrate that HR can also be combined with spectral/hp element methods for solving hyperbolic conservation laws. We show that HR preserves the order of accuracy of spectral/hp element methods for smooth solutions and generate essentially non-oscillatory solution profiles for shock wave problems.