31 research outputs found
Adatom-assisted structural transformations of fullerenes
Microscopic mechanism of autocatalytic structural transformations of fullerenes is investigated by the action-derived molecular dynamics. Dynamic pathways and the corresponding activation energies are obtained for the Stone-Wales transformation in fullerene and the fullerene coalescence, under the presence of extra carbon atoms. The adatom-assisted Stone-Wales transformation is proved to be a highly probable process unit for the structural transformations and annealing treatments of carbon-based graphitic networks. The complex processes of adatom-assisted fullerene coalescence, yielding very low activation energies, are presented.open271
Interaction and ordering of vacancy defects in NiO
By using a first-principles method employing the local density approximation plus Hubbard parameter approach, we study point defects in NiO and interactions between them. The defect states associated with nickel or oxygen vacancies are identified within the energy gap. It is found that nickel vacancies introduce shallow levels in the density of states for the spin direction opposite to that of the removed Ni atom, while the oxygen vacancy creates more localized in-gap states. The interaction profiles between vacancies indicate that specific defect arrangements are strongly favored for both nickel and oxygen vacancies. In the case of nickel vacancies, defect ordering in a simple-cubic style is found to be most stable, leading to a half-metallic behavior. The ionized oxygen vacancies also show a tendency toward clustering, more strongly than neutral pairs. The microscopic origin of vacancy clustering is understood based on overlap integrals between defect states. © 2008 The American Physical Society.open343
HARES: an efficient method for first-principles electronic structure calculations of complex systems
We discuss our new implementation of the Real-space Electronic Structure
method for studying the atomic and electronic structure of infinite periodic as
well as finite systems, based on density functional theory. This improved
version which we call HARES (for High-performance-fortran Adaptive grid
Real-space Electronic Structure) aims at making the method widely applicable
and efficient, using high performance Fortran on parallel architectures. The
scaling of various parts of a HARES calculation is analyzed and compared to
that of plane-wave based methods. The new developments that lead to enhanced
performance, and their parallel implementation, are presented in detail. We
illustrate the application of HARES to the study of elemental crystalline
solids, molecules and complex crystalline materials, such as blue bronze and
zeolites.Comment: 17 two-column pages, including 9 figures, 5 tables. To appear in
Computer Physics Communications. Several minor revisions based on feedbac
Ab initio Pseudopotential Plane-wave Calculations of the Electronic Structure of YBa_2Cu_3O_7
We present an ab initio pseudopotential local density functional calculation
for stoichiometric high-Tc cuprate YBa_2Cu_3O_7 using the plane-wave basis set.
We have overcome well-known difficulties in applying pseudopotential methods to
first-row elements, transition metals, and rare-earth materials by carefully
generating norm-conserving pseudopotentials with excellent transferability and
employing an extremely efficient iterative diagonalization scheme optimized for
our purpose. The self-consistent band structures, the total and site-projected
densities of states, the partial charges and their symmetry-decompositions, and
some characteristic charge densities near E_f are presented. We compare our
results with various existing (F)LAPW and (F)LMTO calculations and establish
that the ab initio pseudopotential method is competitive with other methods in
studying the electronic structure of such complicated materials as high-Tc
cuprates. [8 postscript files in uuencoded compressed form]Comment: 14 pages, RevTeX v3.0, 8 figures (appended in postscript file), SNUTP
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Kinematic Prediction and Experimental Demonstration of Conditioning Process for Controlling the Profile Shape of a Chemical Mechanical Polishing Pad
The uniformity of the wafer in a chemical mechanical polishing (CMP) process is vital to the ultra-fine and high integration of semiconductor structures. In particular, the uniformity of the polishing pad corresponding to the tool directly affects the polishing uniformity and wafer shape. In this study, the profile shape of a CMP pad was predicted through a kinematic simulation based on the trajectory density of the diamond abrasives of the diamond conditioner disc. The kinematic prediction was found to be in good agreement with the experimentally measured pad profile shape. Based on this, the shape error of the pad could be maintained within 10 μm even after performing the pad conditioning process for more than 2 h, through the overhang of the conditioner