Tight-Binding Molecular Dynamics of Ceramic Nanocrystals Using Pc-Based Parallel Machines

1999 ◽  
Vol 581 ◽  
Author(s):  
Kenji Tsuruta ◽  
Hiroo Totsuji ◽  
Chieko Totsuji
Keyword(s):  
Molecular Dynamics ◽  
Electronic Structures ◽  
Parallel Machines ◽  
Tight Binding ◽  
Expansion Method ◽  
Surface Structures ◽  
Electronic Density ◽  
Pc Cluster ◽  
Fermi Operator ◽  
Gap States

ABSTRACTEvolution of atomic and electronic structures of silicon-carbide (SiC) nanocrystals during sintering is investigated by a tight-binding molecular dynamics (TBMD) method. An O(N) algorithm (the Fermi-operator expansion method) is employed for calculating electronic contributions in the energy and forces. Simulations are performed on our eight-node parallel PC cluster. In a sintering simulation of aligned (no tilt or twist) SiC nanocrystals at T = 1000K, we find that a neck is formed promptly without formation of defects. Analyses of local electronic density-of-states (DOS) and effective charges reveal that unsaturated bonds exist only in grain surfaces accompanying the gap states. In the case of tilted (<122>) nanocrystals, surface structures formed before sintering affect significantly the grainboundary formation.

Parallel Tight-Binding Simulations of Nanophase Ceramics: Atomic and Electronic Transport at Grain Boundaries

2000 ◽  
Vol 653 ◽  
Author(s):  
Kenji Tsuruta ◽  
Hiroo Totsuji ◽  
Chieko Totsuji
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Electronic Transport ◽  
Parallel Machines ◽  
Tight Binding ◽  
Expansion Method ◽  
Nanocrystalline Silicon ◽  
Contact Angles ◽  
Neck Formation ◽  
Fermi Operator

AbstractWe report on tight-binding molecular dynamics (TBMD) of neck formation processes and atomistic and electronic diffusivity at grain boundaries of nanocrystalline silicon carbide. The TBMD simulations are based on an O(N) algorithm (the Fermi-operator expansion method) for calculating electronic contributions to energy and forces. The code has been fully parallelized on our PC-based parallel machines. The TBMD simulations of collision of SiC nanospheres show that the processes of neck formation depend strongly on contact angles between the two grains. Atomic diffusions are quite different in the necks formed with different angles. Also, the electronic transport property at grain boundary is investigated via a TB representation of an electronic diffusivity. A preliminary result on the diffusivity at a Σ=9 grain boundary of SiC indicates significant enhancement of electron mobility along the grain boundary.

scholarly journals Initial Stage of Consolidation of Silicon-Carbide Nanocrystals under Pressure: A Tight-Binding Molecular-Dynamics Study

2011 ◽  
Vol 2011 ◽  
pp. 1-6 ◽  
Author(s):  
Kenji Tsuruta
Keyword(s):  
Molecular Dynamics ◽  
Silicon Carbide ◽  
Tight Binding ◽  
Expansion Method ◽  
Nanocrystalline Silicon ◽  
External Pressure ◽  
Fermi Operator ◽  
Initial Stage ◽  
Long Time ◽  
Nanocrystalline Silicon Carbide

Tight-binding molecular-dynamics (TBMDs) simulations are performed to study atomic and electronic structures during high-temperature consolidation processes of nanocrystalline silicon carbide under external pressure. We employ a linear-scaling method (the Fermi-operator expansion method) with a scalable parallel algorithm for efficient calculations of the long time-scale phenomena. The results show that microscopic processes of the consolidation depend strongly on initial orientations of the nanocrystals. It is observed that an orientational rearrangement of the nanocrystals initially misaligned is induced by an instantaneous shearing force between nanocrystals, whereas the aligned system undergoes densification without shearing. Analysis on an effective-charge distribution and an average bond-order distribution reveals electronic-structure evolutions during these processes.

Structural Disorder and Localized Gap States in Silicon Grain Boundaries from a Tight-Binding Model

1997 ◽  
Vol 491 ◽  
Author(s):  
F. Cleri ◽  
P. Keblinski ◽  
L. Colombo ◽  
S. R. Phillpot ◽  
D. Wolf
Keyword(s):  
Molecular Dynamics ◽  
Grain Boundaries ◽  
Tight Binding ◽  
Structural Disorder ◽  
High Energy ◽  
Tight Binding Model ◽  
Binding Model ◽  
Dynamics Simulations ◽  
Gap States ◽  
Forbidden Gap

ABSTRACTTight-binding molecular dynamics simulations of typical high-energy grain boundaries in silicon show that the atomic structure of the interface in thermodynamic equilibrium is similar to that of bulk amorphous silicon and contains coordination defects. The corresponding electronic structure is also amorphous-like, displaying extra states in the forbidden gap mainly localized around the coordination defects, where large changes in the bond-hybridization character are observed. It is proposed that such coordination defects in disordered high-energy grain boundaries are responsible for the experimentally observed gap states in polycrystalline Si.

THE ELECTRONIC STRUCTURE AND MEMORY DEVICE APPLICATIONS OF TETRAHEDRAL AMORPHOUS CARBON

2000 ◽  
Vol 14 (02n03) ◽  
pp. 230-241 ◽  
Author(s):  
D. R. McKENZIE ◽  
E. G. GERSTNER ◽  
A. R. MERCHANT ◽  
D. G. McCULLOCH ◽  
P. E. GOA ◽  
...  
Keyword(s):  
Molecular Dynamics ◽  
Amorphous Carbon ◽  
Density Functional ◽  
Tight Binding ◽  
Memory Device ◽  
Theory Approach ◽  
Operating Characteristics ◽  
Electronic Density ◽  
Tetrahedral Amorphous Carbon ◽  
Device Applications

The introduction of nitrogen dopant sites into tetrahedral amorphous carbon produces changes in the structure and the electronic density of states that can be modelled using molecular dynamics. In this work we use both a tight-binding approach and a Car-Parrinello density functional theory approach. In a comparison of these, we found that the former tends to overestimate the strain energy of 3 membered carbon rings relative to the latter and to experiment, explaining the reduced occurrence of 3 membered rings in networks and simulated using tight-binding. Experiment shows that at approximately 3% of nitrogen, the network begins to change rapidly with nitrogen content. In this form, an additional electronic conduction mode is found experimentally, of the Poole-Frenkel type, which can be turned on and off at will. The conduction is turned on by negative voltage excursion and quenched by a positive one. This conduction bistability can be exploited to produced a simple new type of memory device in which the high conductivity state (''on'') is a digital ''1'' and the low conductivity state (''off'') is a digital ''0''. The operating characteristics of the device are excellent, with more than one million read cycles having been demonstrated without deterioration of the discrimination between the ''on'' and ''off'' states. Molecular dynamics is used to study the configuration of the nitrogen atoms, yielding a possible candidate for the site responsible for the Poole-Frenkel conduction.

A Parallel Implementation of Tight-Binding Molecular Dynamics Based on Reordering of Atoms and the Lanczos Eigen-Solver

1995 ◽  
Vol 408 ◽  
Author(s):  
Luciano Colombot ◽  
William Sawyer ◽  
Djordje Marict
Keyword(s):  
Molecular Dynamics ◽  
Parallel Machines ◽  
Materials Science ◽  
Parallel Implementation ◽  
Tight Binding ◽  
Hamiltonian Matrix ◽  
Time Step ◽  
Full Spectrum ◽  
Eigen Values ◽  
Storage Format

AbstractWe introduce an efficient and scalable parallel implementation of tight-binding molecular dynamics (TBMD) which employs reordering of the atoms in order to maximize datalocality of the distributed tight-binding (TB) Hamiltonian matrix. Reordering of the atom labels allows our new algorithm to scale well on parallel machines since most of the TB hopping integrals for a given atom are local to the processing element (PE) therefore minimizing communication. The sparse storage format and the distribution of the required eigenvectors reduces memory requirements per PE. The sparse storage format and a stabilized parallel Lanczos eigen-solver allow consideration of large problem sizes relevant to materials science. In addition, the implementation allows the calculation of the full spectrum of individual eigen-values/-vectors of the TB matrix at each time-step. This feature is a key issue when the dielectric and optical response must be computed during a TBMD simulation. We present a benchmark of our code and an analysis of the overall efficiency.

Prediction of a Very Hard Triclinic Form of Diamond

1995 ◽  
Vol 408 ◽  
Author(s):  
G. Benedek ◽  
M. Facchinetti ◽  
L. Miglio ◽  
S. Serra
Keyword(s):  
Molecular Dynamics ◽  
Ground State ◽  
Tight Binding ◽  
Cubic Phase ◽  
Ground State Structure ◽  
State Structure ◽  
Carbon Structure ◽  
Electronic Density ◽  
Phonon Spectra ◽  
Triclinic Form

AbstractIn a theoretical search for new hypotetical sp3-bonded carbon structure containing five-fold rings as a possible result of fullerene transformation under pressure, we have found a triclinic form of diamond with 16 atoms per unit cell which we called tcl-16. We have calculated the ground state structure, the cohesive energy, the bulk modulus and the electronic density of states by means of tight binding molecular dynamics (TBMD). Finally we have compared the phonon spectra at F to existing Raman data for a non-cubic- phase of diamond.

Atomic and electronic structures of a-SiC:H from tight-binding molecular dynamics

2003 ◽  
Vol 15 (24) ◽  
pp. 4119-4126 ◽  
Author(s):  
V I Ivashchenko ◽  
P E A Turchi ◽  
V I Shevchenko ◽  
L A Ivashchenko ◽  
G V Rusakov
Keyword(s):  
Molecular Dynamics ◽  
Electronic Structures ◽  
Tight Binding

Origins of the Gap States in Polycrystalline Silicon: Tight-Binding Calculations of Twist Boundaries

1992 ◽  
Vol 262 ◽  
Author(s):  
M. Kohyama ◽  
S. Kose ◽  
R. Yamamoto
Keyword(s):  
Electronic Structures ◽  
Polycrystalline Silicon ◽  
Tight Binding ◽  
Electronic States ◽  
Structural Disorder ◽  
Interfacial Energies ◽  
Amorphous Si ◽  
Gap States ◽  
Twist Boundaries ◽  
Generate Characteristic

ABSTRACTThe atomic and electronic structures of the twist boundaries Σ (=3 (011), Σ=7 (111) and Σ=5 (001)) in Si have been calculated by using the transferable SETB method coupled with the supercell technique. The twist boundaries in Si contain larger structural disorder or more defects and larger interfacial energies than tilt grain boundaries. Several kinds of structural disorder or defects have been found to generate characteristic electronic states inside the gap. The present structural disorder or defects and the gap states are the candidates of the origins of the observed band-tails or mid-gap states in polycrystalline Si as well as those In amorphous Si.

Structural Phase Transition from Fluorite to Orthorhombic FeSi2 by Tight Binding Molecular Dynamics

1995 ◽  
Vol 408 ◽  
Author(s):  
Leo Miglio ◽  
Massimo Celino ◽  
Valeria Meregalli ◽  
Francesca Tavazza
Keyword(s):  
Phase Transition ◽  
Molecular Dynamics ◽  
Density Of States ◽  
Structural Phase Transition ◽  
Structural Phase ◽  
Tight Binding ◽  
Dynamics Simulation ◽  
Fluorite Phase ◽  
Electronic Density ◽  
Jahn Teller

AbstractIn this paper we report a molecular dynamics simulation at constant pressure and constant temperature of the structural phase transition occurring in epitaxial FeSi2 from the fluorite phase (metallic and pseudomorphic) to orthorhombic one (semiconductor and bulk stable). The evolution of the electronic density of states is carefully monitored during the transformation and we can show that the Jahn-Teller coupling between the density of states at the Fermi level and the lattice deformation drives the metal-semiconductor transition.

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