674 research outputs found
Shock Induced Decomposition and Sensitivity of Energetic Materials by ReaxFF Molecular Dynamics
We develop strain-driven compression-expansion technique using molecular dynamics (MD) with reactive force fields (ReaxFF) to study the impact sensitivity of energetic materials. It has been applied to simulation of 1,3,5-trinitrohexahydro-s-triazine (RDX) crystal subjected to high-rate compression typical at the detonation front. The obtained results show that at lower compression ratio x = 1-V/V040%) all molecules decompose very quickly. We have observed both primary and secondary reactions during the decomposition process as well as production of various intermediates (NO2, NO, HONO, OH) and final products (H2O, N2, CO, CO2). The results of strain-driven compression-expansion modeling are in a good agreement with previous ReaxFF-MD shock simulations in RDX. Proposed approach might be useful for a quick test of sensitivity of energetic materials under conditions of high strain rate loading
Energetic Materials at High Compression: First-Principles Density Functional Theory and Reactive Force Field Studies
We report the results of a comparative study of pentaerythritol tetranitrate (PETN) at high compression using classical reactive interatomic potential ReaxFF and first-principles density functional theory (DFT). Lattice parameters of PETN I, the ground state structure at ambient conditions, is obtained by ReaxFF and two different density functional methods (plane wave and LCAO pseudopotential methods) and compared with experiment. Calculated energetics and isothermal equation of state (EOS) upon hydrostatic compression obtained by DFT and ReaxFF are both in good agreement with available experimental data. Our calculations of the hydrostatic EOS at zero temperature are extended to high pressures up to 50 GPa. The anisotropic characteristics of PETN upon uniaxial compression were also calculated by both ReaxFF and DFT
Interplay of superconductivity and magnetism in strong coupling
A model is introduced describing the interplay between superconductivity and
spin-ordering. It is characterized by on-site repulsive electron-electron
interactions, causing antiferromagnetism, and nearest-neighbor attractive
interactions, giving rise to d-wave superconductivity. Due to a special choice
for the lattice, this model has a strong-coupling limit where the
superconductivity can be described by a bosonic theory, similar to the strongly
coupled negative U Hubbard model. This limit is analyzed in the present paper.
A rich mean-field phase diagram is found and the leading quantum corrections to
the mean-field results are calculated. The first-order line between the
antiferromagnetic- and the superconducting phase is found to terminate at a
tricritical point, where two second-order lines originate. At these lines, the
system undergoes a transition to- and from a phase exhibiting both
antiferromagnetic order and superconductivity. At finite temperatures above the
spin-disordering line, quantum-critical behavior is found. For specific values
of the model parameters, it is possible to obtain SO(5) symmetry involving the
spin- and the phase-sector at the tricritical point. Although this symmetry is
explicitly broken by the projection to the lower Hubbard band, it survives on
the mean-field level, and modes related to a spontaneously broken SO(5)
symmetry are present on the level of the random phase approximation in the
superconducting phase.Comment: 16 pages Revtex, 5 figure
Quantum-chemical calculations of dye-sensitized semiconductor nanocrystals
Quantum chemical calculations providing detailed information of dye-sensitized semiconductor nanocrystals are presented. The calculations are used to elucidate both structural and electronic properties of photoelectrochemical devices, such as environmentally friendly Dye-Sensitized Solar Cells (DSSCs), at the molecular level. Quantum chemical calculations have recently been performed on both organic and organometallic dye molecules attached to titanium dioxide (TiO2) nanocrystals via different anchor and spacer groups. Strategies to make accurate quantum chemical calculations, e.g. at the DFT level of theory, on increasingly realistic models of such dye-sensitized semiconductor interfaces are presented. The ability of different anchor and spacer groups to act as mediators of ultrafast photo-induced electron injection from the dye molecules into the semiconductor nanocrystals is, in particular, discussed in terms of calculated electronic coupling strengths, and direct comparisons with experimental information are made whenever possible. Progress in the development of multi-scale simulation techniques using so called reactive force fields is illustrated for dye-sensitized solar cell systems
On the origin of the quantum-critical transition in the bilayer Heisenberg model
The bilayer Heisenberg antiferromagnet is known to exhibit a quantum-critical
transition at a particular value of the inter-layer coupling. Using a new type
of coherent state, appropriate to the special order parameter structure of the
bilayer, we map the problem onto the quantum non-linear sigma model. It is
found that the bare coupling constant diverges at the classical transition of
Chubukov and Morr, so that in any finite dimension the actual transition occurs
inside the ordered phase of the classical theory.Comment: 9 pages Revtex, no figures, submitted to Phys. Rev. Let
Melting of Partially Fluorinated Graphene: From Detachment of Fluorine Atoms to Large Defects and Random Coils
The melting of fluorographene is very unusual and depends strongly on the
degree of fluorination. For temperatures below 1000 K, fully fluorinated
graphene (FFG) is thermo-mechanically more stable than graphene but at
T2800 K FFG transits to random coils which is almost twice lower
than the melting temperature of graphene, i.e. 5300 K. For fluorinated graphene
(PFG) up to 30 % ripples causes detachment of individual F-atoms around 2000 K
while for 40-60 % fluorination, large defects are formed beyond 1500 K and
beyond 60% of fluorination F-atoms remain bonded to graphene until melting. The
results agree with recent experiments on the dependence of the reversibility of
the fluorination process on the percentage of fluorination.Comment: 16 pages, 6 figure
Superconductivity and Quantum Spin Disorder in Cuprates
A fundamental connection between superconductivity and quantum spin
fluctuations in underdoped cuprates, is revealed. A variational calculation
shows that {\em Cooper pair hopping} strongly reduces the local magnetization
. This effect pertains to recent neutron scattering and muon spin rotation
measurements in which varies weakly with hole doping in the poorly
conducting regime, but drops precipitously above the onset of
superconductivity
Nonthermal Emission from a Supernova Remnant in a Molecular Cloud
In evolved supernova remnants (SNRs) interacting with molecular clouds, such
as IC 443, W44, and 3C391, a highly inhomogeneous structure consisting of a
forward shock of moderate Mach number, a cooling layer, a dense radiative shell
and an interior region filled with hot tenuous plasma is expected. We present a
kinetic model of nonthermal electron injection, acceleration and propagation in
that environment and find that these SNRs are efficient electron accelerators
and sources of hard X- and gamma-ray emission. The energy spectrum of the
nonthermal electrons is shaped by the joint action of first and second order
Fermi acceleration in a turbulent plasma with substantial Coulomb losses.
Bremsstrahlung, synchrotron, and inverse Compton radiation of the nonthermal
electrons produce multiwavelength photon spectra in quantitative agreement with
the radio and the hard emission observed by ASCA and EGRET from IC 443. We
distinguish interclump shock wave emission from molecular clump shock wave
emission accounting for a complex structure of molecular cloud. Spatially
resolved X- and gamma- ray spectra from the supernova remnants IC 443, W44, and
3C391 as might be observed with BeppoSAX, Chandra XRO, XMM, INTEGRAL and GLAST
would distinguish the contribution of the energetic lepton component to the
gamma-rays observed by EGRET.Comment: 14 pages, 4 figure, Astrophysical Journal, v.538, 2000 (in press
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