37 research outputs found
Liquid-liquid phase transition in Stillinger-Weber silicon
It was recently demonstrated that the Stillinger-Weber silicon undergoes a
liquid-liquid first-order phase transition deep into the supercooled region
(Sastry and Angell, Nature Materials 2, 739 (2003)). Here we study the effects
of perturbations on this phase transition. We show that the order of the
liquid-liquid transition changes with negative pressure. We also find that the
liquid-liquid transition disappears when the three-body term of the potential
is strengthened by as little as 5 %. This implies that the details of the
potential could affect strongly the nature and even the existence of the
liquid-liquid phase.Comment: 13 page
Modeling of Covalent Bonding in Solids by Inversion of Cohesive Energy Curves
We provide a systematic test of empirical theories of covalent bonding in
solids using an exact procedure to invert ab initio cohesive energy curves. By
considering multiple structures of the same material, it is possible for the
first time to test competing angular functions, expose inconsistencies in the
basic assumption of a cluster expansion, and extract general features of
covalent bonding. We test our methods on silicon, and provide the direct
evidence that the Tersoff-type bond order formalism correctly describes
coordination dependence. For bond-bending forces, we obtain skewed angular
functions that favor small angles, unlike existing models. As a
proof-of-principle demonstration, we derive a Si interatomic potential which
exhibits comparable accuracy to existing models.Comment: 4 pages revtex (twocolumn, psfig), 3 figures. Title and some wording
(but no content) changed since original submission on 24 April 199
Detection of HIV-1 in Saliva: Implications for Case-Identification, Clinical Monitoring and Surveillance for Drug Resistance§
Theoretical investigations of a highly mismatched interface: the case of SiC/Si(001)
Using first principles, classical potentials, and elasticity theory, we
investigated the structure of a semiconductor/semiconductor interface with a
high lattice mismatch, SiC/Si(001). Among several tested possible
configurations, a heterostructure with (i) a misfit dislocation network pinned
at the interface and (ii) reconstructed dislocation cores with a carbon
substoichiometry is found to be the most stable one. The importance of the slab
approximation in first-principles calculations is discussed and estimated by
combining classical potential techniques and elasticity theory. For the most
stable configuration, an estimate of the interface energy is given. Finally,
the electronic structure is investigated and discussed in relation with the
dislocation array structure. Interface states, localized in the heterostructure
gap and located on dislocation cores, are identified
Comparison between classical potentials and ab initio for silicon under large shear
The homogeneous shear of the {111} planes along the direction of bulk
silicon has been investigated using ab initio techniques, to better understand
the strain properties of both shuffle and glide set planes. Similar
calculations have been done with three empirical potentials, Stillinger-Weber,
Tersoff and EDIP, in order to find the one giving the best results under large
shear strains. The generalized stacking fault energies have also been
calculated with these potentials to complement this study. It turns out that
the Stillinger-Weber potential better reproduces the ab initio results, for the
smoothness and the amplitude of the energy variation as well as the
localization of shear in the shuffle set
Temperature effects on dislocation core energies in silicon and germanium
Temperature effects on the energetics of the 90-degree partial dislocation in
silicon and germanium are investigated, using non-equilibrium methods to
estimate free energies, coupled with Monte Carlo simulations. Atomic
interactions are described by Tersoff and EDIP interatomic potentials. Our
results indicate that the vibrational entropy has the effect of increasing the
difference in free energy between the two possible reconstructions of the
90-degree partial, namely, the single-period and the double-period geometries.
This effect further increases the energetic stability of the double-period
reconstruction at high temperatures. The results also indicate that anharmonic
effects may play an important role in determining the structural properties of
these defects in the high-temperature regime.Comment: 8 pages in two-column physical-review format with six figure
Thermodynamic Behavior of a Model Covalent Material Described by the Environment-Dependent Interatomic Potential
Using molecular dynamics simulations we study the thermodynamic behavior of a
single-component covalent material described by the recently proposed
Environment-Dependent Interatomic Potential (EDIP). The parameterization of
EDIP for silicon exhibits a range of unusual properties typically found in more
complex materials, such as the existence of two structurally distinct
disordered phases, a density decrease upon melting of the low-temperature
amorphous phase, and negative thermal expansion coefficients for both the
crystal (at high temperatures) and the amorphous phase (at all temperatures).
Structural differences between the two disordered phases also lead to a
first-order transition between them, which suggests the existence of a second
critical point, as is believed to exist for amorphous forms of frozen water.
For EDIP-Si, however, the unusual behavior is associated not only with the open
nature of tetrahedral bonding but also with a competition between four-fold
(covalent) and five-fold (metallic) coordination. The unusual behavior of the
model and its unique ability to simulation the liquid/amorphous transition on
molecular-dynamics time scales make it a suitable prototype for fundamental
studies of anomalous thermodynamics in disordeered systems.Comment: 48 pages (double-spaced), 13 figure
Human Breast Milk and Antiretrovirals Dramatically Reduce Oral HIV-1 Transmission in BLT Humanized Mice
Currently, over 15% of new HIV infections occur in children. Breastfeeding is a major contributor to HIV infections in infants. This represents a major paradox in the field because in vitro, breast milk has been shown to have a strong inhibitory effect on HIV infectivity. However, this inhibitory effect has never been demonstrated in vivo. Here, we address this important paradox using the first humanized mouse model of oral HIV transmission. We established that reconstitution of the oral cavity and upper gastrointestinal (GI) tract of humanized bone marrow/liver/thymus (BLT) mice with human leukocytes, including the human cell types important for mucosal HIV transmission (i.e. dendritic cells, macrophages and CD4+ T cells), renders them susceptible to oral transmission of cell-free and cell-associated HIV. Oral transmission of HIV resulted in systemic infection of lymphoid and non-lymphoid tissues that is characterized by the presence of HIV RNA in plasma and a gradual decline of CD4+ T cells in peripheral blood. Consistent with infection of the oral cavity, we observed virus shedding into saliva. We then evaluated the role of human breast milk on oral HIV transmission. Our in vivo results demonstrate that breast milk has a strong inhibitory effect on oral transmission of both cell-free and cell-associated HIV. Finally, we evaluated the effect of antiretrovirals on oral transmission of HIV. Our results show that systemic antiretrovirals administered prior to exposure can efficiently prevent oral HIV transmission in BLT mice