904 research outputs found
Doping Dependence of Thermal Oxidation on n-type 4H-SiC
The doping dependence of dry thermal oxidation rates in n-type 4H-SiC was
investigated. The oxidation was performed in the temperature range 1000C to
1200C for samples with nitrogen doping in the range of 6.5e15/cm3 to
9.3e18/cm3, showing a clear doping dependence. Samples with higher doping
concentrations displayed higher oxidation rates. The results were interpreted
using a modified Deal-Grove model. Linear and parabolic rate constants and
activation energies were extracted. Increasing nitrogen led to an increase in
linear rate constant pre-exponential factor from 10-6m/s to 10-2m/s and the
parabolic rate constant pre-exponential factor from 10e9m2/s to 10e6m2/s. The
increase in linear rate constant was attributed to defects from doping-induced
lattice mismatch, which tend to be more reactive than bulk crystal regions. The
increase in the diffusion-limited parabolic rate constant was attributed to
degradation in oxide quality originating from the doping-induced lattice
mismatch. This degradation was confirmed by the observation of a decrease in
optical density of the grown oxide films from 1.4 to 1.24. The linear
activation energy varied from 1.6eV to 2.8eV, while the parabolic activation
energy varied from 2.7eV to 3.3eV, increasing with doping concentration. These
increased activation energies were attributed to higher nitrogen content,
leading to an increase in effective bond energy stemming from the difference in
C-Si (2.82eV) and Si-N (4.26eV) binding energies. This work provides crucial
information in the engineering of SiO2 dielectrics for SiC MOS structures,
which typically involve regions of very different doping concentrations, and
suggests that thermal oxidation at high doping concentrations in SiC may be
defect mediated.Comment: 13 pages. 9 figures, accepted as a transiction in IEEE electron
device. TED MS#8035
Positivity in the presence of initial system-environment correlation
The constraints imposed by the initial system-environment correlation can
lead to nonpositive Dynamical maps. We find the conditions for positivity and
complete positivity of such dynamical maps by using the concept of an
assignment map. Any initial system-environment correlations make the assignment
map nonpositive, while the positivity of the dynamical map depends on the
interplay between the assignment map and the system-environment coupling. We
show how this interplay can reveal or hide the nonpositivity of the assignment
map. We discuss the role of this interplay in Markovian models.Comment: close to the published version. 5 pages, 1 figur
Effect of particle size on thermal conductivity of nanofluid
Nanofluids, containing nanometric metallic or oxide particles, exhibit extraordinarily high thermal conductivity. It is reported that the identity (composition), amount (volume percent), size, and shape of nanoparticles largely determine the extent of this enhancement. In the present study, we have experimentally investigated the impact of Al2Cu and Ag2Al nanoparticle size and volume fraction on the effective thermal conductivity of water and ethylene glycol based nanofluid prepared by a two-stage process comprising mechanical alloying of appropriate Al-Cu and Al-Ag elemental powder blend followed by dispersing these nanoparticles (1 to 2 vol pct) in water and ethylene glycol with different particle sizes. The thermal conductivity ratio of nanofluid, measured using an indigenously developed thermal comparator device, shows a significant increase of up to 100 pct with only 1.5 vol pct nanoparticles of 30- to 40-nm average diameter. Furthermore, an analytical model shows that the interfacial layer significantly influences the effective thermal conductivity ratio of nanofluid for the comparable amount of nanoparticles
Identification of variety-specific ISSR markers in small cardamom (Elettaria cardamomum Maton.)
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Completely Positive Maps and Classical Correlations
We expand the set of initial states of a system and its environment that are
known to guarantee completely positive reduced dynamics for the system when the
combined state evolves unitarily. We characterize the correlations in the
initial state in terms of its quantum discord [H. Ollivier and W. H. Zurek,
Phys. Rev. Lett. 88, 017901 (2001)]. We prove that initial states that have
only classical correlations lead to completely positive reduced dynamics. The
induced maps can be not completely positive when quantum correlations
including, but not limited to, entanglement are present. We outline the
implications of our results to quantum process tomography experiments.Comment: 4 pages, 1 figur
Gauge invariances vis-{\'a}-vis Diffeomorphisms in second order metric gravity: A new Hamiltonian approach
A new analysis of the gauge invariances and their unity with diffeomorphism
invariances in second order metric gravity is presented which strictly follows
Dirac's constrained Hamiltonian approach.Comment: 6 Pages, revTex, paper modified substantiall
Evolutionary Laws, Initial Conditions, and Gauge Fixing in Constrained Systems
We describe in detail how to eliminate nonphysical degrees of freedom in the
Lagrangian and Hamiltonian formulations of a constrained system. Two important
and distinct steps in our method are the fixing of ambiguities in the dynamics
and the determination of inequivalent initial data. The Lagrangian discussion
is novel, and a proof is given that the final number of degrees of freedom in
the two formulations agrees. We give applications to reparameterization
invariant theories, where we prove that one of the constraints must be
explicitly time dependent. We illustrate our procedure with the examples of
trajectories in spacetime and with spatially homogeneous cosmological models.
Finally, we comment briefly on Dirac's extended Hamiltonian technique.Comment: 23 pages; plain TeX. To appear: Classical & Quantum Gravit
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