11,216 research outputs found
Clustering of vacancy defects in high-purity semi-insulating SiC
Positron lifetime spectroscopy was used to study native vacancy defects in
semi-insulating silicon carbide. The material is shown to contain (i) vacancy
clusters consisting of 4--5 missing atoms and (ii) Si vacancy related
negatively charged defects. The total open volume bound to the clusters
anticorrelates with the electrical resistivity both in as-grown and annealed
material. Our results suggest that Si vacancy related complexes compensate
electrically the as-grown material, but migrate to increase the size of the
clusters during annealing, leading to loss of resistivity.Comment: 8 pages, 5 figure
Color Superconductivity from Supersymmetry
A supersymmetric composite model of color superconductivity is proposed.
Quarks and diquarks are dynamically generated as composite fields by a newly
introduced strong gauge dynamics. It is shown that the condensation of the
scalar component of the diquark supermultiplet occurs when the chemical
potential becomes larger than some critical value. We believe that the model
well captures aspects of the diquark condensate behavior and helps our
understanding of the diquark dynamics in real QCD. The results obtained here
might be useful when we consider a theory composed of quarks and diquarks.Comment: 4 pages, 2 figures, An error in Eq.(10) correcte
(Non)-Renormalization of the Chiral Vortical Effect Coefficient
We show using diagramtic arguments that in some (but not all) cases, the
temperature dependent part of the chiral vortical effect coefficient is
independent of the coupling constant. An interpretation of this result in terms
of quantization in the effective 3 dimensional Chern-Simons theory is also
given. In the language of 3D dimensionally reduced theory, the value of the
chiral vortical coefficient is related to the formula . We also show that in the presence of dynamical gauge fields, the CVE
coefficient is not protected from renormalization, even in the large limit.Comment: 11 pages, 3 figures. Version 2 corrects an error and calculates
leading radiative correctio
Characterization of the nitrogen split interstitial defect in wurtzite aluminum nitride using density functional theory
We carried out Heyd-Scuseria-Ernzerhof hybrid density functional theory plane
wave supercell calculations in wurtzite aluminum nitride in order to
characterize the geometry, formation energies, transition levels and hyperfine
tensors of the nitrogen split interstitial defect. The calculated hyperfine
tensors may provide useful fingerprint of this defect for electron paramagnetic
resonance measurement.Comment: 5 pages, 3 figure
Superluminal pions in a hadronic fluid
We study the propagation of pions at finite temperature and finite chemical
potential in the framework of the linear sigma model with 2 quark flavors and
colors. The velocity of massless pions in general differs from that of
light. One-loop calculations show that in the chiral symmetry broken phase
pions, under certain conditions, propagate faster than light.Comment: 8 pages, 3 figures included. Considerably revised, discussions
expanded, one figure added, typos corrected, results unchanged. To be
published in Phys. Rev.
Standard Isotherm Fit Information for Dry CO2 on Sorbents for 4-Bed Molecular Sieve
Onboard the ISS, one of the systems tasked with removal of metabolic carbon dioxide (CO2) is a 4-bed molecular sieve (4BMS) system. In order to enable a 4-person mission to succeed, systems for removal of metabolic CO2 must reliably operate for several years while minimizing power, mass, and volume requirements. This minimization can be achieved through system redesign and/or changes to the separation material(s). A material screening process has identified the most reliable sorbent materials for the next 4BMS. Sorbent characterization will provide the information necessary to guide system design by providing inputs for computer simulations
Diffusion constant of supercharge density in N=4 SYM at finite chemical potential
We compute holographically the diffusion constant of supercharges in N=4 SYM
at finite chemical potential for the R-charge, by solving the equations of
motion for the transverse mode of the gravitino in the STU black hole in 5
dimensions. We consider the case of one charge and three charges, and we
present analytical solutions for small values of the charges and numerical
solutions for arbitrary values. We compare our results with other known results
in 4 dimensions.Comment: 20 pages, 4 figures; v2: typos correcte
Design of Multifunctional Lattice‐Frame Materials for Compact Heat Exchangers
Structured porous materials show great potential as extended surfaces in heat-exchange applications that also require design for load-bearing capability. In particular, lattice-frame materials (LFM) are known for their superior strength-to-weight ratio; this work presents a comprehensive experimental and numerical study of fluid flow and heat transfer in porous LFMs. Flow through a periodic unit cell of the material is simulated to characterize the forced-convection performance under hydraulically and thermally fully developed conditions. The performance of LFMs with a tetrahedral ligament configuration is characterized as a function of Reynolds number in the laminar regime (150 \u3c Re \u3c 1000) in terms of Nusselt number and friction factor; the effect of porosity is studied by changing the ligament diameter. Experiments are performed for a subset of porosities to validate the numerical approach. A method is demonstrated for utilizing the simulation results, which assume perfect surface efficiency, to predict the performance of LFMs with non-ideal surface efficiency, based on the conduction resistance of the ligaments. It is shown that the thermal behavior of the ligaments closely matches that of cylindrical fins in cross flow and that this analogy can be used to calculate the overall surface efficiency. The implications of the current results on the design of compact heat exchangers using LFMs is assessed using several conventional performance metrics. Our analysis illustrates the challenges in defining any one universal performance metric for compact heat exchanger design; an appropriate performance metric must be selected that accounts for the particular multifunctional performance characteristics of interest. LFMs are shown to provide the benefits of high-porosity and high surface area-to-volume ratio of materials such as metal foams, while also incurring lower pressure drops and displaying higher structural integrity. This makes them ideal for heat exchangers in aerospace and other applications demanding such multifunctional capabilities. The characterization provided in this study readily allows LFM designs for heat exchanger applications with combined heat-transfer and pressure-drop constraints
Shear sum rules at finite chemical potential
We derive sum rules which constrain the spectral density corresponding to the
retarded propagator of the T_{xy} component of the stress tensor for three
gravitational duals. The shear sum rule is obtained for the gravitational dual
of the N=4 Yang-Mills, theory of the M2-branes and M5-branes all at finite
chemical potential. We show that at finite chemical potential there are
additional terms in the sum rule which involve the chemical potential. These
modifications are shown to be due to the presence of scalars in the operator
product expansion of the stress tensor which have non-trivial vacuum
expectation values at finite chemical potential.Comment: The proof for the absence of branch cuts is corrected.Results
unchange
Calibration and Uncertainty Analysis of a Fixed-Bed Adsorption Model for CO2 Separation
Fixed-bed adsorption is widely used in industrial gas separation and is the primary method for atmosphere revitalization in space. This paper analyzes the uncertainty of a one-dimensional, fixed-bed adsorption model due to uncertainty in several model inputs, namely, the linear-driving-force (LDF) mass transfer coefficient, axial dispersion, heat transfer coefficients, and adsorbent properties. The input parameter uncertainties are determined from a comprehensive survey of experimental data in the literature. The model is first calibrated against experimental data from intra-bed centerline concentration measurements to find the LDF coefficient. We then use this LDF coefficient to extract axial dispersion coefficients from mixed, downstream concentration measurements for both a small-diameter bed (dominated by wall-channeling) and a large-diameter bed (dominated by pellet-driven dispersion). The predicted effluent concentration and temperature profiles are most strongly affected by uncertainty in LDF coefficient, adsorbent density, and void fraction. The uncertainty analysis further reveals that ignoring the effect of wall-channeling on apparent axial dispersion can cause significant error in the predicted breakthrough times of small-diameter beds
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