2,711 research outputs found
Completing the Connection: Achieving Universal Service Through Municipal Wi-Fi
The federal universal service scheme is designed to ensure that everyone has affordable access to advanced telecommunications and information services. Despite the development of cost-effective technologies that drastically reduce the cost of telephone services vis-Ã -vis the Internet and Wi-Fi networks, federal regulations generally prevent municipalities or private companies from providing wireless Internet access with universal service funds. Federal regulations have replaced technology costs, lack of business incentives, and consumer affordability as the primary barrier to universal service. Competitive neutrality, the pro-competitive and technology-neutral approach to universal service funding, must be fully embraced in order to empower local communities with the choice of technologies that best suits their residents in providing universal and affordable access to advanced telecommunications and information services
Structural and Correlation Effects in the Itinerant Insulating Antiferromagnetic Perovskite NaOsO3
The orthorhombic perovskite NaOsO3 undergoes a continuous metal-insulator
transition (MIT), accompanied by antiferromagnetic (AFM) order at T_N=410 K,
suggested to be an example of the rare Slater (itinerant) MIT. We study this
system using ab initio and related methods, focusing on the origin and nature
of magnetic ordering and the MIT. The rotation and tilting of OsO6 octahedra in
the GdFeO3 structure result in moderate narrowing the band width of the t_{2g}
manifold, but sufficient to induce flattening of bands and AFM order within the
local spin density approximation (LSDA), where it remains metallic but with a
deep pseudogap. Including on-site Coulomb repulsion U, at U_c ~2 eV a MIT
occurs only in the AFM state. Effects of spin-orbit coupling (SOC) on the band
structure seem minor as expected for a half-filled shell, but SOC
doubles the critical value U_c necessary to open a gap and also leads to large
magnetocrystalline energy differences in spite of normal orbital moments no
greater than 0.1. Our results are consistent with a Slater MIT driven by
magnetic order, induced by a combination of structurally-induced band narrowing
and moderate Coulomb repulsion, with SOC necessary for a full picture. Strong
p-d hybridization reduces the moment, and when bootstrapped by the reduced
Hund's rule coupling (proportional to the moment) gives a calculated moment of
~1 , consistent with the observed moment and only a third of the formal
value. We raise and discuss one important question: since this AFM
ordering is at q=0 (in the 20 atom cell) where nesting is a moot issue, what is
the microscopic driving force for ordering and the accompanying MIT?Comment: 9 page
Origins of anomalous electronic structures of epitaxial graphene on silicon carbide
On the basis of first-principles calculations, we report that a novel
interfacial atomic structure occurs between graphene and the surface of silicon
carbide, destroying the Dirac point of graphene and opening a substantial
energy gap there. In the calculated atomic structures, a quasi-periodic
domain pattern emerges out of a larger commensurate
periodic interfacial reconstruction,
resolving a long standing experimental controversy on the periodicity of the
interfacial superstructures. Our theoretical energy spectrum shows a gap and
midgap states at the Dirac point of graphene, which are in excellent agreement
with the recently-observed anomalous angle-resolved photoemission spectra.
Beyond solving unexplained issues of epitaxial graphene, our atomistic study
may provide a way to engineer the energy gaps of graphene on substrates.Comment: Additional references added; published version; 4 pages, 4 figure
Tree indiscernibilities, revisited
We give definitions that distinguish between two notions of indiscernibility
for a set \{a_\eta \mid \eta \in \W\} that saw original use in \cite{sh90},
which we name \textit{\s-} and \textit{\n-indiscernibility}. Using these
definitions and detailed proofs, we prove \s- and \n-modeling theorems and
give applications of these theorems. In particular, we verify a step in the
argument that TP is equivalent to TP or TP that has not seen
explication in the literature. In the Appendix, we exposit the proofs of
\citep[{App. 2.6, 2.7}]{sh90}, expanding on the details.Comment: submitte
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