928 research outputs found
Hall-Effect Sign Anomaly and Small-Polaronic Conduction in (La_{1-x}Gd_x)_{0.67}Ca_{0.33}MnO_3
The Hall coefficient of Gd-doped La_{2/3}Ca_{1/3}MnO_3 exhibits Arrhenius
behavior over a temperature range from 2T_c to 4T_c, with an activation energy
very close to 2/3 that of the electrical conductivity. Although both the doping
level and thermoelectric coefficient indicate hole-like conduction, the Hall
coefficient is electron-like. This unusual result provides strong evidence in
favor of small-polaronic conduction in the paramagnetic regime of the
manganites.Comment: 11 pages, 4 figures, uses revtex.st
Refractory materials for high-temperature thermoelectric energy conversion
Theoretical work of two decades ago adequately explained the transport behavior and effectively guided the development of thermoelectric materials of high conversion efficiencies of conventional semiconductors (e.g., SiGe alloys). The more significant contributions involved the estimation of optimum doping concentrations, the reduction of thermal conductivity by solid solution doping and the development of a variety of materials with ZT approx. 1 in the temperature range 300 K to 1200 K. ZT approx. 1 is not a theoretical limitation although, experimentally, values in excess of one were not achieved. Work has continued with emphasis on higher temperature energy conversion. A number of promising materials have been discovered in which it appears that ZT 1 is realizable. These materials are divided into two classes: (1) the rare-earth chalcogenides which behave as itinerant highly-degenerate n-type semiconductors at room-temperature, and (2) the boron-rich borides, which exhibit p-type small-polaronic hopping conductivity
Scattering theory of nonlinear thermoelectric transport
We investigate nonlinear transport properties of quantum conductors in
response to both electrical and thermal driving forces. Within scattering
approach, we determine the nonequilibrium screening potential of a generic
mesoscopic system and find that its response is dictated by particle and
entropic injectivities which describe the charge and entropy transfer during
transport. We illustrate our model analyzing the voltage and thermal
rectification of a resonant tunneling barrier. Importantly, we discuss
interaction induced contributions to the thermopower in the presence of large
temperature differences.Comment: 5 pages, 3 figures; slightly shortened version to fulfill the
journal's requirement
Thermal conductivity behavior of boron carbides
Knowledge of the thermal conductivity of boron carbides is necessary to evaluate its potential for high temperature thermoelectric energy conversion applications. The thermal diffusivity of hot pressed boron carbide B/sub 1-x/C/sub x/ samples as a function of composition, temperature and temperature cycling was measured. These data in concert with density and specific heat data yield the thermal conductivities of these materials. The results in terms of a structural model to explain the electrical transport data and novel mechanisms for thermal conduction are discussed
Emergence and development of co-operations between co-located creative companies. The case of the Alstom Marketplace on the Ile de Nantes, France
Emergence et développement de dynamiques coopératives entre des entreprises créatives co-localisées : le cas des anciennes Halles Alstom de l’île de Nantes, France
International audienc
Reliable Multi-Path Routing Schemes for Real-Time Streaming
In off-line streaming, packet level erasure resilient Forward Error
Correction (FEC) codes rely on the unrestricted buffering time at the receiver.
In real-time streaming, the extremely short playback buffering time makes FEC
inefficient for protecting a single path communication against long link
failures. It has been shown that one alternative path added to a single path
route makes packet level FEC applicable even when the buffering time is
limited. Further path diversity, however, increases the number of underlying
links increasing the total link failure rate, requiring from the sender
possibly more FEC packets. We introduce a scalar coefficient for rating a
multi-path routing topology of any complexity. It is called Redundancy Overall
Requirement (ROR) and is proportional to the total number of adaptive FEC
packets required for protection of the communication. With the capillary
routing algorithm, introduced in this paper we build thousands of multi-path
routing patterns. By computing their ROR coefficients, we show that contrary to
the expectations the overall requirement in FEC codes is reduced when the
further diversity of dual-path routing is achieved by the capillary routing
algorithm.Comment: Emin Gabrielyan, "Reliable Multi-Path Routing Schemes for Voice over
Packet Networks", ICDT'06, International Conference on Digital
Telecommunications, Cote d'Azur, France, 29-31 August 2006, pp. 65-7
Extensions of Picard 2-Stacks and the cohomology groups Ext^i of length 3 complexes
The aim of this paper is to define and study the 3-category of extensions of
Picard 2-stacks over a site S and to furnish a geometrical description of the
cohomology groups Ext^i of length 3 complexes of abelian sheaves. More
precisely, our main Theorem furnishes
(1) a parametrization of the equivalence classes of objects, 1-arrows,
2-arrows, and 3-arrows of the 3-category of extensions of Picard 2-stacks by
the cohomology groups Ext^i, and
(2) a geometrical description of the cohomology groups Ext^i of length 3
complexes of abelian sheaves via extensions of Picard 2-stacks.
To this end, we use the triequivalence between the 3-category of Picard
2-stacks and the tricategory T^[-2,0](S) of length 3 complexes of abelian
sheaves over S introduced by the second author in arXiv:0906.2393, and we
define the notion of extension in this tricategory T^[-2,0](S), getting a pure
algebraic analogue of the 3-category of extensions of Picard 2-stacks. The
calculus of fractions that we use to define extensions in the tricategory
T^[-2,0](S) plays a central role in the proof of our Main Theorem.Comment: 2 New Appendix: in the first Appendix we compute a long exact
sequence involving the homotopy groups of an extension of Picard 2-stacks,
and in the second Appendix we sketch the proof that the fibered sum of Picard
2-stacks satisfies the universal propert
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