31,441 research outputs found
Tungsten-rhenium alloy thermocouples effective for high-temperature measurement
Tungsten-rhenium alloy thermocouples, specifically, insulated, sheathed W/W plus 26Re and W plus 5 Re/W plus 26 Re thermocouples, are effective for temperature measurement in excess of 2920 degrees C. These thermocouples have a high thermoelectric output and excellent relationship to temperatures up to 2760 degrees C
Electron correlations and single-particle physics in the Integer Quantum Hall Effect
The compressibility of a two-dimensional electron system with spin in a
spatially correlated random potential and a quantizing magnetic field is
investigated. Electron-electron interaction is treated with the Hartree-Fock
method. Numerical results for the influences of interaction and disorder on the
compressibility as a function of the particle density and the strength of the
magnetic field are presented. Localization-delocalization transitions
associated with highly compressible region in the energy spectrum are found at
half-integer filling factors. Coulomb blockade effects are found near integer
fillings in the regions of low compressibility. Results are compared with
recent experiments.Comment: 4 pages, 2 figures, replaced with revised versio
Development of high strength, brazed aluminum, honeycomb sandwich composites adaptable for both elevated and cryogenic temperature applications, volume I - Brazing alloy development and selection Final report, Jul. 1963 - Sep. 1966
Brazing alloy development for high strength aluminum honeycomb sandwich composites for elevated and cryogenic temperature application
Monitoring and control in scenario-based requirements analysis
Scenarios are an effective means for eliciting, validating and documenting requirements. At the requirements level, scenarios describe sequences of interactions between the software-to-be and agents in the environment. Interactions correspond to the occurrence of an event that is controlled by one agent and monitored by another.This paper presents a technique to analyse requirements-level scenarios for unforeseen, potentially harmful, consequences. Our aim is to perform analysis early in system development, where it is highly cost-effective. The approach recognises the importance of monitoring and control issues and extends existing work on implied scenarios accordingly. These so-called input-output implied scenarios expose problematic behaviours in scenario descriptions that cannot be detected using standard implied scenarios. Validation of these implied scenarios supports requirements elaboration. We demonstrate the relevance of input-output implied scenarios using a number of examples
Fractional charges in pyrochlore lattices
A pyrochlore lattice is considered where the average electron number of
electrons per site is half--integer, concentrating on the case of exactly half
an electron per site. Strong on-site repulsions are assumed, so that all sites
are either empty or singly occupied. Where there are in addition strong
nearest--neighbour repulsions, a tetrahedron rule comes into effect, as
previously suggested for magnetite. We show that in this case, there exist
excitations with fractional charge (+/-) e/2. These are intimately connected
with the high degeneracy of the ground state in the absence of kinetic energy
terms. When an additional electron is inserted into the system, it decays into
two point like excitations with charge -e/2, connected by a Heisenberg spin
chain which carries the electron's spin.Comment: 10 pages, 4 eps figures. To appear in Decemeber issue of Annalen der
Physi
Revivals of quantum wave-packets in graphene
We investigate the propagation of wave-packets on graphene in a perpendicular
magnetic field and the appearance of collapses and revivals in the
time-evolution of an initially localised wave-packet. The wave-packet evolution
in graphene differs drastically from the one in an electron gas and shows a
rich revival structure similar to the dynamics of highly excited Rydberg
states.
We present a novel numerical wave-packet propagation scheme in order to solve
the effective single-particle Dirac-Hamiltonian of graphene and show how the
collapse and revival dynamics is affected by the presence of disorder. Our
effective numerical method is of general interest for the solution of the Dirac
equation in the presence of potentials and magnetic fields.Comment: 22 pages, 10 figures, 3 movies, to appear in New Journal of Physic
Surface structure of i-Al(68)Pd(23)Mn(9): An analysis based on the T*(2F) tiling decorated by Bergman polytopes
A Fibonacci-like terrace structure along a 5fold axis of i-Al(68)Pd(23)Mn(9)
monograins has been observed by T.M. Schaub et al. with scanning tunnelling
microscopy (STM). In the planes of the terraces they see patterns of dark
pentagonal holes. These holes are well oriented both within and among terraces.
In one of 11 planes Schaub et al. obtain the autocorrelation function of the
hole pattern. We interpret these experimental findings in terms of the
Katz-Gratias-de Boisseu-Elser model. Following the suggestion of Elser that the
Bergman clusters are the dominant motive of this model, we decorate the tiling
T*(2F) by the Bergman polytopes only. The tiling T*(2F) allows us to use the
powerful tools of the projection techniques. The Bergman polytopes can be
easily replaced by the Mackay polytopes as the decoration objects. We derive a
picture of ``geared'' layers of Bergman polytopes from the projection
techniques as well as from a huge patch. Under the assumption that no surface
reconstruction takes place, this picture explains the Fibonacci-sequence of the
step heights as well as the related structure in the terraces qualitatively and
to certain extent even quantitatively. Furthermore, this layer-picture requires
that the polytopes are cut in order to allow for the observed step heights. We
conclude that Bergman or Mackay clusters have to be considered as geometric
building blocks of the i-AlPdMn structure rather than as energetically stable
entities
- …