69,344 research outputs found
A unified projection formalism for the Al-Pd-Mn quasicrystal Xi-approximants and their metadislocations
The approximants xi, xi' and xi'_n of the quasicrystal Al-Mn-Pd display most
interesting plastic properties as for example phason-induced deformation
processes (Klein, H., Audier, M., Boudard, M., de Boissieu, M., Beraha, L., and
Duneau, M., 1996, Phil. Mag. A, 73, 309.) or metadislocations (Klein, H.,
Feuerbacher, M., Schall, P., and Urban, K., 1999, Phys. Rev. Lett., 82, 3468.).
Here we demonstrate that the phases and their deformed or defected states can
be described by a simple projection formalism in three-dimensional space - not
as usual in four to six dimensions. With the method we can interpret
microstructures observed with electron microscopy as phasonic phase boundaries.
Furthermore we determine the metadislocations of lowest energy and relate them
uniquely to experimentally observed ones. Since moving metadislocations in the
xi'-phase can create new phason-planes, we suggest a dislocation induced phase
transition from xi' to xi'_n. The methods developed in this paper can as well
be used for various other complex metallic alloys.Comment: 25 pages, 12 figure
Quantum theory of large amplitude collective motion and the Born-Oppenheimer method
We study the quantum foundations of a theory of large amplitude collective
motion for a Hamiltonian expressed in terms of canonical variables. In previous
work the separation into slow and fast (collective and non-collective)
variables was carried out without the explicit intervention of the Born
Oppenheimer approach. The addition of the Born Oppenheimer assumption not only
provides support for the results found previously in leading approximation, but
also facilitates an extension of the theory to include an approximate
description of the fast variables and their interaction with the slow ones.
Among other corrections, one encounters the Berry vector and scalar potential.
The formalism is illustrated with the aid of some simple examples, where the
potentials in question are actually evaluated and where the accuracy of the
Born Oppenheimer approximation is tested. Variational formulations of both
Hamiltonian and Lagrangian type are described for the equations of motion for
the slow variables.Comment: 29 pages, 1 postscript figure, preprint no UPR-0085NT. Latex + epsf
styl
Hydrogen-oxygen electrolytic regenerative fuel cells
X ray diffraction analysis of matrices of hydrogen-oxygen electrolytic regenerative fuel cell
Research and development of magnesium/aluminum electroforming process for solar concentrators Final report, 13 May 1966 - 10 May 1967
Magnesium and aluminum electroforming process for solar concentrator
Infrared light emission from atomic point contacts
Gold atomic point contacts are prototype systems to evidence ballistic
electron transport. The typical dimension of the nanojunction being smaller
than the electron-phonon interaction length, even at room temperature,
electrons transfer their excess energy to the lattice only far from the
contact. At the contact however, favored by huge current densities,
electron-electron interactions result in a nano hot electron gas acting as a
source of photons. Using a home built Mechanically Controlled Break Junction,
it is reported here, for the first time, that this hot electron gas also
radiates in the infrared range (0.2eV to 1.2eV). Moreover, in agreement with
the pioneering work of Tomchuk, we show that this radiation is compatible with
a blackbody like spectrum emitted from an electron gas at temperatures of
several thousands of Kelvin given by where ,
and are respectively a fitting parameter, the current flowing and the
applied bias.Comment: 13 pages, 5 figure
Estimating single molecule conductance from spontaneous evolution of a molecular contact
We present an original method to estimate the conductivity of a single
molecule anchored to nanometric-sized metallic electrodes, using a Mechanically
Controlled Break Junction (MCBJ) operated at room temperature in liquid. We
record the conductance through the metal / molecules / metal nanocontact while
keeping the metallic electrodes at a fixed distance. Taking advantage of
thermal diffusion and electromigration, we let the contact naturally explore
the more stable configurations around a chosen conductance value. The
conductance of a single molecule is estimated from a statistical analysis of
raw conductance and conductance standard deviation data for molecular contacts
containing up to 14 molecules. The single molecule conductance values are
interpreted as time-averaged conductance of an ensemble of conformers at
thermal equilibrium.Comment: 25 pages, 6 figure
Spin-Peierls vs. Peierls distortions in a family of conjugated Polymers
Distortions in a family of conjugated polymers are studied within two
complementary approaches, i.e. within a many-body Valence Bond (VB) approach
using a transfer matrix technique to treat the Heisenberg model of the systems,
and also in terms of the tight-binding band-theoretic model with interactions
limited to nearest neighbors. The computations indicate that both methods
predict the presence or absence of the same distortions in most of the polymers
studied.Comment: Latex209 (twocolumn revtex), 11 pages; 9 figures available by mail
from authors; Phys. Rev. B (in press
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