950 research outputs found
Electron pairing: from metastable electron pair to bipolaron
Starting from the shell structure in atoms and the significant correlation
within electron pairs, we distinguish the exchange-correlation effects between
two electrons of opposite spins occupying the same orbital from the average
correlation among many electrons in a crystal. In the periodic potential of the
crystal with lattice constant larger than the effective Bohr radius of the
valence electrons, these correlated electron pairs can form a metastable energy
band above the corresponding single-electron band separated by an energy gap.
In order to determine if these metastable electron pairs can be stabilized, we
calculate the many-electron exchange-correlation renormalization and the
polaron correction to the two-band system with single electrons and electron
pairs. We find that the electron-phonon interaction is essential to
counterbalance the Coulomb repulsion and to stabilize the electron pairs. The
interplay of the electron-electron and electron-phonon interactions, manifested
in the exchange-correlation energies, polaron effects, and screening, is
responsible for the formation of electron pairs (bipolarons) that are located
on the Fermi surface of the single-electron band.Comment: 17 pages, 6 figures, Journal of Physics Communications 201
The Magnetic Ordering of the 3d Wigner Crystal
Using Path Integral Monte Carlo, we have calculated exchange frequencies as
electrons undergo ring exchanges of 2, 3 and 4 electrons in a ``clean'' 3d
Wigner crystal (bcc lattice) as a function of density. We find pair exchange
dominates and estimate the critical temperature for the transition to
antiferromagnetic ordering to be roughly Ry at melting. In
contrast to the situation in 2d, the 3d Wigner crystal is different from the
solid bcc 3He in that the pair exchange dominates because of the softer
interparticle potential. We discuss implications for the magnetic phase diagram
of the electron gas
Melting temperature of screened Wigner crystal on helium films by molecular dynamics
Using molecular dynamics (MD) simulation, we have calculated the melting
temperature of two-dimensional electron systems on \AA-\AA helium
films supported by substrates of dielectric constants
at areal densities varying from cm to cm. Our results are in good agreement with the available
theoretical and experimental results.Comment: 4 pages and 4 figure
BICULTURALISM: AN ENDLESS ENIGMA
Neste trabalho discutimos, de maneira sucinta, o conceito de culturarelacionado ao ensino e à aprendizagem de língua estrangeira. A relaçãoexistente entre bilingüismo e biculturalismo é também levada em consideraçãodentro de uma perspectiva sociolingüística
Configurational entropy of Wigner crystals
We present a theoretical study of classical Wigner crystals in two- and
three-dimensional isotropic parabolic traps aiming at understanding and
quantifying the configurational uncertainty due to the presence of multiple
stable configurations. Strongly interacting systems of classical charged
particles confined in traps are known to form regular structures. The number of
distinct arrangements grows very rapidly with the number of particles, many of
these arrangements have quite low occurrence probabilities and often the
lowest-energy structure is not the most probable one. We perform numerical
simulations on systems containing up to 100 particles interacting through
Coulomb and Yukawa forces, and show that the total number of metastable
configurations is not a well defined and representative quantity. Instead, we
propose to rely on the configurational entropy as a robust and objective
measure of uncertainty. The configurational entropy can be understood as the
logarithm of the effective number of states; it is insensitive to the presence
of overlooked low-probability states and can be reliably determined even within
a limited time of a simulation or an experiment.Comment: 12 pages, 8 figures. This is an author-created, un-copyedited version
of an article accepted for publication in J. Phys.: Condens. Matter. IOP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from it. The definitive
publisher-authenticated version is available online at
10.1088/0953-8984/23/7/075302.
Classical artificial two-dimensional atoms: the Thomson model
The ring configurations for classical two-dimensional atoms are calculated
within the Thomson model and compared with the results from `exact' numerical
simulations. The influence of the functional form of the confinement potential
and the repulsive interaction potential between the particles on the
configurations is investigated. We also give exact results on those eigenmodes
of the system whose frequency does not depend on the number of particles in the
system.Comment: 9 pages, RevTeX, 4 figure
First Flight from Europe to the South Atlantic
The History of the transatlantic flights goes back to 1919 and began with a flight performed from Newfoundland to Lisbon; two weeks later another flight was performed between Newfoundland and Ireland. On 1922, the Portuguese airmen Gago Coutinho and Sacadura Cabral crossed the South Atlantic Ocean by air in a flight performed exclusively with internal means of navigation: a new instrument that consisted in a type of sextant improved with two spirit levels to provide an artificial horizon and also with the help of a “path corrector”. Despite this journey had lasted 79 days to cross South Atlantic Ocean, their flight time was only 62:26 minutes, and they’ve flown 8,383 nautical miles, using 3 different hydroplanes christened: Lusitania, Pátria and Santa Cruz. Despite this journey had lasted 79 days, their flight time was only 62 h 26 m; they’ve flown 8,383 nautical miles using 3 different hydroplanes christened: Lusitania, Pátria and Santa Cruz. The new artificial horizon sextant had proven itself while flying over the ocean, without external references.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Identifying dead regions in the cochlea through the TEN Test
SummaryAn audiogram is not sufficient to indicate cochlear dead regions.AimTo investigate cochlear dead regions in sensorineural hearing loss subjects using the TEN test. Site: CEDALVI/ HRAC-USP-Bauru/Sao Paulo/Brazil, August 2003 to February 2004.Study DesignA contemporary cross-sectional cohort study. Material and Methods: The TEN test was applied in three groups: G1(5 women with pure-tone thresholds within normal limits); G2(4 women and 5 men with moderate sensorineural flat hearing loss); G3(19 women and 24 men with mild to severe sloping sensorineural hearing loss).ResultsIn the G1 group the TEN value required to eliminate the test tone was, on average, close to the absolute threshold for all frequencies. No dead regions were found in the ears tested in group G2. 76 ears were tested in group G3, and six showed no evidence of dead regions in the cochlea.ConclusionsThe TEN test was an effective test to indicate a dead region in the cochlea of subjects with sloping sensorineural hearing loss. There is evidence that pure-tone detection is different for subjects with high frequency sensorineural hearing loss and flat hearing loss; we observed a significant difference between the masked threshold and the absolute threshold only in sloping hearing loss and not for flat hearing loss
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