22,822 research outputs found
Noncommutative D-Brane in Non-Constant NS-NS B Field Background
We show that when the field strength H of the NS-NS B field does not vanish,
the coordinates X and momenta P of an open string endpoints satisfy a set of
mixed commutation relations among themselves. Identifying X and P with the
coordinates and derivatives of the D-brane world volume, we find a new type of
noncommutative spaces which is very different from those associated with a
constant B field background.Comment: 11 pages, Latex, minor modification
Hartree-Fock-Bogoliubov Model and Simulation of Attractive and Repulsive Bose-Einstein Condensates
We describe a model of dynamic Bose-Einstein condensates near a Feshbach
resonance that is computationally feasible under assumptions of spherical or
cylindrical symmetry. Simulations in spherical symmetry approximate the
experimentally measured time to collapse of an unstably attractive condensate
only when the molecular binding energy in the model is correct, demonstrating
that the quantum fluctuations and atom-molecule pairing included in the model
are the dominant mechanisms during collapse. Simulations of condensates with
repulsive interactions find some quantitative disagreement, suggesting that
pairing and quantum fluctuations are not the only significant factors for
condensate loss or burst formation. Inclusion of three-body recombination was
found to be inconsequential in all of our simulations, though we do not
consider recent experiments [1] conducted at higher densities
High-frequency performance of Schottky source/drain silicon pMOS devices
A radio-frequency performance of 85-nm gate-length p-type Schottky barrier (SB) with PtSi source/drain materials is investigated. The impact of silicidation annealing temperature on the high-frequency behavior of SB MOSFETs is analyzed using an extrinsic small-signal equivalent circuit. It is demonstrated that the current drive and the gate transconductance strongly depend on the silicidation anneal temperature, whereas the unity-gain cutoff frequency of the measured devices remains nearly unchanged
Exploiting neutron-rich radioactive ion beams to constrain the symmetry energy
The Modular Neutron Array (MoNA) and 4 Tm Sweeper magnet were used to measure
the free neutrons and heavy charged particles from the radioactive ion beam
induced 32Mg + 9Be reaction. The fragmentation reaction was simulated with the
Constrained Molecular Dynamics model(CoMD), which demonstrated that the
of the heavy fragments and free neutron multiplicities were observables
sensitive to the density dependence of the symmetry energy at sub-saturation
densities. Through comparison of these simulations with the experimental data
constraints on the density dependence of the symmetry energy were extracted.
The advantage of radioactive ion beams as a probe of the symmetry energy is
demonstrated through examination of CoMD calculations for stable and
radioactive beam induced reactions
Developing autonomous learning in first year university students using perspectives from positive psychology
Autonomous learning is a commonly occurring learning outcome from university study, and it is argued that students require confidence in their own abilities to achieve this. Using approaches from positive psychology, this study aimed to develop confidence in firstâyear university students to facilitate autonomous learning. Psychological character strengths were assessed in 214 students on day one at university. Two weeks later their top three strengths were given to them in study skills modules as part of a psychoâeducational intervention designed to increase their selfâefficacy and selfâesteem. The impact of the intervention was assessed against a control group of 40 students who had not received the intervention. The results suggested that students were more confident after the intervention, and that levels of autonomous learning increased significantly compared to the controls. Character strengths were found to be associated with selfâefficacy, selfâesteem and autonomous learning in ways that were theoretically meaningful
Self-Structuring of Granular Media under Internal Avalanches
We study the phenomenon of internal avalanching within the context of
recently proposed ``Tetris'' lattice models for granular media. We define a
recycling dynamics under which the system reaches a steady state which is
self-structured, i.e. it shows a complex interplay between textured internal
structures and critical avalanche behavior. Furthermore we develop a general
mean-field theory for this class of systems and discuss possible scenarios for
the breakdown of universality.Comment: 4 pages RevTex, 3 eps figures, revised version to appear in Phys.
Rev. Let
Noisy traveling waves: effect of selection on genealogies
For a family of models of evolving population under selection, which can be
described by noisy traveling wave equations, the coalescence times along the
genealogical tree scale like , where is the size of the
population, in contrast with neutral models for which they scale like . An
argument relating this time scale to the diffusion constant of the noisy
traveling wave leads to a prediction for which agrees with our
simulations. An exactly soluble case gives trees with statistics identical to
those predicted for mean-field spin glasses in Parisi's theory.Comment: 4 pages, 2 figures New version includes more numerical simulations
and some rewriting of the text presenting our result
Hall Effect of La2/3(Ca,Pb)1/3MnO3 Single Crystals near the Critical Temperature
The Hall resistivity rho_{xy} of a La_{2/3}(Ca,Pb)_{1/3}MnO_3 single crystal
has been measured as a function of temperature and field. The overall behavior
is similar to that observed previously in thin-films. At 5 K, rho_{xy} is
positive and linear in field, indicating that the anomalous contribution
is negligible. However, the effective carrier density in a free electron model
is n_{eff}=2.4 holes/Mn, even larger than the 0.85-1.9 holes/Mn reported for
thin-films and far larger than the 0.33 holes/Mn expected from the doping
level. As temperature increases, a strong, negative contribution to rho_{xy}
appears, that we ascribe to R_S. Using detailed magnetization data, we separate
the ordinary (\propto B) and anomalous (\propto M) contributions. Below T_C,
R_S \propto rho_{xx}, indicating that magnetic skew scattering is the dominant
mechanism in the metallic ferromagnetic regime. At and above the
resistivity-peak temperature, we find that rho_{xy}/rho_{xx}M is a constant,
independent of temperature and field. This implies that the anomalous Hall
coefficient is proportional to the magnetoresistance. A different explanation
based on two fluid model is also presented.Comment: revtex, 11 pages, 4 figure
Giant Enhancement of Surface Second Harmonic Generation in BaTiO_3 due to Photorefractive Surface Wave Excitation
We report observation of strongly enhanced surface SHG in BaTiO_3 due to
excitation of a photorefractive surface electromagnetic wave. Surface SH
intensity may reach 10^{-2} of the incident fundamental light intensity.
Angular, crystal orientation and polarization dependencies of this SHG are
presented. Possible applications of this effect in nonlinear surface
spectroscopy are discussed.Comment: 5 pages, 6 figures, submitted to Physical Review Letters on the
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