485 research outputs found
Self-Organizing Maps Algorithm for Parton Distribution Functions Extraction
We describe a new method to extract parton distribution functions from hard
scattering processes based on Self-Organizing Maps. The extension to a larger,
and more complex class of soft matrix elements, including generalized parton
distributions is also discussed.Comment: 6 pages, 3 figures, to be published in the proceedings of ACAT 2011,
14th International Workshop on Advanced Computing and Analysis Techniques in
Physics Researc
Excitons in quasi-one dimensional organics: Strong correlation approximation
An exciton theory for quasi-one dimensional organic materials is developed in
the framework of the Su-Schrieffer-Heeger Hamiltonian augmented by short range
extended Hubbard interactions. Within a strong electron-electron correlation
approximation, the exciton properties are extensively studied. Using scattering
theory, we analytically obtain the exciton energy and wavefunction and derive a
criterion for the existence of a exciton. We also systematically
investigate the effect of impurities on the coherent motion of an exciton. The
coherence is measured by a suitably defined electron-hole correlation function.
It is shown that, for impurities with an on-site potential, a crossover
behavior will occur if the impurity strength is comparable to the bandwidth of
the exciton, corresponding to exciton localization. For a charged impurity with
a spatially extended potential, in addition to localization the exciton will
dissociate into an uncorrelated electron-hole pair when the impurity is
sufficiently strong to overcome the Coulomb interaction which binds the
electron-hole pair. Interchain coupling effects are also discussed by
considering two polymer chains coupled through nearest-neighbor interchain
hopping and interchain Coulomb interaction . Within the
matrix scattering formalism, for every center-of-mass momentum, we find two
poles determined only by , which correspond to the interchain
excitons. Finally, the exciton state is used to study the charge transfer from
a polymer chain to an adjacent dopant molecule.Comment: 24 pages, 23 eps figures, pdf file of the paper availabl
Energetic ion transport by microturbulence is insignificant in tokamaks
Energetic ion transport due to microturbulence is investigated in magnetohydrodynamic-quiescent plasmas by way of neutral beam injection in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)]. A range of on-axis and off-axis beam injection scenarios are employed to vary relevant parameters such as the character of the background microturbulence and the value of Eb/Te , where Eb is the energetic ion energy and Te the electron temperature. In all cases, it is found that any transport enhancement due to microturbulence is too small to observe experimentally. These transport effects are modeled using numerical and analytic expectations that calculate the energetic ion diffusivity due to microturbulence. It is determined that energetic ion transport due to coherent fluctuations (e.g., Alfvén eigenmodes) is a considerably larger effect and should therefore be considered more important for ITER.United States. Dept. of Energy (DE-FC02-04ER54698)United States. Dept. of Energy (DE-FC02-99ER54512)United States. Dept. of Energy (DE-FG03-97ER54415)United States. Dept. of Energy (DE-FG02-07ER54917)United States. Dept. of Energy (DE-AC02-09CH11466)United States. Dept. of Energy (SC-G903402)United States. Dept. of Energy (DE-FG02-08ER54984)United States. Dept. of Energy ( DE-AC52-07NA27344)United States. Dept. of Energy ( DE-FG02-89ER53296)United States. Dept. of Energy (DE-FG02-08ER54999)United States. Dept. of Energy (DE-AC05-00OR22725
Stabilization of weak ferromagnetism by strong magnetic response to epitaxial strain in multiferroic BiFeO3
Multiferroic BiFeO3 exhibits excellent magnetoelectric coupling critical for magnetic information processing with minimal power consumption. However, the degenerate nature of the easy spin axis in the (111) plane presents roadblocks for real world applications. Here, we explore the stabilization and switchability of the weak ferromagnetic moments under applied epitaxial strain using a combination of first-principles calculations and group-theoretic analyses. We demonstrate that the antiferromagnetic moment vector can be stabilized along unique crystallographic directions ([110] and [-110]) under compressive and tensile strains. A direct coupling between the anisotropic antiferrodistortive rotations and the Dzyaloshinskii-Moria interactions drives the stabilization of the weak ferromagnetism. Furthermore, energetically competing C- and G-type magnetic orderings are observed at high compressive strains, suggesting that it may be possible to switch the weak ferromagnetism "on" and "off" under the application of strain. These findings emphasize the importance of strain and antiferrodistortive rotations as routes to enhancing induced weak ferromagnetism in multiferroic oxides.ope
The Evolution of Compact Binary Star Systems
We review the formation and evolution of compact binary stars consisting of
white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and
BHs are thought to be the primary astrophysical sources of gravitational waves
(GWs) within the frequency band of ground-based detectors, while compact
binaries of WDs are important sources of GWs at lower frequencies to be covered
by space interferometers (LISA). Major uncertainties in the current
understanding of properties of NSs and BHs most relevant to the GW studies are
discussed, including the treatment of the natal kicks which compact stellar
remnants acquire during the core collapse of massive stars and the common
envelope phase of binary evolution. We discuss the coalescence rates of binary
NSs and BHs and prospects for their detections, the formation and evolution of
binary WDs and their observational manifestations. Special attention is given
to AM CVn-stars -- compact binaries in which the Roche lobe is filled by
another WD or a low-mass partially degenerate helium-star, as these stars are
thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure
A negative-U interpretation of the femto-second laser pulse induced crystallographic expansion of a cuprate HTSC material reported recently by Gedik et al
Gedik et al have very recently demonstrated using a pump/probe femto-second
laser technique that the c-axis lattice parameter of LaCuO4+d temporarily
becomes expanded by as much as 2.5% following pulsed laser optical excitation
at 1.55 eV. Access to an out-of-equilibrium metastable excited state is
observed to develop on a time scale of 30 ps. Subsequently the latter state
decays displaying a still longer half-life of just over 300 ps. Observation of
the temperature independence of this laser induced interstate transfer and of
the linear dependence of the production of the metastable population upon the
energy delivered per unit area by the initiating light pulse (beyond a key
threshold fluence) have been interpreted by Gedik et al within the framework of
standard p-to-d, O-to-Cu, charge transfer excitations. By contrast these same
data are reinterpreted here in terms of pumped local pairs, within a negative-U
scenario of cuprate HTSC behaviour long advocated by the current author. The
d8-to-d10 laser-induced augmentation in the negative-U state population
(10CuIII2-) brings marked c-axis expansion by virtue of (i) the local
electrostatic charge imbalance, (ii) the increased antibonding nature of the
electron double-loading d10(p6) configuration created at pair-receptive CuIII
coordination units, and (iii) the layered nature of the cuprate crystal
structure. The new observations are related through to Rohlers striking,
standard crystallographic observations, to the stripe domain formation, and to
previous pump/probe experiments.Comment: 14 pages, 1 figur
Discrete Emotion Effects on Lexical Decision Response Times
Our knowledge about affective processes, especially concerning effects on cognitive demands like word processing, is increasing steadily. Several studies consistently document valence and arousal effects, and although there is some debate on possible interactions and different notions of valence, broad agreement on a two dimensional model of affective space has been achieved. Alternative models like the discrete emotion theory have received little interest in word recognition research so far. Using backward elimination and multiple regression analyses, we show that five discrete emotions (i.e., happiness, disgust, fear, anger and sadness) explain as much variance as two published dimensional models assuming continuous or categorical valence, with the variables happiness, disgust and fear significantly contributing to this account. Moreover, these effects even persist in an experiment with discrete emotion conditions when the stimuli are controlled for emotional valence and arousal levels. We interpret this result as evidence for discrete emotion effects in visual word recognition that cannot be explained by the two dimensional affective space account
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