31,747 research outputs found
Bond patterns and charge order amplitude in 1/4-filled charge-transfer solids
Metal-insulator transition accompanied by charge-ordering has been widely
investigated in quasi-one-dimensional conductors, including in particular
organic charge-transfer solids. Among such materials the 1/4-filled band
charge-transfer solids are of strong interest, because of the commensurate
nature of the charge-ordering in these systems. The period-four charge-order
pattern ...1100... here is accompanied by two distinct bond distortion
patterns, giving rise to bond-charge-density waves (BCDW) of types 1 and 2.
Using quantum Monte Carlo methods, we determine the phase diagram within the
extended Hubbard Hamiltonian that gives both types 1 and 2 BCDW in the
thermodynamic limit. We further investigate the effect of electron-electron and
electron-phonon interactions on the amount of charge disproportionation. Our
results show that between these two bond patterns, one (BCDW2) in general
coexists with a large magnitude charge order, which is highly sensitive to
electron-phonon interactions, while the other (BCDW1) is characterized by weak
charge order. We discuss the relevance of our work to experiments on several
1/4-filled conductors, focusing in particular on the materials (EDO-TTF)_2X and
(DMEDO-TTF)_2X with large amplitude charge-order.Comment: 7 pages, 8 figure
The effects of acceleration stress on human workload and manual control
The effects of +Gz stress on operator task performance and workload were assessed. Subjects were presented a two dimensional maze and were required to solve it as rapidly as possible (by moving a light dot through it via a trim switch on a control stick) while under G-stress at levels from +1 Gz to +6 Gz. The G-stress was provided by a human centrifuge. The effects of this stress were assessed by two techniques; (1) objective performance measures on the primary maze-solving task, and (2) subjective workload measures obtained using the subjective workload assessment technique (SWAT). It was found that while neither moderate (+3 Gz) nor high (+5 Gz and +6 Gz) levels of G-stress affected maze solving performance, the high G levels did increase significantly the subjective workload of the maze task
Planck Scale Cosmology in Resummed Quantum Gravity
We show that, by using resummation techniques based on the extension of the
methods of Yennie, Frautschi and Suura to Feynman's formulation of Einstein's
theory, we get quantum field theoretic predictions for the UV fixed-point
values of the dimensionless gravitational and cosmological constants.
Connections to the phenomenological asymptotic safety analysis of Planck scale
cosmology by Bonanno and Reuter are discussed.Comment: 7 pages, 1 figur
Reduced dynamics of Ward solitons
The moduli space of static finite energy solutions to Ward's integrable
chiral model is the space of based rational maps from \CP^1 to itself
with degree . The Lagrangian of Ward's model gives rise to a K\"ahler metric
and a magnetic vector potential on this space. However, the magnetic field
strength vanishes, and the approximate non--relativistic solutions to Ward's
model correspond to a geodesic motion on . These solutions can be compared
with exact solutions which describe non--scattering or scattering solitons.Comment: Final version, to appear in Nonlinearit
Synthesis and characteristics of polyarylene ether sulfones
A method utilizing potassium carbonate/dimethyl acetamide, as base and solvent respectively, was used for the synthesis of several homopolymers and copolymers derived from various bisphenols. It is demonstrated that this method deviates from simple second order kinetics; this deviation being due to the heterogeneous nature of the reaction. Also, it is shown that a liquid induced crystallization process can improve the solvent resistance of these polymers. Finally, a Monte Carlo simulation of the triad distribution of monomers in nonequilibrium copolycondensation is discussed
Quiescent Cores and the Efficiency of Turbulence-Accelerated, Magnetically Regulated Star Formation
The efficiency of star formation, defined as the ratio of the stellar to
total (gas and stellar) mass, is observed to vary from a few percent in regions
of dispersed star formation to about a third in cluster-forming cores. This
difference may reflect the relative importance of magnetic fields and
turbulence in controlling star formation. We investigate the interplay between
supersonic turbulence and magnetic fields using numerical simulations, in a
sheet-like geometry. We demonstrate that star formation with an efficiency of a
few percent can occur over several gravitational collapse times in moderately
magnetically subcritical clouds that are supersonically turbulent. The
turbulence accelerates star formation by reducing the time for dense core
formation. The dense cores produced are predominantly quiescent, with subsonic
internal motions. These cores tend to be moderately supercritical. They have
lifetimes long compared with their local gravitational collapse time. Some of
the cores collapse to form stars, while others disperse away without star
formation. In turbulent clouds that are marginally magnetically supercritical,
the star formation efficiency is higher, but can still be consistent with the
values inferred for nearby embedded clusters. If not regulated by magnetic
fields at all, star formation in a multi-Jeans mass cloud endowed with a strong
initial turbulence proceeds rapidly, with the majority of cloud mass converted
into stars in a gravitational collapse time. The efficiency is formally higher
than the values inferred for nearby cluster-forming cores, indicating that
magnetic fields are dynamically important even for cluster formation.Comment: submitted to Ap
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