6,702 research outputs found
Harmonic analysis of irradiation asymmetry for cylindrical implosions driven by high-frequency rotating ion beams
Cylindrical implosions driven by intense heavy ions beams should be
instrumental in a near future to study High Energy Density Matter. By rotating
the beam by means of a high frequency wobbler, it should be possible to deposit
energy in the outer layers of a cylinder, compressing the material deposited in
its core. The beam temporal profile should however generate an inevitable
irradiation asymmetry likely to feed the Rayleigh-Taylor instability (RTI)
during the implosion phase. In this paper, we compute the Fourier components of
the target irradiation in order to make the junction with previous works on RTI
performed in this setting. Implementing a 1D and 2D beam models, we find these
components can be expressed exactly in terms of the Fourier transform of the
temporal beam profile. If is the beam duration and its rotation
frequency, "magic products" can be identified which cancel the first
harmonic of the deposited density, resulting in an improved irradiation
symmetry.Comment: 19 pages, 8 figures, to appear in PR
Crystal structure of 3-{(E)-[(3, 4-dichloro-phenyl)imino]methyl}benzene-1, 2-diol
The authors acknowledge the provision of funds for the purchase of diffractometer and encouragement by Dr.Muhammad Akram Chaudhary, Vice Chancellor, University of Sargodha, Pakistan.Peer reviewedPublisher PD
Universal Properties of Cuprate Superconductors: T_c Phase Diagram, Room-Temperature Thermopower, Neutron Spin Resonance, and STM Incommensurability Explained in Terms of Chiral Plaquette Pairing
We report that four properties of cuprates and their evolution with
doping are consequences of simply counting four-site plaquettes arising from
doping, (1) the universal T_c phase diagram (superconductivity between ~0.05 and
~0.27 doping per CuO_2 plane and optimal T_c at ~0.16), (2) the universal doping
dependence of the room-temperature thermopower, (3) the superconducting
neutron spin resonance peak (the “41 meV peak”), and (4) the dispersionless
scanning tunneling conductance incommensurability. Properties (1), (3), and (4)
are explained with no adjustable parameters, and (2) is explained with exactly one.
The successful quantitative interpretation of four very distinct aspects of cuprate
phenomenology by a simple counting rule provides strong evidence for four-site
plaquette percolation in these materials. This suggests that inhomogeneity, percolation,
and plaquettes play an essential role in cuprates. This geometric analysis
may provide a useful guide to search for new compositions and structures with
improved superconducting properties
Frequency and phase modulation performance of an injection-locked CW magnetron.
It is demonstrated that the output of a 2.45-GHz magnetron operated as a current-controlled oscillator through its pushing characteristic can lock to injection signals in times of the order of 100-500 ns depending on injection power, magnetron heater power, load impedance, and frequency offset of the injection frequency from the natural frequency of the magnetron. Accordingly, the magnetron can follow frequency and phase modulations of the injection signal, behaving as a narrow-band amplifier. The transmission of phase-shift-keyed data at 2 Mb/s has been achieved. Measurements of the frequency response and anode current after a switch of phase as a function of average anode current and heater power give new insight into the locking mechanisms and the noise characteristics of magnetrons
The ferroelectric and cubic phases in BaTiO_3 ferroelectrics are also antiferroelectric
Using quantum mechanics (QM, Density Functional Theory) we show that all four phases of barium titanate (BaTiO3) have local Ti distortions toward (an octahedral face). The stable rhombohedral phase has all distortions in phase (ferroelectric, FE), whereas higher temperature phases have antiferroelectric coupling (AFE) in one, two, or three dimensions (orthorhombic, tetragonal, cubic). This FE–AFE model from QM explains such puzzling aspects of these systems as the allowed Raman excitation observed for the cubic phase, the distortions toward observed in the cubic phase using x-ray fine structure, the small transition entropies, the heavily damped soft phonon modes, and the strong diffuse x-ray scattering in all but the rhombohedral phase. In addition, we expect to see additional weak Bragg peaks at the face centers of the reciprocal lattice for the cubic phase. Similar FE–AFE descriptions are expected to occur for other FE materials. Accounting for this FE–AFE nature of these phases is expected to be important in accurately simulating the domain wall structures, energetics, and dynamics, which in turn may lead to the design of improved materials
Reply to “Comment on ‘Phase diagram of MgO from density-functional theory and molecular-dynamics simulations’”
In answer to a Comment by Belonoshko [Phys. Rev. B 63, 096101 (2001)], we show that the B1-liquid melting curve of MgO obtained using two-phase simulations is in good agreement with the published one obtained using the Clausius-Clapeyron equation in conjunction with separate single phase calculations of liquid and solid
Studies of fullerenes and carbon nanotubes by an extended bond order potential
We present a novel approach to combine bond order potentials with long-range nonbond interactions. This extended bond order potential consistently takes into account bond terms and nonbond terms. It not only captures the advantages of the bond order potentials (i.e. simulating bond forming and breaking), but also systematically includes the nonbond contributions to energy and forces in studying the structure and dynamics of covalently bonded systems such as graphite, diamond, nanotubes, fullerenes and hydrocarbons, in their crystal and melt forms. Using this modified bond order potential, we studied the structure and thermal properties (including thermal conductivity) of C60 crystal, and the elastic properties and plastic deformation processes of the single-walled and double-walled nanotubes. This extended bond order potential enables us to simulate large deformations of a nanotube under tensile and compressive loads. The basic formulation in this paper is transferable to other bond order potentials and traditional valence force fields
Morse stretch potential charge equilibrium force field for ceramics: Application to the quartz-stishovite phase transition and to silica glass
To predict phase transitions in ceramics and minerals from molecular dynamics simulations, we have developed a force field in which the charges are allowed to readjust instantaneously to the atomic configurations. These charges are calculated using the charge equilibration (QEq) method. In addition to electrostatics, a two-body Morse stretch potential is included to account for short-range nonelectrostatic interactions. This MS-Q potential is applied herein to SiO_2, where we find that it describes well the fourfold coordinated and sixfold coordinated systems (such as quartz and stishovite), silica glass, and the pressure-induced phase transition from quartz to stishovite
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