30,043 research outputs found
A computer program for anisotropic shallow-shell finite elements using symbolic integration
A FORTRAN computer program for anisotropic shallow-shell finite elements with variable curvature is described. A listing of the program is presented together with printed output for a sample case. Computation times and central memory requirements are given for several different elements. The program is based on a stiffness (displacement) finite-element model in which the fundamental unknowns consist of both the displacement and the rotation components of the reference surface of the shell. Two triangular and four quadrilateral elements are implemented in the program. The triangular elements have 6 or 10 nodes, and the quadrilateral elements have 4 or 8 nodes. Two of the quadrilateral elements have internal degrees of freedom associated with displacement modes which vanish along the edges of the elements (bubble modes). The triangular elements and the remaining two quadrilateral elements do not have bubble modes. The output from the program consists of arrays corresponding to the stiffness, the geometric stiffness, the consistent mass, and the consistent load matrices for individual elements. The integrals required for the generation of these arrays are evaluated by using symbolic (or analytic) integration in conjunction with certain group-theoretic techniques. The analytic expressions for the integrals are exact and were developed using the symbolic and algebraic manipulation language
The Reionization History and Early Metal Enrichment inferred from the Gamma-Ray Burst Rate
Based on the gamma-ray burst (GRB) event rate at redshifts of , which is assessed by the spectral peak energy-to-luminosity relation
recently found by Yonetoku et al., we observationally derive the star formation
rate (SFR) for Pop III stars in a high redshift universe. As a result, we find
that Pop III stars could form continuously at . Using the
derived Pop III SFR, we attempt to estimate the ultraviolet (UV) photon
emission rate at in which redshift range no observational
information has been hitherto obtained on ionizing radiation intensity. We find
that the UV emissivity at can make a noticeable contribution
to the early reionization. The maximal emissivity is higher than the level
required to keep ionizing the intergalactic matter at .
However, if the escape fraction of ionizing photons from Pop III objects is
smaller than 10%, then the IGM can be neutralized at some redshift, which may
lead to the double reionization. As for the enrichment, the ejection of all
metals synthesized in Pop III objects is marginally consistent with the IGM
metallicity, although the confinement of metals in Pop III objects can reduce
the enrichment significantly.Comment: 12 pages, 2 figures, ApJL accepte
Suppression of Dephasing of Optically Trapped Atoms
Ultra-cold atoms trapped in an optical dipole trap and prepared in a coherent
superposition of their hyperfine ground states, decohere as they interact with
their environment. We demonstrate than the loss in coherence in an "echo"
experiment, which is caused by mechanisms such as Rayleigh scattering, can be
suppressed by the use of a new pulse sequence. We also show that the coherence
time is then limited by mixing to other vibrational levels in the trap and by
the finite lifetime of the internal quantum states of the atoms
Raising the critical temperature by disorder in unconventional superconductors mediated by spin fluctuations
We propose a mechanism whereby disorder can enhance the transition
temperature Tc of an unconventional superconductor with pairing driven by
exchange of spin fluctuations. The theory is based on a self-consistent real
space treatment of pairing in the disordered one-band Hubbard model. It has
been demonstrated before that impurities can enhance pairing by softening the
spin fluctuations locally; here, we consider the competing effect of
pair-breaking by the screened Coulomb potential also present. We show that,
depending on the impurity potential strength and proximity to magnetic order,
this mechanism results in a weakening of the disorder-dependent Tc-suppression
rate expected from Abrikosov-Gor'kov theory, or even in disorder-generated Tc
enhancements.Comment: 6 pages, 4 figures + Supplementary Materia
Time-Dependent Random Walks and the Theory of Complex Adaptive Systems
Motivated by novel results in the theory of complex adaptive systems, we
analyze the dynamics of random walks in which the jumping probabilities are
{\it time-dependent}. We determine the survival probability in the presence of
an absorbing boundary. For an unbiased walk the survival probability is
maximized in the case of large temporal oscillations in the jumping
probabilities. On the other hand, a random walker who is drifted towards the
absorbing boundary performs best with a constant jumping probability. We use
the results to reveal the underlying dynamics responsible for the phenomenon of
self-segregation and clustering observed in the evolutionary minority game.Comment: 5 pages, 2 figure
Spin susceptibility of underdoped cuprates: the case of Ortho-II YBa_2Cu_3O_{6.5}
Recent inelastic neutron scattering measurements found that the spin
susceptibility of detwinned and highly ordered ortho-II YBa_2Cu_3O_{6.5}
exhibits, in both the normal and superconducting states, one-dimensional
incommensurate modulations at low energies which were interpreted as a
signature of dynamic stripes. We propose an alternative model based on
quasiparticle transitions between the arcs of a truncated Fermi surface. Such
transitions are resonantly enhanced by scattering to the triplet spin
resonance. We show that the anisotropy in the experimental spin response is
consistent with this model if the gap at the saddle points is anisotropic.Comment: 5 fives, 3 postscript figure
Accuracy control in ultra-large-scale electronic structure calculation
Numerical aspects are investigated in ultra-large-scale electronic structure
calculation. Accuracy control methods in process (molecular-dynamics)
calculation are focused. Flexible control methods are proposed so as to control
variational freedoms, automatically at each time step, within the framework of
generalized Wannier state theory. The method is demonstrated in silicon
cleavage simulation with 10^2-10^5 atoms. The idea is of general importance
among process calculations and is also used in Krylov subspace theory, another
large-scale-calculation theory.Comment: 8 pages, 3 figures. To appear in J.Phys. Condens. Matter. A preprint
PDF file in better graphics is available at
http://fujimac.t.u-tokyo.ac.jp/lses/index_e.htm
Anisotropy study of multiferroicity in the pyroxene NaFeGeO
We present a study of the anisotropy of the dielectric, magnetic and
magnetoelastic properties of the multiferroic clinopyroxene NaFeGeO.
Pyroelectric currents, dielectric constants and magnetic susceptibilities as
well as the thermal expansion and the magnetostriction were examined on large
synthetic single crystals of NaFeGeO. The spontaneous electric
polarization detected below K in an
antiferromagnetically ordered state ( K) is mainly lying
within the plane with a small component along , indicating a triclinic
symmetry of the multiferroic phase of NaFeGeO. The electric
polarization can be strongly modified by applying magnetic fields along
different directions. We derive detailed magnetic-field versus temperature
phase diagrams and identify three multiferroic low-temperature phases, which
are separated by a non-ferroelectric, antiferromagnetically ordered state from
the paramagnetic high-temperature phase.Comment: 14 pages, 8 figures. (minor modifications and corrections of the
text
Electronic Structure of Ladder Cuprates
We study the electronic structure of the ladder compounds (SrCa)CuO 14-24-41
and SrCuO 123. LDA calculations for both give similar Cu 3d-bands near the
Fermi energy. The hopping parameters estimated by fitting LDA energy bands show
a strong anisotropy between the t_perp t_par intra-ladder hopping and small
inter-ladder hopping. A downfolding method shows that this anisotropy arises
from the ladder structure.The conductivity perpendicular to the ladders is
computed assuming incoherent tunneling giving a value close to experiment.Comment: 5 pages, 3 figure
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