8,356 research outputs found
Adding stress plot function to NASTRAN
Stress plot function was developed and added to the NASTRAN level 15.5. Computed stress distribution can be displayed by this function, with vectors showing the principal stresses of the finite elements over the specified portions of the structure. NASTRAN is reviewed in the aspect of plotting capabilities. Stress tensor field is examined in preparation of stress display. Then the stress plot function as added to the NASTRAN is described. A sample plotout by this function is shown
Spin Gap in Two-Dimensional Heisenberg Model for CaVO
We investigate the mechanism of spin gap formation in a two-dimensional model
relevant to Mott insulators such as CaVO. From the perturbation
expansion and quantum Monte Carlo calculations, the origin of the spin gap is
ascribed to the four-site plaquette singlet in contrast to the dimer gap
established in the generalized dimerized Heisenberg model.Comment: 8 pages, 6 figures available upon request (Revtex
Phase diagram of depleted Heisenberg model for CaV4O9
We have numerically investigated the 1/5-depleted Heisenberg square lattice
representing CaV4O9 using the Quantum Monte Carlo loop algorithm. We have
determined the phase diagram of the model as a function of the ratio of the two
different couplings: bonds within a plaquette and dimer bonds between
plaquettes. By calculating both the spin gap and the staggered magnetization we
determine the range of stability of the long range ordered (LRO) phase. At
isotropic coupling LRO survives the depletion. But the close vicinity of the
isotropic point to the spin gap phase leads us to the conclusion that already a
small frustrating next nearest neighbor interaction can drive the system into
the quantum disordered phase and thus explain the spin gap behavior of CaV4O9
An Examination of Chimpanzee Use in Human Cancer Research
Advocates of chimpanzee research claim the genetic similarity of humans and chimpanzees make them an indispensable research tool to combat human diseases. Given that cancer is a leading cause of human death worldwide, one might expect that if chimpanzees were needed for, or were productive in, cancer research, then they would have been widely used. This comprehensive literature analysis reveals that chimpanzees have scarcely been used in any form of cancer research, and that chimpanzee tumours are extremely rare and biologically different from human cancers. Often, chimpanzee citations described peripheral use of chimpanzee cells and genetic material in predominantly human genomic studies. Papers describing potential new cancer therapies noted significant concerns regarding the chimpanzee model. Other studies described interventions that have not been pursued clinically. Finally, available evidence indicates that chimpanzees are not essential in the development of therapeutic monoclonal antibodies. It would therefore be unscientific to claim that chimpanzees are vital to cancer research. On the contrary, it is reasonable to conclude that cancer research would not suffer, if the use of chimpanzees for this purpose were prohibited in the US. Genetic differences between humans and chimpanzees, make them an unsuitable model for cancer, as well as other human diseases
Magnetic Susceptibility for
We examine experimental magnetic susceptibility for
CaVO by fitting with fitting function .
The function is a power series of 1/T and the lowest order
term is fixed as , where is the Curie constant as determined by the
experimental -value (g=1.96). Fitting parameters are , and
expansion coefficients except for the first one in .
We determine and as 0.73 and 0 for an
experimental sample. We interpret as the volume fraction of
CaVO in the sample and as the susceptibility for the
pure CaVO. The result of means that the sample includes
nonmagnetic components. This interpretation consists with the result of a
perturbation theory and a neutron scattering experiment.Comment: 4pages, 4figure
Spin Gaps in a Frustrated Heisenberg model for CaVO
I report results of a density matrix renormalization group (DMRG) study of a
model for the two dimensional spin-gapped system CaVO. This study
represents the first time that DMRG has been used to study a two dimensional
system on large lattices, in this case as large as , allowing
extrapolation to the thermodynamic limit. I present a substantial improvement
to the DMRG algorithms which makes these calculations feasible.Comment: 10 pages, with 4 Postscript figure
First-principles study on scanning tunneling microscopy images of hydrogen-terminated Si(110) surfaces
Scanning tunneling microscopy images of hydrogen-terminated Si(110) surfaces
are studied using first-principles calculations. Our results show that the
calculated filled-state images and local density of states are consistent with
recent experimental results, and the empty-state images appear significantly
different from the filled-state ones. To elucidate the origin of this
difference, we examined in detail the local density of states, which affects
the images, and found that the bonding and antibonding states of surface
silicon atoms largely affect the difference between the filled- and empty-state
images.Comment: 4 pages, and 4 figure
Impurity Effect on Spin Ladder System
Effects of nonmagnetic impurity doping in a spin ladder system with a spin
gap are investigated by the exact diagonalization as well as by the variational
Monte Carlo calculations. Substantial changes in macroscopic properties such as
enhancements in spin correlations and magnetic susceptibilities are observed in
the low impurity concentration region, which are caused by the increase of
low-energy states. These results suggest that small but finite amount of
nonmagnetic impurity doping relevantly causes the reduction or the vanishment
of the spin gap. This qualitatively explains the experimental result of
Zn-doped SrCuO where small doping induces gapless nature. We
propose a possible scenario for this drastic change as a quantum phase
transition in a spin gapped ladder system due to spinon doping effects.Comment: 14 pages LaTeX including 5 PS figure
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