22,609 research outputs found
Magnetic properties of the spin-1 two-dimensional Heisenberg model on a triangular lattice
Motivated by the recent experiment in NiGaS, the spin-1 Heisenberg
model on a triangular lattice with the ferromagnetic nearest- and
antiferromagnetic third-nearest-neighbor exchange interactions,
and , is studied in the range of the parameter . Mori's projection operator technique is used as a method, which retains the
rotation symmetry of spin components and does not anticipate any magnetic
ordering. For zero temperature several phase transitions are observed. At the ground state is transformed from the ferromagnetic order into
a disordered state, which in its turn is changed to an antiferromagnetic
long-range ordered state with the incommensurate ordering vector at . With growing the ordering vector moves along the line to the
commensurate point , which is reached at . The
final state with the antiferromagnetic long-range order can be conceived as
four interpenetrating sublattices with the spin structure on each of
them. Obtained results offer a satisfactory explanation for the experimental
data in NiGaS.Comment: 2 pages, 3 figure
A model for the anisotropic response of fibrous soft tissues using six discrete fibre bundles
The development of accurate constitutive models of fibrous soft-tissues is a challenging problem. Many consider the tissue to be a collection of fibres with a continuous distribution function representing their orientations. A novel discrete fibre model is presented consisting of six weighted fibre bundles. Each bundle is oriented such that they pass through opposing vertices of a regular icosahedron. A novel aspect of the model is the use of simple analytical distribution functions to simulate the undulated collagen fibres. This approach yields a closed form analytical expression for the strain energy function for the collagen fibre bundle that avoids the sometimes costly numerical integration of some statistical distribution functions. The elastin fibres are characterized by a neo-Hookean strain energy function. The model accurately simulates the biaxial stretching of rabbit-skin (error-of-fit 8.7%), the uniaxial stretching of pig-skin (error-of-fit 7.6%), equibiaxial loading of aortic valve cusp (error-of-fit 0.8%), and the simple shear of rat septal myocardium (error-of-fit 9.1%). The proposed model compares favourably with previously published soft-tissue models and alternative methods of representing undulated collagen fibres. The stiffness of collagen fibres predicted by the model ranges from 8.0 MPa to 0.93 GPa. The stiffness of elastin fibres ranges from 2.5 kPa to 154.4 kPa. The anisotropy of model resulting from the representation of the fibre field with a discrete number of fibres is also explored
Magnetic phase diagram of the spin-1 two-dimensional J1-J3 Heisenberg model on a triangular lattice
The spin-1 Heisenberg model on a triangular lattice with the ferromagnetic
nearest, , and antiferromagnetic third-nearest-neighbor,
, exchange interactions is studied in the range of the parameter . Mori's projection operator technique is used as a
method, which retains the rotation symmetry of spin components and does not
anticipate any magnetic ordering. For zero temperature several phase
transitions are observed. At the ground state is transformed
from the ferromagnetic spin structure into a disordered state, which in its
turn is changed to an antiferromagnetic long-range ordered state with the
incommensurate ordering vector at
. With the further growth of the ordering vector moves along
the line to the commensurate point , which is reached at . The final state with an
antiferromagnetic long-range order can be conceived as four interpenetrating
sublattices with the spin structure on each of them. Obtained
results are used for interpretation of the incommensurate magnetic ordering
observed in NiGaS.Comment: 18 pages, 6 figures, accepted for publication in Physics Letters
Comment on ``Dispersion-Independent High-Visibility Quantum Interference ... "
We show in this Comment that the interpretation of experimental data as well
as the theory presented in Atat\"ure et al. [Phys. Rev. Lett. 84, 618 (2000)]
are incorrect and discuss why such a scheme cannot be used to "recover"
high-visibility quantum interference.Comment: Comment on Atat\"ure et al. [Phys. Rev. Lett. 84, 618 (2000)], 2nd
revision, To appear in Phys. Rev. Lett. April, (2001
Solution of three-dimensional afterbody flow using reduced Navier-Stokes equations
The flow over afterbody geometries was investigated using the reduced Navier-Stokes (RNS) approximation. Both pressure velocity flux-split and composites velocity primitive variable formulations were considered. Pressure or pseudopotential relaxation procedures are combined with sparse matrix or coupled strongly implicit algorithms to form a three-dimensional solver for general non-orthogonal coordinates. Three-dimensional subsonic and transonic viscous/inviscid interacting flows were evaluated. Solutions with and without regions of recirculation were obtained
ALLY: An operator's associate for satellite ground control systems
The key characteristics of an intelligent advisory system is explored. A central feature is that human-machine cooperation should be based on a metaphor of human-to-human cooperation. ALLY, a computer-based operator's associate which is based on a preliminary theory of human-to-human cooperation, is discussed. ALLY assists the operator in carrying out the supervisory control functions for a simulated NASA ground control system. Experimental evaluation of ALLY indicates that operators using ALLY performed at least as well as they did when using a human associate and in some cases even better
The spin-1 two-dimensional J1-J2 Heisenberg antiferromagnet on a triangular lattice
The spin-1 Heisenberg antiferromagnet on a triangular lattice with the
nearest- and next-nearest-neighbor couplings, and , ,
is studied in the entire range of the parameter . Mori's projection operator
technique is used as a method which retains the rotation symmetry of spin
components and does not anticipate any magnetic ordering. For zero temperature
four second-order phase transitions are observed. At the
ground state is transformed from the long-range ordered spin
structure into a state with short-range ordering, which in its turn is changed
to a long-range ordered state with the ordering vector at . For
a new transition to a state with a short-range order occurs.
This state has a large correlation length which continuously grows with
until the establishment of a long-range order happens at . In
the range , the ordering vector is incommensurate. With growing
it moves along the line to the point which is reached at . The obtained state with a long-range order can be conceived as three
interpenetrating sublattices with the spin structure on each of
them.Comment: 13 pages, 5 figures, accepted for publication in Physics Letters
Gravitational Wave Bursts from Collisions of Primordial Black Holes in Clusters
The rate of gravitational wave bursts from the mergers of massive primordial
black holes in clusters is calculated. Such clusters of black holes can be
formed through phase transitions in the early Universe. The central black holes
in clusters can serve as the seeds of supermassive black holes in galactic
nuclei. The expected burst detection rate by the LISA gravitational wave
detector is estimated.Comment: 10 pages, 2 figure
Open Questions in Classical Gravity
We discuss some outstanding open questions regarding the validity and
uniqueness of the standard second order Newton-Einstein classical gravitational
theory. On the observational side we discuss the degree to which the realm of
validity of Newton's Law of Gravity can actually be extended to distances much
larger than the solar system distance scales on which the law was originally
established. On the theoretical side we identify some commonly accepted but
actually still open to question assumptions which go into the formulating of
the standard second order Einstein theory in the first place. In particular, we
show that while the familiar second order Poisson gravitational equation (and
accordingly its second order covariant Einstein generalization) may be
sufficient to yield Newton's Law of Gravity they are not in fact necessary. The
standard theory thus still awaits the identification of some principle which
would then make it necessary too. We show that current observational
information does not exclusively mandate the standard theory, and that the
conformal invariant fourth order theory of gravity considered recently by
Mannheim and Kazanas is also able to meet the constraints of data, and in fact
to do so without the need for any so far unobserved non-luminous or dark
matter.Comment: UCONN-93-1, plain TeX format, 22 pages (plus 7 figures - send
requests to [email protected]). To appear in a special issue of
Foundations of Physics honoring Professor Fritz Rohrlich on the occasion of
his retirement, L. P. Horwitz and A. van der Merwe Editors, Plenum Publishing
Company, N.Y., Fall 199
The ionization structure of the Orion nebula: Infrared line observations and models
Observations of the (O III) 52 and 88 micron lines and the (N III) 57 micron line have been made at 6 positions and the (Ne III) 36 micron line at 4 positions in the Orion Nebula to probe its ionization structure. The measurements, made with a -40" diameter beam, were spaced every 45" in a line south from and including the Trapezium. The wavelength of the (Ne III) line was measured to be 36.013 + or - 0.004 micron. Electron densities and abundance ratios of N(++)/O(++) have been calculated and compared to other radio and optical observations. Detailed one component and two component (bar plus halo) spherical models were calculated for exciting stars with effective temperatures of 37 to 40,000K and log g = 4.0 and 4.5. Both the new infrared observations and the visible line measurements of oxygen and nitrogen require T sub eff approx less than 37,000K. However, the double ionized neon requires a model with T sub eff more than or equal to 39,000K, which is more consistent with that inferred from the radio flux or spectral type. These differences in T sub eff are not due to effects of dust on the stellar radiation field, but are probably due to inaccuracies in the assumed stellar spectrum. The observed N(++)/O(++) ratio is almost twice the N(+)/O(+) ratio. The best fit models give N/H = 8.4 x 10 to the -5 power, O/H = 4.0 x 10 to the -4 power, and Ne/H = 1.3 x 10 to the -4 power. Thus neon and nitrogen are approximately solar, but oxygen is half solar in abundance. From the infrared O(++) lines it is concluded that the ionization bar results from an increase in column depth rather than from a local density enhancement
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