12,052 research outputs found
Finer, Better Lime, Please!
During the past 4 years, Iowa farmers have used over 5 million tons of ground limestone, approximately four times as much as in any similar period prior to the AAA Conservation Materials Program. This limestone is helping materially in producing record crops on several million acres of good crop land. But many million acres still need to be mobilized for maximum production by liming
Magnetic relaxation of a system of superparamagnetic particles weakly coupled by dipole-dipole interactions
The effect of long range dipole-dipole interactions on the thermal
fluctuations of the magnetization of an assembly of single-domain ferromagnetic
particles is considered. If orientational correlations between the particles
are neglected, the evolution of the magnetization orientations may be described
by a nonlinear Fokker-Planck equation (FPE) reducing to the usual linear one in
the limit of infinite dilution [W.F. Brown Jr, Phys. Rev. 130, 1677 (1963)].
The thermally activated relaxation time scale of the assembly is estimated,
leading to a simple modification of the axially symmetric asymptotes for the
superparamagnetic relaxation time.Comment: 31 pages, 3 figures, regular articl
Some remarks on quasi-Hermitian operators
A quasi-Hermitian operator is an operator that is similar to its adjoint in
some sense, via a metric operator, i.e., a strictly positive self-adjoint
operator. Whereas those metric operators are in general assumed to be bounded,
we analyze the structure generated by unbounded metric operators in a Hilbert
space. Following our previous work, we introduce several generalizations of the
notion of similarity between operators. Then we explore systematically the
various types of quasi-Hermitian operators, bounded or not. Finally we discuss
their application in the so-called pseudo-Hermitian quantum mechanics.Comment: 18page
Wiggly Relativistic Strings
We derive the equations of motion for general strings, i.e. strings with
arbitrary relation between tension and energy per unit length
. The renormalization of and that results from
averaging out small scale wiggles on the string is obtained in the general case
to lowest order in the amount of wiggliness. For Nambu-Goto strings we find
deviations from the equation of state in
higher orders. Finally we argue that wiggliness may radically modify the gauge
cosmic string scenario.Comment: 10 pages, LaTeX, UFIFT-HEP-92-1
Intermittent permeation of cylindrical nanopores by water
Molecular Dynamics simulations of water molecules in nanometre sized
cylindrical channels connecting two reservoirs show that the permeation of
water is very sensitive to the channel radius and to electric polarization of
the embedding material. At threshold, the permeation is {\emph{intermittent}}
on a nanosecond timescale, and strongly enhanced by the presence of an ion
inside the channel, providing a possible mechanism for gating. Confined water
remains surprisingly fluid and bulk-like. Its behaviour differs strikingly from
that of a reference Lennard-Jones fluid, which tends to contract into a highly
layered structure inside the channel.Comment: 4 pages, 4 figure
Stretching Wiggly Strings
How does the amplitude of a wiggle on a string change when the string is
stretched? We answer this question for both longitudinal and transverse wiggles
and for arbitrary equation of state, {\it i.e.}, for arbitrary relation between
the tension and the energy per unit length of the string.
This completes our derivation of the renormalization of string parameters which
results from averaging out small scale wiggles on a string. The program is
presented here in its entirety.Comment: Written with ReVTeX 3.0 package. Two figures are not included.
Complete paper with postscript figures can be retrieved through anonymous ftp
@quark.phys.ufl.edu. Get /preprints/ifthep94_4.tar.gz, gunzip and tar it.
UFIFT-HEP-94-
Rapid interstellar scintillation of PKS B1257-326: two-station pattern time delays and constraints on scattering and microarcsecond source structure
We report measurements of time delays of up to 8 minutes in the centimeter
wavelength variability patterns of the intra-hour scintillating quasar PKS
1257-326 as observed between the VLA and the ATCA on three separate epochs.
These time delays confirm interstellar scintillation as the mechanism
responsible for the rapid variability, at the same time effectively ruling out
the coexistence of intrinsic intra-hour variability in this source. The time
delays are combined with measurements of the annual variation in variability
timescale exhibited by this source to determine the characteristic length scale
and anisotropy of the quasar's intensity scintillation pattern, as well as
attempting to fit for the bulk velocity of the scattering plasma responsible
for the scintillation. We find evidence for anisotropic scattering and highly
elongated scintillation patterns at both 4.9 and 8.5 GHz, with an axial ratio >
10:1, extended in a northwest direction on the sky. The characteristic scale of
the scintillation pattern along its minor axis is well determined, but the high
anisotropy leads to degenerate solutions for the scintillation velocity. The
decorrelation of the pattern over the baseline gives an estimate of the major
axis length scale of the scintillation pattern. We derive an upper limit on the
distance to the scattering plasma of no more than 10 pc.Comment: 27 pages, 6 figures, accepted for publication in Ap
An implicit method for radiative transfer with the diffusion approximation in SPH
An implicit method for radiative transfer in SPH is described. The diffusion
approximation is used, and the hydrodynamic calculations are performed by a
fully three--dimensional SPH code. Instead of the energy equation of state for
an ideal gas, various energy states and the dissociation of hydrogen molecules
are considered in the energy calculation for a more realistic temperature and
pressure determination. In order to test the implicit code, we have performed
non--isothermal collapse simulations of a centrally condensed cloud, and have
compared our results with those of finite difference calculations performed by
MB93. The results produced by the two completely different numerical methods
agree well with each other.Comment: 25 pages, 9 figure
Space and Ground Based Pulsation Data of Eta Bootis Explained with Stellar Models Including Turbulence
The space telescope MOST is now providing us with extremely accurate low
frequency p-mode oscillation data for the star Eta Boo. We demonstrate in this
paper that these data, when combined with ground based measurements of the high
frequency p-mode spectrum, can be reproduced with stellar models that include
the effects of turbulence in their outer layers. Without turbulence, the l=0
modes of our models deviate from either the ground based or the space data by
about 1.5-4.0 micro Hz. This discrepancy can be completely removed by including
turbulence in the models and we can exactly match 12 out of 13 MOST frequencies
that we identified as l=0 modes in addition to 13 out of 21 ground based
frequencies within their observational 2 sigma tolerances. The better agreement
between model frequencies and observed ones depends for the most part on the
turbulent kinetic energy which was taken from a 3D convection simulation for
the Sun.Comment: 13 pages, 7 figures, ApJ in pres
quantum Heisenberg antiferromagnet on the triangular lattice: a group symmetry analysis of order by disorder
On the triangular lattice, for between and , the classical
Heisenberg model with first and second neighbor interactions presents
four-sublattice ordered ground-states. Spin-wave calculations of Chubukov and
Jolicoeur\cite{cj92} and Korshunov\cite{k93} suggest that quantum fluctuations
select amongst these states a colinear two-sublattice order. From theoretical
requirements, we develop the full symmetry analysis of the low lying levels of
the spin-1/2 Hamiltonian in the hypotheses of either a four or a two-sublattice
order. We show on the exact spectra of periodic samples ( and )
how quantum fluctuations select the colinear order from the four-sublattice
order.Comment: 15 pages, 4 figures (available upon request), Revte
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