623 research outputs found
Two-dimensional modelling of a towed transient magnetic dipole-dipole sea floor EM system
The discovery of massive sulphide deposits along mid-ocean ridges has prompted the development of towed sea floor electromagnetic mapping tools. One suitable configuration of transmitter and receiver is the in-line, coaxial, magnetic dipole-dipole. The step response of this system to a double half-space model has two distinct parts. The position in time of the initial event is indicative of the conductivity of the sea floor. A reduction in dimensionality greatly simplifies the analytic and numerical computation of more complicated cases. The transmitter is reduced to a pair of horizontal line sources carrying equal but opposite currents and separated by a small vertical distance. The transient responses of the simplified system and the coaxial system to the double half-space model are remarkably similar, even though the electromagnetic mode characterised by vertical current flow is excluded by the simplification. The analytic form of the sensitivity function enables a simple expression for a depth of investigation beneath the sea floor to be derived as a function of time. The magnetic effects of currents impressed in a two-dimensional conductive target embedded in the sea floor by a horizontal magnetic point dipole transmitter may be represented approximately by a system of vortex currents only. Since vortex current flow is the type of current flow induced in a two-dimensional target by a two-dimensional magnetic source, the principal characteristics of the three-dimensional problem can be studied by two-dimensional modelling. The scattered fields from a thin conductive dike and a thin conductive sill are evaluated by setting up and solving a boundary element integral equation in the electric field. Transient response curves are presented for a limited range of models. The sea floor conductivity is fixed at 1/30 that of seawater, a typical value for recent basalt. The array size and plate depth extent are fixed at 100 m and 25 m, respectively, while the depth of burial is allowed to vary from 4 m to 25 m. The ratio of the inductive response parameter of the plate to the response parameter of the crust, which determines the degree of influence of the plate conductor on the combined step response, is varied from 0.4 to 10. Increasing the relative effect of the target delays the onset and decreases the gradient of the initial part of the response. Pronounced minima in the response as a function of array location are observed when either the transmitter or the receiver cross over the target. The minimum field over a wide range of times is close to zero for a shallow dike due to the combined shielding effect of the dike and the seawater. The shallow dike may be distinguished from a shallow sill by the shape of the minima.
ARK: https://n2t.net/ark:/88439/y079019
Permalink: https://geophysicsjournal.com/article/123
 
Efficient Reverse Transcription Using Locked Nucleic Acid Nucleotides towards the Evolution of Nuclease Resistant RNA Aptamers
Background: Modified nucleotides are increasingly being utilized in the de novo selection of aptamers for enhancing their drug-like character and abolishing the need for time consuming trial-and-error based post-selection modifications. Locked nucleic acid (LNA) is one of the most prominent and successful nucleic acid analogues because of its remarkable properties, and widely explored as building blocks in therapeutic oligonucleotides. Evolution of LNA-modified RNA aptamers requires an efficient reverse transcription method for PCR enrichment of the selected RNA aptamer candidates. Establishing this key step is a pre-requisite for performing LNA-modified RNA aptamer selection
About the Functional Form of the Parisi Overlap Distribution for the Three-Dimensional Edwards-Anderson Ising Spin Glass
Recently, it has been conjectured that the statistics of extremes is of
relevance for a large class of correlated system. For certain probability
densities this predicts the characteristic large fall-off behavior
, . Using a multicanonical Monte Carlo technique,
we have calculated the Parisi overlap distribution for the
three-dimensional Edward-Anderson Ising spin glass at and below the critical
temperature, even where is exponentially small. We find that a
probability distribution related to extreme order statistics gives an excellent
description of over about 80 orders of magnitude.Comment: 4 pages RevTex, 3 figure
Rotation measure variations for 20 millisecond pulsars
We report on variations in the mean position angle of the 20 millisecond
pulsars being observed as part of the Parkes Pulsar Timing Array (PPTA)
project. It is found that the observed variations are dominated by changes in
the Faraday rotation occurring in the Earth's ionosphere. Two ionospheric
models are used to correct for the ionospheric contribution and it is found
that one based on the International Reference Ionosphere gave the best results.
Little or no significant long-term variation in interstellar RM was found with
limits typically about 0.1 rad m yr in absolute value. In a few
cases, apparently significant RM variations over timescales of a few 100 days
or more were seen. These are unlikely to be due to localised magnetised regions
crossing the line of sight since the implied magnetic fields are too high. Most
probably they are statistical fluctuations due to random spatial and temporal
variations in the interstellar electron density and magnetic field along the
line of sight.Comment: Accepted for publication in Astrophysics & Space Scienc
Spin glass behavior of frustrated 2-D Penrose lattice in the classical planar model
Via extensive Monte Carlo studies we show that the frustrated XY Hamiltonian
on a 2-D Penrose lattice admits of a spin glass phase at low temperature.
Studies of the Edwards-Anderson order parameter, spin glass susceptibility, and
local (linear) susceptibility point unequivocally to a paramagnetic to spin
glass transition as the temperature is lowered. Specific heat shows a rounded
peak at a temperature above the spin glass transition temperature, as is
commonly observed in spin glasses. Our results strongly suggest that the
critical point exponents are the same as obtained by Bhatt and Young in the
Ising model on a square lattice. However, unlike in the latter case,
the critical temperature is clearly finite (nonzero). The results imply that a
quasiperiodic 2-D array of superconducting grains in a suitably chosen
transverse magnetic field should behave as a superconducting glass at low
temperature.Comment: RevTex, 4 pages Including 4 figures. To appear in the June 1 1996
issue of Phys. Rev. B (Rapid Communications). Revised/replaced edition
contains an erratum at the end of the paper, also to appear in Phys. Rev.
Properties of heavy quarkonia and B_c mesons in the relativistic quark model
The mass spectra and electromagnetic decay rates of charmonium, bottomonium
and B_c mesons are comprehensively investigated in the relativistic quark
model. The presence of only heavy quarks allows the expansion in powers of
their velocities. All relativistic corrections of order v^2/c^2, including
retardation effects and one-loop radiative corrections, are systematically
taken into account in the computations of the mass spectra. The obtained wave
functions are used for the calculation of radiative magnetic dipole (M1) and
electric dipole (E1) transitions. It is found that relativistic effects play a
substantial role. Their account and the proper choice of the Lorentz structure
of the quark-antiquark interaction in a meson is crucial for bringing
theoretical predictions in accord with experimental data. A detailed comparison
of the calculated decay rates and branching fractions with available
experimental data for radiative decays of charmonium and bottomonium is
presented. The possibilities to observe the currently missing spin-singlet S
and P states as well as D states in bottomonium are discussed. The results for
B_c masses and decays are compared with other quark model predictions.Comment: 31 pages, 2 figures, minor correction
The Magnetic Field of the Solar Corona from Pulsar Observations
We present a novel experiment with the capacity to independently measure both
the electron density and the magnetic field of the solar corona. We achieve
this through measurement of the excess Faraday rotation due to propagation of
the polarised emission from a number of pulsars through the magnetic field of
the solar corona. This method yields independent measures of the integrated
electron density, via dispersion of the pulsed signal and the magnetic field,
via the amount of Faraday rotation. In principle this allows the determination
of the integrated magnetic field through the solar corona along many lines of
sight without any assumptions regarding the electron density distribution. We
present a detection of an increase in the rotation measure of the pulsar
J18012304 of approximately 160 \rad at an elongation of 0.95 from
the centre of the solar disk. This corresponds to a lower limit of the magnetic
field strength along this line of sight of . The lack of
precision in the integrated electron density measurement restricts this result
to a limit, but application of coronal plasma models can further constrain this
to approximately 20mG, along a path passing 2.5 solar radii from the solar
limb. Which is consistent with predictions obtained using extensions to the
Source Surface models published by Wilcox Solar ObservatoryComment: 16 pages, 4 figures (1 colour): Submitted to Solar Physic
Superconductivity in the two dimensional Hubbard Model.
Quasiparticle bands of the two-dimensional Hubbard model are calculated using
the Roth two-pole approximation to the one particle Green's function. Excellent
agreement is obtained with recent Monte Carlo calculations, including an
anomalous volume of the Fermi surface near half-filling, which can possibly be
explained in terms of a breakdown of Fermi liquid theory. The calculated bands
are very flat around the (pi,0) points of the Brillouin zone in agreement with
photoemission measurements of cuprate superconductors. With doping there is a
shift in spectral weight from the upper band to the lower band. The Roth method
is extended to deal with superconductivity within a four-pole approximation
allowing electron-hole mixing. It is shown that triplet p-wave pairing never
occurs. Singlet d_{x^2-y^2}-wave pairing is strongly favoured and optimal
doping occurs when the van Hove singularity, corresponding to the flat band
part, lies at the Fermi level. Nearest neighbour antiferromagnetic correlations
play an important role in flattening the bands near the Fermi level and in
favouring superconductivity. However the mechanism for superconductivity is a
local one, in contrast to spin fluctuation exchange models. For reasonable
values of the hopping parameter the transition temperature T_c is in the range
10-100K. The optimum doping delta_c lies between 0.14 and 0.25, depending on
the ratio U/t. The gap equation has a BCS-like form and (2*Delta_{max})/(kT_c)
~ 4.Comment: REVTeX, 35 pages, including 19 PostScript figures numbered 1a to 11.
Uses epsf.sty (included). Everything in uuencoded gz-compressed .tar file,
(self-unpacking, see header). Submitted to Phys. Rev. B (24-2-95
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