4,618 research outputs found
Spectral analysis and a closest tree method for genetic sequences
We describe a new method for estimating the evolutionary tree linking a collection of species from their aligned four-state genetic sequences. This method, which can be adapted to provide a branch-and-bound algorithm, is statistically consistent provided the sequences have evolved according to a standard stochastic model of nucleotide mutation. Our approach exploits a recent group-theoretic description of this model
A new dromaeosaurid (Dinosauria: Theropoda) from the Upper Cretaceous Wulansuhai Formation of Inner Mongolia, China
We describe a new dromaeosaurid theropod from the Upper Cretaceous Wulansuhai Formation of Bayan Mandahu, Inner
Mongolia. The new taxon, Linheraptor exquisitus gen. et sp. nov., is based on an exceptionally well-preserved, nearly
complete skeleton. This specimen represents the fifth dromaeosaurid taxon recovered from the Upper Cretaceous
Djadokhta Formation and its laterally equivalent strata, which include the Wulansuhai Formation, and adds to the known
diversity of Late Cretaceous dromaeosaurids. Linheraptor exquisitus closely resembles the recently reported Tsaagan
mangas. Uniquely among dromaeosaurids, the two taxa share a large, anteriorly located maxillary fenestra and a contact
between the jugal and the squamosal that excludes the postorbital from the infratemporal fenestra. These features suggest
a sister-taxon relationship between L. exquisitus and T. mangas, which indicates the presence of a unique dromaeosaurid
lineage in the Late Cretaceous of Asia. A number of cranial and dental features seen in L. exquisitus and T. mangas, and
particularly some postcranial features of L. exquisitus, suggest that these two taxa are probably intermediate in
systematic position between known basal and derived dromaeosaurids. The discovery of Linheraptor exquisitus is thus
important for understanding the evolution of some salient features seen in the derived dromaeosaurids
Flux Jumping and a Bulk-to-Granular Transition in the Magnetization of a Compacted and Sintered MgB2 Superconductor
The recent discovery of intermediate-temperature superconductivity (ITC) in
MgB2 by Akimitsu et al. and its almost simultaneous explanation in terms of a
hole-carrier-based pairing mechanism by Hirsch, has triggered an avalanche of
studies of its structural, magnetic and transport properties. As a further
contribution to the field we report the results of field (H) and temperature
(T) dependent magnetization (M) measurements of a pellet of uniform,
large-grain sintered MgB2. We show that at low temperatures the size of the
pellet and its critical current density, Jc(H) - i.e. its M(H) - ensure low
field flux jumping, which of course ceases when M(H) drops below a critical
value. With further increase of H and T the individual grains decouple and the
M(H) loops drop to lower lying branches, unresolved in the usual full M(H)
representation. After taking into account the sample size and grain size,
respectively, the bulk sample and the grains were deduced to exhibit the same
magnetically determined Jc s (e.g. 105 A/cm2, 20 K, 0T) and hence that for each
temperature of measurement Jc(H) decreased monotonically with H over the entire
field range, except for a gap within the grain-decoupling zone.Comment: 7 pages, 6 figures, Changes: Fig 6 Vertical scale an order of
magnitude out (changed figure and associated text). Also corrected typo in
last sectio
Critical Susceptibility Exponent Measured from Fe/W(110) Bilayers
The critical phase transition in ferromagnetic ultrathin Fe/W(110) films has
been studied using the magnetic ac susceptibility. A statistically objective,
unconstrained fitting of the susceptibility is used to extract values for the
critical exponent (gamma), the critical temperature Tc, the critical amplitude
(chi_o) and the range of temperature that exhibits power-law behaviour. A
fitting algorithm was used to simultaneously minimize the statistical variance
of a power law fit to individual experimental measurements of chi(T). This
avoids systematic errors and generates objective fitting results. An ensemble
of 25 measurements on many different films are analyzed. Those which permit an
extended fitting range in reduced temperature lower than approximately .00475
give an average value gamma=1.76+-0.01. Bilayer films give a weighted average
value of gamma = 1.75+-0.02. These results are in agreement with the
-dimensional Ising exponent gamma= 7/4. Measurements that do not exhibit
power-law scaling as close to Tc (especially films of thickness 1.75ML) show a
value of gamma higher than the Ising value. Several possibilities are
considered to account for this behaviour.Comment: -Submitted to Phys. Rev. B -Revtex4 Format -6 postscript figure
Stability of Transonic Shock Solutions for One-Dimensional Euler-Poisson Equations
In this paper, both structural and dynamical stabilities of steady transonic
shock solutions for one-dimensional Euler-Poission system are investigated.
First, a steady transonic shock solution with supersonic backgroumd charge is
shown to be structurally stable with respect to small perturbations of the
background charge, provided that the electric field is positive at the shock
location. Second, any steady transonic shock solution with the supersonic
background charge is proved to be dynamically and exponentially stable with
respect to small perturbation of the initial data, provided the electric field
is not too negative at the shock location. The proof of the first stability
result relies on a monotonicity argument for the shock position and the
downstream density, and a stability analysis for subsonic and supersonic
solutions. The dynamical stability of the steady transonic shock for the
Euler-Poisson equations can be transformed to the global well-posedness of a
free boundary problem for a quasilinear second order equation with nonlinear
boundary conditions. The analysis for the associated linearized problem plays
an essential role
Entangled light in transition through the generation threshold
We investigate continuous variable entangling resources on the base of
two-mode squeezing for all operational regimes of a nondegenerate optical
parametric oscillator with allowance for quantum noise of arbitrary level. The
results for the quadrature variances of a pair of generated modes are obtained
by using the exact steady-state solution of Fokker-Planck equation for the
complex P-quasiprobability distribution function. We find a simple expression
for the squeezed variances in the near-threshold range and conclude that the
maximal two-mode squeezing reaches 50% relative to the level of vacuum
fluctuations and is achieved at the pump field intensity close to the
generation threshold. The distinction between the degree of two-mode squeezing
for monostable and bistable operational regimes is cleared up.Comment: 7 pages, 4 figures; Content changed: more details added to the
discussion. To be published in Phys. Rev.
Recovery of 150-250 MeV/nuc Cosmic Ray Helium Nuclei Intensities Between 2004-2010 Near the Earth, at Voyager 2 and Voyager 1 in the Heliosheath - A Two Zone Helioshpere
The recovery of cosmic ray He nuclei of energy ~150-250 MeV/nuc in solar
cycle #23 from 2004 to 2010 has been followed at the Earth using IMP and ACE
data and at V2 between 74-92 AU and also at V1 beyond the heliospheric
termination shock (91-113 AU). The correlation coefficient between the
intensities at the Earth and at V1 during this time period is remarkable
(0.921), after allowing for a ~0.9 year delay due to the solar wind propagation
time from the Earth to the outer heliosphere. To describe the intensity changes
and to predict the absolute intensities measured at all three locations we have
used a simple spherically symmetric (no drift) two-zone heliospheric transport
model with specific values for the diffusion coefficient in both the inner and
outer zones. The diffusion coefficient in the outer zone, assumed to be the
heliosheath from about 90 to 120 (130) AU, is determined to be ~5 times smaller
than that in the inner zone out to 90 AU. This means the Heliosheath acts much
like a diffusing barrier in this model. The absolute magnitude of the
intensities and the intensity changes at V1 and the Earth are described to
within a few percent by a diffusion coefficient that varies with time by a
factor ~4 in the inner zone and only a factor of ~1.5 in the outer zone over
the time period from 2004-2010. For V2 the observed intensities follow a curve
that is as much as 25% higher than the calculated intensities at the V2 radius
and at times the observed V2 intensities are equal to those at V1. At least
one-half of the difference between the calculated and observed intensities
between V1 and V2 can be explained if the heliosphere is squashed by ~10% in
distance (non-spherical) so that the HTS location is closer to the Sun in the
direction of V2 compared to V1.Comment: 13 Pages, 8 Figure
The electromagnetic gauge invariance in models of electroweak unification reexamined
Two models with gauge symmetry are
considered. We show that the masslessness of the photon does not prevent the
neutrinos from acquiring Majorana masses. That is, there is no relation between
the VEVs of Higgs fields and the electromagnetic gauge invariance contrary to
what has been claimed recently.Comment: Revtex, 9 pages, no figure
A Zero-attracting Quaternion-valued Least Mean Square Algorithm for Sparse System Identification
Recently, quaternion-valued signal processing has received more and more attention. In this paper, the quaternion-valued sparse system identification problem is studied for the first time and a zero-attracting quaternion-valued least mean square (LMS) algorithm is derived by considering the l1 norm of the quaternion-valued adaptive weight vector. By incorporating the sparsity information of the system into the update process, a faster convergence speed is achieved, as verified by simulation results
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