856 research outputs found
A Computer Simulation Model of Waterhyacinth and Weevil Interactions
A personal computer simulation model termed INSECT has been developed to evaluate biological control of waterhyacinth (Eichhornia crassipes (Mart.) Solms.) by two species of weevil (Neochetina eichhorniae Warner, and N. bruchi Hustache). The model results were compared with the data from three different locations. For each data set, the simulated plant biomass, adult and larva populations were plotted aqainst the 95% confidence intervals of the actual field observations. In many cases, the simulation results were within the 95% confidence intervals, and especially during the growing season, they indicated trends similar to those seen in the field data. However, there were discrepancies in both the magnitude and the trend for early and the late periods of the year. These initial results suggest that development of a model to simulate the impact of a biocontrol agent on waterhyacinth populations is a feasible approach to better understand the interactions within this control system
March Madness: NCAA Tournament Participation and College Alcohol Use
While athletic success may improve the visibility of a university to prospective students and thereby benefit the school, it may also increase risky behavior in the current student body. Using the Harvard School of Public Health College Alcohol Study, we find that a school\u27s participation in the NCAA Basketball Tournament is associated with a 47% increase in binge drinking by male students at that school. Additionally, we find evidence that drunk driving increases by 5% among all students during the tournament. (JEL I12, I23, Z28
Binary continuous random networks
Many properties of disordered materials can be understood by looking at
idealized structural models, in which the strain is as small as is possible in
the absence of long-range order. For covalent amorphous semiconductors and
glasses, such an idealized structural model, the continuous-random network, was
introduced 70 years ago by Zachariasen. In this model, each atom is placed in a
crystal-like local environment, with perfect coordination and chemical
ordering, yet longer-range order is nonexistent. Defects, such as missing or
added bonds, or chemical mismatches, however, are not accounted for. In this
paper we explore under which conditions the idealized CRN model without defects
captures the properties of the material, and under which conditions defects are
an inherent part of the idealized model. We find that the density of defects in
tetrahedral networks does not vary smoothly with variations in the interaction
strengths, but jumps from close-to-zero to a finite density. Consequently, in
certain materials, defects do not play a role except for being thermodynamical
excitations, whereas in others they are a fundamental ingredient of the ideal
structure.Comment: Article in honor of Mike Thorpe's 60th birthday (to appear in J.
Phys: Cond Matt.
Signaling, Polyubiquitination, Trafficking, and Inclusions: Sequestosome 1/p62's Role in Neurodegenerative Disease
Aggregated misfolded proteins are hallmarks of most neurodegenerative diseases. In a chronic disease state, including pathologic situations of oxidative stress, these proteins are sequestered into inclusions. Accumulation of aggregated proteins can be prevented by chaperones, or by targeting their degradation to the UPS. If the accumulation of these proteins exceeds their degradation, they may impair the function of the proteasome. Alternatively, the function of the proteasome may be preserved by directing aggregated proteins to the autophagy-lysosome pathway for degradation. Sequestosome 1/p62 has recently been shown to interact with polyubiquitinated proteins through its UBA domain and may direct proteins to either the UPS or autophagosome. P62 is present in neuronal inclusions of individuals with Alzheimer's disease and other neurodegenerative diseases. Herein, we review p62's role in signaling, aggregation, and inclusion formation, and specifically as a possible contributor to Alzheimer's disease. The use of p62 as a potential target for the development of therapeutics and as a disease biomarker is also discussed
Constant effective mass across the phase diagram of high-T cuprates
We investigate the hole dynamics in two prototypical high temperature
superconducting systems: LaSrCuO and YBaCuO using a combination of DC transport and infrared spectroscopy. By
exploring the effective spectral weight obtained with optics in conjunction
with DC Hall results we find that the transition to the Mott insulating state
in these systems is of the "vanishing carrier number" type since we observe no
substantial enhancement of the mass as one proceeds to undoped phases. Further,
the effective mass remains constant across the entire underdoped regime of the
phase diagram. We discuss the implications of these results for the
understanding of both transport phenomena and pairing mechanism in high-T
systems.Comment: 5 pages, 2 figure
Optical investigations on : Electronic structure evolutions related to the metal-insulator transition
Optical conductivity spectra of cubic pyrochlore
(0.0{\it x}2.0) compounds are investigated. As a metal-insulator
transition (MIT) occurs around {\it x}0.8, large spectral changes are
observed. With increase of {\it x}, the correlation-induced peak between the
lower and the upper Hubbard bands seems to be suppressed, and a strong
mid-infrared feature is observed. In addition, the charge transfer peak
shifts to the lower energies. The spectral changes cannot be explained by
electronic structural evolutions in the simple bandwidth-controlled MIT
picture, but are consistent with those in the filling-controlled MIT picture.
In addition, they are also similar to the spectral changes of
YCaRuO compounds, which is a typical
filling-controlled system. This work suggests that, near the MIT, the Ru bands
could be doped with the easily polarizable Bi cations.Comment: 5 figure
The a-axis optical conductivity of detwinned ortho-II YBa_2Cu_3O_6.50
The a-axis optical properties of a detwinned single crystal of
YBa_2Cu_3O_6.50 in the ortho II phase (Ortho II Y123, T_c= 59 K) were
determined from reflectance data over a wide frequency range (70 - 42 000
cm^-1) for nine temperature values between 28 and 295 K. Above 200 K the
spectra are dominated by a broad background of scattering that extends to 1 eV.
Below 200 K a shoulder in the reflectance appears and signals the onset of
scattering at 400 cm^-1. In this temperature range we also observe a peak in
the optical conductivity at 177 cm^-1. Below 59 K, the superconducting
transition temperature, the spectra change dramatically with the appearance of
the superconducting condensate. Its spectral weight is consistent, to within
experimental error, with the Ferrell-Glover-Tinkham (FGT) sum rule. We also
compare our data with magnetic neutron scattering on samples from the same
source that show a strong resonance at 31 meV. We find that the scattering
rates can be modeled as the combined effect of the neutron resonance and a
bosonic background in the presence of a density of states with a pseudogap. The
model shows that the decreasing amplitude of the neutron resonance with
temperature is compensated for by an increasing of the bosonic background
yielding a net temperature independent scattering rate at high frequencies.
This is in agreement with the experiments.Comment: 13 pages 16 figure
Properties of a continuous-random-network model for amorphous systems
We use a Monte Carlo bond-switching method to study systematically the
thermodynamic properties of a "continuous random network" model, the canonical
model for such amorphous systems as a-Si and a-SiO. Simulations show
first-order "melting" into an amorphous state, and clear evidence for a glass
transition in the supercooled liquid. The random-network model is also extended
to study heterogeneous structures, such as the interface between amorphous and
crystalline Si.Comment: Revtex file with 4 figure
Magneto-optical investigation of the field-induced spin-glass insulator to ferromagnetic metallic transition of the bilayer manganite (LaPr)SrMnO
We measured the magneto-optical response of
(LaPr)SrMnO in order to investigate the
microscopic aspects of the magnetic field driven spin-glass insulator to
ferromagnetic metal transition. Application of a magnetic field recovers the
ferromagnetic state with an overall redshift of the electronic structure,
growth of the bound carrier localization associated with ferromagnetic domains,
development of a pseudogap, and softening of the Mn-O stretching and bending
modes that indicate a structural change. We discuss field- and
temperature-induced trends within the framework of the Tomioka-Tokura global
electronic phase diagram picture and suggest that controlled disorder near a
phase boundary can be used to tune the magnetodielectric response. Remnants of
the spin-glass insulator to ferromagnetic metallic transition can also drive
300 K color changes in (LaPr)SrMnO.Comment: 9 pages, 8 figure
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