2,133 research outputs found
Summer phytoplankton composition and nitrogen limitation of the deep, naturally-acidic (pH∼2.2) Lake Caviahue, Patagonia, Argentina
AbstractDuring the warm seasons of 1998–2004, the naturally-acidic (pH∼2.2) Lake Caviahue was sampled for conductivity, temperature, oxygen, light, nutrients, and phytoplankton (density, biomass and chlorophyll a) with a view to studying the summer phytoplankton population changes with relation to environmental factors, as well as the significance of nitrogen limitation on the phytoplankton yield. Lake Caviahue is characterized by its low transparency, CO2, and N concentration; significant P values; a distinctive vertical distribution of phytoplankton biomass with high values along the water column; and sometimes maximum meta-hypolimnion values. Biodiversity is very low as a result of extreme environmental conditions, Chlorophyceae being the prevailing algae group. Two types of bioassays were carried out to assess nitrogen limitation. For the first bioassay, a solution of ammonium–nitrogen chloride and/or wastewater (rich in ammonium and phosphorus) was used, while one of the lake's sediments was the source of nutrients for the second bioassay. Contrary to the case of acidic mining lakes, N-ammonium proved to be a significant supportive capacity limiting factor as to phytoplankton yield. The present paper provides for the first time information on phytoplankton nitrogen limitation in a naturally-acidic lake
Wigner negativity in the steady-state output of a Kerr parametric oscillator
The output field from a continuously driven linear parametric oscillator may exhibit considerably more squeezing than the intracavity field. Inspired by this fact, we explore the nonclassical features of the steady-state output field of a driven nonlinear Kerr parametric oscillator using a temporal wave packet mode description. Utilizing a new numerical method, we have access to the density matrix of arbitrary wave packet modes. Remarkably, we find that even though the steady-state cavity field is always characterized by a positive Wigner function, the output may exhibit Wigner negativity, depending on the properties of the selected mode
Numerical study of Wigner negativity in one-dimensional steady-state resonance fluorescence
In a numerical study, we investigate the steady-state generation of nonclassical states of light from a coherently driven two-level atom in a one-dimensional waveguide. Specifically, we look for states with a negative Wigner function, since such nonclassical states are a resource for quantum information processing applications, including quantum computing. We find that a waveguide terminated by a mirror at the position of the atom can provide Wigner-negative states, while an infinite waveguide yields strictly positive Wigner functions. Moreover, our paper reveals a connection between the purity of a quantum state and its Wigner negativity. We also analyze the effects of decoherence on the negativity of a state
Microsurgical Techniques in Cerebral Revascularization
The surgical management of patients with cerebrovascular disease is reviewed. Our approach to the management of extracranial cerebral vasculature is discussed first, and increasingly more complex areas are then presented. Our discussion reviews the applications of carotid endarterectomy, extracranial-intracranial bypass procedures, and vertebral extracranial reconstruction
Steady-State Generation of Wigner-Negative States in One-Dimensional Resonance Fluorescence
In this work we demonstrate numerically that the nonlinearity provided by a continuously driven two-level system allows for the generation of Wigner-negative states of the electromagnetic field confined in one spatial dimension. Wigner-negative states, also known as Wigner nonclassical states, are desirable for quantum information protocols beyond the scope of classical computers. Focusing on the steady-state emission from the two-level system, we find the largest negativity at the drive strength where the coherent reflection vanishes
Heterologous Expression of the Phytochelatin Synthase CaPCS2 from Chlamydomonas acidophila and Its Effect on Different Stress Factors in Escherichia coli
Phytochelatins (PCs) are cysteine-rich small peptides, enzymatically synthesized from reduced glutathione (GSH) by cytosolic enzyme phytochelatin synthase (PCS). The open reading frame (ORF) of the phytochelatin synthase CaPCS2 gene from the microalgae Chlamydomonas acidophila was heterologously expressed in Escherichia coli strain DH5 alpha, to analyze its role in protection against various abiotic agents that cause cellular stress. The transformed E. coli strain showed increased tolerance to exposure to different heavy metals (HMs) and arsenic (As), as well as to acidic pH and exposure to UVB, salt, or perchlorate. In addition to metal detoxification activity, new functions have also been reported for PCS and PCs. According to the results obtained in this work, the heterologous expression of CaPCS2 in E. coli provides protection against oxidative stress produced by metals and exposure to different ROS-inducing agents. However, the function of this PCS is not related to HM bioaccumulation
Detailed black hole state counting in loop quantum gravity
We give a complete and detailed description of the computation of black hole
entropy in loop quantum gravity by employing the most recently introduced
number-theoretic and combinatorial methods. The use of these techniques allows
us to perform a detailed analysis of the precise structure of the entropy
spectrum for small black holes, showing some relevant features that were not
discernible in previous computations. The ability to manipulate and understand
the spectrum up to the level of detail that we describe in the paper is a
crucial step towards obtaining the behavior of entropy in the asymptotic (large
horizon area) regime
Tunable Graphene Electronics with Local Ultrahigh Pressure
We achieve fine tuning of graphene effective doping by applying ultrahigh
pressures (> 10 GPa) using Atomic Force Microscopy (AFM) diamond tips. Specific
areas in graphene flakes are irreversibly flattened against a SiO2 substrate.
Our work represents the first demonstration of local creation of very stable
effective p-doped graphene regions with nanometer precision, as unambiguously
verified by a battery of techniques. Importantly, the doping strength depends
monotonically on the applied pressure, allowing a controlled tuning of graphene
electronics. Through this doping effect, ultrahigh pressure modifications
include the possibility of selectively modifying graphene areas to improve
their electrical contact with metal electrodes, as shown by Conductive AFM.
Density Functional Theory calculations and experimental data suggest that this
pressure level induces the onset of covalent bonding between graphene and the
underlying SiO2 substrate. Our work opens a convenient avenue to tuning the
electronics of 2D materials and van der Waals heterostructures through pressure
with nanometer resolution
Thermodynamic analysis of black hole solutions in gravitating nonlinear electrodynamics
We perform a general study of the thermodynamic properties of static
electrically charged black hole solutions of nonlinear electrodynamics
minimally coupled to gravitation in three space dimensions. The Lagrangian
densities governing the dynamics of these models in flat space are defined as
arbitrary functions of the gauge field invariants, constrained by some
requirements for physical admissibility. The exhaustive classification of these
theories in flat space, in terms of the behaviour of the Lagrangian densities
in vacuum and on the boundary of their domain of definition, defines twelve
families of admissible models. When these models are coupled to gravity, the
flat space classification leads to a complete characterization of the
associated sets of gravitating electrostatic spherically symmetric solutions by
their central and asymptotic behaviours. We focus on nine of these families,
which support asymptotically Schwarzschild-like black hole configurations, for
which the thermodynamic analysis is possible and pertinent. In this way, the
thermodynamic laws are extended to the sets of black hole solutions of these
families, for which the generic behaviours of the relevant state variables are
classified and thoroughly analyzed in terms of the aforementioned boundary
properties of the Lagrangians. Moreover, we find universal scaling laws (which
hold and are the same for all the black hole solutions of models belonging to
any of the nine families) running the thermodynamic variables with the electric
charge and the horizon radius. These scale transformations form a one-parameter
multiplicative group, leading to universal "renormalization group"-like
first-order differential equations. The beams of characteristics of these
equations generate the full set of black hole states associated to any of these
gravitating nonlinear electrodynamics...Comment: 51 single column pages, 19 postscript figures, 2 tables, GRG tex
style; minor corrections added; final version appearing in General Relativity
and Gravitatio
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