3,769 research outputs found
Non-Arrhenius ionic conductivities in glasses due to a distribution of activation energies
Previously observed non-Arrhenius behavior in fast ion conducting glasses
[\textit{Phys.\ Rev.\ Lett.}\ \textbf{76}, 70 (1996)] occurs at temperatures
near the glass transition temperature, , and is attributed to changes in
the ion mobility due to ion trapping mechanisms that diminish the conductivity
and result in a decreasing conductivity with increasing temperature. It is
intuitive that disorder in glass will also result in a distribution of the
activation energies (DAE) for ion conduction, which should increase the
conductivity with increasing temperature, yet this has not been identified in
the literature. In this paper, a series of high precision ionic conductivity
measurements are reported for
glasses with compositions ranging from . The impact of the
cation site disorder on the activation energy is identified and explained using
a DAE model. The absence of the non-Arrhenius behavior in other glasses is
explained and it is predicted which glasses are expected to accentuate the DAE
effect on the ionic conductivity.Comment: 2 figure
Nitric oxide and peroxynitrite in health and disease
The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review
Inequalities that test locality in quantum mechanics
Quantum theory violates Bell's inequality, but not to the maximum extent that
is logically possible. We derive inequalities (generalizations of Cirel'son's
inequality) that quantify the upper bound of the violation, both for the
standard formalism and the formalism of generalized observables (POVMs). These
inequalities are quantum analogues of Bell inequalities, and they can be used
to test the quantum version of locality. We discuss the nature of this kind of
locality. We also go into the relation of our results to an argument by Popescu
and Rohrlich (Found. Phys. 24, 379 (1994)) that there is no general connection
between the existence of Cirel'son's bound and locality.Comment: 5 pages, 1 figure; the argument has been made clearer in the revised
version; 1 reference adde
Ideal barriers to polarization reversal and domain-wall motion in strained ferroelectric thin films
The ideal intrinsic barriers to domain switching in c-phase PbTiO_3 (PTO),
PbZrO_3 (PZO), and PbZr_{1-x}Ti_xO_3 (PZT) are investigated via
first-principles computational methods. The effects of epitaxial strain on the
atomic structure, ferroelectric response, barrier to coherent domain reversal,
domain-wall energy, and barrier to domain-wall translation are studied. It is
found that PTO has a larger polarization, but smaller energy barrier to domain
reversal, than PZO. Consequentially the idealized coercive field is over two
times smaller in PTO than PZO. The Ti--O bond length is more sensitive to
strain than the other bonds in the crystals. This results in the polarization
and domain-wall energy in PTO having greater sensitivity to strain than in PZO.
Two ordered phases of PZT are considered, the rock-salt structure and a (100)
PTO/PZO superlattice. In these simple structures we find that the ferroelectric
properties do not obey Vergard's law, but instead can be approximated as an
average over individual 5-atom unit cells.Comment: 9 pages, 13 figure
An Empirical Relation Between The Large-Scale Magnetic Field And The Dynamical Mass In Galaxies
The origin and evolution of cosmic magnetic fields as well as the influence
of the magnetic fields on the evolution of galaxies are unknown. Though not
without challenges, the dynamo theory can explain the large-scale coherent
magnetic fields which govern galaxies, but observational evidence for the
theory is so far very scarce. Putting together the available data of
non-interacting, non-cluster galaxies with known large-scale magnetic fields,
we find a tight correlation between the integrated polarized flux density,
S(PI), and the rotation speed, v(rot), of galaxies. This leads to an almost
linear correlation between the large-scale magnetic field B and v(rot),
assuming that the number of cosmic ray electrons is proportional to the star
formation rate, and a super-linear correlation assuming equipartition between
magnetic fields and cosmic rays. This correlation cannot be attributed to an
active linear alpha-Omega dynamo, as no correlation holds with global shear or
angular speed. It indicates instead a coupling between the large-scale magnetic
field and the dynamical mass of the galaxies, B ~ M^(0.25-0.4). Hence, faster
rotating and/or more massive galaxies have stronger large-scale magnetic
fields. The observed B-v(rot) correlation shows that the anisotropic turbulent
magnetic field dominates B in fast rotating galaxies as the turbulent magnetic
field, coupled with gas, is enhanced and ordered due to the strong gas
compression and/or local shear in these systems. This study supports an
stationary condition for the large-scale magnetic field as long as the
dynamical mass of galaxies is constant.Comment: 23 pages, 4 figures, accepted for publication in the Astrophysical
Journal Letter
The structure, energy, and electronic states of vacancies in Ge nanocrystals
The atomic structure, energy of formation, and electronic states of vacancies
in H-passivated Ge nanocrystals are studied by density functional theory (DFT)
methods. The competition between quantum self-purification and the free surface
relaxations is investigated. The free surfaces of crystals smaller than 2 nm
distort the Jahn-Teller relaxation and enhance the reconstruction bonds. This
increases the energy splitting of the quantum states and reduces the energy of
formation to as low as 1 eV per defect in the smallest nanocrystals. In
crystals larger than 2 nm the observed symmetry of the Jahn-Teller distortion
matches the symmetry expected for bulk Ge crystals. Near the nanocrystal's
surface the vacancy is found to have an energy of formation no larger than 0.5
to 1.4 eV per defect, but a vacancy more than 0.7 nm inside the surface has an
energy of formation that is the same as in bulk Ge. No evidence of the
self-purification effect is observed; the dominant effect is the free surface
relaxations, which allow for the enhanced reconstruction. From the evidence in
this paper, it is predicted that for moderate sized Ge nanocrystals a vacancy
inside the crystal will behave bulk-like and not interact strongly with the
surface, except when it is within 0.7 nm of the surface.Comment: In Press at Phys. Rev.
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