182 research outputs found
Charge and Spin Response of the Spin--Polarized Electron Gas
The charge and spin response of a spin--polarized electron gas is
investigated including terms beyond the random phase approximation. We evaluate
the charge response, the longitudinal and transverse spin response, and the
mixed spin--charge response self--consistently in terms of the susceptibility
functions of a non--interacting system. Exchange--correlation effects between
electrons of spin and are included following Kukkonen and
Overhauser, by using spin--polarization dependent generalized Hubbard local
field factors and . The general
condition for charge--density and spin--density--wave excitations of the system
is discussed.Comment: 4 pages, latex, no figure
Traveling through potential energy landscapes of disordered materials: the activation-relaxation technique
A detailed description of the activation-relaxation technique (ART) is
presented. This method defines events in the configurational energy landscape
of disordered materials, such as a-Si, glasses and polymers, in a two-step
process: first, a configuration is activated from a local minimum to a nearby
saddle-point; next, the configuration is relaxed to a new minimum; this allows
for jumps over energy barriers much higher than what can be reached with
standard techniques. Such events can serve as basic steps in equilibrium and
kinetic Monte Carlo schemes.Comment: 7 pages, 2 postscript figure
Dynamic exchange-correlation potentials for the electron gas in dimensionality D=3 and D=2
Recent progress in the formulation of a fully dynamical local approximation
to time-dependent Density Functional Theory appeals to the longitudinal and
transverse components of the exchange and correlation kernel in the linear
current-density response of the homogeneous fluid at long wavelength. Both
components are evaluated for the electron gas in dimensionality D=3 and D=2 by
an approximate decoupling in the equation of motion for the current density,
which accounts for processes of excitation of two electron-hole pairs. Each
pair is treated in the random phase approximation, but the role of exchange and
correlation is also examined; in addition, final-state exchange processes are
included phenomenologically so as to satisfy the exactly known high-frequency
behaviours of the kernel. The transverse and longitudinal spectra involve the
same decay channels and are similar in shape. A two-plasmon threshold in the
spectrum for two-pair excitations in D=3 leads to a sharp minimum in the real
part of the exchange and correlation kernel at twice the plasma frequency. In
D=2 the same mechanism leads to a broad spectral peak and to a broad minimum in
the real part of the kernel, as a consequence of the dispersion law of the
plasmon vanishing at long wavelength. The numerical results have been fitted to
simple analytic functions.Comment: 13 pages, 11 figures included. Accepted for publication in Phys. Rev.
Glass breaks like metals, but at the nanometer scale
We report in situ Atomic Force Microscopy experiments which reveal the
presence of nanoscale damage cavities ahead of a stress-corrosion crack tip in
glass. Their presence might explain the departure from linear elasticity
observed in the vicinity of a crack tip in glass. Such a ductile fracture
mechanism, widely observed in the case of metallic materials at the micrometer
scale, might be also at the origin of the striking similarity of the
morphologies of fracture surfaces of glass and metallic alloys at different
length scales.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Lett, few minor
corrections, Fig. 1b change
Ionization degree of the electron-hole plasma in semiconductor quantum wells
The degree of ionization of a nondegenerate two-dimensional electron-hole
plasma is calculated using the modified law of mass action, which takes into
account all bound and unbound states in a screened Coulomb potential.
Application of the variable phase method to this potential allows us to treat
scattering and bound states on the same footing. Inclusion of the scattering
states leads to a strong deviation from the standard law of mass action. A
qualitative difference between mid- and wide-gap semiconductors is
demonstrated. For wide-gap semiconductors at room temperature, when the bare
exciton binding energy is of the order of T, the equilibrium consists of an
almost equal mixture of correlated electron-hole pairs and uncorrelated free
carriers.Comment: 22 pages, 6 figure
What are the experimentally observable effects of vertex corrections in superconductors?
We calculate the effects of vertex corrections, of non-constant density of
states and of a (self-consistently determined) phonon self-energy for the
Holstein model on a 3D cubic lattice. We replace vertex corrections with a
Coulomb pseudopotential, mu*, adjusted to give the same Tc, and repeat the
calculations, to see which effects are a distinct feature of vertex
corrections. This allows us to determine directly observable effects ofvertex
corrections on a variety of thermodynamic properties of superconductors. To
this end, we employ conserving approximations (in the local approximation) to
calculate the superconducting critical temperatures, isotope coefficients,
superconducting gaps, free-energy differences and thermodynamic critical fields
for a range of parameters. We find that the dressed value of lambda is
significantly larger than the bare value. While vertex corrections can cause
significant changes in all the above quantities (even whenthe bare
electron-phonon coupling is small), the changes can usually be well-modeled by
an appropriate Coulomb pseudopotential. The isotope coefficient proves to be
the quantity that most clearly shows effects of vertex corrections that can not
be mimicked by a mu*.Comment: 28 pages, 12 figure
Simulation of thermal conductivity and heat transport in solids
Using molecular dynamics (MD) with classical interaction potentials we
present calculations of thermal conductivity and heat transport in crystals and
glasses. Inducing shock waves and heat pulses into the systems we study the
spreading of energy and temperature over the configurations. Phonon decay is
investigated by exciting single modes in the structures and monitoring the time
evolution of the amplitude using MD in a microcanonical ensemble. As examples,
crystalline and amorphous modifications of Selenium and are
considered.Comment: Revtex, 8 pages, 11 postscript figures, accepted for publication in
PR
Local field factors in a polarized two-dimensional electron gas
We derive approximate expressions for the static local field factors of a
spin polarized two-dimensional electron gas which smoothly interpolate between
their small- and large-wavevector asymptotic limits. For the unpolarized
electron gas, the proposed analytical expressions reproduce recent diffusion
Monte Carlo data. We find that the degree of spin polarization produces
important modifications to the local factors of the minority spins, while the
local field functions of the majority spins are less affected.Comment: 8 pages, 10 figure
Antioksidativni potencijal i sposobnost hvatanja slobodnih radikala metanolnog ekstrakta plodova Citrullus colocynthis (L.) Schrad.
Citrullus colocynthis (L.) Schrad. (Cucurbitaceae) is a medicinal plant traditionally used as an abortifacient and to treat constipation, oedema, bacterial infections, cancer and diabetes. Preliminary phytochemical screening of the plant showed the presence of large amounts of phenolics and flavonoids. Subsequent quantification showed the presence of 0.74% phenolics (calculated as gallic acid) and 0.13% flavonoids calculated as catechin equivalents per 100 g of fresh mass. The presence of phenolic compounds prompted us to evaluate its antioxidant activity. In the present study, methanolic fruit extract of C. colocynthis was screened to evaluate its free-radical scavenging ability. The highest antioxidant and free radical scavenging effect of the fruit extract was observed at a concentration of 2500 µg mL1.Citrullus colocynthis (L.) Schrad. (Cucurbitaceae) je ljekovita biljka koja se tradicionalno upotrebljava kao abortiv i za liječenje konstipacije, edema, bakterijskih infekcija, karcinoma i dijabetesa. Preliminarno fitokemijsko pretraživanje ukazalo je na prisutnost velikih količina fenola i flavonoida. Udio fenola bio je 0,74% (preračunato na galnu kiselinu), a flavonoida 0,13% preračunato na ekvivalente katehina na 100 g svježe mase. Zbog prisutnosti fenolnih spojeva ispitivano je antioksidativno djelovanje i sposobnost hvatanja slobodnih radikala metanolnog ekstrakta plodova. Koncentracija 2500 µg mL1 imala je najjači učinak
Knock-Down of Cathepsin D Affects the Retinal Pigment Epithelium, Impairs Swim-Bladder Ontogenesis and Causes Premature Death in Zebrafish
The lysosomal aspartic protease Cathepsin D (CD) is ubiquitously expressed in eukaryotic organisms. CD activity is essential to accomplish the acid-dependent extensive or partial proteolysis of protein substrates within endosomal and lysosomal compartments therein delivered via endocytosis, phagocytosis or autophagocytosis. CD may also act at physiological pH on small-size substrates in the cytosol and in the extracellular milieu. Mouse and fruit fly CD knock-out models have highlighted the multi-pathophysiological roles of CD in tissue homeostasis and organ development. Here we report the first phenotypic description of the lack of CD expression during zebrafish (Danio rerio) development obtained by morpholino-mediated knock-down of CD mRNA. Since the un-fertilized eggs were shown to be supplied with maternal CD mRNA, only a morpholino targeting a sequence containing the starting ATG codon was effective. The main phenotypic alterations produced by CD knock-down in zebrafish were: 1. abnormal development of the eye and of retinal pigment epithelium; 2. absence of the swim-bladder; 3. skin hyper-pigmentation; 4. reduced growth and premature death. Rescue experiments confirmed the involvement of CD in the developmental processes leading to these phenotypic alterations. Our findings add to the list of CD functions in organ development and patho-physiology in vertebrates
- …