421 research outputs found
Equilibrium and Stability of Polarization in Ultrathin Ferroelectric Films with Ionic Surface Compensation
Thermodynamic theory is developed for the ferroelectric phase transition of
an ultrathin film in equilibrium with a chemical environment that supplies
ionic species to compensate its surface. Equations of state and free energy
expressions are developed based on Landau-Ginzburg-Devonshire theory, using
electrochemical equilibria to provide ionic compensation boundary conditions.
Calculations are presented for a monodomain PbTiO (001) film coherently
strained to SrTiO with its exposed surface and its electronically
conducting bottom electrode in equilibrium with a controlled oxygen partial
pressure. The stability and metastability boundaries of phases of different
polarization are determined as a function of temperature, oxygen partial
pressure, and film thickness. Phase diagrams showing polarization and internal
electric field are presented. At temperatures below a thickness-dependent Curie
point, high or low oxygen partial pressure stabilizes positive or negative
polarization, respectively. Results are compared to the standard cases of
electronic compensation controlled by either an applied voltage or charge
across two electrodes. Ionic surface compensation through chemical equilibrium
with an environment introduces new features into the phase diagram. In
ultrathin films, a stable non-polar phase can occur between the positive and
negative polar phases when varying the external chemical potential at fixed
temperature, under conditions where charged surface species are not present in
sufficient concentration to stabilize a polar phase.Comment: 53 pages, 24 figure
Wire scanners in low energy accelerators
Fast wire scanners are today considered as part of standard instrumentation
in high energy synchrotrons. The extension of their use to synchrotrons working
at lower energies, where Coulomb scattering can be important and the transverse
beam size is large, introduces new complications considering beam heating of
the wire, composition of the secondary particle shower and geometrical
consideration in the detection set-up. A major problem in treating these
effects is that the creation of secondaries in a thin carbon wire by a
energetic primary beam is difficult to describe in an analytical way. We are
here presenting new results from a full Monte Carlo simulation of this process
yielding information on heat deposited in the wire, particle type and energy
spectrum of secondaries and angular dependence as a function of primary beam
energy. The results are used to derive limits for the use of wire scanners in
low energy accelerators.Comment: 20 pages, 8 Postscript figures, uses elsart.cl
INO80 and γ-H2AX Interaction Links ATP-Dependent Chromatin Remodeling to DNA Damage Repair
AbstractWhile the role of ATP-dependent chromatin remodeling in transcription is well established, a link between chromatin remodeling and DNA repair has remained elusive. We have found that the evolutionarily conserved INO80 chromatin remodeling complex directly participates in the repair of a double-strand break (DSB) in yeast. The INO80 complex is recruited to a HO endonuclease-induced DSB through a specific interaction with the DNA damage-induced phosphorylated histone H2A (γ-H2AX). This interaction requires Nhp10, an HMG-like subunit of the INO80 complex. The loss of Nhp10 or γ-H2AX results in reduced INO80 recruitment to the DSB. Finally, components of the INO80 complex show synthetic genetic interactions with the RAD52 DNA repair pathway, the main pathway for DSB repair in yeast. Our findings reveal a new role of ATP-dependent chromatin remodeling in nuclear processes and suggest that an ATP-dependent chromatin remodeling complex can read a DNA repair histone code
新収作品 : ジョルジュ・ド・ラ・トゥール《聖トマス》
We present a tomographic technique making use of a gigaelectronvolt electron beam for the determination of the material budget distribution of centimeter-sized objects by means of simulations and measurements. In both cases, the trajectory of electrons traversing a sample under test is reconstructed using a pixel beam-telescope. The width of the deflection angle distribution of electrons undergoing multiple Coulomb scattering at the sample is estimated. Basing the sinogram on position-resolved estimators enables the reconstruction of the original sample using an inverse radon transform. We exemplify the feasibility of this tomographic technique via simulations of two structured cubes—made of aluminium and lead—and via an in-beam measured coaxial adapter. The simulations yield images with FWHM edge resolutions of (177 ± 13) μm and a contrast-to-noise ratio of 5.6 ± 0.2 (7.8 ± 0.3) for aluminium (lead) compared to air. The tomographic reconstruction of a coaxial adapter serves as experimental evidence of the technique and yields a contrast-to-noise ratio of 15.3 ± 1.0 and a FWHM edge resolution of (117 ± 4) μm
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