136 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
Takagi-Taupin Description of X-ray Dynamical Diffraction from Diffractive Optics with Large Numerical Aperture
We present a formalism of x-ray dynamical diffraction from volume diffractive
optics with large numerical aperture and high aspect ratio, in an analogy to
the Takagi-Taupin equations for strained single crystals. We derive a set of
basic equations for dynamical diffraction from volume diffractive optics, which
enable us to study the focusing property of these optics with various grating
profiles. We study volume diffractive optics that satisfy the Bragg condition
to various degrees, namely flat, tilted and wedged geometries, and derive the
curved geometries required for ultimate focusing. We show that the curved
geometries satisfy the Bragg condition everywhere and phase requirement for
point focusing, and effectively focus hard x-rays to a scale close to the
wavelength.Comment: 18 pages, 12 figure
Valence Quark Distribution in A=3 Nuclei
We calculate the quark distribution function for 3He/3H in a relativistic
quark model of nuclear structure which adequately reproduces the nucleon
approximation, nuclear binding energies, and nuclear sizes for small nuclei.
The results show a clear distortion from the quark distribution function for
individual nucleons (EMC effect) arising dominantly from a combination of
recoil and quark tunneling effects. Antisymmetrization (Pauli) effects are
found to be small due to limited spatial overlaps. We compare our predictions
with a published parameterization of the nuclear valence quark distributions
and find significant agreement.Comment: 18pp., revtex4, 4 fig
Machine learning for classifying and interpreting coherent X-ray speckle patterns
Speckle patterns produced by coherent X-ray have a close relationship with
the internal structure of materials but quantitative inversion of the
relationship to determine structure from speckle patterns is challenging. Here,
we investigate the link between coherent X-ray speckle patterns and sample
structures using a model 2D disk system and explore the ability of machine
learning to learn aspects of the relationship. Specifically, we train a deep
neural network to classify the coherent X-ray speckle patterns according to the
disk number density in the corresponding structure. It is demonstrated that the
classification system is accurate for both non-disperse and disperse size
distributions
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