3,275 research outputs found
Robust ab initio calculation of condensed matter: transparent convergence through semicardinal multiresolution analysis
We present the first wavelet-based all-electron density-functional
calculations to include gradient corrections and the first in a solid. Direct
comparison shows this approach to be unique in providing systematic
``transparent'' convergence, convergence with a priori prediction of errors, to
beyond chemical (millihartree) accuracy. The method is ideal for exploration of
materials under novel conditions where there is little experience with how
traditional methods perform and for the development and use of chemically
accurate density functionals, which demand reliable access to such precision.Comment: 4 pages, 3 figures, 4 tables. Submitted to Phys. Rev. Lett. (updated
to include GGA
Sulfur-Switch Ugi Reaction for Macrocyclic Disulfide-Bridged Peptidomimetics
A general strategy is introduced for the efficient synthetic access of disulfide linked artificial macrocycles via a Ugi four-component reaction (U4CR) followed by oxidative cyclization. The double-mercapto input is proposed for use in the Ugi reaction, thereby yielding all six topologically possible combinations. The protocol is convergent and short and enables the production of novel disulfide peptidomimetics in a highly general fashion
The Bekenstein Bound in Asymptotically Free Field Theory
For spatially bounded free fields, the Bekenstein bound states that the
specific entropy satisfies the inequality , where
stands for the radius of the smallest sphere that circumscribes the system. The
validity of the Bekenstein bound on the specific entropy in the asymptotically
free side of the Euclidean self-interacting scalar
field theory is investigated. We consider the system in thermal equilibrium
with a reservoir at temperature and defined in a compact spatial
region without boundaries. Using the effective potential, we presented an
exhaustive study of the thermodynamic of the model. For low and high
temperatures the system presents a condensate. We obtain also the renormalized
mean energy and entropy for the system. With these quantities, we shown
in which situations the specific entropy satisfies the quantum bound
Scalar Quantum Field Theory in Disordered Media
A free massive scalar field in inhomogeneous random media is investigated.
The coefficients of the Klein-Gordon equation are taken to be random functions
of the spatial coordinates. The case of an annealed-like disordered medium,
modeled by centered stationary and Gaussian processes, is analyzed. After
performing the averages over the random functions, we obtain the two-point
causal Green's function of the model up to one-loop. The disordered scalar
quantum field theory becomes qualitatively similar to a
self-interacting theory with a frequency-dependent coupling
Oxygen-deficient perovskite-related (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ as oxygen electrode materials for SOFC/SOEC
Perovskite-related Ln2NiO4+δ (Ln = La, Pr, Nd) nickelates with layered Ruddlesden-Popper combine redox
stability with noticeable oxygen stoichiometry changes, yielding enhanced mixed transport and
electrocatalytic properties. These unique features are promising for applications as oxygen electrodes with
good electrochemical performance in reversible SOFC/SOEC (solid oxide fuel/electrolysis cell) systems.
To date, most efforts were focused on oxygen-hyperstoichiometric Ln2NiO4+δ-based phases, whereas
nickelates with oxygen-deficient lattice remain poorly explored. Recent studies demonstrated that the
highest electrical conductivity in (Ln2-xSrx)2NiO4±δ series at elevated temperatures is observed for the
compositions containing ~ 60 at.% of strontium in A sublattice [1,2]. The present work was focused on the
characterization of (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ (M = Ni, Co, Fe) nickelates for the possible use as materials
for reversible oxygen electrodes.
The ceramic materials were prepared by Pechini method with repeated annealings at 650-1200°C and
sintered at 1250-1300°C for 5 h under oxygen atmosphere. Variable-temperature XRD studies confirmed
that all studied compositions retain tetragonal K2NiF4-type structure in the temperature range 25-900°C.
The results of thermogravimetric analysis showed that the prepared nickelates has oxygen-deficient lattice
under oxidizing conditions at temperatures above 700°C. Partial substitution of nickel by cobalt or iron
results in a decrease of p-type electronic conductivity and the concentration of oxygen vacancies in the
lattice (Fig.1), but also suppresses dimensional changes associated with microcracking effects (due to
anisotropic thermal expansion of tetragonal lattice). Electrochemical performance of porous
(Nd0.4Sr0.6)2Ni0.8M0.2O4-δ electrodes in contact with Ce0.9Gd0.1O2-δ solid electrolyte was evaluated at 600-
800°C employing electrochemical impedance spectroscopy and steady-state polarization (anodic and
cathodic) measurements.publishe
Large Scale Electronic Structure Calculations with Multigrid Acceleration
We have developed a set of techniques for performing large scale ab initio
calculations using multigrid accelerations and a real-space grid as a basis.
The multigrid methods permit efficient calculations on ill-conditioned systems
with long length scales or high energy cutoffs. The technique has been applied
to systems containing up to 100 atoms, including a highly elongated diamond
cell, an isolated C molecule, and a 32-atom cell of GaN with the Ga
d-states in valence. The method is well suited for implementation on both
vector and massively parallel architectures.Comment: 4 pages, 1 postscript figur
Oxygen-deficient Nd0.8Sr1.2Ni0.8M0.2O4-δ (M = Ni, Co, Fe) nickelates as oxygen electrode materials for SOFC/SOEC
Ruddlesden-Popper Nd0.8Sr1.2Ni0.8M0.2O4±δ (M = Ni, Co, Fe)
nickelates have been characterized as prospective oxygen
electrode materials for solid electrolyte cells. XRD studies
showed that these oxides retain tetragonal K2NiF4-type structure
in air until at least 900°C. Average thermal expansion
coefficients of Nd0.8Sr1.2Ni0.8M0.2O4±δ calculated from the
structural data are in the range 14.5-15.8 ppm/K. TGA studies
revealed that these nickelates are oxygen-deficient in air at
temperature above 700°C but tends to oxygen stoichiometry or
minor excess on cooling. Incorporation of cobalt or iron into
nickel sublattice of Nd0.8Sr1.2NiO4-δ reduces oxygen deficiency
and electrical conductivity. Electrochemical impedance
spectroscopy studies of symmetrical cells showed that porous
Nd0.8Sr1.2Ni0.8M0.2O4-δ electrodes applied onto Ce0.9Gd0.1O2-δ
electrolyte exhibit quite similar performance, with lowest values
of polarization resistance (0.8 Ohm×cm2 at 800°C) observed for
M = Ni. The polarization resistance can be further decreased
(down to 0.04 Ohm×cm2 at 800°C for M = Ni) by surface
modification with PrOx.publishe
Fracture in Mode I using a Conserved Phase-Field Model
We present a continuum phase-field model of crack propagation. It includes a
phase-field that is proportional to the mass density and a displacement field
that is governed by linear elastic theory. Generic macroscopic crack growth
laws emerge naturally from this model. In contrast to classical continuum
fracture mechanics simulations, our model avoids numerical front tracking. The
added phase-field smoothes the sharp interface, enabling us to use equations of
motion for the material (grounded in basic physical principles) rather than for
the interface (which often are deduced from complicated theories or empirical
observations). The interface dynamics thus emerges naturally. In this paper, we
look at stationary solutions of the model, mode I fracture, and also discuss
numerical issues. We find that the Griffith's threshold underestimates the
critical value at which our system fractures due to long wavelength modes
excited by the fracture process.Comment: 10 pages, 5 figures (eps). Added 2 figures and some text. Removed one
section (and a figure). To be published in PR
Acceleration Schemes for Ab-Initio Molecular Dynamics and Electronic Structure Calculations
We study the convergence and the stability of fictitious dynamical methods
for electrons. First, we show that a particular damped second-order dynamics
has a much faster rate of convergence to the ground-state than first-order
steepest descent algorithms while retaining their numerical cost per time step.
Our damped dynamics has efficiency comparable to that of conjugate gradient
methods in typical electronic minimization problems. Then, we analyse the
factors that limit the size of the integration time step in approaches based on
plane-wave expansions. The maximum allowed time step is dictated by the highest
frequency components of the fictitious electronic dynamics. These can result
either from the large wavevector components of the kinetic energy or from the
small wavevector components of the Coulomb potential giving rise to the so
called {\it charge sloshing} problem. We show how to eliminate large wavevector
instabilities by adopting a preconditioning scheme that is implemented here for
the first-time in the context of Car-Parrinello ab-initio molecular dynamics
simulations of the ionic motion. We also show how to solve the charge-sloshing
problem when this is present. We substantiate our theoretical analysis with
numerical tests on a number of different silicon and carbon systems having both
insulating and metallic character.Comment: RevTex, 9 figures available upon request, to appear in Phys. Rev.
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