5,119 research outputs found
Energetics of oxygen-octahedra rotations in perovskite oxides from first principles
We use first-principles methods to study oxygen-octahedra rotations in ABO3
perovskite oxides. We focus on the short-period, perfectly antiphase or
in-phase, tilt patterns that characterize most compounds and control their
physical (e.g., conductive, magnetic) properties. Based on an analytical form
of the relevant potential energy surface, we discuss the conditions for the
stability of polymorphs presenting different tilt patterns, and obtain
numerical results for a collection of thirty-five representative materials. Our
results reveal the mechanisms responsible for the frequent occurrence of a
particular structure that combines antiphase and in-phase rotations, i.e., the
orthorhombic Pbnm phase displayed by about half of all perovskite oxides and by
many non-oxidic perovskites. The Pbnm phase benefits from the simultaneous
occurrence of antiphase and in-phase tilt patterns that compete with each
other, but not as strongly as to be mutually exclusive. We also find that
secondary antipolar modes, involving the A cations, contribute to weaken the
competition between different tilts and play a key role in their coexistence.
Our results thus confirm and better explain previous observations for
particular compounds. Interestingly, we also find that strain effects, which
are known to be a major factor governing phase competition in related (e.g.,
ferroelectric) perovskite oxides, play no essential role as regards the
relative stability of different rotational polymorphs. Further, we discuss why
the Pbnm structure stops being the ground state in two opposite limits, for
large and small A cations, showing that very different effects become relevant
in each case. Our work thus provides a comprehensive discussion on these
all-important and abundant materials, which will be useful to better understand
existing compounds as well as to identify new strategies for materials
engineering
Low compressible noble metal carbides with rock-salt structure: ab initio total energy calculations of the elastic stability
We have systematically studied the mechanical stability of all noble metal
carbides with the rock-salt structure by calculating their elastic constants
within the density function theory scheme. It was found that only four carbides
(RuC, PdC, AgC and PtC) are mechanically stable. In particular, we have shown
that RuC, PdC, and PtC have very high bulk modulus, which has been remarkably
observed by the most recent experiment for the case of PtC. From the calculated
density of states, we can conclude that these compounds are metallic, like the
conventional group IV and group V transition metal carbides.Comment: Appl. Phys. Lett. 89, 071913 (2006
Thermal radiation in non-static curved spacetimes: quantum mechanical path integrals and configuration space topology
A quantum mechanical path integral derivation is given of a thermal
propagator in non-static Gui spacetime. The thermal nature of the propagator is
understood in terms of homotopically non-trivial paths in the configuration
space appropriate to tortoise coordinates. The connection to thermal emission
from collapsing black holes is discussed.Comment: 20 pages, major revised version, 9 figures, new titl
Symmetry of Traveling Wave Solutions to the Allen-Cahn Equation in \Er^2
In this paper, we prove even symmetry of monotone traveling wave solutions to
the balanced Allen-Cahn equation in the entire plane. Related results for the
unbalanced Allen-Cahn equation are also discussed
Dielectric nonlinearity of relaxor ferroelectric ceramics at low ac drives
Dielectric nonlinear response of
(PbMgNbO)(PbTiO) (0.9PMN-0.1PT) relaxor
ceramics was investigated under different ac drive voltages. It was observed
that: (i) the dielectric permittivity is independent on ac field amplitude at
high temperatures; (ii) with increasing ac drive, the permittivity maximum
increases, and the temperature of the maximum shifts to lower temperature;
(iii) the nonlinear effect is weakened when the measurement frequency
increases. The influences of increasing ac drive were found to be similar to
that of decreasing frequency. It is believed that the dielectric nonlinearities
of relaxors at low drives can be explained by the phase transition theory of
ergodic space shrinking in succession. A Monte Carlo simulation was performed
on the flips of micro polarizations at low ac drives to verify the theory.Comment: Submitted to J. Phys.: Cond. Matte
Quantum mechanical path integrals and thermal radiation in static curved spacetimes
The propagator of a spinless particle is calculated from the quantum
mechanical path integral formalism in static curved spacetimes endowed with
event-horizons. A toy model, the Gui spacetime, and the 2D and 4D Schwarzschild
black holes are considered. The role of the topology of the coordinates
configuration space is emphasised in this framework. To cover entirely the
above spacetimes with a single set of coordinates, tortoise coordinates are
extended to complex values. It is shown that the homotopic properties of the
complex tortoise configuration space imply the thermal behaviour of the
propagator in these spacetimes. The propagator is calculated when end points
are located in identical or distinct spacetime regions separated by one or
several event-horizons. Quantum evolution through the event-horizons is shown
to be unitary in the fifth variable.Comment: 22 pages, 10 figure
On the structure of phase transition maps for three or more coexisting phases
This paper is partly based on a lecture delivered by the author at the ERC
workshop "Geometric Partial Differential Equations" held in Pisa in September
2012. What is presented here is an expanded version of that lecture.Comment: 23 pages, 6 figure
Symbiotic Bright Solitary Wave Solutions of Coupled Nonlinear Schrodinger Equations
Conventionally, bright solitary wave solutions can be obtained in
self-focusing nonlinear Schrodinger equations with attractive self-interaction.
However, when self-interaction becomes repulsive, it seems impossible to have
bright solitary wave solution. Here we show that there exists symbiotic bright
solitary wave solution of coupled nonlinear Schrodinger equations with
repulsive self-interaction but strongly attractive interspecies interaction.
For such coupled nonlinear Schrodinger equations in two and three dimensional
domains, we prove the existence of least energy solutions and study the
location and configuration of symbiotic bright solitons. We use Nehari's
manifold to construct least energy solutions and derive their asymptotic
behaviors by some techniques of singular perturbation problems.Comment: to appear in Nonlinearit
Effects of ac-field amplitude on the dielectric susceptibility of relaxors
The thermally activated flips of the local spontaneous polarization in
relaxors were simulated to investigate the effects of the applied-ac-field
amplitude on the dielectric susceptibility. It was observed that the
susceptibility increases with increasing the amplitude at low temperatures. At
high temperatures, the susceptibility experiences a plateau and then drops. The
maximum in the temperature dependence of susceptibility shifts to lower
temperatures when the amplitude increases. A similarity was found between the
effects of the amplitude and frequency on the susceptibility.Comment: 8 pages, 7 figures, Phys. Rev. B (in July 1st
Modularization of multi-qubit controlled phase gate and its NMR implementation
Quantum circuit network is a set of circuits that implements a certain
computation task. Being at the center of the quantum circuit network, the
multi-qubit controlled phase shift is one of the most important quantum gates.
In this paper, we apply the method of modular structuring in classical computer
architecture to quantum computer and give a recursive realization of the
multi-qubit phase gate. This realization of the controlled phase shift gate is
convenient in realizing certain quantum algorithms. We have experimentally
implemented this modularized multi-qubit controlled phase gate in a three qubit
nuclear magnetic resonance quantum system. The network is demonstrated
experimentally using line selective pulses in nuclear magnetic resonance
technique. The procedure has the advantage of being simple and easy to
implement.Comment: to appear in Journal of Optics B: Quantum and Semiclassical Optic
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