4,278 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
Two-speed phase dynamics in Si(111) (7x7)-(1x1) phase transition
We propose a natural two-speed model for the phase dynamics of Si(111)
77 phase transition to high temperature unreconstructed phase. We
formulate the phase dynamics by using phase-field method and adaptive mesh
refinement. Our simulated results show that a 77 island decays with its
shape kept unchanged, and its area decay rate is shown to be a constant
increasing with its initial area. LEEM experiments concerned are explained,
which confirms that the dimer chains and corner holes are broken first in the
transition, and then the stacking fault is remedied slowly. This phase-field
method is a reliable approach to phase dynamics of surface phase transitions.Comment: 4 pages with figures include
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
Thermal conduction of carbon nanotubes using molecular dynamics
The heat flux autocorrelation functions of carbon nanotubes (CNTs) with
different radius and lengths is calculated using equilibrium molecular
dynamics. The thermal conductance of CNTs is also calculated using the
Green-Kubo relation from the linear response theory. By pointing out the
ambiguity in the cross section definition of single wall CNTs, we use the
thermal conductance instead of conductivity in calculations and discussions. We
find that the thermal conductance of CNTs diverges with the CNT length. After
the analysis of vibrational density of states, it can be concluded that more
low frequency vibration modes exist in longer CNTs, and they effectively
contribute to the divergence of thermal conductance.Comment: 15 pages, 6 figures, submitted to Physical Review
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
Half-metallic ferromagnetism and structural stability of zincblende phases of the transition-metal chalcogenides
An accurate density-functional method is used to study systematically
half-metallic ferromagnetism and stability of zincblende phases of
3d-transition-metal chalcogenides. The zincblende CrTe, CrSe, and VTe phases
are found to be excellent half-metallic ferromagnets with large half-metallic
gaps (up to 0.88 eV). They are mechanically stable and approximately 0.31-0.53
eV per formula unit higher in total energy than the corresponding
nickel-arsenide ground-state phases, and therefore would be grown epitaxially
in the form of films and layers thick enough for spintronic applications.Comment: 4 pages with 4 figures include
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
Dynamics of Hadronic Molecule in One-Boson Exchange Approach and Possible Heavy Flavor Molecules
We extend the one pion exchange model at quark level to include the short
distance contributions coming from , , and
exchange. This formalism is applied to discuss the possible molecular states of
, , , ,
the pseudoscalar-vector systems with and respectively. The
" function" term contribution and the S-D mixing effects have been
taken into account. We find the conclusions reached after including the heavier
mesons exchange are qualitatively the same as those in the one pion exchange
model. The previous suggestion that
molecule should exist, is confirmed in the one boson exchange model, whereas
bound state should not exist. The system
can accomodate a molecule close to the threshold, the mixing between
the molecule and the conventional charmonium has to be considered to identify
this state with X(3872). For the system, the pseudoscalar-vector
systems with and , near threshold molecular states may exist.
These bound states should be rather narrow, isospin is violated and the I=0
component is dominant. Experimental search channels for these states are
suggested.Comment: 21 pages, 11 figures, 8 table
A scheme for dense coding in the non-symmetric quantum channel
We investigate the dense coding in the case of non-symmetric Hilbert spaces
of the sender and receiver's particles sharing the quantum maximally entangled
state. The efficiency of classical information gain is also considered. We
conclude that when a more level particle is with the sender, she can get a
non-symmetric quantum channel from a symmetric one by entanglement transfer.
Thus the efficiency of information transmission is improved
Potential super-hard Osmium di-nitride with fluorite structure: First-principles calculations
We have performed systematic first-principles calculations on di-carbide,
-nitride, -oxide and -boride of platinum and osmium with the fluorite
structure. It is found that only PtN, OsN and OsO are
mechanically stable. In particular OsN has the highest bulk modulus of
360.7 GPa. Both the band structure and density of states show that the new
phase of OsN is metallic. The high bulk modulus is owing to the strong
covalent bonding between Os 5\textit{d} and N 2\textit{p} states and the dense
packed fluorite structure.Comment: Phys. Rev. B 74,125118 (2006
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