261 research outputs found
A study of local and non-local spatial densities in quantum field theory
We use a one-dimensional model system to compare the predictions of two
different 'yardsticks' to compute the position of a particle from its quantum
field theoretical state. Based on the first yardstick (defined by the
Newton-Wigner position operator), the spatial density can be arbitrarily narrow
and its time-evolution is superluminal for short time intervals. Furthermore,
two spatially distant particles might be able to interact with each other
outside the light cone, which is manifested by an asymmetric spreading of the
spatial density. The second yardstick (defined by the quantum field operator)
does not permit localized states and the time evolution is subluminal.Comment: 29 pages, 3 figure
Structural, electronic, vibrational and dielectric properties of LaBGeO from first principles
Structural, electronic, vibrational and dielectric properties of LaBGeO
with the stillwellite structure are determined based on \textit{ab initio}
density functional theory. The theoretically relaxed structure is found to
agree well with the existing experimental data with a deviation of less than
. Both the density of states and the electronic band structure are
calculated, showing five distinct groups of valence bands. Furthermore, the
Born effective charge, the dielectric permittivity tensors, and the vibrational
frequencies at the center of the Brillouin zone are all obtained. Compared to
existing model calculations, the vibrational frequencies are found in much
better agreement with the published experimental infrared and Raman data, with
absolute and relative rms values of 6.04 cm, and , respectively.
Consequently, numerical values for both the parallel and perpendicular
components of the permittivity tensor are established as 3.55 and 3.71 (10.34
and 12.28), respectively, for the high-(low-)frequency limit
Moving system with speeded-up evolution
In the classical (non-quantum) relativity theory the course of the moving
clock is dilated as compared to the course of the clock at rest (the Einstein
dilation). Any unstable system may be regarded as a clock. The time evolution
(e.g., the decay) of a uniformly moving physical system is considered using the
relativistic quantum theory. The example of a moving system is given whose
evolution turns out to be speeded-up instead of being dilated. A discussion of
this paradoxical result is presented.Comment: 10 pages, LaTe
Time dilation in relativistic two-particle interactions
We study the orbits of two interacting particles described by a fully relativistic classical mechanical Hamiltonian. We use two sets of initial conditions. In the first set (dynamics 1) the system\u27s center of mass is at rest. In the second set (dynamics 2) the center of mass evolves with velocity V. If dynamics 1 is observed from a reference frame moving with velocity-V, the principle of relativity requires that all observables must be identical to those of dynamics 2 seen from the laboratory frame. Our numerical simulations demonstrate that kinematic Lorentz space-time transformations fail to transform particle observables between the two frames. This is explained as a result of the inevitable interaction dependence of the boost generator in the instant form of relativistic dynamics. Despite general inaccuracies of the Lorentz formulas, the orbital periods are correctly predicted by the Einstein\u27s time dilation factor for all interaction strengths
Quasi-chemical study of Be(aq) speciation
Be(aq) hydrolysis can to lead to the formation of multi-beryllium
clusters, but the thermodynamics of this process has not been resolved
theoretically. We study the hydration state of an isolated Be ion using
both the quasi-chemical theory of solutions and ab initio molecular dynamics.
These studies confirm that Be(aq) is tetra-hydrated. The quasi-chemical
approach is then applied to then the deprotonation of Be(H_2O)_4^{2+}} to
give BeOH(H_2O)_3{}^{+}}. The calculated pK of 3.8 is in good agreement
with the experimentally suggested value around 3.5. The calculated energetics
for the formation of BeOHBe are then obtained in fair agreement with
experiments.Comment: 11 pages, 3 figure
Semiempirical Hartree-Fock calculations for KNbO3
In applying the semiempirical intermediate neglect of differential overlap
(INDO) method based on the Hartree-Fock formalism to a cubic perovskite-based
ferroelectric material KNbO3, it was demonstrated that the accuracy of the
method is sufficient for adequately describing the small energy differences
related to the ferroelectric instability. The choice of INDO parameters has
been done for a system containing Nb. Based on the parametrization proposed,
the electronic structure, equilibrium ground state structure of the
orthorhombic and rhombohedral phases, and Gamma-TO phonon frequencies in cubic
and rhombohedral phases of KNbO3 were calculated and found to be in good
agreement with the experimental data and with the first-principles calculations
available.Comment: 7 pages, 2 Postscript figures, uses psfig.tex. To be published in
Phys.Rev.B 54, No.4 (1996
ОРГАНИЗАЦИЯ ПЕРЕВОЗКИ СБОРНЫХ ГРУЗОВ С ИСПОЛЬЗОВАНИЕМ ЛОГИСТИЧЕСКОГО ПОДХОДА
General cargoes – a very convenient way to carry small consignments in a vehicle from several senders to several consignees in one direction, which helps to reduce the cost of transportation. However, freight owners do not know how to use this type of transportation in order to achieve essential frugality.In this article is considered in details all bullet points about property of consolidation breaking bulk of several senders on the consolidation storage used for to be departing point for regular transportation of general cargoes on a given route.Сборные доставки грузов – очень удобный способ перевозки небольших партий грузов в одном транспортном средстве от нескольких отправителей к нескольким получателям в одном направлении, который позволяет сократить расходы на транспортировку. Однако многие грузовладельцы не знают, как правильно использовать этот тип перевозки, чтобы добиться существенной экономии. В данной статье подробно рассмотрены все ключевые моменты по возможности консолидации мелких партий нескольких отправителей на консолидационном складе, служащем отправной точкой для регулярной перевозки сборных грузов по заданному маршруту.
Theory of bound polarons in oxide compounds
We present a multilateral theoretical study of bound polarons in oxide
compounds MgO and \alpha-Al_2O_3 (corundum). A continuum theory at arbitrary
electron-phonon coupling is used for calculation of the energies of thermal
dissociation, photoionization (optically induced release of an electron (hole)
from the ground self-consistent state), as well as optical absorption to the
non-relaxed excited states. Unlike the case of free strong-coupling polarons,
where the ratio \kappa of the photoionization energy to the thermal
dissociation energy was shown to be always equal to 3, here this ratio depends
on the Froehlich coupling constant \alpha and the screened Coulomb interaction
strength \beta. Reasonable variation of these two parameters has demonstrated
that the magnitude of \kappa remains usually in the narrow interval from 1 to
2.5. This is in agreement with atomistic calculations and experimental data for
hole O^- polarons bound to the cation vacancy in MgO. The thermal dissociation
energy for the ground self-consistent state and the energy of the optically
induced charge transfer process (hops of a hole between O^{2-} ions) have been
calculated using the quantum-chemical method INDO. Results obtained within the
two approaches for hole O polarons bound by the cation vacancies (V^-) in
MgO and by the Mg^{2+} impurity (V_{Mg}) in corundum are compared to
experimental data and to each other. We discuss a surprising closeness of the
results obtained on the basis of independent models and their agreement with
experiment.Comment: 13 pages, 2 figures, 2 tables, E-mail addresses:
[email protected], [email protected]
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