197 research outputs found
Statistical Derivation of Basic Equations of Diffusional Kinetics in Alloys with Application to the Description of Diffusion of Carbon in Austenite
Basic equations of diffusional kinetics in alloys are statistically derived
using the master equation approach. To describe diffusional transformations in
substitution alloys, we derive the "quasi-equilibrium" kinetic equation which
generalizes its earlier versions by taking into account possible "interaction
renormalization" effects. For the interstitial alloys Me-X, we derive the
explicit expression for the diffusivity D of an interstitial atom X which
notably differs from those used in previous phenomenological treatments. This
microscopic expression for D is applied to describe the diffusion of carbon in
austenite basing on some simple models of carbon-carbon interaction. The
results obtained enable us to make certain conclusions about the real form of
these interactions, and about the scale of the "transition state entropy" for
diffusion of carbon in austenite.Comment: 26 pages, 5 postscript figures, LaTe
Characteristic features of anharmonic effects in the lattice dynamics of fcc metals
The dispersion in the entire Brillouin zone and the temperature dependence
(right up to the melting temperature) of the anharmonic frequency shift and
phonon damping in a number of fcc metals is investigated on the basis of
microscopic calculations. It is found that the anharmonic effects depend
sharply on the wave vector in the directions -X, X-W, and -L
and, in contrast to bcc metals, the magnitude of the effects is not due to the
softness of the initial phonon spectrum. It is shown that the relative
frequency shifts and the phonon damping near melting do not exceed 10-20%. The
relative role of various anharmonic processes is examined, and the relation
between the results obtained and existing experimental data is discussed.Comment: 4 pages, 5 figures, LaTe
Studies of concentration and temperature dependencies of precipitation kinetics in iron-copper alloys using kinetic monte carlo and stochastic statistical simulations
The earlier-developed ab initio model and the kinetic Monte Carlo method
(KMCM) are used to simulate precipitation in a number of iron-copper alloys
with different copper concentrations x and temperatures T. The same simulations
are also made using the improved version of the earlier-suggested stochastic
statistical method (SSM). The results obtained enable us to make a number of
general conclusions about the dependencies of the decomposition kinetics in
Fe-Cu alloys on x and T. We also show that the SSM describes the precipitation
kinetics in a fair agreement with the KMCM, and employing the SSM in
conjunction with the KMCM enables us to extend the KMC simulations to the
longer evolution times. The results of simulations seem to agree with available
experimental data for Fe-Cu alloys within statistical errors of simulations and
the scatter of experimental results. Comparison of results of simulations to
experiments for some multicomponent Fe-Cu-based alloys enables us to make
certain conclusions about the influence of alloying elements in these alloys on
the precipitation kinetics at different stages of evolution.Comment: 18 pages, 17 postscript figures, LaTe
Transition from a one-dimensional to a quasi-one-dimensional state in interacting quantum wires
Upon increasing the electron density in a quantum wire, the one-dimensional
electron system undergoes a transition to a quasi-one-dimensional state. In the
absence of interactions between electrons, this corresponds to filling up the
second subband of transverse quantization, and there are two gapless excitation
modes above the transition. On the other hand, strongly interacting
one-dimensional electrons form a Wigner crystal, and the transition corresponds
to it splitting into two chains (zigzag crystal). The two chains are locked, so
their relative motion is gapped, and only one gapless mode remains. We study
the evolution of the system as the interaction strength changes, and show that
only one gapless mode exists near the transition at any interaction strength.Comment: 4 pages, 2 figure
Color superconductivity in the static Einstein Universe
We study the behavior of quark and diquark condensates in dense quark matter
under the influence of a gravitational field adopting as a simple model the
static dimensional Einstein Universe. Calculations are performed in the
framework of the extended Nambu--Jona-Lasinio model at finite temperature and
quark density on the basis of the thermodynamic potential and the gap
equations. Quark and diquark condensates as functions of the chemical potential
and temperature at different values of the curvature have been studied. Phase
portraits of the system have been constructed
Peculiarities of phonon spectra and lattice heat capacity in Ir and Rh
A simple pseudopotential model is proposed, which allows the phonon spectra
and temperature dependence of the lattice heat capacity of Ir and Rh be
described with a high enough accuracy. A careful comparison of the calculated
and experimental values of the lattice heat capacity is carried out, with the
procedure of the identification of the phonon contribution to the heat capacity
and determination of the characteristics (momenta) of the phonon density of
states from the experimental values of the total heat capacity of metal at a
constant pressure being described in detail. The results of the theoretical
calculations explain, in particular, such peculiar feature of Ir and Rh,
unusual for cubic metals, as a sharp (more than by a factor of 1.5) decrease in
the effective Debye temperature with increasing termperature. The temperature
dependence of the mean square amplitude of atomic displacements in Ir and Rh
has been calculated. Basing on the band calculations the manifestation of the
Kohn singularities in the phonon spectra of Ir are discussed.Comment: 15 pages, LaTeX2e, 12 figures in postscrip
Anharmonicity of BaTiO_3 single crystals
By analyzing the dielectric non-linearity with the Landau thermodynamic
expansion, we find a simple and direct way to assess the importance of the
eighth order term. Following this approach, it is demonstrated that the eighth
order term is essential for the adequate description of the para/ferroelectric
phase transition of BaTiO_3. The temperature dependence of the quartic
coefficient \beta is accordingly reconsidered and is strongly evidenced by the
change of its sign above 165 C. All these findings attest to the strong
polarization anharmonicity of this material, which is unexpected for classical
displacive ferroelectrics.Comment: 4 figures, to be published in Phys. Rev.
Exact Curie temperature for the Ising model on Archimedean and Laves lattices
Using the Feynman-Vdovichenko combinatorial approach to the two dimensional
Ising model, we determine the exact Curie temperature for all two dimensional
Archimedean lattices. By means of duality, we extend our results to cover all
two dimensional Laves lattices. For those lattices where the exact critical
temperatures are not exactly known yet, we compare them with Monte Carlo
simulations.Comment: 10 pages, 1 figures, 3 table
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