22 research outputs found
Temperature driven to phase-transformation in Ti, Zr and Hf from first principles theory combined with lattice dynamics
Lattice dynamical methods used to predict phase transformations in crystals
typically deal with harmonic phonon spectra and are therefore not applicable in
important situations where one of the competing crystal structures is unstable
in the harmonic approximation, such as the bcc structure involved in the hcp to
bcc martensitic phase transformation in Ti, Zr and Hf. Here we present an
expression for the free energy that does not suffer from such shortcomings, and
we show by self consistent {\it ab initio} lattice dynamical calculations
(SCAILD), that the critical temperature for the hcp to bcc phase transformation
in Ti, Zr and Hf, can be effectively calculated from the free energy difference
between the two phases. This opens up the possibility to study quantitatively,
from first principles theory, temperature induced phase transitions.Comment: 4 pages, 3 figure
MAELAS 2.0: A new version of a computer program for the calculation of magneto-elastic properties
MAELAS is a computer program for the calculation of magnetocrystalline
anisotropy energy, anisotropic magnetostrictive coefficients and magnetoelastic
constants in an automated way. The method originally implemented in version 1.0
of MAELAS was based on the length optimization of the unit cell, proposed by Wu
and Freeman, to calculate the anisotropic magnetostrictive coefficients. We
present here a revised and updated version (v2.0) of MAELAS, where we added a
new methodology to compute anisotropic magnetoelastic constants from a linear
fitting of the energy versus applied strain. We analyze and compare the
accuracy of both methods showing that the new approach is more reliable and
robust than the one implemented in version 1.0, especially for non-cubic
crystal symmetries. This analysis also help us to find that the accuracy of the
method implemented in version 1.0 could be improved by using deformation
gradients derived from the equilibrium magnetoelastic strain tensor, as well as
potential future alternative methods like the strain optimization method.
Additionally, we clarify the role of the demagnetized state in the fractional
change in length, and derive the expression for saturation magnetostriction for
polycrystals with trigonal, tetragonal and orthorhombic crystal symmetry. In
this new version, we also fix some issues related to trigonal crystal symmetry
found in version 1.0
Automated calculations of exchange magnetostriction
We present a methodology based on deformations of the unit cell that allows
to compute the isotropic magnetoelastic constants, isotropic magnetostrictive
coefficients and spontaneous volume magnetostriction associated to the exchange
magnetostriction. This method is implemented in the python package MAELAS
(v3.0), so that it can be used to obtain these quantities by first-principles
calculations and classical spin-lattice models in an automated way. We show
that the required reference state to obtain the spontaneous volume
magnetostriction combines the equilibrium volume of the paramagnetic state and
magnetic order of the ground state. The presented computational tool may be
helpful to provide a better understanding and characterization of the
relationship between the exchange interaction and magnetoelasticity
The superconducting phase of Calcium under the pressure at 200 GPa: the strong-coupling description
The thermodynamic parameters of the superconducting state in Calcium under
the pressure at 200 GPa have been determined. The numerical analysis by using
the Eliashberg equations in the mixed representation has been conducted. It has
been stated, that the critical temperature () decreases from 36.15 K to
20.79 K dependently on the assumed value of the Coulomb pseudopotential
(). Next, the order parameter near the temperature of zero
Kelvin () has been obtained. It has been proven, that the
dimensionless ratio decreases from 4.25 to 3.90
together with the growth of . Finally, the ratio of the electron
effective mass to the electron bare mass () has been
calculated. It has been shown, that takes the high value in
the whole range of the superconducting phase's existence, and its maximum is
equal to 2.23 for T=T_{C}.Comment: 5 pages, 5 figure
Simple Metals at High Pressure
In this lecture we review high-pressure phase transition sequences exhibited
by simple elements, looking at the examples of the main group I, II, IV, V, and
VI elements. General trends are established by analyzing the changes in
coordination number on compression. Experimentally found phase transitions and
crystal structures are discussed with a brief description of the present
theoretical picture.Comment: 22 pages, 4 figures, lecture notes for the lecture given at the Erice
course on High-Pressure Crystallography in June 2009, Sicily, Ital
Extraordinarily complex crystal structure with mesoscopic patterning in barium at high pressure
First-principles calculations of the spontaneous volume magnetostriction based on the magnetoelastic energy
We present a simple methodology to compute the spontaneous volume
magnetostriction with first-principles calculations on the basis of the
magnetoelastic energy. This method makes use of deformations of the unit cell
only at the ferromagnetic state. Hence, it does not require the difficult
first-principles calculation of the equilibrium volume at the paramagnetic
state. To validate this methodology, we apply it to body-centered cubic Fe and
face-centered cubic Ni single crystals, finding consistent results with
experiment and previous first-principles calculations. The simplicity and
reliability of this approach could be exploited in the high-throughput
screening of spontaneous volume magnetostriction, as well as associated
quantities like isotropic magnetoelastic constants and isotropic
magnetostrictive coefficients
Creation Of Information Technologies For Sign Language Modeling And Learning
In the article a conception is developed and new information technology is proposed for sign language modelling on the base of human spatial model. For the transmission of movements of the real human-informant of sign language on a spatial model technology of motion capture is used. The algorithm of synchronization of vocal information with its visualization on the human face is developed. Efficiency of the developed information technology is shown on realization of Ukrainian sign language. The proposed approach is carried by universal character and can be used for the modelling of other sign languages
Effect of the boundary roughness on the conductance of double quantum wire in a magnetic field
We investigate the effect of the boundary roughness scattering on
the conductance of a disordered tunnel-coupled quantum wire in
the presence of a magnetic field. It is shown that the average
distance between the neighboring discontinuities of the boundary
profile plays an important role in the transport properties of
the system and the manifestation of the
localization-delocalization effect. Present studies point out a
new effect consisting in the decrease of the conductance with the
increase of the disorder correlation length