802 research outputs found
Orbital structure and magnetic ordering in stoichiometric and doped crednerite CuMnO2
The exchange interactions and magnetic structure in layered system CuMnO2
(mineral crednerite) and in nonstoichiometric system Cu1.04Mn0.96O2, with
triangular layers distorted due to orbital ordering of the Mn3+ ions, are
studied by ab-initio band-structure calculations, which were performed within
the GGA+U approximation. The exchange interaction parameters for the Heisenberg
model within the Mn-planes and between the Mn-planes were estimated. We explain
the observed in-plane magnetic structure by the dominant mechanism of the
direct d-d exchange between neighboring Mn ions. The superexchange via O ions,
with 90 degree Mn-O-Mn bonds, plays less important role for the in-plane
exchange. The interlayer coupling is largely dominated by one exchange path
between the half-filled 3z^2-r^2 orbitals of Mn3+. The change of interlayer
coupling from antiferromagnetic in pure CuMnO2 to ferromagnetic in doped
material is also explained by our calculations
Electronic structure of VO: charge ordering, metal-insulator transition and magnetism
The low and high-temperature phases of VO have been studied by
\textit{ab initio} calculations. At high temperature, all V atoms are
electronically equivalent and the material is metallic. Charge and orbital
ordering, associated with the distortions in the V pseudo-rutile chains, occur
below the metal-insulator transition. Orbital ordering in the low-temperature
phase, different in V and V chains, allows to explain the
distortion pattern in the insulating phase of VO. The in-chain magnetic
couplings in the low-temperature phase turn out to be antiferromagnetic, but
very different in the various V and V bonds. The V dimers
formed below the transition temperature form spin singlets, but V ions,
despite dimerization, apparently participate in magnetic ordering.Comment: 10 pages, 6 figures, 2 table
Structural transition in AuAgTe4 under pressure
Gold is inert and forms very few compounds. One of the most interesting of
those is calaverite AuTe2, which has incommensurate structure and which becomes
superconducting when doped or under pressure. There exist a "sibling" of AuTe2
the mineral sylvanite AuAgTe4, which properties are almost unknown. In
sylvanite Au and Ag ions are ordered in stripes, and Te6 octahedra around
metals are distorted in such a way that Ag becomes linearly coordinated, what
is typical for Ag^{1+}, whereas Au is square coordinated - it is typical for
d^8 configurations, i.e. one can assign to Au the valence 3+. Our theoretical
study shows that at pressure P_C ~ 5 GPa there should occur in it a structural
transition such that above this critical pressure Te6 octahedra around Au and
Ag become regular and practically identical. Simultaneously Te-Te dimers,
existing at P = 0 GPa, disappear, and material from a bad metal becomes a usual
metal with predominantly Te 5p states at the Fermi energy. We expect that,
similar to AuTe2, AuAgTe4 should become superconducting above P_C.Comment: 8 pages, 4 figure
Laser Diodes with a Wide Flat Modal Gain Spectrum in the (1–3)- μm Range
The gain spectra of multilayer quantum-well heterostructures on the basis of GaInAs/GaInPAs and
GaInAsSb/AlGaAsSb compounds have been analyzed. The possibility of implementing of wide and almost flat
spectrum of modal gain in the near IR range upon inhomogeneous excitation of quantum wells in such compounds
is shown
Щодо розрахунку деформацій зсуву в призматичних стержнях із полімерних матеріалів за умов розтягу з крученням
The process of creep of prismatic rods made of linear-viscoelastic polymeric materials under combined loading is considered. Defining equations that determine the relationship between strains, stresses and time are given in the form of a superposition of shear and bulk strain. The object of study is prismatic bars made of fiberglass ST-1. The area of linearity of the model is substantiated on the basis of the hypothesis of the existence of the creep function, which is built on the yield curves, a single diagram of long-term deformation and the statistical value of the quantile of statistics. The region of linear-elastic deformation is recognized based on the fulfillment of the condition of existence of a single creep function. The defining equations of the model contain a set of functions and coefficients determined from the basic experiments. On the basis of the relations between the kernels of the one-dimensional stress state, the parameters of the kernels under the condition of a complex stress state are determined. The linearity of viscoelastic properties is given by the Boltzmann-Voltaire equations. The fractional-exponential kernels of heredity are chosen as the kernels of heredity. The obtained values of the core parameters are used to calculate the creep deformations of prismatic bars made of ST-1 fiberglass under conditions of simultaneous tensile tension.
Pages of the article in the issue: 115 - 118
Language of the article: UkrainianРозглядається процес повзучості призматичних стержнів з лінійно-в’язкопружних полімерних матеріалів за умов комбінованого навантаження. Визначальні рівняння, що описують залежність між деформаціями, напруженнями і часом, задаються у вигляді суперпозиції зсувної і об’ємної деформації. Об’єктом дослідження являються призматичні стержні зі склопластику СТ-1. Обґрунтовується область лінійності моделі, виходячи з гіпотези існування функції повзучості, побудованої по кривим податливості, єдиної діаграми довготривалого деформування і статистичного значення квантиля статистики. Визначається область лінійно-пружного деформування, виходячи з виконання умови існування єдиної функції повзучості. Визначальні рівняння моделі містять набір функцій і коефіцієнтів, що знаходяться з базових експериментів. На підставі співвідношень між ядрами одновимірного напруженого стану визначаються параметри ядер за умови складного напруженого стану. Лінійність в'язкопружних властивостей задається рівняннями Больцмана-Вольтера з дробово-експоненціальним ядром спадковості. Отримані значення параметрів ядер використовуються для розрахунку деформацій повзучості призматичних стержнів зі склоапластика СТ-1 за умов одночасної дії розтягу з крученням
Crystal field splitting in correlated systems with the negative charge-transfer gap
Special features of the crystal field splitting of levels in the
transition metal compounds with the small or negative charge-transfer gap
are considered. We show that in this case the Coulomb term and
the covalent contribution to the splitting have different signs.
In order to check the theoretical predictions we carried out the ab-initio band
structure calculations for CsAuCl, in which the charge-transfer gap
is negative, so that the electrons predominantly occupy low-lying bonding
states. For these states the -levels lie below ones, which
demonstrates that at least in this case the influence of the covalency on
the total value of the crystal field splitting is stronger than the Coulomb
interaction (which would lead to the opposite level order). We also show that
the states in conduction band are made predominantly of states of ligands
(Cl), with small admixture of states of Au
Energy and Emission Characteristics of Superlattice Quantum-Cascade Structures
Energy levels, wave functions, and matrix elements of optical dipole transitions have been numerically
calculated for superlattice quantum-cascade structures. An expression for the gain has been obtained with
regard to the complete set of energy levels in different models of spectral-line broadening. A universal relation
between the gain and emission spectra for unipolar laser structures has been proposed. The effect of spectral
broadening on the shape of emission spectra is estimated. The electroluminescence spectra are compared with
the calculated spontaneous recombination spectra and good agreement between the results is shown
Optoelectronic properties and characteristics of doping superlattices
Optical and electric properties of doping superlattices, or n-i-p-i crystals, can be varied in a wide range under excitation and through the choice of the thicknesses and doping of the crystal layers. Some basic results concerned the transformation of the electron energy spectrum of doping superlattices are summarized. Parameters and characteristics of doping superlattices related to optoelectronics devices, such as photodetectors, laser diodes, and optical modulators, are presented
Nonlinear optical processes in doped semiconductor superlattices
For doped semiconductor superlattices we have analyzed the effects of absorption saturation and
change in the refractive index depending on the level of their excitation and structure parameters.
The calculations were carried out with account for the tails of the density of states and screening of
the electrostatic potential. It is shown that doped superlattices may display "shading," i.e., the increase
in the absorption coefficient at a fixed frequency with increase in light intensity. For δ-doped
superlattices a stronger nonmonotonicity of the change in the refractive index with increase in the
excitation level in comparison with typical structures can manifest itself
- …