893 research outputs found
Hilbert's 16th Problem for Quadratic Systems. New Methods Based on a Transformation to the Lienard Equation
Fractionally-quadratic transformations which reduce any two-dimensional
quadratic system to the special Lienard equation are introduced. Existence
criteria of cycles are obtained
Correlation strength, Lifshitz transition and the emergence of a two- to three-dimensional crossover in FeSe under pressure
We report a detailed theoretical study of the electronic structure, spectral
properties, and lattice parameters of bulk FeSe under pressure using a fully
charge self-consistent implementation of the density functional theory plus
dynamical mean-field theory method (DFT+DMFT). In particular, we perform a
structural optimization and compute the evolution of the lattice parameters
(volume, ratio, and the internal position of Se) and the electronic
structure of the tetragonal (space group ) paramagnetic FeSe. Our
results for the lattice parameters are in good quantitative agreement with
experiment. The ratio is slightly overestimated by about ~\%,
presumably due to the absence of the van der Waals interactions between the
FeSe layers in our calculations. The lattice parameters determined within DFT
are off the experimental values by a remarkable -~\%, implying a
crucial importance of electron correlations. Upon compression to ~GPa, the
ratio and the lattice volume show a decrease by and ~\%,
respectively, while the Se coordinate weakly increases by ~\%.
Most importantly, our results reveal a topological change of the Fermi surface
(Lifshitz transition) which is accompanied by a two- to three-dimensional
crossover. Our results indicate a small reduction of the quasiparticle mass
renormalization by about ~\% for the and less than ~\% for
the states, as compared to ambient pressure. The behavior of the
momentum-resolved magnetic susceptibility shows no topological
changes of magnetic correlations under pressure, but demonstrates a reduction
of the degree of the in-plane stripe-type nesting. Our results for
the electronic structure and lattice parameters of FeSe are in good qualitative
agreement with recent experiments on its isoelectronic counterpart
FeSeS.Comment: 10 pages, 6 figure
A Frequency-Domain Criterion for Global Stability of Systems with Angular Coordinates
 
EVALUATION OF SUBMERGED HEALING OF COLONIC ANASTOMOSES IN SURGICAL HOSPITAL
Thus, the main causes of «hand technique» anastomosis by invagination are technical defects of the intervention in telescopic anastomosis by invagination and the ischemic disorders in colorectal anastomosis by invagination. 48 patients were operated on the colon and rectum with «hand technique» anastomosis. Invalidated «manual technique» anastomosis after 70 days of the formation were fully functional, and processes of healing and morphological transformation of mucosa were completed
Hidden attractors in fundamental problems and engineering models
Recently a concept of self-excited and hidden attractors was suggested: an
attractor is called a self-excited attractor if its basin of attraction
overlaps with neighborhood of an equilibrium, otherwise it is called a hidden
attractor. For example, hidden attractors are attractors in systems with no
equilibria or with only one stable equilibrium (a special case of
multistability and coexistence of attractors). While coexisting self-excited
attractors can be found using the standard computational procedure, there is no
standard way of predicting the existence or coexistence of hidden attractors in
a system. In this plenary survey lecture the concept of self-excited and hidden
attractors is discussed, and various corresponding examples of self-excited and
hidden attractors are considered
Change in stability of solid solution at radiation influence
Stability of solid solution at radiation influence has been investigated. Expressions for diffusion streams of binary alloy components, which specify the existence of temperature interval in which the phenomenon of ascending diffusion of elements is observed, were received. Vacancy characters of diffusion, configuration entropy, and potential energy of atomic bonds were considered at derivation. The ascending diffusion testifies to stability infringement of homogeneous solid solution - stratification. Influence of radiation is connected with increase in concentration of vacancies which changes the energy of atomic bonds and, simultaneously, accelerates diffusion processes. The condition of alloy stability with regard to stratification at radiating influence was obtaine
Calculation of thermal parameters of SiGe microbolometers
The thermal parameters of a SiGe microbolometer were calculated using
numerical modeling. The calculated thermal conduction and thermal response time
are in good agreement with the values found experimentally and range between
2x10 and 7x10 W/K and 1.5 and 4.5 ms, respectively. High sensitivity
of microbolometer is achieved due to optimization of the thermal response time
and thermal conduction by fitting the geometry of supporting heat-removing legs
or by selection of a suitable material providing boundary thermal resistance
higher than 8x10 cmK/W at the SiGe interface.Comment: 11 pages, 6 figure
Polaron physics and crossover transition in magnetite probed by pressure-dependent infrared spectroscopy
The optical properties of magnetite at room temperature were studied by
infrared reflectivity measurements as a function of pressure up to 8 GPa. The
optical conductivity spectrum consists of a Drude term, two sharp phonon modes,
a far-infrared band at around 600 cm, and a pronounced mid-infrared
absorption band. With increasing pressure both absorption bands shift to lower
frequencies and the phonon modes harden in a linear fashion. Based on the shape
of the MIR band, the temperature dependence of the dc transport data, and the
occurrence of the far-infrared band in the optical conductivity spectrum the
polaronic coupling strength in magnetite at room temperature should be
classified as intermediate. For the lower-energy phonon mode an abrupt increase
of the linear pressure coefficient occurs at around 6 GPa, which could be
attributed to minor alterations of the charge distribution among the different
Fe sites.Comment: 7 pages, 7 figure
Orbital-selective coherence-incoherence crossover and metal-insulator transition in Cu-doped NaFeAs
We study the effects of electron-electron interactions and hole doping on the electronic structure of Cu-doped NaFeAs using the density functional theory plus dynamical mean-field theory (DFT+DMFT) method. In particular, we employ an effective multiorbital Hubbard model with a realistic band structure of NaFeAs in which Cu-doping was modeled within a rigid band approximation and compute the evolution of the spectral properties, orbital-selective electronic mass renormalizations, and magnetic properties of NaFeAs on doping with Cu. In addition, we perform fully charge self-consistent DFT+DMFT calculations for the long-range antiferromagnetically ordered Na(Fe,Cu)As with Cu x=0.5 with a real-space ordering of Fe and Cu ions. Our results reveal a crucial importance of strong electron-electron correlations and local potential difference between the Cu and Fe ions for understanding the k-resolved spectra of Na(Fe,Cu)As. On Cu-doping, we observe a strong orbital-selective localization of the Fe 3d states accompanied by a large renormalization of the Fe xy and xz/yz orbitals. Na(Fe,Cu)As exhibits bad-metal behavior associated with a coherence-to-incoherence crossover of the Fe 3d electronic states and local moments formation near a Mott metal-insulator transition (MIT). For heavily doped NaFeAs with Cu x∼0.5 we obtain a Mott insulator with a band gap of ∼0.3 eV which is characterized by divergence of the quasiparticle effective mass of the Fe xy states. In contrast to this, the quasiparticle weights of the Fe xz/yz and e states remain finite at the MIT. The MIT occurs via an orbital-selective Mott phase to appear at Cu x≃0.375 with the Fe xy states being Mott localized. We propose the possible importance of Fe/Cu disorder to explain the magnetic properties of Cu-doped NaFeAs. © 2021 American Physical Society.The DMFT model calculations of NaFeAs were supported by the state assignment of Minobrnauki of Russia (theme “Electron” No. AAAA-A18-118020190098-5). The electronic structure calculations and magnetic properties analysis of were supported by the Russian Science Foundation (Project No. 19-12-00012)
Theory of vortex states in magnetic nanodisks with induced Dzyaloshinskii-Moriya interactions
Vortex states in magnetic nanodisks are essentially affected by
surface/interface induced Dzyaloshinskii-Moriya interactions. Within a
micromagnetic approach we calculate the equilibrium sizes and shape of the
vortices as functions of magnetic field, the material and geometrical
parameters of nanodisks. It was found that the Dzyaloshinskii-Moriya coupling
can considerably increase sizes of vortices with "right" chirality and suppress
vortices with opposite chirality. This allows to form a bistable system of
homochiral vortices as a basic element for storage applications.Comment: 8 pages, 8 figure
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