arXiv subjects

D. A. Pavlov

Publications and source records attributed to D. A. Pavlov.

11 recordsLinked to original sources

arXiv2023

The non-monotonicity of growth rate of viscous fingers in heterogeneous porous media

The paper presents a stochastic analysis of the growth rate of viscous fingers in miscible displacement in a heterogeneous porous medium. The statistical parameters characterizing the permeability distribution of a reservoir vary over a wide range. The formation of fingers is provided by the mixing of different-viscosity fluids -- water and polymer solution. The distribution functions of the growth rate of viscous fingers are numerically determined and visualized. Careful data processing reveals the non-monotonic nature of the dependence of the front end of the mixing zone on the correlation length of the permeability (describing the medium graininess) of the reservoir formation. It is demonstrated that an increase in graininess up to a certain value causes an expansion of the distribution shape and a shift of the distribution maximum to the region of higher velocities. In addition, an increase in the standard deviation of permeability leads to a slight change in the shape and characteristics of the density distribution of the growth rates of viscous fingers. The theoretical predictions within the framework of the transverse flow equilibrium approximation and the Koval model are contrasted with the numerically computed velocity distributions.

physics.flu-dyn↗

arXiv2022

Estimates of the change rate of solar mass and gravitational constant based on the dynamics of the Solar System

The estimate of the change rate of the solar gravitational parameter $\mathrm{d}(GM_{\odot})/\mathrm{d}t$ is obtained from processing modern positional observations of planets and spacecraft. Observations were processed and parameters were determined basing on the numerical planetary ephemeris EPM2019. The obtained annual decrease in solar mass $M_{\odot}$ accounts for the loss through radiation ${\dot M}_{{\odot}\mathrm{rad}}$, through the outgoing solar wind ${\dot M}_{{\odot}{\mathrm{wind}}}$, and for the material falling on the Sun ${\dot M}_{{\odot}\mathrm{fall}}$. The estimated relative value is within $-13.4\cdot 10^{-14} < (\dot M_{\odot} /M_{\odot})_{\mathrm{rad}+\mathrm{wind}+ \mathrm{fall}} < -8.7\cdot 10^{-14}$ per year. The following range for the change rate of the gravitational constant $G$ was obtained: $-2.9 \cdot 10^{-14} < \dot G /G < + 4.6 \cdot 10^{-14}$ per year $(3σ$). The new result reduces the interval for the change in $G$ and narrows the limits of possible deviations for alternative gravitational theories from general relativity.

astro-ph.EP↗

arXiv2020

High-temperature intrinsic ferromagnetism in heavily Fe-doped GaAs layers

The layers of a high-temperature novel GaAs:Fe diluted magnetic semiconductor (DMS) with an average Fe content up to 20 at. % were grown on (001) i-GaAs substrates using a pulsed laser deposition in a vacuum. The transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy investigations revealed that the conductive layers obtained at 180 and 200 C are epitaxial, do not contain any second-phase inclusions, but contain the Fe-enriched columnar regions of overlapped microtwins. The TEM investigations of the non-conductive layer obtained at 250 C revealed the embedded coherent Fe-rich clusters of GaAs:Fe DMS. The X-ray photoelectron spectroscopy investigations showed that Fe atoms form chemical bonds with Ga and As atoms with almost equal probability and thus the comparable number of Fe atoms substitute on Ga and As sites. The n-type conductivity of the obtained conductive GaAs:Fe layers is apparently associated with electron transport in a Fe acceptor impurity band within the GaAs band gap. A hysteretic negative magnetoresistance was observed in the conductive layers up to room temperature. Magnetoresistance measurements point to the out-of-plane magnetic anisotropy of the conductive GaAs:Fe layers related to the presence of the columnar regions. The studies of the magnetic circular dichroism confirm that the layers obtained at 180, 200 and 250 C are intrinsic ferromagnetic semiconductors and the Curie point can reach up to at least room temperature in case of the conductive layer obtained at 200 C. It was suggested that in heavily Fe-doped GaAs layers the ferromagnetism is related to the Zener double exchange between Fe atoms with different valence states via an intermediate As and Ga atom.

physics.app-ph↗

arXiv2019

Ripplocation in graphite nanoplatelets during sonication assisted liquid phase exfoliation

Defects induced by liquid-phase exfoliation of graphite using sonication were studied. It was shown that localized impact by cavitation shock waves can produce bulk ripplocations and various types of dislocations in graphite nanoplatelets. Formation of ripples is more pronounced in large aspect (length/width) ratio platelets or nanobelts. Quasi-periodical ripple systems were observed in many nanobelts after sonication. Mechanism of formation of ripples and dislocations during sonication was proposed. Surprisingly, fast high-temperature processing was found to anneal most of defects. This is consistent with our observations that defects associated with ripplocations are strongly localized and thus can be fast annealed.

cond-mat.mes-hall↗

arXiv2019

The formation of epitaxial p-i-n structures on the basis of (In,Fe)Sb and (Ga,Fe)Sb layers

Multilayer structures on the basis of n-type (In,Fe)Sb and p-type (Ga,Fe)Sb diluted magnetic semiconductors (DMS) along with separate (In,Fe)Sb and (Ga,Fe)Sb layers were fabricated on GaAs substrates by pulsed laser sputtering of InSb, GaAs, GaSb, Sb and Fe targets in a vacuum. Transmission electron microscopy and energy-dispersive x-ray spectroscopy reveal a strong dependence of the phase composition of the (In,Fe)Sb compound on the growth temperature. An increase of the latter from 220C to 300C leads to a coalescence of Fe atoms and formation of a secondary crystalline phase in the (In,Fe)Sb layer with a total Fe content of ca. 10 at. %. At the same time, the Ga0.8Fe0.2Sb layers obtained at 220C and 300C are single-phase. The separate In0.8Fe0.2Sb and Ga0.8Fe0.2Sb layers grown on i-GaAs at 220C are DMS with Curie temperatures of ca. 190 K and 170 K, respectively. The three-layer p-i-n diode (In,Fe)Sb/GaAs/(Ga,Fe)Sb structure grown on a GaAs substrate at 220C with a Fe content of ca. 10 at. % in the single-phase (In,Fe)Sb and (Ga,Fe)Sb layers has a rather high crystalline quality and can be considered as a prototype of a bipolar spintronic device based on Fe-doped III-V semiconductors.

physics.app-ph↗

arXiv2019

The nature of transport and ferromagnetic properties of the GaAs structures with the Mn δ-doped layer

We reveal the nature of transport and ferromagnetic properties of the GaAs structure with the single Mn δ-doped layer. To modify the properties of the structure the electrically active radiation defects are created by irradiation with helium ions. The investigations show that the transport properties of the structure are determined by two parallel conduction channels (the channel associated with hole transport in the valence band and the channel associated with electron transport in the Mn impurity band) and that the ferromagnetic properties are determined by the electrons localized at the allowed states within the Mn impurity band. The results also help to understand the features of structures with the Mn δ-layer nearby the quantum well.

physics.app-ph↗

arXiv2017

High-temperature intrinsic ferromagnetism in the (In,Fe)Sb semiconductor

The (In,Fe)Sb layers with the Fe content up to 13 at. % have been grown on (001) GaAs substrates using the pulsed laser deposition. The TEM investigations show that the (In,Fe)Sb layers are epitaxial and free of the inclusions of a second phase. The observation of the hysteretic magnetoresistance curves at temperatures up to 300 K reveals that the Curie point is above room temperature. The resonant character of magnetic circular dichroism confirms the intrinsic ferromagnetism in the (In,Fe)Sb layers. We suggest that the ferromagnetism of the (In,Fe)Sb matrix is not carrier-mediated and apparently is determined by the mechanism of superexchange interaction between Fe atoms (This work was presented at the XXI Symposium Nanophysics and Nanoelectronics, Nizhny Novgorod, March, 13-16, 2017 (book of proceedings v.1, p. 195), http://nanosymp.ru/UserFiles/Symp/2017_v1.pdf).

cond-mat.mtrl-sci↗

arXiv2017

The computational complexity of the initial value problem for the three body problem

The paper is concerned with the computational complexity of the initial value problem (IVP) for a system of ordinary dynamical equations. Formal problem statement is given, containing a Turing machine with an oracle for getting the initial values as real numbers. It is proven that the computational complexity of the IVP for the three body problem is not bounded by a polynomial. The proof is based on the analysis of oscillatory solutions of the Sitnikov problem that have complex dynamical behavior. These solutions contradict the existence of an algorithm that solves the IVP in polynomial time.

cs.CC↗

arXiv2013

Anomalous Hall effect in two-phase semiconductor structures: the crucial role of ferromagnetic inclusions

The Hall effect in InMnAs layers with MnAs inclusions of 20-50 nm in size is studied both theoretically and experimentally. We find that the anomalous Hall effect can be explained by the Lorentz force caused by the magnetic field of ferromagnetic inclusions and by an inhomogeneous distribution of the current density in the layer. The hysteretic dependence of the average magnetization of ferromagnetic inclusions on an external magnetic field results in a hysteretic dependence of RH(Hext). Thus we show the possibility of a hysteretic RH(Hext) dependence (i.e. observation of the anomalous Hall effect) in thin conductive layers with ferromagnetic inclusions in the absence of carriers spin polarization.

cond-mat.mes-hall↗

arXiv2004

High pressure effects in fluorinated HgBa2Ca2Cu3O(8+d)

We have measured the pressure sensitivity of Tc in fluorinated HgBa2Ca2Cu3O(8+d) (Hg-1223) ceramic samples with different F contents, applying pressures up to 30 GPa. We obtained that Tc increases with increasing pressure, reaching different maximum values, depending on the F doping level, and decreases for a further increase of pressure. A new high Tc record (166 K +/- 1 K) was achieved by applying pressure (23 GPa) in a fluorinated Hg-1223 sample near the optimum doping level. Our results show that all our samples are at the optimal doping, and that fluorine incorporation decreases the crystallographic $a$-parameter concomitantly increasing the maximum attainable Tc. This effect reveals that the compression of the $a$ axes is one of the keys that controls the Tc of high temperature superconductors.

cond-mat.supr-con↗

arXiv2000

An algorithm for cleaning extended interferences in radio astronomical records

An algorithm is described for removing extended interferences, for instance from a radar, which are shorter than the time of passage of a radio source across the beam of the radio telescope. The algorithm is developed on the basis of robust procedures with the application of extreme statistics. A procedure of removing ``jumps'' in observational records is also described. The results of performance of the algorithms are illustrated in the figures. The cleaning procedures are included in the standard data reduction system of RATAN-600.

astro-ph↗