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Search Results: 1 - 10 of 191598 matches for " D. Semenov "
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Chemical evolution of turbulent protoplanetary disks and the Solar nebula
D. Semenov,D. Wiebe
Physics , 2011, DOI: 10.1088/0067-0049/196/2/25
Abstract: This is the second paper in a series where we study the influence of transport processes on the chemical evolution of protoplanetary disks. Our analysis is based on a flared alpha-model of the DM Tau system, coupled to a large gas-grain chemical network. To account for production of complex molecules, the chemical network is supplied with an extended set of surface reactions and photo-processes in ice mantles. Our disk model covers a wide range of radii, 10-800 AU (from a Jovian planet-forming zone to the outer disk edge). Turbulent transport of gases and ices is implicitly modeled in full 2D along with the time-dependent chemistry. Two regimes are considered, with high and low efficiency of turbulent mixing. The results of the chemical model with suppressed turbulent diffusion are close to those from the laminar model, but not completely. A simple analysis for the laminar chemical model to highlight potential sensitivity of a molecule to transport processes is performed. It is shown that the higher the ratio of the characteristic chemical timescale to the turbulent transport timescale for a given molecule, the higher the probability that its column density will be affected by diffusion. We find that turbulent transport enhances abundances and column densities of many gas-phase species and ices, particularly, complex ones. For such species a chemical steady-state is not reached due to long timescales associated with evaporation and surface photoprocessing and recombination. In contrast, simple radicals and molecular ions, which chemical evolution is fast and proceeds solely in the gas phase, are not much affected by dynamics. All molecules are divided into three groups according to the sensitivity of their column densities to the turbulent diffusion. [Abridged]
Reduction of chemical networks. I. The case of molecular clouds
D. Wiebe,D. Semenov,Th. Henning
Physics , 2002, DOI: 10.1051/0004-6361:20021773
Abstract: We present a new method to analyse and reduce chemical networks and apply this technique to the chemistry in molecular clouds. Using the technique, we investigated the possibility of reducing the number of chemical reactions and species in the UMIST 95 database simultaneously. In addition, we did the same reduction but with the ``objective technique'' in order to compare both methods. We found that it is possible to compute the abundance of carbon monoxide and fractional ionisation accurately with significantly reduced chemical networks in the case of pure gas-phase chemistry. For gas-grain chemistry involving surface reactions reduction is not worthwhile. Compared to the ``objective technique'' our reduction method is more effective but more time-consuming as well.
Inductance, electrically adjusted by semiconductor structure
Semenov А. А.,Usanov D. A.,Kolokin A. A.
Tekhnologiya i Konstruirovanie v Elektronnoi Apparature , 2012,
Abstract: A theoretical model of a passive flat inductor with electronic control is offered. Design charts of tank inductance and Q factor dependence on the forward bias voltage of n—i—p—i—n-structure, used as a specific core, the characteristics of which are regulated under the influence of an applied electric field, are presented. The comparison of design values with experimental features has shown their good correspondence with each other.
Aharonov-Bohm oscillations in disordered nanorings with quantum dots: Effect of electron-electron interactions
Andrew G. Semenov,Andrei D. Zaikin
Physics , 2009, DOI: 10.1016/j.physe.2009.06.063
Abstract: We investigate the effect of electron-electron interactions on Aharonov-Bohm (AB) current oscillations in nanorings formed by a chain of metallic quantum dots. We demonstrate that electron-electron interactions cause electron dephasing thereby suppressing the amplitude of AB oscillations at all temperatures down to T=0. The crossover between thermal and quantum dephasing is found to be controlled by the ring perimeter. Our predictions can be directly tested in future experiments.
Fluctuations of persistent current
Andrew G. Semenov,Andrei D. Zaikin
Physics , 2010, DOI: 10.1088/0953-8984/22/48/485302
Abstract: We theoretically analyze equilibrium fluctuations of persistent current (PC) in nanorings. We demonstrate that these fluctuations persist down to zero temperature provided the current operator does not commute with the total Hamiltonian of the system. For a model of a quantum particle on a ring we explicitly evaluate PC noise power which has the form of sharp peaks at frequencies set by the corresponding interlevel distances. In rings with many conducting channels a much smoother and broader PC noise spectrum is expected. A specific feature of PC noise is that its spectrum can be tuned by an external magnetic flux indicating the presence of quantum coherence in the system.
Persistent currents in nanorings and quantum decoherence by Coulomb interaction
Andrew G. Semenov,Andrei D. Zaikin
Physics , 2009, DOI: 10.1103/PhysRevB.80.155312
Abstract: Employing instanton technique we evaluate equilibrium persistent current (PC) produced by a quantum particle moving in a periodic potential on a ring and interacting with a dissipative environment formed by diffusive electron gas. The model allows for detailed non-perturbative analysis of interaction effects and -- depending on the system parameters -- yields a rich structure of different regimes. We demonstrate that at low temperatures PC is exponentially suppressed at sufficiently large ring perimeters $2\pi R> L_{\varphi}$ where the dephasing length $L_{\varphi}$ is set by interactions and does not depend on temperature. This behavior represents a clear example of quantum decoherence by electron-electron interactions at $T\to 0$.
Decoherence of Cooper pairs and subgap magnetoconductance of superconducting hybrids
Andrew G. Semenov,Andrei D. Zaikin
Physics , 2014, DOI: 10.1088/0031-8949/2015/T165/014041
Abstract: We demonstrate that electron-electron interactions fundamentally restrict the penetration length of superconducting correlations into a diffusive normal metal (N) attached to a superconductor (S). We evaluate the subgap magnetoconductance $G$ of SN hybrids in the presence of electron-electron interactions and demonstrate that the effect of the magnetic field on $G$ is twofold: It includes ($i$) additional temperature independent dephasing of Cooper pairs and ($ii$) Zeeman splitting between the states with opposite spins. The dephasing length of Cooper pairs can be directly extracted from measurements of the subgap magnetoconductance in SN systems at low temperatures.
Chemo-dynamical deuterium fractionation in the early solar nebula: The origin of water on Earth and in asteroids and comets
T. Albertsson,D. Semenov,Th. Henning
Physics , 2014, DOI: 10.1088/0004-637X/784/1/39
Abstract: Formation and evolution of water in the Solar System and the origin of water on Earth constitute one of the most interesting questions in astronomy. The prevailing hypothesis for the origin of water on Earth is by delivery through water-rich small Solar system bodies. In this paper, the isotopic and chemical evolution of water during the early history of the solar nebula, before the onset of planetesimal formation, is studied. A gas-grain chemical model that includes multiply-deuterated species and nuclear spin-states is combined with a steady-state solar nebula model. To calculate initial abundances, we simulated 1 Myr of evolution of a cold and dark TMC1-like prestellar core. Two time-dependent chemical models of the solar nebula are calculated over 1 Myr: (1) a laminar model and (2) a model with 2D turbulent mixing. We find that the radial outward increase of the H2O D/H ratio is shallower in the chemo-dynamical nebular model compared to the laminar model. This is related to more efficient de-fractionation of HDO via rapid gas-phase processes, as the 2D mixing model allows the water ice to be transported either inward and thermally evaporated or upward and photodesorbed. The laminar model shows the Earth water D/H ratio at r ~<2.5 AU, while for the 2D chemo-dynamical model this zone is larger, r ~<9 AU. Similarly, the water D/H ratios representative of the Oort-family comets, ~2.5-10 x 10-4, are achieved within ~2-6 AU and ~2-20 AU in the laminar and the 2D model, respectively. We find that with regards to the water isotopic composition and the origin of the comets, the mixing model seems to be favored over the laminar model.
Modelling of Polarization Properties of Cometary Dust Grains
D. A. Semenov,N. V. Voshchinnikov
Physics , 2000,
Abstract: Observational data for dusty comets are summarized and systemized. The dependence of the linear polarization on various parameters (the phase angle, wavelength, etc.) is analyzed. An ensemble of aligned spheroidal particles having various sizes and chemical compositions is used to fit the linear and circular polarization observed for the comet Halley.
The Temperature of Nonspherical Circumstellar Dust Grains
N. V. Voshchinnikov,D. A. Semenov
Physics , 2000, DOI: 10.1134/1.1316114
Abstract: The temperatures of prolate and oblate spheroidal dust grains in the envelopes of stars of various spectral types are calculated. Homogeneous particles with aspect ratios {\small $a/b \le 10$} composed of amorphous carbon, iron, dirty ice, various silicates, and other materials are considered. The temperatures of spherical and spheroidal particles were found to vary similarly with particle size, distance to the star, and stellar temperature. The temperature ratio {\small $T_{\rm d}({\rm spheroid})/T_{\rm d}({\rm sphere}) $} depends most strongly on the grain chemical composition and shape. Spheroidal grains are generally colder than spherical particles of the same volume; only iron spheroids can be slightly hotter than iron spheres. At {\small $a/b \approx 2$}, the temperature differences do not exceed 10 %. If {\small $a/b \ga 4$}, the temperatures can differ by 30-40 %. For a fixed dust mass in the medium, the fluxes at wavelengths {\small $\lambda \ga 100 \mkm$} are higher if the grains are nonspherical, which gives over estimated dust masses from millimeter observations. The effect of grain shape should also be taken into account when modeling Galactic-dust emission properties, which are calculated when searching for fluctuations of the cosmic microwave background radiation in its Wien wing.
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