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Search Results: 1 - 10 of 382449 matches for " S. Yu. Ovchinnikov "
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Temperature induced in-gap states in the band structure and the insulator-metal transition in LaCoO3
S. Ovchinnikov,Yu. Orlov,I. Nekrasov,Z. Pchelkina
Physics , 2010,
Abstract: For many years a spin-state transition at $T \approx 100 K$ and insulator - metal transition (IMT) at $T_{IMT} \approx 600 K$ in LaCoO$_3 $ remains a mystery. Small low-spin - high-spin spin gap $\Delta _S = E\left( {HS} \right) - E\left( {LS} \right) \sim 100 K$ results in the spin-state transition. The large charge gap $2E_a \approx 2300 K$ ($E_a $ is the activation energy) vs. $\Delta _S $ and $T_{IMT} $ implies that LaCoO$_3 $ is not a simple narrow-gap semiconductor. Here we explain both the spin-state and IMT on the same footing. We obtain strong temperature dependent band structure in LaCoO$_3 $ by the LDA+GTB method that incorporates strong electron correlations, covalence and spin-orbital interaction exactly inside the CoO$_6 $ cluster and the interclaster hopping between different multielectron configurations by perturbation theory for Hubbard X-operators.
Polar Jahn-Teller Centers and Isotope Effect in Copper Oxide high-T_c-Superconductors
A. S. Moskvin,A. S. Ovchinnikov,Yu. D. Panov,M. A. Sidorov
Physics , 1997, DOI: 10.1016/S0921-4534(97)01028-9
Abstract: The $CuO_4$-cluster based copper oxides are considered as generalized quantum lattice bose-gas or a system of the local singlet bosons moving in a lattice of the hole Jahn-Teller centers $[CuO_{4}^{5-}]_{JT}$. The model is illustrated by the qualitative and quantitative description of the various peculiarities of an isotope shift (IS-) effect in a series of 123-like oxides.
Universal metric spaces according to W. Holsztynski
S. Ovchinnikov
Mathematics , 2000,
Abstract: We show, following W. Holsztynski, that there exists a continuous metric d on the set of real numbers R such that any finite metric space is isometrically embeddable into (R,d).
Resonance phenomena in Macroscopic Quantum Tunneling: the small viscosity limit
Yu. N. Ovchinnikov,S. Rombetto,B. Ruggiero,V. Corato,P. Silvestrini
Physics , 2006,
Abstract: We present a new theoretical approach to describe the quantum behavior of a macroscopic system interacting with an external irradiation field, close to the resonant condition. Here we consider the extremely underdamped regime for a system described by a double well potential. The theory includes both: transitions from one well to the other and relaxation processes. We simulate resonant phenomena in a rf-SQUID, whose parameters lie in the range typically used in the experiments. The dependence of the transition probability W on the external drive of the system $\phi_x$ shows three resonance peaks. One peak is connected with the resonant tunneling and the two others with the resonant pumping. The relative position of the two peaks correlated to the resonant pumping depends on the pumping frequency $\nu$ and on the system parameters. The preliminary measurements on our devices show a low dissipation level and assure that they are good candidates in order to realize new experiments on the resonant phenomena in the presence of an external microwave irradiation of proper frequency.
On Spectra of Linearized Operators for Keller-Segel Models of Chemotaxis
S. I. Dejak,P. M. Lushnikov,Yu. N. Ovchinnikov,I. M. Sigal
Physics , 2011, DOI: 10.1016/j.physd.2012.04.003
Abstract: We consider the phenomenon of collapse in the critical Keller-Segel equation (KS) which models chemotactic aggregation of micro-organisms underlying many social activities, e.g. fruiting body development and biofilm formation. Also KS describes the collapse of a gas of self-gravitating Brownian particles. We find the fluctuation spectrum around the collapsing family of steady states for these equations, which is instrumental in derivation of the critical collapse law. To this end we develop a rigorous version of the method of matched asymptotics for the spectral analysis of a class of second order differential operators containing the linearized Keller-Segel operators (and as we argue linearized operators appearing in nonlinear evolution problems). We explain how the results we obtain are used to derive the critical collapse law, as well as for proving its stability.
Effect of Magnetic Impurity Correlations on Josephson Tunneling
A. Bill,S. A. Wolf,Yu. N. Ovchinnikov,V. Z. Kresin
Physics , 1998, DOI: 10.1016/S0921-4534(97)01813-3
Abstract: The ordering trend of magnetic impurities at low temperature results in the frustration of the pair-breaking effect and induces a ``recovery'' of superconducting properties. We show that this effect manifests itself in the deviation of the Josephson current amplitude from the values obtained within the Ambegaokar-Baratoff and the Abrikosov-Gor'kov models. We consider both weak and strong-coupling cases. The theory is applied to describe the experimental data obtained for the low-$T_c$ superconductor SmRh$_4$B$_4$. We further predict a ``recovery'' effect of the Josephson current in high-temperature superconductors.
Quantum Treatment of Continuum Electrons in the Fields of Moving Charges
Teck-Ghee Lee,S. Yu. Ovchinnikov,J. Sternberg,V. Chupryna,D. R. Schultz,J. H. Macek
Physics , 2007, DOI: 10.1103/PhysRevA.76.050701
Abstract: An ab initio, three-dimensional quantum mechanical calculation has been performed for the time-evolution of continuum electrons in the fields of moving charges. Here the essential singularity associated with the diverging phase factor in the continuum wave function is identified and removed analytically. As a result, the continuum components of the regularized wave function are slowly varying with time. Therefore, one can propagate continuum electrons to asymptotically large times and obtain numerically stable, well-converged ejected electron momentum spectra with very low numerical noise. As a consequence, our approach resolves outstanding controversies concerning structures in electron momentum distributions. The main conclusions are general and are illustrated here for ionization of atomic hydrogen by proton impact. Our results show that in order to obtain correct long-time free-particle propagation, the essential singularity identified here should be removed from the continuum components of solutions to the time-dependent Schrodinger equation.
Magnetic mechanisms of pairing in strongly correlated electron system of copper oxides
S. G. Ovchinnikov,I. A. Makarov,E. I. Shneyder,Yu. N. Togushova,V. A. Gavrichkov,M. M. Korshunov
Physics , 2015,
Abstract: The multielectron LDA+GTB approach has been developed to calculate electronic structure of strongly correlated cuprates. At low energies the effective Hamiltonian of the $t - t' - t" - {t_ \bot } - {J^ * } - {J_ \bot }$-model has been derived with parameters coming from the ab initio calculation for LSCO. The electronic structure of LSCO has been calculated self-consistently with the short-range antiferromagnetic order for various doping level. Two Lifshitz-type quantum phase transitions with Fermi surface topology changes have been found at dopings $x_{c1}=0.15$ and $x_{c2}=0.24$. Its effect on normal and superconducting properties has been calculated. The interatomic exchange parameter and its pressure dependence has been calculated within LDA+GTB scheme. The magnetic mechanisms of d-wave pairing induced by static and dynamical spin correlations are discussed. Simultaneous treatment of magnetic and phonon pairing results in the conclusion that both contributions are of the same order. For two layer cuprates like YBCO the interlayer hopping and exchange effects on the electronic structure and doping dependence of $T_c$ is discussed as well as the Coulomb interaction induced mechanism of pairing.
Search for Free Decay of Negative Pions in Water and Light Materials
T. Numao,Yu. I. Davydov,J-M. Poutissou,T. C. Awes,V. Cianciolo,S. Berridge,W. Bugg,Yu. Efremenko,R. Gearhart,S. Ovchinnikov
Physics , 2006, DOI: 10.1103/PhysRevD.73.092004
Abstract: We report on a search for the free decay component of pi- stopped in water and light materials. A non-zero value of this would be an indication of anomalous nu_e contamination to the nu_e and nu_mu_bar production at stopped-pion neutrino facilities. No free decay component of pi- was observed in water, Beryllium, and Aluminum, for which upper limits were established at 8.2E-4, 3.2E-3, and 7.7E-3, respectively.
Cuprates, Manganites, and Cobaltites: Multielectron Approach to the Band Structure
M. M. Korshunov,S. G. Ovchinnikov,E. I. Shneyder,V. A. Gavrichkov,Yu. S. Orlov,I. A. Nekrasov,Z. V. Pchelkina
Physics , 2012, DOI: 10.1142/S0217984912300165
Abstract: High-$T_c$ superconductors with CuO$_2$ layers, manganites La$_{1-x}$Sr$_x$MnO$_3$, and cobaltites LaCoO$_3$ present several mysteries in their physical properties. Most of them are believed to come from the strongly-correlated nature of these materials. From the theoretical viewpoint, there are many hidden rocks in making the consistent description of the band structure and low-energy physics starting from the Fermi-liquid approach. Here we discuss the alternative method -- multielectron approach to the electronic structure calculations for the Mott insulators -- called LDA+GTB (local density approximation + generalized tight-binding) method. Its origin is a straightforward generalization of the Hubbard perturbation theory in the atomic limit and the multiband $p-d$ Hamiltonian with the parameters calculated within LDA. We briefly discuss the method and focus on its applications to cuprates, manganites, and cobaltites.
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