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Now showing items 1 - 16 of 28123374

  • On the exactly solvable pairing models for bosons

    A. A. Ovchinnikov  

    We propose the new exactly solvable model for bosons corresponding to the attractive pairing interaction. Using the electrostatic analogy, the solution of this model in thermodynamic limit is found. The transition from the superfluid phase with the Bose condensate and the Bogoliubov - type spectrum of excitations in the weak coupling regime to the incompressible phase with the gap in the excitation spectrum in the strong coupling regime is observed.
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  • Nonlinear Hall Effect on Metal Surfaces in a Periodic Electric Field

    A. A. Ovchinnikov  

    A theory of nonlinear Hall effect arising in the metal surface layer as a result of the application of an alternating electric field is given. Expressions for the Hall current are derived.
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  • Nonlinear Hall Effect on Metal Surfaces in a Periodic Electric Field

    A. A. Ovchinnikov  

    A theory of nonlinear Hall effect arising in the metal surface layer as a result of the application of an alternating electric field is given. Expressions for the Hall current are derived.
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  • Nonlinear Hall Effect on Metal Surfaces in a Periodic Electric Field

    A. A. Ovchinnikov  

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  • Models of spin structures in Sr2RuO4

    A. A. Ovchinnikov   M. Ya. Ovchinnikova  

    The mean-field method is used to calculate the bands, Fermi surfaces, and spin susceptibilities of a three-band model of the RuO 4 plane of Sr 2RuO 4 rutinate for states with different spin structures. In particular, the spiral state is studied with the "incommensurate" vector Q = 2pi(1/3, 1/3) corresponding to the nesting of bands with the population n = 4. This state proves to be the lowest with respect to energy among other (paramagnetic, ferromagnetic, antiferromagnetic, and periodic) solutions. In the spiral state, in addition to the main alpha, beta, and gamma sheets of the Fermi surface, shadow Fermi boundaries along the Gamma(0, 0)-M(pi, 0) line (previously observed in the ARPES experiments) are revealed and explained. This may change the interpretation of the data on dispersionless peaks in photoemission, previously ascribed to surface states. The spin susceptibilities of the spiral state exhibit peaks in the dependence Imchi(q, omega) at q = Q in accordance with the observed magnetic peak in neutron scattering. The hypothesis of the presence of spin structures with q = Q in the normal state of Sr 2RuO 4 and the methods of checking this hypothesis are discussed
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  • Band splitting and relative spin alignment in bilayer systems

    A. A. Ovchinnikov   M. Ya. Ovchinnikova  

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  • Variational tests of current order parameters in the Hubbard model

    A. A. Ovchinnikov   M. Ya. Ovchinnikova  

    Variational tests are performed for current order parameters as probable sources of the pseudogap normal state of cuprates. The calculations are carried out based on the states with correlations of the valence bond type whose formation can induce in principle both the superconducting order of the d symmetry and current phases. It is shown for the t-t'-U Hubbard models with a large value of U(~8t) and the Hubbard splitting of the conduction band that (1) phases of alternating charge and longitudinal spin currents cannot be realized and (2) transverse spin currents are not compatible with the superconducting order and they could exist against the normal-state background only within a very narrow doping region near the optimal one. This region does not correspond to the region of existence of a pseudogap in cuprates, which refutes the above-mentioned hypothesis of the pseudogap origin. The requirements to the parameters of models for which the consideration of correlations of the valence bond type yields a reasonable phase curve. The existence of current phases in the t-t'-U-V Hubbard models with a strong interaction (V>0.25t) of particles in neighboring sites is predicted when the d-superconductivity is completely suppressed
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  • Asymptotics and functional form of correlators in the XX-spin chain of finite length

    A. A. Ovchinnikov  

    We verify the functional form of the asymptotics of the spin - spin equal - time correlation function for the XX-chain, predicted by the hypothesis of conformal invariance at large distances and by the bosonization procedure. We point out that bosonization also predicts the functional form of the correlators for the chains of finite length. We found the exact expression for the spin- spin equal- time correlator on finite lattice. We find the excellent agreement of the exact correlator with the prediction given by the leading asymptotics result up to the very small distances. We also establish the correspondence between the value of the constant before the asymptotics for the XX- chain with the expression for this constant proposed by Lukyanov and Zamolodchikov. We also evaluate the constant corresponding to the subleading term in the asymptotics in a way which is different from the previous studies.
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  • Experimental study of the generalized synchronization of chaotic oscillations in the presence of noise

    A. A. Ovchinnikov   O. I. Moskalenko   A. A. Koronovski?   A. E. Hramov  

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  • Spin susceptibilities for the Hubbard model in a band approach

    A. A. Ovchinnikov   M. Ya. Ovchinnikova  

    The properties of spin excitations superposed on a uniform ground state with antiferromagnetic (or spiral) spin structure are studied in a 2D Hubbard model. Expressions are derived for the spin susceptibility in the random phase approximation (RPA) using split Hubbard bands as a zeroth approximation. The calculated collective modes with dispersion omega(Q)=c|Q-(pi,pi)| near Q~(pi,pi) reproduce well the characteristics of the spin excitations observed in undoped cuprates. For doped systems with an antiferromagnetic structure of the ground state, calculating chi"(Q,omegararr0) gives the same mode with a peak at Q~(pi,pi), regardless of the type of Fermi surface. It is shown that in doped systems with a spiral ground state spin structure, chi"(Q,omegararr0) peaks occur with incommensurate quasimomenta Q that are coupled to the spirality vector
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  • Pseudogap and symmetry of superconducting order parameter in cuprates

    A. A. Ovchinnikov   M. Ya. Ovchinnikova   E. A. Plekhanov  

    The phase diagram, nature of the normal state pseudogap, type of the Fermi surface, and behavior of the superconducting gap in various cuprates are discussed in terms of a correlated state with valence bonds. The variational correlated state, which is a band analogue of the Anderson (RVB) states, is constructed using local unitary transformations. Formation of valence bonds causes attraction between holes in the d-channel and corresponding superconductivity compatible with antiferromagnetic spin order. Our calculations indicate that there is a fairly wide range of doping with antiferromagnetic order in isolated CuO 2 planes. The shape of the Fermi surface and phase transition curve are sensitive to the value and sign of the hopping interaction t' between diagonal neighboring sites. In underdoped samples, the dielectrization of various sections of the Fermi boundary, depending on the sign of t', gives rise to a pseudogap detected in photoemission spectra for various quasi-momentum directions. In particular, in bismuth- and yttrium-based ceramics (t'>0), the transition from the normal state of overdoped samples to the pseudogap state of underdoped samples corresponds to the onset of dielectrization on the Brillouin zone boundary near k=(0,pi) and transition from ldquolargerdquo to ldquosmallrdquo Fermi surfaces. The hypothesis about s-wave superconductivity of La- and Nd-based ceramics has been revised: a situation is predicted when, notwithstanding the d-wave symmetry of the superconducting order parameter, the excitation energy on the Fermi surface does not vanish at all points of the phase space owing to the dielectrization of the Fermi boundary at k x=plusmnk y. The model with orthorhombic distortions and two peaks on the curve of T c versus doping is discussed in connection with experimental data for the yttrium-based ceramic
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  • Manifestations of the pseudogap and superconducting gap of cuprates in photoemission

    A. A. Ovchinnikov   M. Ya. Ovchinnikova  

    The degree to which the interpretation of the existence of a pseudogap and a superconducting gap in cuprates on the basis of the t-t'-U Hubbard-model corresponds to the data obtained from the photoemission spectra is discussed. The pseudogap in the model is interpreted as the work function of electrons from the insulating parts of the Brillouin zone boundary. On this basis one can explain the angle dependence of the gap measured in the photoemission spectra and its evolution on changes in doping and temperature. In particular, an explanation is found for the decline in the ratio of the angle derivative of the gap near the site, v Delta (1/2)dDelta(phi)/dphi, to the maximum value of the gap, Delta max with decreasing doping. That behavior and the different temperature dependence of the gap Delta(phi) for different angles are due to the presence of two contributions to Delta with different anisotropies-from the pseudogap and from the superconducting gap. The calculation of the spectral functions confirms the sharp Fermi boundary observed in the direction and the smeared edge of the distribution along the path Gamma(0,0)-M(pi,0)-Y(pi,pi)
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  • Separation of liquid-liquid and liquid-solid heterogeneous systems in single-flow vortex separators

    A. A. Ovchinnikov   A. A. Shadrin   D. V. Alekseev   N. A. Nikolaev  

    A method for calculating the efficiency of single-flow vortex separators intended for removing liquid and solid suspensions from industrial waste is proposed. The method reveals a relationship between the efficiency of separation of heterogeneous systems and the particle size distribution of the dispersed phase, its flow rate parameters, and the geometry of the apparatus
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  • Separation of liquid-liquid and liquid-solid heterogeneous systems in single-flow vortex separators

    A. A. Ovchinnikov   A. A. Shadrin   D. V. Alekseev and N. A. Nikolaev  

    A method for calculating the efficiency of single-flow vortex separators intended for removing liquid and solid suspensions from industrial waste is proposed. The method reveals a relationship between the efficiency of separation of heterogeneous systems and the particle size distribution of the dispersed phase, its flow rate parameters, and the geometry of the apparatus.
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  • d-symmetry superconductivity due to valence bond correlations

    A. A. Ovchinnikov   M. Ya. Ovchinnikova   E. A. Plekhanov  

    It is shown that d-symmetry superconductivity due to valence bond correlations is possible. Valence bond correlations are compatible with antiferromagnetic spin order. In order to explicitly construct a homogeneous state with the valence bond structure in the two-dimensional Hubbard model for an arbitrary doping, we have used the variational method based on unitary local transformation. Attraction between holes in the d-channel is due to modulation of hopping by the site population in course of the valence bond formation, and corresponding parameters have been calculated variationally. An important factor for the gap width is the increase in the density of states on the Fermi level due to antiferromagnetic splitting of the band. The gap width and its ratio to the T c are 2Deltasime0.1t and 2Delta/kT csime4.5-4 for U/tsime8. The correspondence between the theoretical phase diagram and experimental data is discussed. The dependence of T c on the doping delta=|n-1| and the Fermi surface shape are highly sensitive to the weak interaction t' leading to diagonal hoppings. In the case of t '>0 and p-doping, the peak on the curve of T c (delta) occurs at the doping delta opt, when the energy of the flattest part of the lower Hubbard subband crosses the Fermi level at k~(pi,0). In underdoped samples with delta
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  • Magnetic susceptibility of multilayered carbon nanotubes

    A. A. Ovchinnikov   V. V. Atrazhev  

    A theoretical study of the magnetic susceptibility of multilayered carbon nanotubes in fields both parallel, Hpar, and perpendicular, Hperp, to the tube axis has been carried out disregarding electron-electron interaction. The temperature dependences of the magnetic susceptibility obtained exhibit a nontrivial form for Hpar, which is related to the quasi-one-dimensionality of such a system as the nanotube. The dependences of the magnetic susceptibility on chemical potential chi(mu) have also been derived. At low temperatures, chi(mu) has sharp peaks in fields Hpar, which is connected with the presence of 1/radicE-type singularities in the density of states of nanotubes. The effect of interlayer coupling on magnetic susceptibility of small-radius tubes has been investigated numerically
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