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A. A. Ovchinnikov

Publications and source records attributed to A. A. Ovchinnikov.

51 records · Page 3Linked to original sources
arXiv2001

Exact ground state of one- and two-dimensional frustrated quantum spin systems

We outline the recent results on the ground state for a class of one- and two-dimensional frustrated quantum spin models with competing ferro(F)- and antiferromagnetic (AF) interactions. Frustrated spin systems are known to have many interesting properties due to large quantum fluctuations. As a result of these fluctuations usual mean-field approach gives quite crude (if not false) description of these systems. Therefore, exactly solvable models are very instructive at investigations of such systems. The exact ground state wave function of proposed models has a structure of the valence-bond state (VBS) type. One of the 1D model describes the transition line between the F and AF phase. The exact singlet ground state on this line has a double-spiral ordering. Using different approximation methods we study the magnetization curve in the AF phase. The second considered set of the 1D and 2D models has an exact non-degenerate ground state with exponentially decaying spin correlations. We also proposed the 1D and 2D electronic models with exact ground state represented in terms of singlet bond functions which are the generalization of the RVB functions including ionic states.

cond-mat.str-el↗
arXiv2000

Direct proof for the Scalar Product with Bethe eigenstate in Spin chains

We present the simple and direct proof of the determinantal formula for the scalar product of Bethe eigenstate with an arbitrary dual state. We briefly review the direct calculation of the general scalar product with the help of the factorizing operator and the construction of the factorizing operator itself. We also comment on the previous determination of the scalar product of Bethe eigenstate with an arbitrary dual state.

math-ph↗
arXiv2000

Construction of Monodromy Matrix in the F- basis and Scalar products in Spin Chains

We present in a simple terms the theory of the factorizing operator introduced recently by Maillet and Sanches de Santos for the spin - 1/2 chains. We obtain the explicit expressions for the matrix elements of the factorizing operator in terms of the elements of the Monodromy matrix. We use this results to derive the expression for the general scalar product for the quantum spin chain. We comment on the previous determination of the scalar product of Bethe eigenstate with an arbitrary dual state. We also establish the direct correspondence between the calculations of scalar products in the F- basis and the usual basis.

math-ph↗
arXiv2000

Rectification of current in ac-driven nonlinear systems and symmetry properties of the Boltzmann equation

We study rectification of a current of particles moving in a spatially periodic potential under the influence of time-periodic forces with zero mean value. If certain time-space symmetries are broken a non-zero directed current of particles is possible. We investigate this phenomenon in the framework of the kinetic Boltzmann equation. We find that the attractor of the Boltzmann equation completely reflects the symmetries of the original one-particle equation of motion. Especially, we analyse the limits of weak and strong relaxation. The dc current increases by several orders of magnitude with decreasing dissipation.

nlin.CD↗
arXiv2000

Tunneling of localized excitations: giant enhancement due to fluctuations

We consider the tunneling of localized excitations (many boson bound states) in the presence of a bosonic bath. We show both analytically and numerically that the bath influence results in a dramatical enhancement of the amplitude of the excitation tunneling. The order of the bosonic flow in the course of the tunneling process is obtained. On the background of the giant tunneling enhancement we observe and describe additional resonant enhancement and {\sl suppression} of tunneling due to avoided level crossings.

cond-mat.stat-mech↗
arXiv2000

Static magnetization induced by time-periodic fields with zero mean

We consider a single spin in a constant magnetic field or an anisotropy field. We show that additional external time-periodic fields with zero mean may generate nonzero time-averaged spin components which are vanishing for the time-averaged Hamiltonian. The reason is a lowering of the dynamical symmetry of the system. A harmonic signal with proper orientation is enough to display the effect. We analyze the problem both with and without dissipation, both for quantum spins (s=1/2,1) and classical spins. The results are of importance for controlling the system's state using high or low frequency fields and for using new resonance techniques which probe internal system parameters, to name a few.

cond-mat.stat-mech↗
arXiv1999

Interplay of pseudo- and superconducting gaps: observable manifestations in cuprates

Angular dependence of gap, seen in photoemission, its evolution with doping and temperature are interpreted on base t-t'-U Hubbard model in which a pseudogap is a working function for electrons removing from dielectric segments of zone boundary. Tunnel spectra in one-particle approximation display only superconducting gap and too large asymmetry in regions aside from optimal doping. Spectral functions confirm sharp well defined Fermi boundary at $k_x=k_y$ direction and smoothed destructed one at the $\G(0,0)-M(π,0)-Y(π,π)$ path. Both tunnel and ARPES data seem require spreading a model to bilayer ones.

cond-mat.str-el↗
arXiv1999

Exact ground states for a class of one-dimensional frustrated quantum spin models

We have found the exact ground state for two frustrated quantum spin-1/2 models on a linear chain. The first model describes ferromagnet- antiferromagnet transition point. The singlet state at this point has double-spiral ordering. The second model is equivalent to special case of the spin-1/2 ladder. It has non-degenerate singlet ground state with exponentially decaying spin correlations and there is an energy gap. The exact ground state wave function of these models is presented in a special recurrent form and recurrence technics of expectation value calculations is developed.

cond-mat.str-el↗
arXiv1999

Two-dimensional frustrated Heisenberg model: Variational study

The stability of the ferromagnetic phase of the 2D quantum spin-1/2 model with nearest-neighbor ferro- and next-nearest neighbor antiferromagnetic interactions is studied. It turns out that values of exchange integrals at which the ferromagnetic state becomes unstable with respect to a creation of one and two magnon are different. This difference shows that the classical approximation is inapplicable to the study of the transition from the ferromagnetic to the singlet state in contrast with 1D case. This problem is investigated using a variational function of new type. It is based on the boson representation of spin operators which is different from the Holstein-Primakoff approximation. This allows us to obtain the accurate estimate of the transition point and to study the character of the phase transition.

cond-mat.str-el↗
arXiv1999

Exact singlet bond ground states for electronic models

We have proposed several 1D and 2D electronic models with the exact ground state. The ground state wave function of these models is represented in terms of "singlet bond" functions consisting of homopolar and ionic configurations. The Hamiltonians of the models include the correlated hopping of electrons, pair hopping terms and the spin interactions. One of the 1D models demonstrates spontaneous breaking of the translational symmetry. We studied also the model describing the transition point between the phases with and without an off-diagonal long-range order.

cond-mat.str-el↗
arXiv1999

Exactly solvable spin ladder model with degenerate ferromagnetic and singlet states

We study the spin ladder model with interactions between spins on neighboring rungs. The model Hamiltonian with the exact singlet ground state degenerated with ferromagnetic state is obtained. The singlet ground state wave function has a special recurrent form and depends on two parameters. Spin correlations in the singlet ground state show double-spiral structure with period of spirals equals to the system size. For special values of parameters they have exponential decay. The spectrum of the model is gapless and there are asymptotically degenerated excited states for special values of parameters in the thermodynamic limit.

cond-mat.str-el↗
arXiv1999

Exactly solvable two-dimensional quantum spin models

A method is proposed for constructing an exact ground-state wave function of a two-dimensional model with spin 1/2. The basis of the method is to represent the wave function by a product of fourth-rank spinors associated with the sites of a lattice and the metric spinors corresponding to bonds between nearest neighbor sites. The function so constructed is an exact wave function of a 14-parameter model. The special case of this model depending on one parameter is analyzed in detail. The ground state is always a nondegenerate singlet, and the spin correlation functions decay exponentially with distance. The method can be generalized for models with spin 1/2 to other types of lattices.

cond-mat.str-el↗
arXiv1999

Exact ground state for 1D electronic models

We have found the exact ground state for two electronic models on a linear chain. The first model describes half-filling electron system in the ferromagnet--antiferromagnet transition point. In the singlet ground state the spin correlators show giant spiral magnetic ordering with the period of a spiral equals to the system size. The second electronic model describes the point where the ground state has giant spiral off-diagonal long-range order and, therefore, superconducting. We suggest the formation of the ground state with giant spiral order (ferromagnetic or off-diagonal) as a probable scenario of the subsequent destruction of the ferromagnetism and the superconductivity.

cond-mat.str-el↗
arXiv1993

Variational bound for the energy of two-dimensional quantum antiferromagnet

We obtain the variational upper bound for the ground- state energy of two-dimensional antiferromagnetic Heisenberg model on a square lattice at arbitrary value of the anisotropy parameter using the two-dimensional generalization of Jordan-Wigner transformation. Our result can be considered as an upper bound for the perturbation theory series about the Ising limit.

cond-mat↗