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A. A. Ovchinnikov

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

At least 37 records · Page 2Linked to original sources
arXiv2003

The antiferromagnetic Ising chain in a mixed transverse and longitudinal magnetic field

We have studied the antiferromagnetic Ising chain in a transverse magnetic field $h_{x}$ and uniform longitudinal field $h_{z}$. Using the density matrix renormalization group calculation combined with a finite-size scaling the ground state phase diagram in ($h_{x},h_{z}$) plane is determined. It is shown that there is an order-disordered transition line in this plane and the critical properties belong to the universality class of the two-dimensional Ising model. Based on the perturbation theory in $h_{z}$ the scaling behavior of the mass gap in the vicinity of the critical point ($h_{x}=1/2,h_{z}=0$) is established. It is found that the form of the transition line near the classical multicritical point ($h_{x}=0,h_{z}=1$) is linear. The connection of the considered quantum model with the quasi-one-dimensional classical Ising model in the magnetic field is discussed.

cond-mat.str-el↗
arXiv2003

Exactly solvable discrete BCS - type Hamiltonians and the Six-Vertex model

We propose the new family of the exactly solvable discrete state BCS - type Hamiltonians based on its relationship to the six-vertex model in the quasiclassical limit both in the rational and the trigonometric cases. We establish the relation of the BCS Hamiltonian and its eigenfunctions to the form of the monodromy matrix in the F-basis. Using the Algebraic Bethe Ansatz method for the standard BCS model with equal coupling the expression for the general scalar product and the determinant expressions for the physically interesting correlation functions for the finite number of sites which can be used in the numerical and analytical computations are obtained. We also compare the correlators with the results obtained by means of the variational method.

math-ph↗
arXiv2002

Asymptotics and functional form of correlators in the XX - spin chain of finite length

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.

math-ph↗
arXiv2002

Nomlinear Hall effect in a time-periodic electric field and related phenomena

A motion of neutral and charged particles located nearby a metal surface under joint action of time-periodic electric field E(t) directed normally to the surface and permanent magnetic field H directed along the surface has been considered. I has been shown that due to a nonlinearity of the system there exists a directed transport of the particles perpendicularly to both magnetic and electric fields with velocity which is proportional to H and a square of E. The mean velocity has been evaluated for charged (ions,electrons etc.) and neutral (atoms, molecules, nano-size clusters etc.) particles adsorbed on a metal surface. Corresponding surface electric current has been found as a function of a frequency of the electric field and material parameters. It has been noted that the similar phenomena appear in a bulk of electrically inhomogeneous media.

cond-mat.mtrl-sci↗
arXiv2002

Coexisting superconducting and spiral spin orders: models of ruthenate

Influence of a spiral spin structure on the superconducting (SC) pairing in the most probable active $γ$ band of $Sr_2RuO_4$ is studied in the mean field approximation. Such structure with "incommensurate" vector $Q=2π(1/3,1/3)$ has been proposed as that which removes the nesting instability in $\alfa, \betta$ bands. The model pairing interaction of adjacent centres is adopted. The mixed singlet and triplet SC order is revealed. The d-wave singlet pairing is accompanied by formation of the p-wave triplet pairs with large total momentum Q and spin projections $\pm 1$ onto axis normal to the spin rotation plane of the spiral structure. Both the superconducting and normal states have a broken time reversal symmetry which might be detected by polarised photoemission.

cond-mat.supr-con↗
arXiv2002

Gap generation in the XXZ model in a transverse magnetic field

The ground state phase diagram of the 1D XXZ model in transverse magnetic field is obtained. It consists of the gapped phases with different types of long range order (LRO) and critical lines at which the gap and the LRO vanish. Using scaling estimations and a mean-field approach as well as numerical results we found critical indices of the gap and the LRO in the vicinity of all critical lines.

cond-mat.str-el↗
arXiv2002

The anisotropic Heisenberg model in a transverse magnetic field

One dimensional spin-1/2 $XXZ$ model in a transverse magnetic field is studied. It is shown that the field induces the gap in the spectrum of the model with easy-plain anisotropy. Using conformal invariance the field dependence of the gap at small fields is found. The ground state phase diagram is obtained. It contains four phases with different types of the long range order (LRO) and a disordered one. These phases are separated by critical lines, where the gap and the long range order vanish. Using scaling estimations and a mean-field approach as well as numerical calculations in the vicinity of all critical lines we found the critical exponents of the gap and the LRO. It is shown that transition line between the ordered and disordered phases belongs to the universality class of the transverse Ising model.

cond-mat.str-el↗
arXiv2002

Broken space-time symmetries and mechanisms of rectification of ac fields by nonlinear (non)adiabatic response

We consider low-dimensional dynamical systems exposed to a heat bath and to additional ac fields. The presence of these ac fields may lead to a breaking of certain spatial or temporal symmetries which in turn cause nonzero averages of relevant observables. Nonlinear (non)adiabatic response is employed to explain the effect. We consider a case of a particle in a periodic potential as an example and discuss the relevant symmetry breakings and the mechanisms of rectification of the current in such a system.

cond-mat.stat-mech↗
arXiv2002

Ring current effects on the dielectric function of cylindrical nano-organic materials

We review recent results on the behaviour of the dielectric function of cylindrical nano-organic materials at very low frequencies in a magnetic field. For cylindrical structures - such as carbon nanotubes - the polarisability is shown to be a discontinuous function of a longitudinal magnetic field where plateau-like regions are separated by sudden jumps or peaks. A relation is pointed out between each discontinuity in the polarisability and the cross-over between ground and first excited states induced by the magnetic field. This one to one correspondence suggests to use measurements of the dielectric function in an applied magnetic field in order to obtain informations about the electronic structures of cylindrical nanostructures. In addition, it is shown, by studying finite graphene layers, that the measurement of the polarisability in a magnetic field could be a powerful way for detecting possible edge-states in amorphous carbon materials such as activated carbon fibres. Finally, the importance of the electron-electron interaction is emphasised by discussing examples of strongly interacting electrons on rings or cylinders, in the limit of infinite interaction.

cond-mat.mes-hall↗
arXiv2002

Models of spin structures of Sr2RuO4

The mean field study of bands and the Fermi surfaces (FS's) are carried out for 3-band model of ruthenate. In particular, the lowest spiral state with incommensurate vector Q=2π(1/3,1/3) corresponding to nesting of bands display a set of FS's around electro pockets and shadow sheets along $Γ-M$ lines. The latter may provide a new interpretation of the dispersionless peaks (so called SS features) observed in photoemission. For spiral state the polarisation asymmetry of the photoemission intensity is revealed. The calculated spin susceptibility for this state describes the low frequency magnetic peak at q=Q observed in neutron scattering.

cond-mat.str-el↗
arXiv2002

Magnetophoresis of neutral particles in AC electric field

It has been shown that under joint action of DC magnetic and AC electric fields neutral particles (atoms,molecules,nano-size particles etc.) nearby interface move with permanent velocity along the surface. The driving force depends on non-linear dynamical properties of polarization of the particles and a coupling between the polarisation and the position of the particles on a surface.

cond-mat.stat-mech↗
arXiv2001

Nonlinear Hall effect in an AC electric field and related phenomena

It has been shown that in metals and semiconductors the joint action of permanent magnetic and AC electric fields leads to arising of DC surface electric current. This is due to essentially non-linear dynamics of electronic gas in surface layer.The surface current is linear with respect to external magnetic field and depends quadratically on an amplitude of AC electric field.The conditions for spontaneous generation of magnetic field have been found.

cond-mat.str-el↗
arXiv2001

AC-driven quantum spins: resonant enhancement of transverse DC magnetization

We consider s=1/2 spins in the presence of a constant magnetic field in z-direction and an AC magnetic field in the x-z plane. A nonzero DC magnetization component in y direction is a result of broken symmetries. A pairwise interaction between two spins is shown to resonantly increase the induced magnetization by one order of magnitude. We discuss the mechanism of this enhancement, which is due to additional avoided crossings in the level structure of the system.

quant-ph↗
arXiv2001

The emergence of charged collective modes from a large N extrapolation of the Hubbard model

We consider a symplectic extrapolation of the Hubbard model of N fold replicated electrons and solve this model exactly in two special cases, at N=infinity in the bosonic sector and for any N on a dimer of two points. At N=infinity we find a multiplet of collective modes that contains neutral spin fluctuations and charged pair fluctuations that are degenerate with each other at zero doping. Our solution of the symplectic model on a dimer of two points for any N interpolates smoothly between N=1 and N=infinity without any visible discontinuity. These results suggest that the inclusion of charged pairing modes in weakly doped antiferromagnets is essential and that an expansion about the N=infinity limit is appropriate in this context.

cond-mat.str-el↗
arXiv2001

Edge-state effects on the magneto-polarizability of nanographite layers

The possibility for carbon materials such as activated carbon fibers or graphite ribbons, to support edge-states could change drastically their magnetic properties. The purpose of this work is to propose a novel way to identify the existence of those states by considering the behaviour of the static electric polarizability under a perpendicular magnetic field. The edge-sates are shown to induce a quadratic dependence on magnetic field detectable experimentally nowaday.

cond-mat↗
arXiv2001

Static magneto-polarizability of cylindrical nanostructures

The static polarizability of cylindrical systems is shown to have a strong dependence on a uniform magnetic field applied parallel to the tube axis. This dependence is demonstrated by performing exact numerical diagonalizations of simple cylinders (rolled square lattices), armchair and zig-zag carbon nanotubes (rolled honeycomb lattices) for different electron-fillings. At low temperature, the polarizability as function of the magnetic field has a discontinuous character where plateau-like region are separated by sudden jumps or peaks. A one to one correspondence is pointed out between each discontinuity of the polarizability and the magnetic-field induced cross-over between the ground state and the first excited state. Our results suggest the possibility to use measurements of the static polarizability under magnetic field to get important informations about excited states of cylindrical systems such as carbon nanotubes.

cond-mat↗
arXiv2001

Band splitting and relative spin alignment in bilayer systems

Influence of relative spin alignment on the band splitting and magnetic excitations in the bilayer correlated systems is studied. Splitting occures to be large or small depending on orientation of staggered spins of layers. Change of the ground state configuration with doping is shown. Behavior of the bilayer splitting in Bi-based cuprate allows to suppose that superconducting transition is accompanied by transformation of spin configuration of system.

cond-mat.str-el↗