arXiv subjects

I. V. Stasyuk

Publications and source records attributed to I. V. Stasyuk.

30 records · Page 2Linked to original sources

arXiv2003

Green's functions of infinite-U asymmetric Hubbard model: Falicov-Kimball limit

The asymmetric Hubbard model is used in investigating the lattice gas of the moving particles of two types. The model is considered within the dynamical mean-field method. The effective single-site problem is formulated in terms of the auxiliary Fermi-field. To solve the problem an approximate analytical method based on the irreducible Green's function technique is used. This approach is tested on the Falicov-Kimball limit (when the mobility of ions of either type is infinitesimally small) of the infinite-U case of the model considered. The dependence of chemical potentials on concentration is calculated using the one-particle Green's functions, and different approximations are compared with the exact results obtained thermodynamically. The densities of states of localized particles are obtained for different temperatures and particle concentrations. The phase transitions are investigated for the case of the Falicov-Kimball limit in different thermodynamic regimes.

cond-mat.str-el↗

arXiv2003

Strong coupling approach in dynamical mean-field theory for strongly correlated electron systems

We review two analytical approaches in Dynamical Mean-Field Theory (DMFT) based on a perturbation theory expansion over the electron hopping to and from the self consistent environment. In the first approach the effective single impurity Anderson model (SIAM) is formulated in terms of the auxiliary Fermi-fields and the projection (irreducible Green's function) technique is used for its solution. A system of the DMFT equations is obtained that includes as simple specific cases a number of known approximations (Hubbard-III, AA, MAA, ...). The second approach is based on the diagrammatic technique (Wick's theorem) for Hubbard operators that allows to construct a thermodynamically consistent theory when SIAM exactly splits into four components (subspaces): two Fermi liquid and two non-Fermi liquid. The results for the density of states, concentration dependences of the band energies, chemical potential and magnetic order parameters are presented for different self-consistent approximations (AA, strong coupling Hartree--Fock and further).

cond-mat.str-el↗

arXiv2002

Thermodynamics of pseudospin-electron model in mean field approximation

The mean field type approach based on the self-consistent consideration of an effective field created by electron transfer is developed for a description of thermodynamics of the Hubbard type models with an infinitely large on-site repulsion. This procedure, formulated by Izyumov et al, is an extension of the recently proposed generalized random phase approximation(GRPA). Within this approach, the thermodynamic properties of the two-sublattice pseudospin-electron model (the Hubbard model with local anharmonicity) are studied. Such a model can be used for a description of dielectric properties of YBaCuO-type superconductors along c-axis; pseudospins represent anharmonic motions of apical oxygens O4. It is shown that there are either phase transitions in the model with jumps of the mean values of a pseudospin and of electron concentration (in the $μ=const$ regime) or the phase separation (in the $n=const$ regime). The phase transitions or phase separation are caused by pseudospin-pseudospin interaction as well as by electron transfer (the latter results in appearing of effective interaction between pseudospins). The possibility of the ferroelectric type ordering of pseudospins is investigated.

cond-mat.str-el↗

arXiv2000

Self-consistent approach for thermodynamics of a simplified pseudospin-electron model

We present the method of the self-consistent calculation of thermodynamical and correlation functions. This approach is based on the GRPA (generalized random phase approximation) scheme with the inclusion of the mean field corrections. Investigation of a pseudospin-electron model (PEM) within the framework of the presented method shows that interaction between the electron and pseudospin subsystems leads to the possibility of either first or second order phase transitions between different uniform phases (bistability) as well as between the uniform and the chess-board ones. In the regime n=const, an instability with respect to phase separation in the electron and pseudospin subsystems can take place.

cond-mat.stat-mech↗

arXiv2000

Self-consistent description of simplified pseudospin-electron model

A method of the self-consistent calculation of the thermodynamical and correlation functions is presented. This approach is based on the GRPA (generalized random phase approximation) scheme with the inclusion of the mean field corrections. The numerical research shows that interaction between the electron and pseudospin subsystems leads to the possibility of the dipole (pseudospin) and charge-density instabilities as well as phase separation into the uniform and/or chess-board phases.

cond-mat.supr-con↗

arXiv2000

Thermodynamics of a pseudospin-electron model without correlations

Thermodynamics of a pseudospin-electron model without correlations is investigated. The correlation functions, the mean values of pseudospin and particle number, as well as the thermodynamic potential are calculated. The calculation is performed by a diagrammatic method in the mean field approximation. Single-particle Green functions are taken in the Hubbard-I approximation. The numerical research shows that an interaction between the electron and pseudospin subsystems leads in the $μ={}$const regime to the possibility of the first order phase transition at the temperature change with the jump of the pseudospin mean value $ $ and reconstruction of the electron spectrum. In the regime $n={}$const, an instability with respect to phase separation in the electron subsystem can take place for certain values of the model parameters.

cond-mat.supr-con↗

arXiv2000

Order-disorder model of phase transitions in the DMAGaS-DMAAlS family crystals

A four-state model is proposed for description of the sequence of phase transitions in ferroelectric crystals of DMAGaS and DMAAlS type. The ordering processes in the subsystems of DMA groups are considered as a main reason of such transformations. The interaction between groups in their various orientational states is taken into account in the dipole-dipole approximation. Obtained thermodynamical characteristics of the model (spontaneous polarization, occupancy of orientational states, dielectric susceptibility, phase diagram) are in good agreement with experimental data. The experimental fact of suppression of ferroelectric phase at increase of hydrostatic pressure is explained under assumption that pressure changes the difference between energies of various orientational states of DMA groups mostly.

cond-mat↗

arXiv2000

Thermodynamics and dielectric anomalies of DMAAS and DMAGaS crystals in the phase transitions region (Landau theory approach)

A simple description of thermodynamics of DMAAS and DMAGaS ferroelectric crystals by means of Landau expansion is proposed. Conditions of occurrence of phase transitions are established and their temperatures are obtained. The influence of external hydrostatic pressure on phase transitions is described. The temperature behaviour of dielectric susceptibility components and their anomalies in the vicinity of phase transition points are investigated. Obtained results are compared with experimental data.

cond-mat.mtrl-sci↗

arXiv1999

Microscopic model of phase transition in the crystals of DMAAlS and DMAGaS type

The four-state model is proposed for description of phase transition in ferroelectric crystals of DMAGaS and DMAAlS type. Thermodynamical functions of the model are obtained in the mean field approximation. The phase transition between paraelectric and ferroelectric phases is investigated. It is established that order of the phase transitions depends on relations between model parameters.

cond-mat.mtrl-sci↗

arXiv1999

Influence of shear stress $σ_6$ on the phase transition and physical properties of KD2PO4 type ferroelectrics

Within the four-particle cluster approximation for the proton ordering model we study the influence of shear stress $σ_6$ on the phase transition, static dielectric, elastic and thermal properties of deuterated KD2PO4 type ferroelectrics. Thermodynamic potentials and physical characteristics of the crystals in the presence of stress $σ_6$ are calculated. Numerical analysis of the obtained results is performed, and the set of the theory parameters providing the best fit to the available experimental data is found. The $Tc-σ_6$ phase diagram is constructed; temperature dependences of the calculated quantities are studied, possible stress dependences are indicated. Influence of $σ_6$ stress is analogous to that of electric field conjugate to the order parameter.

cond-mat↗

arXiv1998

Dynamics and thermodynamics of the pseudospin-electron model in the case of absence of the electron Hubbard correlation. I. The analytical consideration

Dynamics and thermodynamics of the model with local anharmonism in the case of absence of the electron (Hubbard) correlation is investigated in the present work. The correlation functions, mean values of pseudospin and particle number as well as the thermodynamical potential are calculated. The calculation is performed by diagrammatic method in the mean field approximation. Single--particle Green functions are taken in the Hubbard--I approximation.

cond-mat↗

arXiv1998

Pseudospin-Electron Model in Large Dimensions

Energy spectrum and thermodynamics of the pseudospin-electron model introduced at the consideration of the anharmonicity effects in high-$T_c$ superconductors are investigated in the dynamical mean field approximation ($d=\infty$ limit). In the limit of zero electron correlation $U\to 0$ this model is analytically exactly soluble within this approach: in the $μ=const$ regime the first order phase transition with the jump of the pseudospin mean value $ $ and reconstruction of the electron spectrum can realize, while in the $n=const$ regime the phase separation in electron subsystem can take place for certain values of the model parameters. On the basis of the obtained results the applicability of the approximate schemes previously used for the investigation of the pseudospin-electron model are discussed.

cond-mat.str-el↗