The Analysis of Ferroelectric Type Instabilities in the Two-Sublattice Model of High Temperature Superconducting Systems

A review of the present state of investigations of the pseudospin-electron model (PEM), which is used in the theory of strongly correlated electron systems, is given. The model is used to describe the systems with the locally anharmonic elements of structure represented in the model by pseudospins. The consideration is based on the dynamical mean field theory approach and the generalized random phase approximation. Electron spectrum and thermodynamics of the model are investigated; the cases of the simplified model, the model with strong interaction and the twosublattice model are studied more in detail. The phase transitions into other uniform or modulated states as well as superconducting phases are described; the criteria of their realization are established. Based on this, the description of structural and dielectric (ferroelectric type) instabilities, phase separation and bistability phenomena is given. A comparison is made with the thermodynamics of the Falicov-Kimball model (which can be considered as a particular case of PEM). The possibility of applying the PEM to the analysis of thermodynamics of the real HTSC systems is discussed. Attention is paid to the unsolved problems in the study of PEM.

Section: Introductionmentioning

confidence: 99%

Phase Transitions in Pseudospin-Electron Model With Direct Interaction Between Pseudospins

Stasyuk¹,

Havrylyuk²

1999

“…parameter (a < −1) or at the increase of the intracluster interaction constant J [39]. As a result, there appears a line of the first order phase transitions that separates two nonpolar phases.…”

Section: Two-sublattice Pem With Direct Interactionmentioning

confidence: 99%

“…Pseudospin-electron model with direct interaction between pseudospins was generalized in [39] into a two-sublattice case with the aim to take into account the features of real structure of superconducting systems of the YBa 2 Cu 3 O t−δ type (the presence of two superconducting planes and two nonequivalent apical oxygen ions in the unit cell). The unit cell Hamiltoniañ…”

Section: Two-sublattice Pem With Direct Interactionmentioning

confidence: 99%

“…Pseudospin part of the operator An essential addition to this picture can be obtained when the interaction of pseudospins with electrons is taken into account. It was shown [39] that the ground state of the system (which can correspond to the parallel or antiparallel configuration of pseudospins in the unit cell) depends not only on the h and J values but also on the position of the chemical potential µ level. Ferroelectric phase is possible in the narrow range of the µ and h values (the width of the interval of the h values is of the order of J).…”

Section: Two-sublattice Pem With Direct Interactionmentioning

confidence: 99%

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LOCAL ANHARMONIC EFFECTS IN HIGH-T_c SUPERCONDUCTORS. PSEUDOSPIN-ELECTRON MODEL

Stasyuk¹

1999

Some aspects of physical phenomena in high-T c crystals caused by anharmonicity of the crystal lattice are discussed. Appropriate theoretical models are considered. A review is given of the results obtained while investigating the pseudospin-electron model (PEM) when applied to the description of structural and dielectric instabilities, phase separations and bistability phenomena. The theoretical problems are analyzed arising in PEM when the strong short-range correlations and electron transfer are taken into account simultaneously.

“…Within the framework of this model, the Hubbard model (applied to the description of electrons) is supplemented by a pseudospin subsystem [9,10]:…”

Section: The Hamiltonian Of Modelmentioning

confidence: 99%

The Phase Separation Effects in a Pseudospin-Electron Model

Danyliv¹,

Stasyuk²

2002

The two-sublattice pseudospin-electron model of high temperature superconductors is studied with respect to phase separations. Such a model can be used for the description of dielectric properties of YBaCuO-type crystals along c -axis (the pseudospins represent an anharmonic motions of apical oxygen O4). The model is treated within different approximations. It is shown that the model has got phase transitions to the phase with pseudospin ordering as well as phase separation. The effect of separation on the appearance of the ordered state is discussed.