Электронно-дырочные димеры в "родительской" фазе квази-2D-купратов

One of the most important features of the parent cuprates such as La2CuO4, which predetermines their unusual behavior under non-isovalent substitution, is high ionic polarizability and proximity to the «polarization catastrophe». We show that at the same time parent cuprates are characterized by a charge transfer instability with the formation of a system of metastable dipole-active «Mott-Hubbard» excitons, or electron-hole (EH) dimers. Non-isovalent substitution shifts the phase equilibrium in the direction of condensation of EH-dimers and the formation of an inhomogeneous EH-liquid, in the simplest model, equivalent to a system of composite bosons. To effectively describe the electronic state of doped cuprates, it is proposed to use the S = 1 pseudospin formalism, which allows one to consider novel charge states of the Anderson RVB-type phase. The recombination of EH-dimers under a critically small value of the energy of local and nonlocal correlations leads to the transition of the system to the Fermi-liquid state.

Dielectric Properties of Barium Titanate Ceramics with Nano-Sized Domain

Barium titanate (BaTiO3) ceramics with various grain sizes from 0.7 to 13 μm on average were prepared by a conventional sintering method, a two-step sintering method and a rate controlled two-step sintering method. The permittivity of the ceramics was increased with decreasing grain size to 1.1 μm on average. However, the permittivity of the ceramics was decreased when the grain size was below 1 μm. The field emission scanning electron microscope (FE-SEM) observations revealed that the 90º domain width decreased with decreasing the grain size. By ultrawide range dielectric spectra from kHz to THz range of the BaTiO3 ceramics, the domain contribution to the permittivity was investigated. For the BaTiO3 ceramics with grain sizes over 1 μm, the dipole polarizability and the ionic polarizability were enhanced by high domain-wall density. In contrast, for the BaTiO3 ceramics with grain sizes below 1 μm, these polarizabilities were weakened.