The model of the optical switching center and the polarization anomalies in absorption spectra of cupric oxide after fast particles irradiation

We consider the model of the optical switching center - a system with the following properties: It has two (or more) metastable states |1>, |2>, separated by a potential barrier U; it can switch from one state to another by absorbing the photons with energy proportional to U; the transition from |1> to |2> is allowed only for a certain light wave polarization p_1 and the transition from |2> to |1> - for other polarization p_2; these polarizations p_1, p_2 are orthogonal. The optical properties of this system are studied and are found to exhibit unconventional polarization dependence. In particular, the absorption spectrum observed in natural (unpolarized) light can display new features, that are absent in the spectra, obtained in two independent polarizations. We discuss these results in connexion with the (yet unexplained) experimental findings (Ref.1), where the similar anomalous polarization dependence of the absorption spectra of cupric monoxide CuO after the fast particle bombardment is reported.Comment: 7 pages, 4 figure

Field induced antiferromagnetism and 17^{17}O Knight shift anomaly in La2_2CuO4_4

We address the effect of the field induced antiferromagnetism in paramagnetic state of the cuprate weak ferromagnet La$_2$CuO$_4$. The planar oxygen $^{17}$O Knight shift is shown to be an effective tool to inspect the effects of Dzyaloshinsky-Moriya coupling in cuprates in an external magnetic field. Field induced antiferromagnetism and anisotropic antiferromagnetic contribution to $^{17}$K explain the anomalies observed in $^{17}$O NMR in La$_2$CuO$_4$. The experimental observation of antiferromagnetic contribution to the $^{17}$O Knight shift provides probably the only way to find out the problem of the sense of Dzyaloshinsky vector in cuprates.Comment: 4 pages, 1 figure, submitted to PR

The Singlet-Triplet Pseudo-Jahn-Teller Centers in Copper Oxides

One of the most exciting features of the hole centers CuO_{4}^{5-} in doped cuprates is an unusually complicated ground state which is the result of the electronic quasi-degeneracy. An additional hole, doped to the basic CuO_{4}^{6-} cluster with the b_{1g} hole can occupy both the same hybrid Cu3d-O2p orbital state resulting in a Zhang-Rice singlet ^1A_{1g} and the purely oxygen e_u molecular orbital resulting in a singlet or triplet ^{1,3}E_u term with the close energies. We present detailed analysis of the (pseudo)-Jahn-Teller effect driven by the near-degeneracy within the (^1A_{1g},^{1,3}E_u)-manifold.Comment: RevTex, 20 pages, 8 figures; to be published in J.Phys.Chem.So

The singlet-triplet magnetism and induced spin fluctuations in the high-TcT_c copper oxides

High-$T_c$ cuprates like $La_{2-x}Sr_{x}CuO_{4}$ and $YBa_{2-x}Cu_{3}O_{6+x}$ are considered as a system of the electron and hole polar pseudo-Jahn-Teller $CuO_{4}$ centers $[{CuO}_{4}^{7-}]_{JT}$ and $[CuO_{4}^{5-}]_{JT}$, respectively, or a system of the local bosons moving in a lattice of the hole centers. Ground manifold of the polar centers includes three terms $^{1}A_{1g}$ (Zhang-Rice singlet), $^{1}E_{u}$, $^{3}E_{u}$ with different spin multiplicity, orbital degeneracy and parity that provides an unconventional multi-mode behaviour of the cuprates. The spin subsystem of the copper oxides within the polar Jahn-Teller $CuO_{4}$ centers model is a two-component spin liquid and corresponds to a singlet-triplet magnet with possible noncollinear spin configurations. In the framework of a modified mean field approximation some kinds of spin ordering are discussed including as a trivial singlet or triplet states as a pure quantum singlet-triplet mixed state. A local boson movement is accompanied by a modulation of the spin density on the site resulting in the so called induced spin fluctuations. Some unconventional features of the induced spin fluctuations are considered including an appearance of the induced longitudinal ferrimagnetism with an appropriate contribution to the spin susceptibility, a possibility to observe and examine the charge fluctuations with the help of the traditional magnetic methods such as the magnetic inelastic neutron scattering and the spin lattice relaxation experiments. The suggested model, in comparison with the nearly antiferromagnetic Fermi-liquid model, represents new approach to the description of the spin system of the high-$T_c$ cuprates.Comment: 21 pages, 13 figures, (LaTeX

What is the true charge transfer gap in parent insulating cuprates?

A large body of experimental data point towards a charge transfer instability of parent insulating cuprates to be their unique property. We argue that the true charge transfer gap in these compounds is as small as 0.4-0.5\,eV rather than 1.5-2.0\,eV as usually derived from the optical gap measurements. In fact we deal with a competition of the conventional (3d$^9$) ground state and a charge transfer (CT) state with formation of electron-hole dimers which evolves under doping to an unconventional bosonic system. Our conjecture does provide an unified standpoint on the main experimental findings for parent cuprates including linear and nonlinear optical, Raman, photoemission, photoabsorption, and transport properties anyhow related with the CT excitations. In addition we suggest a scenario for the evolution of the CuO$_2$ planes in the CT unstable cuprates under a nonisovalent doping.Comment: 13 pages, 5 figures, submitted to PR

Pseudospin S=1 formalism and skyrmion-like excitations in the three body constrained extended Bose-Hubbard model

We have focused in the paper on the most prominent and intensively studied S=1 pseudospin formalism for extended bosonic Hubbard model (EHBM) with truncation of the on-site Hilbert space to the three lowest occupation states n = 0, 1, 2. The EHBM Hamiltonian is a paradigmatic model for the highly topical field of ultracold gases in optical lattices. Generalized non-Heisenberg effective pseudospin Hamiltonian does provide a deep link with boson system and physically clear description of "the myriad of phases" from uniform Mott insulating phases and density waves to two types of superfluids and supersolids. We argue that the 2D pseudospin system is prone to a topological phase separation and focus on several types of unconventional skyrmion-like topological structures in 2D boson systems, which have not been analysed till now. The structures are characterized by a complicated interplay of insulating and the two superfluid phases with a single boson and two-boson condensation, respectively.Comment: 13 pages, 2 figures. arXiv admin note: text overlap with arXiv:1301.2025, arXiv:0804.1171 by other author

Polar Jahn-Teller Centers and Isotope Effect in Copper Oxide high-T_c-Superconductors

The $CuO_4$-cluster based copper oxides are considered as generalized quantum lattice bose-gas or a system of the local singlet bosons moving in a lattice of the hole Jahn-Teller centers $[CuO_{4}^{5-}]_{JT}$. The model is illustrated by the qualitative and quantitative description of the various peculiarities of an isotope shift (IS-) effect in a series of 123-like oxides.Comment: 4 pages, 1 figure, LaTeX; will be published in Physica C in Materials of Int.Conf. HTSC-V. (1997, Beijing, China