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Spatial electron ordering and uniform mixed-valence behavior, experimentally observed in many transition and rare-earth metals and compounds with narrow d and f bands, can be interpreted in terms of a generalized Hubbard-like two-band model. In the strong interaction limit (U [approaches][infinity]) and exactly at half-filling, an effective Hamiltonian for such a model becomes strictly equivalent to an anisotropic XYZ model in an external field. The problem of forming either an 'antiferromagnetic' orbital crystal with valence density waves (VDW) or a liquid-like mixed valence state (MVS) with strong excitonic correlations is then analogous to the problem of finding the magnetization of the corresponding Heisenberg antiferromagnet. The variation of the orbital occupation number versus the energy difference between the centers of two bands and the conditions for smooth and first-order transitions at half-filling from VDW into MVS have been found using a generalized mean field approximation at large U. The observed anomalies of the valence behavior and compressibility in mixed-valence compounds can be viewed in this context as a diamagnetic response an applied external field. We demonstrate strong evidence for the existence in the ground state of a sequence of spatially modulated phases with periodic arrangement of d or f electrons (commensurate VDW) in the presence of an external field.

Development and application of a new method enables the analysis of the two-point correlation functions of a finite size random isotropic XY spin glass system. The method yields upper bound values for the generalized spin-glass susceptibility factor that elucidates the off-diagonal transverse magnetization in the glass system. The slowest decay rate for the susceptibility in the spin glass is of the Kosterlitz-Thouless-type with a 2D power-law decay.

Use of a generalized Hubbard model helps elucidate the properties of mixed-valence compounds. A generalized mean field theory yields analytical values for partition function and ground-state energy. The influence of local hybridization between the s and f states helps derive the conditions representing the first-order transition.

Circular dichroism and absorption are calculated for one-dimensional chiral soft substance (a cholesteric liquid crystal, CLC) cells with an isotropic defect layer. The photonic density of states (PDS), circular dichroism, absorption and emission dependencies are calculated as functions of the parameters characterising absorption and gain. The absorption and gain effects in chiral photonic systems with a defect layer are established and it is shown that in some cases the subject system can work as: a low threshold laser, a multi-position trigger, a total wide/narrow band absorber, a wide/narrow band filter/mirror, etc. This work demonstrates the effects of absorption and emission in photonic crystal (PC) layers, and offers a novel approach to understanding of tunable soft photonic substances. [ABSTRACT FROM PUBLISHER]

Abstract: There is growing evidence that the unconventional spatial inhomogeneities in the doped high- superconductors are accompanied by the pairing of electrons, subsequent phase transitions and condensation into coherent states. We show that such pairing states can be obtained from phase separation instabilities near level crossings. Conditions for coherent pairing instabilities are examined using exact diagonalization of Hubbard-like pyramid structures under variation of coupling and interaction strengths. We also evaluate the behavior of the energy charge gap in the vicinity of level crossings using a parametrization of coupling to the apical site to represent out-of-plane effects. These results provide a simple microscopic explanation of (correlation induced) supermodulation of the coherent pairing gap observed in scanning tunneling microscopy measurements at atomic scale in Bi2Sr2CaCu2O8+ δ . [Copyright &y& Elsevier]