Structure and properties of alkali cobalt double oxides A(0.6)CoO(2) (A = Li, Na, and K).

Lamellar A(x)CoO(2) cobalt double oxides with A = Li, Na, and K (x approximately 0.6) have been synthesized and their chemical (alkali content, oxidation state, and structure) and physical (resistivity, thermopower, magnetization, and specific heat) properties have been studied. All the three materials exhibit strong electron correlation emphasized by their behavior ranging from Fermi liquid to spin-polarized system.

Sodium ion mobility in Na(x)CoO2 (0.6 < x < 0.75) cobaltites studied by 23Na MAS NMR.

Various P2 and P'3-Na(x)CoO(2) phases, with x ranging approximately from 0.6 to 0.75, have been studied by variable-temperature (23)Na magic angle spinning (MAS) NMR.

Real-space pairing through charge transfer excitons in high-TC cuprates.

While approaching a Mott-Hubbard transition by hole doping of the pristine La(2)CuO(4) cuprate, excitons are created because of exciton-exciton and exciton-doping hole stabilizing interactions. Here, excitons are of charge-transfer Frenkel-type, with effective Cu(+)O(-) electrical dipoles that solvate the doping charges. Assuming a moderate screening by charge carriers, we show that mobile exciton-solvated doping holes should be associated in pairs either by a deep energy well or as thermodynamically stable pairs that can glide in the [100] or [010] direction after Bose condensation.

Status of trivalent copper and charge-transfer excitons in high-TC cuprates.

A chemical bonding approach based on tight-binding cluster and band calculations, taking into account on-site Coulomb repulsion (Hubbard U parameter) to differentiate doubly and singly occupied states, was applied to high- T C superconducting cuprates and related compounds. This work provides rational insight and explanations for issues such as (i) the actual oxidation number Cu (I+) for formally trivalent copper in oxides such as La 2Li 1/2Cu 1/2O 4, (ii) the dominant oxygen character of the doping holes in (CuO 2) ( n- ) planes, (iii) the Mott-Hubbard character of the insulator-to-metal transition triggered by hole doping, leading to an oxygen-to-copper charge transfer of avalanche type, (iv) the occurrence of an excitonic phase with anisotropic Frenkel-type excitons, (v) the role of Coulomb interactions between excitons and between doping holes and their exciton surroundings, and (vi) the on-time pairing of doping holes by means of an "excitonic glue".

First experimental evidence of potassium ordering in layered k4co7o14.

The layered P2-K4Co7O14 oxide has been prepared and characterized by means of X-ray diffraction, electrical conductivity, thermopower, and magnetic measurements. The crystal structure of K4Co7O14 (P6(3)/m space group, Z=2, a=7.5171(1) A, and c=12.