ABSTRACT
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. Our results show that both the dimensionality of the interactions and the nature of the alkali play a determining role on the properties.
ABSTRACT
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. Signal modification versus temperature plots clearly show that Na(+) ions are not totally mobile at room temperature on the NMR time scale. As the temperature increases, the line shape change of the (23)Na MAS NMR signal differs for the P2 and P'3 stackings and is interpreted by the differences of Na(+) ion sites and of sodium diffusion pathways in the two structures.
ABSTRACT
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. Exciton-exciton dipolar interactions constitute thus the "pairing glue" in this model, which is based on instantaneous interactions and intrinsically differs from the previous excitonic models, in which BCS virtual phonons were replaced by virtual excitons.
ABSTRACT
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".
ABSTRACT
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=12.371(1) A) consists of a stacking of slabs of edge-shared CoO6 octahedra with K+ ions occupying ordered positions in the interslab space, leading to a a0 radical7xa0 radical7 supercell. Potential energy calculations at 0 K are in good agreement with the ordered distribution of potassium ions in the (ab) plane. This oxide is metallic, and the magnetic susceptibility is of Pauli-type, which contrasts with the Curie-Weiss behavior of the homologous NaxCoO2 (x approximately 0.6) oxide with close alkali content. The thermopower at room temperature is about one-third that of polycrystalline Na0.6CoO2.
