Electronic structure and properties of the high- Tc superconductors: Tl 2Ba 2CaCu 2O 8 and Tl 2Ba 2Ca 2Cu 3O 10

Abstract

We present results of highly precise local density calculations of the electronic structure of the two high- T c superconductors Tl 2Ba 2CaCuO 8 ( T c ≅112 K) and Tl 2Ba 2Ca 2Cu 3O 10 ( T c ≅125 K), as obtained with the full potential linearized augmented plane wave (FLAPW) method. A relatively simple band structure is found near E F and strong 2D properties are predicted. Again as in the case of the other high- T c materials, Ba and Ca are highly ionic, with the Ca 2+ ions insulating the Cu-O planes. The Tl-O complexes, which are decoupled from the Cu-O planes, produce small electron pockets at E F with predominant O character that contribute to the transport properties. Each Cu-O plane is found to contribute ≃ 1 state/(eV-Cu atom) to the density of states (DOS) at E F, similar to the Bi 2Sr 2CaCu 2O 8 case. Remarkable strong fermi surface nesting is found along the (100) and (010) directions for the 2D Cu-O dpσ bands. Significantly, we show that the usual simple nearest-neighbors only tight-binding model cannot properly describe these states. Crude rigid ion calculations show that a purely electron-phonon mechanism is inadequate to explain the observed high T c. As in the case of the other high- T c superconductors, a charge transfer excitation (excitonic) mechanism appears likely. Interestingly, the partial DOS structure shows that both systems can be viewed as a metal-semiconductor (or metal-semimetal) superlattice indicating a possible relation to the Allender, Bray, and Bardeen model of excitonic superconductivity.