We present results of highly precise local density calculations of the electronic structure of the two high-Tc superconductors Tl2Ba2CaCuO8 (Tc≅112 K) and Tl2Ba2Ca2Cu3O10 (Tc≅125 K), as obtained with the full potential linearized augmented plane wave (FLAPW) method. A relatively simple band structure is found near EF and strong 2D properties are predicted. Again as in the case of the other high-Tc materials, Ba and Ca are highly ionic, with the Ca2+ ions insulating the Cu-O planes. The Tl-O complexes, which are decoupled from the Cu-O planes, produce small electron pockets at EF 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 EF, similar to the Bi2Sr2CaCu2O8 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 Tc. As in the case of the other high-Tc 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.
ASJC Scopus subject areas
- Condensed Matter Physics