THEORETICAL EVIDENCE FOR THE CORRELATION BETWEEN HOLE DENSITY AND T(C) IN HIGH-T(C) TL-BASED SUPERCONDUCTORS

A tight-binding calculation has been performed for determining the electronic structure of the Tl-monolayer superconducting systems TIBa2Can-1CunO2n+3, for n = 1-6 and the Tl-bilayer systems Tl2Ba2Can-1CunO2n+4, for n = 1-4. In the TI-monolayer systems, the antibonding Tl(6s)-O3(p(z)) band appears well above the hybridized Cu(dx2-y2)-O1(px,y)) bands throughout the Brillouin zone and remains unoccupied. On the other hand, in the Tl-bilayer systems, one of the antibonding Tl(6s)-O3(p(z)) band appears well above the hybridized Cu(dx2-y2)-O1(px,y) bands throughout the whole Brillouin zone and is unoccupied. However, some parts of the other Tl(6s)-O3(P(z)) antibonding band either touch the Fermi surface or cross it. The valences of the atoms lying in the Ba-O layers and in the Ca layer are quite similar in both the Tl-monolayer and -bilayer systems. In the Tl-monolayer systems, the number of electrons residing on one Cu atom increases with the number of the CuO2 layers in contrast to the Tl-bilayer systems, where the electrons on the Cu atoms are larger and do not change with the number of Cu-O2 layers. On the other hand, the number of electrons on O atoms of the CuO2 layer are similar in both the systems. The Tl valences in the Tl-monolayer and -bilayer systems are seen to be around 2.4 and 1.3, respectively, which indicates a bias towards the respective occurrence of the nearly trivalent Tl+3 and the monovalent Tl+ ions in the Tl-monplayer and -bilayer systems. The number of electrons in the O atoms lying in the Tl-layers is seen to be larger in the monolayer systems as compared to those in the bilayer systems. In both the systems, the hole density on the Cu-d orbitals is seen to be larger than the O-p orbitals lying in the CuO2 layers. The hole density per unit cell at the Cu atom increases with the number of CuO2 layers, a result that is in conformity with the observed enhancement in the transition temperatures T(c) in these phases. The calculated density of states for the T12Ba2Ca2Cu3O10 is in very good agreement with the only available x-ray photoemission spectroscopy and inverse-photoemission-spectroscopy data in terms of the number of location of peaks.