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 ele ctronic 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)) band s throughout the Brillouin zone and remains unoccupied. On the other h and, 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 thro ughout 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 layer s 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 i n contrast to the Tl-bilayer systems, where the electrons on the Cu at oms are larger and do not change with the number of Cu-O2 layers. On t he other hand, the number of electrons on O atoms of the CuO2 layer ar e 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, whi ch indicates a bias towards the respective occurrence of the nearly tr ivalent Tl+3 and the monovalent Tl+ ions in the Tl-monplayer and -bila yer systems. The number of electrons in the O atoms lying in the Tl-la yers is seen to be larger in the monolayer systems as compared to thos e 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 th e observed enhancement in the transition temperatures T(c) in these ph ases. The calculated density of states for the T12Ba2Ca2Cu3O10 is in v ery good agreement with the only available x-ray photoemission spectro scopy and inverse-photoemission-spectroscopy data in terms of the numb er of location of peaks.