Abstract
For two HTSC materials, Y1−x Ca x Ba2Cu3 O 6.7 and Tl0.8Hg0.2Ba2Ca2Cu3 O 8.15, their crystal structure has been studied earlier in the temperature interval 100–300 K. It is shown that for both compounds, the height of CuO5 pyramids has a minimum in temperature interval from ∼ 150 to ∼ 250 K. The electronic structure is calculated using first principles based on the obtained data of crystal structure. It is revealed that a decrease in the CuO5 pyramid height leads to the appearance of the electronic state density peak with an energy of 0.3–0.4 eV under Fermi level. Localized states of barium and apical oxygen make the main contribution to this peak. By reduction the temperature below ∼ 150 K, these localized electronic states disappear, and under continuous cooling, the superconductivity appears.
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