Unconventional consideration of the symmetry properties—New techniques and computer codes: Application for high‐temperature superconductors

The conventional technique for solving the equations of quantum chemistry is extended to the structures possessing certain symmetries. This proposal allows the release of unoccupied electronic states located lower than the ground‐state Fermi level of a specific system. Such states can be treated as “spectral holes.” Application of this technique, in particular, when calculating the electronic structure of the high‐temperature superconductor (HTSC) compound YBa₂Cu₃O_{7? δ} (0 ≤ δ ≤ 1) results in the following. For all versions of the examined charge distributions over a crystal lattice, spectral holes of high spatial localization are found. The “spatial spectral holes” are located mainly at the p_y‐orbitals of the apex oxygens. These orbitals overlap and form linear chains that are parallel to the known Cu(1)? O chains, disappearing when δ is close to 1. One can suppose that the linear chains of the overlapping hole states form a “superconducting channel.” Some other parameters closely related to the critical characteristics of HTSC materials are also calculated. The calculations show that the superconducting channel is broken when the oxygen chain atoms O(1) are removed (δ > 0). © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002