Key Properties of La2CuO4 and YBa2Cu3O6 - Specific Type of Chemical Bonding and Lenear Magnetoelectric Effect, Promoting the Emergence of Superconductivity in Unconventional High-Temperature Superconductors

Two principal physical properties of La2CuO4 and YBa2Cu3O6 are identified, which can promote the emergence of superconductivity in unconventional high-temperature superconductors. The first property is a specific type of chemical bonding stimulating fluctuations of the charge density. To reveal it, it is necessary to carry out calculations of the electronic band structure and find the parameters of critical points in the charge density distribution in the crystal. For calculation of the electronic band structure of YBa2Cu3O6 the same approach was applied, which we earlier used for the study of the electronic band structure of La2CuO4 and high-temperature superconductors: computer program WIEN2k and exchange potential of Becke and Johnston modified by Tran and Blaha. Such approach allowed us to obtain the antiferromagnetic ground state for the compound YBa2Cu3O6 with a band gap Eg = 1.5 eV and a magnetic moment on copper atoms MCu = 0.71 mu B. It turned out that YBa2Cu3O6 compound has a type of chemical bonding similar to that which we found earlier in La2CuO4 and in high-temperature superconductors. Consideration of the symmetry properties of the crystal structures of the La2CuO4 and YBa2Cu3O6 compounds revealed a second characteristic property of them - the existence of magnetic crystal classes that admit a linear magnetoelectric effect. This effect allows the simultaneous existence of local magnetic fields and electric polarization in the sample. Based on two identified characteristic physical properties of the La2CuO4 and YBa2Cu3O6 compounds, a roadmap is proposed for a search of new parent substances for obtaining unconventional high-temperature superconductors.