LOCAL 2-DIMENSIONAL FERROMAGNETIC ORDERING AS AN ORIGIN OF THE METAL-SEMICONDUCTOR TRANSITION IN LAYERED YBA2CU3O6+X CRYSTALS

It is shown that the metal-semiconductor transition at the concentrati on point x(c) = 0.4 has its origin in a local ferromagnetic (FM) order ing of the copper spins below x(c). As was earlier shown experimentall y [S.L. Gnatchenko et al., Phys. Rev. B 55 (1997) 3876], such FM order ing takes place within long-distance potential wells, where oxygen hol es are concentrated, due to their spin polarization and interaction wi th the copper ions. This phenomenon is restricted to the oxygen conten t region x < x(c). For x > x(c), all the long-distance potential wells , devoid of FM clusters, are nearly identical, so that the hole potent ial profile is regular enough to provide metal properties and supercon ductivity. But in the region x < x(c), where the hole cluster interact ion energy strongly depends on the direction and size of the FM cluste rs, the long-distance potential profile becomes highly irregular, dest roying superconductivity and giving a semiconductor character to the t ransport properties. (C) 1998 Elsevier Science B.V.