Local two-dimensional ferromagnetic ordering as an origin of the metal-semiconductor transition in layered YBa 2Cu 3O 6+ x crystals

Abstract

It is shown that the metal-semiconductor transition at the concentration point x c = 0.4 has its origin in a local ferromagnetic (FM) ordering of the copper spins below x c . As was earlier shown experimentally [S.L. Gnatchenko et al., Phys. Rev. B 55 (1997) 3876], such FM ordering takes place within long-distance potential wells, where oxygen holes are concentrated, due to their spin polarization and interaction with the copper ions. This phenomenon is restricted to the oxygen content 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 potential profile is regular enough to provide metal properties and superconductivity. But in the region x < x c , where the hole cluster interaction energy strongly depends on the direction and size of the FM clusters, the long-distance potential profile becomes highly irregular, destroying superconductivity and giving a semiconductor character to the transport properties.