Chain metallicity and competition between paramagnetism and antiferromagnetism in underdoped YBa2Cu3O6+x: A first principles description

We describe, from advanced first principles calculations, the energetics of oxygen doping and its relation to insulator-metal transitions in underdoped YBa2Cu3O6+x. We find a strong tendency of doping oxygens to order into nonmagnetic Cu1+Ox chains at any x. Ordering produces one-dimensional metallic bands, while configurations with nonaligned oxygens are insulating. The Cu2+O2 planes remain insulating and antiferromagnetic up to a threshold between x=0.25 and 0.5, above which a paramagnetic normal-metal state prevails. The in-plane antiferroparamagnetic competition depends on x but only weakly on the ordering state of the chains.