Spin spirals in underdoped La2-xSrxCuO4 and YBa2Cu3O6+y: Differences and similarities

Contrary to a widely accepted view the phase diagrams of La2-xSrxCuO4 (LSCO) and YBa2Cu3O6+y (YBCO), in spite of similarities are remarkably different. Both the electric conduction properties and the commensurate/incommensurate spin ordering properties differ dramatically. It is argued that the role of disorder in YBCO is insignificant while the bilayer structure is crucial. On the other hand in LSCO the intrinsic disorder to a large extent drives the properties of the system. The developed approach explains the low-temperature magnetic properties of the systems. The most important point is the difference with respect to the incommensurate spin ordering, including the difference in the incommensurate pitches. The understanding of mechanisms for the differences provides a deep insight into generic physics of the systems.