Rotating antiferromagnetism in high-temperature superconductors

In order to address the issue of the puzzling properties of high-temperature superconductors in the underdoped regime, we investigated the effect of on-site Coulomb repulsion on superconductivity by proposing a two-dimensional disordered quantum state that is characterized by rotating antiferromagnetism. The underdoped and overdoped phases are found to be separated by a quantum critical point where the rotating order parameter vanishes. This point is practically equal to the optimal doping density. Our calculations of the doping dependence of the ratio R=2E(d)/k(B)T(C) shows a sharp change at the quantum critical point and accounts for the experimental variations of R in YBCO systems for example. Several other quantities are evaluated and compared to experimental data with fairly good agreement.