Anomalous weak magnetism in superconducting YBa2Cu3O6+x

For some time now, there has been considerable experimental and theoretical effort to understand the role of the normal-state "pseudogap" phase in und erdoped high-temperature cuprate superconductors. Recent debate has centere d on the question of whether the pseudogap is independent of superconductiv ity. We provide evidence from zero-field muon spin relaxation measurements in YBa2Cu3O6+x for the presence of small spontaneous static magnetic fields of electronic origin intimately related to the pseudogap transition. Our m ost significant finding is that, for optimal doping, these weak static magn etic fields appear well below the superconducting transition temperature. T he two compositions measured suggest the existence of a quantum critical po int somewhat above optimal doping.