Magnetism and Superconductivity in YBa2Cu3O6+x Superconductors

Perhaps the most puzzling attribute of the high-temperature cuprate superconductors is that for underdoped materials the energy gap normally associated with superconductivity is observed well above the superconducting transition temperature T_c. Neutron scattering measurements of YBa₂Cu₃O_6.6 have identified small moments that increase in strength as the temperature is reduced below the pseudogap temperature T*. Polarized beam neutron scattering measurements demonstrate that the signal is magnetic in origin. Furthermore, the pattern of neutron intensities found in the experiment is unusual as it is not readily understandable in terms of conventional spin magnetism. Although we have considered the origin of the moment in terms of the orbital bond currents, new experimental data suggest that only part of the signal is consistent with this interpretation. We present detail analysis of polarized neutron data and systematic studies of oxygen chain ordering. Finally, we discuss our most recent results on spin-glass phase of Ca-doped YBa₂Cu₃O₆.