Absence of Static Loop-Current Magnetism at the Apical Oxygen Site in HgBa2CuO4+δ from NMR.

The simple structure of HgBa2CuO4+δ (Hg1201) is ideal among cuprates for study of the pseudogap phase as a broken symmetry state. We have performed 17O nuclear magnetic resonance on an underdoped Hg1201 crystal with a transition temperature of 74 K to look for circulating loop currents proposed theoretically and inferred from neutron scattering. The narrow spectra preclude static local fields in the pseudogap phase at the apical site, suggesting that the moments observed with neutrons are fluctuating. The nuclear magnetic resonance frequency shifts are consistent with a dipolar field from the Cu2+ site. [ABSTRACT FROM AUTHOR]