Absence of static orbital current magnetism at the apical oxygen site in HgBa$_2$CuO$_{4+��}$ from NMR

The simple structure of HgBa$_2$CuO$_{4+��}$ (Hg1201) is ideal among cuprates for study of the pseudogap phase as a broken symmetry state. We have performed $^{17}$O nuclear magnetic resonance (NMR) on an underdoped Hg1201 crystal with transition temperature of 74 K to look for circulating orbital 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 NMR frequency shifts are consistent with a dipolar field from the Cu$^{+2}$ site.
6 pages, 5 figures