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

TL;DR

This study uses $^{17}$O NMR to investigate the presence of static orbital current magnetism at the apical oxygen site in HgBa$_2$CuO$_{4+eta}$, finding no evidence for static order and suggesting observed moments are fluctuating.

Contribution

It provides the first NMR evidence ruling out static orbital currents at the apical oxygen site in Hg1201, challenging some theoretical models of the pseudogap phase.

Findings

No static local fields detected at the apical site in the pseudogap phase.

NMR shifts consistent with dipolar fields from Cu$^{2+}$, not orbital currents.

Moments observed by neutrons are likely fluctuating, not static.

Abstract

The simple structure of HgBa$_2$CuO$_{4+\delta}$ (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.