Coherent charge and spin density waves in underdoped HgBa₂CuO_4+δ

Charge order in cuprate superconductors appears to be a universal characteristic, often associated with pseudogap behavior in the normal state. The central question is whether such charge ordering or the pseudogap are required for the existence of high temperature superconductivity and embody its mechanism. An important but phenomenological approach to this question is to examine whether these phenomena extend over various members of the cuprate family. Recent nuclear magnetic resonance (NMR) measurements on oxygen chain-ordered single crystals of YBa₂Cu₃O_6+y (Y123) have demonstrated temperature and magnetic field induced charge ordering that was confirmed in x-ray experiments. In the present work on high-quality single crystals of the tetragonal compound, HgBa₂CuO_4+δ, we use ¹⁷O NMR to investigate the interplay between charge and spin order deduced from the full quadrupolar-split NMR spectrum over a wide range of temperature and magnetic field. We have found evidence for a coherent modulation of charge and spin order in this compound. However, neither temperature nor magnetic field induced ordering was observed and we infer that this aspect of high temperature superconductivity is not universal.