Two-component uniform spin susceptibility of superconducting HgBa ₂CuO _4+δ single crystals measured using 63Cu and 199Hg nuclear magnetic resonance

63Cu and 199Hg nuclear magnetic resonance shifts for an optimally doped and underdoped HgBa ₂CuO _4+δ single crystal are reported, and the temperature dependence dictates a two-component description of the uniform spin susceptibility. The first component, associated with the pseudogap phenomenon in the NMR shifts, decreases at room temperature and continues to drop as the temperature is lowered, without a drastic change at the transition temperature into the superconducting state. The second component is temperature independent above the superconducting transition temperature and vanishes rapidly below it. It is a substantial part of the total T-dependent susceptibility measured at both nuclei.