Ultraslow fluctuations in the pseudogap states ofHgBa2CaCu2O6+δ

We report the transverse relaxation rates $1/{T}₂$'s of the $⁶³\mathrm{Cu}$ nuclear spin-echo envelope for double-layer high-${T}_{\mathrm{c}}$ cuprate superconductors ${\mathrm{HgBa}}₂{\mathrm{CaCu}}₂{\mathrm{O}}_{6+\ensuremath{\delta}}$ from underdoped to overdoped. The relaxation rate $1/{T}_{2\mathrm{L}}$ of the exponential function (Lorentzian component) shows a peak at $220--240$ K in the underdoped (${T}_{\mathrm{c}}=103$ K) and the optimally doped (${T}_{\mathrm{c}}=127$ K) samples but no peak in the overdoped (${T}_{\mathrm{c}}=93$ K) sample. The enhancement in $1/{T}_{2\mathrm{L}}$ suggests a development of the zero frequency components of local field fluctuations. Ultraslow fluctuations are hidden in the pseudogap states.