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 ${\mathrm{HgBa}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{6+\delta }$ 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.