Pseudo-spin-gap and slow spin fluctuation in La_2-xSr _xCuO₄ (x=0.13 and 0.18) via ⁶³Cu and ¹³⁹La nuclear quadrupole resonance

We analyzed nonexponential ⁶³Cu nuclear spin-lattice relaxation curves for ⁶³Cu-enriched high-T_c superconductors: La _2-xSr_xCuO₄ with x=0.13 (slightly underdoped) and 0.18 (slightly overdoped), and studied the applicability of an impurity-induced nuclear spin-lattice relaxation theory. We found a remnant of pseudo-spin-gap effect on the host ⁶³Cu nuclear spin-lattice relaxation time and slow inhomogeneous spin fluctuation via the impurity-induced relaxation time. The effect of slow spin dynamics was also observed in ¹³⁹La nuclear spin-lattice relaxation. The inhomogeneous electron-spin fluctuation, which is associated with randomly distributed staggered moments on the CuO₂ plane, smears the pseudo-spin-gap. The fact that the optimal T_c ∼38 K is smaller than T _c∼96 K of HgBa₂CuO_4+δ can be attributed to the depairing effect due to the slow spin fluctuation.