Glassy Slowing of Stripe Modulation in (La,Eu,Nd)(2-x)(Sr,Ba)(x)CuO4: A Cu-63 and La-139 NQR Study Down to 350mK

We report a comprehensive Cu and La NQR study down to 350mK in the stripe phase of the hole-doped high temperature superconductors with hole concentration approximately 1/8. We observe the hyperfine broadened NQR lineshape only below 8-20K for x=1/8, indicating that the ordered moments continue to fluctuate slowly even below the temperature range where they appear static in other measurements. Detailed numerical simulations of the Zeeman perturbed Cu NQR lineshape establish that widely accepted models of periodic modulation of spin density waves with maximum moment approxiamtely 0.3mu_B can not account for the NQR lineshape. These results suggest that significant disorder exists at NQR time scales. By fitting a single model to several sets of experimental data, we deduce the spatial distribution of the spin fluctuation frequency scale and its temperature dependence. Our results indicate that as soon as charge dynamics slow down, the spin fluctuationsbegin to slow dramatically with spin stiffness approximately 200K. However, the motional narrowing feature observed in the Zeeman perturbed NQR measurements suggest that the slowing of the spin fluctuations levels off at frequencies somewhat below the NQR frequency. By extending the analysis to other hole concentrations, we find a qualitative difference above and below x=1/8.