Similar glassy features in the L 139 a NMR response of pure and disordered La1.88 Sr0.12 CuO4

High Tc superconductivity in La2-x Srx CuO4 coexists with (striped and glassy) magnetic order. Here, we report NMR measurements of the L 139 a spin-lattice relaxation, which displays a stretched-exponential time dependence, in both pure and disordered x=0.12 single crystals. An analysis in terms of a distribution of relaxation rates T 139 1 -1 indicates that: (i) the spin-freezing temperature is spatially inhomogeneous with an onset at Tg onset =20 K for the pristine samples and (ii) the width of the T1 -1 distribution in the vicinity of Tg onset is insensitive to an ∼1% level of atomic disorder in CuO2 planes. This suggests that the stretched-exponential L 139 a relaxation, considered as a manifestation of the system's glassiness, may not arise from quenched disorder.