Fluctuations and depinning of stripes at acoustic frequencies in La 2− xSr xCuO 4

Abstract

The low frequency elastic response of La 2− x Sr x CuO 4 displays an absorption process in correspondence with the freezing into the cluster spin glass state, that has been identified with fluctuations of the walls between the antiferromagnetic spin clusters. At these low temperatures (<10 K) the domain walls, which are thought to coincide with the charge stripes, are pinned by the Sr dopants. The process of depinning of the stripes is identified with an additional absorption maximum around 80 K, whose intensity: (1) drops at x>0.045, where the stripe order changes from parallel to diagonal; (2) has a local maximum at x=1/8; (3) vanishes in the overdoped region, x=0.2. The effective energy barrier for depinning has been deduced from the frequency dependence of the absorption, and exhibits the same dependence on doping as the logarithm of the absorption intensity. This is taken as evidence that pinning and stripe-strain interaction are mediated by the octahedral tilts, and larger atomic displacements result both in larger relaxation strength and larger pinning energy.