Relaxational lattice dynamics in La2-xSrxCuO4 and possible connection with the stripes in cuprate superconductors

The dynamic Young's modulus of La2-xSrxCuO4 (x = 0.022, 0.032) has been measured down to 1.5 K, in order to see the effect of doping on the intrinsic lattice fluctuations found in stoichiometric La2CuO4, and identified with tilting relaxation modes of the O octahedra. The relaxation process occurring at lower temperature drastically shifts to still lower temperatures, implying an enhancement of the corresponding relaxation rate with doping. This observation is in accordance with the hypothesis of tunnelling-driven pseudodiffusive switching of the octahedra between minima of a multiwell potential, with the distances and barriers between minima decreasing upon doping. The present measurements, together with a model of the excitations of a planar array of octahedra, provide a possible scenario for the evolution of the stripe phase in the cuprate superconductors at low doping.