Tunneling-driven tilt modes of the O octahedra in La2-xSrxCuO4:: Strong dependence on doping

The anelastic spectrum of La2-xSrxCuO4 (x = 0, 0.008, 0.019, 0.032) has been measured down to 1.5 K, in order to see the effect of doping on the intrinsic lattice fluctuations already found in stoichiometric La2CuO4, and identified with tunneling driven tilt modes of the O octahedra. Slight doping with Sr causes a drastic increase of the transition rates and relaxation strength of the: tunneling systems. The influence of doping on the relaxation rate is interpreted in terms of direct coupling between the tilts of the octahedra and the hole excitations. However, the observed fast dependence of the rate on temperature cannot be explained in terms of the usual models of coupling between a tunneling system and the conduction electrons.