Tilt-wave dynamics of the oxygen octahedra in La2CuO4 from anelastic and 139La NQR relaxation

The anharmonic vibrational dynamics in nearly stoichiometric La2CuO4+delta is studied by means of anelastic and La-139 NQR relaxation. Tn the absorption component of the elastic susceptibility as well as in the nuclear relaxation rate a peak is detected as a function of temperature, and a relaxation time tau=1.7 x 10(-12) exp[(2800 K)/T] a is derived. The relaxation processes are attributed to tilt motion of the CuO6 octahedra in double-well potentials, whose cooperative character increases the effective energy barrier to the observed value; The analysis of the relaxation mechanisms has been carried out by reducing the dynamics of the interacting octahedra to a one-dimensional equation of motion. The solitonlike solutions correspond to parallel walls separating domains of different tilt patterns and give rise to pseudodiffusive modes which appear as a central component in the spectral density of the motion of the octahedra. The tilt waves may be considered to correspond to the dynamical lattice stripes observed in La-based and Bi-based high-T-c superconductors. [S0163-1829(99)04218-6].