Anelastic and 139La NQR relaxation study of La2-xSrxCuO4 around the critical Sr content x = 0.02

Anelastic relaxation and 139La NQR relaxation measurements in La2-xSrxCuO4 with x ? 0.02 are discussed in terms of spin and lattice excitations, ordering processes and thermal depinning of charged stripes. Two possible scenarios are considered. On one hand, the phase diagram of La2-xSrxCuO4 at the boundary xc ~ 0.02 between the antiferromagnetic (AF) and the spin-glass phase could be more complicate than previously thought, and a peak of magnetic origin in the NQR rate near 150 K could signal an unconventional transition to a quasi-long range ordered state. On the other hand, the 139La NQR spectra favour a transition to a conventional AF phase around 50 K. In this case, the peak of the NQR rate around 150 K at 12 MHz would correspond to an anelastic peak around 80 K in the kHz range, and they would constitute the first direct evidence of the freezing of charged stripes involving simultaneously lattice and spin excitations. The barrier of thermal depinning and the frequencies of the stripes motions are thus estimated.