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

Anelastic relaxation and La-139 NQR relaxation measurements in La2-xSrxCuO4 with x similar or equal to 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 La 2-xSrxCuO4 at the boundary x(c) similar to 0.02 between the antiferromagnet ic (AF) and the spin-glass phase could be more complicate than previously t hought, and a peak of magnetic origin in the NQR rate near 150 K could sign al an unconventional transition to a quasi-long range ordered state. On the other hand, the La-139 NQR spectra favour a transition to a conventional A F 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 cha rged stripes involving simultaneously lattice and spin excitations. The bar rier of thermal depinning and the frequencies of the stripes motions are th us estimated.