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

Anelastic relaxation and (139)LaNQR relaxation rates in La2-xSrxCuO4 for Sr content around 2 and 3 percent are discussed in terms of spin and lattice excitations and of the related ordering processes. It is argued how the phase diagram of La2-xSrxCuO4 at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previously thought, with a transition to a quasi-long range ordered state at T similar or equal to 150 K, as indicated by neutron scattering data. On the other hand, the (139)LaNQR spectra are compatible with a transition to an AF phase around T similar or equal to 50 K, in agreement with the phase diagram commonly accepted in literature. In this case the peaks in NQR and anelastic relaxation rates around 150 K and 80 K respectively in La1.98Sr0.02CuO4, yield the first evidence of freezing process involving simultaneously lattice and spin excitations, possibly corresponding to motion of charged stripes.