Anelastic relaxation and ${}^{139}$La NQR in La${}_{2-x}$Sr${}_x$CuO${}_4$ around the critical Sr content x=0.02

Abstract

Anelastic relaxation and ${}^{139}$La NQR relaxation measurements in La${}_{2-x}$Sr${}_x$CuO${}_4$ for Sr content x around 2 and 3 percent, are presented and discussed in terms of spin and lattice excitations and ordering processes. It is discussed how the phase diagram of La${}_{2-x}$Sr${}_x$CuO${}_4$ at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previous thought, with a transition to a quasi-long range ordered state at T = 150 K, as indicated by recent neutron scattering data. On the other hand, the ${}^{139}$La NQR spectra are compatible with a transition to a conventional AF phase around T = 50 K, in agreement with the phase diagram commonly accepted in the literature. In this case the relaxation data, with a peak of magnetic origin in the relaxation rate around 150 K at 12 MHz and the anelastic counterparts around 80 K in the kHz range, yield the first evidence in La${}_{1.98}$Sr${}_{0.02}$CuO${}_4$ of freezing involving simultaneously lattice and spin excitations. This excitation could correspond to the motion of charged stripes.

10 pages, 8 figures