Low frequency spin dynamics in undoped and Sr-dopedLa2CuO4

Abstract

We report ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ NQR and NMR studies of low frequency spin dynamics in the paramagnetic state of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ for 0≤x≤0.15 in the temperature range up to 900 K. For x=0, the results were in good agreement with the theoretical predictions for an S=1/2 two-dimensional quantum Heisenberg antiferromagnet. For Sr-doped samples, we found that d spins exhibit, above 300 K, the characteristic behavior expected for a local moment system even for the highest-${\mathit{T}}_{\mathit{c}}$ phase ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$, and cross over to an itinerant regime before superconductivity sets in.