Two-magnon Raman scattering in (${\mathrm{La}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${)}_2$${\mathrm{CuO}}_4$

The two-dimensional two-magnon peak in (${\mathrm{La}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${)}_2$ ${\mathrm{CuO}}_4$ decreases rapidly from x=0 to x=0.01. For x>0.03 scattering is observed over a very wide energy region from low energy to over 4000 ${\mathrm{cm}}^{\mathrm{-}1}$. This indicates that the correlation length of the two-dimensional spin order decreases rapidly from x=0 to x=0.01. This decrease can be related to the rapid decrease of the three-dimensional antiferromagnetic transition temperature ($T_N$), while at x>0.03 the correlation still retains a short length and a short time. At x<0.01 the scattering is in a resonant state with narrow electronic levels of which transition energy is about 18 000 ${\mathrm{cm}}^{\mathrm{-}1}$. The narrowness of these energy levels, and the simultaneous extinction of resonance and the two-magnon scattering, suggest that these are localized levels relating to the Cu spins.