Two-magnon Raman scattering in cuprate superconductors: Evolution of magnetic fluctuations with doping.

Abstract

We report observation and resonance Raman studies of spin-pair excitations in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ (0.37x0.7) and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ superconductors. These excitations at energy \ensuremath{\sim}3J (2700\ifmmode\pm\else\textpm\fi{}150 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$), similar to two-magnon excitations in insulators, are evidence that antiferromagnetic fluctuations with spatial extent of at least three lattice constants are not overdamped in the underdoped superconductors. With optimal hole dopings, where ${\mathit{T}}_{\mathit{c}}$>80 K, the two-magnon peak can no longer be seen above the flat Raman continuum. Photoluminescence and the origin of the broad-spectrum Raman continuum are also discussed.