2-MAGNON RAMAN-SCATTERING IN CUPRATE SUPERCONDUCTORS - EVOLUTION OF MAGNETIC FLUCTUATIONS WITH DOPING

We report observation and resonance Raman studies of spin-pair excitations in YBa2Cu3O6+x (0.37 < x < 0.7) and YBa2Cu4O8 superconductors. These excitations at energy approximately 3J (2700 +/-150 cm-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 doping, where T(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.