Two-magnon Raman scattering in cuprates: Evidence for antiferromagnetic fluctuations in cuprate superconductors

Abstract

We report observation and resonance-Raman studies of spin-pair excitations in Y Ba 2 Cu 3 O 6+ x (0.37 < x < 0.7) and YBa 2 Cu 4 O 8 superconductors. These citations at energy ∼ 3 J (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, and in the overdoped TlBa 2 CuO 6 superccconductors the two-magnon peak can no longer be seen above the flat Raman continuum.