Raman Probes of Spin Fluctuation Dynamics in Superconducting Cuprates

Magnetic Raman measurements in a wide range of cuprate materials have manifested the presence of high energy spin fluctuations, with a characteristic energy near 3000 cm sup (-1), corresponding to an exchange interaction energy near 1500 K. Two of the key questions surrounding these measurements concern (i) whether the spin fluctuations coexist with superconductivity and (ii) whether any anomalies in the associated scattering may be observed at T sub c. Spectra of carefully annealed samples of Ba sub 2 YCu sub 3 O sub (6+x) (x~0.7, T sub c~60 K) show simultaneous presence of the superconducting gap and the spin fluctuations at low temperatures. This together with IR reflectivity measurements on the same samples, provides evidence for the coexistence of spin correlations and superconductivity. Moreover, the spectra reveal a surprising decrease in the scattering intensity of a broad continuum scattering component on cooling through T sub c. Recent studies of insulating samples to higher energy shift have revealed the presence of A sub (1g) modes of still higher energy, up to 5500 cm sup (-1), which might be related to the observed continuum scattering in the superconductor. related to the observed continuum scattering in the superconductors.