Spin fluctuations in superconducting cuprates.

Our recent reports of highly energetic spin fluctuations in cuprate insulators closely related to high-T sub c superconductors have raised the possibility that the strong spin interactions thus manifested may mediate the superconducting pairing. In La sub 2 CuO sub 4 the exchange measured by light scattering is 1100 cm sup (-1), while in Ba sub 2 YCu sub 3 O sub 6 it is 950 cm sup (-1). In the case of Ba sub 2 YCu sub 3 O sub (6+x) it was also shown that the fluctuations persist at room temperature when carriers have been introduced to make the material superconducting. The fluctuations in this case exhibit a broad spectrum shifted to lower frequency, with no clearly defined peak in the spectrum. In this paper, we (1) review the theory of two-magnon light scattering and the selection rules applicable to the cuprates, (2) present the data described above, and (3) present data on other related systems, including Eu sub 2 CuO sub 4, La sub 2 NiO sub 4, and the superconducting phase of Ba sub 2 YCu sub 3 O sub (6.7) (for T