Spin fluctuations in superconducting cuprates (invited)

Publisher Website
Google Scholar
Abstract
Our recent reports of highly energetic spin fluctuations in cuprate insulators closely related to high‐Tc superconductors have raised the possibility that the strong spin interactions thus manifested may mediate the superconducting pairing. In La2CuO4 the exchange measured by light scattering is 1100 cm−1, while in Ba2YCu3O6 it is 950 cm−1. In the case of Ba2YCu3O6+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 shall (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 Eu2CuO4, La2NiO4, and the superconducting phase of Ba2YCu3O6.7 (for T<Tc =60 K). Although the newest data show quite unequivocally that the spin fluctuations persist in the superconductor, there is as yet no detailed explanation of the observed line shape. Certainly, a simple first‐order calculation for a spin‐half Heisenberg square lattice gives a peak that is far narrower than that observed. Although a calculation based on a resonating valence bond model describes our line shape in insulating La2CuO4, other evidence contravenes this interpretation. It is thus not yet possible to say definitively whether the magnetic interaction is the root cause of the high‐Tc phenomena. What is clear is that our data represent a clear and well‐defined challenge for any detailed theoretical description of the cuprates.