Spin fluctuations in YBa2Cu3O6.6

An important feature of the high-transition-temperature (high-T_c) copper oxide superconductors is the magnetism that results from the spins associated with the incomplete outer electronic shells (3d⁹) of the copper ions. Fluctuations of these spins give rise to magnetic excitations of the material, and might mediate the electron pairing that leads to superconductivity. If the mechanism for high-T_c superconductivity is the same for all copper oxide systems, their spin fluctuations should be universal. But so far, theopposite has seemed to be the case: neutron scattering data reveal clear differences between the spin fluctuations for two major classes of high-T_c materials, La_2-xSr_xCuO₄ (refs 1-3) and YBa₂Cu₃O_7-x (refs 4-6), whose respective building blocks are CuO₂ layers and bilayers. Here we report two-dimensional neutron-scattering imaging of YBa₂Cu₃O_6.6, which reveals that the low-frequency magnetic excitations are virtually identical to those of similarly doped La_2-xSr_xCuO₄. Thus, the high-temperature (Tc <~ 92K) superconductivity of the former materials may be related to spatially coherent low-frequency spin excitations that were previously thought to be unique to the lower-T_c (<40K) single-layer La_2-xSr_xCuO₄ family.