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 (3d(9)) 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, the opposite 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, La2-xSrxCuO4 (refs 1-3) and YBa2Cu3O7-x (refs 4-6), whose respective building blocks are CuO2 layers and bilayers. Here we report two-dimensional neutron-scattering imaging of YBa2Cu3O6.6, which reveals that the low-frequency magnetic excitations are virtually identical to those of similarly doped La2-xSrxCuO4. Thus, the high-temperature (T-c less than or similar to 92 K) 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 (<40 K) single-layer La2-xSrxCuO4 family.