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, 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− x Sr x CuO 4 ( 1 - 3 ) and YBa 2 Cu 3 O 7− x ( 4 - 6 ), whose respective building blocks are CuO 2 layers and bilayers. Here we report two-dimensional neutron-scattering imaging of YBa 2 Cu 3 O 6.6 , which reveals that the low-frequency magnetic excitations are virtually identical to those of similarly doped La 2− x Sr x CuO 4 . Thus, the high-temperature ( T c ≲ 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 (