Incommensurate Magnetic Fluctuations in YBa2Cu3O6.6

We use inelastic neutron scattering to demonstrate that the low-frequency magnetic fluctuations in YBa2Cu3O6.6 (T-c = 62.7 K) change from commensurate to incommensurate on cooling with the incommensurability first appearing at temperatures above T-c. For the energies studied, the susceptibility at incommensurate positions increases on cooling below T-c, accompanied by a suppression of the spin fluctuations at the commensurate points. These results suggest that incommensurate spin fluctuations may be a common feature for all cuprate superconductors.