INCOMMENSURATE MAGNETIC FLUCTUATIONS IN YBA2CU3O6.6

We use inelastic neutron scattering to demonstrate that the low-freque ncy magnetic fluctuations in YBa2Cu3O6.6 (T-c = 62.7 K) change from co mmensurate to incommensurate on cooling with the incommensurability fi rst 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 com mensurate points. These results suggest that incommensurate spin fluct uations may be a common feature for all cuprate superconductors.