The spin excitation spectrum in superconducting YBa2Cu3O6.85

A comprehensive inelastic neutron scattering study of magnetic excitations in the near optimally doped high-temperature superconductor YBa2Cu3O6.85 is presented. The spin correlations in the normal state are commensurate with the crystal Lattice, and the intensity is peaked around the wave vector ch aracterizing the antiferromagnetic state of the insulating precursor, YBa2C u3O6. Profound modifications of the spin excitation spectrum appear abruptl y below the superconducting transition temperature T-c, where a commensurat e resonant mode and a set of weaker incommensurate peaks develop. The data are consistent with models that are based on an underlying two-dimensional Fermi surface, predicting a continuous, downward dispersion relation connec ting the resonant mode and the incommensurate excitations. The magnetic inc ommensurability in the YBa2Cu3O6+x system is thus not simply related to tha t of another high-temperature superconductor, La2-xSrxCuO4, where incommens urate peaks persist well above T-c. The temperature-dependent incommensurab ility is difficult to reconcile with interpretations based on charge stripe formation in YBa2Cu3O6+x near optimum doping.