A comprehensive inelastic neutron scattering study of magnetic excitations in the near-optimally doped high-temperature superconductor ${ \rm YBa_{ 2}Cu_{ 3}O_{ 6.85}}$ is presented. The spin correlations in the normal state are commensurate with the crystal lattice, and the intensity is peaked around the wave vector characterizing the antiferromagnetic state of the insulating precursor ${ \rm YBa_{ 2}Cu_{ 3}O_{ 6}}$. Profound modifications of the spin excitation spectrum appear abruptly below the superconducting transition temperature, $\rm T_C$, where a commensurate resonant mode and a set of weaker incommensurate peaks develop. The data are consistent with models based on an underlying two-dimensional Fermi surface that predict a continuous, downward dispersion relation connecting the resonant mode and the incommensurate excitations. The magnetic incommensurability in the ${ \rm YBa_{ 2}Cu_{ 3}O_{ 6+x}}$ system is thus not simply related to that of another high temperature superconductor, ${ \rm La_{ 2-x}Sr_xCuO_{ 4}}$, where incommensurate peaks persist well above $\rm T_C$. The temperature dependent incommensurability is difficult to reconcile with interpretations based on charge stripe formation in ${ \rm YBa_{ 2}Cu_{ 3}O_{ 6+x}}$ near optimum doping.
Categories: cond-mat.supr-con cond-mat.str-el