PHYSICA Doping Dependence ofthe Magnetic Resonance Peak in YBa2Cu3OB+x

We report inelastic neutron scattering experiments on the doping dependence of the energy and spectral weight of the sharp magnetic resonance peak in YBa~CusO6+~. These measurements also shed light on the relationship between the magnetic excitations in the normal and superconducting states. Magnetic excitations in high temperature su-perconductors have been intensively studied ex-perimentally and theoretically for a number of years as they provide a direct and incisive probe of correlation effects in the cuprates. These ef-forts have been redoubled after the discovery of a sharp magnetic ollective mode in YBa2CusOr by inelastic neutron scattering [1-3]. This mode is strongly coupled to superconductivity n this material; in fact, it is only present in the super-conducting state and disappears in the normal state [1]. Two different mechanisms, with vari-ous modifications, have been proposed to explain this observation. First, it may be a consequence of the pileup of electronic states above the super-conducting energy gap which compensates for the loss of states below the gap. Both a d-wave BCS gap function with strong Coulomb correlations [4-6] and the (non-BCS) gap function resulting from the interlayer pair tunneling model of su-perconductivity [7]can account for the sharpness of the mode in both wavevector q and energy tun. Second, superconductivity may provide a matrix element (through particle-hole mixing) that cou-ples a preexisting collective mode to the external probe, magnetic neutron scattering [8]. Further experimental information is clearly necessary in order to distinguish between these fundamentally different interpretations. Since the doping dependence of the superconducting energy gap has recently been determined independently *Supported by NSF-DMR94-00362