Doping Dependence of the Magnetic Resonance Peak in YBa2Cu3O6+x

Magnetic excitations in high temperature superconductors have been intensively studied experimentally and theoretically for a number of years as they provide a direct and incisive probe of correlation effects in the cuprates. These efforts have been redoubled after the discovery of a sharp magnetic collective mode in YBa2Cu3O7 by inelastic neutron scattering [1–3]. This mode is strongly coupled to superconductivity in this material; in fact, it is only present in the superconducting state and disappears in the normal state [1]. Two different mechanisms, with various modifications, have been proposed to explain this observation. First, it may be a consequence of the pileup of electronic states above the superconducting 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 superconductivity [7] can account for the sharpness of the mode in both wavevector q and energy h̄ω. Second, superconductivity may provide a matrix element (through particle-hole mixing) that couples 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