Anisotropic damping and wave vector dependent susceptibility of the spin fluctuations in La2-xSrxCuO4 studied by resonant inelastic x-ray scattering

We report high-resolution resonant inelastic x-ray scattering (RIXS) measurements of the collective spin fluctuations in three compositions of the superconducting cuprate system La2-xSrxCuO4. We have mapped out the excitations throughout much of the two-dimensional (h,k) Brillouin zone. The spin fluctuations in La2-xSrxCuO4 are found to be fairly well described by a damped harmonic oscillator model, thus our data allows us to determine the full wave vector dependence of the damping parameter. This parameter increases with doping and is largest along the (h,h) line, where it is peaked near (0.2,0.2). We have used a new procedure to determine the absolute wave vector dependent susceptibility for the doped compositions La2-xSrxCuO4(x=0.12,0.16) by normalizing our data to La2CuO4 measurements made with inelastic neutron scattering (INS). We find that the evolution with doping of the intensity of high-energy excitations measured by RIXS and INS is consistent. For the doped compositions, the wave vector dependent susceptibility is much larger at (14,14) than at (12,0). It increases rapidly along the (h,h) line towards the antiferromagnetic wave vector of the parent compound (12,12). Thus, the strongest magnetic excitations, and those predicted to favor superconductive pairing, occur towards the (12,12) position as observed by INS.