Comparison of the High-Frequency Magnetic Fluctuations in Insulating and SuperconductingLa2−xSrxCuO4

Inelastic neutron scattering performed at a spallation source is used to make absolute measurements of the dynamic susceptibility of insulating L${\mathrm{a}}_2$Cu${\mathrm{O}}_4$ and superconducting L${\mathrm{a}}_{1.86}$S${\mathrm{r}}_{0.14}$Cu${\mathrm{O}}_4$ over the energy range $15\le ħ\omega \le 350$ meV. The effect of Sr doping on the magnetic excitations is to cause a large broadening in the wave vector and a substantial change in the spectrum of the local spin fluctuations. Comparison of the two compositions reveals a new energy scale $ħ\gamma =22\mathrm{}\pm 5$ meV in L${\mathrm{a}}_{1.86}$S${\mathrm{r}}_{0.14}$Cu${\mathrm{O}}_4$.