Two energy scales in the spin excitations of the high-temperature superconductor La_2-xSr_xCuO₄

The excitations responsible for producing high-temperature superconductivity in the copper oxides have yet to be identified. Two promising candidates are collective spin excitations and phonons1. A recent argument against spin excitations is based on their inability to explain structures observed in electronic spectroscopies such as photoemission and optical conductivity. Here, we use inelastic neutron scattering to demonstrate that collective spin excitations in optimally doped La_2-xSr_xCuO₄ are more structured than previously thought. The excitations have a two-component structure with a low-frequency component strongest around 18 meV and a broader component peaking near 40-70 meV. The second component carries most of the spectral weight and its energy matches structures observed in photoemission in the range 50-90 meV. Our results demonstrate that collective spin excitations can explain features of electronic spectroscopies and are therefore likely to be strongly coupled to the electron quasiparticles.