Development of spin-wave-like dispersive excitations below the pseudogap temperature in the high-temperature superconductor La2-xSrxCuO4

Inelastic neutron scattering measurements of magnetic excitations in hole-doped high-Tc cuprates La2-xSrxCuO4 (x=0 and 0.075) have been performed over a wide temperature range of 5-500 K extending beyond the pseudogap temperature of ∼360 K for x=0.075. For x=0, the two-dimensional spin-wave excitations at low temperatures are thermally robust and the dispersion is slightly softened on heating up to T=400 K. For x=0.075, the spin-wave-like excitations in the upper part of the hourglass-shaped dispersion remain at T=300 K. However, further heating above T=400 K induces a broad ridge centered at (1/2,1/2); thereby the hourglass-shaped dispersion becomes blurred above the pseudogap temperature. The localized spin nature of magnetic excitations below T∗ suggests that the pseudogap is related to the proximity to a Mott insulator rather than being a precursor of Cooper pairs.