New Magnetic Coherence Effect in SuperconductingLa2−xSrxCuO4

We have used inelastic neutron scattering to examine the magnetic fluctuations at intermediate frequencies in the simplest high temperature superconductor, ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. The suppression of the low energy magnetic response in the superconducting state is accompanied by an increase in the response at higher energies. Just above a threshold energy of $\sim 7\mathrm{meV}$ there is additional scattering present below $T_c$ which is characterized by an extraordinarily long coherence length, in excess of $50Å$.