Direct observation of a magnetic gap in superconducting La1.85Sr0.15CuO4 (Tc=37.3 K)

Neutron inelastic scattering experiments have been performed on homogeneous single crystals of La1.85Sr0.15CuO4 with Tc=37.3 K (at onset), higher than any previously studied single crystals. The temperature dependence of the low energy incommensurate peak intensity at (π(1±δ),π) and (π,π(1±δ)) exhibits a pronounced maximum near Tc. In contrast to the results reported on lower Tc crystals, the intensity below 3.5 meV dramatically decreases as the temperature decreases below Tc, vanishing into the background below ∼15 K. The behavior is consistent with predictions based on a dx2-y2 superconducting order parameter.