Low-energy incommensurate spin excitations in superconducting La1.85Sr0.15CuO4

Neutron-scattering experiments with markedly improved signal-to-noise ratios have been performed on single-crystal superconducting La1.85Sr0.15CuO4 (Tc=33 K) at energies down to 1.5 meV. The measurements provide quantitative information on the temperature dependence of χ''(q,ω). Surprisingly, χ''(qpeak,ω) is approximately constant below Tc for the measured energies 1.5 meV≤ω≤6 meV, that is, for energies much less than the mean-field BCS gap energy of ∼10 meV, hence implying other than conventional s-wave superconductivity.