Spin fluctuations in superconducting La1.85Sr0.15CuO4

Neutron-scattering experiments have been performed to study the dynamic spin properties of La1.85Sr0.15CuO4 (Tc=33 K) in the temperature range 1.9 K≤T≤150 K and energy range 1.5 meV≤ω≤20 meV; these span, respectively, Tc and the weak-coupling BCS gap energy 2Δ∼10 meV. The width of S(q,ω) in momentum space decreases with decreasing temperature at fixed energy as well as with decreasing energy at fixed T, becoming very sharp at both low ω and low temperature. It is found that both in the normal state at T=35 K and in the superconducting state at T=10 K the integrated generalized susceptibility χ''(ω)=F(π,π) dq2Dχ''(q,ω) is constant above ω∼10 meV with an amplitude about three times that in La1.98Sr0.02CuO4 and begins to decrease with decreasing ω at ω∼10 meV, which corresponds to the BCS superconducting gap energy 2Δ. However, we observe nonzero magnetic scattering well below the pseudogap energy at temperatures as low as 2 K. These results are compared with those in La1.98Sr0.02CuO4 and YBa2Cu3O6.6.