SPIN FLUCTUATIONS IN SUPERCONDUCTING LA1.85SR0.15CUO4

Neutron-scattering experiments have been performed to study the dynamic spin properties of La1.85Sr0.15CuO4 (T(c)=33 K) in the temperature range 1.9 K less-than-or-equal-to T less-than-or-equal-to 150 K and energy range 1.5 meV less-than-or-equal-to omega less-than-or-equal-to 20 meV; these span, respectively, T(c) and the weak-coupling BCS gap energy 2DELTA approximately 10 meV. The width of S(q,omega) in momentum space decreases with decreasing temperature at fixed energy as well as with decreasing energy at fixed T(c) becoming very sharp at both low omega 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 chi''(omega)= integral (pi,pi) dq2Dchi''(q,omega) is constant above omega approximately 10 meV with an amplitude about three times that in La1.98Sr0.02CuO4 and begins to decrease with decreasing omega at omega approximately 10 meV, which corresponds to the BCS superconducting gap energy 2DELTA. 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.