  The energy spectrum of incommensurate spin fluctuations in superconducting
Sr-doped La2CuO4 has been studied by inelastic neutron scattering experiments.
An energy gap in the spin excitation spectrum is observed in the
superconducting state of optimally doped (x=0.15) and slightly overdoped
(x=0.18) samples. At temperatures well below Tc, the incommensurate peaks
diminish rapidly in intensity with decreasing energy below ~8meV and merge into
the background below ~3.5meV for x=0.15 and ~4.5meV for x=0.18. For both
samples, the energy spectrum of the q-integrated spin susceptibility, x"(w),
exhibits an enhancement around 7meV, which is caused by a broadening in the
momentum width of the incommensurate peaks. The gap-like structure in the
energy spectrum and the enhancement of x"(w) survive at Tc. On the other hand,
for both underdoped (x=0.10) and heavily overdoped (x=0.25) samples there is
neither a clear energy gap nor an enhancement of x"(w) below Tc.
