Energy spectrum of spin fluctuations in superconducting La_2-xSr_xCuO₄ (0.10 ≤ x ≤ 0.25)

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 T_c, the incommensurate peaks diminish rapidly in intensity with decreasing energy below ∼8 meV 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. χx″(ω), exhibits an enhancement around 7 meV, 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 χ″(ω) survive at T_c. 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 χ″(ω) below T_c.