Abstract
Motivated by recent attention to a potential antiferromagnetic quantum critical point at xc∼0.19, we have used inelastic neutron scattering to investigate the low-energy spin excitations in crystals of La2−xSrxCuO4 bracketing xc. We observe a peak in the normal-state spin-fluctuation weight at ∼20meV for both x=0.21 and 0.17, inconsistent with quantum critical behavior. The presence of the peak raises the question of whether low-energy spin fluctuations limit the onset of superconducting order. Empirically evaluating the spin gap Δspin in the superconducting state, we find that Δspin is equal to the coherent superconducting gap Δc determined by electronic spectroscopies. To test whether this is a general result for other cuprate families, we have checked through the literature and find that Δc≤Δspin for cuprates with uniform d-wave superconductivity. We discuss the implications of this result.
