Spin fluctuations in superconducting La1.85Sr0.15CuO4

Abstract

Neutron-scattering experiments have been performed to study the dynamic spin properties of ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$ (${\mathit{T}}_{\mathit{c}}$=33 K) in the temperature range 1.9 K≤T≤150 K and energy range 1.5 meV≤ω≤20 meV; these span, respectively, ${\mathit{T}}_{\mathit{c}}$ 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)=${\mathcal{F}}_{(\mathrm{\pi },\mathrm{\pi })}$ d${\mathit{q}}_{2\mathrm{D}}$χ’ ’(q,ω) is constant above ω∼10 meV with an amplitude about three times that in ${\mathrm{La}}_{1.98}$ ${\mathrm{Sr}}_{0.02}$ ${\mathrm{CuO}}_4$ 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 ${\mathrm{La}}_{1.98}$ ${\mathrm{Sr}}_{0.02}$ ${\mathrm{CuO}}_4$ and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.6}$.