Spin fluctuations and superconductivity in ${\mathrm{Ba}}_2$${\mathrm{YCu}}_3$${\mathrm{O}}_{6+\mathrm{\delta }}$

We report observation of spin-pair excitations in ${\mathrm{Ba}}_2$ ${\mathrm{YCu}}_3$ ${\mathrm{O}}_{6+\mathrm{\delta }}$, with δ in the range 0.0–0.9. The frequency of the excitations, and hence the value of the exchange constant, J=950 ${\mathrm{cm}}^{\mathrm{-}1}$, is similar to that reported recently for (nonsuperconducting) ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$. The present observation of magnetic excitations in a superconducting material, and of their evolution as a function of δ, demonstrate a large influence of carrier concentration on the spin-fluctuation dynamics.