Spin fluctuations and superconductivity in Ba sub 2 YCu sub 3 0 sub (6-delta).

We report observation of two dimensional spin fluctuation excitations in Ba sub 2 YCu sub 3 0 sub (6-delta), with delta in the range 0.0-0.9. The frequency of the excitations, and hence the value of the exchange constant, J=950 cm sup (-1), is similar to that reported recently for (non-superconducting) La sub 2 Cu0 sub 4. The present observation of magnetic excitations in a superconducting material, and their evolution as a function of delta, demonstrates a large influence of carrier concentration on the spin fluctuation dynamics. This indicates a strong coupling between the carriers and the spin excitations, which could lead to carrier pairing and thus to superconductivity.