We show that, independent of the choice of values for the carrier concentration n and specific heat in the normal state, existing experiments on the superconducting properties of polycrystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.9}$ are consistent with calculations carried out in the clean limit. We find, for a wide choice of n, a substantial exchange enhancement of the Pauli susceptibility. For n<5×${10}^{21}$ ${\mathrm{cm}}^{\mathrm{-}3}$, long wavelength spin fluctuations, with a characteristic temperature, $T_{\mathrm{SF}\mathrm{\sim }600}$ K, can give rise to a strong-coupling superconducting transition to a triplet Balian-Werthamer phase. On the other hand, if n${>10\mathrm{}}^{22}$ ${\mathrm{cm}}^{\mathrm{-}3}$ antiferromagnetic spin fluctuations must be invoked if the pairing is of magnetic origin.

• Received 25 September 1987

DOI:https://doi.org/10.1103/PhysRevB.37.3730