A 10% substitution of Al on the Cu(1) (chain) site of single crystals of ${\mathrm{Ba}}_2$Y${\mathrm{Cu}}_3$${\mathrm{O}}_7$ drives the material to the edge of the orthorhombic-tetragonal transition and strongly decreases the resistivity and upper critical field, compared to the unsubstituted parent compound, but has little effect on $T_c$. The coherence length in the â-$\hat{\mathrm{b}}$ plane increases twofold, while the coherence length along the ĉ axis is unchanged. These results can be reconciled with the conventional relationships between superconducting and normal-state properties only if a twofold change in the Fermi-surface area is postulated.

• Received 27 July 1988

DOI:https://doi.org/10.1103/PhysRevB.39.2932