Extraordinary effect of aluminum substitution on the upper critical field of Ba2YCu3O7

A 10% substitution of Al on the Cu(1) (chain) site of single crystals of Ba2YCu3O7 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 Tc. The coherence length in the ĝ-b^ plane increases twofold, while the coherence length along the 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.