We have artificially varied the coupling between ultrathin ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ layers by interposing insulating planes of ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ in multilayers and studied the consequences of decoupling 12-Å ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ unit cells on the superconducting properties. We find that the ${\mathrm{T}}_{\mathrm{c}}$ (midpoint) of a 12-Å/12-Å multilayer is 55 K, twice that of the corresponding alloy. ${\mathrm{T}}_{\mathrm{c}}$ decreases linearly with further separation of the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ unit cells, the transition width increases and shows evidence of 2D fluctuations, and the influence of a magnetic field parallel to the layers is markedly reduced.

• Received 20 November 1989

DOI:https://doi.org/10.1103/PhysRevLett.64.804