Enhanced Kosterlitz-Thouless transition in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$/${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ multilayers as a measure of two-dimensionality

We have observed enhanced Kosterlitz-Thouless transitions in multilayers composed of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ with a thickness of two and four unit cells, separated by ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ sufficient to isolate them from one another. The parameter ${\mathrm{\tau }}_{\mathrm{KT}}$=(${\mathit{T}}_{\mathit{c}0}$-${\mathit{T}}_{\mathrm{KT}}$)/${\mathit{T}}_{\mathit{c}0}$ is up to an order of magnitude larger than in single crystals or in films, showing that two-dimensional behavior is approached more closely in the multilayers. The fact that the enhancement of two-dimensionality is accompanied by a lowering of ${\mathit{T}}_{\mathit{c}0}$ demonstrates the importance of interlayer coupling.