The electrical transport properties of $[{\mathrm{BaCuO}}_x{]}_2/[{\mathrm{CaCuO}}_2{]}_n(\mathrm{C}\mathrm{B}\mathrm{C}\mathrm{C}\mathrm{O}-2\mathrm{}\times n)$ underdoped high-temperature superconducting superlattices grown by pulsed laser deposition have been investigated. Starting from the optimally doped $\mathrm{C}\mathrm{B}\mathrm{C}\mathrm{C}\mathrm{O}-2\mathrm{}\times 2$ superlattice, having three ${\mathrm{CuO}}_2$ planes and $T_c$ around 80 K, we have systematically increased the number n up to 15 moving toward the underdoped region and hence decreasing $T_c.$ For $n>\mathrm{}11\mathrm{}$ the artificial structures are no longer superconducting, as expected, for a uniformly distributed charge carrier density inside the conducting block layer. The sheet resistance of such artificial structures $\left(n\approx 11\right)$ turns out to be quite temperature independent and close to the two-dimensional quantum resistance 26 kΩ. A further increase of the number of ${\mathrm{CuO}}_2$ planes results in an insulator-type dependence of $R\left(T\right)\mathrm{}$ in the wide range of temperatures from room temperature to 1 K. The value of the sheet resistance separating the superconducting and the insulating regimes supports the fermionic scenario of the superconductor-insulator transition in these systems.

• Received 5 May 2000

DOI:https://doi.org/10.1103/PhysRevB.62.9835