Superconducting properties of [ BaCuO x ] 2 / [ CaCuO 2 ] n artificial structures with ultrathick CaCuO 2 blocks

The electrical transport properties of $[{\mathrm{BaCuO}}_{x}{]}_{2}/[{\mathrm{CaCuO}}_{2}{]}_{n}(\mathrm{C}\mathrm{B}\mathrm{C}\mathrm{C}\mathrm{O}\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}n)$ underdoped high-temperature superconducting superlattices grown by pulsed laser deposition have been investigated. Starting from the optimally doped $\mathrm{C}\mathrm{B}\mathrm{C}\mathrm{C}\mathrm{O}\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}2$ 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 $ng11$ 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 $(n\ensuremath{\approx}11)$ turns out to be quite temperature independent and close to the two-dimensional quantum resistance 26 k\ensuremath{\Omega}. A further increase of the number of ${\mathrm{CuO}}_{2}$ planes results in an insulator-type dependence of $R(T)$ 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.