c-axis response of single- and double-layered cuprates

We report on an infrared study of the c-axis response of the single-layered superconductor ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ (${\mathit{T}}_{\mathit{c}}$=34 K) and analyze the results in comparison with our earlier data for the double-layered ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ [Basov et al., Phys. Rev. B 50, 3511 (1994)]. We find that in both materials nonmetallic temperature dependence of the dc resistivity along the c axis results from the transfer of the spectral weight from the far-infrared region to much higher energies. The energy scale associated with this transfer in the normal state is about 200 ${\mathrm{cm}}^{\mathrm{-}1}$ in the double-layered system and exceeds 5000 ${\mathrm{cm}}^{\mathrm{-}1}$ in the single-layered material. No well-defined pseudogap is found in the latter case. The superconducting state in both systems is highly unusual since the spectral weight of the superfluid δ function peak is accumulated from the frequencies as high as 20 ${\mathit{kT}}_{\mathit{c}}$.