We report on the temperature dependence of the far-infrared conductivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ single crystals for E∥c. With decreasing temperature, the conductivity shows a transition from a Drude-like behavior to a pseudogap at 180 ${\mathrm{cm}}^{\mathrm{-}1}$ which grows deeper below 180 K without any abrupt changes at the superconducting transition at ${\mathit{T}}_{\mathit{c}}$=80 K. In the superconducting state the formation of the superfluid condensate can be seen. We show that in a variety of high-${\mathit{T}}_{\mathit{c}}$ superconductors the value of the London penetration depth for E∥c correlates with the conductivity along the c direction. With respect to this correlation we discuss the possibility of Josephson coupling between the ${\mathrm{CuO}}_2$ layers.

• Received 16 May 1994

DOI:https://doi.org/10.1103/PhysRevB.50.3511