c-axis response of YBa₂Cu₄O₈: A pseudogap and possibility of Josephson coupling of CuO₂ planes

We report on the temperature dependence of the far-infrared conductivity of YBa₂Cu₄O₈ single crystals for E∥c. With decreasing temperature, the conductivity shows a transition from a Drude-like behavior to a pseudogap at 180 cm^-1 which grows deeper below 180 K without any abrupt changes at the superconducting transition at T_c=80 K. In the superconducting state the formation of the superfluid condensate can be seen. We show that in a variety of high-T_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 CuO₂ layers.