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 parallel-to 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 parallel-to 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.