c-axis penetration depth and interlayer conductivity in the thallium-basedcuprate superconductors

The c-axis Josephson plasmons in optimally doped single-layer and bilayer h igh-T-c cuprates Tl2Ba2CuO6 and Tl2Ba2CaCu2O8 have been investigated using infrared spectroscopy. We observed the plasma frequencies for these two com pounds at 27.8 and 25.6 cm(-1) respectively, which we interpret as Josephso n resonances across the TlO blocking layers. No maximum in the temperature dependence of the c-axis conductivity was observed below T-c, indicating th at even in the superconducting state a coherent quasiparticle contribution to the c-axis conductivity is absent or very weak. [S0163-1829(99)51346-5].