Invited Paper Interlayer tunnelling mechanism: experimental test of single layer compounds

The c-axis optical properties of cuprate superconductors have been studied by grazing angle reflectivity technique. We tested the interlayer tunnelling (ILT) model of high temperature superconductivity and showed that the basic relation between the condensation energy of the superconducting state and the interlayer Josephson coupling does not hold for T12Ba2CuO6. Measuring the reflectivity at an oblique angle of incidence on the ab-surface of T12Ba2CuO6, we found the c-axis plasma resonance at 28 cm. The corresponding Josephson coupling energy is at least an order of magnitude lower than the condensation energy determined from the specific heat measurements. In the double layer compound Tl2Ba2CaCu2O8 the c-axis plasma resonance was observed at 26 cm1. The c-axis penetration depth and the c-axis conductivity follow the Ambegaokar-Baratoff or "dirty limit " superconductor relation if a very small energy gap is assumed