Far-infrared reflectivity along the c axis in La2CuO4+delta, La1.89Ca1.11Cu2O6+delta, and Nd2CuO4-xF4 single crystals

The far-infrared reflectivity along the c axis in La2CuO4+delta, La1.89Ca1.11Cu2O6+delta (double pyramidal planes), and Nd2CuO4-xFx (T' phase) single crystals is measured down to 9 cm(-1). All the insulating spectra in the normal states change to show a sharp plasma edge in the superconducting state, which confirms our earlier report of sphere resonance of powder samples [Shibata and Yamada, Phys. Rev. B 54, 7500 (1996)]. The edge of La1.89Ca1.11Cu2O6+delta is broader than that of La2CuO4+delta and Nd2CuO4-xFx, indicating the existence of a large number of unpaired carriers in the superconducting state. The obtained c-axis penetration depth lambda(c) is discussed based on the Josephson-coupled layer model.