Optical response of the Josephson coupled CuO2-layers in high-T-c superconductors

Anomalous spectral behavior of the c-axis far-infrared conductivity at low- temperatures has been investigated in high-T-c superconducting cuprates (HT SC). As the temperature is lowered, a broad bump grows around 450 cm(-1) an d the in-plane oxygen phonon mode around 350 cm(-1) softens and loses its i ntensity. This anomaly had been observed in underdoped YBa2Cu3Oy, YBa2Cu4O8 and Pb2Sr2RCu3O8 but not in (La,Sr)(2)CuO4. Recently we observed a similar anomaly in underdoped Bi2Sr2CaCu2Oz, and confirmed that this is a common p roperty in all high-T-c cuprates with a double CuO-pyramid in a unit cell. The observed spectral change can be well described by the stacked supercond ucting layer model taking into account a local field effect. This indicates that the charge response in the c-direction of HTSC cannot be treated by a ny conventional model based on the assumption of a uniform internal electri c field. (C) 2001 Elsevier Science B.V. All rights reserved.