Optical response of the Josephson coupled CuO 2-layers in high- Tc superconductors

Abstract

Anomalous spectral behavior of the c-axis far-infrared conductivity at low-temperatures has been investigated in high- T c superconducting cuprates (HTSC). As the temperature is lowered, a broad bump grows around 450 cm −1 and the in-plane oxygen phonon mode around 350 cm −1 softens and loses its intensity. This anomaly had been observed in underdoped YBa 2Cu 3O y , YBa 2Cu 4O 8 and Pb 2Sr 2RCu 3O 8 but not in (La,Sr) 2CuO 4. Recently we observed a similar anomaly in underdoped Bi 2Sr 2CaCu 2O z , and confirmed that this is a common property 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 superconducting 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 any conventional model based on the assumption of a uniform internal electric field.