Far-infrared investigation of the pseudogap in underdoped Pb2Sr2(Y/Ca)Cu3O8

The optical conductivity of underdoped Pb2Sr2(Y/Ca)Cu3O8 has been obtained from 40 to 700 cm-1 via Kramers-Kronig analysis of the c-axis-polarized reflectance measured at temperatures between 10 and 300 K. In the normal state at the lowest and highest frequencies where it is least obscured by a large phonon contribution, the background c-axis optical conductivity of underdoped Pb2Sr2(Y/Ca)Cu3O8 is observed to decrease systematically as the temperature is lowered from 300 to near 150 K. Upon lowering the temperature further, the optical conductivity remains constant until the superconducting transition temperature of 65 K is reached, whereupon, at the lowest frequencies only, a further depression is observed due to the removal of spectral weight into the superconducting condensate. The spectral weight which is redistributed into the δ-function condensate is estimated to derive from frequencies below 425 cm-1. The depressed conductivity in the normal state is attributed to the formation of a pseudogap similar to that previously observed for underdoped YBa2Cu3O7-y, YBa2Cu4O8, and La2-xSrxCuO4. The temperature dependence suggests that the pseudogap is fully formed at a temperature of the order 150 K. This corresponds closely to the temperature range where a change in slope of the ab-plane dc resistivity, and a decrease in the 63Cu NMR Knight shift are observed.