FAR-INFRARED INVESTIGATION OF THE PSEUDOGAP IN UNDERDOPED PB2SR2(Y CA)CU3O8/

The optical conductivity of underdoped Pb2Sr2(Y/Ca)Cu3O8 has been obta ined from 40 to 700 cm(-1) via Kramers-Kronig analysis of the c-axis-p olarized reflectance measured at temperatures between 10 and 300 K. In the normal state at the lowest and highest frequencies where it is le ast obscured by a large phonon contribution, the background c-axis opt ical conductivity of underdoped Pb2Sr2(Y/Ca)Cu3O8 is observed to decre ase systematically as the temperature is lowered from 300 to near 150 K. Upon lowering the temperature further, the optical conductivity rem ains constant until the superconducting transition temperature of 65 K is reached, whereupon, at the lowest frequencies only, a further depr ession is observed due to the removal of spectral weight into the supe rconducting condensate. The spectral weight which is redistributed int o the delta-function condensate is estimated to derive from frequencie s 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 temp erature range where a change in slope of the ab-plane de resistivity, and a decrease in the Cu-63 NMR Knight shift are observed.