Dynamics of charge carriers in cuprates

Summary form only given. New results of infrared photoinduced absorption experiments on undoped La/sub 2/CuO/sub 4/ and Nd/sub 2/CuO/sub 4/ and insulating YBa/sub 2/Cu/sub 3/O/sub 6+/spl delta/ are reported. We observed two broad electronic absorptions within the charge transfer energy gap one at /spl tilde/ 0.15 eV and the other at /spl tilde/ 0.5 eV for all three systems. Detailed temperature dependence measurements show that the peak position of the low energy electronic absorption becomes abruptly red-shifted by 200 cm/sup -1/ at T /spl tilde/ 40 K for La/sub 2/CuO/sub 4/ and Nd/sub 2/CuO/sub 4/ and by /spl tilde/ 480 cm/sub -1/ at T /spl tilde/ 90 K for YBa/sub 2/Cu/sub 3/O/sub 6+/spl delta/. Charge recombination dynimics studies suggest that the ground state of photocarriers in the CuO/sub 2/ plane at T < T/sub c/ is remarkably different from that observed at T >T/sub c/.