THE PSEUDOGAP STATE IN HIGH-T-C SUPERCONDUCTORS - AN INFRARED STUDY

We report on a study of the electromagnetic response of three differen t families of high-T-c superconductors that in combination allowed us to cover the whole doping range from under- to overdoped. The discussi on is focused on the nb-plane charge dynamics in the pseudogap stare w hich is realized in underdoped materials below a characteristic temper ature T, a temperature that can significantly exceed the superconduct ing transition temperature T-c. We explore the evolution of the pseudo gap response by changing the doping level, by varying the temperature from above to below T, or by introducing impurities in the underdoped compounds. We employ a memory function analysis of the ab-plane optic al data that allows us to observe the effect of the pseudogap most cle arly. We compare the infrared data with other experimental results, in cluding the c-axis optical response, de transport, and angle-reserved photoemission.