The pseudogap state in high-T-c superconductors: An infrared study

We report on a study of the electromagnetic response of three different families of high-T-c superconductors that in combination allowed us to cover the whole doping range from under- to overdoped. The discussion is focused on the nb-plane charge dynamics in the pseudogap stare which is realized in underdoped materials below a characteristic temperature T*, a temperature that can significantly exceed the superconducting transition temperature T-c. We explore the evolution of the pseudogap 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 optical data that allows us to observe the effect of the pseudogap most clearly. We compare the infrared data with other experimental results, including the c-axis optical response, de transport, and angle-reserved photoemission.