The pseudogap state in high-Tc superconductors: an infrared study

We report on a study of the electromagnetic response of three different families of high-Tc superconductors that in combination allowed us to cover the whole doping range from under- to overdoped. The discussion is focused on the ab-plane charge dynamics in the pseudogap state which is realized in underdoped materials below a characteristic temperature T*, a temperature that can significantly exceed the superconducting transition temperature Tc. 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, dc transport, and angle-resolved photoemission. (author)