The pseudogap state in high- superconductors: an infrared study

We report on a study of the electromagnetic response of three different families of high- 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 , a temperature that can significantly exceed the superconducting transition temperature . We explore the evolution of the pseudogap response by changing the doping level, by varying the temperature from above to below , 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.