Investigating the Phase Diagram of the High Temperature Superconductors by Angle-Resolved Photoemission (ARPES)

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The superconducting phase transition in the underdoped high temperature superconductors is rather unusual, in that it is not a mean-field transition as other superconducting transitions are. Instead, it has been observed that a pseudo-gap in the electronic excitation spectrum appears at a temperature T* higher than Tc, while phase coherence, and superconductivity, are established at Tc. This individual investigator project will probe the relationship of the pseudogap to the superconducting gap. As doping is increased, the project will investigate if T* continues to decrease below the superconducting transition temperature Tc - making the pseudogap independent of the superconducting gap - or whether T* becomes Tc beyond optimal doping. Determining the behavior of the pseudogap will provide experimental evidence for which of several theories apply to these materials. It is very difficult to determine whether there are two gaps of different origin below Tc if the gaps have the same magnitude. This project will use angle resolved photoemission with circularly polarized light. The PI recently showed that this technique provides evidence that the state below T* is rather unusual, it appears to break time reversal symmetry. Therefore, using this technique it would be possible to separate the two gaps, and help determine their origin. The students involved with this project will be exposed to international collaborations and to performing research at national facilities. They will receive training that will prepare them for future scientific careers.High temperature superconductors (HTSCs) were discovered in 1987. They loose all resistance to electrical current at temperatures near that of liquid air. It is still not understood how they work. Superconducting materials discovered earlier are understood, and when they superconduct, a gap appears in the energies available to the electrons. This gap suppresses the energy loss mechanism in current transport. However, in the HTSCs, the gap appears before the material becomes a superconductor, and understanding this phenomenon is essential to understanding how the new HTSCs work. This gap, called a pseudogap, since it is not yet a superconducting gap, changes size according to how many charge carriers have been put in the material. Two of the most significant theories for the HTSCs propose different roles for the pseudogap. In one, the pseudogap is a direct manifestation of the superconducting gap that will appear at lower temperatures, while in the other it is a manifestation of a change in some unusual order, which indirectly leads to superconductivity. This work aims to determine which point of view applies. In this new order, time reversal symmetry is broken, which can be explained in terms of a movie: normally, when a movie of the electrons moving in the solid is played backwards, the electrons will return to their starting position. But when time reversal is broken and the movie is played backwards, the electrons will not return to the starting position, but will end at a different place. This very strange behavior appears to be found in the pseudogap state, and its study will allow the researchers to determine the origin of the pseudogap. The students involved with this project will be exposed to international collaborations and to performing research at national facilities. They will receive training that will prepare them for future scientific careers.