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

欠掺杂高温超导体中的超导相变是相当不寻常的，因为它不是像其他超导相变那样的平均场相变。相反，已经观察到电子激发光谱中的赝能隙出现在高于Tc的温度T* 处，而相位相干性和超导性在Tc处建立。这个个人研究项目将探讨赝能隙与超导能隙的关系。随着掺杂的增加，该项目将研究T* 是否继续降低到超导转变温度Tc以下-使赝能隙独立于超导能隙-或者T* 是否超过最佳掺杂。 确定赝能隙的行为将为几种理论中哪一种适用于这些材料提供实验证据。 如果两个能隙具有相同的量值，则很难确定在Tc以下是否存在两个起源不同的能隙。这个项目将使用角分辨光电发射与圆偏振光。PI最近表明，这种技术提供的证据表明，低于T* 的状态是相当不寻常的，它似乎打破了时间反演对称性。 因此，使用这种技术将有可能分离这两个间隙，并帮助确定它们的来源。 参与该项目的学生将接触到国际合作，并在国家设施进行研究。 他们将接受培训，为未来的科学事业做好准备。高温超导体（HTSC）于1987年被发现。 在接近液态空气的温度下，它们对电流失去了所有的电阻。目前还不清楚它们是如何工作的。人们已经了解了早期发现的超导材料，当它们超导时，电子可用的能量会出现一个缺口。该间隙抑制了电流传输中的能量损失机制。然而，在高温超导体中，差距在材料成为超导体之前出现，理解这种现象对于理解新的高温超导体如何工作至关重要。这个间隙称为赝间隙，因为它还不是超导间隙，它的大小会根据材料中的载流子数量而变化。两个最重要的理论的高温超导体提出了不同的角色的pseudogap。在一种情况下，赝能隙是在较低温度下出现的超导能隙的直接表现，而在另一种情况下，它是某种不寻常顺序变化的表现，这间接导致了超导性。这项工作旨在确定哪种观点适用。在这种新的秩序中，时间反演对称性被打破，这可以用电影来解释：通常，当电子在固体中移动的电影向后播放时，电子将返回到它们的起始位置。但是当时间反转被打破，电影倒着播放时，电子不会回到起始位置，而是会在不同的地方结束。这种非常奇怪的行为似乎存在于pseudogap状态中，对它的研究将使研究人员能够确定pseudogap的起源。参与该项目的学生将接触到国际合作，并在国家设施进行研究。 他们将接受培训，为未来的科学事业做好准备。

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.