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，因为它不像其他超导相变那样是平均场转变。相反，在温度T*高于Tc时，电子激发谱中出现了伪间隙，而在Tc处则建立了相相干性和超导性。这个个人研究项目将探讨赝隙与超导隙的关系。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-使赝隙与超导间隙无关-或者T*是否超过最佳掺杂成为Tc。确定赝隙的行为将为几种理论中的哪一种适用于这些材料提供实验证据。如果两个间隙的大小相同，则很难确定在Tc以下是否存在两个不同来源的间隙。这个项目将使用角分辨光发射与圆偏振光。PI最近表明，这种技术提供了证据，证明T*以下的状态相当不寻常，它似乎打破了时间反转对称性。因此，使用这种技术将有可能分离两个间隙，并帮助确定它们的起源。参与该项目的学生将接触到国际合作，并在国家机构进行研究。他们将接受培训，为将来的科学事业做准备。高温超导体（HTSCs）于1987年被发现。在接近液态空气的温度下，它们会失去对电流的抵抗力。它们的工作原理尚不清楚。早先发现的超导材料是已知的，当它们超导时，电子可用的能量会出现一个缺口。这个间隙抑制了电流输运中的能量损失机制。然而，在高温超导材料中，这种间隙在材料成为超导体之前就出现了，理解这种现象对于理解新的高温超导材料是如何工作的至关重要。这个间隙被称为赝隙，因为它还不是超导间隙，它的大小取决于材料中有多少载流子。HTSCs的两个最重要的理论提出了赝隙的不同作用。在一种情况下，赝隙是在较低温度下出现的超导隙的直接表现，而在另一种情况下，它是一些不寻常顺序变化的表现，间接导致超导性。这项工作旨在确定哪种观点适用。在这个新顺序中，时间反转对称性被打破了，这可以用电影来解释：通常，当电子在固体中运动的电影向后播放时，电子会回到它们的起始位置。但是当时间反转被打破，倒放影片时，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是在赝隙状态中发现的，对它的研究将使研究人员能够确定赝隙的起源。参与该项目的学生将接触到国际合作，并在国家机构进行研究。他们将接受培训，为将来的科学事业做准备。

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.