Exploring connections between superconductivity, unconventional quantum order, and Fermi surface reconstruction

我们正处于一场材料革命中，这场革命正在改变计算、太阳能、强材料等技术的面貌。超导在发电和输电、量子计算、交通和磁铁技术等领域有着重要的未来作用。然而，这一未来取决于我们探索新材料体系的能力，以及更好地理解非常规超导的能力。*重费米子、高T_c氧化铜和铁-磷酸盐超导体的发现使超导体物理学取得了巨大的进步，但这些发现揭示了一些我们不了解的深层次物理问题，涉及超导令人惊讶地相互交织的物理、低温磁相变，特别是费米曲面重建。对这些问题的探索产生了新的、意想不到的物理学。我自己的研究小组，在我们最近在这个领域的研究中，已经被带入了一系列广泛的主题：焦绿石铁酸盐中的拓扑半金属状态，受挫磁性金属中的非金属行为，与费米表面转变相关的奇异量子磁性，以及将核电子有序参数与Pr基超导体中的一种新的量子临界点相结合。*同时，我们增加了新的实验能力，因此我们现在有了可以达到400千巴、温度低至10 MK的压力室，磁场高达18特斯拉。我们用它们在复杂的物质中创造新的状态。现在我们想要增加新的能力：能够在高压体积内施加非常大的单轴应力；使用电子结构计算来指导我们的压力或单轴应力测量；以及使用现代聚焦离子束样品制备来快速可靠地准备高压测量。这些发展为研究型学生提供了一个极好的培训场所，也将使我们能够快速进步。*在这次资助期间，我们将把我们的能力应用于各种有希望的材料，同时我们也将继续努力了解我们团队的最新发现。前者的一个例子是Sr2RhO4，它是高温和拓扑超导体的近亲。它有一个扭曲的钙钛矿结构，我们将使用电子结构计算来了解如何最好地施加压力来驱动一个非常平坦的带，在费米能量附近，跨越费米能量，导致重大的费米表面重建，我预测，超导，甚至高温超导。正在进行的项目的一个例子是与英国的一个小组进行的新合作，以探索将核电子秩序降至1mlikelvin以下。*我们的最终目标是新的和改进的超导技术，但在这个过程中，有很多新的物理问题需要探索。

We are in the midst of a materials revolution that is changing the face of technologies such as computation, solar energy, strong materials, and beyond. Superconductivity has an important future role to play, in power generation and transmission, quantum computing, transportation, and magnet technology. This future, however, depends on our ability to explore new materials systems, and to better understand unconventional superconductivity. ******A huge advance in the physics of superconductors resulted from the discovery of heavy fermion, high-Tc copper-oxide, and iron-pnictide superconductors, but these discoveries revealed that there are deep physics issues that we don't understand, regarding the surprisingly intertwined physics of superconductivity, low-temperature magnetic phase transitions and, especially, Fermi surface reconstruction. The exploration of these issues has produced new and unexpected physics. My own research group, in our recent studies in this field, has been led into a broad range of topics: topological semi-metallic states in pyrochlore iridates, 'non-metallic metal' behaviour in frustrated magnetic metals, exotic quantum magnetism associated with Fermi surface transitions, and combined nuclear-electronic order parameters with a new kind of quantum critical point in praseodymium-based superconductors.******At the same time we have added new experimental capabilities, so that we now have pressure cells that can reach 400 kilobar, and temperatures down to 10 mK, in magnetic fields up to 18 tesla. We use these to create novel states in complex matter. Now we want to add new capabilities: the ability to apply very large uniaxial stress within a high-pressure volume; to use electronic structure calculations to guide our pressure or uniaxial stress measurements; and to use modern focussed-ion-beam sample preparation to rapidly and reliably prepare high-pressure measurements. Such developments offer an excellent training ground for research students, and they will also allow us to advance rapidly.******During this grant we will apply our capabilities to a variety of promising materials, as we also continue to work to understand recent discoveries in our group. An example of the former is Sr2RhO4, which is a cousin to high-temperature and topological superconductors. It has a distorted perovskite structure, and we will use electronic-structure calculations to understand how best to apply pressure to drive a very flat band, near the Fermi energy, across the Fermi energy, causing a major Fermi surface reconstruction and, I predict, superconductivity, perhaps even high temperature superconductivity. An example of an ongoing project is a new collaboration with a group in the UK to explore combined nuclear-electronic order to below 1 millikelvin. ******Our ultimate goal is new and improved superconducting technologies, but along the way there is a lot of new physics to explore.