High pressure studies of quantum criticality in unconventional superconductors

超导是一种有可能改变社会的现象。例如，电力通过电线的无损传输能够在我们的配电基础设施中节省大量资金。超导体的独特特性使其他几项关键技术成为可能，例如医疗保健领域磁共振成像所需的超大、超稳定磁场的产生。超导约瑟夫森结的独特性质可能是开发量子计算机最有希望的途径，这可能会彻底改变我们的数字经济。尽管超导体的许多应用已经进入市场，但仍有巨大的潜力。为了实现这一点，我们需要更好地理解超导现象，特别是它的微观起源，即过去25年来发现的所谓的非常规高温超导体。尽管在发展非常规超导的微观理论方面已经取得了很大的进展，但答案仍然没有找到。一个非常有前途的、相对较新的理论来解释这些效应，它依赖于物质的高度量子力学纠缠态的物理学，这种纠缠态存在于接近量子临界点的地方，在那里，有序/无序相变被一些外部参数（如压力）调谐到绝对零度。在本提案中，我们计划详细研究量子临界性对几种不同候选系统的超导性的影响。这些材料涵盖了过去25年来发现的超导体的所有主要家族（铜、有机物、重费米子、铁-镍），它们都可以在外部压力下调谐到量子临界点。不幸的是，超导性本身阻碍了几乎所有用于研究量子临界性的既定方法。解决这个问题的一种方法是施加一个大的外部磁场来淬灭超导性。然而，这也可能有改变感兴趣的量子临界行为的效果。幸运的是，磁穿透深度为零外加磁场和极低温度下潜在的正常电子结构提供了一个敏感和独特的探针。我们的建议是首次测量压力电池中的绝对穿透深度。这将提供独特的信息，将指导理论如何（或是否）量子临界导致高温超导。

Superconductivity is a phenomena that has the potential to transform society. For example, the lossless transport of electricity through wires has the ability to make tremendous savings in our electrical distribution infrastructure. The unique properties of superconductors enable several other key technologies, such as the generation of large, ultra stable magnetic fields needed for magnetic resonance imaging in healthcare. The unique properties of superconducting Josephson junctions are perhaps the most promising route towards the development of a quantum computer which could revolutionize our digital economy.Although many applications of superconductors are already in the marketplace there is still enormous potential. To realise this we need to gain a better understanding of the phenomena of superconductivity, in particular its microscopic origin in so-called unconventional high temperature superconductors which have been discovered in the last 25 years. Although there has been much progress towards the goal of developing a microscopic theory of unconventional superconductivity the answer has still not been found. A highly promising, relatively recent, theory to explain these effect relies on physics of the highly quantum mechanically entangled state of matter which exists close to a quantum critical point, where an order/disorder phase transition is tuned to absolute zero by some external parameter such as pressure.In this proposal we plan to study in detail the effect of quantum criticality on the superconductivity in several different candidate systems. These materials encompass all the major families of superconductors which have been discovered in the last 25 years (cupates, organics, heavy fermions, iron-pnictides) can all be tuned towards an quantum critical point with external pressure. Unfortunately, superconductivity itself prevents almost all established methods used to study quantum criticality. One way round this problem is to apply a large external magnetic field to quench the superconductivity. However, this may also have the effect of modifying the quantum critical behaviour of interest.Fortunately, the magnetic penetration depth provides a sensitive and unique probe of the underlying normal state electronic structure with zero applied magnetic field and at very low temperature. Our proposal seeks to measure the absolute penetration depth in a pressure cell for the first time. This will provide unique information which will guide theories of how (or if) quantum criticality causes high temperature superconductivity.