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，通过电线无损传输电力能够在我们的配电基础设施中节省大量资金。超导体的独特性质使其他几项关键技术成为可能，例如产生医疗保健中磁共振成像所需的大型超稳定磁场。超导约瑟夫森结的独特性质可能是量子计算机发展的最有前途的途径，量子计算机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在市场上，但仍然有巨大的潜力。为了实现这一点，我们需要更好地理解超导现象，特别是在过去25年中发现的所谓非常规高温超导体中的微观起源。尽管在发展非常规超导的微观理论方面已经取得了很大进展，但答案仍然没有找到。一个非常有前途的，相对较新的解释这些效应的理论依赖于存在于量子临界点附近的物质的高度量子力学纠缠态的物理学，其中有序/无序相变被某些外部参数如压力调谐到绝对零度。在这个提议中，我们计划详细研究量子临界性对几个不同候选系统中超导性的影响。这些材料包含了在过去25年中发现的所有主要的超导体家族（铜，有机物，重费米子，铁磷族化合物），都可以通过外部压力调整到量子临界点。不幸的是，超导性本身阻止了几乎所有用于研究量子临界性的既定方法。解决这个问题的一个方法是施加一个大的外部磁场来淬灭超导性。幸运的是，磁穿透深度提供了一个灵敏而独特的探测器，可以在零外加磁场和极低温度下探测潜在的正常态电子结构。我们的建议旨在测量绝对渗透深度的压力电池的第一次。这将提供独特的信息，指导量子临界如何（或是否）导致高温超导性的理论。

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.