Quantum Critical Superconductivity

从提高电力传输效率到在聚变反应堆中产生更强的磁场，高温超导体有可能在工业上产生重大影响。然而，只有当我们能够在室温和环境（或非常低）压力下处理超导体材料时，它们才会被广泛使用。解决这个问题的一个主要部分是更好地理解非常规超导体中超导状态背后的基本物理原理，这样我们就可以提高发现新超导体的能力。从理论上讲，这些材料中的超导性由于量子临界涨落的存在而增强，这些涨落通常来自其他竞争磁态。在这个项目中，我将研究这些超导材料中量子临界点（通过调谐参数抑制到0K的二阶相变）的存在，以确定它们对超导状态的影响。通过测量超导状态下磁穿透深度的变化，可以研究量子临界点和超导隙本身的结构。我将使用隧道二极管振荡器测量这个参数，它将振荡器电路中的频率变化与穿透深度的变化联系起来。跟踪该参数如何随温度变化可以深入了解超导间隙的性质（是否为节点），但我也将研究穿透深度如何随相互作用强度的变化而变化。为了做到这一点，我将使用圆柱形活塞电池调整施加的压力，并跟踪超导临界温度和磁穿透深度如何随压力变化。我将特别关注磁穿透深度的发散，这为量子临界点的存在提供了证据，以及这些点与超导状态的增强（临界温度的增加）之间的关系。我的目标是在一系列高Tc超导体上使用这种方法，但特别关注铜酸盐。然后，目标将是确认或拒绝量子临界点与增强超导性之间的联系。测量穿透深度的TDO技术已被该集团广泛使用，正如活塞压力电池的使用一样。然而，这两种技术的结合是一个新的发展，它将使具有穿透深度和Tc的压力相图的构建成为可能，这将有望增加我们对铜酸盐中高Tc超导性背后机制的理解。该项目属于EPSRC超导研究领域。该项目50%由EPSRC资助，50%由布里斯托尔物理系波特捐赠基金资助。

Summary: High temperature superconductors have the potential to have a significant impact in industry from increasing efficiency of electrical power transmission to generating stronger magnetic fields in fusion reactors. However, they will only be of widespread use when we are able to work with materials that are superconductors at room temperature and ambient (or just very low) pressure. A main part of solving this problem is to better understand the fundamental physics behind this superconducting state in the unconventional superconductors such that we can improve the ability to find new ones. It has been theorised that superconductivity in these materials is enhanced by the presence of quantum critical fluctuations often from other competing magnetic states. In this project I will be investigating the presence of quantum critical points (second order phase transitions supressed to 0K by a tuning parameter) in these superconducting materials in order to ascertain the impact they have on the superconducting state. Quantum critical points and the structure of the superconducting gap itself can be investigated by measuring the variation of the magnetic penetration depth in the superconducting state. I will be measuring this parameter using a Tunnel Diode Oscillator which relates a change in frequency in the oscillator circuit to a change in penetration depth. Tracking how this parameter varies with temperature can provide insight into the nature of the superconducting gap (whether it is nodal), but I will also be investigating how the penetration depth varies with a change in interaction strength. To do this I will be tuning the applied pressure using a cylindrical piston cell and tracking how both the superconducting critical temperature, and the magnetic penetration depth vary with pressure. I will be looking specifically for divergences in the magnetic penetration depth which give evidence for quantum critical points, and the relationship between these points and the enhancing of the superconducting state (increase of critical temperature). I aim to employ this method on a range of high Tc superconductors but specifically focussing on cuprates. The goal will then be to either confirm or reject a link between quantum critical points and enhanced superconductivity. The TDO technique for measuring penetration depth has been used extensively by the group, as has use of piston pressure cells. The combining of these two techniques, however, is a new development that will enable the construction of pressure phase diagrams with penetration depth and Tc which will hopefully increase our understanding of the mechanisms behind high Tc superconductivity in cuprates. This project falls within the EPSRC superconductivity research area. It is 50% funded by the EPSRC and 50% by Bristol Department of Physics Potter Endowment fund.