Normal and superconducting state electronic structure of iron based superconductors

超导性是一种令人着迷的现象，它能在很远的距离(几百公里长的电线)上产生量子相干。它也有许多有价值的实际应用--基本上，任何使用电力的东西原则上都可以受益于超导技术。最明显的应用是无损输电电缆、非常高效的电力变压器和发电机。这些应用从20世纪初超导早期就被承诺了，但直到发现了高于液氮沸点(77开尔文)的超导材料20年后，这些应用才开始实用。新超导材料的发现和完善极大地得益于我们对其基本物理的理解--最重要的是形成相干超导态的原因。尽管经过了二十多年的研究，但对于著名的氧化铜(铜酸盐)材料的高温超导机理仍未达成共识。一年多前在含有铁元素的材料中发现高温超导电性是非常令人惊讶的，因为通常与铁有关的磁性对超导状态的形成非常不利。从基础物理和潜在应用的角度来看，这些材料都具有极大的吸引力。前者源于与铜酸盐超导体的几个关键的相似之处(和不同之处)--也许使这些材料成为可以用来理解电子机制如何产生高温超导状态的“罗塞塔石头”。后者是因为有些材料即使在世界上最强的磁场超过60T的情况下也会继续超导。本方案的研究旨在通过两条相关的研究路径来加深我们对高温超导，特别是铁基超导体的理解。其中一条途径将探讨超导电性本身的性质。通过测量温度对超导电子密度的影响，通过测量磁穿透深度(即超导体屏蔽外加磁场的基本能力)等性质，我们可以了解携带超导电流的电子的微观性质。另一条途径是研究“正常”金属状态的性质。通过施加强磁场，可以抑制(有效地关闭)超导电性，从而可以研究材料的非超导正常金属状态。在足够高的磁场中，磁化作为场的函数的特征周期振荡(德哈斯-范·阿尔芬效应)揭示了携带电流的电子的准确动量，以及这种动量如何作为方向的函数变化。这种对金属‘电子结构’的测定类似于知道它的DNA。通过研究这种正常态电子结构在不同材料中的变化及其对超导性质的影响，将有可能建立起高温超导机制的理论图景。

Superconductivity is a fascinating phenomenon giving rise to quantum coherence over vast distances (several hundred kilometres of wire). It also has many valuable practical applications - basically anything which uses electricity can in principle benefit from superconducting technology. The most obvious applications are lossless power transmission cables, very high efficiency power transformers and generators. These applications have been promised since the early days of superconductivity at the start of the 20th century, but are only now becoming practical 20 years after the discovery of materials which superconduct above the boiling point of liquid nitrogen (77 Kelvin). The discovery and refinement of new superconducting materials benefits immensely from improving our understanding of their fundamental physics - most importantly the reason for the formation of the coherent superconducting state. Despite more than twenty years of research there is still no consensus as to the mechanism of high temperature superconductivity in the famous copper oxide (cuprate) materials. The discovery just over one year ago of high temperature superconductivity in material containing the element iron was very surprising as the magnetism normally associated with iron is highly detrimental to the formation of the superconducting state. These materials are of great interest from a point of view of both the fundamental physics and potential applications. The former stems from several key similarities (and differences) with the cuprate superconductors - perhaps making these materials the 'Rosetta stone' that can be used to understand how electronic mechanisms can produce a high temperature superconducting state. The latter from the fact that some materials continue to superconduct even when subjected to the world most intense magnetic fields in excess of 60T.The research in this proposal aims to further our understanding of high temperature superconductivity and in particular iron-based superconductors by pursuing two related research paths. One path will address the nature of the superconductivity itself. By measuring the influence of temperature on the density of superconducting electrons, through measurement of properties such as the magnetic penetration depth (i.e., the fundamental ability of a superconductor to screen out an applied magnetic field) we can learn the microscopic properties of the electrons which carry the superconducting current. The other path will be to study the properties of the 'normal' metal state. By applying large magnetic field, the superconductivity can be suppressed (effectively turned off) and then we can study the non-superconducting normal metallic state of the materials. In sufficiently high field, characteristic periodic oscillations of the magnetisation as a function of field (the de Haas-van Alphen effect) reveal the exact momenta of the electrons that carry current and how this momentum varies as a function of direction. This determination of the 'electronic structure' of a metal is akin to knowing its DNA. By studying how this normal state electronic structure varies in the different materials and how this influences the superconducting properties it will be possible to build up a theoretical picture of the mechanism of high temperature superconductivity.