What stabilizes unconventional superconductivity?

抵抗是徒劳的：除了灯泡和加热器之外，大多数电子元件在其电阻最小化时效率最高。在目前的气候下，能源可持续性经常成为国际议程的首要问题，减少传导装置或输电线路的功率损失具有全球重要性。因此，研究新型导电材料的性质对确保全球能源的未来至关重要。超导性，即某些材料在临界温度以下出现的零电阻现象，仍然没有得到充分的解释。目前，这些临界温度通常非常低，低于140开尔文（-133摄氏度），但更全面地了解导致超导状态形成的原因可能会导致设计在大规模技术开发所需的增强温度下显示超导性的材料。不幸的是，最有可能将我们引向这一目的的材料，即所谓的非常规超导体，却最不为人所知。在这样的材料中，超导状态似乎与物质的至少两个其他相竞争：磁性和正常的金属导电性。一个微妙的平衡控制着低温下的主导相;基态。通过对材料的组成进行轻微的调整，或者通过施加适度的压力，化合物中电子之间的某些相互作用可以以牺牲其他电子为代价而得到加强，从而使平衡倾向于特定的基态。如何做到这一点的技术细节是相对众所周知的。尚待解释的是为什么会发生这种情况，在超导性战胜磁性或金属相的关键临界点上发生了什么--简而言之，究竟是什么稳定了非常规的超导状态？拟议项目试图回答的正是这个问题。我将利用磁场来探讨三种非传统超导体所展现的基态：著名的铜酸盐超导体（其在20世纪80年代的发现彻底改变了超导领域，并且仍然是最高临界温度的记录保持者）;最近发现的一些超导体是基于磁性最强的原子铁（这些新材料在2008年春天的发现有些令人惊讶，磁性通常被认为是与超导性竞争）;以及基于有机分子超导层的材料家族。我建议测量相互作用的强度，这些相互作用是负责这些材料的磁性和电子特性的系统被推，使用施加的压力，通过临界点，超导性成为主导。特别是，像这里考虑的分层材料中的电子相互作用只能通过一种称为角度相关磁电阻的技术可靠且完整地确定。这项技术仍然适用于大多数非常规超导体，特别是在高压下，很可能是因为它在实验上具有挑战性，只有少数研究人员熟悉。然而，进行这样的实验的回报是更深入地了解在超导状态的边缘发生的相互作用的变化。追踪稳定非常规超导性的机制是一个雄心勃勃的目标，许多传统的实验技术已经被证明是不合适的。根据该领域的最新进展，越来越清楚的是，成功的途径在于使高质量的样品经受现有的最极端的探针，即高磁场和高施加压力的组合。

Resistance is futile: lightbulbs and heaters aside, the majority of electronic components are at their most efficient when their electrical resistance is minimized. In the present climate, with energy sustainability regularly topping the international agenda, reducing the power lost in conducting devices or transmission lines is of worldwide importance. Research into the nature of novel conducting materials is hence vital to secure the global energy future.Superconductivity, the phenomenon of zero electrical resistance which occurs below a critical temperature in certain materials, remains inadequately explained. At present, these critical temperatures are typically very low, less than 140 Kelvin (-133 Celsius), but a more complete understanding of what causes the superconducting state to form could result in the design of materials that display superconductivity at the enhanced temperatures required for mass technological exploitation. Unfortunately, it is the very materials which are most likely to lead us to this end, the so-called unconventional superconductors, that are the least understood. In such materials, the superconducting state appears to be in competition with at least two other phases of matter: magnetism and normal, metallic conductivity. A delicate balance governs which is the dominant phase at low temperatures; the ground-state. By making slight adjustments to the composition of the materials or by applying moderate pressures certain interactions between the electrons in the compound can be strengthened at the expense of others causing the balance to tip in favour of a particular ground-state. The technicalities of how to do this are relatively well-known. What remains to be explained is why it happens, what it is that occurs at the vital tipping point where the superconductivity wins out over the magnetic or the metallic phases - in short, exactly what stabilizes the unconventional superconducting state? It is this question that the proposed project seeks to answer. I will use magnetic fields to explore the ground-states exhibited by three families of unconventional superconductor: the famous cuprate superconductors (whose discovery in the 1980s revolutionized the field of superconductivity and which remain the record-holders for the highest critical temperature); some recently discovered superconductors based on the most magnetic of atoms - iron (the discovery of these new materials in the spring of 2008 came as somewhat of a surprise, magnetism often being thought as competing with superconductivity); and a family of material based on superconducting layers of organic molecules. I propose to measure the strength of the interactions that are responsible for the magnetic and electronic properties of these materials as the systems are pushed, using applied pressure, through the tipping point at which the superconductivity becomes dominant. In particular, the electronic interactions in layered materials like those considered here can only be reliably and completely determined via a technique known as angle-dependent magnetoresistance. This technique remains to be applied to most unconventional superconductors, particularly at elevated pressures, mostly likely because it is experimentally challenging and familiar only to a handful of researchers. However, the rewards of performing such experiments are a far greater insight into what changes in interactions occur at the very edge of the superconducting state. Chasing the mechanism responsible for stabilizing unconventional superconductivity is an ambitious aim, and many traditional experimental techniques have proved inadquate. It is becoming clear, in the light of recent advances in the field, that the route to success lies in subjecting high-quality samples to the most extreme probes available, a combination of high magnetic fields and high applied pressures.