What stabilizes unconventional superconductivity?

电阻是无用的：除了灯泡和加热器，大多数电子元件在电阻最小时效率最高。在当前的气候下，能源可持续性经常成为国际议程的首要议题，减少导电设备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于确保全球能源的未来至关重要。超导现象，即某些材料在低于临界温度时出现的零电阻现象，至今仍未得到充分解释。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出在大规模技术开发所需的更高温度下显示超导性的材料。不幸的是，正是那些最有可能把我们引向这一终点的材料，即所谓的非常规超导体，却最不为人所知。在这类材料中，超导状态似乎至少与其他两种物质相竞争：磁性和正常的金属导电性。一种微妙的平衡支配着低温下的主导相；极化子。通过对材料的组成进行轻微的调整或施加适度的压力，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得到加强，从而使平衡向有利于特定基态的方向倾斜。如何做到这一点的技术细节相对来说是众所周知的。有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转折点上发生的是什么——简而言之，到底是什么稳定了非常规的超导状态？拟议的项目试图回答的正是这个问题。我将用磁场来探索三种非常规超导体所表现出的基态：著名的铜超导体（它在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）；一些最近发现的超导体是基于磁性最强的原子——铁（这些新材料的发现在2008年春天有些令人惊讶，磁性通常被认为是与超导性竞争的）；以及一系列基于有机分子超导层的材料。我建议测量相互作用的强度，这些相互作用负责这些材料的磁性和电子特性，当系统被推动时，使用施加的压力，通过超导性成为主导的临界点。特别是，层状材料中的电子相互作用，如这里所考虑的，只能通过一种称为角相关磁电阻的技术来可靠和完全地确定。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，而且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。追求稳定非常规超导的机制是一个雄心勃勃的目标，许多传统的实验技术已被证明是不够的。鉴于该领域最近的进展，越来越清楚的是，通往成功的途径在于将高质量的样品置于可用的最极端的探针下，即高磁场和高压的结合。

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.