What stabilizes unconventional superconductivity?

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.