High pressure studies of quantum criticality in unconventional superconductors

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Superconductivity is a phenomena that has the potential to transform society. For example, the lossless transport of electricity through wires has the ability to make tremendous savings in our electrical distribution infrastructure. The unique properties of superconductors enable several other key technologies, such as the generation of large, ultra stable magnetic fields needed for magnetic resonance imaging in healthcare. The unique properties of superconducting Josephson junctions are perhaps the most promising route towards the development of a quantum computer which could revolutionize our digital economy.Although many applications of superconductors are already in the marketplace there is still enormous potential. To realise this we need to gain a better understanding of the phenomena of superconductivity, in particular its microscopic origin in so-called unconventional high temperature superconductors which have been discovered in the last 25 years. Although there has been much progress towards the goal of developing a microscopic theory of unconventional superconductivity the answer has still not been found. A highly promising, relatively recent, theory to explain these effect relies on physics of the highly quantum mechanically entangled state of matter which exists close to a quantum critical point, where an order/disorder phase transition is tuned to absolute zero by some external parameter such as pressure.In this proposal we plan to study in detail the effect of quantum criticality on the superconductivity in several different candidate systems. These materials encompass all the major families of superconductors which have been discovered in the last 25 years (cupates, organics, heavy fermions, iron-pnictides) can all be tuned towards an quantum critical point with external pressure. Unfortunately, superconductivity itself prevents almost all established methods used to study quantum criticality. One way round this problem is to apply a large external magnetic field to quench the superconductivity. However, this may also have the effect of modifying the quantum critical behaviour of interest.Fortunately, the magnetic penetration depth provides a sensitive and unique probe of the underlying normal state electronic structure with zero applied magnetic field and at very low temperature. Our proposal seeks to measure the absolute penetration depth in a pressure cell for the first time. This will provide unique information which will guide theories of how (or if) quantum criticality causes high temperature superconductivity.