High pressure studies of quantum criticality in unconventional superconductors

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.

The work outlined in this proposal is fundamental research into the mechanism of unconventional superconductivity. Superconductors are already used in numerous applications; the most important (by economic value) is currently magnetic resonance imaging (MRI) scanners. However, the future use could be much wider-ranging if the properties of the available materials could be improved. The key parameters are the critical temperature and the ability of the material to carry a high electrical current. If these parameters could be suitably enhanced the societal and economic benefits would be immense as these materials could then be used to improve the vast majority of devices which currently use electricity as well as facilitate the development of completely new devices. Possible applications range from the generation, transmission and distribution of electricity to improving the speed of computers by reducing the power dissipation in integrated circuits, improving the diagnostic abilities of MRI scanners (by increasing the available magnetic field and possibly reducing the need for cryogenic refrigeration) and also mass people transport through the development of superconducting levitating trains, such as the Linear Chuo Shinkansen Project presently under development in Japan.

A crucial step on the pathway to achieving this goal is to gain a more comprehensive picture of the fundamental physics that causes materials to superconduct at high temperature. If these were known, it would clarify the route towards optimising current materials as well as inform the search for new materials. Our proposed programme to investigate the nature of superconductivity close to quantum critical points using measurements of the magnetic penetration depth will help to guide theoretical understanding of how quantum criticality boosts the superconducting transition temperature. Therefore it moves towards a better understanding and eventual control of the fundamental physics that drives these materials fascinating and unique properties.

A recent comprehensive report(*), from a panel commissioned by the US Department of Energy on the basic research needs for superconductivity, identifies the following as priority research directions: i) Understand and exploit competing electronic phases, ii) Identify the essential interactions that give rise to high-Tc superconductivity, iii) Develop a comprehensive and predictive theory of superconductivity. Our programme impacts strongly on all three of these themes.
* Available from http://science.energy.gov/~/media/bes/pdf/reports/files/sc_rpt.pdf