High pressure studies of quantum criticality in unconventional superconductors

Lead Research Organisation: University of Bristol
Department Name: Physics


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.

Planned Impact

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
Description This project aimed to study an aspect of superconductivity in particular materials tuned (by material or other parameters) to be close to a so-called quantum critical point. Such points exist where an ordered phase of a material, for example anti-ferromagnetism, is tuned so that the phase transition temperature is pushed down to zero kelvin. It is widely thought that close to such quantum-critical points the materials can have very unusual properties, such as high temperature superconductivity. In this project we studied a particular material, CeCu_2Si_2 which is close to such a quantum critical point at ambient pressure. This material was first discovered to be superconducting in 1979 and since then has been thought to be the prototypical example of a superconductor where the pairing between the electrons occurs through an unconventional mechanism (i.e., a mechanism different from the standard electron-phonon mechanism). Our re-evaluation of the properties of this material involved a new generation of higher purity materials and also new techniques to measure the magnetic penetration depth to very low temperatures (0.05K). We found, unexpectedly, that the electrons in this material seem to have condensed into an isotropic pairing state - similar to that found for conventional superconductors.
Another aspect of this project was to study the magnetic penetration depth as a function of pressure. The technical demands in achieving this were very high and a working apparatus was only achieved towards the end of the award. Experiments are continuing and further high impact outcomes are anticipated in the forthcoming year.
Exploitation Route The results help to guide out understanding of the fundamental reasons why materials superconduct, and hence eventually will help guide the search for new materials with better superconducting properties, such as a higher superconducting transition temperature.
Sectors Energy