What stabilizes unconventional superconductivity?

Lead Research Organisation: University of Warwick
Department Name: Physics

Abstract

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.

Publications

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Related Projects

Project Reference Relationship Related To Start End Award Value
EP/H00324X/1 01/11/2009 30/06/2013 £831,622
EP/H00324X/2 Transfer EP/H00324X/1 01/07/2013 31/10/2014 £275,479
 
Description Note: this grant is the same as EP/H00324X/1 but with a different host organisation. The results here are duplicated. This research was aimed at understanding what mechanisms drive the high temperature superconducting state, as well as understanding low dimensional magnetism by studying self-assembly molecular materials. As part of this project we developed a new measurement technique for measuring the magnetic properties of materials in the restrictive confines of a high field magnet and/or a cell for the application of high pressures. This method has since been used by other groups. Several studies of the electronic properties of unconventional superconductors were made and reported in high-impact journals. In particular, an analysis of the data resulting from several experiments in the high temperature superconductor YBCO enabled us to give the fullest identification so far of the electronic structure of this material and correlate this with recently identified details in the charge structure, which are currently suspected to play an important role in the formation of the superconducting state. This study was published in Nature in 2014. Further measurements on other, so far hidden, aspects of the electronic properties were made and are now in analysis. The other aspect of the proposal - the low-dimensional magnets - has also been successful. So much so that an additional New Directions award was made midway through the grant to further evolve this aspect of the research. Our key findings in this area are an experimental recipe for establishing the magnetic properties of these compounds, methods by which the magnetic interactions in molecular materials can be controlled and tuned, the testing of theoretical models of quantum magnetism, and the identification of exotic magnetic ground states.

Update 2017: Angle-dependent magnetoresistance data taken on an under-doped curate (a high-temperature superconductor) during the project lifetime were extensively analysed and modelled. In this way we detected a breaking of fourfold rotational symmetry on the Fermi surface (the most important element of the electronic structure of any metallic system). This symmetry breaking is consistent with that seen in other aspects of these materials, but, despite previous experiments, was not observed at the Fermi surface until now. These results were published in npj Quantum Materials in February 2017.
Exploitation Route Amongst other achievements, we have identified the Fermi surface of the underdoped cuprates. This represents a fixed point against which the success of future and existing theories of the high-temperature superconducting state can be tested. We also detected a breaking of fourfold symmetry on the Fermi surface for the first time, which places constraints on the nature of the reconstruction that takes place in the high temperature superconductors close to optimal doping. The area of molecular magnetism is burgeoning and more and more researchers are building on the results we have obtained in the last five years. We and others are presently working on extending the our findings to other materials and using other techniques.
Sectors Electronics,Energy,Environment,Transport

URL http://rdcu.be/pgq2
 
Description EPSRC New Directions for Developing Leaders
Amount £282,333 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2012 
End 10/2014
 
Description High-temperature superconductivity 
Organisation Los Alamos National Laboratory
Country United States 
Sector Public 
PI Contribution High magnetic field measurements
Collaborator Contribution Experimental data, samples, experimental results
Impact This collaboration is multidisciplinary involving physics, chemistry, materials science. The collaboration has resulting in several high-impact publications as outlined in the relevant section.
Start Year 2010
 
Description High-temperature superconductivity 
Organisation University of British Columbia
Country Canada 
Sector Academic/University 
PI Contribution High magnetic field measurements
Collaborator Contribution Experimental data, samples, experimental results
Impact This collaboration is multidisciplinary involving physics, chemistry, materials science. The collaboration has resulting in several high-impact publications as outlined in the relevant section.
Start Year 2010
 
Description High-temperature superconductivity 
Organisation University of Cambridge
Country United Kingdom 
Sector Academic/University 
PI Contribution High magnetic field measurements
Collaborator Contribution Experimental data, samples, experimental results
Impact This collaboration is multidisciplinary involving physics, chemistry, materials science. The collaboration has resulting in several high-impact publications as outlined in the relevant section.
Start Year 2010
 
Description Quantum magnetism in molecular materials 
Organisation Durham University
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaboration to explore low-dimensional magnetism via the creation and characterization of magnetic systems using coordination chemistry, high magnetic fields and applied pressures.
Collaborator Contribution Providing samples and experimental data.
Impact Many of the entries under 'publications' are a result of this collaboration. Disciplines: chemistry, physics, materials science.
Start Year 2007
 
Description Quantum magnetism in molecular materials 
Organisation Eastern Washington University
Country United States 
Sector Academic/University 
PI Contribution Collaboration to explore low-dimensional magnetism via the creation and characterization of magnetic systems using coordination chemistry, high magnetic fields and applied pressures.
Collaborator Contribution Providing samples and experimental data.
Impact Many of the entries under 'publications' are a result of this collaboration. Disciplines: chemistry, physics, materials science.
Start Year 2007
 
Description Quantum magnetism in molecular materials 
Organisation Los Alamos National Laboratory
Country United States 
Sector Public 
PI Contribution Collaboration to explore low-dimensional magnetism via the creation and characterization of magnetic systems using coordination chemistry, high magnetic fields and applied pressures.
Collaborator Contribution Providing samples and experimental data.
Impact Many of the entries under 'publications' are a result of this collaboration. Disciplines: chemistry, physics, materials science.
Start Year 2007
 
Description Quantum magnetism in molecular materials 
Organisation University of Oxford
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaboration to explore low-dimensional magnetism via the creation and characterization of magnetic systems using coordination chemistry, high magnetic fields and applied pressures.
Collaborator Contribution Providing samples and experimental data.
Impact Many of the entries under 'publications' are a result of this collaboration. Disciplines: chemistry, physics, materials science.
Start Year 2007
 
Description Outreach 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Development of workshops for pre-GCSE schoolchildren on the subject of Superconductivity and magnetism. Several hundred children in the Oxford area have now taken part.

More schools have requested the workshop. There are plans for the workshops to be implemented in Midlands and North East.
Year(s) Of Engagement Activity 2012,2013,2014
URL http://www.physics.ox.ac.uk/Users/edwardsm/Site/Welcome.html