Magnetic flux line structures and phase transitions in unconventional and conventional superconductors

Lead Research Organisation: University of Birmingham
Department Name: School of Physics and Astronomy

Abstract

When a magnetic field is applied to any superconductor, its initial response is to exclude the field: this leads to the phenomenon of levitation of magnets by superconductors. However, many superconductors allow larger magnetic fields to penetrate, which they do as individual lines of magnetic flux. One of the practically important aspects of these flux lines is that currents passed through the superconductor may force them to move. If this occurs, energy will be dissipated, and the superconductor will become resistive. However, if the flux lines are pinned to impurities in the material, the superconductor may carry large currents without dissipation. Some of our research is directed towards understanding pinning, by measuring how the flux lines interact with the pinning centres and with each other. Another important aim is to use the observation of flux lines to tell us more about the superconductors themselves. A well-established example of this is the amount of magnetic flux in each line, which is given by the ratio of Planck's constant (which relates the wavelength of quantum particles to their momentum) to twice the electronic charge (which demonstrates that in superconductors, the electrons are coupled together in pairs). Electrons pair up in all known superconductors, but the type of pairing can be greatly different in different materials. For instance, in high-temperature superconductors, the electrons in each pair are circulating around each other. This should cause the cores of the flux lines to have a four-leaved clover-like structure, arising from the standing electron waves making up the pairs. When the flux lines are squeezed by increasing the magnetic field, the interaction between these odd-shaped cores becomes important. We are using neutron diffraction to observe these flux lines; if on a rainy night, you view the light from a sodium street lamp through the material of an umbrella, it is found to be surrounded by a pattern of diffraction spots,. These arise from the interference of light waves passing through the regularly-spaced fibres in the umbrella material. In a similar way, if one views a source of slow neutrons, such as a research reactor, through a superconductor containing flux lines, the main neutron beam is found to be surrounded by diffracted neutrons. The neutron diffraction pattern tells us about the arrangement of flux lines in the material. In the simplest case, this would be a hexagonal packing, like that adopted by a handful of pencils. In practice, we are finding many other arrangements, which change with the density of flux lines and tell us about their interactions with each other and with the underlying crystal structure of the superconductor. Our measurements rely on two properties of neutrons: firstly, that they have quantum-wave-like properties, and secondly, that they behave like microscopic magnets, so that they are diffracted by something as apparently insubstantial as a magnetic field. In this research, we are applying these methods to a wide variety of superconductors, and extending them to new extremes of high magnetic field at ultra-low temperatures. Here we have indications of a new kind of flux line, with a magnetic core like the lead in a pencil. We are also interested in the role of increasing temperature, which will tend to make the flux lines move from their equilibrium positions, and possibly get into an arrangement like cooked spaghetti, which is certainly associated with the appearance of electrical resistance. This phenomenon has been clearly observed at elevated temperatures in High-Tc materials, but so far it has evaded clear demonstration in conventional superconductors, which suggests that our understanding is imperfect. In summary, we are pushing forward an interlocking programme, which will increase our understanding of the many ways in which the fascinating and useful phenomenon of superconductivity can occur.

Publications

10 25 50

 
Description Flexible design of cryomagnet proved extremely successful and has enabled high-field studies of High-Temperature and Pnictide superconductors, NdFeB high field nanomagnets and colloidal suspensions in aqueous solution - all with neutron scattering, plus X-ray scattering from superconductors. None of these would have been possible without this unique equipment.
In the follow-up grant, he main goal was to develop a dilution refrigerator system that could be fitted into the 17 T cryomagnet, for use in X-ray and neutron scattering. This goal has been achieved.

A secondary goal of the new grant was to use the magnet in its previous configuration to carry out further experiments on the behaviour of a range of materials in extreme conditions of field and temperature. Most notably, we have discovered a 'Charge Density Wave' - a new ordered state - common to the high temperature superconductors using this magnet, and have been busy exploring this with high-profile publications resulting.
Exploitation Route The use of this magnet is an international advertisement for UK Industry The equipment is easily transportable to research establishments in Europe, which enables its use by us and collaborators in a variety of contexts.
The Helmholtz Zentrum Berlin is interested in adopting our methodology to build a similar dilution refrigeration option for their own horizontal magnet for neutron scattering. We are currently discussing a joint project to do this with them.
The Helmholtz Zentrum Berlin is interested in adopting our methodology to build a similar dilution refrigeration option for their own horizontal magnet for neutron scattering. We are currently discussing a joint project to do this with them.

Several weeks of experimental time at the Institut Laue-Langevin and SINQ, Paul Scherrer Institute, are planned for later in 2016 to investigate unconventional superconductors at dilution temperatures, by other users.
Sectors Manufacturing, including Industrial Biotechology,Other

URL http://www.cm.ph.bham.ac.uk/publications/publications.html
 
Description The success of our 17 T magnet has led to enquiries from overseas researchers and further orders for the UK manufacturer. As a result of the more recent technical developments during the follow-up grant from the EPSRC, we are in discussions with a group at the Helmholtz Zentrum Berlin about adapting the expertise developed in dilution refrigeration in high magnetic fields to a specific problem that they have. Negotiations on a joint project are at an advanced stage. Part of this developing expertise has involved collaboration with local Birmingham jewellery expertise, for manufacturing pieces from bespoke silver alloys.
First Year Of Impact 2014
Sector Other
Impact Types Economic

 
Description UK Neutron Strategy Panel
Geographic Reach Europe 
Policy Influence Type Participation in a advisory committee
 
Description EPSRC
Amount £616,478 (GBP)
Funding ID EP/J016977/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 06/2012 
 
Description Leverhulme Emeritus Fellowships
Amount £21,830 (GBP)
Organisation The Leverhulme Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 12/2015 
End 11/2017
 
Description Magnetic flux line structures and phase transitions in unconventional and conventional superconductors
Amount £755,389 (GBP)
Funding ID EP/G027161/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2009 
End 06/2012
 
Description Scattering studies of emergent phenomena at extreme conditions of magnetic field and temperature
Amount £625,257 (GBP)
Funding ID EP/J016977/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 07/2012 
 
Description Cryogenic Ltd 
Organisation Cryogenic
Country United Kingdom 
Sector Private 
PI Contribution Joint design of Variable-Temperature Insert for cryomagnet. Local design of in-vacuum manipulator which can be licensed to Cryogenic
Collaborator Contribution Modification of cryomagnet design to meet our requirements. Joint design of Variable-Temperature Insert for cryomagnet
Impact Publicity for UK Cryogenic Industry and sales of similar products abroad
Start Year 2009
 
Description HZB High Field Beamline 
Organisation Helmholtz Association of German Research Centres
Department Helmholtz-Zentrum Berlin for Materials and Energy
Country Germany 
Sector Academic/University 
PI Contribution Member of High Field magnet Steering Cttee. Proposed a novel design of a dilution refrigerator insert for the High Field magnet
Collaborator Contribution Discussion of contract for us to supply the DR with financial and technical support from HZB
Impact Not yet
Start Year 2015