̽��ֱ�� of Cambridge - National High Magnetic Field Laboratory

To conduct, or to insulate? That is the question

sc604 — Thu, 02 Jul 2015 18:00:00 +0000

A new study has discovered mysterious behaviour of a material that acts like an insulator in certain measurements, but simultaneously acts like a conductor in others. In an insulator, electrons are largely stuck in one place, while in a conductor, the electrons flow freely. ̽��ֱ��results, published today (2 July) in the journal Science, challenge current understanding of how materials behave.

Conductors, such as metals, conduct electricity, while insulators, such as rubber or glass, prevent or block the flow of electricity. But by tracing the path that electrons follow as they move through a material, researchers led by the ̽��ֱ�� of Cambridge found that it is possible for a single material to display dual metal-insulator properties at once – although at the very lowest temperatures, it completely disobeys the rules that govern conventional metals. While it’s not known exactly what’s causing this mysterious behaviour, one possibility is the existence of a potential third phase which is neither insulator nor conductor.

̽��ֱ��duelling metal-insulator properties were observed throughout the interior of the material, called samarium hexaboride (SmB6). There are other recently-discovered materials which behave both as a conductor and an insulator, but they are structured like a sandwich, so the surface behaves differently from the bulk. But the new study found that in SmB6, the bulk itself can be both conductor and insulator simultaneously.

“ ̽��ֱ��discovery of dual metal-insulator behaviour in a single material has the potential to overturn decades of conventional wisdom regarding the fundamental dichotomy between metals and insulators,” said Dr Suchitra Sebastian of the ̽��ֱ��’s Cavendish Laboratory, who led the research.

In order to learn more about SmB6 and various other materials, Sebastian and her colleagues traced the path that the electrons take as they move through the material: the geometrical surface traced by the orbits of the electrons leads to a construction which is known as a Fermi surface. In order to find the Fermi surface, the researchers used a technique based on measurements of quantum oscillations, which measure various properties of a material in the presence of a high magnetic field to get an accurate ‘fingerprint’ of the material. For quantum oscillations to be observed, the materials must be as close to pure as possible, so that there are minimal defects for the electrons to bump into. Key experiments for the research were conducted at the National High Magnetic Field Laboratory in Tallahassee, Florida.

SmB6 belongs to the class of materials called Kondo insulators, which are close to the border between insulating and conducting behaviour. Kondo insulators are part of a larger group of materials called heavy fermion materials, in which complex physics arises from an interplay of two types of electrons: highly localised ‘f’ electrons, and ‘d’ electrons, which have larger orbits. In the case of SmB6, correlations between these two types of electrons result in insulating behaviour.

“It’s a dichotomy,” said Sebastian. “ ̽��ֱ��high electrical resistance of SmB6 reveals its insulating behaviour, but the Fermi surface we observed was that of a good metal.”

But the mystery didn’t end there. At the very lowest temperatures, approaching 0 degrees Kelvin (-273 Celsius), it became clear that the quantum oscillations for SmB6 are not characteristic of a conventional metal. In metals, the amplitude of quantum oscillations grows and then levels off as the temperature is lowered. Strangely, in the case of SmB6, the amplitude of quantum oscillations continues to grow dramatically as the temperature is lowered, violating the rules that govern conventional metals.

̽��ֱ��researchers considered several reasons for this peculiar behaviour: it could be a novel phase, neither insulator nor conductor; it could be fluctuating back and forth between the two; or because SmB6 has a very small ‘gap’ between insulating and conducting behaviour, perhaps the electrons are capable of jumping that gap.

“ ̽��ֱ��crossover region between two different phases – magnetic and non-magnetic, for example – is where the really interesting physics happens,” said Sebastian. “Because this material is close to the crossover region between insulator and conductor, we found it displays some really strange properties – we’re exploring the possibility that it’s a new quantum phase.”

Tim Murphy, the head of the National High Magnetic Field Laboratory’s DC Field Facility, where most of the research was conducted, said: “This work on SmB6 provides a vivid and exciting illustration of emergent physics resulting from MagLab researchers refining the quality of the materials they study and pushing the sample environment to the extremes of high magnetic fields and low temperatures.”

̽��ֱ��Cambridge researchers were funded by the Royal Society, the Winton Programme for the Physics of Sustainability, the European Research Council and the Engineering and Physical Sciences Research Council (UK).

Researchers have identified a material that behaves as a conductor and an insulator at the same time, challenging current understanding of how materials behave, and pointing to a new type of insulating state.

̽��ֱ��discovery of dual metal-insulator behaviour in a single material has the potential to overturn decades of conventional wisdom regarding the fundamental dichotomy between metals and insulators

Suchitra Sebastian

PhD student Maria Kiourlappou holding a piece of SmB6

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Cambridge team breaks superconductor world record

sc604 — Thu, 26 Jun 2014 23:01:00 +0000

A world record that has stood for more than a decade has been broken by a team led by ̽��ֱ�� of Cambridge engineers, harnessing the equivalent of three tonnes of force inside a golf ball-sized sample of material that is normally as brittle as fine china.

̽��ֱ��Cambridge researchers managed to ‘trap’ a magnetic field with a strength of 17.6 Tesla - roughly 2000 times stronger than the field generated by a typical fridge magnet - in a high temperature gadolinium barium copper oxide (GdBCO) superconductor, beating the previous record by 0.4 Tesla. ̽��ֱ��results are published today in the journal Superconductor Science and Technology.

̽��ֱ��research demonstrates the potential of high-temperature superconductors for applications in a range of fields, including flywheels for energy storage, ‘magnetic separators’, which can be used in mineral refinement and pollution control, and in high-speed levitating monorail trains.

Superconductors are materials that carry electrical current with little or no resistance when cooled below a certain temperature. While conventional superconductors need to be cooled close to absolute zero (zero degrees on the Kelvin scale, or –273 °C) before they superconduct, high temperature superconductors do so above the boiling point of liquid nitrogen (–196 °C), which makes them relatively easy to cool and cheaper to operate.

Superconductors are currently used in scientific and medical applications, such as MRI scanners, and in the future could be used to protect the national grid and increase energy efficiency, due to the amount of electrical current they can carry without losing energy.

̽��ֱ��current carried by a superconductor also generates a magnetic field, and the more field strength that can be contained within the superconductor, the more current it can carry. State of the art, practical superconductors can carry currents that are typically 100 times greater than copper, which gives them considerable performance advantages over conventional conductors and permanent magnets.

̽��ֱ��new record was achieved using 25 mm diameter samples of GdBCO high temperature superconductor fabricated in the form of a large, single grain using an established melt processing method and reinforced using a relatively simple technique. ̽��ֱ��previous record of 17.24 Tesla, set in 2003 by a team led by Professor Masato Murakami from the Shibaura Institute of Technology in Japan, used a highly specialised type of superconductor of a similar, but subtly different, composition and structure.

“ ̽��ֱ��fact that this record has stood for so long shows just how demanding this field really is,” said Professor David Cardwell of Cambridge’s Department of Engineering and Fitzwilliam College, who led the research, in collaboration with Boeing and the National High Field Magnet Laboratory at the Florida State ̽��ֱ��. “There are real potential gains to be had with even small increases in field.”

To contain such a large field, the team used materials known as cuprates: thin sheets of copper and oxygen separated by more complex types of atoms. ̽��ֱ��cuprates were the earliest high temperature superconductors to be discovered, and have the potential to be used widely in scientific and medical applications.

While they are high quality superconductors with outstanding potential for practical applications, the cuprates can be as brittle as dried pasta when fabricated in the form of sintered ceramics, so trying to contain a strong magnetic field within bulk forms of the cuprates tends to cause them to explode.

In order to hold in, or trap, the magnetic field, the researchers had to modify both the microstructure of GdBCO to increase its current carrying and thermal performance, and reinforce it with a stainless steel ring, which was used to ‘shrink-wrap’ the single grain samples. “This was an important step in achieving this result,” said Dr John Durrell who led the experiment in Florida.

̽��ֱ��lines of magnetic flux in a superconductor repel each other strongly, making containing such a large field difficult. But, by engineering the bulk microstructure, the field is retained in the sample by so-called ‘flux pinning centres’ distributed throughout the material. “ ̽��ֱ��development of effective pinning sites in GdBCO has been key to this success,” said Dr Yun-Hua Shi, who has been responsible for developing the melt process fabrication technique at Cambridge for the past 20 years.

̽��ֱ��result was the biggest ever trapped field achieved in a bulk, standalone material at any temperature.

“This work could herald the arrival of superconductors in real-world applications,” said Professor Cardwell. “In order to see bulk superconductors applied for everyday use, we need large grains of superconducting material with the required properties that can be manufactured by relatively standard processes.”

A number of niche applications are currently being developed by the Cambridge team and its collaborators, and it is anticipated that widespread commercial applications for superconductors could be seen within the next five years.

“This record could not have been achieved without the support of our academic and industrial colleagues and partners,” said Professor Cardwell, who is the next Head of the Department of Engineering. “It was a real team effort, and one which we hope will bring these materials a significant step closer to practical applications.”

“Boeing continues to see practical applications for this superconducting material research and we are excited about the possibilities being enabled by the recent advances achieved by the Cambridge team,” said Patrick Stokes, who leads the Boeing-funded research portfolio with Cambridge ̽��ֱ��.

̽��ֱ��research was funded by ̽��ֱ��Boeing Company and by the UK Engineering and Physical Sciences Research Council (EPSRC). ̽��ֱ��National High Magnetic Field Laboratory, where the measurements were performed, is funded the National Science Foundation and the State of Florida.

New record for a trapped field in a superconductor, beating a record that has stood for more than a decade, could herald the arrival of materials in a broad range of fields.

̽��ֱ��fact that this record has stood for so long shows just how demanding this field really is

David Cardwell

̽��ֱ�� of Cambridge

A bulk superconductor levitated by a permanent magnet

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