��Ī - superconductor

Some superconductors can also carry currents of ‘spin’

sc604 — Mon, 16 Apr 2018 15:00:00 +0000

Spin is a particle’s intrinsic angular momentum, and is normally carried in non-superconducting, non-magnetic materials by individual electrons. Spin can be ‘up’ or ‘down’, and for any given material, there is a maximum length that spin can be carried. In a conventional superconductor electrons with opposite spins are paired together so that a flow of electrons carries zero spin.

A few years ago, researchers from the ��Ī showed that it was possible to create electron pairs in which the spins are aligned: up-up or down-down. The spin current can be carried by up-up and down-down pairs moving in opposite directions with a net charge current of zero. The ability to create such a pure spin supercurrent is an important step towards the team’s vision of creating a superconducting computing technology which could use massively less energy than the present silicon-based electronics.

Now, the same researchers have found a set of materials which encourage the pairing of spin-aligned electrons, so that a spin current flows more effectively in the superconducting state than in the non-superconducting (normal) state. Their results are reported in the journal Nature Materials.

“Although some aspects of normal state spin electronics, or spintronics, are more efficient than standard semiconductor electronics, their large-scale application has been prevented because the large charge currents required to generate spin currents waste too much energy,” said Professor Mark Blamire of ��Ī’s Department of Materials Science and Metallurgy, who led the research. “A fully-superconducting method of generating and controlling spin currents offers a way to improve on this.”

In the current work, Blamire and his collaborators used a multi-layered stack of metal films in which each layer was only a few nanometres thick. They observed that when a microwave field was applied to the films, it caused the central magnetic layer to emit a spin current into the superconductor next to it.

“If we used only a superconductor, the spin current is blocked once the system is cooled below the temperature when it becomes a superconductor,” said Blamire. “The surprising result was that when we added a platinum layer to the superconductor, the spin current in the superconducting state was greater than in the normal state.”

Although the researchers have shown that certain superconductors can carry spin currents, so far these only occur over short distances. The next step for the research team is to understand how to increase the distance and how to control the spin currents.

The research was funded by the Engineering and Physical Sciences Research Council (EPSRC).

Reference:
Kun-Rok Jeon et al. ‘Enhanced spin pumping into superconductors provides evidence for superconducting pure spin currents.’ Nature Materials (2018). DOI: 10.1038/s41563-018-0058-9

Researchers have shown that certain superconductors – materials that carry electrical current with zero resistance at very low temperatures – can also carry currents of ‘spin’. The successful combination of superconductivity and spin could lead to a revolution in high-performance computing, by dramatically reducing energy consumption.

Jason Robinson

Conceptual image of spin current flow in a superconductor

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The Electron Manifesto: transforming high performance computing with 'spintronics'

tdk25 — Wed, 26 Jul 2017 11:54:19 +0000

In the early days of the computer, calculators were room-sized and public demand was low. Now, it’s the reverse. Digital technology has become smaller and faster, and our dependence on it has grown.

We are almost desensitised to a stream of facts about the startling rate at which this is occurring. In 2016, IBM found that humans now create 2.5 quintillion bytes of data daily. From the start of this decade to its end, the world’s data will increase 50 times over.

The basic building blocks of electronic devices, such as the transistor, work by moving packets of charge around a circuit. A single unit of charge is an electron, and its movement is governed by semiconductors, commonly made from silicon. But technology based on these principles is now reaching a point where it cannot get much smaller or faster. A paradigm shift is due.

“There have been many failed attempts to oust silicon from its predominance,” reflects Professor Mark Blamire, Head of Materials Science at ��Ī. “Something has to be done because the technology can’t be scaled to smaller sizes for very much longer. It’s already a major source of power consumption. There’s no obvious competitor, so in a sense the opportunity is there.”

Blamire and his colleague Dr Jason Robinson are leading several major programmes investigating one such competitor, known as superconducting spintronics.

The launch of a UK-based programme last year provoked excitement within the scientific community. “��Ī Uni spins up green and beefy supercomputer project,” announced British tech site The Register, for example. One reason in particular is because superconducting spintronics might address the eye-watering energy consumption of the huge server farms that handle internet traffic. Data centres account for 3% of the world’s electricity supply and about 2% of greenhouse gas emissions.

The project combines two phenomena: superconductivity and spin. Superconductivity refers to the fact that at low temperatures some materials carry a charge with zero resistance. Unlike, for example, copper wires, which lose energy as heat, superconductors are therefore extremely energy efficient.

‘Spin’ is the expression for electrons’ intrinsic source of magnetism. Originally it was thought that this existed because electrons were indeed spinning, which turned out to be wrong, but the name stuck, and it is still used to describe the property in particles that makes them behave a bit like tiny bar magnets. Like a magnet, this property makes the electrons point a certain way; the spin state is therefore referred to as ‘up’ or ‘down’.

Researchers have been using the magnetic moments of electrons to store and read data since the 1980s. At their most basic, spintronic devices use the up/down states instead of the 0 and 1 in conventional computer logic.

Spintronics could also transform the way in which computers process information. The researchers envisage that instead of the devices moving packets of charge around, they will transmit information using the relative spin of a series of electrons, known as a ‘pure spin current’, and sense these using magnetic elements within a circuit.

By eliminating the movement of charge, any such device would need less power and be less prone to overheating – removing some of the most significant obstacles to further improving computer efficiency. Spintronics could therefore give us faster, energy-efficient computers, capable of performing more complex operations than at present.

To generate large enough spin currents for memory and logic devices, significant charge is required as an input, and the power requirements of this currently outweigh many of the benefits. Using a superconductor to provide that charge, given its energy efficiency, would present a solution. But the magnetic materials used to control spin within spintronic devices also interfere with superconductivity.

This problem was thought insurmountable until, in 2010, Robinson discovered how to combine superconductors and spintronics so that they can work together in complete synergy. His team added an intervening magnetic layer (a material called holmium). By using this interface, they were able to preserve the delicate balance of electron pairing that’s needed to achieve superconductivity, but still managed to create a bias within the overall spin of the electrons.

This, explains Robinson, “created a marriage that opens up the emerging field of superconducting spintronics.” Over the next five years, he and Blamire developed the field, and last year were awarded a major grant from the Engineering and Physical Sciences Research Council: “To lead the world in understanding the coupling of magnetism and superconductivity to enable future low energy computing technologies.” Robinson has since been awarded a second grant with Professor Yoshi Maeno, from the University of Kyoto, to broaden materials research on superconducting spintronics.

Although still at an experimental stage, the project – which includes collaborators from Imperial College London, University College London and Royal Holloway London – is tackling questions such as how to generate and control the flow of spin in a superconducting system. And its scope is already expanding. “We have found more ways of achieving what we are trying to do than we originally dreamed up,” Robinson says.

One example involves making potentially innovative use of superconductivity itself. In ‘conventional’ spintronics, spin is manipulated through the interactions between magnetic materials within the device. But Blamire has found that when a superconductor is placed between two ferromagnets, its intrinsic energy depends on the orientation of those magnetic layers. “Turning that on its head, if you can manipulate the superconducting state, you can control the orientation of the magnetic layers, and therefore the spin,” he says.

Meanwhile, Robinson has led a study that for the first time enabled graphene, a material already recognised for its potential to revolutionise the electronics industry, to superconduct. This raises the possibility of using this extraordinary material, and other two-dimensional materials like it, in superconducting spintronics.

Although approaches like this are still being tested, Blamire says that by 2021 the team will have developed sample logic and memory devices that fuse superconductivity and spin. These proof-of-concept models could, perhaps, be incorporated into a new type of computer processor. “It would be a huge step to get from there to a device that could be competitive,” he admits. “It’s not necessarily difficult, but it would require considerable investment.”

The project is set up to enable industrial collaboration in the years to come. A key partner is the Hitachi Lab in ��Ī, while the project’s advisory board also features representatives from the ��Ī-based semiconductor firm ARM, and HYPRES, a digital superconductor company in the USA.

Robinson points out that the UK – and ��Ī in particular – has historical strengths in research into superconductivity and spintronics, but adds that a “grand challenge” has long been needed to focus academic investigation on a meaningful partnership with industry.

Leading low-energy computing into a post-semiconductor age is certainly grand. Silicon’s domination, after all, stretches from its eponymous valley in California, to a fen in ��Ī, a gulf in the Philippines and an island in Japan.

Can the unlikely – not to say still primitive – marriage of spintronics and superconductivity really replace an electronic empire on which the sun never sets? “I suspect people had similar questions at the dawn of the semiconductor,” Robinson observes. “One shouldn’t lose sight of what we are doing here. We aren’t just trying to do something better; we are offering something entirely different and new.”

Electron ‘spin’ could hold the key to managing the world’s growing data demands without consuming huge amounts of energy. Now, researchers have shown that energy-efficient superconductors can power devices designed to achieve this. What once seemed an impossible marriage of superconductivity and spin may be about to transform high performance computing.

One shouldn’t lose sight of what we are doing here. We aren’t just trying to do something better; we are offering something entirely different and new.

Jason Robinson

Creativity103

Spinning top

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��Ī alumni win 2016 Nobel Prize in Physics

sjr81 — Tue, 04 Oct 2016 10:05:15 +0000

David Thouless (Trinity Hall, 1952), Duncan Haldane (Christ’s, 1970) and Michael Kosterlitz (Gonville and Caius, 1962) discovered unexpected behaviours of solid materials - and devised a mathematical framework to explain their properties. Their discoveries have led to new materials with an array of unique properties.

The Prize was divided, one half awarded to Thouless, the other half jointly to Haldane and Kosterlitz. The trio become the 93rd, 94th and 95th Nobel Affiliates of ��Ī to be awarded a Nobel Prize.

“This prize is richly deserved,” said Professor Nigel Cooper of ��Ī’s Cavendish Laboratory. “Through the great breakthroughs they’ve made, Thouless, Haldane and Kosterlitz took a visionary approach to understanding how topology plays a role in novel materials.”

Topology is a mathematical concept that accounts for how certain physical properties are related by smooth deformations: a football can be smoothly deformed into a rugby ball (so these have the same topology), but neither of these can be smoothly deformed into a bicycle tube (which therefore has different topology). The Laureates recognized how novel states of matter could arise due to the differing topologies of how the underlying particles arrange themselves at the microscopic level.

The Nobel Assembly made their announcement this morning (October 4), saying: “This year’s Laureates opened the door on an unknown world where matter can assume strange states. They have used advanced mathematical methods to study unusual phases, or states, of matter, such as superconductors, superfluids or thin magnetic films. Thanks to their pioneering work, the hunt is now on for new and exotic phases of matter. Many people are hopeful of future applications in both materials science and electronics.

“The three Laureates’ use of topological concepts in physics was decisive for their discoveries. Topology is a branch of mathematics that describes properties that only change step-wise. Using topology as a tool, they were able to astound the experts. In the early 1970s, Michael Kosterlitz and David Thouless overturned the then current theory that superconductivity or suprafluidity could not occur in thin layers. They demonstrated that superconductivity could occur at low temperatures and also explained the mechanism, phase transition, that makes superconductivity disappear at higher temperatures.

"In the 1980s, Thouless was able to explain a previous experiment with very thin electrically conducting layers in which conductance was precisely measured as integer steps. He showed that these integers were topological in their nature. At around the same time, Duncan Haldane discovered how topological concepts can be used to understand the properties of chains of small magnets found in some materials.

"We now know of many topological phases, not only in thin layers and threads, but also in ordinary three-dimensional materials. Over the last decade, this area has boosted frontline research in condensed matter physics, not least because of the hope that topological materials could be used in new generations of electronics and superconductors, or in future quantum computers. Current research is revealing the secrets of matter in the exotic worlds discovered by this year’s Nobel Laureates.”

Professor Haldane is the current Eugene Higgins Professor of Physics at Princeton University. Born in London in 1951, he came to Christ’s as an undergraduate in 1970 to read Natural Sciences. His PhD was conferred in 1978.

Professor Kosterlitz is the Harrison E. Farnsworth Professor of Physics at Brown University, where he joined the faculty in 1982. He was born to German Jewish emigres in 1942 and his father was the pioneering biochemist Hans Walter Kosterlitz. Professor Kosterlitz, who came to ��Ī in 1965, is the 14th Nobel Laureate affiliated to Gonville and Caius.

Professor Thouless, born in 1934, is Emeritus Professor of Physics at the University of Washington. An undergraduate at Trinity Hall, he was also previously a Visiting Fellow at Clare Hall, where he was awarded a Doctorate of Science in 1985. He has been a Life Member of the college since 1986.

Professor Thouless was also a Fellow of Churchill College from 1961-65, and in 1961 became its first Director of Studies for Physics. He has also held the position of Visiting Fellow at Churchill. He is Churchill's 31st Nobel Affiliate and Trinity Hall's first.

The Master of Caius, Professor Sir Alan Fersht, today warmly congratulated Prof Kosterlitz, who was his exact contemporary at Caius, coming up to ��Ī to read Natural Sciences in 1962. "This is fantastic news," Sir Alan said. "Mike was obviously an exceptionally clever guy. We went to physics lectures together in our first year, and he continued to specialise in Physics in the second year while I specialised in Chemistry. He was a very good physicist, and moved from the UK to America fairly rapidly.

"He was an absolutely mad climber - he disappeared every weekend to go mountain climbing in the Peak District. He lived on Tree Court, and he built a traverse around the room where he would climb using his fingers and hanging on to the picture rail."

More details on previous ��Ī winners can be found here: /research/research-at-cambridge/nobel-prize.

The first Nobel Prize in Physics was awarded in 1901.

This is great, groundbreaking materials science. The work is "beautiful and deep" with big applications in future electronics #NobelPrize
— Dr Paul Coxon (@paulcoxon) October 4, 2016

Check out our story on #NobelPrize -winning Caian Michael Kosterlitz and find out why he used to climb the walls https://www.cai.cam.ac.uk/news/caian-wins-nobel-prize-physics-2016
— Gonville & Caius (@CaiusCollege) October 4, 2016

Congratulations to Christ's alumnus Duncan Haldane! /research/news/cambridge-alumni-win-2016-nobel-prize-in-physics
— Christ's College (@christs_college) October 4, 2016

David Thouless (1952) has been awarded the #NobelPrize in Physics. /research/news/cambridge-alumni-win-2016-nobel-prize-in-physics
— Trinity Hall (@TrinityHallCamb) October 4, 2016

If you wanted to have a quick explanation of topological phases. ☺️ #NobelPrize #Physics #Topology https://twitter.com/nobelprize/status/783245808611135489
— Mete Atature (@MeteAtature) October 4, 2016

It all started at Caius... Co-winner of 2016 #NobelPrize for #Physics Michael Kosterlitz in his Matriculation photo at Caius in 1962 pic.twitter.com/M9p3D15fcl
— Gonville & Caius (@CaiusCollege) October 4, 2016

Three alumni of the ��Ī were today awarded the 2016 Nobel Prize in Physics for their pioneering work in the field of condensed matter physics.

The trio become the 93rd, 94th and 95th Nobel Affiliates of ��Ī to be awarded a Nobel Prize.

Nobelprize.org

Left to right: David Thouless, Duncan Haldane, and Michael Kosterlitz

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Quantum effects at work in the world’s smelliest superconductor

sc604 — Mon, 28 Mar 2016 15:00:00 +0000

The quantum behaviour of hydrogen affects the structural properties of hydrogen-rich compounds, which are possible candidates for the elusive room temperature superconductor, according to new research co-authored at the ��Ī.

New theoretical results, published online in the journal Nature, suggest that the quantum nature of hydrogen – meaning that it can behave like a particle or a wave – strongly affects the recently discovered hydrogen sulphur superconductor, a compound that when subjected to extremely high pressure, is the highest-temperature superconductor yet identified. This new step towards understanding the underlying physics of high temperature superconductivity may aid in the search for a room temperature superconductor, which could be used for applications such as levitating trains, lossless electrical grids and next-generation supercomputers.

Superconductors are materials that carry electrical current with zero electrical resistance. Low-temperature, or conventional, superconductors were first identified in the early 20th century, but they need to be cooled close to absolute zero (zero degrees on the Kelvin scale, or -273 degrees Celsius) before they start to display superconductivity. For the past century, researchers have been searching for materials that behave as superconductors at higher temperatures, which would make them more suitable for practical applications. The ultimate goal is to identify a material which behaves as a superconductor at room temperature.

Last year, German researchers identified the highest temperature superconductor yet – hydrogen sulphide, the same compound that gives rotten eggs their distinctive odour. When subjected to extreme pressure – about one million times higher than the Earth’s atmospheric pressure – this stinky compound displays superconducting behaviour at temperatures as high as 203 Kelvin (-70 degrees Celsius), which is far higher than any other high temperature superconductor yet discovered.

Since this discovery, researchers have attempted to understand what it is about hydrogen sulphide that makes it capable of superconducting at such high temperatures. Now, new theoretical results suggest that the quantum behaviour of hydrogen may be the reason, as it changes the structure of the chemical bonds between atoms. The results were obtained by an international collaboration of researchers led by the University of the Basque Country and the Donostia International Physics Center, and including researchers from the ��Ī.

The behaviour of objects in our daily life is governed by classical, or Newtonian, physics. If an object is moving, we can measure both its position and momentum, to determine where an object is going and how long it will take to get there. The two properties are inherently linked.

However, in the strange world of quantum physics, things are different. According to a rule known as Heisenberg’s uncertainty principle, in any situation in which a particle has two linked properties, only one can be measured and the other must be uncertain.

Hydrogen, being the lightest element of the periodic table, is the atom most strongly subjected to quantum behaviour. Its quantum nature affects structural and physical properties of many hydrogen compounds. An example is high-pressure ice, where quantum fluctuations of the proton lead to a change in the way that the molecules are held together, so that the chemical bonds between atoms become symmetrical.

The researchers behind the current study believe that a similar quantum hydrogen-bond symmetrisation occurs in the hydrogen sulphide superconductor.

Theoretical models that treat hydrogen atoms as classical particles predict that at extremely high pressures – even higher than those used by the German researchers for their record-breaking superconductor – the atoms sit exactly halfway between two sulphur atoms, making a fully symmetrical structure. However, at lower pressures, hydrogen atoms move to an off-centre position, forming one shorter and one longer bond.

The researchers have found that when considering the hydrogen atoms as quantum particles behaving like waves, they form symmetrical bonds at much lower pressures – around the same as those used for the German-led experiment, meaning that quantum physics, and symmetrical hydrogen bonds, were behind the record-breaking superconductivity.

“That we are able to make quantitative predictions with such a good agreement with the experiments is exciting and means that computation can be confidently used to accelerate the discovery of high temperature superconductors,” said study co-author Professor Chris Pickard of ��Ī’s Department of Materials Science & Metallurgy.

According to the researcher’s calculations, the quantum symmetrisation of the hydrogen bond has a tremendous impact on the vibrational and superconducting properties of hydrogen sulphide. “In order to theoretically reproduce the observed pressure dependence of the superconducting critical temperature the quantum symmetrisation needs to be taken into account,” said the study’s first author, Ion Errea, from the University of the Basque Country and Donostia International Physics Center.

The discovery of such a high temperature superconductor suggests that room temperature superconductivity might be possible in other hydrogen-rich compounds. The current theoretical study shows that in all these compounds, the quantum motion of hydrogen can strongly affect the structural properties, even modifying the chemical bonding, and the electron-phonon interaction that drives the superconducting transition.

“Theory and computation have played an important role in the hunt for superconducting hydrides under extreme compression,” said Pickard. “The challenges for the future are twofold - increasing the temperature towards room temperature, but, more importantly, dramatically reducing the pressures required.”

Reference:
Ion Errea et. al. ‘Quantum hydrogen-bond symmetrization in the superconducting hydrogen sulfide system.’ Nature (2016).DOI: 10.1038/nature17175.

Researchers have found that quantum effects are the reason that hydrogen sulphide – which has the distinct smell of rotten eggs –behaves as a superconductor at record-breaking temperatures, which may aid in the search for room temperature superconductors.

That we are able to make quantitative predictions with such a good agreement with the experiments is exciting and means that computation can be confidently used to accelerate the discovery of high temperature superconductors.

Chris Pickard

UPV/EHU

Structure with symmetric hydrogen bonds induced by the quantum behavior of the protons, represented by the fluctuating blue spheroids

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��Ī 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 ��Ī 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.

The ��Ī 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. The results are published today in the journal Superconductor Science and Technology.

The 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.

The 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.

The 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. The 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.

“The fact that this record has stood for so long shows just how demanding this field really is,” said Professor David Cardwell of ��Ī’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 University. “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. The 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.

The 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. “The 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 ��Ī for the past 20 years.

The 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 ��Ī 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 ��Ī team,” said Patrick Stokes, who leads the Boeing-funded research portfolio with ��Ī University.

The research was funded by The Boeing Company and by the UK Engineering and Physical Sciences Research Council (EPSRC). The 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.

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

David Cardwell

��Ī

A bulk superconductor levitated by a permanent magnet

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Superconducting secrets solved after 30 years

sc604 — Sun, 15 Jun 2014 17:00:00 +0000

Harnessing the enormous technological potential of high-temperature superconductors – which could be used in lossless electrical grids, next-generation supercomputers and levitating trains – could be much more straightforward in future, as the origin of superconductivity in these materials has finally been identified.

Superconductors, materials which can carry electric current with zero resistance, could be used in a huge range of applications, but a lack of understanding about where their properties originate from has meant that the process of identifying new materials has been rather haphazard.

Researchers from the ��Ī have found that ripples of electrons, known as charge density waves or charge order, create twisted ‘pockets’ of electrons in these materials, from which superconductivity emerges. The results are published in the June 15th issue of the journal Nature.

Low-temperature, or conventional, superconductors were first identified in the early 20th century, but they need to be cooled close to absolute zero (zero degrees on the Kelvin scale, or -273 degrees Celsius) before they start to display superconductivity. So-called high-temperature superconductors however, can display the same properties at temperatures up to 138 Kelvin (-135 degrees Celsius), making them much more suitable for practical applications.

Since they were first identified in the mid-1980s, the process of discovering new high-temperature superconductors could be best described as random. While researchers have identified the ingredients that make for a good low-temperature superconductor, high-temperature superconductors have been more reluctant to give up their secrets.

In a superconductor, as in any electronic device, current is carried via the charge on an electron. What is different about superconductors is that the electrons travel in tightly bound pairs. When travelling on their own, electrons tend to bump into each other, resulting in a loss of energy. But when paired up, the electrons move smoothly through a superconductor’s structure, which is why superconductors can carry current with no resistance. As long as the temperature is kept sufficiently low, the electron pairs will keep moving through the superconductor indefinitely.

Key to conventional superconductors are the interactions of electrons with the lattice structure of the material. These interactions generate a type of ‘glue’ which holds the electrons together. The strength of the glue is directly related to the strength of the superconductor, and when the superconductor is exposed to an increase in temperature or magnetic field strength, the glue is weakened, the electron pairs break apart and superconductivity is lost.

“One of the problems with high-temperature superconductors is that we don’t know how to find new ones, because we don’t actually know what the ingredients are that are responsible for creating high-temperature superconductivity in the first place,” said Dr Suchitra Sebastian of the Cavendish Laboratory, lead author of the paper. “We know there’s some sort of glue which causes the electrons to pair up, but we don’t know what that glue is.”

In order to decode what makes high-temperature superconductors tick, the researchers worked backwards: by determining what properties the materials have in their normal, non-superconducting state, they might be able to figure out what was causing superconductivity.

“We’re trying to understand what sorts of interactions were happening in the material before the electrons paired up, because one of those interactions must be responsible for creating the glue,” said Dr Sebastian. “Once the electrons are already paired up, it’s hard to know what made them pair up. But if we can break the pairs apart, then we can see what the electrons are doing and hopefully understand where the superconductivity came from.”

Superconductivity tends to override other properties. For example, if in its normal state a superconductor was a magnet, suppressing that magnetism has been found to result in superconductivity. “So by determining the normal state of a superconductor, it would make the process of identifying new ones much less random, as we’d know what sorts of materials to be looking for in the first place,” said Dr Sebastian.

Working with extremely strong magnetic fields, the researchers were able to kill the superconducting effect in cuprates - thin sheets of copper and oxygen separated by more complex types of atoms.

Previous attempts to determine the origins of superconductivity by determining the normal state have used temperature instead of magnetic field to break the electron pairs apart, which has led to inconclusive results.

As cuprates are such good superconductors, it took the strongest magnetic fields in the world – 100 Tesla, or roughly one million times stronger than the Earth’s magnetic field – in order to suppress their superconducting properties.

These experiments were finally able to solve the mystery surrounding the origin of pockets of electrons in the normal state that pair to create superconductivity. It was previously widely held that electron pockets were located in the region of strongest superconductivity. Instead, the present experiments using strong magnetic fields revealed a peculiar undulating twisted pocket geometry -similar to Jenga bricks where each layer goes in a different direction to the one above or beneath it.

These results pinpointed the pocket locations to be where superconductivity is weakest, and their origin to be ripples of electrons known as charge density waves, or charge order. It is this normal state that is overridden to yield superconductivity in the family of cuprate superconductors studied.

“By identifying other materials which have similar properties, hopefully it will help us find new superconductors at higher and higher temperatures, even perhaps materials which are superconductors at room temperature, which would open up a huge range of applications,” said Dr Sebastian.

A breakthrough has been made in identifying the origin of superconductivity in high-temperature superconductors, which has puzzled researchers for the past three decades.

By identifying other materials which have similar properties, hopefully it will help us find new superconductors at higher and higher temperatures

Suchitra Sebastian

Nicolle R Fuller

Map of superconducting copper oxide structure.

Yes

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