̽��ֱ�� of Cambridge - electrons

Switching ‘spin’ on and off (and up and down) in quantum materials at room temperature

sc604 — Wed, 16 Aug 2023 15:00:00 +0000

Spin is the term for the intrinsic angular momentum of electrons, which is referred to as up or down. Using the up/down spin states of electrons instead of the 0 and 1 in conventional computer logic could transform the way in which computers process information. And sensors based on quantum principles could vastly improve our abilities to measure and study the world around us.

An international team of researchers, led by the ̽��ֱ�� of Cambridge, has found a way to use particles of light as a ‘switch’ that can connect and control the spin of electrons, making them behave like tiny magnets that could be used for quantum applications.

̽��ֱ��researchers designed modular molecular units connected by tiny ‘bridges’. Shining a light on these bridges allowed electrons on opposite ends of the structure to connect to each other by aligning their spin states. Even after the bridge was removed, the electrons stayed connected through their aligned spins.

This level of control over quantum properties can normally only be achieved at ultra-low temperatures. However, the Cambridge-led team has been able to control the quantum behaviour of these materials at room temperature, which opens up a new world of potential quantum applications by reliably coupling spins to photons. ̽��ֱ��results are reported in the journal Nature.

Almost all types of quantum technology – based on the strange behaviour of particles at the subatomic level – involve spin. As they move, electrons usually form stable pairs, with one electron spin up and one spin down. However, it is possible to make molecules with unpaired electrons, called radicals. Most radicals are very reactive, but with careful design of the molecule, they can be made chemically stable.

“These unpaired spins change the rules for what happens when a photon is absorbed and electrons are moved up to a higher energy level,” said first author Sebastian Gorgon, from Cambridge’s Cavendish Laboratory. “We’ve been working with systems where there is one net spin, which makes them good for light emission and making LEDs.”

Gorgon is a member of Professor Sir Richard Friend’s research group, where they have been studying radicals in organic semiconductors for light generation, and identified a stable and bright family of materials a few years ago. These materials can beat the best conventional OLEDs for red light generation.

“Using tricks developed by different fields was important,” said Dr Emrys Evans from Swansea ̽��ֱ��, who co-led the research. “ ̽��ֱ��team has significant expertise from a number of areas in physics and chemistry, such as the spin properties of electrons and how to make organic semiconductors work in LEDs. This was critical for knowing how to prepare and study these molecules in the solid state, enabling our demonstration of quantum effects at room temperature.”

Organic semiconductors are the current state-of-the-art for lighting and commercial displays, and they could be a more sustainable alternative to silicon for solar cells. However, they have not yet been widely studied for quantum applications, such as quantum computing or quantum sensing.

“We’ve now taken the next big step and linked the optical and magnetic properties of radicals in an organic semiconductor,” said Gorgon. “These new materials hold great promise for completely new applications, since we’ve been able to remove the need for ultra-cold temperatures.”

“Knowing what electron spins are doing, let alone controlling them, is not straightforward, especially at room temperature,” said Friend, who co-led the research. “But if we can control the spins, we can build some interesting and useful quantum objects.”

̽��ֱ��researchers designed a new family of materials by first determining how they wanted the electron spins to behave. Using this bottom-up approach, they were able to control the properties of the end material by using a building block method and changing the ‘bridges’ between different modules of the molecule. These bridges were made of anthracene, a type of hydrocarbon.

For their ‘mix-and-match’ molecules, the researchers attached a bright light-emitting radical to an anthracene molecule. After a photon of light is absorbed by the radical, the excitation spreads out onto the neighbouring anthracene, causing three electrons to start spinning in the same way. When a further radical group is attached to the other side of the anthracene molecules, its electron is also coupled, bringing four electrons to spin in the same direction.

“In this example, we can switch on the interaction between two electrons on opposite ends of the molecule by aligning electron spins on the bridge absorbing a photon of light,” said Gorgon. “After relaxing back, the distant electrons remember they were together even after the bridge is gone.

“In these materials we’ve designed, absorbing a photon is like turning a switch on. ̽��ֱ��fact that we can start to control these quantum objects by reliably coupling spins at room temperature could open up far more flexibility in the world of quantum technologies. There’s a huge potential here to go in lots of new directions.”

“People have spent years trying to get spins to reliably talk to each other, but by starting instead with what we want the spins to do and then the chemists can design a molecule around that, we’ve been able to get the spins to align,” said Friend. “It’s like we’ve hit the Goldilocks zone where we can tune the spin coupling between the building blocks of extended molecules.”

̽��ֱ��advance was made possible through a large international collaboration – the materials were made in China, experiments were done in Cambridge, Oxford and Germany, and theory work was done in Belgium and Spain.

̽��ֱ��research was supported in part by the European Research Council, the European Union, the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI), and the Royal Society. Richard Friend is a Fellow of St John’s College, Cambridge.

Reference:
Sebastian Gorgon et al. ‘Reversible spin-optical interface in luminescent organic radicals.’ Nature (2023). DOI: 10.1038/s41586-023-06222-1

Researchers have found a way to control the interaction of light and quantum ‘spin’ in organic semiconductors, that works even at room temperature.

These new materials hold great promise for completely new applications, since we’ve been able to remove the need for ultra-cold temperatures

Sebastian Gorgon

Artist's impression of aligned spins in an organic semiconductor

̽��ֱ��text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Photosynthesis ‘hack’ could lead to new ways of generating renewable energy

sc604 — Wed, 22 Mar 2023 15:57:53 +0000

Researchers have ‘hacked’ the earliest stages of photosynthesis, the natural machine that powers the vast majority of life on Earth, and discovered new ways to extract energy from the process, a finding that could lead to new ways of generating clean fuel and renewable energy.

Light used to detect quantum information stored in 100,000 nuclear quantum bits

sc604 — Mon, 15 Feb 2021 15:18:20 +0000

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge, were able to inject a ‘needle’ of highly fragile quantum information in a ‘haystack’ of 100,000 nuclei. Using lasers to control an electron, the researchers could then use that electron to control the behaviour of the haystack, making it easier to find the needle. They were able to detect the ‘needle’ with a precision of 1.9 parts per million: high enough to detect a single quantum bit in this large ensemble.

̽��ֱ��technique makes it possible to send highly fragile quantum information optically to a nuclear system for storage, and to verify its imprint with minimal disturbance, an important step in the development of a quantum internet based on quantum light sources. ̽��ֱ��results are reported in the journal Nature Physics.

̽��ֱ��first quantum computers – which will harness the strange behaviour of subatomic particles to far outperform even the most powerful supercomputers – are on the horizon. However, leveraging their full potential will require a way to network them: a quantum internet. Channels of light that transmit quantum information are promising candidates for a quantum internet, and currently there is no better quantum light source than the semiconductor quantum dot: tiny crystals that are essentially artificial atoms.

However, one thing stands in the way of quantum dots and a quantum internet: the ability to store quantum information temporarily at staging posts along the network.

“ ̽��ֱ��solution to this problem is to store the fragile quantum information by hiding it in the cloud of 100,000 atomic nuclei that each quantum dot contains, like a needle in a haystack,” said Professor Mete Atatüre from Cambridge’s Cavendish Laboratory, who led the research. “But if we try to communicate with these nuclei like we communicate with bits, they tend to ‘flip’ randomly, creating a noisy system.”

̽��ֱ��cloud of quantum bits contained in a quantum dot don’t normally act in a collective state, making it a challenge to get information in or out of them. However, Atatüre and his colleagues showed in 2019 that when cooled to ultra-low temperatures also using light, these nuclei can be made to do ‘quantum dances’ in unison, significantly reducing the amount of noise in the system.

Now, they have shown another fundamental step towards storing and retrieving quantum information in the nuclei. By controlling the collective state of the 100,000 nuclei, they were able to detect the existence of the quantum information as a ‘flipped quantum bit’ at an ultra-high precision of 1.9 parts per million: enough to see a single bit flip in the cloud of nuclei.

“Technically this is extremely demanding,” said Atatüre, who is also a Fellow of St John’s College. “We don’t have a way of ‘talking’ to the cloud and the cloud doesn’t have a way of talking to us. But what we can talk to is an electron: we can communicate with it sort of like a dog that herds sheep.”

Using the light from a laser, the researchers are able to communicate with an electron, which then communicates with the spins, or inherent angular momentum, of the nuclei.

By talking to the electron, the chaotic ensemble of spins starts to cool down and rally around the shepherding electron; out of this more ordered state, the electron can create spin waves in the nuclei.

“If we imagine our cloud of spins as a herd of 100,000 sheep moving randomly, one sheep suddenly changing direction is hard to see,” said Atatüre. “But if the entire herd is moving as a well-defined wave, then a single sheep changing direction becomes highly noticeable.”

In other words, injecting a spin wave made of a single nuclear spin flip into the ensemble makes it easier to detect a single nuclear spin flip among 100,000 nuclear spins.

Using this technique, the researchers are able to send information to the quantum bit and ‘listen in’ on what the spins are saying with minimal disturbance, down to the fundamental limit set by quantum mechanics.

“Having harnessed this control and sensing capability over this large ensemble of nuclei, our next step will be to demonstrate the storage and retrieval of an arbitrary quantum bit from the nuclear spin register,” said co-first author Daniel Jackson, a PhD student at the Cavendish Laboratory.

“This step will complete a quantum memory connected to light – a major building block on the road to realising the quantum internet,” said co-first author Dorian Gangloff, a Research Fellow at St John’s College.

Besides its potential usage for a future quantum internet, the technique could also be useful in the development of solid-state quantum computing.

̽��ֱ��research was supported in part by the European Research Council (ERC), the Engineering and Physical Sciences Research Council (EPSRC) and the Royal Society.

Reference:
D. M. Jackson et al. ‘Quantum sensing of a coherent single spin excitation in a nuclear ensemble.’ Nature Physics (2021). DOI: 10.1038/s41567-020-01161-4

Researchers have found a way to use light and a single electron to communicate with a cloud of quantum bits and sense their behaviour, making it possible to detect a single quantum bit in a dense cloud.

We don’t have a way of ‘talking’ to the cloud and the cloud doesn’t have a way of talking to us. But what we can talk to is an electron: we can communicate with it sort of like a dog that herds sheep

Mete Atatüre

Gerd Altmann from Pixabay

Quantum particles

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̽��ֱ��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.

̽��ֱ��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 Cambridge. “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.

̽��ֱ��launch of a UK-based programme last year provoked excitement within the scientific community. “Cambridge Uni spins up green and beefy supercomputer project,” announced British tech site ̽��ֱ��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.

̽��ֱ��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. ̽��ֱ��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 ̽��ֱ�� of Kyoto, to broaden materials research on superconducting spintronics.

Although still at an experimental stage, the project – which includes collaborators from Imperial College London, ̽��ֱ�� 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.”

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

Robinson points out that the UK – and Cambridge 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 Cambridge, 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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Researchers road-test powerful method for studying singlet fission

tdk25 — Mon, 17 Oct 2016 15:00:00 +0000

Physicists have successfully employed a powerful technique for studying electrons generated through singlet fission, a process which it is believed will be key to more efficient solar energy production in years to come.

Their approach, reported in the journal Nature Physics, employed lasers, microwave radiation and magnetic fields to analyse the spin of excitons, which are energetically excited particles formed in molecular systems.

These are generated as a result of singlet fission, a process that researchers around the world are trying to understand fully in order to use it to better harness energy from the sun. Using materials exhibiting singlet fission in solar cells could make energy production much more efficient in the future, but the process needs to be fully understood in order to optimize the relevant materials and design appropriate technologies to exploit it.

In most existing solar cells, light particles (or photons) are absorbed by a semiconducting material, such as silicon. Each photon stimulates an electron in the material's atomic structure, giving a single electron enough energy to move. This can then potentially be extracted as electrical current.

In some materials, however, the absorption of a single photon initially creates one higher-energy, excited particle, called a spin singlet exciton. This singlet can also share its energy with another molecule, forming two lower-energy excitons, rather than just one. These lower-energy particles are called spin "triplet" excitons. Each triplet can move through the molecular structure of the material and be used to produce charge.

̽��ֱ��splitting process - from one absorbed photon to two energetic triplet excitons - is singlet fission. For scientists studying how to generate more solar power, it represents a potential bargain - a two-for-one offer on the amount of electrical current generated, relative to the amount of light put in. If materials capable of singlet fission can be integrated into solar cells, it will become possible to generate energy more efficiently from sunlight.

But achieving this is far from straightforward. One challenge is that the pairs of triplet excitons only last for a tiny fraction of a second, and must be separated and used before they decay. Their lifespan is connected to their relative "spin", which is a unique property of elementary particles and is an intrinsic angular momentum. Studying and measuring spin through time, from the initial formation of the pairs to their decay, is essential if they are to be harnessed.

In the new study, researchers from the ̽��ֱ�� of Cambridge and the Freie Universität Berlin (FUB) utilised a method that allows the spin properties of materials to be measured through time. ̽��ֱ��approach, called electron spin resonance (ESR) spectroscopy, has been used and improved since its discovery over 50 years ago to better understand how spin impacts on many different natural phenomena.

It involves placing the material being studied within a large electromagnet, and then using laser light to excite molecules within the sample, and microwave radiation to measure how the spin changes over time. This is especially useful when studying triplet states formed by singlet fission as these are difficult to study using most other techniques.

Because the excitons' spin interacts with microwave radiation and magnetic fields, these interactions can be used as an additional way to understand what happens to the triplet pairs after they are formed. In short, the approach allowed the researchers to effectively watch and manipulate the spin state of triplet pairs through time, following formation by singlet fission.

̽��ֱ��study was led by Professor Jan Behrends at the Freie Universität Berlin (FUB), Dr Akshay Rao, a College Research Associate at St John's College, ̽��ֱ�� of Cambridge, and Professor Neil Greenham in the Department of Physics, ̽��ֱ�� of Cambridge.

Leah Weiss, a Gates-Cambridge Scholar and PhD student in Physics based at Trinity College, Cambridge, was the paper's first author. "This research has opened up many new questions," she said. "What makes these excited states either separate and become independent, or stay together as a pair, are questions that we need to answer before we can make use of them."

̽��ֱ��researchers were able to look at the spin states of the triplet excitons in considerable detail. They observed pairs had formed which variously had both weakly and strongly-linked spin states, reflecting the co-existence of pairs that were spatially close and further apart. Intriguingly, the group found that some pairs which they would have expected to decay very quickly, due to their close proximity, actually survived for several microseconds.

"Finding those pairs in particular was completely unexpected," Weiss added. We think that they could be protected by their overall spin state, making it harder for them to decay. Continued research will focus on making devices and examining how these states can be harnessed for use in solar cells."

Professor Behrends added: "This interdisciplinary collaboration nicely demonstrates that bringing together expertise from different fields can provide novel and striking insights. Future studies will need to address how to efficiently split the strongly-coupled states that we observed here, to improve the yield from singlet fission cells."

Beyond trying to improve photovoltaic technologies, the research also has implications for wider efforts to create fast and efficient electronics using spin, so-called "spintronic" devices, which similarly rely on being able to measure and control the spin properties of electrons.

̽��ֱ��research was made possible with support from the UK Engineering and Physical Sciences Research Council (EPSRC) and from the Freie Universität Berlin (FUB). Weiss and colleague Sam Bayliss carried out the spectroscopy experiments within the laboratories of Professor Jan Behrends and Professor Robert Bittl at FUB. ̽��ֱ��work is also part of the Cambridge initiative to connect fundamental physics research with global energy and environmental challenges, backed by the Winton Programme for the Physics of Sustainability.

̽��ֱ��study, Strongly exchange-coupled triplet pairs in an organic semiconductor, is published in Nature Physics. DOI: 10.1038/nphys3908.

In a new study, researchers measure the spin properties of electronic states produced in singlet fission – a process which could have a central role in the future development of solar cells.

Future research will focus on making devices and examining how these states can be harnessed for use in solar cells

Leah Weiss

Spin, an intrinsic property of electrons, is related to the dynamics of electrons excited as a result of singlet fission – a process which could be used to extract energy in future solar cell technologies.

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A tight squeeze for electrons – quantum effects observed in ‘one-dimensional’ wires

sc604 — Thu, 15 Sep 2016 07:00:00 +0000

Scientists have controlled electrons by packing them so tightly that they start to display quantum effects, using an extension of the technology currently used to make computer processors. ̽��ֱ��technique, reported in the journal Nature Communications, has uncovered properties of quantum matter that could pave a way to new quantum technologies.

̽��ֱ��ability to control electrons in this way may lay the groundwork for many technological advances, including quantum computers that can solve problems fundamentally intractable by modern electronics. Before such technologies become practical however, researchers need to better understand quantum, or wave-like, particles, and more importantly, the interactions between them.

Squeezing electrons into a one-dimensional ‘quantum wire’ amplifies their quantum nature to the point that it can be seen, by measuring at what energy and wavelength (or momentum) electrons can be injected into the wire.

“Think of a crowded train carriage, with people standing tightly packed all the way down the centre of the carriage,” said Professor Christopher Ford of the ̽��ֱ�� of Cambridge’s Cavendish Laboratory, one of the paper’s co-authors. “If someone tries to get in a door, they have to push the people closest to them along a bit to make room. In turn, those people push slightly on their neighbours, and so on. A wave of compression passes down the carriage, at some speed related to how people interact with their neighbours, and that speed probably depends on how hard they were shoved by the person getting on the train. By measuring this speed, one could learn about the interactions.”

“ ̽��ֱ��same is true for electrons in a quantum wire – they repel each other and cannot get past, so if one electron enters or leaves, it excites a compressive wave like the people in the train,” said the paper’s first author Dr Maria Moreno, also from the Cavendish Laboratory.

However, electrons have another characteristic, their angular momentum or ‘spin’, which also interacts with their neighbours. Spin can also set off a wave carrying energy along the wire, and this spin wave travels at a different speed to the charge wave. Measuring the wavelength of these waves as the energy is varied is called tunnelling spectroscopy. ̽��ֱ��separate spin and charge waves were detected experimentally by researchers from Harvard and Cambridge Universities.

Now, in the paper published in Nature Communications, the Cambridge researchers have gone one stage further, to test the latest predictions of what should happen at high energies, where the original theory breaks down.

A flurry of theoretical activity in the past decade has led to new predictions of other ways of exciting waves among the electrons — it’s as if the person entering the train pushes so hard some people fall over and knock into others much further down the carriage. These new ‘modes’ are weaker than the spin and charge waves and so are harder to detect.

̽��ֱ��collaborators of the Cambridge researchers from the ̽��ֱ�� of Birmingham predicted that there would be a hierarchy of modes corresponding to the variety of ways in which the interactions can affect the quantum-mechanical particles, and the weaker modes should be strongest in very short wires.

To make a set of such short wires, the Cambridge group set about devising a way of making contact to a set of 6000 narrow strips of metal that are used to create the quantum wires from the semiconducting material gallium arsenide (GaAs). This required an extra layer of metal in the shape of bridges between the strips.

By varying the magnetic field and voltage, the tunnelling from the wires to an adjacent sheet of electrons could be mapped out, and this revealed evidence for the extra curves predicted, where it can be seen as an upside-down replica of the spin curve.

These results will now be applied to better understand and control the behaviour of electrons in the building blocks of a quantum computer.

Reference:
Moreno et al. ‘Nonlinear spectra of spinons and holons in short GaAs quantum wires.’ Nature Communications (2016).DOI: 10.1038/ncomms12784

Researchers have observed quantum effects in electrons by squeezing them into one-dimensional ‘quantum wires’ and observing the interactions between them. ̽��ֱ��results could be used to aid in the development of quantum technologies, including quantum computing.

Dr Maria Moreno

Regime of a single 1D wire subband filled

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New state of matter detected in a two-dimensional material

sc604 — Mon, 04 Apr 2016 15:04:40 +0000

An international team of researchers have found evidence of a mysterious new state of matter, first predicted 40 years ago, in a real material. This state, known as a quantum spin liquid, causes electrons – thought to be indivisible building blocks of nature – to break into pieces.

̽��ֱ��researchers, including physicists from the ̽��ֱ�� of Cambridge, measured the first signatures of these fractional particles, known as Majorana fermions, in a two-dimensional material with a structure similar to graphene. Their experimental results successfully matched with one of the main theoretical models for a quantum spin liquid, known as a Kitaev model. ̽��ֱ��results are reported in the journal Nature Materials.

Quantum spin liquids are mysterious states of matter which are thought to be hiding in certain magnetic materials, but had not been conclusively sighted in nature.

̽��ֱ��observation of one of their most intriguing properties — electron splitting, or fractionalisation — in real materials is a breakthrough. ̽��ֱ��resulting Majorana fermions may be used as building blocks of quantum computers, which would be far faster than conventional computers and would be able to perform calculations that could not be done otherwise.

“This is a new quantum state of matter, which has been predicted but hasn’t been seen before,” said Dr Johannes Knolle of Cambridge’s Cavendish Laboratory, one of the paper’s co-authors.

In a typical magnetic material, the electrons each behave like tiny bar magnets. And when a material is cooled to a low enough temperature, the ‘magnets’ will order themselves over long ranges, so that all the north magnetic poles point in the same direction, for example.

But in a material containing a spin liquid state, even if that material is cooled to absolute zero, the bar magnets would not align but form an entangled soup caused by quantum fluctuations.

“Until recently, we didn’t even know what the experimental fingerprints of a quantum spin liquid would look like,” said paper co-author Dr Dmitry Kovrizhin, also from the Theory of Condensed Matter group of the Cavendish Laboratory. “One thing we’ve done in previous work is to ask, if I were performing experiments on a possible quantum spin liquid, what would I observe?”

Knolle and Kovrizhin’s co-authors, led by Dr Arnab Banerjee and Dr Stephen Nagler from Oak Ridge National Laboratory in the US, used neutron scattering techniques to look for experimental evidence of fractionalisation in alpha-ruthenium chloride (α-RuCl₃). ̽��ֱ��researchers tested the magnetic properties of α-RuCl₃ powder by illuminating it with neutrons, and observing the pattern of ripples that the neutrons produced on a screen when they scattered from the sample.

A regular magnet would create distinct sharp lines, but it was a mystery what sort of pattern the Majorana fermions in a quantum spin liquid would make. ̽��ֱ��theoretical prediction of distinct signatures by Knolle and his collaborators in 2014 match well with the broad humps instead of sharp lines which experimentalists observed on the screen, providing for the first time direct evidence of a quantum spin liquid and the fractionalisation of electrons in a two dimensional material.

“This is a new addition to a short list of known quantum states of matter,” said Knolle.

“It’s an important step for our understanding of quantum matter,” said Kovrizhin. “It’s fun to have another new quantum state that we’ve never seen before – it presents us with new possibilities to try new things.”

Reference:
A. Banerjee et al. ‘Proximate Kitaev quantum spin liquid behaviour in a honeycomb magnet.’ Nature Materials (2016). DOI: 10.1038/nmat4604

Researchers have observed the ‘fingerprint’ of a mysterious new quantum state of matter in a two-dimensional material, in which electrons break apart.

It’s an important step for our understanding of quantum matter.

Dmitry Kovrizhin

Genevieve Martin, Oak Ridge National Laboratory

Excitation of a spin liquid on a honeycomb lattice with neutrons.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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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

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes