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Topological materials for Green ICT, spintronics , terahertz and quantum technology

The  discovery of topological materials whose properties  remain intact even when an object is stretched, twisted or deformed, led to the awarding of the Nobel Prize in physics in 2016 to F. Duncan M. Haldane,  and others. Their topological nature means these states are resistant to change, and so stable to temperature fluctuations and physical distortion — features that could make them useful in devices.


“Imagine a rope identified by a number of knots,” Suyang Xu, assistant professor of chemical biology, said. “No matter how much the shape of the rope is changed, the number of knots — known as the topological number — cannot be changed without altering its fundamental identity by adding or undoing knots.” It is this robustness that potentially makes topological materials particularly useful.


Topological materials, hold promise for a wide range of technological applications due to their exotic electronic properties such as ultralow-energy transistors, cancer-scanning lasers­, and free-space communication beyond 5G.


“In everyday life we are familiar with conducting materials, such as copper and insulating materials, such as plastic or glass. However there are also topological insulators with very peculiar properties,” says the Utrecht University professor. “These materials are insulating in the bulk, but current can flow along the edges. Furthermore, the conductivity is quantised and varies in discrete steps. This special property, of being both a conductor and an insulator, has had semiconductor researchers excited for computers that operate on ultra-low power, while also being much faster and more reliable.


Just like graphene, electric currents can flow in topological semimetals with virtually zero dissipation of energy, potentially making them useful for ultralow-power electronics, says physicist Masaki Uchida at the Tokyo Institute of Technology. At the same time, researchers can theoretically vary the thicknesses of topological semimetals to tune their properties, whereas atomically thin graphene has finite thickness and thus less flexibility for design ­purposes, says physicist Yee Sin Ang at the Singapore University of Technology and Design.


A new study represents a significant advance in topological transistors and beyond-CMOS electronics. First time that the topological state in a topological insulator has been switched on and off using an electric field. Researchers proved this is possible at room temperature, which is necessary for any viable replacement to CMOS technology in everyday applications.


Topological materials

Topology examines the properties of objects and solids that are protected against perturbations and deformations. Materials known so far include topological insulators, which are crystals that insulate on the inside but conduct electrical current on their surface. The conducting surfaces are topologically protected, which means that they cannot easily be brought into an insulating state.


Topological materials can be classified into topological insulators (TIs), topological crystalline insulators, topological Dirac semimetals, topological Weyl semimetals, topological nodal-line semimetals, and others. The topologically nontrivial nature is tied to the appearance of inverted bands in the electronic structure.


Theoretical physicists at the University of Zurich have now predicted a new class of topological insulators with conducting properties on the edges of crystals rather than on the surface. The research team, made up of scientists from UZH, Princeton University, the Donostia International Physics Center and the Max Planck Institute of Microstructure Physics in Halle, dubbed the new material class “higher-order topological insulators.” The extraordinary robustness of the conducting edges makes them particularly interesting: The current of topological electrons cannot be stopped by disorder or impurities. If an imperfection gets in the way of the current, it simply flows around the impurity.


In addition, the crystal edges do not have to be specially prepared to conduct electrical current. If the crystal breaks, the new edges automatically also conduct current. “The most exciting aspect is that electricity can at least in theory be conducted without any dissipation,” says Titus Neupert, professor at the Department of Physics at UZH. “You could think of the crystal edges as a kind of highway for electrons. They can’t simply make a U-turn.” This property of dissipationless conductance, otherwise known from superconductors at low temperatures, is not shared with the previously known topological insulator crystals that have conducting surfaces, but is specific to the higher-order topological crystals.


The Australian Research Council’s Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET) is  realising new types of electronic conduction without resistance in solid-state systems at room temperature. These concepts will form the basis of new types of switching devices (transistors) with vastly lower energy consumption per computation than silicon CMOS. Electronic conduction without resistance will be realised in topological insulators that conduct only along their edges, and in semiconductors that support superflow of electrons strongly coupled to photons. These pathways are enabled by the new science of atomically thin materials.”


Topological materials are full of surprises for physicists. For example, it is not just electrons that can freely move along the edges. The spin information can also move along the edges of these materials. Morais Smith: “If all electrons with an upward spin move in one direction and those with a downward spin in the other direction, then on balance no charge has moved, but spin information has. This phenomenon is important in spintronics. Due to this quantised conductivity, topological materials make quantum spintronics possible. Furthermore, the topological protection is so robust that small impurities exert no influence on it.


The physicists’ study still mostly relies on theoretical aspects. They have proposed tin telluride as the first compound to show these novel properties. “More material candidates have to be identified and probed in experiments,” says Neupert. The researchers hope that in the future nanowires made of higher-order topological insulators may be used as conducting paths in electric circuits. They could be combined with magnetic and superconducting materials and used for building quantum computers.


Scientists have developed the world’s best-performing pure spin current source made of bismuth-antimony (BiSb) alloys, which they report as the best candidate for the first industrial application of topological insulators. The achievement represents a big step forward in the development of spin-orbit torque magnetoresistive random-access memory (SOT-MRAM) devices with the potential to replace existing memory technologies.


A research team led by Pham Nam Hai at the Department of Electrical and Electronic Engineering, Tokyo Institute of Technology (Tokyo Tech), has developed thin films of BiSb for a topological insulator that simultaneously achieves a colossal spin Hall effect and high electrical conductivity. Their study, published in Nature Materials, could accelerate the development of high-density, ultra-low power, and ultra-fast non-volatile memories for Internet of Things (IoT) and other applications now becoming increasingly in demand for industrial and home use.


Topological semimetals can also display unexpected properties—for example, physicist Ken Burch at Boston College and his colleagues found that tantalum arsenide can intrinsically generate more than 10 times as much electric current from light as any other material. This effect occurs with mid-­infrared light, which suggests that tantalum arsenide could find use in chemical and thermal imaging. “You could also imagine taking infrared radiation emitted as waste energy off hot objects and converting it to useful electricity,” Burch says.


Topological material switched off and on for the first time

Now, FLEET researchers at Monash University, Australia, have for the first time successfully ‘switched’ a material between these two states of matter via application of an electric-field. This is the first step in creating a functioning topological transistor — a proposed new generation of ultra-low energy electronic devices. Ultra-low energy electronics such as topological transistors would allow computing to continue to grow, without being limited by available energy as we near the end of achievable improvements in traditional, silicon-based electronics (a phenomenon known as the end of Moore’s Law).


“Ultra-low energy topological electronics are a potential answer to the increasing challenge of energy wasted in modern computing,” explains study author Professor Michael Fuhrer. “Information and Communications Technology (ICT) already consumes 8% of global electricity, and that’s doubling every decade.” This new study is a major advance towards that goal of a functioning topological transistor.


Topological insulators are novel materials that behave as electrical insulators in their interior, but can carry a current along their edges. “In these edge paths, electrons can only travel in one direction,” explains lead author Dr Mark Edmonds. “And this means there can be no ‘back-scattering,’ which is what causes electrical resistance in conventional electrical conductors.” Unlike conventional electrical conductors, such topological edge paths can carry electrical current with near-zero dissipation of energy, meaning that topological transistors could burn much less energy than conventional electronics. They could also potentially switch must faster.


Topological materials would form a transistor’s active, ‘channel’ component, accomplishing the binary operation used in computing, switching between open (0) and closed (1). “This new switch works on a fundamentally different principle than the transistors in today’s computers,” explains Dr Edmonds. “We envision such switches facilitating a completely new computing technology, which uses lower energy.”  The electric field induces a quantum transition from ‘topological’ insulator to conventional insulator.


To be a viable alternative to current, silicon-based technology (CMOS), topological transistors must:

  • operate at room temperature (without the need for expensive supercooling),
  • ‘switch’ between conducting (1) and non-conducting (0), and
  • switch extremely rapidly, by application of an electric field.”

While switchable topological insulators have been proposed in theory, this is the first time that experiment has proved that a material can switch at room temperature, which is crucial for any viable replacement technology. (In this study, experiments were conducted at cryogenic temperatures, but the large band-gap measured confirms that the material will switch properly at room temperatures.)

BiSb expands the potential of topological insulators for ultra-low-power electronic devices

A research team led by Pham Nam Hai at the Department of Electrical and Electronic Engineering, Tokyo Institute of Technology (Tokyo Tech), has developed thin films of BiSb for a topological insulator that simultaneously achieves a colossal spin Hall effect and high electrical conductivity.


The BiSb thin films achieve a colossal spin Hall angle of approximately 52, conductivity of 2.5 x 105 and spin Hall conductivity of 1.3×107 at room temperature. Notably, the spin Hall conductivity is two orders of magnitude greater than that of bismuth selenide (Bi2Se3), reported in Nature in 2014.


Until now, the search for suitable spin Hall materials for next-generation SOT-MRAM devices has been faced with a dilemma: First, heavy metals such as platinum, tantalum and tungsten have high electrical conductivity but a small spin Hall effect. Second, topological insulators investigated to date have a large spin Hall effect but low electrical conductivity.


The BiSb thin films satisfy both requirements at room temperature. This raises the real possibility that BiSb-based SOT-MRAM could outperform the existing spin-transfer torque (STT) MRAM technology. “As SOT-MRAM can be switched one order of magnitude faster than STT-MRAM, the switching energy can be reduced by at least two orders of magnitude,” says Pham. “Also, the writing speed could be increased 20 times and the bit density increased by a factor of ten.”


If scaled up successfully, BiSb-based SOT-MRAM could drastically improve upon its heavy metal-based counterparts and even become competitive with dynamic random access memory (DRAM), the dominant technology of today.


The thin films were grown using a high-precision method called molecular beam epitaxy (MBE). The researchers discovered a particular surface orientation named BiSb(012), which is thought to be a key factor behind the large spin Hall effect. Pham points out that the number of Dirac cones on the BiSb(012) surface is another important factor, which his team is now investigating.


A team of researchers led by the University of Minnesota has developed a new material that could potentially improve the efficiency of computer processing and memory. “We used a quantum material that has attracted a lot of attention by the semiconductor industry in the past few years, but created it in unique way that resulted in a material with new physical and spin-electronic properties that could greatly improve computing and memory efficiency,” said lead researcher Jian-Ping Wang, a University of Minnesota Distinguished McKnight Professor and Robert F. Hartmann Chair in electrical engineering.


In this study, researchers started with bismuth selenide (Bi2Se3), a compound of bismuth and selenium. They then used a thin film deposition technique called “sputtering,” which is driven by the momentum exchange between the ions and atoms in the target materials due to collisions. While the sputtering technique is common in the semiconductor industry, this is the first time it has been used to create a topological insulator material that could be scaled up for semiconductor and magnetic industry applications.



However, the fact that the sputtering technique worked was not the most surprising part of the experiment. The nano-sized grains of less than 6 nanometers in the sputtered topological insulator layer created new physical properties for the material that changed the behavior of the electrons in the material. After testing the new material, the researchers found it to be 18 times more efficient in computing processing and memory compared to current materials.


“As the size of the grains decreased, we experienced what we call ‘quantum confinement’ in which the electrons in the material act differently giving us more control over the electron behavior,” said study co-author Tony Low, a University of Minnesota assistant professor of electrical and computer engineering.



Researchers say this is only the beginning and that this discovery could open the door to more advances in the semiconductor industry as well as related industries, such as magnetic random access memory (MRAM) technology. “With the new physics of these materials could come many new applications,” said Mahendra DC (Dangi Chhetri), first author of the paper and a physics Ph.D. student in Professor Wang’s lab.


Wang agrees that this cutting-edge research could make a big impact. “Using the sputtering process to fabricate a quantum material like a bismuth-selenide-based topological insulator is against the intuitive instincts of all researchers in the field and actually is not supported by any existing theory,” Wang said. “Four years ago, with a strong support from Semiconductor Research Corporation and the Defense Advanced Research Projects Agency, we started with a big idea to search for a practical pathway to grow and apply the topological insulator material for future computing and memory devices. Our surprising experimental discovery led to a new theory for topological insulator materials.


Topological light makes a quantum connection

Single photons have also been emitted into topological edge states by physicists in the US. The research provides a direct link between quantum optics and topological photonics, and could prove important for a wide range of applications, including quantum communication and quantum computing.


The possibility of moving single photons around without scattering has attracted particular interest for potential applications in quantum-information processing. However, quantum optician Mohammad Hafezi of the Joint Quantum Institute at the University of Maryland, College Park explains that although topological photonics is an inherently quantum phenomenon, it has only ever been demonstrated with classical light.


Now, Maryland researchers led by Hafezi and Edo Waks have created a heterostructure containing two adjoining periodic optical nanostructures called photonic crystals, each comprising distorted gallium arsenide honeycomb lattices. The perfectly regular lattice transmits photons of any frequency: “You can think of it basically as graphene,” says Hafezi. “Any system that has this honeycomb symmetry has this Dirac cone that does not have any band gap.” Distorting the lattice structure, however, opens up an optical band gap. In the researchers’ hereterostructure, photons around 950 nm wavelength would not propagate.


In one photonic crystal the researchers moved the triangular holes of each hexagon closer to the hexagon’s centre. In the other they moved the holes further apart. The optical band gaps created by these two distortions are such that the energy band that sits above the energy gap in one photonic crystal sits underneath it in the other, and vice versa. At the edge where the two photonic crystals meet, therefore, the two bands have to touch and cross over. This produces an edge state with an energy that lies in the middle of each photonic crystal’s band gap. Photons with this energy can therefore travel between the two photonic crystals but never scatter into the bulk. Symmetry considerations mean that photons with one circular polarization travel in one direction, whereas photons with the opposite circular polarization travel the opposite way.


“It’s definitely exciting,” says Peter Lodahl of the Niels Bohr Institute at the University of Copenhagen in Denmark. “It’s a major experimental step forward to demonstrate true topological phenomena in a quantum regime.” His own group first demonstrated that the direction of photons from single quantum emitters in waveguides can depend on their spin. “It’s too early to tell to what extent this topological addition gives you practical advantages,” he says, “but it’s a really exciting thing to investigate further.”


“This [work] is a big step forward in the implementation of new optical properties in materials,” agrees Alberto Amo of the University of Lille. “The next step would be to go further with these ideas of quantum optics in topologically protected circuits and connect two or more single-photon emitters such that they can start to interact. When you have that, you can implement quantum gates and other quantum optics protocols.”

New photonic chip promises more robust quantum computers

Scientists have developed a topological photonic chip to process quantum information, promising a more robust option for scalable quantum computers. The research team, led by RMIT University’s Dr Alberto Peruzzo, has for the first time demonstrated that quantum information can be encoded, processed and transferred at a distance with topological circuits on the chip. The research is published in Science Advances.


The breakthrough could lead to the development of new materials, new generation computers and deeper understandings of fundamental science. In collaboration with scientists from the Politecnico di Milano and ETH Zurich, the researchers used topological photonics – a rapidly growing field that aims to study the physics of topological phases of matter in a novel optical context – to fabricate a chip with a ‘beamsplitter’ creating a high precision photonic quantum gate.


“We anticipate that the new chip design will open the way to studying quantum effects in topological materials and to a new area of topologically robust quantum processing in integrated photonics technology,” says Peruzzo, Chief Investigator at the ARC Centre of Excellence for Quantum Computation and Communication Technology (CQC2T) and Director, Quantum Photonics Laboratory, RMIT.


“Topological photonics have the advantage of not requiring strong magnetic fields, and feature intrinsically high-coherence, room-temperature operation and easy manipulation” says Peruzzo. “These are essential requirements for the scaling-up of quantum computers.” Replicating the well known Hong-Ou-Mandel (HOM) experiment – which takes two photons, the ultimate constituents of light, and interfere them according to the laws of quantum mechanics – the team was able to use the photonic chip to demonstrate, for the first time, that topological states can undergo high-fidelity quantum interference.


HOM interference lies at the heart of optical quantum computation which is very sensitive to errors. Topologically protected states could add robustness to quantum communication, decreasing noise and defects prevalent in quantum technology. This is particularly attractive for optical quantum information processing. “Previous research had focussed on topological photonics using ‘classical’ -laser- light, which behaves as a classical wave. Here we use single photons, which behave according to quantum mechanics” says lead-author Jean-Luc Tambasco, PhD student at RMIT.


Demonstrating high-fidelity quantum interference is a precursor to transmitting accurate data using single photons for quantum communications – a vital component of a global quantum network. “This work intersects the two thriving fields of quantum technology and topological insulators and can lead to the development of new materials, new generation computers and fundamental science” says Peruzzo. The research is part of the Photonic Quantum Processor Program at CQC2T. The Centre of Excellence is developing parallel approaches using optical and silicon processors in the race to develop the first quantum computation system


Microsoft building quantum computer based on topological qubit

Researchers at Microsoft are working on an entirely new topological quantum computer, which uses exotic materials to limit errors. Microsoft’s new hires include Leo Kouwenhoven, a professor at the Delft University of Technology in the Netherlands; Charles Marcus, a professor at the University of Copenhagen; Matthias Troyer, a professor at ETH Zurich; and David Reilly, a professor at the University of Sydney in Australia.


Microsoft’s approach to building a quantum computer is based on a type of qubit – or unit of quantum information – called a topological qubit. The Microsoft team believes that topological qubits are better able to withstand challenges such as heat or electrical noise, allowing them to remain in a quantum state longer. That, in turn, makes them much more practical and effective. “A topological design is less impacted by changes in its environment,” Holmdahl said.

Topological Materials Boost the Efficiency of Thermoelectric Devices Threefold

Thermoelectric devices are made from materials that can convert a temperature difference into electricity, without requiring any moving parts — a quality that makes thermoelectrics a potentially appealing source of electricity. Now researchers at MIT have discovered a way to increase that efficiency threefold, using “topological” materials, which have unique electronic properties.


In a paper published in the Proceedings of the National Academy of Sciences, the MIT researchers identify the underlying property that makes certain topological materials a potentially more efficient thermoelectric material, compared to existing devices.


“We’ve found we can push the boundaries of this nanostructured material in a way that makes topological materials a good thermoelectric material, more so than conventional semiconductors like silicon,” says Te-Huan Liu, a postdoc in MIT’s Department of Mechanical Engineering. “In the end, this could be a clean-energy way to help us use a heat source to generate electricity, which will lessen our release of carbon dioxide.”


Scientists have observed that some topological materials can be made into efficient thermoelectric devices through nanostructuring, a technique scientists use to synthesize a material by patterning its features at the scale of nanometers. Scientists have thought that topological materials’ thermoelectric advantage comes from a reduced thermal conductivity in their nanostructures.


The team aimed to understand the effect of nanostructuring on tin telluride’s thermoelectric performance, by simulating the way electrons travel through the material. The researchers found that decreasing tin telluride’s average grain size to about 10 nanometers produced three times the amount of electricity that the material would have produced with larger grains.


That is, with smaller grain sizes, higher-energy electrons contribute much more to the material’s electrical conduction than lower-energy electrons, as they have shorter mean free paths and are less likely to scatter against grain boundaries. This results in a larger voltage difference that can be generated.


Tin telluride is just one example of many topological materials that have yet to be explored. If researchers can determine the ideal grain size for each of these materials, Liu says topological materials may soon be a viable, more efficient alternative to producing clean energy.


“I think topological materials are very good for thermoelectric materials, and our results show this is a very promising material for future applications,” Liu says.


This research was supported in part by the Solid-State Solar Thermal Energy Conversion Center, an Energy Frontier Research Center of U.S. Department of Energy; and the Defense Advanced Research Projects Agency (DARPA).


Quantum materials of the topological insulator family can efficiently upconvert electromagnetic radiation in the terahertz (THz) regime.

In the last decade, a number of research groups have focused their attention on identifying techniques and materials to efficiently generate THz electromagnetic waves: among them is graphene, which, however, does not provide the desired results. In particular, the generated terahertz output power is limited.

Better performance has now been achieved by topological insulators (TIs) – quantum materials that behave as insulators in the bulk while exhibiting conductive properties on the surface – according to a paper recently published in Light: Science & Applications (“Milliwatt terahertz harmonic generation from topological insulator metamaterials”).


Earlier studies had shown that materials which host electrons with zero effective mass enable efficient generation of terahertz harmonics, including the aforementioned graphene and topological insulators. The phenomenon of harmonic generation occurs when photons of the same frequency and energy interact non-linearly with matter, leading to the emission of photons whose energy is a multiple of that of the incident ones. This can be exploited, for example, to upconvert electronically generated signals in the high GHz regime into signals in the THz regime.


Dr Tielrooij and colleagues investigated the behaviour of two topological insulators – the prototypical Bi2Se3 and Bi2Te3 – in direct comparison with a reference graphene sample. They observed that, while the maximum power of the harmonics generated in graphene is limited by saturation effects (which arise at high incident powers), in these quantum materials it continued to increase with the incident fundamental power. The performed experiments revealed an improvement in generated output power by orders of magnitude over graphene, approaching the milliwatt regime.


This significant divergence in behaviour is due to the fact that topological insulators can rely on a highly efficient cooling mechanism, in which the massless charges on the surface dissipate their electronic heat to those in the rest of the thin film. In other words, bulk electrons lend a helping hand to the surface-state electrons by sinking electronic heat. The highest output power for the terahertz third-harmonic – i.e. radiation with three times the same energy – was achieved in a metamaterial that contained a topological insulator film together with a metallic grating – consisting of metal strips separated by gaps on the surface of the material.
“In this work we demonstrate that the saturation effect occurring in graphene is much less detrimental in topological insulators. This occurs thanks to a novel cooling mechanism between surface and bulk electrons of topological insulators,” explains Dr Klaas-Jan Tielrooij, first author of the paper. “These quantum metamaterials thus bring nonlinear terahertz photonics technology a big step closer.”


Thousands of exotic ‘topological’ materials discovered through sweeping search

Now for the first time, researchers have systematically scoured through entire databases of materials in search of ones that harbour topological states. The results show that thousands of known materials probably have topological properties — and perhaps up to 24% of materials in all. Previously, researchers knew of just a few hundred topological materials, and only around a dozen have been studied in detail.


In July 2018, several teams posted preprints  detailing their scans of tens of thousands of materials and their predicted topological classifications, which are based on algorithms that use a material’s chemistry and symmetry to calculate their properties. Two teams have already integrated their algorithms into searchable databases. “You can put in a compound name and, with one click, get whether there is topology or not. For me, this is wonderful,” says Chandra Shekhar, a condensed-matter physicist at the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany.


The resulting haul of topological materials could bring scientists closer to finding practical applications for these exotic phases — which have the potential to revolutionize electronics and catalysis. “The more materials with unusual properties we know, the more chance there will be of a breakthrough,” says Oleg Yazyev, a physicist at the Swiss Federal Institute of Technology in Lausanne.


Not every new topological material will prove interesting. So even more useful, says Judy Cha, an experimental physicist at Yale University in New Haven, Connecticut, would be if theorists could factor into the databases other practical information about the materials, such as how defects in the crystal affect the flow of electrons through it; this would help to whittle the list down to only the most practical. “That would be really fantastic,” she says.


UCLA-led team opens new avenues to research on topological insulators

However, before they could be widely used researchers needed to demonstrate that TIs, as topological insulators are commonly known, could operate at room temperature rather than only at near absolute-zero conditions.


The work led by Wang, who holds UCLA’s Raytheon Chair in Electrical Engineering, will open new ways for more groups to study them, with the long-term goal of getting the insulators to operate at room temperature. The researchers came up with a new approach by pairing the insulators with an antiferromagnetic material, which also has special properties. This pairing of two distinct layers with a well-defined interface between the two makes the new material a “heterostructure.”


Using topological insulators that are magnetized could dramatically improve the energy efficiency and operating speed of computers. There are two ways to do this. One way infuses magnetic materials into the TI. The other stacks thin layers of alternating magnetic materials between the insulators. Both methods can magnetize the TIs, but both also can disrupt the insulator’s desirable properties if the magnetism of these magnetic materials is too strong. Additionally, these two methods can work only in temperatures near absolute zero.


Instead of stacking alternating layers, the UCLA-led team uses an “antiferromagnetic material”  in conjunction with TI. An antiferromagnetic will have one layer of the spin of its atoms point one way, the next layer’s point another and so on: In effect the material’s overall magnetism is cancelled out, however one atomic layer is still magnetic at the interface. This type of magnetic structure can be exploited for data storage because of its stability and robustness. These are the properties that the researchers looked to exploit. “Using antiferromagnetic materials with topological insulators provides a new avenue to bring the latter material into applications in addition to exploring new physics,” Wang said.


Collaborators on the study include researchers at the National Institutes of Standards and Technology, in Gaithersburg, Maryland, Stanford University, and the Beijing University of Technology.


“When we combined topological insulators with antiferromagnetic layers we found they could operate at 90 Kelvin, still a low temperature, but warm enough that now many research groups can use nitrogen to keep them cool to study them, rather than under super-cold conditions only available in specially equipped laboratories,” said Qing Lin He, the study’s lead author, a UCLA postdoctoral scholar, who is a member of Wang’s research group. “This is an enormous opportunity for opening up a totally new direction. While it’s still not room-temperature, it’s a very promising way forward.”


Additionally, when TIs conduct electricity, all of the electrons flowing in one direction have the same spin direction, a particularly useful property that could be used in quantum computers without dissipation.


The research received its primary funding from the Army Research Office. It was also supported by the Department of Energy, the National Science Foundation and by the Focus Center on Function Accelerated nanoMaterial Engineering, a research center based at UCLA Engineering and funded by the Defense Advanced Research Projects Agency and the Semiconductor Research Corporation.


A breakthrough in magnetic materials research could lead to novel ways to manipulate electron flow with much less energy loss, reported in August 2022

Scientists from the U.S. Department of Energy’s Ames National Laboratory and Oak Ridge National Laboratory conducted an in-depth investigation of TbMn6Sn6 to better understand the material and its magnetic characteristics. These results could impact future technology advancements in fields such as quantum computing, magnetic storage media, and high-precision sensors.


Kagomes are a type of material whose structure is named after a traditional Japanese basket weaving technique. The weave produces a pattern of hexagons surrounded by triangles and vice-versa. The arrangement of the atoms in Kagome metals reproduces the weaving pattern. This characteristic causes electrons within the material to behave in unique ways.


Solid materials have electronic properties controlled by the characteristics of their electronic band structure. The band structure is strongly dependent on the geometry of the atomic lattice, and sometimes bands may display special shapes such as cones. These special shapes, called topological features, are responsible for the unique ways electrons behave in these materials. The Kagome structure in particular leads to complex and potentially tunable features in the electronic bands.

Using magnetic atoms to construct the lattice of these materials, such as Mn in TbMn6Sn6, can further help inducing topological features. Rob McQueeney, a scientist at Ames Lab and the project leader, explained that topological materials “have a special property where under the influence of magnetism, you can get currents which flow on the edge of the material, which are dissipationless, which means that the electrons don’t scatter, and they don’t dissipate energy.”

The team set out to better understand the magnetism in TbMn6Sn6 and used calculations and neutron scattering data collected from the Oak Ridge Spallation Neutron Source to conduct their analysis. Simon Riberolles, a postdoc research associate at Ames Lab and member of the project team, explained the experimental technique the team used. The technique involves a beam of neutron particles which is used to test how rigid the magnetic order is. “The nature and strength of the different magnetic interactions present in the materials can all be mapped out using this technique,” he said.

They discovered that TbMn6Sn6 has competing interactions between the layers, or what is called frustrated magnetism. “So the system has to make a compromise,” McQueeney said, “Usually what that means is that if you poke at it, you can get it to do different things. But what we found out in this material is that even though those competing interactions are there, there are other interactions that are dominant.”

This is the first detailed investigation of the magnetic properties of TbMn6Sn6 to be published. “In research, it’s always exciting when you figure out you understand something new, or you measure something that has not been seen before, or was understood partially or in a different manner,” Riberolles said.



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