Category Archives: Process Materials

Rinchem recently announced the expansion of its chemical distribution center in Pyeongtaek, South Korea.

The new site provides four additional warehouses capable of storing a wide variety of materials including: general commodities (non-DG), flammable, toxic, corrosive, oxidizer and miscellaneous dangerous goods with strict temperature requirements ranging from -30 to 30 C. The new additions add 12,350 pallet positions to the Korea campus.

“This recent investment in South Korea marks Rinchem’s largest investment in the Asian Pacific market to date,” said Chris Wright, Rinchem’s Vice President of Sales & Marketing. “The Pyeongtaek distribution center is state-of-the-art in every way and is positioned as the largest semiconductor grade chemical storage facility in Korea.”

Each new warehouse will be optimized for efficiency and safety consistent with Rinchem’s global standards. The new warehouses will boast a very narrow aisle (VNA) racking system with wire-guided forklift steering controls. Additionally, the Korea campus now operates the newest version of Rinchem’s proprietary Chem-Star® customer interface. The system update showcases a streamlined and easy-to-navigate customer interface granting access to inventory data in real-time.

Rinchem will host a grand opening event to celebrate the recent completion of this expansion project on the new Korea campus on 9 August 2017 at 10:00 AM. Rinchem executives and staff will be on site to participate in a ribbon cutting ceremony, remarks on the new expansion and site tours.

Rinchem provides a wide range of logistics services to support the semiconductor, chemical, gas, life sciences and paint and coatings industries, including dedicated and multi-client warehousing, on-site services, over-the-road and local transportation, freight forwarding, empty container return management and supply chain consulting.

Topological insulators, a class of materials which has been investigated for just over a decade, have been heralded as a new ‘wonder material’, as has graphene. But so far, topological insulators have not quite lived up to the expectations fueled by theoretical studies. University of Groningen physicists now have an idea about why. Their analysis was published on 27 July in the journal Physical Review B.

Topological insulators are materials that are insulating in the bulk but allow charge to flow across the surface. These conducting states at the surface originate from ordering patterns in the states where electrons reside that are different from ordinary materials. This ordering is linked to the physical concept of ‘topology’, analogous to that used in mathematics. This property gives rise to very robust states with some special properties.

Heavy atoms

For one, their spin — a magnetic property of electrons which can have the values ‘up’ or ‘down’ — is locked to their movement. ‘This means that electrons moving to the right have spin down, and those moving to the left have spin up’, explains first author of the study Eric de Vries, PhD student in the ‘Spintronics of Functional Materials’ research group led by his supervisor prof. dr. Tamalika Banerjee. This is group is part of the Zernike Institute for Advanced Materials. ‘But it also means that when you inject electrons with spin up into such a topological insulator, they will travel to the left!’ Topological insulators might therefore be very useful in the realization of spintronics: electronics based on the quantized spin value rather than the charge of electrons.

The special properties of topological insulators are predicted by the theoretical analysis of the surface structures of these materials, made from crystals of heavy atoms. But experiments show mixed results, which don’t quite live up to the theoretical predictions. ‘We wondered why, so we devised experiments to investigate the behaviour of the surface state electrons. Specifically, we wanted to see if transport is really limited to the surface, or if it is also present in the bulk of the material.’

Surprising

Earlier experiments by the group, in which they used ferromagnets to detect the spins of electrons generated in the topological insulator, were surprising, says De Vries. ‘We demonstrated that a voltage presumably originating from spin detection can originate in factors other than the locking of electron spin to its movement. Using different geometries, we showed that artefacts related to stray magnetic fields generated by the ferromagnets can mimic similar spin voltages.’ This observation may lead to a re-evaluation of some published results.

This time, they used a different approach. ‘We analyzed the topological insulators using strong magnet fields. This causes electrons to oscillate in transport channels.’ De Vries went to the national High Field Magnet Laboratory at the Radboud University Nijmegen, where a 33-Tesla magnet is available, one of the stronger magnets in the world. ‘Others have done similar tests with weaker magnets, but these are not sensitive enough to reveal the additional transport channels that coexist with the surface states.’ De Vries’s experiments showed that a considerable part of the charge transport occurred in the bulk phase of the material, and not only at the surface.

Transport channels

The reason for this, explains De Vries, is the imperfect crystal structure of the topological insulator. ‘Sometimes there are atoms missing in the crystal structure. This results in freely moving electrons. These start to conduct as new transport channels, generating electric current in the bulk of the material.’

So why has no one noted this before? De Vries stresses that interpreting transport measurements made on topological insulators can be difficult. ‘We experienced this in our previous experiments. Our message is that extreme care is needed in the interpretation of experimental observations for devices based on these materials.’ Also, experiments which might lead to clearer conclusions require very high magnetic fields in specialized labs.

Glitches

The results point to a way to improve topological insulators. ‘The key is to grow the crystals without any missing atoms. Another solution is to fill the holes, for example with calcium ions that bind the free electrons. But that might cause other disturbances to the electrons’ mobility.’ For ten years, topological insulators were all the rage. They were compared to the wonder material graphene. The discovery that, in practice, topological insulators have glitches serves as a reality check. De Vries: ‘We need to study and understand the interaction between the surface states and the bulk material in much more detail.’

Advances in modern electronics has demanded the requisite hardware, transistors, to be smaller in each new iteration. Recent progress in nanotechnology has reduced the size of silicon transistors down to the order of 10 nanometers. However, for such small transistors, other physical effects set in, which limit their functionality. For example, the power consumption and heat production in these devices is creating significant problems for device design. Therefore, novel quantum materials and device concepts are required to develop a new generation of energy-saving information technology. The recent discoveries of topological materials — a new class of relativistic quantum materials — hold great promise for use in energy saving electronics.

Researchers in the Louisiana Consortium for Neutron Scattering, or LaCNS, led by LSU Department of Physics & Astronomy Chair and Professor John F. DiTusa and Tulane University Professor Zhiqiang Mao, with collaborators at Oak Ridge National Lab, the National High Magnetic Field Laboratory, Florida State University, and the University of New Orleans, recently reported the first observation of this topological behavior in a magnet, Sr1-yMn1-zSb2 (y, z < 0.1). These results were published this week in Nature Materials(doi:10.1038/nmat4953).

“This first observation is a significant milestone in the advancement of novel quantum materials and this discovery opens the opportunity to explore its consequences. The nearly massless behavior of the charge carriers offers possibilities for novel device concepts taking advantage of the extremely low power dissipation,” DiTusa said.

The phrase “topological materials” refers to materials where the current carrying electrons act as if they have no mass similar to the properties of photons, the particles that make up light. Amazingly, these electronic states are robust and immune to defects and disorder because they are protected from scattering by symmetry. This symmetry protection results in exceedingly high charge carrier mobility, creating little to no resistance to current flow. The result is expected to be a substantial reduction in heat production and energy saving efficiencies in electronic devices.

This new magnet displays electronic charge carriers that have almost no mass. The magnetism brings with it an important symmetry breaking property – time reversal symmetry, or TRS, breaking where the ability to run time backward would no longer return the system back to its starting conditions. The combination of relativistic electron behavior, which is the cause of much reduced charge carrier mass, and TRS breaking has been predicted to cause even more unusual behavior, the much sought after magnetic Weyl semimetal phase. The material discovered by this collaboration is thought to be an excellent one to investigate for evidence of the Weyl phase and to uncover its consequences.

A newly discovered collective rattling effect in a type of crystalline semiconductor blocks most heat transfer while preserving high electrical conductivity – a rare pairing that scientists say could reduce heat buildup in electronic devices and turbine engines, among other possible applications.

A team led by scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) discovered these exotic traits in a class of materials known as halide perovskites, which are also considered promising candidates for next-generation solar panels, nanoscale lasers, electronic cooling, and electronic displays.

Rattling structures of halide perovskites: cesium tin iodide (left) and cesium lead iodide (right). Credit: UC Berkeley

Rattling structures of halide perovskites: cesium tin iodide (left) and cesium lead iodide (right). Credit: UC Berkeley

These interrelated thermal and electrical (or “thermoelectric”) properties were found in nanoscale wires of cesium tin iodide (CsSnI3). The material was observed to have one of the lowest levels of heat conductivity among materials with a continuous crystalline structure.

This so-called single-crystal material can also be more easily produced in large quantities than typical thermoelectric materials, such as silicon-germanium, researchers said.

“Its properties originate from the crystal structure itself. It’s an atomic sort of phenomenon,” said Woochul Lee, a postdoctoral researcher at Berkeley Lab who was the lead author of the study, published the week of July 31 in the Proceedings of the National Academy of Sciencesjournal. These are the first published results relating to the thermoelectric performance of this single crystal material.

Researchers earlier thought that the material’s thermal properties were the product of “caged” atoms rattling around within the material’s crystalline structure, as had been observed in some other materials. Such rattling can serve to disrupt heat transfer in a material.

“We initially thought it was atoms of cesium, a heavy element, moving around in the material,” said Peidong Yang, a senior faculty scientist at Berkeley Lab’s Materials Sciences Division who led the study.

Jeffrey Grossman, a researcher at the Massachusetts Institute of Technology, then performed some theory work and computerized simulations that helped to explain what the team had observed. Researchers also used Berkeley Lab’s Molecular Foundry, which specializes in nanoscale research, in the study.

“We believe there is essentially a rattling mechanism, not just with the cesium. It’s the overall structure that’s rattling; it’s a collective rattling,” Yang said. “The rattling mechanism is associated with the crystal structure itself,” and is not the product of a collection of tiny crystal cages. “It is group atomic motion,” he added.

Within the material’s crystal structure, the distance between atoms is shrinking and growing in a collective way that prevents heat from easily flowing through.

But because the material is composed of an orderly, single-crystal structure, electrical current can still flow through it despite this collective rattling. Picture its electrical conductivity is like a submarine traveling smoothly in calm underwater currents, while its thermal conductivity is like a sailboat tossed about in heavy seas at the surface.

Yang said two major applications for thermoelectric materials are in cooling, and in converting heat into electrical current. For this particular cesium tin iodide material, cooling applications such as a coating to help cool electronic camera sensors may be easier to achieve than heat-to-electrical conversion, he said.

A challenge is that the material is highly reactive to air and water, so it requires a protective coating or encapsulation to function in a device.

Cesium tin iodide was first discovered as a semiconductor material decades ago, and only in recent years has it been rediscovered for its other unique traits, Yang said. “It turns out to be an amazing gold mine of physical properties,” he noted.

To measure the thermal conductivity of the material, researchers bridged two islands of an anchoring material with a cesium tin iodide nanowire. The nanowire was connected at either end to micro-islands that functioned as both a heater and a thermometer. Researchers heated one of the islands and precisely measured how the nanowire transported heat to the other island.

They also performed scanning electron microscopy to precisely measure the dimensions of the nanowire. They used these dimensions to provide an exacting measure of the material’s thermal conductivity. The team repeated the experiment with several different nanowire materials and multiple nanowire samples to compare thermoelectric properties and verify the thermal conductivity measurements.

“A next step is to alloy this (cesium tin iodide) material,” Lee said. “This may improve the thermoelectric properties.”

Also, just as computer chip manufacturers implant a succession of elements into silicon wafers to improve their electronic properties – a process known as “doping” – scientists hope to use similar techniques to more fully exploit the thermoelectric traits of this semiconductor material. This is relatively unexplored territory for this class of materials, Yang said.

A key step in unlocking the potential for greener, faster, smaller electronic circuitry was taken recently by a group of researchers led by UAlberta physicist Robert Wolkow.

The research team found a way to delete and replace out-of-place atoms that had been preventing new revolutionary circuitry designs from working. This unleashes a new kind of silicon chips for used in common electronic products, such as our phones and computers.

“For the first time, we can unleash the powerful properties inherent to the atomic scale,” explained Wolkow, noting that printing errors on silicon chips are inevitable when working at the atomic scale. “We were making things that were close to perfect but not quite there. Now that we have the ability to make corrections, we can ensure perfect patterns, and that makes the circuits work. It is this new ability to edit at the atom scale that makes all the difference.”

Think of a typing mistake and the ability to go back and white it out and type it again perfectly. Now imagine that the white out is actually single hydrogen atoms, allowing a level of precision previously unattainable.

“We can precisely erase any errors and reprint that atom in the correct place. It’s not even a compromise like white out where you either have a gooey layer or indentation. It’s actually perfect,” said Wolkow, who worked with fellow scientists from the University of Alberta, the National Research Council, and Quantum Silicon Inc.

Scientists have seen many hints that atomic circuitry was within reach. However, the necessary precision was previously possible only for simple materials that had to be maintained at ultra-low temperatures, impractical for everyday applications demanded in computers and personal digital devices. Wolkow and his team have discovered methods and material to ensure stability at room temperature, challenges that he and other scientists the world over have been working for decades on overcoming.

Wolkow’s graduate students Roshan Achal and Taleana Huff together with postdoc Moe Rashidi showed they can overcome these obstacles with a modified approach to the same silicon chips that are used in today’s circuitry. While they had previously improved accuracy of atomic silicon printing, errors in the form of misplaced atoms always occurred at the one percent level. Though the placement errors were small–about one third of a nanometer–they nevertheless large enough to upset circuit operation.

The students created a reliable procedure for picking up single hydrogen atoms with their atomically sharp probe and replacing one or more hydrogen atoms to perfectly erase atomic misprints.

With their new discovery, many remaining challenges to ultra-low power atomic circuitry have also been erased. Wolkow, Achal, and Huff’s discovery has been captured in the academic paper “Atomic Whiteout,” appearing in the scientific journal ACS Nano.

 

A team of physicists featuring researchers from MIPT and ITMO University has conducted a comparative analysis of a range of materials to determine if they are applicable to dielectric nanophotonics. Their systematic study produced results that can optimize the use of known materials for building optical nanodevices, as well as encourage the search for new materials with superior properties.

In order to send, receive, and process electromagnetic signals, antennas are used. An antenna is a device capable of effectively transmitting, picking up, and redirecting electromagnetic radiation. Typically, one thinks of antennas as macroscopic devices operating in the radio and microwave range. However, there are similar optical devices. The wavelengths of visible light amount to several hundred nanometers. As a consequence, optical antennas are, by necessity, nanosized devices. Optical nanoantennas, which can focus, direct, and effectively transmit light, have a wide range of applications, including information transmission over optical channels, photodetection, microscopy, biomedical technology, and even speeding up chemical reactions.

For an antenna to pick up and transmit signals efficiently, its elements need to be resonant. In the radio band, such elements are pieces of wire. In the optical range, silver and gold nanoparticles with plasmonic resonances have long been used for this purpose. Electromagnetic fields in such particles can be localized on a scale of 10 nanometers or less, but most of the energy of the field is wasted due to Joule heating of the conducting metal. There is an alternative to plasmonic nanoparticles, which has been studied extensively for the last five years, namely particles of dielectric materials with high refractive indices at visible light frequencies, such as silicon. When the size of the dielectric particle and the wavelength of light are just right, the particle supports optical resonances of a particular kind, called Mie resonances. Because the material properties of dielectrics are different from those of metals, it is possible to significantly reduce resistive heating by replacing plasmonic nanoantennas with dielectric analogs.

The key characteristic of a material determining Mie resonance parameters is the refractive index. Particles made of materials with high refractive indices have resonances characterized by high quality factors. This means that in these materials electromagnetic oscillations last longer without external excitation. In addition, higher refractive indices correspond to smaller particle diameters, allowing for more miniature optical devices. These factors make high-index materials — i.e., those with high indices of refraction — more suitable for the implementation of dielectric nanoantennas.

In their paper published in Optica, the researchers systematically examine the available high-index materials in terms of their resonances in the visible and infrared spectral ranges. Materials of this kind include semiconductors and polar crystals, such as silicon carbide. To illustrate the behavior of various materials, the authors present their associated quality factors, which indicate how quickly oscillations excited by incident light die out. Theoretical analysis enabled the researchers to identify crystalline silicon as the best currently available material for the realization of dielectric antennas operating in the visible range, with germanium outperforming other materials in the infrared band. In the mid-infrared part of the spectrum, which is of particular interest due to possible applications, such as radiative cooling, i.e., the cooling of a heated body by means of radiating heat in the form of electromagnetic waves into the environment; and thermal camouflage — reducing thermal radiation given off by an object, thus making it invisible to infrared cameras, the compound of germanium and tellurium took the pedestal.

There are fundamental limitations on the value of the quality factor. It turns out that high refractive indices in semiconductors are associated with interband transitions of electrons, which inevitably entail the absorption of energy carried by the incident light. This absorption in turn leads to a reduction of the quality factor, as well as heating, and that is precisely what the researchers are trying to get rid of. There is, therefore, a delicate balance between a high index of refraction and energy losses.

“This study is special both because it offers the most complete picture of high-index materials, showing which of them is optimal for fabricating a nanoantenna operating in this spectral range, and because it provides an analysis of the manufacturing processes involved,” notes Dmitry Zuev, research scientist at the Metamaterials laboratory of the Faculty of Physics and Engineering, ITMO University. “This enables a researcher to select a material, as well as the desired manufacturing technique, taking into account the requirements imposed by their specific situation. This is a powerful tool furthering the design and experimental realization of a wide range of dielectric nanophotonic devices.”

According to the overview of manufacturing techniques, silicon, germanium, and gallium arsenide are the most thoroughly studied high-index dielectrics used in nanophotonics. A wide range of methods are available for manufacturing resonant nanoantennas based on these materials, including lithographic, chemical, and laser-assisted methods. However, in the case of some materials, no technology for fabrication of resonant nanoparticles has been developed. For example, researchers have yet to come up with ways of making nanoantennas from germanium telluride, whose properties in the mid-infrared range were deemed the most attractive by the theoretical analysis.

“Silicon is currently, beyond any doubt, the most widely used material in dielectric nanoantenna manufacturing,” says Denis Baranov, a PhD student at MIPT. “It is affordable, and silicon-based fabrication techniques are well established. Also, and this is important, it is compatible with the CMOS technology, an industry standard in semiconductor engineering. But silicon is not the only option. Other materials with even higher refractive indices in the optical range might exist. If they are discovered, this would mean great news for dielectric nanophotonics.”

The research findings obtained by the team could be used by nanophotonics engineers to develop new resonant nanoantennas based on high-index dielectric materials. Besides, the paper suggests further theoretical and experimental work devoted to the search for other high-index materials with superior properties to be used in new improved dielectric nanoantennas. Such materials could, among other things, be used to considerably boost the efficiency of radiative cooling of solar cells, which would constitute an important technological advance.

The ‘wonder material’ graphene has many interesting characteristics, and researchers around the world are looking for new ways to utilise them. Graphene itself does not have the characteristics needed to switch electrical currents on and off and smart solutions must be found for this particular problem. “We can make graphene structures with atomic precision. By selecting certain precursor substances (molecules), we can code the structure of the electrical circuit with extreme accuracy,” explains Peter Liljeroth from Aalto University, who conceived the research project together with Ingmar Swart from Utrecht University.

Seamless integration

The electronic properties of graphene can be controlled by synthesizing it into very narrow strips (graphene nanoribbons). Previous research has shown that the ribbon’s electronic characteristics are dependent on its atomic width. A ribbon that is five atoms wide behaves similarly to a metallic wire with extremely good conduction characteristics, but adding two atoms makes the ribbon a semiconductor. “We are now able to seamlessly integrate five atom-wide ribbons with seven atom-wide ribbons. That gives you a metal-semiconductor junction, which is a basic building block of electronic components,” according to Ingmar Swart.

Chemistry on a surface

The researchers produced their electronic graphene structures through a chemical reaction. They evaporated the precursor molecules onto a gold crystal, where they react in a very controlled way to yield new chemical compounds. “This is a different method from that currently used to produce electrical nanostructures, such as those on computer chips. For graphene, it is so important that the structure is precise at the atomic level and it is likely that the chemical route is the only effective method,” Ingmar Swart concludes.

Electronic characteristics

The researchers used advanced microscopic techniques to also determine the electronic and transport characteristics of the resulting structures. It was possible to measure electrical current through a graphene nanoribbon device with an exactly known atomic structure. “This is the first time where we can create e.g. a tunnel barrier and really know its exact atomic structure. Simultaneous measurement of electrical current through the device allows us to compare theory and experiment on a very quantitative level,” says Peter Liljeroth.

Veeco Instruments Inc. (NASDAQ: VECO) announced today that CrayoNano AS, research company for ultraviolet short wavelength light emitting diodes (UV-C LEDs), has ordered the Propel Power Gallium Nitride (GaN) Metal Organic Chemical Vapor Deposition (MOCVD) System. CrayoNano will use the system to grow semiconductor nanowires on graphene for water disinfection, air purification, food processing and life science applications.

UV-C LEDs are free of harmful mercury compared to typically 20-200 milligrams of mercury found in traditional UV lamps used in these applications. They also require minimal energy to operate and have longer life cycles compared to other purification and disinfection lighting methods. The value of the global market for UV-C LEDs used in sterilization and purification equipment is growing at a CAGR of 56% from US$28 million in 2016 to US$257 million in 2021, according to the 2016~2021 UV LED and IR LED Application Market Report by LEDinside, a division of TrendForce.

“We see enormous opportunity in our focused markets and we need superior MOCVD technology to accomplish our goals,” said Mr. Morten Froseth, Chief Executive Officer, CrayoNano. “Veeco’s Propel system offers us the unique opportunity to scale to 200 mm graphene wafer sizes while maintaining superior uniformity, low manufacturing costs and long run campaigns.”

Veeco’s Propel Power GaN MOCVD system is capable of processing single 200 mm wafers or smaller (e.g., two-inch) in batch mode. The system is based on Veeco’s TurboDisc® technology including the IsoFlange™ and SymmHeat™ breakthrough technologies, which provide homogeneous laminar flow and uniform temperature profile across each wafer, up to 200 mm in size.

“The Propel Power GaN system is the best choice to deposit advanced GaN-based structures, including complex semiconductor nanowires on graphene substrates with strict process demands,” said Peo Hansson, Ph.D., Veeco’s Senior Vice President, General Manager, MOCVD. “Our Propel system offers industry leading uniformity and process cycle time, therefore providing superior productivity compared to other technologies. As a global supplier of MOCVD systems, we look forward to supporting CrayoNano and their research activities.”

An international team of physicists, materials scientists and string theoreticians have observed a phenomenon on Earth that was previously thought to only occur hundreds of light years away or at the time when the universe was born. This result could lead to a more evidence-based model for the understanding the universe and for improving the energy-conversion process in electronic devices.

Using a recently discovered material called a Weyl semimetal, similar to 3D graphene, scientists at IBM Research (NYSE: IBM) have mimicked a gravitational field in their test sample by imposing a temperature gradient. The study was supervised by Prof. Kornelius Nielsch, Director at the Leibniz Institute for Materials and Solid State Research Dresden (IFW) and Prof. Claudia Felser, Director at the Max Planck Institute for Chemical Physics of Solids in Dresden.

After conducting the experiment in a cryolab at the University of Hamburg with high magnetic fields, a team of theoreticians from TU Dresden, UC Berkeley and the Instituto de Fisica Teorica UAM/CSIC confirmed with detailed calculations that they observed a quantum effect known as an axial-gravitational anomaly, which breaks one of the classical conservation laws, such as charge, energy and momentum.

This law-breaking anomaly had previously been derived in purely theoretical reasoning with methods based on string theory. It was believed to exist only at extremely high temperatures of trillions of degrees, as an exotic form of matter, called a quark-gluon plasma, at the early stages of the universe deep within the cosmos or created using particle colliders. But to their surprise, the researchers discovered that it also exists on Earth in the properties of solid-state physics, on which much of the computing industry is based on, spanning from tiny transistors to cloud data centers. This discovery is appearing today in the peer-reviewed journal Nature.

“For the first time, we have experimentally observed this fundamental quantum anomaly on Earth which is extremely important towards our understanding of the universe,” said Dr. Johannes Gooth, an IBM Research scientist and lead author of the paper. “We can now build novel solid-state devices based on this anomaly that have never been considered before to potentially circumvent some of the problems inherent in classical electronic devices, such as transistors.”

“This is an incredibly exciting discovery. We can clearly conclude that the same breaking of symmetry can be observed in any physical system, whether it occurred at the beginning of the universe or is happening today, right here on Earth,” said Prof. Dr. Karl Landsteiner, a string theorist at the Instituto de Fisica Teorica UAM/CSIC and co-author of the paper.

IBM scientists predict this discovery will open up a rush of new developments around sensors, switches and thermoelectric coolers or energy-harvesting devices, for improved power consumption.

An international team of researchers has found a way to determine whether a crystal is a topological insulator — and to predict crystal structures and chemical compositions in which new ones can arise. The results, published July 20 in the journal Nature, show that topological insulators are much more common in nature than currently believed.

Topological materials, which hold promise for a wide range of technological applications due to their exotic electronic properties, have attracted a great deal of theoretical and experimental interest over the past decade, culminating in the 2016 Nobel Prize in physics. The materials’ electronic properties include the ability of current to flow without resistance and to respond in unconventional ways to electric and magnetic fields.

Until now, however, the discovery of new topological materials occurred mainly by trial and error. The new approach described this week allows researchers to identify a large series of potential new topological insulators. The research represents a fundamental advance in the physics of topological materials and changes the way topological properties are understood.

The team included: at Princeton University, Barry Bradlyn and Jennifer Cano, both associate research scholars at the Princeton Center for Theoretical Science, Zhijun Wang, a postdoctoral research associate, and B. Andrei Bernevig, professor of physics; professors Luis Elcoro and Mois Aroyo at the University of the Basque Country in Bilbao; assistant professor Maia Garcia Vergniory of University of the Basque Country and Donostia International Physics Center (DIPC) in Spain; and Claudia Felser, professor at the Max Planck Institute for Chemical Physics of Solids in Germany.

“Our approach allows for a much easier way to find topological materials, avoiding the need for detailed calculations,” Felser said. “For some special lattices, we can say that, regardless of whether a material is an insulator or a metal, something topological will be going on,” Bradlyn added.

Until now, of the roughly 200,000 materials catalogued in materials databases, only around a few hundred are known to host topological behavior, according to the researchers. “This raised the question for the team: Are topological materials really that scarce, or does this merely reflect an incomplete understanding of solids?” Cano said.

To find out, the researchers turned to the nearly century-old band theory of solids, considered one of the early landmark achievements of quantum mechanics. Pioneered by Swiss-born physicist Felix Bloch and others, the theory describes the electrons in crystals as residing in specific energy levels known as bands. If all the states in a group of bands are filled with electrons, then the electrons cannot move and the material is an insulator. If some of the states are unoccupied, then electrons can move from atom to atom and the material is capable of conducting an electrical current.

Because of the symmetry properties of crystals, however, the quantum states of electrons in solids have special properties. These states can be described as a set of interconnected bands characterized by their momentum, energy and shape. The connections between these bands, which on a graph resemble tangled spaghetti strands, give rise to topological behaviors such as those of electrons that can travel on surfaces or edges without resistance.

The team used a systematic search to identify many previously undiscovered families of candidate topological materials. The approach combined tools from such disparate fields as chemistry, mathematics, physics and materials science.

First, the team characterized all the possible electronic band structures arising from electronic orbitals at all the possible atomic positions for all possible crystal patterns, or symmetry groups, that exist in nature, with the exception of magnetic crystals. To search for topological bands, the team first found a way to enumerate all allowed non-topological bands, with the understanding that anything left out of the list must be topological. Using tools from group theory, the team organized into classes all the possible non-topological band structures that can arise in nature.

Next, by employing a branch of mathematics known as graph theory — the same approach used by search engines to determine links between websites — the team determined the allowed connectivity patterns for all of the band structures. The bands can either separate or connect together. The mathematical tools determine all the possible band structures in nature — both topological and non-topological. But having already enumerated the non-topological ones, the team was able to show which band structures are topological.

By looking at the symmetry and connectivity properties of different crystals, the team identified several crystal structures that, by virtue of their band connectivity, must host topological bands. The team has made all of the data about non-topological bands and band connectivity available to the public through the Bilbao Crystallographic Server. “Using these tools, along with our results, researchers from around the world can quickly determine if a material of interest can potentially be topological,” Elcoro said.

The research shows that symmetry, topology, chemistry and physics all have a fundamental role to play in our understanding of materials, Bernevig said. “The new theory embeds two previously missing ingredients, band topology and orbital hybridization, into Bloch’s theory and provides a prescriptive path for the discovery and characterization of metals and insulators with topological properties.”

David Vanderbilt, a professor of physics and astronomy at Rutgers University who was not involved in the study, called the work remarkable. “Most of us thought it would be many years before the topological possibilities could be catalogued exhaustively in this enormous space of crystal classes,” Vanderbilt said. “This is why the work of Bradlyn and co-workers comes as such a surprise. They have developed a remarkable set of principles and algorithms that allow them to construct this catalogue at a single stroke. Moreover, they have combined their theoretical approach with materials database search methods to make concrete predictions of a wealth of new topological insulator materials.”

The theoretical underpinnings for these materials, called “topological” because they are described by properties that remain intact 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, Princeton University’s Sherman Fairchild University Professor of Physics, J. Michael Kosterlitz of Brown University, and David J. Thouless of the University of Washington.

Chemistry and physics take different approaches to describing crystalline materials, in which atoms occur in regularly ordered patterns or symmetries. Chemists tend to focus on the atoms and their surrounding clouds of electrons, known as orbitals. Physicists tend to focus on the electrons themselves, which can carry electric current when they hop from atom to atom and are described by their momentum.

“This simple fact — that the physics of electrons is usually described in terms of momentum, while the chemistry of electrons is usually described in terms of electronic orbitals — has left material discovery in this field at the mercy of chance,” Wang said.

“We initially set out to better understand the chemistry of topological materials — to understand why some materials have to be topological,” Vergniory said.

Aroyo added, “What came out was, however, much more interesting: a way to marry chemistry, physics and mathematics that adds the last missing ingredient in a century-old theory of electronics, and in the present-day search for topological materials.”