Tag Archives: letter-pulse-tech

LG Innotek succeeded in mass-producing ultraviolet (UV) LED module that sterilizes the inside of water purifier faucet aerators.

The company started to mass-produce the UV LED module for sterilizing water purifier faucet aerators at the end of the last month. This product is built in LG Electronics’ new direct water purifier “PuriCare Slim Updown” launched in March in the Republic of Korea.

A water purifiers faucet aerator always holds a small amount of water. This part is prone to contamination due to the growth of germs that come in with the influx of air. However, it was difficult to install a sterilizer inside a faucet aerator because its space is too narrow.

LG Innotek developed a UV LED module customized for the faucet aerator that has strong sterilizing power and is harmless.

This module directly sterilizes the water inside faucet aerator with ultraviolet rays. The product is 1.5cm in width and 3.7cm in length and can be mounted in the small space inside the water purifier.

The product kills 99.98% of germs when a faucet aerator is exposed to ultraviolet rays for 5 minutes. This result was obtained by sterilizing a faucet aerator with 278nm wavelength.

UV LED module is also harmless since it uses only ultraviolet rays for sterilization without any chemicals or heavy metals. Also, unlike a mercury UV lamp, you don’t need to worry about breaking it.

This product is convenient to use as it allows you to control ultraviolet rays quickly and accurately. As soon as its sterilization function is activated, ultraviolet rays are released at peak performance. On the contrary, a mercury UV lamp requires about 2 minutes of warming period.

LG Electronics’ direct water purifier installed with this module allows you to sterilize faucets in an instant by pressing the “Self Care” button anytime. It also performs automatic sterilization every 1 hour.

LG Innotek plans to actively expand the application of UV LEDs to various products. The company already developed a 280nm UV-C LED that has the power of 70mW for the first time in the world.

The company has already secured a product line-up with products that are optimized for different applications, including 365nm, 385nm, 395nm and 405nm UV-A LEDs for general industry and 305nm UV-B LED for biomedical field as well as 280nm UV-C LED for sterilization.

Ho-rim Jung, the vice president of LED marketing division, said, “Our UV LEDs will increase the value of the end products that are installed with them and allow us to care for the health of users in a smart way.”

According to Yole Development, a market research firm, the global market for UV LEDs is expected to grow more than seven folds from USD 130 million in 2015 to USD 1 billion in 2021. Especially, the UV LEDs for water purification is expected to occupy 60% of the said UV LED market.

Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a few layers of atoms. Graphene is the best-known 2D material. Molybdenum disulphide (a layer consisting of molybdenum and sulphur atoms that is three-atoms thick) also falls in this category, although, unlike graphene, it has semiconductor properties. With his team, Dr Thomas Mueller from the Photonics Institute at TU Wien is conducting research into 2D materials, viewing them as a promising alternative for the future production of microprocessors and other integrated circuits.

Stefan Wachter, Dmitry K. Polyushkin and Thomas Mueller (f.l.t.r.). Credit: TU Wien, Marco Furchi

Stefan Wachter, Dmitry K. Polyushkin and Thomas Mueller (f.l.t.r.). Credit: TU Wien, Marco Furchi

The whole and the sum of its parts

Microprocessors are an indispensable and ubiquitous component in the modern world. Without their continued development, many of the things we take for granted these days, such as computers, mobile phones and the internet, would not be possible at all. However, while silicon has always been used in the production of microprocessors, it is now slowly but surely approaching its physical limits. 2D materials, including molybdenum disulphide, are showing promise as potential replacements. Although research into individual transistors – the most basic components of every digital circuit – made of 2D materials has been under way since graphene was first discovered back in 2004, success in creating more complex structures has been very limited. To date, it has only been possible to produce individual digital components using a few transistors. In order to achieve a microprocessor that operates independently, however, much more complex circuits are required which, in addition also need to interact flawlessly.

Thomas Mueller and his team have now managed to achieve this for the first time. The result is a 1-bit microprocessor consisting of 115 transistors over a surface area of around 0.6 mm2 that can run simple programs. “Although, this does of course seem modest when compared to the industry standards based on silicon, this is still a major breakthrough within this field of research. Now that we have a proof of concept, in principle there is no reason that further developments can’t be made,” says Stefan Wachter, a doctoral student in Dr Mueller’s research group. However, it was not just the choice of material that resulted in the success of the research project. “We also gave careful consideration to the dimensions of the individual transistors,” explains Mueller. “The exact relationships between the transistor geometries within a basic circuit component are a critical factor in being able to create and cascade more complex units.”

Future prospects

It goes without saying that much more powerful and complex circuits with thousands or even millions of transistors will be required for this technology to have a practical application. Reproducibility continues to be one of the biggest challenges currently being faced within this field of research along with the yield in the production of the transistors used. After all, both the production of 2D materials in the first place as well as the methods for processing them further are still at the very early stages. “As our circuits were made more or less by hand in the lab, such complex designs are of course pretty much beyond our capability. Every single one of the transistors has to function as planned in order for the processor to work as a whole,” explains Mueller, stressing the huge demands placed on state-of-the-art electronics. However, the researchers are convinced that industrial methods could open up new fields of application for this technology over the next few years. One such example might be flexible electronics, which are required for medical sensors and flexible displays. In this case, 2D materials are much more suitable than the silicon traditionally used owing to their significantly greater mechanical flexibility.

The first fully functional microprocessor logic devices based on few-atom-thick layered materials have been demonstrated by researchers from the Graphene Flagship, working at TU Vienna in Austria. The processor chip consists of 115 integrated transistors and is a first step toward ultra-thin, flexible logic devices. Using transistors made from layers of molybdenum disulphide (MoS2), the microprocessors are capable of 1-bit logic operations and the design is scalable to multi-bit operations.

With the drive towards smart objects and the Internet of Things, the microprocessors hold promise for integrating computational power into everyday objects and surfaces. The research is published this week in Nature Communications.

The Graphene Flagship is developing novel technologies based on graphene and related materials (GRMs) such as transition metal dichalcogenides (TMDs) like MoS2, semiconductor materials that can be separated into ultra-thin sheets just a few atoms thick. GRMs are promising for compact and flexible electronic devices due to their thinness and excellent electrical properties.

The ultra-thin MoS2 transistors are inherently flexible and compact, so this result could be directly translated into microprocessors for fully flexible electronic devices, for example, wearable phones or computers, or for wider use in the Internet of Things. The MoS2 transistors are highly responsive, and could enable low-powered computers to be integrated into everyday objects without adding bulk. “In principle, it’s an advantage to have a thin material for a transistor. The thinner the material, the better the electrostatic control of the transistor channel, and the smaller the power consumption,” said Thomas Mueller (TU Vienna), who led the work.

Mueller added “In general, being a flexible material there are new opportunities for novel applications. One could combine these processor circuits with light emitters that could also be made with MoS2 to make flexible displays and e-paper, or integrate them for logic circuits in smart sensors. Our goal is to realise significantly larger circuits that can do much more in terms of useful operations. We want to make a full 8-bit design – or even more bits – on a single chip with smaller feature sizes.”

Talking about increasing the computing power, Stefan Wachter (TU Vienna), first author of the work, said “Adding additional bits of course makes everything much more complicated. For example, adding just one bit will roughly double the complexity of the circuit.”

Compared to modern processors, which can have billions of transistors in a single chip, the 115-transistor devices are very simple. However, it is a very early stage for a new technology, and the team have concrete plans for the next steps: “Our approach is to improve the processing to a point where we can reliably make chips with a few tens of thousands of transistors. For example, growing directly onto the chip would avoid the transfer process, which would give higher yield so that we can go to more complex circuits,” said Dmitry Polyushkin (TU Vienna), an author of the work.

Semiconductors are used for myriad optoelectronic devices. However, as devices get smaller and smaller and more demanding, new materials are needed to ensure that devices work with greater efficiency. Now, researchers at the USC Viterbi School of Engineering have pioneered a new class of semiconductor materials that might enhance the functionality of optoelectronic devices and solar panels–perhaps even using one hundred times less material than the commonly used silicon.

Researchers at USC Viterbi, led by Jayakanth Ravichandran, an assistant professor in the Mork Family Department of Chemical Engineering and Material Sciences and including Shanyuan Niu, Huaixun Huyan, Yang Liu, Matthew Yeung, Kevin Ye, Louis Blankemeier, Thomas Orvis, Debarghya Sarkar, Assistant Professor of Electrical Engineering Rehan Kapadia, and David J. Singh, a professor of physics from University of Missouri, have developed a new class of materials that are superior in performance and have reduced toxicity. Their process, documented in “Bandgap Control via Structural and Chemical Tuning of Transition Metal Perovskite Chalcogenide,” is published in Advanced Materials.

Ravichandran, the lead on this research, is a materials scientist, who has always been interested in understanding the flow of electrons and heat through materials, as well as the how electrons interact within materials. This deep knowledge of how material composition affects electron movement was critical to Ravichandran’s and his colleagues’ most recent innovation.

Computers and electronics have been getting better, but according to Jayakanth Ravichandran, the principal investigator of this study, “the performance of the most basic device–the transistors –are not getting better.” There is a plateau in terms of performance, as noted by what is considered the “end of Moore’s law.” Similar to electronics, there is a lot of interest to develop high performance semiconductors for opto-electronics. The collaborative team of material scientists and electrical engineers wanted to develop new materials which could showcase the ideal optical and electrical properties for a variety of applications such as displays, light detectors and emitters, as well as solar cells.

The researchers developed a class of semiconductors called “transition metal perovskite chalcogenides.” Currently, the most useful semiconductors don’t hold enough carriers for a given volume of material (a property which is referred to as “density of states”) but they transport electrons fast and thus are known to have high mobility. The real challenge for scientists has been to increase this density of states in materials, while maintaining high mobility. The proposed material is predicted to possess these conflicting properties.

As a first step to show its potential applications, the researchers studied its ability absorb and emit light. “There is a saying,” says Ravichandran of the dialogue among those in the optics and photonics fields, “that a very good LED is also a very good solar cell.” Since the materials Ravichandran and his colleagues developed absorb and emit light effectively, solar cells are a possible application.

Solar cells absorb light and convert it into electricity. However, solar panels are made of silicon, which comes from sand via a highly energy intensive extraction process. If solar cells could be made of a new, alternative semiconductor material such as the one created by the USC Viterbi researchers– a material that could fit more electrons for a given volume (and reducing the thickness of the panels), solar cells could be more efficient–perhaps using one hundred times less material to generate the same amount of energy. This new material, if applied in the solar energy industry, could make solar energy less expensive.

While it is a long road to bring such a class of materials to market, the next step is to recreate this material in an ultra-thin film form to make solar cells and test their performance. “The key contribution of this work,” says Ravichandran, “is our new synthesis method, which is a drastic improvement from earlier studies. Also, our demonstration of wide tunability in optical properties (especially band gap) is promising for developing new optoelectronic devices with tunable optical properties.”

NXP Semiconductors N.V. (NASDAQ:NXPI) today announced a new laterally diffused metal oxide semiconductor (LDMOS) technology for RF power transistors designed for operation up to 65 volts (V). This extra-high voltage LDMOS process will give rise to a new generation of products: the MRFX series.

As RF becomes more pervasive in various industrial applications, NXP is providing RF power engineers with a means to reduce design cycle time:

  • More power – Higher voltage enables higher output power, which helps decrease the number of transistors to combine, simplifying power amplifiers complexity and reducing their size.
  • Faster development time – With higher voltage, the output power can be increased while retaining a reasonable output impedance. This simplifies the matching to 50 ohms, especially in wideband applications. Faster matching dramatically speeds up the development time.
  • Design reuse – This impedance benefit also ensures pin-to-pin compatibility with current 50 V LDMOS transistors, making it possible for RF designers to reuse existing printed circuit board (PCB) designs for even shorter time to market.
  • Manageable current level – A higher voltage lowers the current in the system, limiting the stresses on DC power supplies and reducing magnetic radiation.
  • Wide safety margin – The NXP 65 V LDMOS technology has a breakdown voltage of 182 V, which improves reliability and enables higher efficiency architectures.

The first product in the MRFX series is the MRFX1K80, the industry’s most powerful continuous wave (CW) RF transistor. It is designed to deliver 1800 watts (W) CW at 65 V for applications from 1 to 470 megahertz (MHz) and is capable of handling 65:1 voltage standing wave ratio (VSWR).

“The drop-in compatibility between our 1250, 1500 and our new 1800 W transistors enables our customers to create a single scalable platform for multiple end products,” said Pierre Piel, senior director and general manager for multi-market RF power at NXP. “With this new generation, we help our customers deliver on their commitment of higher performing, more rugged products in a shorter amount of time.”

The MRFX1K80 is targeted for industrial, scientific and medical (ISM) applications such as laser generation, plasma etching, magnetic-resonance imaging (MRI), skin treatment and diathermy, as well as particle accelerators and other scientific applications. The MRFX1K80 is also designed for radio and very high frequency (VHF) TV broadcast transmitters. Industrial heating, welding, curing or drying machines currently using vacuum tubes will also benefit from the higher level of control that solid state enables.

ON Semiconductor (Nasdaq: ON) announced it is expanding its portfolio of Interline Transfer Electron Multiplication CCD (IT-EMCCD) image sensors with new options that target not only low-light industrial applications such as medical and scientific imaging, but also commercial and military applications for high-end surveillance.

The new 4 megapixel KAE-04471 uses larger 7.4 micron pixels than those found in existing IT-EMCCD devices, doubling the light gathering capability of the new device and improving image quality under light starved conditions. The KAE-04471 is pin and package compatible with the existing 8 megapixel KAE-08151, allowing camera manufacturers to easily leverage existing camera designs to support the new device.

The new KAE-02152 shares the same 1080p resolution and 2/3” optical format as the existing KAE-02150, but incorporates an enhanced pixel design that increases sensitivity in near-infrared (NIR) wavelengths – an improvement that can be critical in applications such as surveillance, microscopy and ophthalmology. The KAE-02152 is fully drop-in compatible with the existing KAE-02150, and both devices are available in packages that incorporate an integrated thermoelectric cooler, simplifying the work required by camera manufacturers to develop a cooled camera design.

“As the need for sub-lux imaging solutions expands in surveillance, medical, scientific and defense markets, customers are looking for new options that provide the critical performance required in these applications,” said Herb Erhardt, Vice President and General Manager, Industrial Solutions Division, Image Sensor Group at ON Semiconductor. “The new products allow customers to choose from a variety of resolutions, pixel sizes, sensitivities, color configurations and packaging options in our IT-EMCCD portfolio to meet their low-light imaging needs.”

Interline Transfer EMCCD devices combine two established imaging technologies with a unique output structure to enable a new class of low-noise, high-dynamic range imaging. While Interline Transfer CCDs provide excellent image quality and uniformity with a highly efficient electronic shutter, this technology is not always ideal for very low-light imaging. And while EMCCD image sensors excel under low-light conditions, they historically have only been available as low resolution devices with limited dynamic range. Combining these technologies allows the low-noise architecture of EMCCD to be extended to multi-megapixel resolutions, and an innovative output design allows both standard CCD (normal-gain) and EMCCD (high-gain) outputs to be utilized for a single image capture – extending dynamic range and scene detection from sunlight to starlight in a single image.

Engineering grade versions of the KAE-04471 are now available, with production versions available in 2Q17. Engineering grade versions of the KAE-02152 in both a standard package as well as a package incorporating an integrated thermoelectric cooler are also available, with production versions of both configurations available in 3Q17. All IT-EMCCD devices ship in ceramic micro-PGA packages, and are available in both Monochrome and Bayer Color configurations.

Evaluation kits for devices in the IT-EMCCD portfolio allow the full performance of this technology to be examined and reviewed under real-world conditions. Customers can purchase an evaluation kit, or inquire about an on-site demonstration of IT-EMCCD devices, by contacting their local ON Semiconductor sales representative.

Computer electronics are shrinking to small-enough sizes that the very electrical currents underlying their functions can no longer be used for logic computations in the ways of their larger-scale ancestors. A traditional semiconductor-based logic gate called a majority gate, for instance, outputs current to match either the “0” or “1” state that comprise at least two of its three input currents (or equivalently, three voltages). But how do you build a logic gate for devices too small for classical physics?

One recent experimental demonstration, the results of which are published this week in Applied Physics Letters, from AIP Publishing, uses the interference of spin-waves — synchronous waves of electron spin alignment observed in magnetic systems. The spin-wave majority gate prototype, made of Yttrium-Iron-Garnet, comes out of a new collaborative research center funded by the German Research Foundation, named Spin+X. The work has also been supported by the European Union within the project InSpin and has been conducted in collaboration with the Belgian nanotechnology research institute IMEC.

The brass block serves as an electric ground plate ensuring an efficient insertion of the RF currents to the antennae and, on the other hand, microwave connectors mounted to the block allow for the embedding of the device into our microwave setup. Credit: Fischer/Kewenig/Meyer

The brass block serves as an electric ground plate ensuring an efficient insertion of the RF currents to the antennae and, on the other hand, microwave connectors mounted to the block allow for the embedding of the device into our microwave setup. Credit: Fischer/Kewenig/Meyer

“The motto of the research center Spin+X is ‘spin in its collective environment,’ so it basically aims at investigating any type of interaction of spins — with light and matter and electrons and so on,” said Tobias Fischer, a doctoral student at the University of Kaiserslautern in Germany, and lead author of the paper. “More or less the main picture we are aiming at is to employ spin-waves in information processing. Spin waves are the fundamental excitations of magnetic materials.”

So instead of using classical electric currents or voltages to send input information to a logic gate, the Kaiserslautern-based international team uses vibrations in a magnetic material’s collective spin — essentially creating nanoscale waves of magnetization that can then interfere to produce Boolean calculations.

“You have atomic magnetic moments in your magnetic material which interact with each other and due to this interaction, there are wave-like excitations that can propagate in magnetic materials,” Fischer said. “The particular device we were investigating is based on the interference of these waves. If you use wave excitations instead of currents […] then you can make use of wave interference, and that comes with certain advantages.”

Using wave interference to produce the majority gate’s output provides two parameters to use in controlling information: the wave’s amplitude, and phase. In principle, that makes this concept more efficient also since a majority gate can substitute up to 10 transistors in modern electronic devices.

“The device we were investigating consists of three inputs where we excite waves and they combine,” Fischer said. “Depending on the input phases where you encode the information, that determines the phase of the output signal, hence, defining the logic output state ‘0’ or ‘1’. That is actually information processing and that’s what we want.”

This first device prototype, though physically larger than what Fischer and his colleagues see for eventual large-scale use, clearly demonstrates the applicability of spin-wave phenomena for reliable information processing at GHz frequencies.

Because the wavelengths of these spin waves are easily reduced to the nanoscale, so too (though perhaps not quite as easily) can be the gate device itself. Doing so may actually improve the functionality, reducing its sensitivity to unwanted field fluctuations. Besides, nano-scaling will increase spin-wave velocities that will allow for an increase in computing speed.

“What we aim for is the miniaturization of the device, and the smaller you make the device, the less sensitive it becomes to these influences,” Fischer said. “If you look at how many wavelengths fit into this propagation length, the fewer there are, the less influence a change of the wavelength has on the output. So basically downscaling the device would also come with more benefits.”

Furthermore, much like antennae, a single device can be operated at multiple frequencies simultaneously. This will allow for parallel computing using the same “core” of a future spin-wave processor.

“One of my colleagues in Kaiserslautern is into spin-wave multiplexing and de-multiplexing,” Fischer said. “We are also going in that direction, to use multiple frequencies and that would be a good compliment […] to this majority gate.”

Graphene Flagship researchers from AMBER at Trinity College Dublin have fabricated printed transistors consisting entirely of layered materials. Published today in the leading journal Science, the team’s findings have the potential to cheaply print a range of electronic devices from solar cells to LEDs with applications from interactive smart food and drug labels to next-generation banknote security and e-passports.

Led by Professor Jonathan Coleman from AMBER (the Science Foundation Ireland-funded materials science research centre hosted in Trinity College Dublin), in collaboration with the groups of Professor Georg Duesberg (AMBER) and Professor Laurens Siebbeles (TU Delft, Netherlands), the team used standard printing techniques to combine graphene flakes as the electrodes with other layered materials, tungsten diselenide and boron nitride as the channel and separator (two important parts of a transistor) to form an all-printed, all-layered materials, working transistor.

All of these are flakes are a few nanometres thick but hundreds of nanometres wide. Critically, it is the ability of flakes made from different layered materials to have electronic properties that can be conducting (in the case of graphene), insulating (boron nitride) or semiconducting (tungsten diselenide) that enable them to create the building blocks of electronics. While the performance of these printed layered devices cannot yet compare with advanced transistors, the team believe there is a wide scope to improve the performance of their printed TFTs beyond the current state-of-the-art.

Professor Coleman, who is an investigator in AMBER and Trinity’s School of Physics, said, “In the future, printed devices will be incorporated into even the most mundane objects such as labels, posters and packaging. Printed electronic circuitry will allow consumer products to gather, process, display and transmit information: for example, milk cartons will send messages to your phone warning that the milk is about to go out-of-date. We believe that layered materials can compete with the materials currently used for printed electronics.”

All of the layered materials were printed from inks created using the liquid exfoliation method previously developed by Professor Coleman and already licensed. Using liquid processing techniques to create the layered materials inks is especially advantageous in that it yields large quantities of high quality layered materials which helps to enable the potential to print circuitry at low cost.

Optomechanical devices, which simultaneously confine light waves and mechanical waves to permit interaction between them, can be used both to study fundamental questions in physics and to sense motion in a way similar to electromechanical accelerometers. In smartphones, these electronic components switch the touchscreen between portrait and landscape when they detect rotation by the user.

According to experts in the field, however, the use of optomechanical devices to study macroscopic quantum phenomena – in which the large-scale properties of matter such as mechanical vibration are subject to the laws that govern atoms (quantum mechanics) – or to identify very subtle movements requires extremely high levels of interaction, or coupling, between light waves and mechanical waves.

A group of researchers led by Thiago Pedro Mayer Alegre and Gustavo Silva Wiederhecker at the University of Campinas’s Gleb Wataghin Physics Institute (IF-UNICAMP) in São Paulo State, Brazil, have developed an optomechanical device with a novel design that boosts the coupling between light waves and mechanical waves to higher levels than those reported for similar devices developed in the laboratory. Their work was part of research projects supported by FAPESP.

The new optomechanical device and an experimental demonstration of its functioning are described in an article published in the Optical Society of America’s journal Optics Express.

“The way we designed the device allows the levels of interaction between light waves and mechanical waves to be increased,” Alegre told.

“This means the device can both have practical applications and assist us in our basic research by helping us answer certain questions, such as what happens in the transition between the quantum microscopic world and the classical macroscopic world.”

The device created by the researchers, based on a 24-micron silicon disk supported by a silicon dioxide central pedestal so that the disk can vibrate, has a similar shape to a bullseye at the center of a shooting target, with concentric circular grooves.

Thanks to this shape, light waves and mechanical waves can be confined within the device by separate mechanisms.

The light waves are confined only at the edge of the disk by total internal reflection, an optical phenomenon whereby light within a medium such as water or glass is completely reflected from the surrounding surfaces (such as the air interface) back into the medium, provided the angle of incidence is greater than a certain limiting angle called the critical angle.

Light waves are therefore compressed near the disk edge and travel around the rings for a long time, whereas mechanical vibrations can propagate throughout the material.

However, the concentric rings create frequency regions in which mechanical waves cannot propagate, so that they are confined to the outside edge of the disk, where they interact directly with the light waves.

“Confining light waves and mechanical waves to the disk edge enables us to boost their interaction, which is useful for exploring quantum phenomena in macroscopic objects,” Alegre explained.

In devices developed by other research groups, the concentric circular grooves are used to confine light waves in the central region and not at the edge, as in the case of the device designed by the researchers at IF-UNICAMP.

Based on the finding that, like optical vibrations, mechanical vibrations can be understood as waves, Alegre’s group had the idea of using the concentric rings to confine mechanical waves at the edge of the device and make them interact more intensely with light waves in the same region.

“The point of developing the disk with this bullseye design was to prevent the mechanical mode from ‘seeing’ the central pedestal that supports the disk and allow the entire structure to vibrate, eliminating mechanical losses,” he said.

The device is highly customizable, he added, and compatible with existing industrial fabrication processes, making it a solution for the enhancement of sensors that detect force and motion, for example.

One of its potential applications is in telecommunications as an optical modulator, Alegre explained. Because the device can sense and excite mechanical vibration, it could be used as an optical switch, turning on or off a laser beam that passes through it far more efficiently than the modulating technologies used today in optical telecommunications networks.

“It was fabricated according to current industrial processes, so any group in the world could reproduce it,” he said.

An international team of researchers have created a new structure that allows the tuning of topological properties in such a way as to turn on or off these unique behaviors. The structure could open up possibilities for new explorations into the properties of topological states of matter.

“This is an exciting new direction in topological matter research,” said M. Zahid Hasan, professor of physics at Princeton University and an investigator at Lawrence Berkeley National Laboratory in California who led the study, which was published March 24th in the journal Science Advances. “We are engineering new topological states that do not occur naturally, opening up numerous exotic possibilities for controlling the behaviors of these materials.”

The new structure consists of alternating layers of topological and normal, or trivial, insulators, an architecture that allows the researchers to turn on or off the flow of current through the structure. The ability to control the current suggests possibilities for circuits based on topological behaviors, but perhaps more importantly presents a new artificial crystal lattice structure for studying quantum behaviors.

Theories behind the topological properties of matter were the subject of the 2016 Nobel Prize in physics awarded to Princeton University’s F. Duncan Haldane and two other scientists. One class of matter is topological insulators, which are insulators on the inside but allow current to flow without resistance on the surfaces.

In the new structure, interfaces between the layers create a one-dimensional lattice in which topological states can exist. The one-dimensional nature of the lattice can be thought of as if one were to cut into the material and remove a very thin slice, and then look at the thin edge of the slice. This one-dimensional lattice resembles a chain of artificial atoms. This behavior is emergent because it arises only when many layers are stacked together.

By changing the composition of the layers, the researchers can control the hopping of electron-like particles, called Dirac fermions, through the material. For example, by making the trivial-insulator layer relatively thick – still only about four nanometers – the Dirac fermions cannot travel through it, making the entire structure effectively a trivial insulator. However, if the trivial-insulator layer is thin – about one nanometer – the Dirac fermions can tunnel from one topological layer to the next.

To fashion the two materials, the Princeton team worked with researchers at Rutgers University led by Seongshik Oh, associate professor of physics, who in collaboration with Hasan and others showed in 2012 in work published in Physical Review Letters that adding indium to a topological insulator, bismuth selenide, caused it to become a trivial insulator. Prior to that bismuth selenide (Bi2Se3) was theoretically and experimentally identified as a topological insulator by Hasan’s team which was published in Nature in 2009.

“We had shown that, depending on how much indium you add, the resulting material had this nice tunable property from trivial to topological insulator,” Oh said, referring to the 2012 study.

Graduate students Ilya Belopolski of Princeton and Nikesh Koirala of Rutgers combined two state-of-the-art techniques with new instrumentation development and worked together on layering these two materials, bismuth selenide and indium bismuth selenide, to design the optimal structure. One of the challenges was getting the lattice structures of the two materials to match up so that the Dirac fermions can hop from one layer to the next. Belopolski and Suyang Xu worked with colleagues at Princeton University, Lawrence Berkeley National Laboratory and multiple institutions to use high resolution angle-resolved photoemission spectroscopy to optimize the behavior of the Dirac fermions based on a growth to measurement feedback loop.

Although no topologically similar states exist naturally, the researchers note that analogous behavior can be found in a chain of polyacetylene, which is a known model of one-dimensional topological behavior as described by the 1979 Su-Schrieffer-Heeger’s theoretical model of an organic polymer.

The research presents a foray into making artificial topological materials, Hasan said. “In nature, whatever a material is, topological insulator or not, you are stuck with that,” Hasan said. “Here we are tuning the system in a way that we can decide in which phase it should exist; we can design the topological behavior.”

The ability to control the travel of light-like Dirac fermions could eventually lead future researchers to harness the resistance-less flow of current seen in topological materials. “These types of topologically tunable heterostructures are a step toward applications, making devices where topological effects can be utilized,” Hasan said.

The Hasan group plans to further explore ways to tune the thickness and explore the topological states in connection to the quantum Hall effect, superconductivity, magnetism, and Majorana and Weyl fermion states of matter.