Tag Archives: letter-pulse-tech

Developing materials suitable for use in optoelectronic devices is currently a very active area of research. The search for materials for use in photoelectric conversion elements has to be carried out in parallel with developing the optimal film formation process for each material, and this can take a few years for just one material. Until now there has been a trade-off, balancing electronic properties and material morphology. Researchers at Osaka University have developed a two-step process that can produce materials with good morphological properties in addition to excellent photoresistor performance. Their findings were published in the Journal of Physical Chemistry Letters.

The powder sample is insoluble, therefore fabrication of devices using wet processes is not possible. Credit: Osaka University

Bismuth sulfide, Bi2S3, belongs to a class of materials known as metal chalcogenides, which show significant promise owing to their optical and electronic properties. However, the performance of Bi2S3-based photoresponsive devices is dependent on the method used to process the film, and many of the reported approaches are hampered by low film crystallinity. Even when high crystallinity is achieved, the nature of the grains can have a negative effect on performance, therefore films with low surface roughness and large grain size are desirable.

“We searched more than 200 materials using a unique, ultra high-speed screening method that can evaluate performance, even when only powdered samples are available,” study corresponding author Akinori Saeki says. “We found that bismuth sulfide, which is inexpensive and less toxic than conventional inorganic solar cell materials, can be processed in a way that does not compromise its excellent photoelectrical properties.”

The technique used produces a 2D layered film in two treatment steps; solution spin-coating followed by crystallization. The photo response performance of the resulting film showed improvements of 6-100 times compared with those of films prepared using other processing methods. Owing to the non-toxic and abundant nature of bismuth and sulfur, the findings are expected to influence the development of commercial optoelectronic devices including solar cells.

“We demonstrated a facile processing technique that does not compromise material performance,” lead author Ryosuke Nishikubo says. “We believe that solution-processable bismuth-based semiconductors are viable alternatives to commercially available inorganic solar cells and show promise for widespread future use. The fact that they are non-toxic also sets them apart from other alternative optoelectronic materials, such as lead halide perovskites.”

Processing materials for device applications without compromising their electronic properties is important for making materials commercially relevant. The reported process has been used to successfully prepare other metal sulfide semiconductors such as lead sulfide, demonstrating the versatility of the approach.

Leti, a research institute of CEA Tech, today announced the launch of the REDFINCH consortium to develop the next generation of miniaturized, portable optical sensors for chemical detection in both gases and liquids. Initial target applications are in the petrochemical and dairy industries.

The consortium of eight European research institutes and companies will focus on developing novel, high-performance, cost-effective chemical sensors, based on mid-infrared photonic integrated circuits (MIR PICs). Silicon PICs — integrating optical circuits onto millimeter-size silicon chips — create extremely robust miniature systems, in which discrete components are replaced by on-chip equivalents. This makes them easier to use and reduces their cost dramatically, expected at least by a factor of 10.

To develop these chemical sensors, the consortium must overcome the significant challenge of implementing these capabilities in the important mid-infrared region (2-20 μm wavelength range), where many important chemical and biological species have strong absorption fingerprints. This allows both the detection and concentration measurement of a wide range of gases, liquids and biomolecules, which is crucial for applications such as health monitoring and diagnosis, detection of biological compounds and monitoring of toxic gases.

Initially, REDFINCH will focus on three specific applications:

  • Process gas analysis in refineries
  • Gas leak detection in petrochemical plants and pipelines
  • Protein analysis in liquids for the dairy industry.

Silicon photonics leverages the advantages of high-performance CMOS technology, providing low-cost mass manufacturing, high-fidelity reproduction of designs and access to high-refractive index contrasts that enable high-performance nanophotonics.

“Despite the mid-infrared wavelength region’s importance for a wide range of applications, current state-of-the-art sensing systems in the MIR tend to be large and delicate. This significantly limits their spreading in real-world applications,” said Jean-Guillaume Coutard, an instrumentation engineer at Leti, which is coordinating the project. “By harnessing the power of photonic integrated circuits, using hybrid and monolithic integration of III-V diode and interband cascade and quantum cascade materials with silicon, the consortium will create high-performance, cost-effective sensors for a number of industries.” 

In addition to Leti, whose expertise includes the design and manufacture of PICs on a 200mm pilot line and integrated photoacoustic cells on silicon, the consortium members and contributions include:

  • Cork Institute of Technology (Ireland) – PIC design & fabrication, hybrid integration
  • Université de Montpellier (France) – Laser growth on Si, photodetector growth
  • Technische Universität Wien (Austria) – Liquid spectroscopy, assembly/test of sensors
  • mirSense (France) – MIR sensor products, laser module integration
  • Argotech a.s. (Czech Republic) Assembly/packaging of PICs
  • Fraunhofer IPM (Germany) – Gas spectroscopy, instrument design/assembly
  • Endress+Hauser (Germany) Process gas analysis and expertise, testing validation.

Quantum dots are nanometer-sized boxes that have attracted huge scientific interest for use in nanotechnology because their properties obey quantum mechanics and are requisites to develop advanced electronic and photonic devices. Quantum dots that self-assemble during their formation are particularly attractive as tunable light emitters in nanoelectronic devices and to study quantum physics because of their quantized transport behavior. It is important to develop a way to measure the charge in a single self-assembled quantum dot to achieve quantum information processing; however, this is difficult because the metal electrodes needed for the measurement can screen out the very small charge of the quantum dot. Researchers at Osaka University have recently developed the first device based on two self-assembled quantum dots that can measure the single-electron charge of one quantum dot using a second as a sensor.

The device was fabricated using two indium arsenide (InAs) quantum dots connected to electrodes that were deliberately narrowed to minimize the undesirable screening effect.

This is a scanning electron micrograph of InAs self-assembled quantum dot transistor device. Credit: Osaka University

“The two quantum dots in the device showed significant capacitive coupling,” says Haruki Kiyama. “As a result, the single-electron charging of one dot was detected as a change in the current of the other dot.”

The current response of the sensor quantum dot depended on the number of electrons in the target dot. Hence the device can be used for real-time detection of single-electron tunneling in a quantum dot. The tunneling events of single electrons in and out of the target quantum dot were detected as switching between high and low current states in the sensor quantum dot. Detection of such tunneling events is important for the measurement of single spins towards electron spin qubits.

“Sensing single charges in self-assembled quantum dots is exciting for a number of reasons,” explains Akira Oiwa. “The ability to achieve electrical readout of single electron states can be combined with photonics and used in quantum communications. In addition, our device concept can be extended to different materials and systems to study the physics of self-assembled quantum dots.”

An electronic device using self-assembled quantum dots to detect single-electron events is a novel strategy for increasing our understanding of the physics of quantum dots and to aid the development of advanced nanoelectronics and quantum computing.

Keysight Technologies, Inc. (NYSE: KEYS), a technology company that helps enterprises, service providers, and governments accelerate innovation to connect and secure the world, today announced that the company’s 3D planar electromagnetic (EM) simulator, Momentum, has been certified for GLOBALFOUNDRIES (GF) 22FDX®, 22nm Fully-Depleted Silicon-On-Insulator (FD-SOI) technology.

Keysight’s Momentum is a 3D planar EM simulator used for passive circuit modeling and analysis. It accepts arbitrary design geometries (including multi-layer structures) and uses frequency-domain Method of Moments (MoM) technology to accurately simulate complex EM effects including coupling and parasitics.

As a result of the certification, designers can now perform accurate EM simulation with GF’s cutting-edge 22FDX technology, facilitating analysis of electromagnetic effect and behavior in today’s ever shrinking and complex designs. Momentum stack-up files are integrated with the latest 22FDX PDK available from GF.

“The certification of GF 22FDX for Keysight’s EM simulator is testimony to the continuous collaboration between GF and Keysight under Keysight’s Foundry Program,” said Punmark Ngangom, RFIC Foundry Program Manager for Keysight Technologies. “Our mutual customers will now be able to leverage Keysight’s GF certified Momentum stack-up files, which are available with GF’s standard 22FDX PDK package.”

Keysight’s Momentum has also been certified for RF/mmWave-optimized metal options with different inductors, attaining highly precise correlation with silicon measurements and circuit models up to 100 GHz per GF’s certification standards.

When 80 microns is enough


September 17, 2018

Should you care that scientists can control a baffling current? Their research results could someday affect your daily living.

Physicists have managed to send and control a spin current across longer distances than ever before – and in a material that was previously considered unsuitable for the task.

We’ll return to what that strange sentence really means. But a spin current is a current that is kept going without relying on a simultaneous current of electrical charges.

“We’ve transferred spins more than 80 microns in an antiferromagnet,” says Arne Brataas, a professor at the Norwegian University of Science and Technology’s (NTNU) Department of Physics, and head of the university’s recently launched Center for Quantum Spintronics (QuSpin).

Spin current is initiated with an electric field at one end of the material, an antiferromagnet. The spin in the antiferromagnet alternates direction (yellow and blue arrows). The signal spreads as a wave (green arrows) through the antiferromagnet. At the other end of the material, the spin current is transferred to an electric current again. Credit: Illustration: Kolbjørn Skarpnes/NTNU

Eighty microns – a mere 8/100 000th of a metre – is that so impressive?

“We’re not exactly sending signals to the other side of the city. But this is far in the world of nanoelectronics,” says Brataas.

Nanoelectronics forms the basis for all the smart technology we surround ourselves with.

Right about now you can start doing your happy dance. That’s because 80 microns is getting to be a great enough distance to matter to people besides the scientists who are interested in knowledge for its own sake.

QuSpin has been collaborating with international physicists, including several in Germany and the Netherlands. The results are so intriguing that they are being published in the latest issue of the journal Nature.

So what is spintronics?

The technology of the future may depend on spintronics. If you don’t know what it is, you might as well learn about it. But you can also jump ahead to the next section if you just want to learn about its practical uses.

Atoms have several parts. Electrons are the negatively charged particles, as many of us learned in science class.

But electrons don’t only have a charge, they also have spin, an apparent internal rotation.

The spin has a direction, which is the basis of magnetism. A ferromagnetic material has a preference to align the spins in one particular direction. These materials are the magnets that you put on the refrigerator door.

Antiferromagnetic materials are also magnetic, but you don’t notice their magnetic quality. The atoms in the material alternate between spins in opposite directions. These alterations effectively zero out the total spin so that the material itself does not have a magnetic moment. These materials don’t work on your refrigerator door.

How spins are organized can therefore have very clear consequences for how a material behaves. The spin can be exploited.

Magnetism can transfer signals

Brataas and his colleagues aren’t currently focusing so much on practical uses, so that’s up to us to do.

Today’s technology transmits signals in a computer’s microchips by means of an electric charge. In an electrical current, electrons and spin both flow through the material.

But in the future a spin current will be able do parts of the same job, using magnetism to transmit the signals instead, without electrons passing through with the current.

What are the benefits of spin currents?

Well, for one, a spin current can sometimes flow more readily than an electrical charge current, since only the spin moves and not the electrons. This results in less energy loss in transmitting the signal.

A spin current does not generate a lot of heat. As the transistors on microchips have become ever smaller, overheating has become a growing problem, causing microchips to melt. Using spin current means smaller transistors can be used -another practical feature as new electronic gizmos pop up everywhere.

Spin current can also be controlled much more quickly. So all the new gizmos can be a lot faster.

Controlling spin current

These results would not be nearly as exciting if physicists couldn’t control the spin current at the same time. But they can.

Physicists can start the spin current by applying an electric field at one end of the material. The signal flows through the material without the electrons moving from one end to the other.

Physicists can also do the opposite at the other end, and transfer the spin current to an electrical current.

They have managed to do this at temperatures approaching room temperature. Granted, 200 degrees Kelvin – or -73.15 degrees Celsius – is a bit chilly for a room, but it falls within the range of naturally occurring temperatures on Earth. The researchers expect that they will be able do this experiment at a more comfortable room temperature pretty soon as well.

The research group used the antiferromagnet hematite, an iron oxide (Fe2O3), in their experiments.

The results this time are clearly just a step along the way. The research team will continue to test other materials and look at how these materials respond to different types of influences.

High risk, but important

NTNU’s QuSpin was awarded Norwegian Centre of Excellence (SFF) status last year, a highly regarded recognition. The centre was created to combine theory with experimental physics in the spintronics field and can already show world-leading results.

“The centre focuses on high risk projects of major importance in many different directions,” says Brataas.

The status that SFF confers provides more stable research funding, since QuSpin is guaranteed support for ten years. The funding facilitates high-risk research that can fail too. And they do, all the time.

Many of their experiments do not match the theories or vice versa, and that is important in its own way. But some experiments are spot on, and they can have particularly great significance.

GOWIN Semiconductor Corp., a developer of programmable logic devices, announces 2 new additions to the current families of embedded memory FPGA devices, the GW1NR-LV4MG81 and GW1NSR-LX2CQN48.  As computing functions are being distributed to edge locations, the need for silicon to adapt to these new uses is becoming prevalent.  The 2 new embedded FPGA devices were designed with low power, small package size, and low cost in mind.

Adopting an edge to cloud infrastructure is challenging.  Each portion of the chain has its own unique characteristics in design.  For the edge, size of sensor or data gatherer affects product real estate; power consumption affects the power source, especially battery life.  The new embedded memory FPGA devices solve these issues by enhancing the integration of multiple devices into a nice, single package device.

“GOWIN’s vision has always been one of developing new products for customer’s needs,” said Jason Zhu, CEO of GOWIN Semiconductor.  “We saw a lack of product integration at the edge and aimed to fix this with easy to use solutions at cost-effective price points.”

The GW1NR-LV4MG81 is a 4K LUT FPGA fabric with 64Mb internal high-speed memory.  The package size is ultra-small, 4.5mm x 4.5 mm PBGA and .83mm thick.  A great logic device for applications where the thickness is an issue.  Power consumption has been optimized to the lowest possible using TSMC’s 55nm LP process.  And up to 69 user IO’s are available supporting GOWIN’s flexible IO structures.

In a 5mm x 5mm QFN package, the GW1NSR-LX2CQN48 is GOWIN’s first device that combines a 2K LUT FPGA fabric with 32Mb internal high-speed memory and an Arm Cortex M3 microprocessor.  With additional user programmable flash, internal SRAM, ADC, and both USB2.0 and MIPI D-PHY interfaces, this makes the GW1NSR-LX2CQN48 a true SoC to solve low power requirements at the edge and elsewhere.

GOWIN offers a complete all-in-one toolchain for both FPGA fabric programming and Cortex M3 programming.  In addition, a complete library of IP cores and reference designs are available to assist in developing platform solutions.  All of these resources are available for download on GOWIN’s website, www.gowinsemi.com.

Mentor, a Siemens business, today announced LightSuite™ Photonic Compiler – the industry’s first integrated photonic automated layout system. This new tool enables companies designing integrated photonic layouts to describe designs in the Python language, from which the tool then automatically generates designs ready for fabrication. The resulting design is “Correct by Calibre” – with the implementation precisely guided by Mentor’s Calibre® RealTime Custom verification tool. LightSuite Photonic Compiler enables designers to generate as well as update large photonic layouts in minutes versus weeks.

With this breakthrough technology, companies can dramatically speed the development of integrated photonic designs that will bring speed-of-light communications directly into high-speed networking and high-performance computing (HPC) systems. It also speeds the development of more cost-effective LiDAR technology, which is seen as essential to enabling the mass deployment of autonomous vehicles.

“Mentor’s LightSuite Photonic Compiler represents a quantum leap in automating what has up to now been a highly manual, full-custom process that required deep knowledge of photonics as well as electronics,” said Joe Sawicki, vice president and general manager of the Design-to-Silicon Division at Mentor, a Siemens business. “With the new LightSuite Photonic Compiler, Mentor is enabling more companies to push the envelope in creating integrated photonic designs.”

“LightSuite Photonic Compiler fixes the biggest roadblocks preventing industry-wide adoption of electro-optical design and simulation of photonic chips,” said M. Ashkan Seyedi, Ph.D., senior research scientist, Hewlett Packard Enterprise. “Photonic chips promise amazing performance, but designing circuits today is just too difficult and requires specialized knowledge. LightSuite Photonic Compiler circumvents those challenges and enables scalability. I’m thrilled to have worked with Mentor to develop this tool to make it possible for anyone to design and build photonic circuits as easily as designing electronic circuits.”

Until now, photonic designers have been forced to use analog, full-custom IC tools to create photonic designs. In this flow, designers manually place components from a process design kit (PDK) and then interconnect those components manually. Photonic components must be interconnected with curved waveguides. After they have manually placed and interconnected the components, they typically perform a full Calibre physical verification run to check for design rule violations, as Calibre DRC can find violations even in photonic designs.

Mentor designed the new LightSuite Photonic Compiler specifically for photonic layout so that engineers have complete control of their layouts and can use the tool to automatically perform the placement and interconnecting of both photonic and electrical components. The designers create a Python script that is used to drive the LightSuite Photonic Compiler. Initial placement can also be defined in Python, or come from a pre-placed OpenAccess design. Next, the tool interconnects photonics components with curved wave guides. As some of the components might contain built-in electrical elements, the tool will route these electrical connections simultaneously along with the curved waveguides.

LightSuite Photonic Compiler uses Calibre RealTime Custom during the inner placement and routing loop, resulting in a layout that is design-rule correct. The tool enables designers to perform “what-if” design exploration for photonics designs, which was prohibitively time consuming with manual layout. With this new level of automation, designers can generate a new layout in minutes versus weeks for large designs.

Mentor will demonstrate LightSuite Photonic Compiler at ECOC in Rome, September 24 – 26 at Stand 436. LightSuite Photonic Compiler will be available on October 1.

SUNY Polytechnic Institute (SUNY Poly) announced today that Professor of Nanobioscience Dr. Nate Cady has been awarded $500,000 in funding from the National Science Foundation to develop advanced computing systems based on a novel approach to the creation of non-volatile memory architecture. This research, which will also support student opportunities, aims to advance today’s typical computing model, in which processing and memory are separate, by bringing them together to make the entire process faster and more energy efficient.

“I am proud to congratulate Professor Cady on this National Science Foundation (NSF) award which is focused on enabling advanced computing capabilities, and notably, has important implications for advances in artificial intelligence,” said SUNY Poly Interim President Dr. Grace Wang. “The NSF’s selection of Dr. Cady’s research for this funding exemplifies the quality and impact of SUNY Poly’s research where our faculty and students leverage our world-class high-tech resources, explore new frontiers, and develop critical technologies for our society.”

The research will enable the design of a scalable computing infrastructure that uses nanoscale non-volatile memory (NVM) devices for both storage and computation. One of the current limits to computing speed is the result of current personal computing architecture, which separates the processor and memory and leads to a cap on data throughput, known as the “von Neumann bottleneck.” By combining storage and computation on the same device, the project circumvents this barrier and creates scalable solutions for extreme-scale computing—computing that is up to one thousand times more capable than current comparable computing—based on wires that cross each other to form memory cells at every intersection. This more powerful capability is made possible because each memory cell, acting like a synapse of the human brain, can be switched on or off, similar to the 1’s and 0’s of current computing, but it can also store many other values between the on or off states, increasing the amount of information that a given memory cell can store exponentially.

“This grant showcases the incredible potential of our faculty to tackle real-world problems with high-tech solutions that stem from the SUNY Poly’s advanced labs, cleanrooms, and capabilities. This news is especially exciting for a number of our graduate students who will be able to focus on this promising research area where they will be at the cutting-edge,” said SUNY Poly Interim Provost Dr. Steven Schneider.

“Dr. Cady’s research is a powerful example of the kind of expertise that SUNY Poly’s faculty possess as our innovation-centered ecosystem provides us with unique opportunities to move the technologies of the future forward,” said SUNY Poly Interim Dean of the College of Nanoscale Sciences; Empire Innovation Professor of Nanoscale Science; and Executive Director, Center for Nanoscale Metrology Dr. Alain Diebold.

“I look forward to advancing this non-volatile memory research at SUNY Poly, using the institution’s cutting-edge fabrication facilities in order to address current computing bottlenecks that slow computing capability and waste energy,” said Dr. Cady. “This grant will drive the development of computing and memory infrastructure that will be evaluated using high-performance simulations and experimental benchmarking within our state-of-the-art laboratory at SUNY Poly where we are eager to develop the architecture that can help revolutionize processing and memory capabilities for next-gen computers.”

Dr. Cady’s research will support SUNY Poly graduate students who will be able to obtain hands-on experience developing the computing/memory structures. The materials for this project will be developed, demonstrated, and then integrated with traditional complementary metal-oxide-semiconductor (CMOS) computer chips as part of a larger production, which will utilize SUNY Poly’s 200mm and 300mm state-of-the-art fabrication facilities. The University of Central Florida is receiving its own funds for collaborative research related to this effort.

Computing using multiple parallel flows of current through data stored in nanoscale “crossbars” is often fast and more energy-efficient, but the design of such crossbars is highly unintuitive for human designers. More specifically, this project explores formal methods for more efficiently conducting Boolean searches and using artificial intelligence techniques such as best-first search, in addition to automatically synthesizing non-volatile memory crossbar designs from specifications written in a high-level programming language.

By Serena Brischetto

SEMI’s Serena Brischetto caught up with Zimmer and Peacock Director Martin Peacock to discuss sensor opportunities and challenges ahead of the European MEMS & Sensors and Imaging & Sensors Summits.

SEMI: Sensors  enable  a  myriad  of  sensors  and  applications,  from  measuring  caffeine  in  coffee and  the  hotness  of  chillies  and  ions  in  the  blood  of  patients,  to  the  detecting sulfite  levels  in  wine. But  what is,  in  your  opinion,  is  the  hottest  application  today?

Peacock: The  hot  topic  now  is  point-of-care  testing  for  medical  diagnostics  and  wearable  biosensors  including  continuous  glucose  monitoring  sensors  for  Type  1  Diabetics.  At  the  moment,  there  are  three  CGM  market leaders:  Dexcom,  Abbott  and  Medtronic. But in  addition several  companies  are currently  developing  CGM  technologies.

SEMI: What are engineers working on to improve sensors’ efficiency?

Peacock: Though  many  groups  are  working  on  increasing  sensor  sensitivity,  the  big  issues  are  manufacturing  and  the  repeatability  of  manufacturing.  Our  engineers  are  currently  working  on  making  our  manufacturing  repeatable.

The  issue  with  biosensors  and  medical  diagnostics  is  that  the  volumes  of  sensors  are  much  lower  than  the  manufacturing  volumes  traditionally  experienced  in  the  semi-conductor  industry. This  is  simply  due  to  the  fact  the  human  health  market  is  a  very  fragmented  market  and  so,  outside  of  diabetes,  it  is  hard  to  identify  a  high-volume  biosensor  or  medical  diagnostic  that  is  required  at  the  volumes  that  the  semiconductor  industry  would  consider  high  volume.

SEMI: And what are the main challenges?

Peacock: Making  biosensors  at  high  volume,  with  a  tight  tolerance  and  at  a  low  cost.  As  discussed  above,  the  issue  with  biosensors  is  they  are  not  necessarily  required  art  high  volumes,  so  a  manufacture  is  trying  to  produce  high-quality  products  but  where  the  manufacturing  volumes  are  relatively  low – all  the  while trying  to  do  this  at  a  price  point  that  the  market  can  bear.  To  summarise,  the  main  challenge  in  biosensors  one  would  say  ‘this  is  a  very  fragmented  market.’

SEMI: What techniques are currently being deployed by Zimmer and Peacock to overcome those challenges?

Peacock: Zimmer  and  Peacock  has  a  proprietary  database  system  for  organizing  our  development  and  manufacturing  data  so  we  can  track  manufacturing  quality  and  determine  how  we  are  performing. We are  dealing  with  the  fragmented  market  by  having  a  platform  approach  where  we  are  ensuring  that  all  our  clients  are  sharing  the  same  supply  chain  up  to  the  point  where  we  functionalise the  biosensors  with  their  specific  biochemistry. This  means  that  our  clients  are  getting  the  economies  of  scale,  even  though  they  require  their  products  in  relatively  small  volume.

SEMI: What do you expect from SEMI European MEMS & Sensors Summit 2018 and why do you recommend attending in Grenoble?

Peacock: Zimmer  and  Peacock  expects  to  meet  inspiring  experts  who  share  our  own  vision. This  vision  is  that  MEMs  and  Sensors  are  a  critical  part  of  a  number  of  social  and  commercial  revolutions,  including  the  Internet  of  Things  (IoT),  Sensor  Web  and  the  growth  of  the  Invitro  Diagnostics  Market  (IVD). We  are  also  interested  in  finding  supplier  who  can  be  part  of  our  supply  chain.

Serena is a marketing and communications manager at SEMI Europe.

A new technique makes it possible to obtain an individual fingerprint of the current-carrying edge states occurring in novel materials such as topological insulators or 2D materials. Physicists of the University of Basel present the new method together with American scientists in Nature Communications.

Measured tunneling current and its dependence on the two applied magnetic fields: The fans of red/yellow curves each correspond to a fingerprint of the conducting edge states. Each individual curve separately shows one of the edge states. Credit: University of Basel, Department of Physics

While insulators do not conduct electrical currents, some special materials exhibit peculiar electrical properties: though not conducting through their bulk, their surfaces and edges may support electrical currents due to quantum mechanical effects, and do so even without causing losses.

Such so-called topological insulators have attracted great interest in recent years due to their remarkable properties. In particular, their robust edge states are very promising since they could lead to great technological advances.

Currents flowing only along the edges

Similar effects as the edge states of such topological insulators also appear when a two-dimensional metal is exposed to a strong magnetic field at low temperatures. When the so-called quantum Hall effect is realized, current is thought to flow only at the edges, where several conducting channels are formed.

Probing individual edge states

Until now, it was not possible to address the numerous current carrying states individually or to determine their positions separately. The new technique now makes it possible to obtain an exact fingerprint of the current carrying edge states with nanometer resolution.

This is reported by researchers of the Department of Physics and the Swiss Nanoscience Institute of the University of Basel in collaboration with colleagues of the University of California, Los Angeles, as well as of Harvard and Princeton University, USA.

In order to measure the fingerprint of the conducting edge states, the physicists lead by Prof. Dominik Zumbühl have further developed a technique based on tunneling spectroscopy.

They have used a gallium arsenide nanowire located at the sample edge which runs in parallel to the edge states under investigation. In this configuration, electrons may jump (tunnel) back and forth between a specific edge state and the nanowire as long as the energies in both systems coincide. Using an additional magnetic field, the scientists control the momentum of tunneling electrons and can address individual edge states. From the measured tunneling currents, the position and evolution of each edge state may be obtained with nanometer precision.

Tracking the evolution

This new technique is very versatile and can also be used to study dynamically evolving systems. Upon increasing the magnetic field, the number of edge states is reduced, and their distribution is modified. For the first time, the scientists were able to watch the full edge state evolution starting from their formation at very low magnetic fields.

With increasing magnetic field, the edge states are first compressed towards the sample boundary until eventually, they move towards the inside of the sample and then disappear completely. Analytical and numerical models developed by the research team agree very well with the experimental data.

“This new technique is not only very useful to study the quantum Hall edge states,” Dominik Zumbühl comments the results of the international collaboration. “It might also be employed to investigate new exotic materials such as topological insulators, graphene or other 2D materials.”