Computers Math

Amorphous oxide semiconductors (AOS) are a promising option for the next generation of display technologies due to their low costs and high electron (charge carrier) mobility. The high mobility, in particular, is essential for high-speed images. But AOSs also have a distinct drawback that is hampering their commercialization — the mobility-stability tradeoff.
One of the core tests of stability in TFTs is the “negative-bias temperature stress” (NBTS) stability test. Two AOS TFTs of interest are indium gallium zinc oxide (IGZO) and indium tin zinc oxide (ITZO). IGZO TFTs have high NBTS stability but poor mobility while ITZO TFTs have the opposite characteristics. The existence of this tradeoff is well-known, but thus far there has been no understanding of why it occurs.
In a recent study published in Nature Electronics, a team of scientists from Japan have now reported a solution to this tradeoff. “In our study, we focused on NBTS stability which is conventionally explained using ‘charge trapping.’ This describes the loss of accumulated charge into the underlying substrate. However, we doubted if this could explain the differences we see in IGZO and ITZO TFTs, so instead we focused on the possibility of a change in carrier density or Fermi level shift in the AOS itself,” explains Assistant Professor Junghwan Kim of Tokyo Tech, who headed the study.
To investigate the NBTS stability, the team used a “bottom-gate TFT with a bilayer active-channel structure” comprising an NBTS-stable AOS (IGZO) layer and an NBTS-unstable AOS (ITZO) layer. They then characterized the TFT and compared the results with device simulations performed using the charge-trapping and the Fermi-level shift models.
They found that the experimental data agreed with the Fermi-level shift model. “Once we had this information, the next question was, ‘What is the major factor controlling mobility in AOSs?'” says Prof. Kim.
The fabrication of AOS TFTs introduces impurities, including carbon monoxide (CO), into the TFT, especially in the ITZO case. The team found that charge transfer was occurring between the AOSs and the unintended impurities. In this case the CO impurities were donating electrons into the active layer of the TFT, which caused the Fermi-level shift and NBTS instability. “The mechanism of this CO-based electron donation is dependent on the location of the conduction band minimum, which is why you see it in high-mobility TFTs such as ITZO but not in IGZO,” elaborates Prof. Kim.
Armed with this knowledge, the researchers developed an ITZO TFT without CO impurities by treating the TFT at 400°C and found that it was NBTS stable. “Super-high vision technologies need TFTs with an electron mobility above 40 cm2 (Vs)-1. By eliminating the CO impurities, we were able to fabricate an ITZO TFT with a mobility as high as 70 cm2 (Vs)-1,” comments an excited Prof. Kim.
However, CO impurities alone do not cause instability. “Any impurity that induces a charge transfer with AOSs can cause gate-bias instability. To achieve high-mobility oxide TFTs, we need contributions from the industrial side to clarify all possible origins for impurities,” asserts Prof. Kim.
The results could pave the way for fabrication of other similar AOS TFTs for use in display technologies, as well as chip input/output devices, image sensors and power systems. Moreover, given their low cost, they might even replace more expensive silicon-based technologies.
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Materials provided by Tokyo Institute of Technology. Note: Content may be edited for style and length. More

Team’s findings could help the design of industrially relevant quantum materials for sensing, computing and communication.
“Vacancy” is a sign you want to see when searching for a hotel room on a road trip. When it comes to quantum materials, vacancies are also something you want to see. Scientists create them by removing atoms in crystalline materials. Such vacancies can serve as quantum bits or qubits, the basic unit of quantum technology.
Researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory and the University of Chicago have made a breakthrough that should help pave the way for greatly improved control over the formation of vacancies in silicon carbide, a semiconductor.
Semiconductors are the material behind the brains in cell phones, computers, medical equipment and more. For those applications, the existence of atomic-scale defects in the form of vacancies is undesirable, as they can interfere with performance. According to recent studies, however, certain types of vacancies in silicon carbide and other semiconductors show promise for the realization of qubits in quantum devices. Applications of qubits could include unhackable communication networks and hypersensitive sensors able to detect individual molecules or cells. Also possible in the future are new types of computers able to solve complex problems beyond the reach of classical computers.
“Scientists already know how to produce qubit-worthy vacancies in semiconductors such as silicon carbide and diamond,” said Giulia Galli, a senior scientist at Argonne’s Materials Science Division and professor of molecular engineering and chemistry at the University of Chicago. ?”But for practical new quantum applications, they still need to know much more about how to customize these vacancies with desired features.”
In silicon carbide semiconductors, single vacancies occur upon the removal of individual silicon and carbon atoms in the crystal lattice. Importantly, a carbon vacancy can pair with an adjacent silicon vacancy. This paired vacancy, called a divacancy, is a key candidate as a qubit in silicon carbide. The problem has been that the yield for converting single vacancies into divacancies has been low, a few percent. Scientists are racing to develop a pathway to increase that yield. More

Across a vast array of robotic hands and clamps, there is a common foe: the heirloom tomato. You may have seen a robotic gripper deftly pluck an egg or smoothly palm a basketball — but, unlike human hands, one gripper is unlikely to be able to do both and a key challenge remains hidden in the middle ground.
“You’ll see robotic hands do a power grasp and a precision grasp and then kind of imply that they can do everything in between,” said Wilson Ruotolo, PhD ’21, a former graduate student in the Biomimetics and Dextrous Manipulation Lab at Stanford University. “What we wanted to address is how to create manipulators that are both dexterous and strong at the same time.”
The result of this goal is “farmHand,” a robotic hand developed by engineers Ruotolo and Dane Brouwer, a graduate student in the Biomimetics and Dextrous Manipulation Lab, at Stanford (aka “the Farm”) and detailed in a paper published Dec. 15 in Science Robotics. In their testing, the researchers demonstrated that farmHand is capable of handling a wide variety of items, including raw eggs, bunches of grapes, plates, jugs of liquids, basketballs and even an angle grinder.
FarmHand benefits from two kinds of biological inspiration. While the multi-jointed fingers are reminiscent of a human hand — albeit a four-fingered one — the fingers are topped with gecko-inspired adhesives. This grippy but not sticky material is based on the structure of gecko toes and has been developed over the last decade by the Biomimetics and Dextrous Manipulation Lab, led by Mark Cutkosky, the Fletcher Jones Professor in the School of Engineering, who is also senior author of this research.
Using the gecko-adhesive on a multi-fingered, anthropomorphic gripper for the first time was a challenge, which required special attention to the tendons controlling the fingers of farmHand and the design of the finger pads below the adhesive.
From the Farm to space and back again
Like gecko’s toes, the gecko adhesive creates a strong hold via microscopic flaps. When in full contact with a surface, these flaps create a Van der Waals force — a weak intermolecular force that results from subtle differences in the positions of electrons on the outsides of molecules. As a result, the adhesives can grip strongly but require little actual force to do so. Another bonus: they don’t feel sticky to the touch or leave a residue behind. More

Searching for molecules that could act as effective therapies for devastating diseases requires extensive time, money and resources — and it often ends in failure.
Researchers at the USC Dornsife College of Letters, Arts and Sciences have created a process that increases the chances of finding effective drugs in a fraction of the time and at significantly less expense than current methods of drug discovery.
The research was published Dec. 15 in the journal Nature.
Puzzling together new, effective drug therapies
Scientists working to create new drugs are equal parts puzzle-solvers and construction workers.
Having peered into a cell and identified a protein that, if manipulated, could help ease or avoid disease, they search for chemical molecules with a specific shape and size, as well as the right features, to fit a target pocket on that protein. More

Achieving consensus among countries in global climate negotiations is a long and complicated process. Researchers at Linköping University have developed a mathematical model that describes the achievement of the 2015 Paris Agreement and that may contribute to more efficient negotiations when striving for unanimity.
Global climate targets have been in focus this autumn as world leaders met at COP26 in Glasgow. The intention was that countries would negotiate how to work together to keep the global temperature rise below two degrees Celsius, and preferably below 1.5 degrees.
Climate agreements need unanimity, and achieving unanimity takes time, commitment, and a good organisation structure. The Paris Agreement in 2015, for example, was the result of complex diplomatic negotiations that took more than a decade to complete. And the time aspect is something that researchers at Linköping University have examined in depth.
“Our model investigates how the negotiations should be organised in order to achieve unanimity, and what factors can slow down or speed up convergence,” says Claudio Altafini, professor in the Division for Automatic Control, Department of Electrical Engineering, at Linköping University.
The results have been published in the scientific journal Science Advances.
Based on observed data
The model is dynamical and based on observed data from the documents and the minutes of nearly 300 UN-sponsored climate conferences in the period 2001-2015, leading to the Paris Agreement. The documents have enabled the LiU researchers to investigate, among other things, the pattern according to which countries participated and expressed their opinions in the sequence of meetings. More

Semiconductors are moving away from rigid substrates, which are cut or formed into thin discs or wafers, to more flexible plastic material and even paper thanks to new material and fabrication discoveries. The trend toward more flexible substrates has led to fabrication of numerous devices, from light-emitting diodes to solar cells and transistors.
Georgia Tech researchers have created a material that acts like a second skin layer and is up to 200% more stretchable than its original dimension without significantly losing its electric current. The researchers say the soft flexible photodetectors could enhance the utility of medical wearable sensors and implantable devices, among other applications. The research will be published on Dec. 15 in the journal Science Advances.
Georgia Tech researchers from both mechanical and computing engineering labs collaborated over three years to demonstrate a new level of stretchability for a photodetector, a device made from a synthetic polymer and an elastomer that absorbs light to produce an electrical current.
Photodetectors today are used as wearables for health monitoring, such as rigid fingertip pulse oximeter reading devices. They convert light signals into electrical ones and are commonly used on wearable electronics.
Stretchable like a Rubber Band
Given that conventional flexible semiconductors break under a few percentages of strain, the Georgia Tech findings are “an order-of-magnitude improvement,” said Olivier Pierron, professor in the George W. Woodruff School of Mechanical Engineering, whose lab measures the mechanical properties and reliability of flexible electronics under extreme conditions. More

Exotic quantum particles and phenomena are like the world’s most daring elite athletes. Like the free solo climbers who scale impossibly steep cliff faces without a rope or harness, only the most extreme conditions will entice them to show up. For exotic phenomena like superconductivity or particles that carry a fraction of the charge of an electron, that means extremely low temperatures or extremely high magnetic fields.
But what if you could get these particles and phenomena to show up under less extreme conditions? Much has been made of the potential of room-temperature superconductivity, but generating exotic fractionally charged particles at low-to-zero magnetic field is equally important to the future of quantum materials and applications, including new types of quantum computing.
Now, a team of researchers from Harvard University led by Amir Yacoby, Professor of Physics and of Applied Physics at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and Ashvin Vishwanath, Professor of Physics in the Department of Physics, in collaboration with Pablo Jarillo-Herrero at the Massachusetts Institute of Technology, have observed exotic fractional states at low magnetic field in twisted bilayer graphene for the first time.
The research is published in Nature.
“One of the holy grails in the field of condensed matter physics is getting exotic particles with low to zero magnetic field,” said Yacoby, senior author of the study. “There have been theoretical predictions that we should be able to see these bizarre particles with low to zero magnetic field, but no one has been able to observe it until now.”
The researchers were interested in a specific exotic quantum state known as fractional Chern insulators. Chern insulators are topological insulators, meaning they conduct electricity on their surface or edge, but not in the middle. More

Physicists construct theories to describe nature. Let us explain it through an analogy with something that we can do in our everyday life, like going on a hike in the mountains. To avoid getting lost, we generally use a map. The map is a representation of the mountain, with its houses, rivers, paths, etc. By using it, it is rather easy to find our way to the top of the mountain. But the map is not the mountain. The map constitutes the theory we use to represent the mountain’s reality.
Physical theories are expressed in terms of mathematical objects, such as equations, integrals or derivatives. During history, physics theories evolved, making use of more elaborate mathematical concepts to describe more complicated physics phenomena. Introduced in the early 20th century to represent the microscopic world, the advent of quantum theory was a game changer. Among the many drastic changes it brought, it was the first theory phrased in terms of complex numbers.
Invented by mathematicians centuries ago, complex numbers are made of a real and imaginary part. It was Descartes, the famous philosopher considered as the father of rational sciences, who coined the term “imaginary,” to strongly contrast it with what he called “real” numbers. Despite their fundamental role in mathematics, complex numbers were not expected to have a similar role in physics because of this imaginary part. And in fact, before quantum theory, Newton’s mechanics or Maxwell’s electromagnetism used real numbers to describe, say, how objects move, as well as how electro-magnetic fields propagate. The theories sometimes employ complex numbers to simplify some calculations, but their axioms only make use of real numbers.
Schrödinger’s bewilderment
Quantum theory radically challenged this state of affairsbecause its building postulates were phrased in terms of complex numbers. The new theory, even if very useful for predicting the results of experiments, and for instance perfectly explains the hydrogen atom energy levels, went against the intuition in favor of real numbers. Looking for a description of electrons, Schrödinger was the first to introduce complex numbers in quantum theory through his famous equation. However, he could not conceive that complex numbers could actually be necessary in physics at that fundamental level. It was as though he had found a map to represent the mountains but this map was actually made out of abstract and non-intuitive drawings. Such was his bewilderment that he wrote a letter to Lorentz on June 6, 1926, stating “What is unpleasant here, and indeed directly to be objected to, is the use of complex numbers. ? is surely fundamentally a real function.” Several decades later, in 1960, Prof. E.C.G. Stueckelberg, from the University of Geneva, demonstrated that all predictions of quantum theory for single-particle experiments could equally be derived using only real numbers. Since then, the consensus was that complex numbers in quantum theory were only a convenient tool.
However, in a recent study published in Nature, ICFO researchers Marc-Olivier Renou and ICREA Prof. at ICFO Antonio Acín, in collaboration with Prof. Nicolas Gisin from the University of Geneva and the Schaffhausen Institute of Technology, Armin Tavakoli from the Vienna University of Technology, and David Trillo, Mirjam Weilenmann, and Thinh P. Le, led by Prof. Miguel Navascués, from the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences in Vienna have proven that if the quantum postulates were phrased in terms of real numbers, instead of complex, then some predictions about quantum networks would necessarily differ. Indeed, the team of researchers came up with a concrete experimental proposal involving three parties connected by two sources of particles where the prediction by standard complex quantum theory cannot be expressed by its real counterpart.
Two sources and three nodes
To do this, they thought of a specific scenario that involves two independent sources (S and R), placed between three measurement nodes (A, B and C) in an elementary quantum network. The source S emits two particles, say photons, one to A, and the second to B. The two photons are prepared in an entangled state, say in polarization. That is, they have correlated polarization in a way which is allowed by (both complex and real) quantum theory but impossible classically. The source R does exactly the same, emits two other photons prepared in an entangled state and sends them to B and to C, respectively. The key point in this study was to find the appropriate way to measure these four photons in the nodes A, B, C in order to obtain predictions which cannot be explained when quantum theory is restricted to real numbers.
As ICFO researcher Marc-Olivier Renou comments “When we found this result, the challenge was to see if our thought experiment could be done with current technologies. After discussing with colleagues from Shenzhen-China, we found a way to adapt our protocol to make it feasible with their state-of-the-art devices. And, as expected, the experimental results match the predictions!.” This remarkable experiment, realized in collaboration with Zheng-Da Li,Ya-Li Mao,Hu Chen, Lixin Feng, Sheng-Jun Yang, Jingyun Fan from the Southern University of Science and Technology, and Zizhu Wang from the University of Electronic Science and Technology is published at the same time as the Nature paper in Physical Review Letters.
The results published in Nature can be seen as a generalization of Bell’s theorem, which provides a quantum experiment which cannot be explained by any local physics formalism. Bell’s experiment involves one quantum source S that emits two entangled photons, one to A, and the second to B, prepared in an entangled state. Here, in contrast, one needs two independent sources, the assumed independence is crucial and was carefully designed in the experiment.
The study also shows how outstanding predictions can be when combining the concept of a quantum network with Bell’s ideas. For sure, the tools developed to obtain this first result are such that they will allow physicists to achieve a better understanding of quantum theory, and will one day trigger the realization and materialization of so far unfathomable applications for the quantum internet. More