Aurelia Butler

Whether for use in cybersecurity, gaming or scientific simulation, the world needs true random numbers, but generating them is harder than one might think. But a group of Brown University physicists has developed a technique that can potentially generate millions of random digits per second by harnessing the behavior of skyrmions — tiny magnetic anomalies that arise in certain two-dimensional materials.
Their research, published in Nature Communications, reveals previously unexplored dynamics of single skyrmions, the researchers say. Discovered around a half-decade ago, skyrmions have sparked interest in physics as a path toward next-generation computing devices that take advantage of the magnetic properties of particles — a field known as spintronics.
“There has been a lot of research into the global dynamics of skyrmions, using their movements as a basis for performing computations,” said Gang Xiao, chair of the Department of Physics at Brown and senior author of the research. “But in this work, we show that purely random fluctuations in the size of skyrmions can be useful as well. In this case, we show that we can use those fluctuations to generate random numbers, potentially as many as 10 million digits per second.”
Most random numbers produced by computers aren’t random in the strictest sense. Computers use an algorithm to generate random numbers based on an initial starting place, a seed number. But because the algorithm used to generate the number is deterministic, the numbers aren’t truly random. With enough information about the algorithm or its output, it could be possible for someone to find patterns in the numbers that the algorithm produces. While pseudorandom numbers are sufficient in many settings, applications like data security — which uses numbers that can’t be guessed by an outside party — require true random numbers.
Methods of producing true random numbers often draw on the natural world. Random fluctuations in electrical current flowing through a resistor, for example, can be used to generate random numbers. Other techniques harness the inherent randomness in quantum mechanics — the behavior of particles at the tiniest scale.
This new study adds skyrmions to the list of true random number generators. More

Smaller chips, faster computers, less energy consumption. Novel concepts based on semiconductor nanowires are expected to make transistors in microelectronic circuits better and more efficient. Electron mobility plays a key role in this: The faster electrons can accelerate in these tiny wires, the faster a transistor can switch and the less energy it requires. A team of researchers from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the TU Dresden and NaMLab has now succeeded in experimentally demonstrating that electron mobility in nanowires is remarkably enhanced when the shell places the wire core under tensile strain. This phenomenon offers novel opportunities for the development of ultrafast transistors.
Nanowires have a unique property: These ultra-thin wires can sustain very high elastic strains without damaging the crystal structure of the material. And yet the materials themselves are not unusual. Gallium arsenide, for example, is widely used in industrial manufacturing, and is known to have a high intrinsic electron mobility.
Tension creates speed
To further enhance this mobility, the Dresden researchers produced nanowires consisting of a gallium arsenide core and an indium aluminum arsenide shell. The different chemical ingredients result in the crystal structures in the shell and the core having slightly different lattice spacings. This causes the shell to exert a high mechanical strain on the much thinner core. The gallium arsenide in the core changes its electronic properties. “We influence the effective mass of electrons in the core. The electrons become lighter, so to speak, which makes them more mobile,” explained Dr. Emmanouil Dimakis, scientist at the HZDR’s Institute of Ion Beam Physics and Materials Research and initiator of the recently published study.
What started out as a theoretical prediction has now been proven experimentally by the researchers in the recently published study. “We knew that the electrons in the core ought to be even more mobile in the tensile-strained crystal structure. But what we did not know was the extent to which the wire shell would affect electron mobility in the core. The core is extremely thin, allowing electrons to interact with the shell and be scattered by it,” remarked Dimakis. A series of measurements and tests demonstrated this effect: Despite interaction with the shell, electrons in the core of the wires under investigation moved approximately thirty percent faster at room temperature than electrons in comparable nanowires that were strain-free or in bulk gallium arsenide.
Revealing the core
The researchers measured electron mobility by applying contactless optical spectroscopy: Using an optical laser pulse, they set electrons free inside the material. The scientists selected the light-pulse energy such that the shell seems practically transparent to the light, and free electrons are only produced in the wire core. Subsequent high-frequency terahertz pulses caused the free electrons to oscillate. “We practically give the electrons a kick and they start oscillating in the wire,” explained PD Dr. Alexej Pashkin, who optimized the measurements for testing the core-shell nanowires under investigation in collaboration with his team at the HZDR.
Comparing the results with models reveals how the electrons move: The higher their speed and the fewer obstacles they encounter, the longer the oscillation lasts. “This is actually a standard technique. But this time we did not measure the whole wire — comprising the core and the shell — but only the tiny core. This was a new challenge for us. The core accounts for around one percent of the material. In other words, we excite about a hundred times fewer electrons and get a signal that is a hundred times weaker,” stated Pashkin.
Consequently, the choice of sample was also a critical step. A typical sample contains an average of around 20,000 to 100,000 nanowires on a piece of substrate measuring roughly one square millimeter. If the wires are spaced even closer together on the sample, an undesirable effect can occur: Neighboring wires interact with each other, creating a signal similar to that of a single, thicker wire, and distorting the measurements. If this effect is not detected, the electron velocity obtained is too low. To rule out such interference, the Dresden research team carried out additional modelling as well as a series of measurements for nanowires with different densities.
Prototypes for fast transistors
Trends in microelectronics and the semiconductor industry increasingly demand smaller transistors that switch ever faster. Experts anticipate that novel nanowire concepts for transistors will also make inroads into industrial production over the next few years. The development achieved in Dresden is particularly promising for ultra-fast transistors. The researchers’ next step will be to develop the first prototypes based on the studied nanowires and to test their suitability for use. To do this, they intend to apply, test and enhance metallic contacts on the nanowires, as well as testing the doping of nanowires with silicon and optimizing manufacturing processes.
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Inside every cellphone lies a tiny mechanical heart, beating several billion times a second. These micromechanical resonators play an essential role in cellphone communication. Buffeted by the cacophony of radio frequencies in the airwaves, these resonators select just the right frequencies for transmitting and receiving signals between mobile devices.
With the growing importance of these resonators, scientists need a reliable and efficient way to make sure the devices are working properly. That’s best accomplished by carefully studying the acoustic waves that the resonators generate.
Now, researchers at the National Institute of Standards and Technology (NIST) and their colleagues have developed an instrument to image these acoustic waves over a wide range of frequencies and produce “movies” of them with unprecedented detail.
The researchers measured acoustic vibrations as rapid as 12 gigahertz (GHz, or billions of cycles per second) and may be able to extend those measurements to 25 GHz, providing the necessary frequency coverage for 5G communications as well as for potentially powerful future applications in quantum information.
The challenge of measuring these acoustic vibrations is likely to increase as 5G networks dominate wireless communications, generating even tinier acoustic waves.
The new NIST instrument captures these waves in action by relying on a device known as an optical interferometer. The illumination source for this interferometer, ordinarily a steady beam of laser light, is in this case a laser that pulses 50 million times a second, which is significantly slower than the vibrations being measured. More

A hundred years of physics tells us that collective atomic vibrations, called phonons, can behave like particles or waves. When they hit an interface between two materials, they can bounce off like a tennis ball. If the materials are thin and repeating, as in a superlattice, the phonons can jump between successive materials.
Now there is definitive, experimental proof that at the nanoscale, the notion of multiple thin materials with distinct vibrations no longer holds. If the materials are thin, their atoms arrange identically, so that their vibrations are similar and present everywhere. Such structural and vibrational coherency opens new avenues in materials design, which will lead to more energy efficient, low-power devices, novel material solutions to recycle and convert waste heat to electricity, and new ways to manipulate light with heat for advanced computing to power 6G wireless communication.
The discovery emerged from a long-term collaboration of scientists and engineers at seven universities and two U.S. Department of Energy national laboratories. Their paper, Emergent Interface Vibrational Structure of Oxide Superlattices, was published January 26 in Nature.
Eric Hoglund, a postdoctoral researcher at the University of Virginia School of Engineering and Applied Science, took point for the team. He earned his Ph.D. in materials science and engineering from UVA in May 2020 working with James M. Howe, Thomas Goodwin Digges Professor of materials science and engineering. After graduation, Hoglund continued working as a post-doctoral researcher with support from Howe and Patrick Hopkins, Whitney Stone Professor and professor of mechanical and aerospace engineering.
Hoglund’s success illustrates the purpose and potential of UVA’s Multifunctional Materials Integration Initiative, which encourages close collaboration among different researchers from different disciplines to study material performance from atoms to applications.
“The ability to visualize atomic vibrations and link them to functional properties and new device concepts, enabled by collaboration and co-advising in materials science and mechanical engineering, advances MMI’s mission,” Hopkins said. More

Metal halide perovskites (MHPs) are a class of materials with promising properties for semiconductor applications, such as thin-film transistors (TFTs). In particular, tin (Sn)-based MHPs could be an environmentally benign alternative to lead-based ones, which are toxic. However, some critical issues need to be resolved before Sn-based MHPs can be leveraged in planar semiconductor devices.
When arranged into a 2D structure (or quasi-2D structure with a few layers), defects in the crystal structure of Sn-based MHPs called “grain boundaries” hamper the mobility of charge carriers throughout the material. If used in a TFT, this phenomenon results in a large series resistance that ultimately degrades performance. In contrast, a TFT made using an Sn-based MHP arranged into a 3D structure faces a different yet still crippling problem. The extremely high carrier density of the 3D material causes the transistor to be permanently ON unless very high voltages are applied. Needless to say, this renders such a device useless for many applications.
Fortunately, a team of scientists from Tokyo Tech, Japan, have found a solution to these limitations. In a recent study published in Advanced Scienceand led by Assistant Professor Junghwan Kim and Honorary Professor Hideo Hosono, the researchers proposed a novel concept based on a hybrid structure for Sn-based MHPs, called the “2D/3D core-shell structure.” In this structure, 3D MHP cores are fully isolated from one another and connected only through short 2D MHP strips (or “shells”). This alternating arrangement solves both of the abovementioned drawbacks simultaneously. But how?
The trick to lowering the series resistance of 2D MHPs is to eliminate the carrier mobility problems at grain boundaries, which are caused by misalignments between the conductive octahedra of the perovskite. Thanks to the way in which the 3D cores connect to the 2D segments, these misalignments disappear and the series resistance is greatly lowered. As for the high carrier density of 3D MHPs, this problem is simply not present when using the 2D/3D core-shell structure. Since the 3D cores are isolated, their carrier density is no longer relevant; instead, the 2D segments act as a bottleneck and limit the effective carrier density of the overall material.
To demonstrate the effectiveness of this novel structure, the team fabricated a complementary metal-oxide-semiconductor (CMOS) inverter by combining 2D/3D TFTs with a standard indium gallium zinc oxide TFT. “Our device exhibited a high voltage gain of 200 V/V at a drain voltage of 20 V. This performance is the best reported so far for a CMOS inverter made using Sn-MHP TFTs,” highlights Prof. Kim.
The innovative 2D/3D structure presented in this study will help scientists worldwide take advantage of the attractive electronic properties of perovskites. Moreover, their approach is not limited to a narrow class of materials or device types. “The proposed strategy could be applied to various solution-derived semiconductor systems, opening doors to flexible and printable electronics,” says Prof. Kim.
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Researchers at the National Institute of Standards and Technology (NIST) have revived and improved a once-reliable technique to identify and count defects in transistors, the building blocks of modern electronic devices such as smartphones and computers. Over the past decade, transistor components have become so small in high-performance computer chips that the popular method, known as charge pumping, could no longer count defects accurately. NIST’s new and improved method is sensitive enough for the most modern, minuscule technology, and can provide an accurate assessment of defects that could otherwise impair the performance of transistors and limit the reliability of the chips in which they reside.
The new, modified charge pumping technique can detect single defects as small as the diameter of a hydrogen atom (one-tenth of a billionth of a meter) and can indicate where they’re located in the transistor. Researchers could also use the new capability to detect and manipulate a property in each electron known as quantum spin. The ability to manipulate individual spins has applications in both basic research and quantum engineering and computing.
Transistors act as electrical switches. In the on position, which represents the “1” of binary digital information, a designated amount of current flows from one side of a semiconductor to the other. In the off position, representing the “0” of binary logic, current ceases to flow.
Defects in a transistor can interfere with the reliable flow of current and significantly degrade the performance of transistors. These defects could be broken chemical bonds in the transistor material. Or they could be atomic impurities that trap electrons in the material. Scientists have devised several ways to categorize defects and minimize their impact, tailored to the structure of the transistor under study.
In the traditional design known as the metal oxide semiconductor field effect transistor (MOSFET), a metal electrode called the gate sits atop a thin insulating layer of silicon dioxide. Below the insulating layer lies the interface region that separates the insulating layer and the main body of the semiconductor. In a typical transistor, current travels through a narrow channel, only one billionth of a meter thick, that extends from the source, which lies on one side of the gate, to a “drain” on the other side. The gate controls the amount of current in the channel.
Charge pumping is a two-step process in which the examiner alternately pulses the gate with a positive test voltage, then a negative one. (The transistor does not act as an on/off switch during this testing mode.) In traditional charge pumping, the alternating voltage pulses are applied at a single, set frequency. More

The opening lines of the great Chinese historical novel Romance of the Three Kingdoms condense its complex and spectacular stories into a coherent pattern, that is, power blocs divide and unite cyclically in turbulent battle years.
A good philosophy or theorem has general implications. Now, published in the journal Nature Synthesis, scientists from Australia, New Zealand, and the US reported a new type of solidification patterns that resembles the plots in the Chinese classic, but this time appearing on the surface of solidifying liquid metals.
The team dissolved a small amount of metals such as silver (Ag) in low-melting-point solvent metals such as gallium (Ga), and investigated how the metallic components interact and separate to form patterns when the metallic liquid mixtures (alloys) solidify.
The researchers found that a single silver-gallium system can produce distinct patterns such as particles or bundle-like structures of a Ag2Ga compound.
The individual Ag2Ga structures that build the patterns are small, with micrometre or nanometre thicknesses, tens or hundreds of times less than a human hair. More

Recommendation algorithms can make a customer’s online shopping experience quicker and more efficient by suggesting complementary products whenever the shopper adds a product to their basket. Did the customer buy peanut butter? The algorithm recommends several brands of jelly to add next.
These algorithms typically work by associating purchased items with items other shoppers have frequently purchased alongside them. If the shopper’s habits, tastes, or interests closely resemble those of previous customers, such recommendations might save time, jog the memory, and be a welcome addition to the shopping experience.
But what if the shopper is buying peanut butter to stuff a dog toy or bait a mousetrap? What if the shopper prefers honey or bananas with their peanut butter? The recommendation algorithm will offer less useful suggestions, costing the retailer a sale and potentially annoying the customer.
New research led by Negin Entezari, who recently received a doctoral degree in computer science at UC Riverside, Instacart collaborators, and her doctoral advisor Vagelis Papalexakis, brings a methodology called tensor decomposition — used by scientists to find patterns in massive volumes of data — into the world of commerce to recommend complementary products more carefully tailored to customer preferences.
Tensors can be pictured as multi-dimensional cubes and are used to model and analyze data with many different components, called multi-aspect data. Data closely related to other data can be connected in a cube arrangement and related to other cubes to uncover patterns in the data.
“Tensors can be used to represent customers’ shopping behaviors,” said Entezari. “Each mode of a 3-mode tensor can capture one aspect of a transaction. Customers form one mode of the tensor and the second and third mode captures product-to-product interactions by considering products co-purchased in a single transaction.”
For example, three hypothetical shoppers — A, B, and C — make the following purchases: More