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Power/Performance Bits: May 10

Probabilistic bit; 2D COF for low-k dielectrics; cellular sensing circuits.

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Probabilistic bit
Researchers at Tohoku University are working on building probabilistic computers by developing a spintronics-based probabilistic bit (p-bit).

The researchers utilized magnetic tunnel junctions (MTJs). Most commonly used in MRAM technology, where thermal fluctuation typically poses a threat to the stable storage of information, in this case it was a benefit.

The p-bits function with the thermal fluctuations in thermally unstable (stochastic) MTJs. To be effective, stochastic MTJs need much shorter relaxation times which reduces the fluctuation timescale of the p-bit. Doing so would effectively increase the computation speed/accuracy.

The researchers built a nanoscale MTJ device with an in-plane magnetic easy axis. The magnetization direction updates every 8 nanoseconds on average, which they said is 100 times faster than the previous world record.


A top-view scanning electron microscopy image of a magnetic tunnel junction device. Source: © K. Hayakawa et al. / Tohoku University

The development relies on the way entropy behaves in magnetization dynamics, rapidly increasing in MTJs with in-plane easy axis with larger magnitudes of perpendicular magnetic anisotropy. The group intentionally employed an in-plane magnetic easy axis for achieving shorter relaxation times.

“The developed MTJ is compatible with current semiconductor back-end-of-line processes and shows substantial promise for the future realization of high-performance probabilistic computers,” said Shun Kanai, professor at the Research Institute of Electrical Communication at Tohoku University. “Our theoretical framework of magnetization dynamics including entropy also has broad scientific implication, ultimately showing the potential of spintronics to contribute to debatable issues in statistical physics.”

2D COF for low-k dielectrics
Researchers from the University of Virginia, Northwestern University, Georgia Institute of Technology, Carnegie Mellon University, University of California Berkeley, and University of Arizona are investigating using 2D covalent organic frameworks (COFs) as low-k dielectrics in an effort to minimize crosstalk and keep electronics cool.

“Scientists have been in search of a low-k dielectric material that can handle the heat transfer and space issues inherent at much smaller scales,” said Patrick E. Hopkins, a professor in the University of Virginia’s Department of Mechanical and Aerospace Engineering. “Although we’ve come a long way, new breakthroughs are just not going to happen unless we combine disciplines. For this project we’ve used research and principles from several fields – mechanical engineering, chemistry, materials science, electrical engineering — to solve a really tough problem that none of us could work out on our own.”

“The heart of the project was when the chemical team realized the thermal functionality of their material, understanding a new dimension about their work, and when the mechanical and materials team understood the level of molecular engineering possible with chemistry,” said Ashutosh Giri, now a mechanical, industrial and systems engineering assistant professor at Rhode Island University.

“We’re taking sheets of polymer that are only one atom thick – we call this 2D – and controlling their properties by layering the sheets in a specific architecture,” said Will Dichtel, a professor in Northwestern University’s Department of Chemistry. “Our efforts on improving the methods to produce high-quality 2D polymer films enabled this collaborative work.” Generally, known low-k dielectrics exhibit low thermal conductivities, which complicate heat dissipation in high-power-density chips.

The team is applying this new material class to try to meet the requirements of miniaturizing transistors on a dense chip, Dichtel added. “This has enormous potential for use in the semiconductor industry, the industry that that manufactures chips. The material has both low electrical conductivity, or ‘low-k,’ and high heat transfer capability,” he said.

“For this project, we are focusing on the thermal properties of this new material class, which is fantastic, but even more exciting is that we are just scratching the surface,” said Austin Evans, a Ph.D. student at Northwestern. “Developing new classes of materials with unique combinations of properties has amazing technological potential.”

The researchers add that their results show that oriented, layered 2D polymers are promising next-generation dielectric layers and that these molecularly precise materials offer tunable combinations of useful properties, including chemical sensing.

Cellular sensing circuits
Researchers at Pompeu Fabra University propose a way to make manufacturing cellular ‘computers’ cheaper and easier.

Devices using biological cells are able to detect multiple markers and generate complex responses, such as for chemical sensors and disease detection. However, they are generally limited to laboratory use by molecular biologists. The team developed a way to print these cell-based sensors on paper that can be used outside of lab settings.

Circuits are created by drawing an ‘ink’ of different cell types on paper. The cells remain trapped in the paper, alive and functional, and there, they continue growing and are able to release signals that travel through the paper and reach other cells.

Additionally, these devices on paper can be kept in the refrigerator or can even be frozen, since the cellular ink incorporates cryoprotectors. Thus, they can be stored for long periods of time before use.

“We wanted to design a scalable model and we thought about using a printing system like the one for printing T-shirts,” said Sira Mogas-Díez of Pompeu Fabra University. “We make moulds with our drawing, we soak it with the different cellular inks like a buffer, put it on paper and the cells are deposited.”

Each element of the device is a group of cells, in this case bacteria, with minimal genetic modifications that can detect different signals. The cells live in the strip of paper and communicate with each other, integrate signals and generate one response or another depending on the different combinations of signals detected. The elements do not vary, but changing their arrangement in space by means of the drawing they make on paper, devices can be built with different functionalities. “Therefore, the order in which the cells are placed is the software, the cells are the hardware, and the paper is the physical substrate hosting these cells,” said Javier Macía of Pompeu Fabra University.

The team designed several different biosensors, including one to detect and estimate concentrations of mercury. Depending on the amount of mercury present, more or less dots appear on the reactive strip that can be counted with the naked eye. The team is also working on a device to detect cholera in water.

“Certainly there is much work to do, but these initial results suggest that the methodology developed may be the means to facilitate the creation of commercial products based on living devices,” added Macía.



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