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Tag: Tohoku University

  • Graphene nanodevice readout at high speed

    Graphene nanodevice readout at high speed

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    Newswise — Stacking two layers of graphene with atomic layer thickness produces bilayer graphene, which possesses excellent electrical, mechanical, and optical properties. As such, bilayer graphene has attracted significant attention and is being utilized in a host of next-generation devices, including quantum computers.

    But complicating their application in quantum computing comes in the form of gaining accurate measurements of the quantum bit states. Most research has primarily used low-frequency electronics to overcome this. However, for applications that demand faster electronic measurements and insights into the rapid dynamics of electronic states, the need for quicker and more sensitive measurement tools has become evident.

    Now, a group of researchers from Tohoku University have outlined improvements to radio-frequency (rf) reflectometry to achieve a high-speed readout technique. Remarkably, the breakthrough involves the use of graphene itself.

    Rf reflectometry works by sending radio frequency signals into a transmission line and then measuring the reflected signals to obtain information about samples. But in devices employing bilayer graphene, the presence of significant stray capacitance in the measurement circuit leads to rf leakage and less-than-optimal resonator properties. Whilst various techniques have been explored to mitigate this, clear device design guidelines are still awaited.

    “To circumvent this common shortfall of rf reflectometry in bilayer graphene, we employed a microscale graphite back-gate and an undoped silicon substrate,” says Tomohiro Otsuka, corresponding author of the paper and associate professor at Tohoku University’s Advanced Institute for Materials Research (WPI-AIMR). “We successfully realized good rf matching conditions, calculated the readout accuracy numerically, and compared these measurements with direct current measurements to confirm its consistency. This allowed us to observe Coulomb diamonds through rf reflectometry, a phenomenon indicating the formation of quantum dots in the conduction channel, driven by potential fluctuations caused by bubbles.”

    Otsuka and his team’s proposed improvements to rf reflectometry provide important contributions to the development of next-generation devices such as quantum computers, and the exploration of physical properties using two-dimensional materials, such as graphene.

    The details of their study were reported in the journal Physical Review Applied.

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  • New platform empowers high-entropy alloy electrocatalysis study

    New platform empowers high-entropy alloy electrocatalysis study

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    Newswise — Introduced in 2004, high-entropy alloys (HEAs) are alloys composed of multiple principal elements in nearly equiatomic proportions. Their unique chemical composition results in a high degree of chemical disorder, i.e. entropy, and produces remarkable properties such as high strength, ductility, and strong wear-and-tear resistance even at high temperatures. Scientists have dedicated a significant amount of attention to developing novel HEAs to help improve the performance of various electrocatalyst materials.

    Because they are made up of differing constituent elements, HEAs’ atomic-level surface designs can be complex. But unravelling this complexity is crucial, since the surface properties of materials often dictate their catalytic activity. Hence why researchers are seeking to understand the correlation between the atomic arrangement and the catalytic properties exhibited by HEAs.

    Now, a collaborative research team has created a new experimental platform that enables the control of the atomic-level structure of HEAs’ surfaces and the ability to test their catalytic properties. Their breakthrough was reported in the journal Nature Communications on July 26, 2023.

    “In our study we made thin layers of an alloy called a Cantor alloy, which contains a mix of elements (Cr-Mn-Fe-Co-Ni), on platinum (Pt) substrates,” explains Toshimasa Wadayama, co-author of the paper and a professor at Tohoku University’s Graduate School of Environmental Studies. “This produced a model surface for studying a specific reaction called the oxygen reduction reaction (ORR).”

    Using advanced imaging techniques, the group examined the atomic-level structure of the Pt-HEAs’ surfaces and studied their ORR properties. They discovered that the Pt-HEAs’ surfaces performed better in ORR compared to surfaces made of a platinum-cobalt alloy. This indicates that the atomic arrangement and distribution of elements near the surface, which creates a ‘pseudo-core-shell-like structure,’ contributes to the excellent catalytic properties of Pt-HEAs.

    Wadayama and his group stress the wide applicability of their findings, both for any constituent elements and to other nanomaterials.

    “Our newly constructed experimental study platform provides us with a powerful tool to elucidate the detailed relationship between multi-component alloy surface microstructures and their catalytic properties. It is valid for clarifying the precise correlations among the atomic-level, surface microstructure and electrocatalytic properties of HEAs of any constituent elements and ratios and, thus, would provide reliable training datasets for materials informatics. The platform is applicable not only to electrocatalysis but also in various fields of functional nanomaterials.”

    Looking ahead, the group hopes to expand this platform into practical electrocatalysis by using Pt-HEA-nanoparticles that seek to increase electrochemical surface areas.

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  • Eddies: Impact on World’s Hottest Oceans

    Eddies: Impact on World’s Hottest Oceans

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    Newswise — Water from the Pacific Ocean flows into the Indian Ocean via the Indonesia Archipelago Seas thanks to a vast network of currents dubbed the Indonesian Throughflow (ITF). The ITF acts as a heat and moisture conveyer belt, transporting warm and nutrient waters. Yet the ITF is neither a steady nor a straight path, but experiences fluctuations and turbulence as it passes through the various sea regions, straits, and passages.

    Currents can sometimes formulate into circular motions, forming a whirlpool-like phenomena. These are known as eddies, and they are prominent in areas where there are strong gradients in temperature, salinity, or velocity. Their rotating motion can cause nutrients from the colder, deeper waters to rise to the surface.

    To investigate the role eddies play in determining the path of the ITF, an international research group has harnessed a high-resolution ocean general circulation model that reproduces eddies. The group featured researchers from Tohoku University, JAMSTEC, Kyushu University, the University of Hawai`i at Mānoa, and the National Research and Innovation Agency of Indonesia.

    Details of their research were reported in the Journal of Geophysical Research – Oceans on May 14, 2023.

    The group’s model enabled them to calculate the transport of simulated particles in a daily-averaged flow field with eddies and a monthly-averaged flow field with smoothed eddy currents, respectively, and estimate the flow rate transported by the simulated particles.

    In the Sulawesi Sea, which is situated along the northeastern coast of Borneo and also borders the southern Filipino island of Mindanao, the Sulu archipelago, and Sulawesi Island’s western coast, the group found that large flow fluctuations occur, and seawater circulates over a wider area for an extended period. Seawater also rises from the middle to near the surface, which may cause significant changes in the water when flowing through due to turbulent mixing.
    On the eastern side of Sulawesi Island sits the Banda Sea, which surrounds the Maluku Islands and borders the islands of New Guinea and Timor. Here, the current fluctuation is slight, and the model predicted negligible influence from the eddies on the Indonesian Current.

    “Our results indicate that the path and residence time of the ITF, along with the mixing process of seawater, must be appropriately reproduced by an ocean general circulation model to gain further insights into and better predict sea surface temperature fluctuations in each region of the Indonesian Archipelago,” points out Toshio Suga, professor of physical oceanography at Tohoku University’s Graduate School of Science and co-author of the paper.

    Global warming’s progression is expected to change the ITF. Such changes could have profound repercussions for water temperatures in the Indonesia Archipelago and the Indian Ocean, El Niño and the Indian Ocean Dipole, and the frequency and scale of marine heatwaves that affect marine ecosystems and local weather. Therefore, it is vital to predict accurately such phenomena.

    Looking ahead, the group hopes to improve the accuracy of future predictions by clarifying the degree to which eddies impact the path and residence time of the ITF, something quantitatively linked to the determination of water temperature in these areas.

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  • Mushrooms communicate with electricity after rain

    Mushrooms communicate with electricity after rain

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    Newswise — Certain fungi play a critical role in the ecological sustenance of forest trees. Ectomycorrhizal fungi are one such example. Commonly found on pine, oak, and birch trees, ectomycorrhizal fungi form a sheath around the outside of tree roots, and their mycelial body develops into vast underground networks that absorb vital nutrients from the soil and transfer it to the trees.

    Scientists have been studying the possibility of electrical signal transfer between mushrooms and across trees via the mycelial networks. It is thought that fungi generate electrical signals in response to external stimuli and use these signals to communicate with each other, coordinating growth and other behavior. It has even been hypothesized that these signals can be used to help transfer nutrients to plants and trees.

    Still, current scientific evidence remains sparse. Moreover, many studies have been limited to the laboratory, failing to recreate what happens in the wild.

    Now, a group of researchers has recently headed to the forest floor to examine small, tan-colored ectomycorrhizal mushrooms known as Laccaria bicolor. Attaching electrodes to six mushrooms in a cluster, the researchers discovered that the electrical signals increased after rainfall.

    “In the beginning, the mushrooms exhibited less electrical potential, and we boiled this down to the lack of precipitation,” says Yu Fukasawa from Tohoku University, who lead the project along with Takayuki Takehi and Daisuke Akai from the National Institute of Technology, Nagaoka College, and Masayuki Ushio from the Hakubi Center, Kyoto University (presently at the Hong Kong University of Science and Technology). “However, the electrical potential began to fluctuate after raining, sometimes going over 100 mV.”

    The researcher correlated this fluctuation with precipitation and temperature, and causality analysis revealed that the post-rain electric potential showed signal transport among mushrooms. This transport was particularly strong between spatially close mushrooms and demonstrated directionality.

    “Our results confirm the need for further studies on fungal electrical potentials under a true ecological context,” adds Fukasawa.

    Details of their research were reported in the journal Fungal Ecology on March 14, 2023.

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  • Acid glia in REM sleep: Stronger acid response in epileptic mice

    Acid glia in REM sleep: Stronger acid response in epileptic mice

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    Newswise — Researchers at Tohoku University have shown that astrocytes – star-shaped glial cells that control the local ionic and metabotropic environment of the brain – exhibit an acid response with REM sleep in mice. They theorize that the acid response could be the underlying drive for specific information processing and generating plasticity during sleep. 

    They further discovered that the REM response in astrocytes intensified in the epileptic brain, meaning studying brain environmental changes associated with REM sleep could potentially be employed as a biomarker for the severity of epileptogenesis. 

    The findings were detailed in the journal Brain on March 3, 2023. 

    Neurons are undoubtedly responsible for information processing in the brain. Astrocytes were not thought to be an essential component of the neural information circuit. However, recent findings suggest that the state of the mind, such as consciousness, sleep, memory formation, and meta-plasticity may all be controlled by astrocytes’ actions. 

    To understand the role of astrocytes in brain function, fluorescent sensor proteins were genetically expressed in the astrocytes of mice. The researchers implanted an optical fiber into the mice’s lateral hypothalamus, a part of the brain vital for controlling our state of being asleep or awake and whole-body metabolism. 

    Excitation light was sent through this fiber and the emitted fluorescence signals were recorded. Using a newly devised method, the researchers dissected the calcium concentration and pH of the astrocytes and the local brain blood volume changes from the recorded optical signals. 

    A clear change in the optical signals associated with REM sleep was observed. A calcium decrease, pH decrease (i.e., acidification), and increase in local brain blood volume occurred. The researchers identified that acidification and blood volume changes produce a strong effect on the optical signals; thus, many of the previous studies using fiber photometry could have misinterpreted their recorded data. 

    Acidification was especially unexpected, as the intracellular solution of cells is highly buffered for pH. Strong acidification occurs upon ischemia but changes in pH were not assumed to occur under physiological conditions. This astrocyte acidification may drive the amplification of synaptic signals and may underlie memory formation during REM sleep. 

    Interestingly, changes in the local brain environment detected with the optical recordings preceded the signature change of the ensemble neuronal electrical activity detected with electroencephalogram by nearly 20 seconds. This suggests that astrocytes and vascular changes control the state of neuronal activity. Transition to REM sleep can also be predicted from these local brain environmental changes. 

    “During REM sleep, prior experiences are sorted and remembered or forgotten, and this process is likely perceived as dreams,” says Professor Ko Matsui of the Super-network Brain Physiology lab at Tohoku University, who led the research. “Acidification in astrocytes may control the likelihood of plasticity to occur in the neural circuits.” 

    The researchers further studied how the properties of REM sleep change with epilepsy. Repeated stimulus to a mouse’s hippocampus produces a brain prone to hyperactivity and this “kindling” method has been used as a model of epileptogenesis. After kindling, spontaneously occurring REM sleep episodes were recorded. Surprisingly, very little astrocytic calcium decreases and local brain blood volume increases occurred with REM sleep, and a strong acid response was recorded from astrocytes. 

    “Our previous study has shown an increased acid response of astrocytes associated with intensified epileptic seizures,” says the lead study investigator, Dr. Yoko Ikoma. “Information is transmitted and processed with electrical signals in neurons. The pH of astrocytes may have control over these neuronal activities both in physiology and in disease.” 

    Monitoring of the bulk pH and local brain blood flow is possible in humans using fMRI. Ikoma says that these local brain environmental changes associated with REM sleep can potentially be used to diagnose the severity of epilepsy in human patients. “A therapeutic strategy designed to control astrocytes’ pH could potentially be used for preventing exacerbation of epilepsy.”

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