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Category: materials – Page 141

Magnetorotational instability—a process that might explain the dynamics of astrophysical accretion disks—has finally been observed in the laboratory.

What do black holes, forming stars, and a tank of liquid metal in Princeton, New Jersey, have in common? The first two might and the third one definitely does play host to an important process in magnetized-fluid dynamics called magnetorotational instability (MRI). MRI has been well studied theoretically and computationally, and related processes have been seen experimentally [1]. But until now, there has not been an unambiguous laboratory confirmation of its existence. Yin Wang and his colleagues at Princeton University have demonstrated MRI in an ingenious liquid-metal experiment—the culmination of more than 20 years of work [2].

The team’s discovery is significant because MRI has long been suspected of being at the heart of accretion [3]. Accretion, in which material spirals inward in a flattened disk around a black hole or a young star, is a major source of the light coming from those objects. For accretion to occur, the material in the disk must lose its angular momentum. However, angular momentum is conserved: much like the trash we generate in our daily lives, it does not cease to exist when it is not wanted. Instead, angular momentum must be passed from the inner parts of the disk to the outer parts. What drives this angular-momentum transport has long been a mystery.

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Scientists from the Faculty of Pure and Applied Sciences at The University of Tsukuba created scanning tunneling microscopy (STM) “snapshots” with a delay between frames much shorter than previously possible. By using ultrafast laser methods, they improved the time resolution from picoseconds to tens of femtoseconds, which may greatly enhance the ability of condensed matter scientists to study extremely rapid processes.

One picosecond, which is a mere trillionth of a second, is much shorter than the blink of an eye. For most applications, a movie camera that could record frames in a picosecond would be much faster than necessary. However, for scientists trying to understand the ultrafast dynamics of materials using STM, such as the rearrangement of atoms during a phase transition or the brief excitation of electrons, it can be painfully slow.

Now, a team of researchers at the University of Tsukuba designed an STM system based on a pump-probe method that can be used over a wide range of delay times as short as 30 femtoseconds (ACS Photonics, “Subcycle mid-infrared electric-field-driven scanning tunneling microscopy with a time resolution higher than 30 fs”).

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Two-dimensional (2D) semiconductors, like transition-metal dichalcogenides, have become a competitive alternative to traditional semiconducting materials in the post-Moore era, and caused worldwide interest. However, before they can be used in practical applications, some key obstacles must be resolved. One of them is the large electrical contact resistances at the metal-semiconductor interfaces. Researchers have proposed a brand-new contact resistance lowering strategy of 2D semiconductors with a good feasibility, a wide generality and a high stability.

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Two-dimensional van der Waals materials have been the focus of work by numerous research groups for some time. Standing just a few atomic layers thick, these structures are produced in the laboratory by combining atom-thick layers of different materials (in a process referred to as “atomic Lego”).

Interactions between the stacked layers allow the heterostructures to exhibit properties that the individual constituents lack.

The coupling of two different electron-hole pairs leads to a fusion of their properties. (Image: L. Sponfeldner, SNI and Department of Physics, University of Basel)

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The adoption rate of fuel cells has increased owing to the rising need for clean energy.

In a research that could jump-start the work on a range of technologies, including fuel cells, which are key to storing solar and wind energy, MIT researchers have found a simple way to significantly increase the lifetimes of fuel cells and other devices – changing the pH of the system.

Fuel/electrolysis cells made of materials known as solid metal oxides are in interest for several reasons. In electrolysis mode, they are very efficient at converting electricity from a renewable source into a storable fuel like hydrogen or methane. This storable fuel can be used in the fuel cell mode to generate electricity when the sun is not shining, or the wind isn’t blowing.

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Two-dimensional van der Waals materials have been the focus of work by numerous research groups for some time. Standing just a few atomic layers thick, these structures are produced in the laboratory by combining atom-thick layers of different materials (in a process referred to as “atomic Lego”). Interactions between the stacked layers allow the heterostructures to exhibit properties that the individual constituents lack.

Two-layered molybdenum disulfide is one such van der Waals material, in which electrons can be excited using a suitable experimental setup. These negatively charged particles then leave their position in the , leaving behind a positively charged hole, and enter the conduction band. Given the different charges of electrons and holes, the two are attracted to one another and form what is known as a quasiparticle. The latter is also referred to as an electron-hole pair, or exciton, and can move freely within the material.

In two-layered molybdenum disulfide, excitation with light produces two different types of electron-hole pairs: intralayer pairs, in which the electron and hole are localized in the same layer of the material, and interlayer pairs, whose hole and electron are located in different layers and are therefore spatially separate from one another.

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A new theoretical model explains why chiral molecules favour transporting electrons in one spin state over the other, providing a quantitative fit to experiments.1

Many current-carrying materials show no bias towards the spin state of the electrons they transport, allowing spin-up and spin-down species to flow in equal numbers. But chiral molecules can be more discriminating, offering easier passage to one spin orientation than to its counterpart.

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A multidisciplinary team of Indiana University researchers have discovered that the motion of chromatin, the material that DNA is made of, can help facilitate effective repair of DNA damage in the human nucleus — a finding that could lead to improved cancer diagnosis and treatment. Their findings were recently published in the Proceedings of the National Academy of Sciences.

DNA damage happens naturally in human body and most of the damage can be repaired by the cell itself. However, unsuccessful repair could lead to cancer.

“DNA in the nucleus is always moving, not static. The motion of its high-order complex, chromatin, has a direct role in influencing DNA repair,” said Jing Liu, an assistant professor of physics in the School of Science at IUPUI. “In yeast, past research shows that DNA damage promotes chromatin motion, and the high mobility of it also facilitates the DNA repair. However, in human cells this relationship is more complicated.”

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Astronomers may soon have the answer to what is perhaps the greatest mystery of modern science –is dark energy a uniform force across space and time, or has its strength evolved over eons?

The universe is not only expanding – it is accelerating outward, driven by what is commonly referred to as “dark energy.” The term is a poetic analogy to the label for dark matter, the mysterious material that dominates the matter in the Universe and that really is dark because it does not radiate light (it reveals itself via its gravitational influence on galaxies).

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