Iowa State researchers create nanoscale sensors to better see how high pressure affects materials

Researchers have developed new nanoscale technology to image and measure more of the stresses and strains on materials under high pressures.

As the researchers reported in the journal Science, that matters because, "Pressure alters the physical, chemical and electronic properties of matter."

Understanding those changes could lead to new materials or new phases of matter for use in all kinds of technologies and applications, said Valery Levitas, a paper co-author and Anson Marston Distinguished Professor in Engineering at Iowa State University, the Vance Coffman Faculty Chair and professor in aerospace engineering. CAPTION Krishan Kumar Pandey, Valery Levitas and Mehdi Kamrani, left to right, study materials subject to high pressures in Levitas' Iowa State University laboratory.  CREDIT Photo by Christopher Gannon/Iowa State University{module INSIDE STORY}

Levitas - whose lab specializes in experimental testing and computational modeling of high-pressure sciences - said the new sensing technology could also advance high-pressure studies in chemistry, mechanics, geology and planetary science.

Development and demonstration of the technology is described in a paper, "Imaging stress and magnetism at high pressures using a nanoscale quantum sensor," just published by Science. The lead author is Norman Yao, an assistant professor of physics at the University of California, Berkeley. Iowa State's Mehdi Kamrani, a doctoral student in aerospace engineering, is also a co-author.

The paper describes how the researchers fit a series of nanoscale sensors - they call them nitrogen-vacancy color centers - into diamonds used to exert high pressures on tiny material samples. Typically, those "diamond anvil" experiments with materials squeezed between two diamonds have allowed researchers to measure pressure and changes in volume.

The new system allows researchers to image, measure and calculate six different stresses - a much more comprehensive and realistic measure of the effects of high pressure on materials. The new tests also allow researchers to measure changes in a material's magnetism.

"This has been one of the key problems in high-pressure science," Levitas said. "We need to measure all six of these stresses across a diamond and sample. But it's hard to measure all of them under high pressure."

Levitas' lab has done unique experiments by putting materials under high pressure and then giving them a twist, allowing researchers to drastically reduce phase transformation pressure and search for new phases of matter, which may have technological applications.

The lab also does multiscale computer modeling for high-pressure diamond anvil experiments - Levitas says it's the only lab in the world doing such simulations. He said that experience with high-pressure simulations was why he was invited to collaborate with Yao's sensor project. Simulations made it possible to reconstruct fields of all six stresses in the entire diamond anvil, where they could not be measured, as well as verify experimental results. Levitas plans to use this sensor in his lab.

The sensor enables "pursuit of two complementary objectives in high-pressure science: understanding the strength and failure of materials under pressure (e.g., the brittle-ductile transition) and discovering and characterizing exotic phases of matter (e.g., pressure-stabilized high-temperature superconductors)," the researchers wrote in their paper.

The nitrogen-vacancy sensing technology described in the paper has also been used to measure other material properties - for example, electric and thermal characteristics. The researchers wrote it "can now straightforwardly be extended to high-pressure environments, opening up a large range of experiments for quantitatively characterizing materials at such extreme conditions."

Russian scientists use supercomputing to produce a better description of how the graphite melting curve behaves

Physicists from the Moscow Institute of Physics and Technology and the Institute for High-Pressure Physics of the Russian Academy of Sciences have used supercomputer modeling to refine the melting curve of graphite that has been studied for over 100 years, with inconsistent findings. They also found that graphene "melting" is in fact sublimation. The results of the study came out in the journal Carbon.

Graphite is a material widely used in various industries -- for example in heat shields for spacecraft -- so accurate data on its behavior at ultrahigh temperatures is of paramount importance. Graphite melting has been studied since the early 20th century. About 100 experiments have placed the graphite melting point at various temperatures between 3,000 and 7,000 kelvins. With a spread so large, it is unclear which number is true and can be considered the actual melting point of graphite. The values returned by different supercomputer models are also at variance with each other. 7403E746 814B 404F AD9B 57ADE11F2BE6 c9613{module INSIDE STORY}

A team of physicists from MIPT and HPPI RAS compared several supercomputer models to try and find the matching predictions. Yuri Fomin and Vadim Brazhkin used two methods: classical molecular dynamics and ab initio molecular dynamics. The latter accounts for quantum mechanical effects, making it more accurate. The downside is that it only deals with interactions between a small number of atoms on short time scales. The researchers compared the obtained results with prior experimental and theoretical data.

Fomin and Brazhkin found the existing models to be highly inaccurate. But it turned out that comparing the results produced by different theoretical models and finding overlaps can provide an explanation for the experimental data.

As far back as the 1960s, the graphite melting curve was predicted to have a maximum. Its existence points to complex liquid behavior, meaning that the structure of the liquid rapidly changes on heating or densification. The discovery of the maximum was heavily disputed, with a number of studies confirming and challenging it over and over. Fomin and Brazhkin's results show that the liquid carbon structure undergoes changes above the melting curve of graphene. The maximum, therefore, has to exist.

The second part of the study is dedicated to studying the melting of graphene. No graphene melting experiments have been conducted. Previously, supercomputer models predicted the melting point of graphene at 4,500 or 4,900 K. Two-dimensional carbon was therefore considered to have the highest melting point in the world.

"In our study, we observed a strange 'melting' behavior of graphene, which formed linear chains. We showed that what happens is it transitions from a solid directly into a gaseous state. This process is called sublimation," commented Associate Professor Yuri Fomin of the Department of General Physics, MIPT. The findings enable a better understanding of phase transitions in low-dimensional materials, which are considered an important component of many technologies currently in development, in fields from electronics to medicine.

The researchers produced a more precise and unified description of how the graphite melting curve behaves, confirming a gradual structural transition in liquid carbon. Their calculations show that the melting temperature of graphene in an argon atmosphere is close to the melting temperature of graphite.

The study reported in this story was supported by the Russian Science Foundation and used the supercomputing resources of the Complex for Simulation and Data Processing for Mega-Science Facilities, a federal center of shared research facilities at Kurchatov Institute.

ALMA supercomputer simulations of two cosmic peacocks show violent history of the magellanic clouds

Two peacock-shaped gaseous clouds were revealed in the Large Magellanic Cloud (LMC) by observations with the Atacama Large Millimeter/submillimeter Array (ALMA). A team of astronomers found several massive baby stars in the complex filamentary clouds, which agrees well with supercomputer simulations of giant collisions of gaseous clouds. The researchers interpret this to mean that the filaments and young stars are telltale evidence of violent interactions between the LMC and the Small Magellanic Cloud (SMC) 200 million years ago.

ALMA images of two molecular clouds: N159E-Papillon Nebula (left) and N159W South (right)

ALMA images of two molecular clouds: N159E-Papillon Nebula (left) and N159W South (right). Red and green show the distributions of molecular gas with different velocities mapped by 13CO emissions. The blue region in N159E-Papillon Nebula shows the ionized hydrogen gas observed with the Hubble Space Telescope. The blue part in N159W South shows the emissions from dust particles obtained with ALMA.
Credit: ALMA (ESO/NAOJ/NRAO)/Fukui et al./Tokuda et al./NASA-ESA Hubble Space Telescope 

{module INSIDE STORY} Astronomers know that stars are formed in collapsing clouds in space. However, the formation processes of giant stars, 10 times or more massive than the Sun, are not well understood because it is difficult to pack such a large amount of material into a small region. Some researchers suggest that interactions between galaxies provide a perfect environment for massive star formation. Due to the colossal gravity, clouds in the galaxies are stirred, stretched, and often collide with each other. A huge amount of gas is compressed in an unusually small area, which could form the seeds of massive stars.

A research team used ALMA to study the structure of dense gas in N159, a bustling star formation region in the LMC. Thanks to ALMA’s high resolution, the team obtained a detailed map of the clouds in two sub-regions, N159E-Papillon Nebula and N159W South.

Interestingly, the cloud structures in the two regions look very similar: fan-shaped filaments of gas extending to the north with the pivots in the southernmost points. The ALMA observations also found several massive baby stars in the filaments in the two regions.

“It is unnatural that in two regions separated by 150 light-years, clouds with such similar shapes were formed and that the ages of the baby stars are similar,” says Kazuki Tokuda, a researcher at Osaka Prefecture University and the National Astronomical Observatory of Japan. “There must be a common cause of these features. Interaction between the LMC and SMC is a good candidate.”

In 2017, Yasuo Fukui, a professor at Nagoya University and his team revealed the motion of hydrogen gas in the LMC and found that a gaseous component right next to N159 has a different velocity than the rest of the clouds. They suggested a hypothesis that the starburst is caused by a massive flow of gas from the SMC to the LMC and that this flow originated from a close encounter between the two galaxies 200 million years ago.

The pair of peacock-shaped clouds in the two regions revealed by ALMA fits nicely with this hypothesis. Supercomputer simulations show that many filamentary structures are formed in a short time after a collision of two clouds, which also backs this idea.

“For the first time, we uncovered a link between massive star formation and galaxy interactions in very sharp detail,” says Fukui, the lead author of one of the research papers. “This is an important step in understanding the formation process of massive star clusters in which galaxy interactions have a big impact.”

Artist’s impression of the formation process of peacock-shaped clouds{module INSIDE STORY}

Artist’s impression of the formation process of peacock-shaped clouds. After the collision of two clouds (left), complicated filamentary structures with a pivot in the bottom are formed in the boundary region (center), and a massive star is formed in the dense part with the ionized region shown in blue (right).
Credit: NAOJ