Fermi surfaces in metals are a boundary between energy states that are occupied and unoccupied by electrons. Fermi surfaces are normally closed contours forming shapes such as spheres, ovoids, etc. Electrons at the Fermi surface control many properties of materials such as electrical and thermal conductivity, optical properties, etc. In extremely rare occasions, the Fermi surface contains disconnected segments that are known as Fermi arcs and often are associated with exotic states like superconductivity.

Adam Kaminski, leader of the research team, explained that newly discovered Fermi arcs are the result of electron band splitting, which results from the magnetic order of Nd atoms that make up 50% of the sample. However, the electron splitting that the team observed in NdBi was not typical band splitting behavior.

There are two established types of band splitting, Zeeman and Rashba. In both cases the bands retain their original shape after splitting. The band splitting that the research team observed resulted in two bands of different shapes. As the temperature of the sample decreased, the separation between these bands increased and the band shapes changed, indicating a change in fermion mass.

“This splitting is very, very unusual, because not only is the separation between those bands increasing, but they also change the curvature,” Kaminski said. “This is very different from anything else that people have observed to date.”

The previously known cases of Fermi arcs in Weyl semimetals persist because they are caused by the crystal structure of the material which is difficult to control. However, the Fermi arcs that the team discovered in NdBi are induced by magnetic ordering of the Nd atoms in the sample. This order can be readily changed by applying a magnetic field, and possibly by changing the Nd ion for another rare earth ion such as Cerium, Praseodymium, or Samarium (Ce, Pr, or Sm). Since Ames Lab is a world leader in rare earth research, such changes in composition can be easily explored.

“This new type of Fermi arcs appears whenever the sample becomes antiferromagnetic. So when the sample develops magnetic order, these arcs just appear seemingly out of nowhere,” said Kaminski.

According to Kaminski, another important characteristic of these new Fermi arcs is that they have what is called spin texture. In normal metals, each electronic state is occupied by two electrons, one with a spin up, one with a spin down, so there is no net spin. The newly discovered Fermi arcs have single orientation of spin at each of their points. Since they exist only in a magnetically ordered state, the arcs can be switched on and off very quickly by applying a magnetic pulse, for example from an ultrafast laser.

“Having such a spin decoration or spin texture is important because one of the quests in electronics is to move away from the charge-based electronics. Everything that you use now is based on moving electrons in wires and that causes dissipation,” Kaminski said.

The ability to control the spin of electrons relates to a new branch of information technology called spintronics, which is based on electron spin rather than on moving charges along wires.

“Instead of moving a charge, we either flip the orientation of the spin or cause the propagation of the spin along the wire,” Kaminski explained. “These spin changes technically should not dissipate energy, so it doesn’t cost a lot of energy to store information as spin or to move information as spin.”

Kaminski emphasized the importance of this finding to the field, but he said there is still a lot of work to be done before these findings can be used in new technology.

Crystal growth and characterization were supported by Center for the Advancement of Topological Semimetals (CATS), an Energy Frontier Research Center funded by the U.S. DOE, Office of Basic Energy Sciences.

The new discovery could accelerate the widespread use of hydrogen fuel cells, which hold great promise as efficient, clean energy sources for vehicles and other applications.

It’s one of a string of discoveries for the Héctor D. Abruña lab in their ongoing search for active, inexpensive, durable catalysts for use in alkaline fuel cells.

“This finding makes progress toward using efficient, clean hydrogen fuel cells in place of fossil fuels,” said Abruña, professor in the department of chemistry and chemical biology at Cornell University.

The results published March 21 in “A Completely Precious-Metal-Free Alkaline Fuel Cell With Enhanced Performance Using a Carbon-Coated Nickel Anode,” in the Proceedings of the National Academy of Sciences.

Expensive precious metals, such as platinum, are currently required in hydrogen fuel cells to efficiently catalyze the reactions they employ to produce electricity. Although alkaline polymer electrolyte membrane fuel cells (APEMFCs) enable the use nonprecious metal electrocatalysts, they lack the necessary performance and durability to replace precious metal-based systems.

A fuel cell produces electricity through the hydrogen oxidation reaction (HOR) and an oxygen reduction reaction (OOR). Platinum, in particular, is a model catalyst for both reactions because it catalyzes them efficiently, and is durable in the acidic environment of a PEM fuel cell, Abruña said.

But what about other materials?

Recent experiments with nonprecious-metal HOR electrocatalysts needed to overcome two major challenges, the researchers wrote: low intrinsic activity from too strong a hydrogen binding energy, and poor durability due to rapid passivation from metal oxide formation.

To overcome these challenges, the researchers designed a nickel-based electrocatalyst with a 2 nanometer shell made of nitrogen-doped carbon.

Their hydrogen fuel cell has an anode (where hydrogen is oxidized) catalyst consisting of a solid nickel core surrounded by the carbon shell. When paired with a cobalt-manganese cathode (where oxygen is reduced), the resulting completely precious-metal-free hydrogen fuel cell outputs more than 200 milliwatts per square centimeter.

The presence of nickel oxide species on the surface of the nickel electrode slows the hydrogen oxidation reaction dramatically, Abruña said. The nitrogen-doped carbon coating serves as a protection layer and enhances the HOR kinetics, making the reaction quicker and much more efficient.

In addition, the presence of the graphene coating on the nickel electrode prevents the formation of nickel oxides — resulting in electrodes with dramatically enhanced lifetimes. These electrodes are also much more tolerant to carbon monoxide, which rapidly poisons platinum.

“The use of this novel anode would dramatically lower prices enabling the application of alkaline fuel cells in a wide variety of areas,” Abruña said.

In February, Abruña and colleagues found that a cobalt nitride catalyst is nearly as efficient as platinum in catalyzing the oxygen reduction reaction.

Funding for this research was provided by the Center for Alkaline-Based Energy Solutions, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, and the Zhuang research group at Wuhan University, China, supported by the National Natural Science Foundation of China.

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Materials provided by Cornell University. Original written by Kate Blackwood, courtesy of the Cornell Chronicle. Note: Content may be edited for style and length.

In the new study, researchers combined diffusion MRI data from the Human Connectome Project with histological cross-sections of the corpus callosum, the major tract that connects the left and right sides of the brain. The Human Connectome maps the strength of all connections in the brain but does not provide the actual number of axons, while the histological cross-sections allow estimates for how many axons are packed into a given volume. Combining the connection strengths with the axon densities yielded estimates for the number of axons in the cerebral cortex.

The analysis indicated that there are almost 2.5 billion long-range axons traversing the cerebral cortex. However, despite this large number, they found that the numbers connecting different functional brain regions were quite low. For example, among the estimated 130 million axons in the arcuate fasciculus tract, only about 1 to 2 million (less than 2%) directly connected Broca’s and Wernicke’s areas, a connection which is necessary for normal language ability. The model predicts that other long connections, like those from the hippocampus to the frontal cortex that are needed for memory retrieval, are actually made in multiple steps. The findings will thus improve models of cognition, especially processes that rely on connections between distal regions of the brain.

“A major unsolved problem is how the human cortex integrates information processing by its 16 billion neurons across its surface to unify awareness,” Rosen adds. “Our finding that cortical areas are sparsely connected implies that this integration is accomplished either via linkage of the dense local connections or by rare, extraordinarily privileged long-range axons.”

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The research, published in the journal Environmental Science and Technology, suggests that wildfire emissions even 10 days old can affect the properties of aerosols — suspended liquid or particles that are key to cloud formation.

Research in aerosols and particulate matter pollution related to wildfires has most often focused on the early hours of smoke plumes, not several days later after smoke has traveled to other areas.

Enhancing modeling

This research helps fill in a knowledge gap and can inform future predictions about the climate and atmospheric effects of wildfire over the lifetime of aerosols, particularly in rural or pristine areas with relatively clean air, said Qi Zhang, an environmental toxicology professor and lead author of the study.

“These parameters are really useful for atmospheric and chemical models,” she said. “It’s a really important component to solving the effects on climate. To capture those characteristics is super critical.”

Zhang, Ph.D. student Ryan Farley and others spent time in 2019 at the Mount Bachelor Observatory atop a volcanic mountain in Oregon. That year was relatively calm in terms of wildfire, but smoke plumes and aerosols were still observed. Some were at least 10 days old and came from as close as Northern California and as far as Siberia, Russia.

The properties and chemical composition of aerosols can do a number of things: scatter or absorb solar radiation affecting temperature, seed clouds to produce rain or snow, or change the reflectivity of clouds — all of which affect climate.

Aerosol properties change with age

Scientists found that particulate matter concentrations were low, but oxidized organic aerosols from burning biomass — such as trees, grasses and shrubs — were detected throughout the samples.

The aerosols, which have a life cycle of about two weeks, were larger in aged samples compared to those found shortly after a fire starts.

“The properties of the smoke determine the effects on the climate,” Zhang said. “The really aged aerosols can behave very differently than the fresh ones. You want to capture these aerosols over the lifetime to properly account for the effects.”

Aerosols in the background

Older aerosols produced by wildfires can be present but not obvious and still affect climate.

“It’s not something you just notice but it’s in the background,” she said.

Knowing that information becomes ever more important as “biomass burning has become more and more frequent,” Zhang said.

Shan Zhou and Sonya Collier from UC Davis also participated in the research as did scientists from University of Montana and University of Washington.

The National Science Foundation, the U.S. Department of Energy’s Atmospheric System Research Program, the California Agricultural Experiment Station and UC Davis funded the research.

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Materials provided by University of California – Davis. Original written by Emily Dooley. Note: Content may be edited for style and length.

“Our results underscore the need to strengthen air pollutionregulations and prevention implementation in tribal territories and areas where AI populations live,” said Maggie Li, PhD candidate in the Department of Environmental Health Sciences at Columbia Mailman School, and first author.”Until now, insight into PM_2.5 exposure burdens in AI communities has been limited, in turn contributing to the lack of air pollution epidemiological studies which include this population in the contiguous U.S.”

The researchers compared ambient PM_2.5 average concentrations and trends in AI-populated vs. non-AI-populated counties in the contiguous U.S. from 2000 to 2018. They conducted their analysis at the county level, the most relevant unit to inform regulatory action. Population data and household income were obtained from the U.S. 2010 Census.

Average concentrations were 1.46 ?g/m³ lower for modeled PM_2.5 in AI-populated counties vs. non-AI-populated counties in 2000; however, towards the end of the study period, PM_2.5 concentrations in AI-populated counties became significantly higher compared to non-AI-populated counties. By 2018, concentrations were on average 0.66µg/m³higher in AI-populated counties than non-AI-populated counties. “Although we observed that average PM_2.5 concentrations were higher in non-AI-populated counties at baseline and across most of the study period, the gap between AI- and non-AI-populated counties decreased over time; after approximately 2015, average PM_2.5 concentrations were higher in AI-populated counties,” observed Li.

Short- and long-term exposure to particulate matter ? 2.5 µm (PM_2.5) increases the risk of cardiovascular and respiratory disease, among other health outcomes. In the U.S., socioeconomically disadvantaged communities are often exposed to higher PM_2.5 exposure levels. They bear a disproportionate burden of disease, even at levels well below air quality standards set by the U.S. Environmental Protection Agency.

“Our findings build upon existing studies showing that socioeconomically disadvantaged communities experience disproportionate burdens of environmental hazards, such as ambient air pollution. Native Americans may be particularly susceptible to the adverse health effects of PM_2.5 and air pollution, but research quantifying air pollution exposures and impacts on health among this population is lacking,” said Marianthi-Anna Kioumourtzoglou, ScD, assistant professor of environmental health sciences at Columbia Mailman School, and senior author. “These communities already face a large disease burden attributable to environmental pollution due, for instance, to extensive mining and water contamination on Tribal Lands.”

Cardiovascular disease, one of the outcomes associated with exposure to PM_2.5, and the leading cause of death in AI populations, occurs at significantly higher rates than in white populations. “There is a critical need for future investigations of health impacts associated with air pollution exposure in AI populations, as well as of interventions to ensure the observed inequalities can be eliminated,” noted Ana Navas-Acien, MD, PhD, professor of environmental health sciences at Columbia Mailman School, and a senior co-author.

Co-authors are Markus Hilpert, Jenni A. Shearston, and Jeff Goldsmith, Columbia University Mailman School of Public Health; Jada Brooks, the University of North Carolina School of Nursing at Chapel Hill; Steven Chillrud, Lamont-Doherty Earth Observatory, Columbia University; Tauqeer Ali, University of Oklahoma Health Sciences Center; Jason Umans, Georgetown-Howard Universities Center for Clinical and Translational Sciences; Lyle Best and Joseph Yracheta, Missouri Breaks Industries Research, Inc.; Aaron van Donkelaar and Randall Martin, Department of Energy, Environmental & Chemical Engineering, Washington University at St. Louis.

Support for the study was provided by the National Institute of Environmental Health Sciences, grants T32 ES007322, P30 ES09089, R01 ES030616, and P42ES010349.