(Press-News.org) UPTON, NY--Electrons in a solid occupy distinct energy bands separated by gaps. Energy band gaps are an electronic "no man's land," an energy range where no electrons are allowed. Now, scientists studying a compound containing iron, tellurium, and selenium have found that an energy band gap opens at a point where two allowed energy bands intersect on the material's surface. They observed this unexpected electronic behavior when they cooled the material and probed its electronic structure with laser light. Their findings, reported in the Proceedings of the National Academy of Sciences, could have implications for future quantum information science and electronics.
The particular compound belongs to the family of iron-based high-temperature superconductors, which were initially discovered in 2008. These materials not only conduct electricity without resistance at relatively higher temperatures (but still very cold ones) than other classes of superconductors but also show magnetic properties.
"For a while, people thought that superconductivity and magnetism would work against each other," said first author Nader Zaki, a scientific associate in the Electron Spectroscopy Group of the Condensed Matter Physics and Materials Science (CMPMS) Division at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory. "We have explored a material where both develop at the same time."
Aside from superconductivity and magnetism, some iron-based superconductors have the right conditions to host "topological" surface states. The existence of these unique electronic states, localized at the surface (they do not exist in the bulk of the material), reflects strong interactions between an electron's spin and its orbital motion around the nucleus of an atom.
"When you have a superconductor with topological surface properties, you're excited by the possibility of topological superconductivity," said corresponding author Peter Johnson, leader of the Electron Spectroscopy Group. "Topological superconductivity is potentially capable of supporting Majorana fermions, which could serve as qubits, the information-storing building blocks of quantum computers."
Quantum computers promise tremendous speedups for calculations that would take an impractical amount of time or be impossible on traditional computers. One of the challenges to realizing practical quantum computing is that qubits are highly sensitive to their environment. Small interactions cause them to lose their quantum state and thus stored information becomes lost. Theory predicts that Majorana fermions (sought-after quasiparticles) existing in superconducting topological surface states are immune to environmental disturbances, making them an ideal platform for robust qubits.
Seeing the iron-based superconductors as a platform for a range of exotic and potentially important phenomena, Zaki, Johnson, and their colleagues set out to understand the roles of topology, superconductivity and magnetism.
CMPMS Division senior physicist Genda Gu first grew high-quality single crystals of the iron-based compound. Then, Zaki mapped the electronic band structure of the material via laser-based photoemission spectroscopy. When light from a laser is focused onto a small spot on the material, electrons from the surface are "kicked out" (i.e., photoemitted). The energy and momentum of these electrons can then be measured.
When they lowered the temperature, something surprising happened.
"The material went superconducting, as we expected, and we saw a superconducting gap associated with that," said Zaki. "But what we didn't expect was the topological surface state opening up a second gap at the Dirac point. You can picture the energy band structure of this surface state as an hourglass or two cones attached at their apex. Where these cones intersect is called the Dirac point."
As Johnson and Zaki explained, when a gap opens up at the Dirac point, it's evidence that time-reversal symmetry has been broken. Time-reversal symmetry means that the laws of physics are the same whether you look at a system going forward or backward in time--akin to rewinding a video and seeing the same sequence of events playing in reverse. But under time reversal, electron spins change their direction and break this symmetry. Thus, one of the ways to break time-reversal symmetry is by developing magnetism--specifically, ferromagnetism, a type of magnetism where all electron spins align in a parallel fashion.
"The system is going into the superconducting state and seemingly magnetism is developing," said Johnson. "We have to assume the magnetism is in the surface region because in this form it cannot coexist in the bulk. This discovery is exciting because the material has a lot of different physics in it: superconductivity, topology, and now magnetism. I like to say it's one-stop shopping. Understanding how these phenomena arise in the material could provide a basis for many new and exciting technological directions."
As previously noted, the material's superconductivity and strong spin-orbit effects could be harnessed for quantum information technologies. Alternatively, the material's magnetism and strong spin-orbit interactions could enable dissipationless (no energy loss) transport of electrical current in electronics. This capability could be leveraged to develop electronic devices that consume low amounts of power.
Coauthors Alexei Tsvelik, senior scientist and group leader of the CMPMS Division Condensed Matter Theory Group, and Congjun Wu, a professor of physics at the University of California, San Diego, provided theoretical insights on how time reversal symmetry is broken and magnetism originates in the surface region.
"This discovery not only reveals deep connections between topological superconducting states and spontaneous magnetization but also provides important insights into the nature of superconducting gap functions in iron-based superconductors--an outstanding problem in the investigation of strongly correlated unconventional superconductors," said Wu.
In a separate study with other collaborators in the CMPMS Division, the experimental team is examining how different concentrations of the three elements in the sample contribute to the observed phenomena. Seemingly, tellurium is needed for the topological effects, too much iron kills superconductivity, and selenium enhances superconductivity.
In follow-on experiments, the team hopes to verify the time-reversal symmetry breaking with other methods and explore how substituting elements in the compound modifies its electronic behavior.
"As materials scientists, we like to alter the ingredients in the mixture to see what happens," said Johnson. "The goal is to figure out how superconductivity, topology, and magnetism interact in these complex materials."
INFORMATION:
This research was supported by the DOE Office of Science and the Air Force Office of Scientific Research.
Brookhaven National Laboratory is supported by the U.S. Department of Energy's Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science.
Follow @BrookhavenLab on Twitter or find us on Facebook.
March 17, 2021 - In recent years, hospital charges and Medicare payments for patients with hip fractures have increased much more rapidly than charges and payments for orthopaedic surgeons, reports a study in the Journal of Orthopaedic Trauma. The journal is published in the Lippincott portfolio by Wolters Kluwer.
The gap in Medicare reimbursements to hospitals compared to surgeons widened substantially in the last decade - even as patient outcomes improved and healthcare resource use decreased, according to a new analysis by Brian Werner, MD, and colleagues of UVA Health, Charlottesville, Va. "The results confirm our hypothesis that hospital charges and payments contribute significantly more to the increasing cost of treating a hip fracture patient than surgeon ...
He and his research group have found a way to more precisely determine the properties of these materials, because they can better account for the underlying disorder. Their article has been designated "ACS Editors' Choice" by the editors of the American Chemical Society journals, who recognise the "importance to the global scientific community" of the Leipzig researchers' work and see it as a breakthrough in the accurate description of phase transition phenomena in disordered porous materials.
In mesoporous materials, the pore openings are far smaller than in a normal sponge: their diameters range from 2 to 50 nanometres and are invisible to the naked eye. Nevertheless, they have a number of interesting properties, including with ...
The abundance and other characteristics of rocks partially extending above the water surface could be important for improving the recovery of aquatic insect populations in restored streams.
Nearly three quarters of stream insects reproduce on large rocks that sit above the water surface by crawling underneath to attach their eggs. Increasing the number of large and stable emergent rocks in streams could provide more egg-laying habitat and allow insects to quickly repopulate restored streams.
"We found that restored streams had fewer emergent rocks for egg-laying and fewer total eggs than naturally intact streams," says Samantha Jordt, first author of the paper and an M.Sc. student at NC State's Department of Applied Ecology.
The study also ...
Simple systems can reproduce faster than complex ones. So, how can the complexity of life have arisen from simple chemical beginnings? Starting with a simple system of self-replicating fibres, chemists at the University of Groningen have discovered that upon introducing a molecule that attacks the replicators, the more complex structures have an advantage. This system shows the way forward in elucidating how life can originate from lifeless matter. The results were published on 10 March in the journal Angewandte Chemie.
The road to answering the question of how life originated is guarded by Spiegelman's monster, named after the American molecular biologist Sol Spiegelman, who some 55 years ago ...
Many people who embrace social welfare programs vote against their own interests, according to new UC Riverside research.
The mitigating factor is education: The more education one has, the more likely one is to stick to one's policy preferences.
"It means candidates who employ tactics such as fear and attaching patriotism to certain concepts can persuade people to vote for candidates who are in opposition to their social beliefs," Diogo Ferrari, a professor of political science at UC Riverside, wrote in his recently published paper, "Education, Belief Structures, Support for Welfare Polices, and Vote," published in the journal Education & Society.
For the study, Ferrari looked at public opinion ...
Brazilian researchers have published a systematic review of the scientific literature showing that some warm-up strategies such as dynamic stretching can effectively prepare soccer players to maintain kicking accuracy, whereas intense physical exercises have a negative effect on the velocity of the ball when kicked, and consumption of carbohydrate beverages during a match can enable players to maintain adequate kicking performance in the concluding moments of prolonged physical exercise such as a sudden-death playoff.
The review, published in the journal Sports Medicine in December 2020, ...
COLUMBUS, Ohio - Electricity may slow - and in some cases, stop - the speed at which breast cancer cells spread through the body, a new study indicates.
The research also found that electromagnetic fields might hinder the amount of breast cancer cells that spread. The findings, published recently in the journal Bioelectricity, suggest that electromagnetic fields might be a useful tool in fighting cancers that are highly metastatic, which means they are likely to spread to other parts of the body, the authors said.
"We think we can hinder metastasis by applying these fields, but we also think it may be possible to even destroy tumors using this approach," said Vish Subramaniam, senior author of the paper and former professor of mechanical ...
PHILADELPHIA (March 17, 2021) - Stroke remains a leading cause of death worldwide and one of the most common reasons for disability. While a wide variety of factors influence stroke outcomes, data show that avoiding readmissions and long lengths of stay among ischemic stroke patients has benefits for patients and health care systems alike. Although reduced readmission rates among various medical patients have been associated with better nurse work environments, it is unknown how the work environment might influence readmissions and length of stay for ischemic stroke patients.
In a new study from the University of Pennsylvania School of Nursing's (Penn Nursing) Center for Health Outcomes ...
The NASA-funded Seismometer to Investigate Ice and Ocean Structure (SIIOS) performed well in seismic experiments conducted in snowy summer Greenland, according to a new study by the SIIOS team led by the University of Arizona published this week in Seismological Research Letters.
SIIOS could be a part of proposed NASA spacecraft missions to the surface of Europa or Enceladus. These moons of Jupiter and Saturn are encrusted by an icy shell over subsurface liquid oceans, and seismic data could be used to better define the thickness and depth of these layers. Other seismic points of interest on these worlds could include ice volcanoes, drainage events below the ice shell and possibly even ...
East Hanover, NJ. March 17, 2021. Kessler Foundation researchers have identified several practical and technical barriers to the widespread use of surface electromyography (sEMG) in clinical neurorehabilitation. Based on their holistic analysis of these factors, the researchers suggest a collaborative, interdisciplinary, and unified approach to enable rehabilitation professionals to routinely use sEMG. The article, "Use of Surface EMG in Clinical Rehabilitation of Individuals With SCI: Barriers and Future Considerations" (doi: 10.3389/fneur.2020.578559), was published December 18, 2020, in Frontiers in Neurology. It is available open access at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7780850/
The authors are Rakesh ...
