Hexbyte Glen Cove Magnetism drives metals to insulators in new experiment thumbnail

Hexbyte Glen Cove Magnetism drives metals to insulators in new experiment

Hexbyte Glen Cove

An illustration of two domains (blue and orange) divided by a domain wall (white area) in a material. The magnetic order is designated with organized arrows (electron spins) while the colors represent two different domains (but the same magnetic order). In the material pictured here, the domain walls are conductive and the domains are insulating. Credit: Yejun Fang

Like all metals, silver, copper, and gold are conductors. Electrons flow across them, carrying heat and electricity. While gold is a good conductor under any conditions, some materials have the property of behaving like metal conductors only if temperatures are high enough; at low temperatures, they act like insulators and do not do a good job of carrying electricity. In other words, these unusual materials go from acting like a chunk of gold to acting like a piece of wood as temperatures are lowered. Physicists have developed theories to explain this so-called metal-insulator transition, but the mechanisms behind the transitions are not always clear.

“In some cases, it is not easy to predict whether a material is a or an insulator,” explains Caltech visiting associate Yejun Feng of the Okinawa Institute for Science and Technology Graduate University. “Metals are always good conductors no matter what, but some other so-called apparent metals are insulators for reasons that are not well understood.” Feng has puzzled over this question for at least five years; others on his team, such as collaborator David Mandrus at the University of Tennessee, have thought about the problem for more than two decades.

Now, a new study from Feng and colleagues, published in Nature Communications, offers the cleanest experimental proof yet of a theory proposed 70 years ago by physicist John Slater. According to that theory, magnetism, which results when the so-called “spins” of electrons in a material are organized in an orderly fashion, can solely drive the metal-insulator transition; in other previous experiments, changes in the lattice structure of a material or based on their charges have been deemed responsible.

“This is a problem that goes back to a theory introduced in 1951, but until now it has been very hard to find an experimental system that actually demonstrates the spin-spin interactions as the driving force because of confounding factors,” explains co-author Thomas Rosenbaum, a professor of physics at Caltech who is also the Institute’s president and the Sonja and William Davidow Presidential Chair.

“Slater proposed that, as the temperature is lowered, an ordered magnetic state would prevent electrons from flowing through the material,” Rosenbaum explains. “Although his idea is theoretically sound, it turns out that for the vast majority of materials, the way that electrons interact with each other electronically has a much stronger effect than the magnetic interactions, which made the task of proving the Slater mechanism challenging.”

The research will help answer fundamental questions about how different materials behave, and may also have applications in technology, for example in the field of spintronics, in which the spins of electrons would form the basis of electrical devices instead of the electron charges as is routine now. “Fundamental questions about metal and insulators will be relevant in the upcoming technological revolution,” says Feng.

Interacting Neighbors

Typically, when something is a good conductor, such as a metal, the electrons can zip around largely unimpeded. Conversely, with insulators, the electrons get stuck and cannot travel freely. The situation is comparable to communities of people, explains Feng. If you think of materials as communities and electrons as members of the households, then “insulators are communities with people who don’t want their neighbors to visit because it makes them feel uncomfortable.” Conductive metals, however, represent “close-knit communities, like in a college dorm, where neighbors visit each other freely and frequently,” he says.

Yejun Feng (left), Yishu Wang (right), and Daniel Silevitch (bottom), are pictured here setting up an experiment in the Rosenbaum lab at Caltech. Credit: California Institute of Technology

Likewise, Feng uses this metaphor to explain what happens when some metals become insulators as temperatures drop. “It’s like winter time, in that people—or the electrons—stay home and don’t go out and interact.”

In the 1940s, physicist Sir Nevill Francis Mott figured out how some metals can become insulators. His theory, which garnered the 1977 Nobel Prize in Physics, described how “certain metals can become insulators when the electronic density decreases by separating the atoms from each other in some convenient way,” according to the Nobel Prize press release. In this case, the repulsion between the electrons is behind the transition.

In 1951, Slater proposed an alternate mechanism based on spin-spin interactions, but this idea has been hard to prove experimentally because the other processes of the metal-insulator transition, including those proposed by Mott, can swamp the Slater mechanism, making it hard to isolate.

Challenges of Real Materials

In the new study, the researchers were able at last to experimentally demonstrate the Slater mechanism using a compound that has been studied since 1974, called pyrochlore oxide or Cd2Os2O7. This compound is not affected by other metal-insulator transition mechanisms. However, within this material, the Slater mechanism is overshadowed by an unforeseen experimental challenge, namely the presence of “” that divide the material into sections.

“The domain walls are like the highways or bigger roads between communities,” says Feng. In pyrochlore oxide, the domain walls are conductive, even though the bulk of the material is insulating. Although the domain walls started out as an experimental challenge, they turned out to be essential to the team’s development of a new measurement procedure and technique to prove the Slater mechanism.

“Previous efforts to prove the Slater metal-insulator transition theory did not account for the fact that the domain walls were masking the magnetism-driven effects,” says Yishu Wang (Ph.D. ’18), a co-author at the Johns Hopkins University who has continuously worked on this study since her graduate work at Caltech. “By isolating the domain walls from the bulk of the insulating materials, we were able to develop a more complete understanding of the Slater mechanism.” Wang had previously worked with Patrick Lee, a visiting professor at Caltech from MIT, to lay the basic understanding of conductive domain walls using symmetry arguments, which describe how and if electrons in materials respond to changes in the direction of a magnetic field.

“By challenging the conventional assumptions about how electrical conductivity measurements are made in magnetic through fundamental symmetry arguments, we have developed new tools to probe spintronic devices, many of which depend on transport across domain walls,” says Rosenbaum.

“We developed a methodology to set apart the domain-wall influence, and only then could the Slater mechanism be revealed,” says Feng. “It’s a bit like discovering a diamond in the rough.”

More information:
Yejun Feng et al, A continuous metal-insulator transition driven by spin correlations, Nature Communications (2021). DOI: 10.1038/s41467-021-23039-6

Magnetism drives metals to insulators in new experiment (2021, June 4)
retrieved 5 June 2021
from https://phys.org/news/2021-06-magnetism-metals-insulators.html

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Hexbyte Glen Cove Magnetism meets topology on a superconductor's surface thumbnail

Hexbyte Glen Cove Magnetism meets topology on a superconductor’s surface

Hexbyte Glen Cove

An illustration depicting a topological surface state with an energy band gap (an energy range where electrons are forbidden) between the apices of the top and corresponding bottom cones (allowed energy bands, or the range of energies electrons are allowed to have). A topological surface state is a unique electronic state, only existing at the surface of a material, that reflects strong interactions between an electron’s spin (red arrow) and its orbital motion around an atom’s nucleus. When the electron spins align parallel to each another, as they do here, the material has a type of magnetism called ferromagnetism. Credit: Dan Nevola, Brookhaven National Laboratory

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 , 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 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 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 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.”

More information:
Nader Zaki et al. Time-reversal symmetry breaking in the Fe-chalcogenide superconductors, Proceedings of the National Academy of Sciences (2021). DOI: 10.1073/pnas.2007241118

Magnetism meets topology on a superconductor’s surface (2021, March 17)
retrieved 17 March 2021
from https://phys.org/news/2021-03-magnetism-topology-superconductor-surface.html

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