Magnetism drives metals to insulators in new experiment


An illustration of two domains (blue and orange) divided by a website wall (white space) in a fabric. The magnetic order is designated with organized arrows (electron spins) whereas the colours symbolize two completely different domains (however the identical magnetic order). In the fabric pictured right here, the area partitions are conductive and the domains are insulating. Credit: Yejun Fang

Like all metals, silver, copper, and gold are conductors. Electrons circulate throughout them, carrying warmth and electrical energy. While gold is an efficient conductor beneath any circumstances, some supplies have the property of behaving like metallic conductors provided that temperatures are excessive sufficient; at low temperatures, they act like insulators and don’t do a very good job of carrying electrical energy. In different phrases, these uncommon supplies go from appearing like a bit of gold to appearing like a chunk of wooden as temperatures are lowered. Physicists have developed theories to clarify this so-called metal-insulator transition, however the mechanisms behind the transitions should not at all times clear.


“In some cases, it is not easy to predict whether a material is a metal or an insulator,” explains Caltech visiting affiliate 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 query for at the least 5 years; others on his workforce, reminiscent of collaborator David Mandrus on the University of Tennessee, have considered the issue for greater than twenty years.

Now, a new research from Feng and colleagues, revealed in Nature Communications, affords the cleanest experimental proof but of a metal-insulator transition principle proposed 70 years in the past by physicist John Slater. According to that principle, magnetism, which ends up when the so-called “spins” of electrons in a fabric are organized in an orderly vogue, can solely drive the metal-insulator transition; in different earlier experiments, adjustments in the lattice construction of a fabric or electron interactions based mostly on their expenses have been deemed accountable.

“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 can be 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 analysis will assist reply basic questions on how completely different supplies behave, and can also have purposes in technology, for instance in the sphere of spintronics, in which the spins of electrons would kind the premise {of electrical} gadgets as a substitute of the electron expenses as is routine now. “Fundamental questions about metal and insulators will be relevant in the upcoming technological revolution,” says Feng.

Interacting Neighbors

Typically, when one thing is an efficient conductor, reminiscent of a metallic, the electrons can zip round largely unimpeded. Conversely, with insulators, the electrons get caught and can’t journey freely. The state of affairs is comparable to communities of individuals, explains Feng. If you consider supplies 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, nevertheless, symbolize “close-knit communities, like in a college dorm, where neighbors visit each other freely and frequently,” he says.

Magnetism drives metals to insulators in new experiment
Yejun Feng (left), Yishu Wang (proper), and Daniel Silevitch (backside), are pictured right here organising an experiment in the Rosenbaum lab at Caltech. Credit: California Institute of Technology

Likewise, Feng makes use of this metaphor to clarify what occurs when some metals turn into 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 Nineteen Forties, physicist Sir Nevill Francis Mott discovered how some metals can turn into insulators. His principle, 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,” in accordance to the Nobel Prize press launch. In this case, the repulsion between the electrons is behind the transition.

In 1951, Slater proposed an alternate mechanism based mostly on spin-spin interactions, however this concept has been laborious to show experimentally as a result of the opposite processes of the metal-insulator transition, together with these proposed by Mott, can swamp the Slater mechanism, making it laborious to isolate.

Challenges of Real Materials

In the new research, the researchers had been ready finally to experimentally show the Slater mechanism utilizing a compound that has been studied since 1974, referred to as pyrochlore oxide or Cd2Os2O7. This compound will not be affected by different metal-insulator transition mechanisms. However, inside this materials, the Slater mechanism is overshadowed by an unexpected experimental problem, specifically the presence of “domain walls” that divide the fabric into sections.

“The domain walls are like the highways or bigger roads between communities,” says Feng. In pyrochlore oxide, the area partitions are conductive, despite the fact that the majority of the fabric is insulating. Although the area partitions began out as an experimental problem, they turned out to be important to the workforce’s growth of a new measurement process and approach to show 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 on the Johns Hopkins University who has constantly labored on this research 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 beforehand labored with Patrick Lee, a visiting professor at Caltech from MIT, to lay the fundamental understanding of conductive area partitions utilizing symmetry arguments, which describe how and if electrons in supplies reply to adjustments in the path of a magnetic discipline.

“By challenging the conventional assumptions about how electrical conductivity measurements are made in magnetic materials 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.”


New quantum change turns metals into insulators


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

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Magnetism drives metals to insulators in new experiment (2021, June 4)
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