Like all metals, silver, copper and gold are conductors. Electrons pass through them, carrying heat and electricity. While gold is a good conductor under all conditions, some materials have the property of behaving like metallic conductors only if the temperatures are high enough; at low temperatures, they act as insulators and do not carry electricity well. In other words, these unusual materials range from acting like a piece of gold to acting like a piece of wood when 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’s not easy to predict whether a material is a metal or an insulator,” says Yejun Feng, visiting associate of Caltech, Okinawa Institute for Science and Technology Graduate University. “Metals are still good conductors no matter what, but some other so-called apparent metals are insulators for reasons that are not well understood.” Feng has been puzzled over this issue for at least five years; other members of his team, such as collaborator David Mandrus of the University of Tennessee, have been pondering the problem for more than two decades.
Now, a new study by Feng and colleagues, published in Nature Communication, offers the clearest experimental proof to date of a theory of the metal-insulator transition proposed 70 years ago by physicist John Slater. According to this theory, magnetism, which results from the orderly organization of the so-called “spins” of electrons in a material, can only cause the metal-insulator transition; in other previous experiments, changes in the lattice structure of a material or interactions of electrons depending on their charges have been held responsible.
“This is a problem that dates back to a theory introduced in 1951, but until now it has been very difficult to find an experimental system that actually demonstrates spin-spin interactions as the driving force due to confounding factors.” says co-author Thomas Rosenbaum, a physics professor at Caltech who is also the Institute’s president and the Sonja and William Davidow presidential chair.
“Slater proposed that when the temperature is lowered, an ordered magnetic state would prevent electrons from passing through the material,” says Rosenbaum. “While his idea is theoretically valid, it turns out that for the vast majority of materials, the way electrons electronically interact with each other has a much stronger effect than magnetic interactions, which made it difficult to determine. try to prove Slater’s mechanism. “
The research will help answer fundamental questions about the behavior of different materials and could also have technological applications, for example in the field of spintronics, in which the spins of electrons would form the basis of electrical devices instead of electronic charges like this is the case for the routine. now. “The fundamental questions about metal and insulators will be relevant in the next technological revolution,” Feng said.
Typically, when something is a good conductor, like a metal, electrons can flow largely unimpeded. Conversely, with insulators, electrons get stuck and cannot travel freely. The situation is comparable to that of communities of people, explains Feng. If you think of materials as communities and electrons as members of households, then “insulators are communities with people who don’t want their neighbors to come because it makes them uncomfortable.” Conductive metals, however, represent “tight-knit communities, like in a college dormitory, where neighbors visit each other freely and frequently,” he says.
Likewise, Feng uses this metaphor to explain what happens when certain metals become insulators when temperatures drop. “It’s like winter, in that people – or electrons – stay home and don’t go out and interact.”
In the 1940s, physicist Sir Nevill Francis Mott discovered how certain metals can become insulators. His theory, which won him the Nobel Prize for Physics in 1977, described how “certain metals can become insulators when the electron density decreases by separating atoms from each other in a practical way,” according to the Nobel Prize’s press release. . In this case, the repulsion between the electrons is at the origin of the transition.
In 1951, Slater proposed an alternative mechanism based on spin-spin interactions, but this idea has been difficult to prove experimentally because other metal-insulator transition processes, including those proposed by Mott, can overwhelm Slater’s mechanism. , making it difficult to isolate.
The challenges of real materials
In the new study, the researchers were finally able to demonstrate the Slater mechanism experimentally using a compound studied since 1974, called pyrochlore oxide or Cd2Os2O7. This compound is not affected by other metal-insulator transition mechanisms. However, within this material, Slater’s mechanism is eclipsed by an unforeseen experimental challenge, namely the presence of “domain walls” which divide the material into sections.
“The walls of the estate are like highways or major roads between communities,” explains Feng. In pyrochlore oxide, the domain walls are conductive, even though most of the material is insulating. Although the domain walls started out as an experimental challenge, they were found to be essential in the team’s development of a new measurement procedure and technique to prove Slater’s mechanism.
“Previous efforts to prove Slater’s metal-insulator transition theory failed to account for the fact that domain walls masked magnetism-induced effects,” says Yishu Wang (PhD ’18), co-author at the Johns Hopkins University who has continuously worked on this study since graduating from Caltech. “By isolating the walls of the mass domain of insulating materials, we were able to develop a more complete understanding of the Slater mechanism.” Wang had previously worked with Patrick Lee, visiting professor at Caltech at MIT, to establish the basic understanding of conductive domain walls using symmetry arguments, which describe how and whether electrons in materials respond to changes in direction of a magnetic field.
“By challenging 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 through the walls of the estate, ”explains Rosenbaum.
“We developed a methodology to set aside the influence of the domain wall, and only then could Slater’s mechanism be revealed,” says Feng. “It’s a bit like finding a diamond in the rough.”