Scientists at the US Department of Energy’s Brookhaven National Laboratory have identified a “antiferromagnetic excitonic insulator,” a long-foreseen magnetic state of matter.
“Broadly speaking, this is a unique form of magnet,” said Mark Dean, a physicist at Brookhaven Lab who is the senior author of an article detailing the findings that was just published in Nature Communications. “New forms of magnets are both fundamentally intriguing and exciting for future uses, since magnetic materials sit at the center of much of the technology around us.”
Strong magnetic attraction between electrons in a layered material causes the electrons to try to organize their magnetic moments, or “spins,” in a regular up-and-down “antiferromagnetic” pattern. In the 1960s, when physicists investigated the characteristics of metals, semiconductors, and insulators, the concept that antiferromagnetism may be caused by unusual electron coupling in an insulating material was initially proposed.
“Physicists were only beginning to investigate how the principles of quantum mechanics relate to the electrical characteristics of materials sixty years ago,” said Daniel Mazzone, a former Brookhaven Lab physicist who is now at the Paul Scherrer Institut in Switzerland, who led the work. “They were attempting to determine what occurs when the electronic ‘energy gap’ between an insulator and a conductor shrinks. Do you just convert a basic insulator to a simple metal, allowing electrons to freely migrate, or do you get anything more interesting?”
The idea was that under certain circumstances, you’d get something more fascinating, such as the Brookhaven team’s recently found “antiferromagnetic excitonic insulator.”
What is it about this substance that makes it so strange and intriguing? Let’s take a closer look at those concepts and see how this new condition of matter emerges.
In an antiferromagnet, the magnetic polarization axes (spins) of neighbouring atoms are aligned in alternating directions: up, down, up, down, and so on. On a larger scale, such alternating internal magnetic orientations cancel each other out, leaving the overall material with no net magnetism. These materials can be swiftly altered between states. They’re also immune to information loss owing to external magnetic field interference. Antiferromagnetic materials are appealing for current communication systems because of these features.
Then there’s excitonic. Excitons emerge when electrons are allowed to move about and interact intensely with one another to produce bound states under particular circumstances. With “holes,” the vacancies left behind when electrons hop to a new place or energy level in a material, electrons may also create bound states. When it comes to electron-electron interactions, magnetic attractions are strong enough to overcome the repulsive force between the two like-charged particles. The attraction between electrons and holes must be strong enough to overcome the material’s “energy gap,” which is a property of insulators.
“An insulator is the polar opposite of a metal; it is a substance that does not carry electricity,” Dean said. Electrons in the material tend to remain in a low-energy state, or “ground,” most of the time. “The electrons are all squished together, like spectators in a crowded amphitheater; they can’t move,” he said. You must give the electrons enough energy to overcome a distinctive gap between the ground state and a higher energy level in order for them to travel.
The energy gain from magnetic electron-hole interactions may sometimes surpass the energy cost of electrons leaping over the energy gap under unusual conditions.
Physicists may now investigate such unusual settings using modern tools to discover how the antiferromagnetic excitonic insulator state originates.
A team of researchers used a substance called strontium iridium oxide (Sr3Ir2O7), which is only marginally insulating at high temperatures. At the Advanced Photon Source, a DOE Office of Science user facility at Argonne National Laboratory, Daniel Mazzone, Yao Shen (Brookhaven Lab), Gilberto Fabbris (Argonne National Laboratory), and Jennifer Sears (Brookhaven Lab) used x-rays to measure the magnetic interactions and associated energy cost of moving electrons. The University of Tennessee’s Jian Liu and Junyi Yang, as well as Argonne experts Mary Upton and Diego Casa, all made significant contributions.
The scientists began their experiment by heating the material to a high degree and then progressively lowering it. The energy gap narrowed as the temperature dropped. At 285 Kelvin (approximately 53 degrees Fahrenheit), electrons began leaping across the material’s magnetic layers, but formed bonded pairs with the holes they left behind, inducing antiferromagnetic alignment of nearby electron spins. Hidemaro Suwa and Christian Batista of the University of Tennessee used calculations to build a model based on the expected antiferromagnetic excitonic insulator, and they demonstrated that this model fully explains the experimental findings.
“We showed using x-rays that the bond produced by electron-hole attraction actually sends back more energy than when the electron jumps across the band gap,” Yao Shen revealed. “Because this process saves energy, all electrons desire to participate. The material then appears different from the high-temperature state in terms of the overall arrangement of electrons and spins once all electrons have completed the transition. The electron spins are organized in an antiferromagnetic pattern in the new configuration, while the bonded pairs generate a ‘locked-in’ insulating state.”
The discovery of the antiferromagnetic excitonic insulator brings to a close a protracted investigation into the interesting ways electrons organize themselves in materials. Understanding the links between spin and charge in such materials might lead to the development of new technologies in the future.