Discovered a long-predicted magnetic state of matter called an “antiferromagnetic excitonic insulator”. “In general, this is a new type of magnet,” said Brookhaven Lab physicist Mark Dean, senior author of a paper describing the research just published in Nature Communications. “Because magnetic materials are at the heart of much of the technology around us, the new types of magnets are fundamentally fascinating and promising for future applications.”
The new state involves a strong magnetic attraction between electrons in a layered material that causes the electrons to want to organize their magnetic moments, or “spins,” in a normal up-down “antiferromagnetic” pattern. The idea that such antiferromagnetism could be driven by bizarre electron coupling in an insulating material was first predicted in 1960 when physicists explored the different properties of metals, semiconductors, and insulators.
“Sixty years ago, physicists were just starting to consider how the rules of quantum mechanics apply to the electronic properties of materials,” explained Daniel Mazzone, a former Brookhaven Lab physicist who led the study and is now at Paul Scherrer. Institut in Switzerland.
The prediction was that, under certain conditions, something more interesting could be achieved: namely, the “antiferromagnetic excitonic insulator” just discovered by the Brookhaven team. Why is this material so exotic and interesting? To understand, let’s dive into these terms and explore how this new state of matter is formed.
In an antiferromagnet, the electrons on adjacent atoms have their axes of magnetic polarization (spin) aligned in alternating directions: up, down, up, down, and so on. On the scale of the entire material these alternating internal magnetic orientations cancel each other out, resulting in the absence of net magnetism of the overall material. Such materials can be quickly switched between different states. They are also resistant to information loss due to interference from external magnetic fields. These properties make antiferromagnetic materials attractive for modern communication technologies.
Then we have the excitonic. Excitons arise when certain conditions allow electrons to move and interact strongly with each other to form bonded states. Electrons can also form bonded states with “holes,” the vacancies left behind when electrons jump to a different position or energy level in a material. In the case of electron-electron interactions, the bond is driven by magnetic attractions that are strong enough to overcome the repulsive force between the two similarly charged particles. In the case of electron-hole interactions, the attraction must be strong enough to overcome the “energy gap” of the material, a characteristic of an insulator.
“An insulator is the opposite of a metal; it is a material that does not conduct electricity, ”explained Dean. The electrons in the material generally remain in a low energy state, or “earth”. “The electrons are all stuck in place, like people in a full amphitheater; they cannot move “. To make electrons move, you need to give them an energy boost large enough to bridge a characteristic gap between the ground state and a higher energy level.
Under very special circumstances, the energy gain from electron-hole magnetic interactions can outweigh the energy cost of electrons jumping across the energy gap. Now, thanks to advanced techniques, physicists can explore those special circumstances to learn how the antiferromagnetic excitonic isolating state emerges.
One team worked with a material called strontium and iridium oxide (Sr3Ir2O7), which is barely insulating at high temperatures. Daniel Mazzone, Yao Shen (Brookhaven Lab), Gilberto Fabbris (Argonne National Laboratory), and Jennifer Sears (Brookhaven Lab) used X-rays at the Advanced Photon Source, a user facility of the DOE Office of Science at Argonne National Laboratory , to measure magnetic interactions and the associated energy cost of moving electrons. Jian Liu and Junyi Yang of the University of Tennessee and Argonne scientists Mary Upton and Diego Casa also made important contributions.
The team began the high-temperature investigation and gradually cooled the material. With cooling, the energy gap gradually narrowed. At 285 Kelvin, the electrons started jumping between the magnetic layers of the material, but they immediately formed bonded pairs with the holes they had left, simultaneously triggering the antiferromagnetic alignment of the spins of the adjacent electrons. Hidemaro Suwa and Christian Batista of the University of Tennessee performed calculations to develop a model using the predicted antiferromagnetic excitonic insulator concept and demonstrated that this model fully explains the experimental results.
“Using X-rays, we observed that the bond triggered by the attraction between electrons and holes actually returns more energy than when the electron jumped over the band gap,” explained Yao Shen. “Since energy is saved from this process, all electrons tend to do it. Hence, after all electrons have made the transition, the material appears to be different from the high-temperature state in terms of the overall arrangement of electrons and spin. The new configuration provides that the spins of the electrons are ordered in an antiferromagnetic pattern while the bonded pairs create a “locked” insulating state.
The identification of the antiferromagnetic excitonic insulator completes a long journey to discover the fascinating ways in which electrons choose to arrange themselves in materials. In the future, understanding the connections between spin and charge in such materials could have the potential to realize new technologies.
