Scientists discover a new magnetic state of matter

Scientists at the Brookhaven National Laboratory in the US have discovered a long-predicted magnetic state of matter called an “antiferromagnetic excitonic insulator.”

“Generally speaking, this is a novel type of magnet,” Mark Dean, a physicist at Brookhaven Lab, lead author of a paper describing the research that was just published in Nature Communications, said in a statement. “Since magnetic materials are at the heart of much of the technology around us, new types of magnets are fundamentally exciting and show promise for future applications.”

The new magnetic state means that it makes the electrons want to arrange their magnetic moments, or “spins,” in a regular top-down “antiferromagnetic” pattern. The idea that such antiferromagnetism could be driven by peculiar electron coupling in an insulating material was first predicted in the 1960s when physicists explored the different properties of metals, semiconductors, and insulators.

“Sixty years ago, physicists were just beginning to consider how the rules of quantum mechanics apply to the electronic properties of materials,” Daniel Mazzone, a former Brookhaven Laboratory physicist who led the study and now works at the Paul Scherrer Institute in Switzerland, said in a statement. “They were trying to figure out what happens when you make the electronic ‘energy gap’ between an insulator and a conductor smaller and smaller. Do you just change a simple insulator into a simple metal where electrons can move freely, or do you do something more interesting?”

The prediction was that, under certain conditions, you might get something more interesting: namely, the newly discovered by the Brookhaven team.

In an antiferromagnetic, electrons from adjacent atoms have their magnetic polarization axes (spins) aligned in alternating directions: up, down, up, down, etc. On the scale of the entire material, those alternating internal magnetic orientations cancel each other out, resulting in no net magnetism in the material as a whole. Such materials can be quickly changed between different states. They are also resistant to data loss due to interference from external magnetic fields. These properties make antiferromagnetic materials attractive for modern communication technologies.

Next, there is the excitonic. Excitons arise when certain conditions allow electrons to move and strongly interact with each other to form bound states. Electrons can also form bonded states with “holes,” vacancies left when electrons jump to a different position or energy level in a material. In the case of electron-electron interactions, the bonding is driven by magnetic attractions that are strong enough to overcome the repulsive force between the two like-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; , Dean said. The electrons in the material generally remain in a low energy or “ground” state. “All the electrons are stuck in place, like people in a packed amphitheater; they can’t move“, said. To get electrons moving, you have to give them an energy boost that is 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 offset 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 insulator state arises.

A collaborative team worked with a material called strontium iridium oxide (Sr3Ir2O7), which is poorly insulating at high temperatures. Daniel Mazzone, Yao Shen (Brookhaven Laboratory), Gilberto Fabbris (Argonne National Laboratory), and Jennifer Sears (Brookhaven Laboratory) used X-rays at the Advanced Photon Source, a DOE Office of Science user facility at the National Laboratory of Argonne, to measure the 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 their investigation at a high temperature and gradually cooled the material. With cooling, the energy gap gradually narrowed. At 285 Kelvin (about 53 degrees Fahrenheit), the electrons began to jump between the magnetic layers of the material, but immediately formed bonded pairs with the holes they had left behind, simultaneously causing the

Hidemaro Suwa and Christian Batista of the University of Tennessee performed calculations to develop a model using the concept of the predicted antiferromagnetic exciton insulator and showed that this model comprehensively explains the experimental results.

“Using X-rays, we see that the bonding caused by the attraction between electrons and holes actually returns more energy than when the electron jumps over the band gap,” Yao Shen explained. “Because this process saves energy, all the electrons want to do this. Then, after all the electrons have made the transition, the material looks different from the high-temperature state in terms of the The new configuration implies that the electron spins are ordered in an antiferromagnetic pattern, while the bonded pairs create a ‘locked’ isolation state”.

The identification of the antiferromagnetic exciton insulator completes a long journey that explores the fascinating ways electrons choose to organize themselves in materials. In the future, understanding the connections between spin and charge in such materials could have potential for realizing new technologies.