Hidden and unexpected quantum behavior discovered in simple iron iodide material

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According to quantum research, some simpler materials may already have advanced properties that scientists couldn’t see.

Recently, such advanced and unexpected quantum behavior has been discovered in a simple iron iodide (FeI2) material. The study by scientists at Georgia Tech and the University of Tennessee-Knoxville reported hidden quantum fluctuations using a combination of neutron scattering experiments and theoretical physics calculations at Oak Ridge National Laboratory (ORNL) of the Department of Energy (DOE).

The material iron iodide was discovered in 1929. It was the subject of intensive studies during the 1970s and 1980s: scientists observed peculiarities or unconventional patterns of behavior. At that time, they didn’t have the resources to understand why they were seeing him fully.

Scientists in this study were aware that there was something unresolved that was strange and interesting about this material. Therefore, they decided to reconsider this issue and hoped to provide new information.

Martin Mourigal, professor of physics at Georgia Tech, said: “For a long time our quest in quantum materials has been to find exotic phases, but the question we asked ourselves in this research is, ‘Maybe the phase itself is not exotic, but what if its excitations are they? And indeed, this is what we found.

The neutron has a magnetic moment which couples to spatial variations in magnetization on the atomic scale. Neutrons are therefore perfectly suited to the study of magnetic structures and of fluctuations and excitations of spin systems.

When the scientists exposed the iron iodide material to a neutron beam, they saw not one but two different quantum fluctuations emanating simultaneously. At first, scientists expected to see a single excitation or band of energy associated with a magnetic moment from a single electron. Yet they were amazed to see two different quantum fluctuations that allowed them to see the hidden fluctuations very clearly.

Xiaojian Bai, the first author of the article, said: “We could measure its full spectrum of excitement, but we still didn’t understand why we were seeing such abnormal behavior in a classical phase.”

To seek answers, the scientists collaborated with theoretical physicist Cristian Batista, Lincoln Professor at the University of Tennessee-Knoxville and deputy director of ORNL’s Shull Wollan Center. The collaboration modeled the behavior of the mysterious quantum fluctuation. After carrying out additional neutron experiments using the CORELLI and SEQUOIA instruments at SNS, they were able to identify the mechanism behind its appearance.

Batista said, “What the theory predicted and what we were able to confirm with neutrons is that this exotic fluctuation occurs when the direction of spin between two electrons is reversed and their magnetic moments tilt in opposite directions. When the neutrons interact with the spins of the electrons, the spins rotate in synchronicity along a certain direction in space. This choreography, triggered by the scattering of neutrons, creates a spin wave.

“In different materials, the electronic spins can take on many different spin orientations and choreographies that create different types of spin waves. In quantum mechanics, this concept is known as “wave-particle duality”, in which new waves are considered new particles and are usually hidden from neutron scattering under normal conditions.

“In a sense, we’re looking for dark particles. We can’t see them, but we know they’re there because we can see their effects or the interactions they have with the particles that we can see.

Bai said, “In quantum mechanics, there is no distinction between waves and particles. We understand the behavior of the particle as a function of wavelength, and this is what neutrons allow us to measure.

“Now that we understand how this exotic behavior works in a relatively simple material, we can imagine what we might find in more complicated materials. This new understanding has motivated us and hopefully will encourage the scientific community to further study this type of material, which will undoubtedly lead to more exciting physics. “

Journal reference:
  1. Xiaojian Bai et al., Hybrid quadrupole excitations in the frustrated spin-anisotropic magnet FeI2, Nature Physics (2021). DOI: 10.1038 / s41567-020-01110-1


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